Abstract

New 40Ar/39Ar dates from the Jemez Mountain volcanic field (JMVF) reveal formerly unrecognized shifts in the loci of pre-caldera volcanic centers across the northern Jemez Mountains; these shifts are interpreted to coincide with episodes of Rio Grande rift faulting. Early activity in the field includes two eruptive pulses: 10.8–9.2 Ma basaltic to dacitic volcanism on Lobato Mesa in the northeastern JMVF and 12–9 Ma mafic to silicic volcanism in the southwestern JMVF. While 9–7 Ma eruptions persisted in the southern JMVF, a new eruptive center developed on the La Grulla Plateau in the northwestern JMVF (8.7–7.2 Ma), corresponding with a period of rift widening caused by reactivation of Laramide faults in this area. The older 8.7–7.8 Ma mafic lavas emitted from Encino Point and the younger 7.7–7.2 Ma trachyandesite and dacite erupted on the La Grulla Plateau are assigned to a new unit called the La Grulla Formation. The chemical composition of a 640 m stack of lava flows exposed in the northern margin of the Valles caldera changes from dacite to andesite, then back to dacite upsection, becoming slightly more alkalic upward. The shift to more alkalic compositions occurs across a sedimentary break, marking a subtle change in magma source for the older Paliza Canyon Formation and the younger La Grulla Formation lavas. New age constraints from a rhyolite intrusion in the southern JMVF and pumiceous rhyolite deposits in the northern JMVF suggest an episode of localized, 7.6–7.8 Ma rhyolitic volcanism that occurred in the central part of the JMVF between 12–8 Ma Canovas Canyon Rhyolite and 7–6 Ma peak Bearhead Rhyolite volcanism. Younger Bearhead Rhyolite intrusions (7.1–6.5 Ma) are more widespread than previously documented, extending into the northeastern JMVF. Tschicoma Formation dacite erupted at 5 Ma in the Sierra de los Valles and then erupted throughout the northeastern JMVF 5–2 Ma. The more refined geochronology of the JMVF indicates that pre-caldera volcanic centers were characterized by geographically and chemically distinct, relatively short-lived, episodes of activity. Volcanism generally migrated eastward through time in the southern JMVF, but the pattern in the northern JMVF had a more complex east (10–9 Ma) to west (9–7 Ma) to east (5–2 Ma) pattern that reflects the timing of motion on faults. The new ages, coupled with detailed mapping of both volcanic rocks and the Santa Fe Group, document significant pulses of faulting, erosion, and deposition during middle Miocene time and during late Miocene time across the Cañones fault zone in the northern JMVF.

INTRODUCTION

The Jemez Mountains volcanic field (JMVF) in north-central New Mexico (Fig. 1) is the site of the Valles caldera, the type example of a continental resurgent caldera (Smith and Bailey, 1968). More than 20 million years of volcanic and associated hydrothermal activity in the JMVF on the western margin of the Rio Grande rift preceded caldera formation. The volcanic field has been the subject of intense geologic study, as well as geothermal and mineral exploration, for several decades. Recently, the JMVF was mapped at a 1:24,000 scale between 1998 and 2006 as part of the STATEMAP program to create updated versions of the classic Smith et al. (1970) geologic map of the area (e.g., Goff et al., 2011). During the course of this mapping, a number of previously unrecognized volcanic units and stratigraphic relationships were found. A program of 40Ar/39Ar dating accompanied the mapping effort. Sampling for geochronology was primarily focused in the northern Jemez Mountains, a portion of the JMVF that has not received as much attention as the eastern and southern sections. In this paper, we combine the new 40Ar/39Ar dates on volcanic rocks with field observations and some recently acquired geochemical data to highlight important insights into the development of this much-studied volcanic field.

The general history of volcanism in the Jemez Mountains region is well established (e.g., Doell et al., 1968; Bailey et al., 1969; Smith et al., 1970; Gardner et al., 1986). Basaltic eruptions began at ca. 25 Ma in the southeastern part of the Jemez Mountains, with sporadic basaltic activity continuing between 21 and 11 Ma (WoldeGabriel et al., 2006, 2007). Significant eruptions of basaltic and rhyolitic lavas and rhyolitic tuffs in the southern mountains and basaltic to dacitic lavas in the northeastern mountains began at ca.10 Ma (Bailey et al., 1969; Gardner et al., 1986). Intermediate to felsic composition eruptions in the southern, central, northwestern, and northeastern parts of the field continued after 10 Ma until caldera-forming eruptions occurred at 1.61 Ma (Izett and Obradovich, 1994) and 1.25 Ma (Phillips et al., 2007) in the middle of the field. Subsequent rhyolitic eruptions were concentrated within the Valles caldera, first on the resurgent dome and then along the ring fracture zone (Gardner et al., 2010). The youngest unit in the JMVF is the ca. 37–45 ka Banco Bonito flow (Ogoh et al., 1993; Phillips et al., 1997), which was erupted along the southwestern ring fracture of the caldera.

Bailey et al. (1969) and Smith et al. (1970), using K-Ar geochronologic data available at the time, divided the pre-caldera volcanic rocks into two groups, the older Keres Group rocks located mainly in the southern Jemez Mountains and the slightly younger Polvadera Group rocks of the northern Jemez Mountains. Bailey et al. (1969) further subdivided the Polvadera Group in the northern JMVF into an older unit called Lobato Basalt and younger units called the Tschicoma Formation and the El Rechuelos Rhyolite. Smith et al. (1970) mapped Lobato Basalt (later changed to Lobato Formation by Goff et al., 1989), which is predominantly composed of basalt flows, on Lobato Mesa, Clara Peak, and Cerro Roman to the east, on the La Grulla Plateau to the west, and on several mesas on the northern edge of the Jemez Mountains volcanic field (e.g., Escoba Mesa and Polvadera Mesa, Fig. 2). Smith et al. (1970) mapped Tschicoma Formation, which is composed mainly of coarsely porphyritic dacite to rhyodacite, in the Sierra de los Valles east of the caldera, on the La Grulla Plateau northwest of the caldera, and in the highlands in between (Fig. 2). The El Rechuelos Rhyolite includes a young group of domes (ca. 2 Ma, Bailey et al., 1969) in the northeastern part of the field and an older set of domes (5.6–7.0 Ma; Gardner et al., 1986; Loeffler et al., 1988) in the headwaters of Cañoncito Seco (Fig. 2).

We use the new 40Ar/39Ar dates presented here, as well as other recently published 40Ar/39Ar dates (McIntosh and Quade, 1995; Smith, 2001; Justet, 2003; WoldeGabriel et al., 2006, 2007; Broxton et al., 2007), to demonstrate that the age distinction between the Polvadera Group in the northern Jemez Mountains and Keres Group in the southern Jemez Mountains is blurred (Gardner et al., 1986). The new data have been instrumental in identifying previously unrecognized temporal eruption patterns across the JMVF, particularly in the northern Jemez Mountains (Kelley et al., 2007a; Kempter et al., 2007). We document the westward migration of volcanism between 9 and 8 Ma across the northern JMVF, followed by eastward migration between 7 and 5 Ma. In this paper, we propose assigning all pre-caldera volcanic rocks in the JMVF to the Keres Group and applying the name La Grulla Formation to 7.3–8.7 Ma andesitic to dacitic centers in the northwestern Jemez Mountains previously mapped as Lobato Basalt and Tschicoma Formation by Smith et al. (1970). The Polvadera Group name is abandoned.

In addition to formulating a slightly revised volcanic stratigraphy for the JMVF, the new dates, coupled with detailed mapping of both the volcanic rocks and the underlying Santa Fe Group, are used to constrain the timing of movement of major Rio Grande rift faults crossing the volcanic field. Although the link between volcanic and fault activity in the JMVF has been discussed previously, the earlier studies focused only on the southern part of the field (Gardner et al., 1986) or a small portion of the northern field (Baldridge et al., 1994). Here we present the first comprehensive analysis of the interplay between nearly continuous eruptions in the JMVF during the past 10 Ma and faulting across the entire JMVF. These data are used to document pulses of middle Miocene and late Miocene faulting and sedimentation. We describe in some detail the temporal and spatial distribution of pre-caldera volcanic centers through the history of the JMVF and discuss the connections between the eruptive history and structural evolution of the area. Generally, basins in the Rio Grande rift have narrowed and deepened as rifting has progressed (Chapin and Cather, 1994), and such a pattern has been recognized in the southern JMVF (Gardner et al., 1986). However, the new dates record a late Miocene (7.3–8.7 Ma) episode of rift widening along reactivated Laramide faults in the northwestern Jemez Mountains. During this time frame, a break in activity prior to 7.6 Ma and a subtle shift to more alkalic compositions are recorded in the major-element geochemistry of 640 m of andesitic to dacitic lava exposed in the northern wall of the Valles caldera.

STRUCTURAL SETTING

The JMVF straddles the boundary between the mildly deformed Colorado Plateau and the basement-cored Laramide uplift of the Sierra Nacimiento to the west and the Neogene Rio Grande rift to the east (Fig. 1). The volcanic field is part of a northeast-striking alignment of <10 Ma volcanic centers that extends from east-central Arizona to southeastern Colorado, referred to as the Jemez lineament (e.g., Aldrich, 1986). This alignment of volcanic centers appears to coincide with a broad Proterozoic suture zone separating the 1.8–1.7 Ga southern Yavapai terrane from the 1.7–1.6 Ga Mazatzal terrane (Karlstrom et al., 2004). The Rio Grande rift consists of a series of en-echelon basins; in the vicinity of the Jemez Mountains, the right-stepping transition between the northern Albuquerque Basin and the Española Basin is accommodated by northeast-striking faults in the Santo Domingo Basin along the southeast margin of the volcanic field (Figs. 1 and 3; Smith et al., 2001; Chamberlin, 2007; Chamberlin and McIntosh, 2007; Smith and Lynch, 2007). The right-stepping transition between the southern San Luis Basin and the Española Basin is accommodated by the northeast-striking Embudo–Santa Clara fault system near the middle of the field (Fig. 1; Aldrich, 1986).

Major rift-bounding normal fault systems along the southern margin of the field in the northern Albuquerque Basin and the Santo Domingo Basin project northward beneath the volcanic field; from west to east, these systems are the Sierrita, Jemez, Cat Mesa, Jose, Cañada de Cochiti, Camada, and Pajarito (Fig. 3). The close relationship between volcanism and normal faulting in the field has long been recognized (e.g., Gardner et al., 1986). Eruptive centers generally are located at intersections of northeast-striking and north-striking fault systems or are aligned along fault strands (Gardner et al., 1986; Lynch et al., 2004). Lava flows commonly thicken adjacent to faults, demonstrating syneruptive displacement along these fault systems (Gardner et al., 1986; Kempter et al., 2004). Fault activity has clearly migrated eastward through time in the southern Jemez Mountains. For example, the Cat Mesa fault zone in the southwestern Jemez Mountains (Fig. 3) offsets Permian Yeso Formation down to the east ∼240 m; but ca. 9 Ma Keres Group andesite is offset <10 m, and 1.25 Ma Tshirege Member of the Bandelier Tuff is displaced <2 m (Kelley et al., 2003, 2007b). Thus, the Cat Mesa fault zone is primarily an early rift fault. In contrast, the Pajarito fault zone in the southeastern Jemez Mountains (Fig. 3) displaces Keres Group rocks by as much as 300 m (Goff et al., 1990) and 1.25 Ma Bandelier Tuff by 90 m (Lynch et al., 2004). The Pajarito fault zone in the southeastern Jemez Mountains also cuts middle Pleistocene terrace gravel (Lynch et al., 2004). Displacement of the Bandelier Tuff along the Pajarito fault is ∼200 m near Los Alamos, where Holocene displacements associated with three inferred M7 earthquakes have been documented (Lewis et al., 2009).

Significant rift-bounding structures along the northern margin of the field across the Colorado Plateau–Rio Grande rift boundary between the Laramide Chama Basin on the west and the rift-related Abiquiu embayment and Española Basin to the east include the Coyote, Largo, Gonzales, Cañones, Garcia, Cerrito Blanco, Madera Canyon, Pajarito, and Santa Clara (Fig. 3). These structures likely project southward beneath the volcanic field. Some of these faults, including the Coyote, Largo, and Cañones fault systems, were reverse faults or monoclines during Laramide deformation and have been reactivated as normal faults by rift extension (Lawrence, 1979; Smith, 1995; Kelley et al., 2005a, 2005b). Most of the faults in the northwestern JMVF are down to the east, but faults on Lobato Mesa in the northeastern JMVF are down to the west. A prominent gravity low underlies the northern Jemez Mountains south of Abiquiu (Fig. 3; Ferguson et al., 1995). Steep gravity gradients coincide with the east-down Coyote fault zone on the west, the southeast-down Gonzalez and Cañones fault zones on the northwest, and west-down faults on Lobato Mesa (Koning et al., 2007a). The gravity gradients associated with the Coyote, Cañones, and Gonzales fault zones project south under the Valles caldera toward the western border faults of the Albuquerque basin, probably as relatively continuous structures (Fig. 3; Koning et al., 2007a).

METHODS

Geochronology

The 40Ar/39Ar dates were determined at the New Mexico Geochronology Research Laboratory using methods similar to those described in McIntosh and Chapin (2004). An age of 28.02 Ma for the Fish Canyon Tuff (Renne et al., 1998) was used in age calibration and to adjust previously published dates. Many of the dates are incremental-heating plateau ages for biotite, hornblende, and plagioclase phenocrysts and for groundmass concentrates from rocks lacking datable phenocryst phases. The rhyolite tuff and ash-fall tephra dates are single-crystal laser-fusion ages on sanidine, biotite, and hornblende. Analytical uncertainties for individual samples are reported to two standard deviations (95% confidence level). Detailed procedures are described in the Supplemental File1. The new dates are summarized in Table 1, and previously published 40Ar/39Ar dates are compiled in Table 2. The dates are plotted in map form on Figure 4. Table 1 is organized temporally, from oldest to youngest, and geographically. The samples are numbered sequentially. Table 2 is also arranged temporally and geographically; sample numbers for these published data are preceded by a “P.”

The analytical age data, age spectra, quality of the analyses, and probability plots for pre-caldera rocks, which are the focus of this paper, are presented in the Supplemental File (DRIntro, Figs. DR1–DR4, and Tables DR1 and DR2 [see footnote 1]). Analytical data, age spectra, and probability plots for 15 caldera and postcaldera samples collected during mapping of the Valles Caldera National Preserve (Goff et al., 2011; Fig. 4) are also included in the Supplemental File (see footnote 1). These data are not discussed in detail in the main body of the paper but are described and interpreted in the Supplemental File (see footnote 1).

Geochemistry

Major elements and selected trace elements for seven lavas and one dike (Table 3) were analyzed by X-ray fluorescence (XRF) in the Geoanalytical Laboratory at Washington State University using the methods of Johnson et al. (1999). Rare-earth and other trace elements were analyzed in the same laboratory using an Agilent model 4500 quadrupole inductively coupled mass spectrometer (ICP-MS); method description is available at http://www.sees.wsu.edu/Geolab/note/icpms.html). In addition, one sample of pumice was analyzed using the electron microprobe at the New Mexico Institute of Mining and Technology (Table 3).

DISTRIBUTION OF VOLCANIC CENTERS THROUGH TIME IN THE JMVF

Late Oligocene to Early Miocene Mafic Activity (25–15 Ma)

Late Oligocene to early Miocene mafic lavas and intrusions in the southeastern and northwestern Jemez Mountains are associated with a widespread, long-lived record of sporadic, alkaline to tholeiitic small-volume magmatism that occurred during early extension in the Española Basin. Many of the lavas are primitive, with up to 16% MgO, and are compositionally related; these lavas are interpreted to be from a common mantle source or set of sources (Gibson et al., 1993; Wolff et al., 2000, 2005). The most well-known outcrops of these older lavas include three or four small-volume mafic lava flows that are discontinuously exposed in the southeastern JMVF east and southeast of Boundary Peak in the footwall of the Pajarito fault zone (Goff et al., 1990; Figs. 2 and 3). These flows are intercalated with volcaniclastic conglomeratic sandstone containing clasts derived from local and distal volcanic sources (Kelley et al., 2013). Typically, the mafic lavas are nephelinite to basanite that, in places, are deeply altered to a green color. Several attempts have been made to determine the age of these flows. Early efforts to date the basanite in Medio Canyon (Fig. 2) resulted in K-Ar and 40Ar/39Ar ages of 16.5 ± 1.4 Ma and 15.44 ± 0.30 Ma (Gardner et al., 1986; Justet, 2003; P6, Table 2). More recently WoldeGabriel et al. (2006) measured an age of 18.79 ± 0.64 Ma for this same basanite (P3, Table 2). In addition, WoldeGabriel et al. (2006) determined 20.95 ± 0.63 Ma and 25.63 ± 0.87 Ma 40Ar/39Ar ages on two basanites east of Boundary Peak ∼2 km north of the Medio Canyon locality (P4 and P5, Table 2). These basanites are the oldest flows in the immediate vicinity of the Jemez Mountains. Modeling of geochemical data from this area suggests that the magma that generated these lavas is chemically linked to <10 Ma basalts in the main JMVF (Wolff et al., 2000, 2005). Alternatively, the upper Oligocene to lower Miocene mafic flows in the southeastern JMVF may have come from eruptive centers in the southern and eastern Española Basin near La Cienega and Santa Fe (Smith, 2004; Myers and Smith, 2006).

During this study, early Miocene mafic intrusive rocks were identified west of the village of Abiquiu in the northern and northwestern Jemez Mountains near the Cañones fault zone (Figs. 2 and 4). One dike and associated plug on the west side of Arroyo de Frijoles (AdlF, Fig. 2) intrudes a north-striking fault splay east of the main Cañones fault zone, cutting the upper Oligocene to lower Miocene Abiquiu Formation (Kelley et al., 2005b). Dated at 19.72 ± 0.33 Ma (2, Table 1), this dike is not deformed by subsequent faulting. Related dikes near Abiquiu ∼3.2 km north and 4 km east, respectively, of the Arroyo de Frijoles dike were dated by Maldonado and Miggins (2007); these authors determined that a north-striking dike in nearby Red Wash Canyon (RWC, Fig. 2; P2, Table 2) has a 40Ar/39Ar age of 19.63 ± 0.40 Ma and the Cerrito de la Ventana (CdlV, Fig. 2; P1, Table 2) dike near Abiquiu has a 40Ar/39Ar age of 19.22 ± 0.30 Ma. The similarity of age and phenocryst content implies a genetic relationship among the three dike splays. Another early Miocene intrusion is exposed for ∼1 km along the east side of Encino Point (EP, Fig. 2) within the Cañones fault zone (Fig. 3). Splays of the Cañones fault zone cut this equigranular to porphyritic intrusion, which has a sill-like geometry. This intrusion was dated twice (3, Table 1) because the 40Ar/39Ar age results of 18.94 ± 0.33–19.00 ± 0.35 Ma were unexpectedly old in this area where 7.0–8.5 Ma ages prevail. The 60–90-m-thick intrusion is within the Chama–El Rito Member of the Tesuque Formation of the Santa Fe Group (Fig. 5; Lawrence et al., 2004). The Abiquiu area and Encino Point dikes and sills are likely related to middle Miocene volcanic centers of similar age (ca.14–26 Ma) in the northern Española Basin near El Rito and Ojo Caliente (Fig. 3; Baldridge et al., 1980; May, 1980, 1984; Manley and Mehnert, 1981; Ekas et al., 1984; Gibson et al., 1993; Koning et al., 2011).

Middle to Late Miocene Activity (14–9 Ma)

Small volumes of lava or tuff of mafic to dacitic to rhyolitic composition erupted from three distinct areas in the Jemez Mountains between 14 and 9.7 Ma. First, 14–11 Ma mafic lava flows are preserved in the vicinity of Santa Clara Canyon and in drill holes on the Pajarito Plateau on the northeast side of the JMVF (Fig 2; Aldrich, 1986; WoldeGabriel et al., 2006). Second, a buried 11–13 Ma dacitic center may underlie younger volcanic rocks of the northeastern JMVF (Koning et al., 2007b). Westward increase in the thickness of ash beds in the Chamita Formation of the Santa Fe Group (Fig. 5) near Española, the distribution of dacitic lag gravel beneath the ca. 9–10 Ma Lobato Formation, and tuffaceous volcaniclastic sediments below a 10.16 ± 0.06 Ma Lobato Formation flow in Santa Clara Canyon (P62, WoldeGabriel et al., 2006, 2007) all point to a now-buried dacitic center. Third, recent 40Ar/39Ar dating of Canovas Canyon rhyolites in the southwestern Jemez Mountains suggests activity began there at ca. 12 Ma (Padmore and Spell, 2008). Although a previous K-Ar date for Canovas Canyon rhyolite lava in Sanchez Canyon (Fig. 2) suggested ca. 12 Ma activity in the southeastern JMVF, more recent 40Ar/39Ar dating on the same outcrop yielded a 9.69 ± 0.12 Ma age (P15, WoldeGabriel et al., 2006).

Following 14–9.7 Ma localized eruptions in the northeastern and southwestern quadrants of the Jemez Mountains, more voluminous eruptions in the JMVF started at ca. 10 Ma. Early activity was focused in the southwestern and southern Jemez Mountains between Borrego Mesa and Boundary Peak (Paliza Canyon Formation and Canovas Canyon Rhyolite) and in the northeastern Jemez Mountains on Lobato Mesa (Lobato Formation; Figs. 2 and 5). The next sections discuss the 9–10 Ma volcanism in detail.

Southern Jemez Mountains

The oldest succession of mafic flows in the southwestern Jemez Mountains is exposed on Chamisa Mesa and on Borrego Mesa (Fig. 2). The basalt of Chamisa Mesa (Bailey et al., 1969) consists of sparsely porphyritic Paliza Canyon Formation basalt flows with a distinctive ophitic texture. The flows overlie the Miocene eolian and fluvial Chamisa Mesa Member of the Zia Formation or a conglomeratic sandstone with Proterozoic pebbles that is likely correlative to Cerro Conejos Formation of the Santa Fe Group (Fig. 5). The basalt of Chamisa Mesa consists of extensive flows that are present as far east as Borrego Dome, as far south as the south end of Borrego Mesa, and as far north as Paliza Canyon (Fig. 2). A flow that is texturally similar to the basalt of Chamisa Mesa sits on Abiquiu Formation in Church Canyon northeast of Jemez Springs (Kelley et al., 2003; Fig. 2). The basalt of Chamisa Mesa has been dated using both K-Ar and 40Ar/39Ar methods, and the results vary over a fairly wide range of 9.02 ± 0.78 to 13.2 ± 1.2 Ma (Luedke and Smith, 1978; Gardner et al., 1986; Chamberlin et al., 1999; Justet, 2003; Chamberlin and McIntosh, 2007; e.g., P7–P9, Table 2). Chamberlin and McIntosh (2007) determined an average 40Ar/39Ar age of 9.9 ± 0.9 Ma for the basalt of Chamisa Mesa. The basalt of Chamisa Mesa samples analyzed during this study are from the middle of a stack of flows on Borrego Mesa sitting above Zia Formation near the Jose fault zone (4, Table 1; Fig. 4), and from the lowest flow in the mesa escarpment south of the Jose fault (5, Table 1). The 40Ar/39Ar ages of these two samples are 9.45 ± 0.22 Ma and 9.39 ± 0.31 Ma, respectively. A basalt flow in Paliza Canyon that probably correlates with the basalt of Chamisa Mesa, dated by K/Ar at 13.2 ± 1.2 (Gardner et al., 1986), gave a groundmass 40Ar/39Ar age of 9.43 ± 0.14 Ma (6, Table 1). Another Paliza Canyon Formation basalt flow exposed on the east side of San Juan Canyon just southwest of Cerro del Pino that overlies the Abiquiu Formation (CdP, Fig. 2) gave a groundmass age of 9.45 ± 0.07 Ma (8, Table 1).

Canovas Canyon rhyolitic tuffs and intercalated volcaniclastic and eolian sediment generally lie above the basalt of Chamisa Mesa; however, one Canovas Canyon ash-fall bed does crop out below basalt sample 4 (Table 1) on Borrego Mesa. One of the tuffs above the basalt of Chamisa Mesa, an ignimbrite with ∼10% lithic fragments of flow-banded rhyolite and obsidian, is similar to tuffs between basalt flows in Paliza Canyon (9, Table 1), in upper Peralta Canyon, and beneath Boundary Peak (Gardner et al., 1986; Goff et al., 1990; Goff et al., 2005a). The tuff in Paliza Canyon yielded a hornblende 40Ar/39Ar age of 9.22 ± 0.36 Ma (9, Table 1), within error of the ages of the basalt flows just below and above it (9.43 ± 0.14 and 9.37 ± 0.12 Ma; 6 and 7, Table 1) and the ages of the dated basalts on Borrego Mesa.

A Canovas Canyon plug intruding Zia Formation and an associated flow of dacitic to rhyolitic composition form a prominent knob east of the village of Ponderosa (PD, Fig. 2). The dominant lithology is a crystal-rich, biotite-hornblende dacite with no quartz that grades into crystal-poor rhyolite with small quartz phenocrysts. A chilled glassy margin is present at the base of the rhyolite flow on the east side of the knob. Two 40Ar/39Ar ages of 9.49 ± 0.18 and 9.47 ± 0.13 Ma were determined for the intrusion (10b, Table 1) and glassy margin (10a, Table 1), respectively. The locus of felsic Canovas Canyon eruptive activity in the southeastern Jemez Mountains is located at Bear Springs Peak and in the area south and west of Tres Cerros (TC, Fig. 2). A dacite vent cluster that trends SW-NE is located near Cerrito Yelo (Fig. 2; Kempter et al., 2003). Borrego Dome to the southwest of Bear Springs Peak is another significant Canovas Canyon center. Several Canovas Canyon rhyolite domes in the southern Jemez Mountains have been dated at 12.4–8.2 Ma by Padmore and Spell (2008), with the ages clustering in the 9.9 to 9.2 Ma range. Justet (2003) determined an age of 9.36 ± 0.05 Ma for Borrego Dome (P14, Table 2). We determined that a rhyolite flow from the Bear Springs Peak center exposed in Canovas Canyon has a biotite age of 9.79 ± 0.09 Ma (12, Table 1), compared to 8.70 ± 0.35 Ma for a flow on the northwest side of the Bear Springs Peak center (P13, Table 2) and 8.05 ± 0.60 Ma for a flow on the southeast side of the peak (P19) determined by Justet (2003). A thick, crystal-poor rhyolite flow from a center west of Tres Cerros is 9.54 ± 0.16 Ma (13, Table 1).

The Canovas Canyon Rhyolite on Borrego Mesa is overlain by an olivine basalt in the Paliza Canyon Formation that contains up to 6% olivine altered to iddingsite. The basalt has a distinctive red speckled appearance on weathered surfaces and is aerially extensive, covering an area from Church Canyon near Jemez Springs to Bodega Butte on Zia Pueblo (Fig. 2). This unit likely originated from a north-trending line of vents on the west flank of Bear Springs Peak (Kempter et al., 2003). The olivine basalt also caps a series of buttes and small mesas to the south and was informally named the basalt of Bodega Butte by Chamberlin and McIntosh (2007). Chamberlin and McIntosh (2007) determined an average age of 9.14 ± 0.12 Ma from four samples of the basalt of Bodega Butte collected on the Loma Creston quadrangle (P41–P44, Table 2). The groundmass 40Ar/39Ar date from a sample of olivine basalt collected for this study at the confluence of Hondo and West Fork Canyons is 9.11 ± 0.13 Ma (14, Table 1). A stack of at least four 1–2-m-thick, south-southwest–dipping trachyandesite and trachybasalt flows intercalated with volcaniclastic sediment and rhyolitic tephra are exposed in Hondo Canyon below the basalt of Bodega Butte. The oldest lava in this stack is 9.58 ± 0.08 Ma (15, Table 1), and a wide E-striking Canovas Canyon Rhyolite intrusion cuts the basal part of this section.

Most of the 9–10 Ma volcanic rocks in the southern Jemez Mountains are basalt, basaltic trachyandesite, andesite, trachyandesite, or rhyolite in composition (Rowe et al., 2007), but units with dacitic compositions also erupted during this time frame, especially in upper La Jara Canyon (Kempter et al., 2003). Justet (2003) determined 40Ar/39Ar ages of 8.96 ± 0.06 Ma and 9.16 ± 0.07 Ma (P20 and P30, Table 2) for an aerially extensive Paliza Canyon Formation trachyandesite flow between Paliza Canyon and Hondo Canyon that caps southwest-dipping Paliza Canyon Formation basalt flows and intercalated volcaniclastic sediments. This flow thickens dramatically on the downthrown side of a north-striking fault on the NW side of Guacamalla Canyon (Fig. 2), indicating ca. 9 Ma motion on this particular fault (Kempter et al., 2003). A porphyritic biotite-hornblende dacite that flowed east from the Cerro del Pino center (CdP, Fig. 2) has a 40Ar/39Ar age of 9.48 ± 0.22 Ma (P18, Table 2, Justet, 2003).

The Bland monzonite stock in the southern JMVF stock is notable because this stock is the only exposure of a pluton that sourced early JMVF lavas. The Bland stock, which is discernable through a combination of structural uplift in a horst block and erosion, was originally assigned an Eocene age (Smith et al., 1970). Stein (1983) later determined a K-Ar age of 11.3 ± 0.3 Ma from the monzonite; however, subsequent attempts to date the intrusive using K-Ar and 40Ar/39Ar methods have yielded complex results. Multiple intrusive events and pervasive hydrothermal alteration, particularly during emplacement of 6.5–7.2 Ma Bearhead Rhyolite, have affected most of the volcanic rocks in this part of the Jemez Mountains (WoldeGabriel and Goff, 1989; Goff et al., 2005b). Despite the fact that the monzonite is challenging to date, field relationships clearly demonstrate that the stock is younger than older successions in the Paliza Canyon Formation (Goff et al., 2005b). Basalt, trachyandesite, and volcaniclastic rocks (oldest to youngest) that belong to the Paliza Canyon Formation are intruded by monzonite and quartz monzonite of the Bland stock in Colle Canyon (Fig. 2; Goff et al., 2005b).

Northeastern Jemez Mountains

The oldest voluminous lavas erupted in the northeastern Jemez Mountains belong to the Lobato Formation, which is well exposed on Lobato Mesa. Eruptions in the northeastern Jemez Mountains were primarily basaltic with minor trachyandesite and dacite (Bailey et al., 1969; Goff et al., 1989; Justet, 2003; Wolff et al., 2005), in contrast to the broad spectrum of felsic to mafic eruptions that occurred during the same time frame in the southern Jemez Mountains. The 9–10 Ma volcanic deposits in the southwestern JMVF contain significant amounts of volcaniclastic sediments interbedded with the lavas, whereas the Lobato Formation contains very little sediment. The few <1-m-thick deposits intercalated with the Lobato Formation are hydromagmatic or eolian in origin. The Lobato basalts were erupted from broad shield volcanoes (Baldridge in Goff et al., 1989). The dacite of the Lobato Formation is variably crystal-rich porphyritic to fine-grained aphyric lava. The crystal-rich dacite exposed along the Rio del Oso and on the north side of Los Cerritos along Forest Road 144 (LCe, Fig. 2) contains plagioclase laths 1–2 cm across and quartz phenocrysts, superficially resembling lavas of the younger Tschicoma Formation (Bailey et al., 1969; Goff et al., 1989); however, the Rio del Oso flows are chemically and chronologically more similar to Lobato Formation (Rowe et al., 2007; Kelley et al., 2007a). We determined a biotite fusion 40Ar/39Ar age of 10.45 ± 0.05 Ma (20, Table 1) for coarsely porphyritic dacite in the Lobato Formation in Rio del Oso and a 9.80 ± 0.15 Ma (19, Table 1) groundmass age for a dacite flow that emanated from Los Cerritos on the southern end of the Lobato Mesa. Groundmass ages of 9.57 ± 0.07 Ma (21, Table 1) and 9.73 ± 0.21 Ma (22, Table 1) for basalt flows and dikes in the vicinity of Rio del Oso confirm earlier observations that the bulk of the Lobato Formation erupted during a short time interval between 9.2 ± 0.2 and 10.8 ± 0.3 Ma (Gardner, 1985; Goff et al., 1989). Justet (2003) determined ages of 8.82 ± 0.19–10.59 ± 0.10 Ma for Lobato Formation basalts and dacites (P52–P57, Table 2). Outcrops of basalt flows in Arroyo de los Frijoles (23, Table 1; 10.08 ± 0.16 Ma) and in the vicinity of Abiquiu (P58, Table 2; 9.51 ± 0.21 Ma; Maldonado and Miggins, 2007) mark the northwestern and northern extent of the Lobato Formation. WoldeGabriel et al. (2006) assign the less voluminous older (11–14 Ma) basaltic flows interbedded with Chama–El Rito Formation exposed in Santa Clara Canyon to the Lobato Formation based on geochemical similarities (Aldrich and Dethier, 1990).

Miocene basalts, andesites, and dacites that were erupted between 9.5 and 13.2 Ma are present in the subsurface and in canyon bottoms of the northern Pajarito Plateau. A dacite (Samuels et al., 2007) outcrop located in the bottom of Guaje Canyon has a groundmass age of 9.46 ± 0.07 Ma (24, Table 1), and an 11.25 ± 0.13 Ma (P66, Table 2) andesite is exposed at Pine Springs in Garcia Canyon (WoldeGabriel et al., 2006). The Guaje Canyon exposure can be tied temporally and chemically to the Lobato Formation, but the affinity of the Pine Springs flow is less certain. WoldeGabriel et al. (2006) note that the 11.62 ± 0.09 Ma to 13.30 ± 0.22 Ma basalt and basaltic andesite flows encountered in drill holes in Guaje Canyon (P195–P204, Table 2; Fig. 2) on the Pajarito Plateau are chemically distinct from Lobato Formation mafic flows.

Late Miocene Activity (9–7 Ma)

Volcanic activity in the southern Jemez Mountains shifted toward the southeast between 9 and 7 Ma. In contrast, volcanic activity in the northern Jemez Mountains shifted toward the northwest during this same time frame. Andesitic to dacitic composition lavas were erupted from multiple centers in the southern and central Jemez Mountains (Paliza Canyon Formation) and from centers on the La Grulla Plateau (lavas of Encino Point and La Grulla Plateau) between 9 and 7 Ma. Miocene mafic flows in the subsurface beneath the Pajarito Plateau between Bayo Canyon on the north to Ancho Canyon on the south (Fig. 2) range in age from 8.4 to 9.3 Ma, and a trachyandesite flow in upper Los Alamos Canyon is 8.71 ± 0.1 Ma (Broxton and Vaniman, 2005; WoldeGabriel et al., 2006; Broxton et al., 2007).

Southern and Central Jemez Mountains

The younger porphyritic to fine-grained trachyandesites and dacites of the Paliza Canyon Formation in the southern Jemez Mountains were not extensively sampled during this study because these rocks have been the focus of previous work (Gardner, 1985; Gardner et al., 1986; Goff et al., 1990; Justet, 2003). Justet (2003) sampled the extensive flows of black porphyritic trachyandesite along the crests of Cerro Pelado and Peralta Ridge that were probably vented in the vicinity of Cerro Pelado (CPel, Fig. 2; Goff et al., 2005a). Two samples from the ridge crests have 40Ar/39Ar dates of 8.83 ± 0.14 (P28, Table 2) and 8.90 ± 0.17 Ma (P22, Table 2); platy basaltic trachyandesite midway down west flank of Cerro Pelado is dated at 9.50 ± 0.21 Ma (P29, Table 2, Justet, 2003).

We obtained new 40Ar/39Ar hornblende ages of 8.53 ± 0.63 Ma (16, Table 1) for a porphyritic hornblende dacite flow capping the summits of Las Conchas and Los Griegos (LC and LG, Fig. 2) and 8.66 ± 0.22 Ma (17, Table 1) for a porphyritic hornblende-biotite dacite dome and flow south of Rabbit Mountain (RM, Figs. 2 and 4). Justet (2003) determined a 40Ar/39Ar age of 7.91 ± 0.14 Ma for a hornblende dacite dome at the head of Sanchez Canyon (P26, Table 2). A dacitic ash bed in the Pojoaque Member of the Santa Fe Group (Koning and Maldonado, 2001) in Ancho Canyon on the southeast side of the Jemez Mountains that probably was derived from a Paliza Canyon Formation dacite dome yielded an age of 8.48 ± 0.14 Ma (18, Table 1). We also acquired a relatively young 40Ar/39Ar age of 7.20 ± 0.68 Ma for a porphyritic biotite-dacite flow capping a hill east of Aspen Ridge, but the latter date may be affected by hydrothermal alteration (because of the large error, this data is not reported in Table 1). These dates lie within the typical range of 7 to 10 Ma for the Paliza Canyon Formation in the southern Jemez Mountains.

Western Jemez Mountains

Paliza Canyon Formation basaltic andesite and trachyandesitic lava flows filling paleovalleys cut into the Ojo Caliente Sandstone of the Tesuque Formation are preserved on Fenton Hill on the western topographic rim of the Valles caldera (Fig. 5). Four samples of basaltic andesite, two from outcrops and two clasts from the overlying volcaniclastic deposit, gave consistent groundmass 40Ar/39Ar ages of 8.98 ± 0.28–9.00 ± 0.13 Ma (26–29, Table 1). A basalt dike that presumably was the source vent for some of the mafic flows is exposed along Forest Road 376 south of San Antonio Hot Spring (Kelley et al., 2004). Porphyritic trachyandesite that lies above the basaltic andesite and is intercalated with volcaniclastic sediments has a plagioclase 40Ar/39Ar age of 8.26 ± 0.09 Ma (30, Table 1).

A biotite-hornblende dacite flow that has been assigned to the Tschicoma Formation (Smith et al., 1970) rests on and is inset against the Paliza Canyon Formation on the west side of the caldera because the dacite fills a paleovalley cut into the older flows. This paleovalley, which roughly parallels modern San Antonio Creek, has been backfilled by lava and tuff and subsequently excavated many times (Kelley et al., 2004). The vent for the dacite is not exposed and may have collapsed into the caldera. The flow was dated previously by Gardner et al. (1986; K-Ar, 4.21 ± 1.3 Ma) and Justet (2003; P229, 3.86 ± 0.08 Ma). A biotite age of 4.36 ± 0.07 Ma (31, Table 1) was determined for the dacite during this investigation.

Northwestern Jemez Mountains

A 7.3–8.7 Ma intermediate composition volcanic highland forms the La Grulla Plateau in the northwestern to north-central Jemez Mountains. The Cañones fault zone, one of the western border faults of the Rio Grande rift (Figs. 3 and 6), controlled the location of volcanic vents in the northwestern Jemez Mountains, and the fault zone continued to be active after the older volcanic units were emplaced. As mentioned earlier, Smith et al. (1970) assigned the older, more mafic volcanic rocks in the northwestern Jemez Mountains to the Lobato Basalt and the younger, more silica-rich rocks to the Tschicoma Formation.

Encino Point. Encino Point is located on the northern tip of the La Grulla Plateau (Figs. 2 and 6). As many as eight andesite to basaltic-andesite lava flows are exposed in the escarpment on the west side of Encino Point (Lawrence, 1979; Singer, 1985; Singer and Kudo, 1986). A deposit preserving a transition from a phreatomagmatic to a strombolian style of eruption is present at the base of the flow sequence above the Ojo Caliente Sandstone at the north end of the mesa (Kelley et al., 2007c). The flows were likely derived from an eroded volcanic center that occupied the low-lying area known as Banco Largo to the west of Encino Point (Fig. 6). Rubbly debris to the west of Encino Point was originally mapped as a landslide deposit by Smith et al. (1970); however, intact deposits of interbedded andesite lava and andesitic pyroclastic material, as well as northeast-striking andesitic to dacitic dikes, were mapped by Lawrence (1979), Singer (1985), Lawrence et al. (2004), and Kelley et al. (2005a), and are interpreted to be the remnants of an eroded cone. One andesitic dike yields groundmass 40Ar/39Ar ages of 7.99 ± 0.10 and 7.94 ± 0.10 Ma (32, Table 1). The western escarpment at Encino Point exposes a dacite cryptodome (first recognized by Singer, 1985) and overlying andesite flows, flow breccia, and pyroclastic beds that were deformed as the dome was emplaced. The margin of the dome is glassy, and the core has an equigranular crystalline texture. Singer (1985) determined a K-Ar age of 7.85 ± 0.22 Ma for an andesite flow overlying the south side of the dome. Thus, the Encino Point volcanic center had at least two episodes of activity—one andesitic phase ca. 7.8–8.0 Ma and a younger dacitic phase of uncertain age.

Basalt and andesite flows that cap Cerro Pedernal, Mesa Escoba, and Polvadera Mesa to the east of Encino Point have previously been assigned to the Lobato Formation (Smith et al., 1970), but the origin of the flows has never been discussed. Based on available age and geochemical data (Singer, 1985; Justet, 2003; Rowe et al., 2007), the flows likely originated from Encino Point. The flows preserved on these mesas were erupted within a short time interval between 7.7 and 7.9 Ma, based on K-Ar ages of Manley and Mehnert (1981) and 40Ar/39Ar ages (P67–P68, Table 2) of Justet (2003) and Maldonado and Miggins (2007). The mesa-capping basalt ages are within the age range of flows and dikes at Encino Point. Previous workers (Manley and Mehnert, 1981; Baldridge et al., 1994; and Gonzalez, 1995) have documented ∼670 m of down-to-the-east faulting of the 7.7–7.9 Ma lava flows across the Cañones fault zone.

Flows on Mesa Escoba and Polvadera Mesa (Figs. 2 and 7) were sampled to clarify the prior results. A previously undocumented tephra fall bed is poorly exposed between the mafic lava flows (between CM21 and CM23 on Polvadera Mesa on Fig. 7). In addition, we mapped the thickness of synrift Santa Fe Group sediments between the top of the upper Oligocene to lower Miocene Abiquiu Formation and Miocene gravels preserved on top of the lava flows. The mesa-capping basalts dated during this study range from 7.74 ± 0.21 to 8.33 ± 0.11 Ma (33–38, Table 1). Pumice from the interbedded tephra within the succession on Polvadera Mesa, which contains phenocrysts of biotite and hornblende, is low-silica rhyolite (72.7% SiO2; Table 3) and is 7.7–7.9 Ma, based on the ages of the basalt flows above and below. This age is older than most Bearhead Rhyolite ages (see below), and Bearhead Rhyolite is generally a high-silica rhyolite; thus, the source of this ash-fall tephra is unknown.

Sandy conglomerate that includes rounded clasts of andesite, dacite, and Amalia Tuff underlies the 7–8 Ma basalt and overlies the Ojo Caliente Sandstone on Mesa del Medio on the east side of Cañones Canyon. One of the andesite clasts gives a 40Ar/39Ar age of 29.29 ± 0.50 Ma (1, Table 1), suggestive of a source in the San Juan volcanic field to the north (Fig. 1). The age of the clast is older than the oldest ages of 22.7–28.5 Ma volcanic rocks in the Latir volcanic field to the northeast (Fig. 1; Zimmerer and McIntosh, 2012). Consequently, the presence of clasts with a San Juan source suggests these gravels most likely belong to the Hernandez Member of the Chamita Formation, an ancestral Rio Chama deposit (Koning et al., 2005).

Detailed mapping of the Santa Fe Group below the basalts has provided new insights into the evolution of the western margin of the rift in this area. Santa Fe Group sediments thicken dramatically across the Cañones fault zone from ∼90 m on Cerro Pedernal to >300 m on Polvadera Mesa (Fig. 7). Thickening of the 19–13.5 Ma Tesuque Formation, which includes, from oldest to youngest, the fluvial Chama–El Rito Member, the eolian Ojo Caliente Sandstone Member, and an unnamed fluvial unit, is particularly pronounced (Kelley and Koning, 2007; Koning et al., 2007b). For example, the Ojo Caliente Sandstone Member appears to be absent on Cerro Pedernal, but is on average ∼120 m thick on Mesa Escoba in the hanging wall of the Gonzales fault, a splay in the Cañones fault zone (Figs. 6 and 7). The 7–8 Ma basalt and andesite lava flows rest on the Chama–El Rito Member on Cerro Pedernal, on the Ojo Caliente Sandstone and younger fluvial Hernandez Member (Chamita Formation; 13.5–7.5 Ma) on Mesa Escoba, and solely on Hernandez Member farther east on Polvadera Mesa (Fig. 7). The unnamed fluvial unit is only found on Polvadera Mesa. The ca. 7.8 Ma lava flow on Mesa Escoba is capped by Hernandez Member gravel. Thus prior to eruption of the lava, the Ojo Caliente Member was completely eroded from Cerro Pedernal and was partially stripped from Mesa Escoba. Some of the eolian sand from the Ojo Caliente was redeposited as a local fluvial unit now preserved on Polvadera Mesa. Consequently, some of the erosion and associated faulting had to take place prior to the deposition of the 13.5–7.5 Ma Hernandez Formation during middle Miocene time. Some of the ancestral Rio Chama deposition recorded by the Hernandez Formation overlapped in time with eruption of lavas from Encino Point that flowed into a low spot in the area now occupied by Cerro Pedernal, Mesa Escoba, and Polvadera Mesa. The lava flows were then disrupted by faulting sometime between 7.8 and 3 Ma (Baldridge et al., 1994; Gonzalez, 1995).

Another basalt flow (34, Table 1; 7.79 ± 0.03 Ma) lies in the bottom of Cañones Canyon between Mesa Escoba and Polvadera Mesa. Manley and Mehnert (1981) determined a K-Ar age of 7.6 ± 0.4 Ma for this same flow. Reddish sedimentary rocks that appear to be Hernandez Formation overlie the thick flow. The age and geometry of this flow relative to the flows on the mesas to the north and south suggest that (1) the flow is downdropped along a narrow graben within the Cañones fault zone, as illustrated in Fig. 7, or (2) the flow filled a narrow valley that occupied the general position of modern Cañones Canyon. At least three other thin (1–2.5 m) flows of basalt and andesite are present in the bottom of the Cañones Canyon east of Encino Point (Figs. 2 and 6). These flows are interbedded with poorly exposed Santa Fe Group sandstone and conglomerate that include rounded clasts of Proterozoic granite and quartzite. An 8.75 ± 0.06 Ma (39, Table 1) basalt flow is separated from a younger (40, Table 1, 7.42 ± 0.13 Ma) overlying andesite flow by ∼15 m of poorly exposed Santa Fe Group (Fig. 6; Lawrence et al., 2004). Other basaltic flows intercalated with Santa Fe Group sediments in Cañones Canyon yield 40Ar/39Ar ages of 8.10 ± 0.13 Ma to 8.17 ± 0.08 Ma (Fig. 6; 41–43, Table 1; Kempter et al., 2004; Lawrence et al., 2004). The spatial distribution of the basalts in the bottom of the canyon, which are not aligned along the Cañones fault zone, appears to favor a paleotopographic rather than a structural explanation for the relative vertical position of the flows in the vicinity of Cañones Canyon.

Flows south of Cerro Pelón (west). (Note: Two widely separated hills in the northern Jemez Mountains are named Cerro Pelón on topographic maps [Fig. 4]. We distinguish the hills by adding the terms “west” and “east.”) Flows just to the south of Cerro Pelón (west), a younger andesitic center on the southwest edge of the La Grulla Plateau (Figs. 2 and 6), include a basal basalt flow, an overlying crystal-rich andesite with sparse biotite (Goff et al., 2006), and a capping weakly porphyritic basalt. These flows, exposed on the northwest rim of the caldera, yield 40Ar/39Ar groundmass ages of 7.70 ± 0.07–7.80 ± 0.08 Ma (44–46, Table 1). Based on the age and mafic composition of the units, these flows may correlate to flows on Encino Point. Alternatively, these mafic flows may have come from a center that was active at the same time as the Encino Point center, and has since collapsed into the Valles caldera.

La Grulla Plateau. The dark-colored andesite and basalt lavas of Encino Point are overlain by lighter-colored, porphyritic to fine-grained trachyandesite, dacite, and trachyte. Generally, the trachyandesites on the La Grulla Plateau are stratigraphically older than the dacites in the same area. These flows originated from northerly-aligned volcanic centers on the plateau that parallel the projection of the Cañones fault zone into this area (Lawrence, 1979). The centers are, from north to south: Barrancones Hill, Four Hills, Cerro Pavo, Cerro del Grant, Cerro Pelón (west), Hill 33, and Cerro de la Garita (Fig. 6).

Barrancones Hill is a hornblende-biotite–bearing trachyte to dacite (Lawrence, 1979; Lawrence et al., 2004; Rowe et al., 2007). No age data are available for this dome. Three domes, two of dacitic composition and one of trachyandesitic composition, are clustered at the Four Hills center. The K-Ar age of 7.35 ± 0.21 Ma for one of the hornblende-biotite dacite domes (Singer, 1985; Fig. 6) is indistinguishable from the 7.36 ± 0.16 Ma plagioclase 40Ar/39Ar age of the crystal-rich trachyandesite dome (47, Table 1). A light-gray, fine-grained rhyolite to trachyte that shares chemical affinity to the Bearhead Rhyolite (Rowe et al., 2007) laps onto the south side of the Four Hills andesite. This unit from an unknown vent yields a 6.51 ± 0.21 Ma 40Ar/39Ar groundmass age, although the spectrum is discordant and the error is large (48, Table 1). The undated trachyte to dacite of Cerro Pavo is a flow-banded, weakly porphyritic lava with a few mafic enclaves (Lawrence, 1979; Justet, 2003; Rowe et al., 2007). The two-pyroxene trachyandesite to trachyte of Cerro Pelón (west) also contains mafic enclaves (Lawrence, 1979; Justet, 2003; Rowe et al., 2007) and has a 40Ar/39Ar age of 7.47 ± 0.14 Ma (P70, Table 2; Justet, 2003). The Cerro del Grant center is composed mainly of porphyritic dacite overlying trachyandesite on the south side of the hill. The 40Ar/39Ar biotite age of the older trachyandesite from the north side of Cerro del Grant is 7.68 ± 0.04 Ma (49, Table 1). The crystal-rich dacite of “Hill 33,” so named because the top of the dome lies near the center of section 33 (T. 21 N., R. 4 E.), yields a 40Ar/39Ar biotite age of 7.27 ± 0.07 Ma (50, Table 1). Andesite from the base of the west side of Hill 33 has a groundmass 40Ar/39Ar age of 7.81 ± 0.09 Ma (51, Table 1); this age suggests a correlation of this andesite with the older flows to the south of Cerro Pelón (west) in the northwestern margin of the caldera. Justet (2003; P69, Table 2) determined a 40Ar/39Ar age of 7.21 ± 0.12 Ma for a trachyandesite at the southeast base of Hill 33.

Cerro de la Garita traverse, northern rim of the Valles caldera. Approximately 640 m of andesitic to dacitic flows that span the transition between lavas mapped as Paliza Canyon Formation and lavas mapped as La Grulla Formation are exposed on the south side of Cerro de la Garita (CdlG) along the northern margin of the Valles caldera (Figs. 2 and 6). A stack of flows separated by basal glassy flow breccias and vesicular flow tops with little sediment between the flows underlies the lower half of the ridge (Fig. 8). The lowest flow in the sequence is a biotite-bearing dacite, and the overlying flows are porphyritic andesite (Figs. 8 and 9). The character of the flows changes in the upper half of the ridge above an elevation of ∼2990 m (Fig. 8). Tan, very fine-grained sandstone that is <3 m thick separates the lower sequence from the upper sequence. Flows are increasingly mafic upsection below the break and increasingly felsic upsection above the break. The flows below the sandstone are intruded by an altered, undated rhyolite dike interpreted to be Bearhead Rhyolite (Goff et al., 2006). The flow just above the sandstone is crystal-rich andesite porphyry with trace biotite, which in turn is overlain by a porphyritic two-pyroxene andesite. A rhyolite pumice-covered slope that may include ∼70 m of pumiceous volcaniclastic sediment or ash-fall deposits lies upslope of the two-pyroxene flow. All flows below the pumice-covered slope are interpreted to be Paliza Canyon Formation andesite and dacite, although the subtle shift toward more alkaline compositions occurs at the sedimentary break (Figs. 8 and 9). The pumiceous interval is overlain by a crystal-rich, two-pyroxene, hornblende-biotite dacite. This dacite to trachyandesite (Justet, 2003) flow yields a biotite 40Ar/39Ar age of 7.61 ± 0.07 Ma (52, Table 1). The sequence is capped by a sparsely porphyritic dacite that is derived from a vent on Cerro de la Garita, which is the southernmost of the preserved La Grulla Plateau centers. The biotite 40Ar/39Ar age of the CdlG dacite flow is 7.34 ± 0.14 Ma (53, Table 1). The pumice, which is older than 7.61 Ma, is older than typical Bearhead Rhyolite and may correlate to the pumice on Polvadera Mesa.

Rowe et al. (2007) found that La Grulla Plateau lavas are geochemically distinct from both Paliza Canyon Formation lavas from the southern Jemez Mountains and younger Tschicoma Formation lavas to the east (Fig. 10). We analyzed the trace-element data from the La Garita section (Fig. 8 and Table 3) to see if sharp breaks or gradational trends in chemical evolution are preserved in the flows on the ridge leading to CdlG and to further test our unit assignments. The CdlG samples do not have unusually high Ba concentrations, and the Ba values overlap those of both the Paliza Canyon Formation and the La Grulla Plateau lavas (Fig. 10A). Nb concentrations of all CdlG samples are similar to concentrations in the La Grulla Plateau lavas, but Nb for the CdlG units assigned to Paliza Canyon remains constant with increasing SiO2, while Nb in the CdlG La Grulla Plateau flows decreases (Fig. 10B). In contrast to the pronounced increase in Nb with increasing silica content measured in the Paliza Canyon Formation elsewhere in the JMVF, the Nb in the CdlG Paliza Canyon flows does not increase with increasing silica content (Fig. 10B). All CdlG samples, except for the highest sample on the traverse, have MgO contents that are more similar to La Grulla Plateau lavas than Paliza Canyon Formation lavas (Fig. 10C). The TiO2 concentrations for all of the CdlG samples overlap both Paliza Canyon Formation and La Grulla Plateau values (Fig. 10D). The Pb/Ce ratios for the CdlG samples lie within a relatively narrow range (0.15 to 0.22), and the U/Nb ratios are 0.14–0.21, which are generally higher than the range for both the Paliza Canyon and La Grulla Plateau lavas (Fig. 10E). Sample F06-23, the highest of the lavas assigned to the Paliza Canyon Formation, has the highest U/Nb (0.21), but the underlying Paliza Canyon Formation flow (F06-24) has the lowest U/Nb (0.09) of the whole sequence. The Zr contents of both the Paliza Canyon Formation flows and the La Grulla flows on CdlG are positively correlated with SiO2 and the CdlG La Grulla lavas have slightly lower Zr concentrations (Fig 10F). In summary, despite erosional breaks and differences in the mineralogy and major-element concentrations of the flows upsection, trace-element distinctions between the older flows and the younger flows along the Cerro de la Garita traverse are not clear, with the exception of a small decrease in Zr content and perturbations in U concentration at the top of the Paliza Canyon Formation. In some respects, the geochemical affinity of the whole succession is like that of the Paliza Canyon Formation (TiO2), but in other respects, the trace-element data suggest that all of the lavas in the CdlG may be closely related, with an affinity that is more similar to La Grulla Plateau lavas (MgO and U/Pb).

Northeastern caldera. Several dacitic flows in the northeastern part of the caldera that had been previously mapped as Paliza Canyon Formation and Tschicoma Formation may, in fact, correlate to La Grulla Plateau flows. These dacite flows have biotite 40Ar/39Ar ages 7.66 ± 0.04 Ma and 7.78 ± 0.10 Ma (54–55, Table 1) and appear to be interlayered with Santa Fe Group sandstone (Gardner et al., 2006), as is the case with the La Grulla mafic flows in the Cerro Pedernal area (e.g., Fig. 7). The flows are to the east of a significant E-side-down fault that juxtaposes the Paliza Canyon Formation andesite and the dacites of the La Grulla Plateau on CdlG against the Santa Fe Group sandstones (dotted and queried projection of Cañones fault zone on Fig. 6). The fault in the northeast caldera wall is covered by colluvium. Interestingly, the capping 7.42 ± 0.05 Ma (56, Table 1) porphyritic biotite-hornblende dacite flow is not obviously affected by the fault, and Bandelier Tuff to the north is unaffected by the fault.

Latest Miocene Activity (7–6 Ma)

Bearhead Rhyolite

The intimately associated Bearhead Rhyolite and Peralta Tuff were mainly emplaced during an episode of intense rhyolitic volcanism peaking between 6.5 and 7 Ma (McIntosh and Harlan, 1991; Justet and Spell, 2001) and located primarily in the southeastern Jemez Mountains. At least 20 vents that extruded rhyolite domes, flows, and pyroclastic deposits of Bearhead Rhyolite are aligned along north- and northeast-striking structures in the southern Jemez Mountains (Smith and Lynch, 2007). The rhyolite lavas and associated lithic-rich pyroclastic deposits are typically crystal poor (<3%), with sparse phenocrysts of quartz, sanidine, biotite, ± plagioclase. We identified one more Bearhead vent in the southeastern Jemez Mountains during this project. An unnamed hill north of Paliza Canyon in the southern Jemez Mountains that was previously mapped as a Canovas Canyon Rhyolite dome gave a biotite 40Ar/39Ar age of 6.51 ± 0.48 Ma (57, Table 1). In addition, a rhyolite lava located east of Tres Cerros has a sanidine age of 6.86 ± 0.28 Ma (58, Table 1). A faulted, aphyric rhyolite intrusion that was previously mapped as Paliza Canyon andesite (Smith et al., 1970) was found just southeast of Cerro Pelado. An obsidian sample from the intrusion yielded a glass 40Ar/39Ar age of 7.62 ± 0.44 Ma (59, Table 1), and a sample of devitrified rhyolite yielded a groundmass age of 7.83 ± 0.26 Ma (60, Table 1; see quality assignment in Supplemental File [footnote 1]). The rhyolite petrographically resembles Bearhead Rhyolite intrusive rocks in the southern Jemez Mountains, but the ages are older than the typical Bearhead Rhyolite age range (P72–P121, Table 2).

An important outcome of this investigation is the observation that the geographic distribution of Bearhead Rhyolite is greater than has been previously described. Several rhyolite vents of Bearhead Rhyolite age have been mapped in the northern Jemez Mountains (Smith et al., 1970; Gardner et al., 1986; Loeffler et al., 1988; Kempter et al., 2004). Loeffler et al. (1988) determined K-Ar ages of 5.6–7.0 Ma for a cluster of domes near the northern caldera margin. We found another small, previously unmapped, rhyolite outcrop in nearby Cañon de la Mora. This rhyolite, which has a typical Bearhead phenocryst assemblage, yielded a sanidine 40Ar/39Ar age of 7.09 ± 0.13 Ma (61, Table 1). Thus the Bearhead Rhyolite appears to have erupted along a northerly striking zone on the east-central side of the volcanic field. The Bearhead Rhyolite footprint is therefore comparable in extent to the Quaternary Valles and Toledo calderas, suggesting a rhyolitic magma system similar in size to the system that produced the Bandelier Tuff (Justet and Spell, 2001). Justet and Spell (2001) speculate that a caldera did not form in late Miocene time because extensional faulting associated with the Rio Grande rift may have kept the system frequently vented, preventing the buildup of fluid and gas pressure that often triggers caldera eruptions.

Several intrusive bodies and dikes interpreted to be composed of Bearhead Rhyolite were also discovered in the northeastern wall of the Valles caldera (Gardner et al., 2006); however, one north-striking rhyolite dike has a 4.81 ± 0.04 Ma biotite 40Ar/39Ar age (62, Table 1). This rhyolite is petrographically similar to the Cañon de la Mora rhyolite located ∼4 km to the north, but both the U and Th content of this rhyolite (JG05-14; Table 3) and the age are more similar to the low-silica rhyolite of Rendija Canyon (Tschicoma Formation; Broxton et al., 2007) discussed in the next section. The exact affinity of this young dike remains unclear.

Pliocene (5.3–2.6 Ma)

Tschicoma Formation

Tschicoma Formation dacitic eruptive centers are most prevalent in the northeastern and eastern part of the field, ranging in age from 5.6 to 2.7 Ma (Goff et al., 1989; Broxton et al., 2007; Samuels et al., 2007). Most centers correspond to the prominent peaks that form Sierra de los Valles in the northern and eastern Jemez Mountains, including Tschicoma Peak, Polvadera Peak, Cerro Pelón (east), Caballo Mountain, Pajarito Mountain, and Sawyer Dome (Fig. 2). The sources of a few of the large flows such as the rhyodacite of Rendija Canyon (low-silica rhyolite) in the Los Alamos area are less clear; the vents have been partially or totally destroyed by the formation of the Toledo and Valles calderas. Tschicoma Formation flows are commonly coarsely porphyritic with plagioclase and sanidine phenocrysts up to 2–3 cm. Pink quartz phenocrysts are found in one of the older flows in the Los Alamos area, the rhyodacite of Rendija Canyon (low- silica rhyolite). Mafic enclaves are common. Variable amounts of hornblende, biotite, green pyroxene, and quartz may be present; the presence or absence of these phenocrysts can be used to distinguish among the flows and eruptive centers in the field. Geochemically, the Tschicoma Formation lavas are predominantly dacite (Fig. 11); andesite and low-silica rhyolite flows are relatively rare in the Tschicoma Formation (Broxton et al., 2007; Rowe et al., 2007). The eruption of significant volumes of dacite in a short amount of time led to the deposition of voluminous debris flows and fluvial sedimentary rocks composed of Tschicoma Formation clasts. These deposits form the Puye Formation in the northeastern part of the JMVF.

Northern Jemez Mountains. Tschicoma Formation dacites in the north-central Jemez Mountains appear to have flowed from the northwestern rim of the Toledo embayment toward the north and may have originated from vents within the embayment prior to its collapse and formation of the Toledo caldera (Gardner and Goff, 1996). The 40Ar/39Ar dates for three samples of porphyritic dacite between Cañoncito Seco to the south and lower Chihuahueños Canyon to the north yielded nearly identical ages (3.26 ± 0.04–3.37 ± 0.05 Ma; 63–65, Table 1). Consequently, these three samples may represent a single dacite flow that extends almost 7 km from the rim of the Toledo embayment to the distal edge of Mesa del Medio. Biotite-bearing andesite along the west fork of Polvadera Creek (66, Table 1; 3.36 ± 0.06 Ma) and a rhyodacite southwest of Polvadera Peak (P128, Table 2; 3.23 ± 0.35 Ma; WoldeGabriel et al., 2006) may also be related to this eruptive episode.

The Tschicoma Peak, Polvadera Peak, and Cerro Pelón (east) centers are part of a north-trending line of eruptive vents. These centers produced viscous, voluminous flows with steep-sided flow lobes. The oldest unit along the main Tschicoma axis, a dacite located ∼1 km south of Polvadera Peak, has a 40Ar/39Ar date of 5.37 ± 0.36 Ma (P129, Table 2; WoldeGabriel et al., 2006). Lavas from the Tschicoma Peak center, including the massive Gallina flow, yield K-Ar ages between 3.2 ± 0.1 and 4.46 ± 0.58 Ma (Goff et al., 1989). The Gallina flow, which covers an area of ∼10 km2, is a flow-banded, crystal-rich, porphyritic lava (Kempter et al., 2005) that preserves impressive lobe morphology where it spilled over Puye Formation deposits, flowing more than 3 km from its source vent high on the eastern flank of Tschicoma Peak. Age estimates for the Gallina flow range from 3.90 ± 0.15 Ma (K-Ar, Goff et al., 1989) to 4.49 ± 0.21 Ma (67, Table 1). A flow that is topographically below the Gallina flow has a groundmass 40Ar/39Ar age of 3.66 ± 0.09 Ma (68, Table 1). Younger lavas and domes form Polvadera Peak (ca. 3.13 ± 0.07 Ma; K-Ar, Goff et al., 1989).

Cerro Pelón (east; Fig. 4) is composed of a porphyritic dacite. In general, craters and collapse features are rare in Tschicoma Formation landforms, but a summit crater, breached to the south, is still preserved on Cerro Pelón (east). The biotite 40Ar/39Ar age of Cerro Pelón (east) is 3.64 ± 0.03 Ma (69, Table 1), much older than the K-Ar age of 2.96 ± 0.27 Ma determined by Goff et al. (1989) for this unit.

Cañones Mesa is a broad mesa located to the northwest of Cerro Pelón (east). Smith et al. (1970) and Manley (1982) mapped the lava capping Cañones Mesa as Tschicoma Formation. This lava contains olivine and is generally more crystal poor (<5%) than typical Tschicoma Formation lavas. The lava is composed of at least two flows separated by basal breccia, and is platy in outcrop. Previous mappers have distinguished a different flow, in addition to Tschicoma Formation lava, at the northwest end of the mesa. Smith et al. (1970) assigned the flow to the Lobato Basalt, and Manley (1982) assigned the flow to El Alto Basalt; however, we did not find this flow to be distinctly different than the basaltic andesite of Cañones Mesa (Kelley et al., 2005b). Manley and Mehnert (1981) and Manley (1982) obtained two K-Ar ages of 2.8 Ma on this flow.

Several key stratigraphic relationships are preserved on the north side of Cerro Pelón (east) (Fig. 4). The ca. 2.8 Ma basaltic andesite of Cañones Mesa overlies the ca. 3.6 Ma dacite of Cerro Pelón (east) and underlies a flow that is clearly El Alto basalt that erupted from a vent on the north side of Cerro Pelón (east). The basaltic andesite of Cañones Mesa may have come from the same general vent area as the El Alto Basalt (Kelley et al., 2005b). A cone of El Alto Basalt on El Alto Mesa just east-southeast of Cerro Pelón has a 40Ar/39Ar age of 2.87 ± 0.02 Ma (71, Table 1), comparable to the age of 2.86 ± 0.05 Ma determined by Maldonado and Miggins (2007) for basalt samples farther north. A second, small-volume Tschicoma Formation dacite vent is exposed at the south end of Cañones Mesa. The dacite of Cañones Mesa has a biotite 40Ar/39Ar age of 3.61 ± 0.05 Ma (70, Table 1), which is virtually the same age as Cerro Pelón (east).

Another eruptive crater, El Lagunito Palo Quemador, is located ∼1.5 km southwest of Cerro Pelón (east). Pumice covers the floor of the crater, which has previously been mapped as part of the El Rechuelos Rhyolite (Smith et al., 1970; Kempter et al., 2004). This pumice has been dated with poor agreement at 5.21 ± 0.25 Ma (K-Ar, Loeffler et al., 1988) and 2.92 ± 0.70 Ma (40Ar/39Ar; P146, Table 2, Justet, 2003). Nonetheless, these ages, along with geochemical data presented by Loeffler (1984) suggest that pumices from this vent are more closely related to the Tschicoma Formation than to the El Rechuelos Rhyolite.

A dacite dome east of Arroyo de la Frijoles and west of Agua Caliente Spring has a 40Ar/39Ar date of 3.81 ± 0.21 Ma (P123, Table 2, Justet, 2003). Interestingly, this flow is faulted down to the east ∼80 m by the Garcia fault zone (Kelley et al., 2007c; Maldonado, 2008), in contrast to the basaltic andesite of Cañones Mesa, which is not cut by the Cañones fault system.

Northeastern Jemez Mountains.Broxton et al. (2007) summarized 40Ar/39Ar dates, the geochemistry, and the spatial distribution of Tschicoma Formation lava flows in the Sierra del los Valles of the northeastern JMVF. The voluminous Tschicoma Formation lava flows east of Los Alamos include dacites from Cerro Rubio (2.18 ± 0.09–3.59 ± 0.36 Ma; Stix et al., 1988), the dacite of Caballo Mountain (3.06 ± 0.15–4.66 ± 0.17 Ma), the low-silica rhyolite of Rendija Canyon (4.98 ± 0.05–5.36 ± 0.02 Ma), the dacite of Pajarito Mountain (2.93 ± 0.06–3.09 ± 0.08 Ma), the dacite of Cerro Grande (2.88 ± 0.02 Ma–3.35 ± 0.17 Ma), and the dacite of Sawyer Dome (3.18 ± 0.20–3.67 ± 0.29 Ma). By comparing our results discussed above with those of Broxton et al. (2007), we note that the older domes and flows (3.1–5.4 Ma Rendija Canyon, Tschicoma Peak, and Caballo Mountain) are the most voluminous and are clustered along the northeastern side of the Jemez Mountains. Broxton et al. (2007) also pointed out that the oldest flow (Rendija Canyon) is the most silicic. The remaining domes and flows in the northern and northeastern Jemez Mountains are in the 2.2 to 3.8 Ma age range.

A few additional samples of Tschicoma Formation dacite were analyzed as part of this study. A rhyodacite dome north of Cerro Rubio overlaps in age with the older cluster of domes and flows (40Ar/39Ar age of 4.21 ± 0.12 Ma; 72, Table 1). The 4.81 ± 0.04 Ma rhyolite dike in the northeastern wall of the caldera described earlier could be associated with this phase of Tschicoma Formation eruptions. The plagioclase age of 3.50 ± 0.23 Ma (73, Table 1) for the low silica rhyolite of Rendija Canyon is much younger than the ages determined by Broxton et al. (2007), but the sample was collected from the highest (youngest) flow at the vent. The hornblende age of 3.44 ± 0.30 for the dacite of Sawyer Dome (74, Table 1), which is from the summit of the dome, is within the range of ages determined by Broxton et al. (2007) for this lava. Broxton et al. (2007) describe a new unit called the dacite of upper Quemazon Canyon, a flow-banded sparsely porphyritic dacite that sits on the low-silica rhyolite of Rendija Canyon. The groundmass age of the sample collected for this investigation is 2.92 ± 0.03 Ma (75, Table 1).

Pleistocene (<2.6 Ma)

El Rechuelos Rhyolite

Three rhyolitic domes of El Rechuelos Rhyolite that were emplaced at ca. 2.1 Ma (Dalrymple et al., 1967; Loeffler et al., 1988; Justet, 2003) are located just west of Polvadera Peak. These domes erupted along a north-south–striking fracture system west of Polvadera Peak onto the underlying Tschicoma Formation surface. The rhyolite is generally aphyric, contains obsidian horizons that are light gray because of numerous small bubbles in the glass, and is commonly brecciated along its margins. The northernmost dome, ∼1.5 km northwest of Polvadera Peak, has landslide scarps along its northwestern flank, and two satellite outcrops of rhyolite in Cañada de Ojitos Creek may represent the detached remnants from the original dome. The middle dome is the most voluminous and produced a short lava flow. The southernmost dome lies ∼1.5 km southwest of Polvadera Peak, where the ground surface is littered with obsidian and crystalline lava fragments. This dome has a 40Ar/39Ar age of 2.09 ± 0.02 Ma (77, Table 1).

DISCUSSION

Trends in Volcanic and Fault Activity across the Northern Jemez Mountains

Published (Manley and Mehnert, 1981; Singer, 1985; Gardner et al., 1986; Goff et al., 1989; Justet, 2003) and new geochronology data, as well as geochemical data (Fig. 11), reveal a pattern of migrating volcanism across the northern Jemez Mountains that is not captured by the current stratigraphic nomenclature for the Polvadera Group, as revised by Gardner et al. (1986). The Lobato Mesa volcanic center in the northeastern part of the JMVF consists mainly of basaltic shield volcanoes that were active 10.8–9.2 Ma (Fig. 12; Goff et al., 1989; this study). Dacitic flows interbedded with basalt flows are more common at the south end of Lobato Mesa and are present high in the sequence of flows. Following eruptions on Lobato Mesa, the locus of volcanism shifted to the northwestern part of the JMVF, where volcanic centers on La Grulla Plateau were active between 8.75 and 7.30 Ma along reactivated Laramide structures (Largo and Cañones faults). Older (8.7–7.8 Ma) flows on La Grulla Plateau, which had been assigned to “Lobato” Formation, are basaltic andesites, basalts, and andesites (Fig. 12) that erupted from a center at Encino Point and a possible center near the northwestern Valles caldera margin. The more mafic lavas erupted from Encino Point generally flowed northeastward. Remnants of these flows cap Cerro Pedernal, Mesa Escoba, and Polvadera Mesa. The early mafic lavas are overlain by 7.7–7.3 Ma trachyandesite and dacite erupted from N-S aligned centers south of Encino Point along projected strands of the Cañones fault zone; these rocks had been assigned to the “Tschicoma” Formation by Smith et al. (1970). Rhyolitic domes in the Cañoncito Seco area were emplaced 7.1–5.6 Ma along the eastern margin of the La Grulla Plateau volcanic center in the vicinity of the projected main strand of the Cañones fault zone (dotted line, Figs. 3 and 6). This rhyolitic volcanism is, for the most part, temporally equivalent to Bearhead Rhyolite volcanism in the southeastern Jemez Mountains (Justet and Spell, 2001). Eastward migration of volcanism continued with the eruption of the voluminous dacitic to rhyodacitic domes of the Tschicoma Formation in the northeastern Jemez Mountains between ca. 2.2 and 5.4 Ma. Some of the youngest Tschicoma Formation dacites (2.47 ± 0.14 Ma to 2.56 ± 0.06 Ma; Samuels et al., 2007) lie beneath the Pajarito Plateau. Eruption of the 2.8 Ma (Manley and Mehnert, 1981) El Alto basalt, the basaltic andesite of Cañones Mesa, and the 2.1 Ma El Rechuelos Rhyolite (Loeffler et al., 1988; Justet, 2003) marks migration of volcanism that is more bimodal in nature into the northern margin of the field along the projections of fault splays between the Garcia fault, which was active <3.8 Ma, and the Cerrito Blanco fault beneath the JMVF.

Revisions to the Stratigraphic Nomenclature of the Northern Jemez Mountains

Here, we argue that the terms “Lobato” and “Tschicoma” should no longer be applied to the volcanic rocks of the La Grulla Plateau because these rocks are temporally and geochemically distinct from volcanic rock units exposed to the east. Mafic rocks on the La Grulla Plateau are younger and are more siliceous than mafic rocks on Lobato Mesa (Figs. 4 and 11). Andesitic and dacitic rocks of the La Grulla Plateau are older and are more alkalic than Tschicoma Formation to the east (Fig. 11). Furthermore, Tschicoma Formation dacites are significantly more alkalic than Lobato Formation dacites (Fig. 11). Singer and Kudo (1986), Justet et al. (2002), and Rowe et al. (2007) also noticed the temporal and geochemical differences between the “Tschicoma” in the northwestern Jemez Mountains and the Tschicoma Formation of the northeastern Jemez Mountains.

We formally propose applying the name La Grulla Formation to the mafic and intermediate composition lava flows and volcanic centers on the La Grulla Plateau and in the northwestern JMVF as far east as Polvadera Mesa. Informal member-level names will be attached to each volcanic center associated with the La Grulla Formation (e.g., andesite of Cerro Pelón (west), dacite of Cerro Pavo, and andesite of Encino Point). We suggest using the name La Grulla Plateau volcanic complex for the entire succession of volcanic rocks in the northwestern Jemez Mountains. The term El Rechuelos Rhyolite should be restricted to the ca. 2.1 Ma domes in the northeast part of the volcanic field (e.g., Loeffler et al., 1988), and the name Bearhead Rhyolite should be applied to the older 5.6–7.0 Ma rhyolites in Cañoncito Seco.

Based on the large number of geochronologic and geochemical data now available for the Jemez Mountains, we propose formally abandoning the name the Polvadera Group because the spatial and temporal patterns of pre-caldera volcanic activity are complex. The age groupings do not separate cleanly into younger northern JMVF versus older southern JMVF activity, which was the basis for the original designation. All post–14 Ma, pre-Bandelier Tuff (as defined by Gardner et al., 2010) volcanic and volcaniclastic rocks in the JMVF are assigned to the Keres Group (Fig. 5), and all Bandelier Tuff and younger volcanic and volcaniclastic units in the JMVF belong to the Tewa Group.

Geochemical Patterns

Rocks of the JMVF display a wide variation in major- and trace-element abundances and radiogenic isotope ratios (Justet, 2003; Wolff et al., 2005; Rowe et al., 2007). This variation is thought to reflect partial melting of a heterogeneous lithospheric mantle to produce parental basalts that then interacted with heterogeneous crust (Wolff et al., 2000, 2005; Rowe et al., 2007). The volcanic field is constructed on a complex Proterozoic lithosphere that was formed during the assembly of North America by collision of the Yavapai and Mazatzal provinces between 1.6 and 1.7 Ga (Shaw and Karlstrom, 1999; Karlstrom et al., 2004) and partly overprinted by intrusion of granitoid plutons at 1.4 Ga. Proterozoic oceanic lithosphere is the likely source of primitive magmas that are represented among the pre-JMVF Santa Fe Group lavas and intrusions and is inferred to be parental to the main volcanic field (Wolff et al., 2005). As the field evolved, the crust beneath the JMVF became an increasingly hybridized mix of Proterozoic crust and solidified Miocene to Pliocene magma chambers (Rowe et al., 2007).

Regional and temporal chemical differences within the JMVF are due partly to different styles of magma-crust interaction. For example, Tschicoma Formation dacites of the northeastern Jemez Mountains and mafic-intermediate lavas of the Cerros del Rio volcanic field (Fig. 4) appear to be related by magma mixing between basalt and silicic crustal melt, whereas Paliza Canyon Formation intermediate and silicic rocks are the products of fractional crystallization, assimilation-fractional crystallization, and simple mixing between basalts and crustal melts, possibly in numerous magma chambers (Rowe et al., 2007). An additional key observation is that the chemistry of the crustal component is geographically distinguishable (Wolff et al., 2005; Rowe et al., 2007). Thus, each major stratigraphic grouping dominated by intermediate and silicic rocks as defined in this paper (Paliza Canyon, La Grulla, and Tschicoma) has its own geochemical signature.

The new major- and trace-element data presented in this paper from an approximately 600-m-thick stack of intermediate-composition lava flows on Cerro de la Garita reveal a subtle shift in chemistry through time across the Paliza Canyon Formation to La Grulla Formation transition. The La Grulla Formation lavas have higher total alkalis (Fig. 9) and lower Sr, Y, Zr (Fig. 10F), and Nb (Fig. 10B) contents than the underlying Paliza Canyon lavas, with the two uppermost Paliza Canyon Formation samples (above the sandstone, Fig. 8) exhibiting transitional chemistry. These data are significant in light of Rowe et al.’s (2007) conclusion that shifts in JMVF chemistry are associated with the establishment of new magma systems. Based on our mapping, the locus of faulting has migrated from east to west, then east, through time, causing the establishment of new magma systems and geographically controlled changes in lava chemistry. More detailed sampling and analysis of lavas exposed in the northern wall of the caldera is needed to assess the importance of this particular transition for our overall understanding of the development of the volcanic field.

VOLCANISM, FAULTING, AND SEDIMENTATION HISTORY OF THE JMVF

This section integrates new and previously published observations of patterns of Keres Group volcanic activity, fault movement, and sedimentation history in the JMVF; new observations concerning the Tewa Group are summarized by Gardner et al. (2010). Sporadic, but widespread, small-volume early Miocene mafic volcanism was localized and focused on the site of the future JMVF by ca. 14 Ma, coinciding temporally with the 14 to 16 Ma deepening of basins elsewhere in the rift (Chapin and Cather, 1994). Olivine basalts erupted in the northeast JMVF in the vicinity of Guaje and Santa Clara canyons at 11–14 Ma (WoldeGabriel et al., 2006). This mafic volcanism was followed by a short-lived, voluminous pulse of basaltic to dacitic volcanism on Lobato Mesa that started at ca. 10.8 Ma and continued to 9.2 Ma (Goff et al., 1989; this study). The oldest mafic flows on Lobato Mesa are olivine basalt, but younger flows are composed of trachybasalt (Goff et al., 1989). Coarse- to fine-grained dacites erupted at the south end of the Lobato volcanic center between 10.4 and 9.8 Ma. This activity was focused along down-to-the-west faults that coincide with the eastern margin of a gravity low in the northern Jemez Mountains (Fig. 3).

At virtually the same time, mafic to felsic centers were active in the southwestern Jemez Mountains, as noted by Bailey et al. (1969), Gardner et al. (1986), and many subsequent researchers. The initial eruptions in the southwestern Jemez Mountains were basalt, trachybasalt, and rhyolite, but as the system evolved, trachyandesite, trachydacite, and dacite became the dominant lava compositions. Most of the Paliza Canyon Formation and Canovas Canyon Rhyolite centers in the southwestern Jemez Mountains are located to the east of the Jose fault (Fig. 3; Kelley, 1977), a significant NE-striking rift structure that separates Santa Fe Group sediments (Zia Formation) to the southeast from Triassic sedimentary rocks to the northwest; fault slivers of steeply-dipping Jurassic units are caught up in the fault zone (Osburn et al., 2002). The Jose fault zone appears to have influenced the location of the dated ca. 9.4 Ma Canovas Canyon rhyolite to dacite plug on the western edge of Borrego Mesa east of Ponderosa. The Jose fault can be traced to the north-northeast where Triassic and remnants of Jurassic rocks are found on the east side of San Juan Canyon just west of the Cerro del Pino dome (Goff et al., 2005a), which may have also vented along this fault. The Canovas Canyon Rhyolite plug at Ponderosa and a younger Paliza Canyon Formation dacite that laps across the Jose fault are not deformed by the fault (Osburn et al., 2002; Kempter et al., 2004).

Many of the oldest Paliza Canyon Formation basalt and trachyandesite flows in the southwestern Jemez Mountains between Borrego Mesa, Bear Springs Peak, and Paliza Canyon are thin (<3 m) and are separated by 5–10 m of locally-derived, volcaniclastic conglomeratic sandstone and sandy conglomerate containing well-rounded boulders and <1-m-thick Canovas Canyon pyroclastic-fall deposits. The thick volcaniclastic succession is interpreted to represent a persistent 9–10 Ma fluvial system on the west side of the Bear Spring Peak–Tres Cerros–Canovas Canyon Rhyolite centers that was periodically disrupted by lava flows erupted from unidentified vents to the north. The sediments and flows were subsequently tilted to the southwest, and then the sequence was capped by relatively flat-lying 8.9–9.1 Ma (Justet, 2003) Paliza Canyon Formation trachyandesite (Kempter et al., 2003).

At about the same time, small basins with southeast-flowing fluvial systems were developing in the area of what are now upper Cochiti, Sanchez, and Capulin canyons (Fig. 4). The rocks consist mainly of debris, block and ash, and hyperconcentrated flows and locally reworked ash beds (Lavine et al., 1996). Structural uplift of St. Peter’s Dome since 6 Ma has exposed a spectacular section of this volcaniclastic material northwest of Boundary Peak. Here, the beds, which are underlain by Santa Fe Group and an 11.3 Ma basalt flow, lap onto a dacite dome and flow complex dated at 9.5 Ma (K-Ar, Goff et al., 1990; Goff et al., 2006). The upper part of the sequence is interlayered with and overlain by two-pyroxene andesite flows, including the flow capping St. Peter’s Dome (8.7 Ma, K-Ar; Goff et al., 1990). Layers of dacitic ash collected from the volcaniclastic beds from all three of the above-mentioned canyons are dated at 9.2–9.6 Ma (P48–P51; Lavine et al., 1996).

In the southeastern Jemez Mountains, deep canyons and ravines cut a faulted, north-trending horst of Keres Group rocks exposed by the Cañada de Cochiti fault system (Goff et al., 2005b, Fig. 3). Within the Cochiti Mining District, medium-grained monzonite to quartz monzonite of the Bland stock intrudes lower Paliza Canyon Formation flows and volcaniclastic beds, and <100 m of white to pale-pink, well-sorted sandstone tentatively assigned to the Santa Fe Group (Bundy, 1958; Stein, 1983; Goff et al., 2005b). All layered units dip 5°–15° to the east and are altered to epidote rank. The well-sorted quartzose sandstone, which superficially resembles a roof pendant, is locally metamorphosed to quartzite and is cut by several dikes related to the Bland stock. The sandstone was apparently deposited at ca. 10 Ma in a small basin in the evolving Rio Grande rift. The white sandstone, which deserves considerably more scrutiny, disappears to the east across the down-to-the east Bland fault and other north-trending faults. The Bland stock consists of multiple intrusions and dikes that were likely the source of the upper Paliza Canyon Formation trachyandesite to dacite flows exposed on surrounding ridge crests. North of the Bland stock, a series of E-W–trending monzonite dikes cuts Paliza Canyon basalt in upper Bland Canyon. The source of these dikes is not known, but they probably originate from an igneous center down-faulted to the east, and now covered by the Bandelier Tuff. In southern Medio Dia Canyon, yet another set of monzonite dikes cuts Paliza Canyon mafic to intermediate composition rocks, and apparently radiates from a small, shallow monzonite plug exposed west of the canyon (Goff et al., 2005b).

While eruptions of intermediate to silicic lavas (Rowe et al., 2007) continued in the south-central to southeastern Jemez Mountains between 9 and 7 Ma, a new volcanic center developed on the La Grulla Plateau on the northwestern side of the JMVF. In contrast to the volcanism along the southern margin of the field that is generally younger to the east, the La Grulla volcanic center formed to the west of the older Lobato Formation center along reactivated Laramide faults, including the Cañones fault zone. Both the Cañones fault zone in the north-central Jemez Mountains and the Jemez fault zone in the southwestern Jemez Mountains (Fig. 2) are old rift-bounding structures that were active starting ca. 25–30 Ma, although the Oligocene offset was modest (<100 m; Aldrich, 1986; Smith et al., 2002; Kelley et al., 2013). The NE-striking Cañones fault zone curves to a more N-S orientation beneath the JMVF, so that the Cañones fault zone, which at some point in its history may have connected to the Jemez fault zone, now aligns with the Cañada de Cochiti fault zone in the southern Jemez Mountains.

The Cañones fault zone was an east-side-up reverse fault during Laramide deformation (Smith, 1995). Smith et al. (2002) determined that the reverse fault experienced east-side-down normal faulting in late Oligocene time (28–25 Ma). Deformation across the rift margin continued during Miocene deposition of the Santa Fe Group and into Pliocene time. Eastward thickening of the Tesuque Formation of the Santa Fe Group (19–13.5 Ma) beneath the La Grulla Formation is particularly pronounced. Lavas on the La Grulla Plateau in the northwestern JMVF erupted over a relatively short interval of time, starting at 8.75 Ma with thin flows intercalated with Santa Fe Group sediments. The activity evolved to more voluminous basaltic to andesitic eruptions at Encino Point with most of the lava flowing to the northeast. Subsequent eruptions were south and east of the older La Grulla Plateau centers, giving rise to more andesitic to dacitic eruptions between 7.7 and 7.4 Ma. Although the La Grulla Formation is offset 670 m down-to-the-east, 2.8–3.0 Ma basaltic andesite and dacite lavas that flowed across the Cañones fault zone are generally not offset. The Garcia fault located 3 km east of the Cañones fault zone displaces a 3.8 Ma dacite by 80 m, and the Madera fault 10 km to the east offsets 5.6 Ma basalt by 11 m (Fig. 3; Koning et al., 2007a). Variations in the thickness of rift-fill sediments and offset of lava flows of different ages imply high displacement rates across this fault zone in middle and late Miocene time and a progressive decrease in displacement rates during late Miocene to Pliocene time.

Evidence for an episode of rhyolitic volcanism that postdates the Canovas Canyon Rhyolite and predates the peak of Bearhead Rhyolite activity was found during this investigation. A rhyolite dome dated at 7.6–7.8 Ma was mapped in the south-central Jemez Mountains (Goff et al., 2005b). Rhyolitic pumice deposits that are 7.8–7.9 Ma and >7.6 Ma were found in the northern Jemez Mountains on Polvadera Mesa and on Cerro de la Garita, respectively. Although a few scattered Bearhead Rhyolite domes are located throughout the southwestern and south-central JMVF, including the two domes discovered during this study, the locus of Bearhead Rhyolite activity was in the southeastern Jemez Mountains, occurring primarily between 6.5 and 7.1 Ma (Justet and Spell, 2001). Erosion of the Bearhead Rhyolite domes is recorded in the volcaniclastic Cochiti Formation in the southern Jemez Mountains. Bearhead-age eruptions also occurred in the northern Jemez Mountains, including numerous dikes interpreted to be Bearhead Rhyolite in the northeastern margin of the Valles caldera (Gardner et al., 2006), a rhyolite dome found in Cañon de la Mora, and a rhyolite to trachyte with a poorly constrained age of 6.51 Ma south of the Four Hills Center on the La Grulla Plateau. Bearhead volcanism generally migrated eastward in both the southern and northern Jemez Mountains along projections of the Cañones–Canada de Cochiti fault systems.

A brief lull in volcanic activity occurred between 6.5 and 5 Ma. Dacitic volcanism incorporating a large crustal component (Rowe et al., 2007) migrated into the northeastern JMVF at ca. 5 Ma. The oldest Tschicoma Formation centers are in the Sierra de los Valles, and then this activity became more widespread at 3.5–2.2 Ma across the northeastern Jemez Mountains, stretching from west of Abiquiu to the Pajarito Plateau. Rowe et al. (2007) noted that the “eastern dacites” of the Tschicoma Formation (Pajarito Mountain and Cerro Rubio) have lower Nb, 143Nd/144Nd, and 206Pb/204Pb compositions compared to the “northern dacites” of the Tschicoma Formation (Tschicoma Peak, Mesa Gallina, and Polvadera Peak). The lack of correlation of age with chemistry and other factors suggests the difference is due to the composition of the melted crustal component (Rowe et al., 2007). Erosion of Tschicoma Formation domes is preserved in the volcaniclastic Puye Formation.

A large alluvial fan complex that overlies the Paliza Canyon Formation and Peralta Tuff of the Bearhead Rhyolite, but underlies the Bandelier tuffs, is exposed in the middle to lower reaches of several canyons of the southeastern Jemez Mountains (from Alamo Canyon south to Bland Canyon). The fans were shed into the evolving Rio Grande rift and form part of the Cochiti Formation (Smith et al., 1970; Smith and Lavine, 1996), which was deposited primarily between ca. 6.5 and 2.0 Ma. The fan materials consist of a pinkish-gray mixture of sands reworked out of the Santa Fe Group and volcanics of the Keres Group. Locally, the beds are tuffaceous. In drill holes on the Pajarito Plateau, volcaniclastic fan deposits made up of reworked and primary Bearhead Rhyolite tephras overlie mostly dacitic volcaniclastic deposits that contain interbedded 8.4–9 Ma basalt flows (Broxton and Vaniman, 2005; Broxton et al., 2012). A significant unconformity separates the Miocene fan deposits from the overlying Pliocene Puye Formation in drill holes on the Pajarito Plateau.

The final phase of volcanic activity just prior to and slightly overlapping eruption of the Bandelier Tuff occurred on the northern, southeastern, and southern margins of the field along north- to northeast-striking faults. Bimodal volcanism migrated to the northern margin of the field with the eruption 2.8–2.9 Ma El Alto basalt, 2.8 basaltic andesite of Cañones Mesa, and 2.1 Ma El Rechuelos Rhyolite (Bailey et al., 1969; Gardner et al., 1986; this study). The Cerros del Rio volcanic field on the southeastern periphery of the JMVF is chemically related to the Tschicoma Formation and overlaps the dacite in age (Thompson et al., 2006; Rowe et al., 2007). Thompson et al. (2006) found three distinct pulses of activity in the Cerros del Rio volcanic field: (1) an early, large-volume, phase that consists of 2.8–2.6 Ma flows of basalt, basaltic andesite, and minor dacite, (2) a middle, smaller-volume phase composed of 2.5–2.2 Ma basalt and andesite; and (3) a late phase that erupted 1.5–1.1 Ma basaltic andesite and minor dacite that is intercalated with the Bandelier Tuff. The Santa Ana Mesa volcanic field on the southern periphery of the JMVF consists of basalts erupted from structurally controlled shield volcanoes and cinder cones. 40Ar/39Ar ages of 2.6–2.4 Ma for the basalts have been determined by Smith and Kuhle (1998a, 1998b) and Smith et al. (2001).

SUMMARY

New field data, geochemistry, and geochronology are synthesized with existing data to provide an updated view of the evolution of the JMVF and the relationship between tectonism and volcanic activity. Volcanism along a northeast trend between Lobato Mesa to the northeast and Borrego Mesa to the southwest at 10–9 Ma shifted to a more northerly striking trend between the La Grulla Plateau to the north and the south-central Jemez Mountains to the south at 8–7 Ma. The northerly striking pattern of volcanism shifted east and became more rhyolitic 7–6 Ma. After a brief lull, voluminous dacitic eruptions covered the northeastern JMVF 5–2 Ma. Finally, mafic and minor felsic magmas leaked to the surface on the margins on the field at 3–1.1 Ma, while related magmas beneath the field coalesced into large magma bodies, ultimately resulting in the caldera eruptions at 1.61 and 1.25 Ma. The pre-caldera volcanic centers are geographically and chemically distinct, with locations and durations that are likely controlled by relatively short-lived episodes of fault activity. The 40Ar/39Ar dates, when coupled with detailed mapping of both JMVF rocks and the Santa Fe Group, document at least two significant pulses of faulting, erosion, and deposition across the Cañones fault zone in the northern JMVF during middle Miocene time and late Miocene time. As a consequence, the Santa Fe Group is ∼300 m thicker on the hanging wall on the east side of the zone compared to the footwall to the west.

This paper would not be possible without the efforts of the members of the mapping team who worked in the Jemez Mountains during the STATEMAP project. We thank Steven Reneau, Cathy Goff, G. Robert Osburn, Charles Ferguson, Mike Rampey, Rick Lawrence, Gary Smith, Scott Lynch, and Dan Koning for their help in collecting the samples and discussing the interpretation of the results. Nelia Dunbar evaluated the pumice chemistry using the electron microprobe. Conversations with David Broxton and David Sawyer are gratefully acknowledged. Thanks to Richard Kelley and Shannon Williams for their help with the maps presented in this paper. This work was done as part of the New Mexico component of the STATEMAP Program of the National Cooperative Geologic Mapping Program of 1992, and was jointly funded by the State of New Mexico and the U.S. Geological Survey. We appreciate the support of Paul Bauer and J. Michael Timmons, program managers at the New Mexico Bureau of Geology and Mineral Resources. We also thank the Valles Caldera National Preserve for allowing us access to research the geologic history of this special place. Thanks also to Lisa Peters, Matt Heizler, and students who participated in age determinations and mineral separations at the New Mexico Geochronology Research Laboratory at the New Mexico Bureau of Geology and Mineral Resources. David Broxton and an anonymous reviewer shared constructive comments that improved the quality of the paper.

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Supplemental File. Description and interpretation of 40Ar/39Ar dates collected within the Valles caldera. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00897.S1 or the full-text article on www.gsapubs.org to view the Supplemental File.