Palinspastic reconstruction of Proterozoic basement-related aeromagnetic features in north-central New Mexico: Implications for Mesoproterozoic to late Cenozoic tectonism

Documentation of strike-slip tectonism is often difficult, particularly for ancient orogenies. It took over a half century for a consensus to be reached concerning the nature of slip on the San Andreas fault (Hill, 1981), and there is continuing debate concerning the Baja British Columbia hypothesis (e.g., Cowan, 1994; Maxson and Tikoff, 1996; Hollister and Andronicos, 1997). The issue of the extent and timing of dextral strike-slip faulting in New Mexico is a major controversy in southwestern U.S. geology (see Karlstrom and Daniel, 1993; Woodward et al., 1997; Yin and Ingersoll, 1997; Cather, 1999, 2004; Woodward et al., 1999; Fankhauser and Erslev, 2004; Ault et al., 2005). All workers agree that the Proterozoic basement terranes of northern New Mexico are dextrally offset by north-striking faults, but there is much disagreement concerning the magnitude of the separations and the timing of the dextral slip that produced them. This paper uses three approaches. First, we identify major basement-related aeromagnetic lineaments in north-central New Mexico and interpret their character where rocks that correspond to these lineaments crop out. Second, we reconstruct these lineaments across north-trending faults, and we use them as piercing lines to delimit the extent of dextral strike separations in northern New Mexico. Third, to understand the timing of dextral slip, we summarize the regional tectonic events that could have formed or reactivated these north-striking fault systems.

Aeromagnetic maps have long been used to interpret basement lithotypes and structure in New Mexico (Ramberg and Smithson, 1975; Cordell, 1976; Chapin, 1983; Cordell and Keller, 1984; Cordell and Grauch, 1985; Grauch and Keller, 2004). Aeromagnetic data reflect variations in the strength of Earth's magnetic field caused by induced and remanent magnetizations of crustal rocks, largely igneous and metamorphic crystalline rocks (Cordell, 1976; Cordell and Grauch, 1985; Grauch and Keller, 2004). Northern New Mexico is one of the best areas in the southern Rocky Mountains for aeromagnetic studies of basement because complications related to Late Cretaceous to Cenozoic magmatism are not widespread and aeromagnetic anomalies can be attributed mostly to variations in the magnetic characteristics of Proterozoic basement rocks.

Evidence for dextral slip on north-striking faults in northern New Mexico has been noted for decades (Kelley, 1955; Miller et al., 1963; Baltz, 1967; Chapin and Cather, 1981). Using the regional aeromagnetic map of Zietz (1982), Chapin (1983) was the first to attempt the reconstruction of dextrally displaced aeromagnetic trends along the eastern margin of the Colorado Plateau. Chapin (1983) estimated ∼100 km of dextral separation of regional aeromagnetic anomalies, which he attributed largely to Laramide strike slip. Numerous subsequent workers have utilized basement-related aeromagnetic anomalies in northern New Mexico to interpret strike-slip offsets (Cordell and Keller, 1984; Cordell and Grauch, 1985; Karlstrom and Daniel, 1993; Daniel et al., 1995; Woodward et al., 1999; Karlstrom et al., 2004; Cather et al., 2005).

North-central New Mexico encompasses part of the regional Paleoproterozoic suture zone between the 1.76–1.72 Ga Yavapai Province and the 1.7–1.6 Ga Mazatzal Province (Wooden and DeWitt, 1991; Karlstrom et al., 2004). Suturing of these provinces occurred during the ca. 1.65 Ga Mazatzal orogeny. Seven major tectonic events have modified the crust in the study area since 1.65 Ga. These are the ca. 1.45–1.35 Ga magmatic and orogenic event, the ca. 1.1 Ga Grenville orogeny, the ca. 0.7 Ga rifting of western Rodinia, Cambrian extension and magmatism related to the southern Oklahoma aulacogen, the late Paleozoic Ancestral Rocky Mountain orogeny, the Late Cretaceous–Eocene Laramide orogeny, and Oligocene–Holocene extension related to the Rio Grande rift. Other than that related to uplift and erosion on local Phanerozoic positive areas, regional exhumation was complete prior to deposition of the Mississippian sediments that overlie the Great Unconformity in many places.

In northern New Mexico (Figs. 1 and 2,²⁰²¹), several major north-striking faults exhibit evidence for dextral components of slip. These include the Picuris-Pecos fault (Miller et al., 1963; Bauer and Ralser, 1995), the Tusas-Picuris fault (Karlstrom and Daniel, 1993; Daniel et al., 1995), the Nacimiento fault system (Baltz, 1967; Pollock et al., 2004) and its strike-linked extensions to the north and south, which include the Sand Hill fault (Cather, 1999), the Salado-Cumbres discontinuity (Baltz, 1967), and the Del Norte fault (Brister and Chapin, 1994), and the dextral oblique frontal thrusts of the Sangre de Cristo Mountains (Erslev et al., 2004; Wawrzyniec et al., 2004; Magnani et al., 2005a). The northeast-striking Tijeras-Cañoncito fault also shows evidence of Laramide dextral slip (Lisenbee et al., 1979; Abbott et al., 2004) and transferred slip northeastward from the Montosa–Paloma–Hubble Springs fault system to the Sangre de Cristo uplift (Yin and Ingersoll, 1997; Cather et al., 2005).

The regional aeromagnetic map of northern New Mexico (Fig. 3,³⁰², see ¹) shows several northeast-trending linear features that appear to be broken by dextral steps. The geologically best-defined of these steps coincides with the Picuris-Pecos fault, which has long been recognized as having accommodated about 37 km of net dextral separation of Proterozoic folds, ductile thrust faults, metasedimentary stratigraphic successions, and distinctive marker horizons (Miller et al., 1963; Grambling, 1979; Karlstrom and Daniel, 1993; Daniel et al., 1995). This separation is larger than that for any other known fault (dip slip or strike slip) in the central and southern Rocky Mountains. Because rocks on each side of the fault show evidence of similar pressure-temperature (P-T) conditions during peak metamorphism (∼3.5 kbar, 510 °C; Grambling, 1981; Daniel et al., 1995), the 37 km of dextral separation on the fault must be largely the result of strike slip. Additional evidence against a dip-slip origin for the dextral separation on the Picuris-Pecos fault includes: (1) the ⁴⁰Ar/³⁹Ar thermochronologic history of Proterozoic rocks, which is similar on both sides of the fault (Erslev et al., 2004; Sanders et al., 2006), and (2) local preservation of Pennsylvanian strata on both sides of the fault, showing that post-Mississippian dip separation on the fault cannot exceed the thickness of the Pennsylvanian section (∼0.3–2.0 km; Baltz and Myers, 1999). Geologic separations on the Picuris-Pecos fault are reasonably matched by a similar dextral step (∼42 km) in the aeromagnetic anomaly pattern (Fig. 3,³⁰²; Karlstrom and Daniel, 1993; Daniel et al., 1995). So, at least for the Picuris-Pecos fault, dextral separation of aeromagnetic anomalies is attributable mostly to strike slip, although it should be emphasized that the present separation may be the cumulative result of multiple tectonic events with various slip directions.

Reconstruction of a Proterozoic orogenic belt in northern New Mexico and its associated aeromagnetic anomalies led Karlstrom and Daniel (1993) and Daniel et al. (1995) to invoke ∼15 km dextral separation on the Tusas-Picuris fault, an inferred structure between the Tusas and Picuris Mountains that is now largely buried within the Rio Grande rift. Peak P-T conditions in the Picuris Range (3.5 kbar, 510 °C) and the Tusas Mountains (4 kbar, 650 °C; Williams et al., 1999) suggest that somewhat greater exhumation has occurred in the latter region. This west-up component of slip on the Tusas-Picuris fault would impart a sinistral separation on the south-dipping ductile structures across the fault. Thus, the present ∼15 km dextral separation across this fault is probably somewhat less than the net dextral slip.

The Nacimiento fault system (Fig. 1), which bounds the western margin of both the Laramide Nacimiento uplift (Baltz, 1967) and the late Paleozoic Peñasco uplift (Woodward, 1996), has been interpreted to have accommodated significant dextral strike slip. Geologically based slip estimates range from 2 km to 20–33 km (Baltz, 1967; Woodward et al., 1992; Cather, 1999, 2004; Pollock et al., 2004). The Nacimiento fault is part of a larger system of strike-linked structures (Sand Hill fault, Salado-Cumbres structural discontinuity, Del Norte fault) that extends from southern Colorado to central New Mexico (Baltz, 1967; Brister and Chapin, 1994; Cather, 1999, 2004).

In a geophysical study of northern New Mexico, Cordell and Keller (1984) argued that dextral separation on the Nacimiento fault system is small. Their interpretation is supported by the relatively continuous aeromagnetic anomalies that cross the fault system southwest of Chama (Fig. 3),³⁰². Such an interpretation is compatible with models of limited Laramide lateral slip (e.g., Woodward et al., 1992; Erslev, 2001). Limited strike slip also favors vertical-uplift models for development of the Laramide Nacimiento uplift (e.g., Woodward, 1987) because, in the absence of bounding tear faults, localized uplifts such as the Nacimiento and Peñasco are difficult to reconcile with a thrust-block origin.

Alternatively, ∼24 km of dextral separation of basement-related aeromagnetic anomalies along the Nacimiento fault system also produces well-matched aeromagnetic patterns (Fig. 4,⁴⁰², see ¹). Several criteria are in accord with such a model: (1) the Nacimiento (and Peñasco?) uplift is explained by a localized restraining bend in the regional Nacimiento fault system in which the resultant east-west shortening (∼7 km) is compatible with the thrust-wedge model of Pollock et al. (2004). (2) The proposed restoration (Fig. 4),⁴⁰² realigns the northern margin of an area of low aeromagnetic value ∼35 km northwest of Albuquerque. This restored alignment was the basis for the ∼25 km dextral separation proposed by Karlstrom and Daniel (1993). (3) The Nacimiento fault system likely is contiguous with the Hot Springs fault system of southern New Mexico (Fig. 1). The Hot Springs fault system dextrally separates Lower Paleozoic pinch-outs and isopach lines by ∼26 km (Harrison and Chapin, 1990; Cather and Harrison, 2002; Harrison and Cather, 2004), similar to the ∼24 km separation proposed for the Nacimiento fault system (Fig. 4),⁴⁰².

A few kilometers of dextral separation of mapped Proterozoic units appears to exist on the Borrego fault in the southern Sangre de Cristo Mountains (Moench et al., 1988; Metcalf, 1995; Daniel, 1995), although quantification of separation on this fault has not yet been adequately addressed. An additional ∼5–10 km of dextral separation may be present on the oblique frontal thrusts of the Sangre de Cristo Mountains, depending on the direction of slip on these faults: east-northeastward (e.g., Erslev et al., 2004; Magnani et al., 2005a) or northeastward (Wawrzyniec et al., 2004). Shortening on these faults diminishes southward as the Sangre de Cristo uplift plunges out (Fig. 2),²⁰². This is modeled as a step-wise decrease, accommodated for the sake of simplicity by two fictional northeast-striking faults. In reality, this southward diminishment of thrust slip was probably accommodated by penetrative brittle deformation near the plunge-out––not easily represented in our simplistic model. Dextral slip on the Tijeras-Cañoncito fault system provides the mechanism by which slip is transferred northeastward from the Montosa–Paloma–Hubble Spring fault system to the frontal thrusts of the Sangre de Cristo uplift (e.g., Yin and Ingersoll, 1997).

Retrodeformation of the aeromagnetic map of New Mexico (Fig. 4),⁴⁰² was accomplished by: (1) cutting the map along major structural trends that correspond to the mapped faults discussed already, across which dextral discontinuities in the aeromagnetic data are present (Fig. 3),³⁰²; (2) restoring contractional deformation across major Laramide and/or Ancestral Rocky Mountain uplifts using published cross sections and seismic reflection data where possible (e.g., Baltz and Myers, 1999; Pollock et al., 2004; Magnani et al., 2005a); and (3) rematching dextrally separated segments of the geologically best-constrained one of the regional aeromagnetic lineaments, the Chama lineament (see below). Correlation of other aeromagnetic lineaments to the south is largely a consequence of restoration of the Chama aeromagnetic lineament. Our interpretive restoration of dextrally separated segments of aeromagnetic lineaments across major faults is comparable to the correlation of beds among well logs in a basin: individual fault blocks were “hung” on a prominent marker (in this case the Chama aeromagnetic lineament), then resulting potential correlations were examined.

The reconstruction in Figure 4,⁴⁰² shows our interpretation of the magnitude of restored dextral separation (values in kilometers are shown in red numerals at north edge of map) as well as areas of thrust overlap in Phanerozoic uplifts (white areas). This reconstruction is simplistic for several reasons: (1) We have only attempted to depict deformation between, but not within, major structural blocks; (2) slip on numerous lesser faults was ignored; (3) zones of subparallel faults, such as the contractional faults of the Sangre de Cristo Mountains and the Montosa–Paloma–Hubbell Springs fault system, are represented simplistically by a single cut-line; and (4) no attempt was made to model deformation associated with the Rio Grande rift.

The palinspastic restoration of the aeromagnetic map of north-central New Mexico (Fig. 4),⁴⁰² realigns several regional, curvilinear gradient trends or lineaments in the aeromagnetic data. From north to south, we term these the Chama, Cuba, Belen, Socorro, and Corona aeromagnetic lineaments. Most of these aeromagnetic lineaments cross areas of Proterozoic outcrop, which provide a basis for geological interpretation of the aeromagnetic lineaments. In the following sections, we attempt to define the geological origin of the aeromagnetic lineaments, using outcrop, seismic reflection, and borehole data. We then discuss the implications of our results to the Proterozoic assembly of the southwestern United States and to subsequent Proterozoic and Phanerozoic tectonism.

The geologically best-defined of the aero-magnetic lineaments described herein is the Chama aeromagnetic lineament (Fig. 4),⁴⁰². The Chama aeromagnetic lineament is characterized by the juxtaposition of Proterozoic rocks associated with low aeromagnetic values on the north (primarily the metasedimentary Hondo Group) with rocks associated with high aero-magnetic values on the south. In the southeastern Tusas Mountains near Cerro Colorado, the Chama aeromagnetic lineament corresponds to the faulted southern limb of the La Madera syncline, a large, nearly isoclinal, ductile fold that initially formed ca. 1.65 Ga in the middle crust but was probably reactivated ca. 1.4 Ga (Williams, 1991; Williams et al., 1999). Tectonically thickened in the syncline, the Ortega Quartzite (basal Hondo Group; ca. 1.70–1.69 Ga) consists of >95% quartz with accessory muscovite, tourmaline, rutile, hematite, zircon, and aluminum silicates (Williams, 1991, p. 173). Because the aeromagnetic signature of rocks is largely a function of magnetite content (Cordell, 1976, p. 65), the mineralogic contrast between the magnetite-poor Ortega Quartzite and the meta-rhyolite, schist, and amphibolite in the southern limb of the syncline probably produced the aero-magnetic gradient associated with the Chama aeromagnetic lineament near Cerro Colorado. In the southern Picuris Mountains (Fig. 2,²⁰², control point 2), the Chama aeromagnetic lineament coincides approximately with the Plomo shear zone, a ductile reverse fault that divides exposures of the Hondo Group on the north from metavolcanic and metavolcaniclastic rocks of the Vadito Group and 1.68–1.45 Ga granites to the south (Fig. 5; Bauer, 1993, 2004; Karlstrom et al., 2004). Southeast of the Truchas Peaks area, the Chama aeromagnetic lineament corresponds closely to juxtaposed metasedimentary and granitic rocks exposed along the upper reaches of the Pecos River (Fig. 2,²⁰², control point 3; Miller et al., 1963; Robertson and Moench, 1979).

In the Rio Mora uplift, the Chama aeromagnetic lineament coincides with a major syncline north of the ductile Pecos thrust (Fig. 2,²⁰², control point 4). The Pecos thrust places Proterozoic granitic gneiss and mafic igneous rocks over the metasedimentary Hondo Group and the metaplutonic and metavolcaniclastic Vadito Group to the north (Fig. 5; Read et al., 1999). In the Rincon Range, the Chama aeromagnetic lineament coincides with a large nappe (Fig. 5), where the ca. 1.68 Ga Guadalupita pluton and Vadito Group were folded and thrusted over a tectonically thickened sequence of the Hondo Group (Fig. 2,²⁰², control point 5; Read et al., 1999). In the southern Raton Basin (Fig. 2),²⁰², northeast of the Rincon Range, the Chama aeromagnetic lineament corresponds to a nappe structure imaged in the CD-ROM seismic reflection profile that is of similar geometry to that exposed in the Rincon Range (Fig. 6, CDP 13000) (Magnani et al., 2005b).

The principal ambiguity with reconstruction of the Chama aeromagnetic lineament is its correlation west of the Nacimiento fault system. There, aeromagnetic anomaly patterns allow interpretations of little or no dextral offset (Fig. 3),³⁰², as was suggested by Cordell and Keller (1984). Alternatively, because of the nonunique nature of aeromagnetic lineaments, a dextral separation of ∼24 km on the Nacimiento fault system also produces plausible correlations of aeromagnetic patterns (Fig. 4),⁴⁰².

The Cuba aeromagnetic lineament is best manifested in the eastern and western parts of the study area. Cordell and Grauch (1985) noted a prominent zone of low aeromagnetic values in the San Juan Basin that extends west-southwest from Cuba, New Mexico, and they concluded (p. 196) that the basin is “traversed by an east-northeast–trending zone, 40 to 70 km wide and at least 250 km long, of quartzite and related metasupracrustal rocks. The zone is bounded on the south by a predominantly granitic terrane. …” The relatively sharp, southern boundary of the zone of low aeromagnetic values in the southern San Juan Basin corresponds to the Cuba aeromagnetic lineament.

In the eastern part of the study zone, the Cuba aeromagnetic lineament was transected by the CD-ROM seismic reflection profile (Magnani et al., 2005b) near CDP 8200 (Fig. 6). There, imaging of the Cuba aeromagnetic lineament is complicated by seismically transparent upper crust that may be due the presence of 1.4 Ga granitic plutons (Magnani et al., 2005b). At CDP 9000, a south-dipping reflection may be the Pecos thrust or a north-vergent frontal structure related to the Cuba aeromagnetic lineament.

Identification of the Cuba aeromagnetic lineament in the region between the San Juan and Raton Basins is problematic. Where it crosses the Nacimiento, Santa Fe, and Sangre de Cristo uplifts, the contrast in aeromagnetic value that characterizes the Cuba aeromagnetic lineament in the basins is weak or absent. East of the Sierra Nacimiento, the Cuba aeromagnetic lineament passes beneath the Jemez volcanic field and the Española Basin of the Rio Grande rift. In these areas, the effects of thick accumulations of Oligocene–Pleistocene volcanic rocks preclude unambiguous interpretation of basement-related aeromagnetic patterns.

We suggest that the effects of uplift and erosion may explain the discontinuous nature of the Cuba aeromagnetic lineament. The Cuba aeromagnetic lineament is obscure or absent where it crosses the Sierra Nacimiento, the Santa Fe Range, and the southeastern Sangre de Cristo Mountains. All of these areas have experienced Laramide and late Paleozoic uplift and erosion (Baltz, 1967; Woodward, 1987, 1996; Baltz and Myers, 1999), and the Sangre de Cristo area was active during the Grenville orogeny (ca. 1.0 Ga) and Neoproterozoic rifting (ca. 0.7 Ga) (Erslev et al., 2004; Sanders et al., 2006). Apatite fission-track (AFT) ages of Proterozoic rocks exposed in these uplifts generally record Laramide (Late Cretaceous–Eocene) or younger cooling (Kelley, 1990; Kelley et al., 1992; Kelley and Chapin, 1995) and indicate that these rocks were at 3–5 km paleodepth during the Laramide orogeny. Regional cross sections (Fig. 5) and the CD-ROM seismic reflection profile (Fig. 6) suggest that substantial contrasts in aeromagnetic value may result from structural juxtaposition of 2–5 km thicknesses of quartzite and other weakly magnetic metasedimentary rocks with plutonic and metavolcanic rocks. It is thus plausible that Phanerozoic (and Proterozoic?) erosion removed infolded metasedimentary rocks from uplifts, which may explain the lack of major aeromagnetic contrast on uplifted blocks.

The Jemez lineament (Mayo, 1958; Aldrich et al., 1986) was originally defined largely as a northeast-trending volcanic lineament, although others have applied the term Jemez lineament to a zone of aeromagnetic highs that underlies the volcanic lineament and extends east-north-eastward across New Mexico (e.g., Karlstrom and Daniel, 1993; Karlstrom et al., 2004; Magnani et al., 2005b). In this paper, we distinguish the discrete Jemez volcanic lineament from the broad zone of aeromagnetic highs, which we term the Santa Fe aeromagnetic zone (Fig. 4),⁴⁰². According to Magnani et al. (2005b), this zone of aeromagnetic highs corresponds to a deep crustal root of a bivergent orogen that is interpreted to broadly coincide with the Proterozoic Yavapai-Mazatzal Province boundary. A simplistic reconstruction of the Santa Fe zone of high aeromagnetic values was the basis for C.E. Chapin's (1983) estimate of ∼100 km of dextral separation, which he attributed largely to Laramide tectonism. The Santa Fe aeromagnetic zone is bounded on the north and south by the Cuba and Belen aeromagnetic lineaments, respectively, and it corresponds to a broad zone of distributed deformation. It was reactivated by pluton emplacement at ca. 1.4 Ga, and locally leaked magmas during the Neogene to form the Jemez volcanic lineament.

Like the Cuba aeromagnetic lineament, the Belen aeromagnetic lineament is best expressed in the western and eastern parts of the study area. Unlike other aeromagnetic lineaments described in this study, however, the Belen aeromagnetic lineament forms the southern boundary of a region of high aeromagnetic values and thus is of opposite gradient relative to the other lineaments.

Southeast of Albuquerque (Fig. 2,²⁰², control point 6), the Belen aeromagnetic lineament crosses the uplifted rift shoulder of the Albuquerque Basin. There, the Belen aeromagnetic lineament coincides closely with the Manzanita shear zone, a several-kilometer-wide zone that accommodated top-to-the-north ductile shearing during syntectonic emplacement of the 1.65 Ga Manzanita granite (Brown et al., 1999). Along with the Vincent Moore shear zone, it represents the northernmost frontal thrust of the Manzano thrust belt (Karlstrom et al., 2004). The lineament is also just south of the sheared margin of the Sandia Granite, which is marked by the Seven Springs shear zone, a major zone (1–2 km wide) of northwest-dipping, top-to-the-northwest, 1.44 Ga ductile deformation Fig. 7; Kirby et al., 1995). The zone of lower aeromagnetic values south of the Belen lineament corresponds to Proterozoic rocks juxtaposed within thrust sheets of the Manzano thrust belt, which was formed ca. 1.65 Ga (Brown et al., 1999) and was reactivated ca. 1.4 Ga (Rogers, 2001). Northvergent ductile thrust faults and associated folds of the Manzano thrust belt tectonically interleave quartzite, schist, phyllite, amphibolite, granite, and metarhyolite (Karlstrom et al., 2004).

The Belen aeromagnetic lineament crosses the northern Pedernal Hills where Proterozoic rocks are exposed in an exhumed part of the late Paleozoic Pedernal uplift. The Belen aeromagnetic lineament in the northern Pedernal Hills coincides closely with a contact between undated (but probably ca. 1.4 Ga) granite on the north and quartzite, schist, and minor phyllite on the south (Fig. 2,²⁰², control point 7; Gonzalez, 1968; Gonzalez and Woodward, 1972; Armstrong and Holcombe, 1982).

The Belen aeromagnetic lineament intersects the CD-ROM seismic reflection line (Magnani et al., 2005b) at CDP 1800, where it may correspond to the frontal thrust of the Manzano thrust belt or the intrusive contact of 1.4(?) Ga granite with thrusted, largely metasedimentary rocks to the south (Fig. 6; Magnani et al., 2005b). Alternatively, The Belen aeromagnetic lineament may represent the southern margin of a large, bivergent, contractional orogen that may represent part of the 1.65 Ga suture between the Mazatzal island arc and the Yavapai protocontinent.

The proposed palinspastic restoration of the aeromagnetic map of north-central New Mexico (Fig. 4),⁴⁰² realigns a narrow zone of low aeromagnetic values northeast of Socorro. This geophysical feature, herein named the Socorro aeromagnetic lineament, crosses the Manzano Mountains (Fig. 2,²⁰², control point 8) where it coincides closely with the Monte Largo thrust, a major ductile fault within the Manzano thrust belt (Karlstrom et al., 2004). The Manzano thrust belt may have been imaged at the south end of the CD-ROM seismic reflection profile (Magnani et al., 2005b), where it is expressed near CDP 1000 as a series of south-dipping reflectors (Fig. 6).

The Corona aeromagnetic lineament is the least understood of the geophysical features described in this report. Proterozoic rocks are nowhere exposed along its trace. The Corona aeromagnetic lineament lies within a zone of short wavelength (5–15 km), northeast-trending aeromagnetic anomalies, the northernmost of which correspond to imbricate, south-dipping reflectors in the CD-ROM seismic reflection profile that we and Magnani et al. (2005b) interpret as the northern part of the Manzano thrust belt. The Corona aeromagnetic lineament is dextrally separated by ∼13 km across the Tecolote fault (new name; Fig. 3,³⁰²), which is a mostly subsurface structure that corresponds approximately to the western edge of the late Paleozoic Pedernal uplift (Meyer, 1966; Broadhead and Jones, 2004).

The palinspastic restoration of the regional aeromagnetic map of north-central New Mexico (Fig. 4),⁴⁰² may be regarded as a first-order geophysical representation of a subhorizontal crustal slice at 10–15 km depth following ca. 1.4 Ga magmatism and deformation. Geologic data suggest that the regional aeromagnetic lineaments that transect the study area were related to north-vergent contractile deformation initiated during ca. 1.65 Ga collisional suturing and reactivation at ca. 1.4 Ga. The prominent zone of high aeromagnetic values (the Santa Fe aeromagnetic zone) bounded by the Cuba and Belen aeromagnetic lineaments may represent a structurally high, bivergent, contractional orogen that represents suturing of the Yavapai and Mazatzal Provinces (Magnani et al., 2005b; Fig. 6), with possible magmatic and structural reactivations during the 1.4 Ga event. The CD-ROM seismic reflection profile suggests that the aeromagnetic lineaments represent thrust-sense ductile shear zones that may be kinematically linked on a crustal scale. The resulting geometry is one of east-northeast–elongated lenses of penetratively deformed rocks that are bounded by interconnected, top-to-the-north ductile shear zones. This geometry of middle crustal duplexes is shown for the entire Southwest by Karlstrom and Williams (2006) and is analogous to complex deep crustal cross sections drawn for the Alps (Schmid et al., 1996).

Cordell and Keller (1984) noted an apparent dextral discontinuity in aeromagnetic patterns across the frontal faults of the Sangre de Cristo north of Las Vegas, which was subsequently interpreted by Karlstrom and Daniel (1993) and Erslev et al. (2004, p. 36) to have possibly resulted from major dextral slip on these faults. We interpret this apparent dextral step as a product of two separate aeromagnetic lineaments (the Chama and Cuba aeromagnetic lineaments; Fig. 4,⁴⁰²), without major strike slip. The CD-ROM seismic reflection line suggests that a nappe structure in the Rincon Range area (Fig. 2,²⁰², control point 5; Read et al., 1999) extends approximately along strike to the northeast beneath the Raton Basin (Fig. 6, CDP 13000). This northeast-striking structure and its associated geophysical signature (the Chama aeromagnetic lineament) thus are not greatly separated along strike by the frontal faults of the Sangre de Cristo Mountains. Cordell and Keller (1984, p. 22) also noted that the apparent dextral step in aeromagnetic patterns may not be the result of strike slip, “… the repetition of blue anomalies east of the rift could be explained as ∼30 km wave-length synformal infolds of quartzite and other weakly magnetic rocks in an isoclinal Precambrian terrane.”

A further argument for no major strike slip on the frontal structures of the Sangre de Cristo Mountains derives from a zone of related faults and folds that continues on-trend southward through Vaughn into southern New Mexico (Figs. 1, 2,²⁰², and 3,³⁰²; Kelley, 1972b). These structures transect the Belen, Socorro, and Corona aeromagnetic lineaments but do not offset them laterally, and thus are mostly dip slip. We interpret the contrasting aeromagnetic patterns across the frontal faults of the Sangre de Cristo Mountains to have resulted from Phanerozoic and Proterozoic dip slip and differential erosion between the Sangre de Cristo Mountains and the southern Raton Basin.

Several previous workers have invoked significant Phanerozoic dextral slip on faults now concealed beneath basins of the Rio Grande rift (Chapin and Cather, 1981; Karlstrom and Daniel, 1993; Woodward et al., 1999). Such slip is probable on faults in the Española and San Luis Basins (the Tusas-Picuris and Picuris-Pecos fault systems) and in the Socorro Basin and the western Albuquerque Basin (the Nacimiento fault system). Aeromagnetic patterns in most other parts of the Albuquerque Basin, however, seemingly prohibit major lateral separations. North of Albuquerque, a broad aeromagnetic low encompasses much of the northern Albuquerque Basin. Except where transected by the Nacimiento fault, the margins of this anomaly show only minor (<5 km) lateral offsets. The Socorro aeromagnetic lineament shows no significant lateral offset in the southeastern Albuquerque Basin.

Net dextral separation on major structures in the study area is at most ∼90 km, although consideration of dextral separation across unmodeled, lesser structures such as the Borrego fault may allow the total maximum separation to approach ∼100 km, similar to C.E. Chapin's (1983) original estimate. We consider the minimum plausible dextral separation to be ∼55 km, resulting from the combined separation on the Picuris-Pecos and Tusas-Picuris faults. Our reconstruction indicates the net Phanerozoic east-west contraction near the latitude of Santa Fe is ∼22 km. Restoration of extension in the Rio Grande rift would subtract a few kilometers from this shortening estimate.

In the second part of this paper we interpret the slip history of the major north-striking faults in northern New Mexico that exhibit evidence for dextral strike separations. All workers agree that major dextral separations exist in the basement rocks of northern New Mexico. This section addresses the continued debate about when they occurred. Because it is the best-exposed and best-studied of these faults, the slip history of the Picuris-Pecos fault is emphasized.

To constrain the time of initiation and the slip history of the Picuris-Pecos and related faults, we evaluate the regional kinematics of seven tectonic events from 1.4 Ga to Holocene in the southwestern United States. For each tectonic episode, we summarize existing data for the orientation of contractional and/or extensional deformation in the region to interpret the sense of lateral shear on the north-striking faults in northern New Mexico. It cannot be demonstrated that all of the tectonic events described herein produced slip on the Picuris-Pecos or related faults in the area. Our objective is to infer the most probable sense, and possibly the magnitude, of lateral slip on these faults, if slip occurred. We recognize that numerous minor tectonic events have affected northern New Mexico. It is unlikely, however, that these lesser events contributed in an important way to the observed dextral separations.

The Picuris-Pecos and Tusas-Picuris faults dextrally separate a series of overturned folds and ductile faults within the Paleoproterozoic rocks of northern New Mexico (Fig. 5; Miller et al., 1963; Karlstrom and Daniel, 1993; Daniel et al., 1995; Read et al., 1999). The folds and ductile faults involve, and are thus younger than, the 1.70–1.69 Ga Hondo Group (Karlstrom et al., 2004, p. 8) and have long been interpreted to have originated by north-south contraction during the ca. 1.65 Ga Mazatzal orogeny (e.g., Williams, 1991; Karlstrom et al., 2004). This interpretation is supported by the dating of 1.67–1.68 Ga metamorphic rims on pre–1.70 Ga detrital monazite cores from the northern Tusas Mountains, which are interpreted to date the initiation of thrusting (Kopera, 2003). There is also evidence for formation of new ductile fabrics and reactivation of older fabrics during major ductile deformation at ca. 1.43–1.35 Ga. For example: (1) a ductile thrust fault in the Tusas Mountains (Spring Creek shear zone of Davis, 2003) has granitic mylonites that yield zircon and monazite ages of 1.43 Ga, and it accommodated ∼3 km of north-directed thrust offset of ca. 1.43 Ga isograds (Davis, 2003). (2) In the Picuris Mountains, 1.45 Ga metamorphic monazite inclusions are found within syn-S₃ porphyroblasts of garnet, cordierite, and andalusite (Wingsted, 1997; Williams et al., 1999). (3) Metamorphic studies indicate that the S_3,dominant east-west subvertical ductile fabric in the range, formed at >500 °C (Williams et al., 1999) and is now truncated by the Picuris-Pecos fault. Thus, we infer that first movement on the Picuris-Pecos fault is younger than ca. 1.45 Ga.

Miller et al. (1963) described apparent ductile “drag” of bedding in metasedimentary rocks adjacent to the Picuris-Pecos fault, which they used to infer a probable Proterozoic origin for the dextral separation on the fault. Such folding of layered rocks, however, also occurs commonly in brittle deformational regimes (e.g., monoclines) where macroscopic “ductility” is achieved by brittle deformation acting on numerous slip planes, and thus does not require metamorphic conditions. Moreover, the deflection of contacts and foliations in the metasedimentary units near the Picuris-Pecos fault has locally been attributed to brittle deformation mechanisms (McDonald and Nielsen, 2004, p. 225). Elsewhere along the fault, foliations in Proterozoic granitegneiss show no systematic dextral deflection near the fault, but instead are commonly truncated by it at moderate to high angles (Booth, 1976; Moench et al., 1988; Ilg et al., 1997; S.M. Cather and A.S. Read, 2004, unpublished mapping; K.E. Karlstrom, 2004, unpublished mapping).

The 1.48–1.35 Ga period has become recognized as an episode of major tectonism and metamorphism in the southwestern United States (Nyman et al., 1994; Ralser, 2000; Williams et al., 1999; Karlstrom et al., 2004). Kinematic studies of ca. 1.4 Ga pluton aureoles, metamorphic fabrics, shear zones, and vein and dike arrays show that the horizontal component of shortening was generally oriented northwest or west-northwest (Nyman et al., 1994; Kirby et al., 1995; Karlstrom and Humphreys, 1998, Figure 2,²⁰²; Sims and Stein, 2003). Adjacent to the Picuris-Pecos fault, Melis (2001) documented a ca. 1.48–1.37 Ga mylonitic foliation (S₂) that strikes east-northeast and records dextral strike slip; this is consistent with regional evidence for west-northwest shortening. Such shortening orientations should produce sinistral components of slip on north-striking faults such as the Picuris-Pecos (Fig. 8A).

No evidence exists, however, to suggest that the Picuris-Pecos fault had formed by ca. 1.4 Ga. Peak metamorphic conditions in north-central New Mexico were attained ca. 1.4 Ga (Williams et al., 1999; Karlstrom et al., 2004; Shaw et al., 2005), with temperatures of 500–550 °C and pressures of 3.5–4.0 kbar. Deformation under these conditions has produced kilometer-wide zones of penetrative mylonitic foliations elsewhere in the Southwest (e.g., Karlstrom and Bowring, 1988; Shaw et al., 2001, 2005; McCoy et al., 2005; Karlstrom and Williams, 2006). To date, however, no north-striking zones of mylonitic rocks have been reported anywhere along the Nacimiento or Picuris-Pecos faults.

The southwestern United States was affected by ca. 1.3–1.0 Ga deformation, metamorphism, and magmatism related to the Grenville collisional orogeny along the southeastern margin of Laurentia. The effects of Grenville deformation varied with distance from the orogenic front. In Texas, crustal contraction and associated metamorphism were prevalent (Mosher, 1998; Bickford et al., 2000), giving way to a mix of far-field shortening and extension in New Mexico and Arizona. Contractional structures show that the orientation of Grenville shortening in the Van Horn area of west Texas was north-northwest (Mosher, 1998; Bickford et al., 2000), and the extension direction during Grenville rifting and magmatism nearby in the Central Basin Platform region of southeastern New Mexico was east-northeast (Adams and Miller, 1995). An imprecisely dated, northwest-striking, ca. 1.1 Ga dike in the Zuni Mountains of New Mexico (Strickland et al., 2003) is indicative of northeast-southwest extension. Northwest-striking dikes that fed ca. 1.1 Ga diabase sills in southern Arizona (Howard, 1991) suggest northeast-southwest extension. In the Grand Canyon area of northwestern Arizona, early Grenville (ca. 1.25 Ga) northwest-southeast shortening was followed by later (ca. 1.1 Ga) northeast-southwest extension (Timmons et al., 2001, 2005). In northern Colorado, the regionally extensive ca. 1.3 Ga Iron Dike strikes northwest (Wahlstrom, 1956; Braddock and Peterman, 1989), thus indicating that the prevailing extension direction was northeast-southwest.

Regional Grenville kinematics suggest that the major north-striking faults of northern New Mexico, if they existed at ca. 1.1 Ga, would have accommodated sinistral components of slip (Fig. 8B). The magnitude of such potential slip is unknown. There is no direct evidence that the Picuris-Pecos, Tusas-Picuris, or Nacimiento faults were active during Grenville deformation. However, thermochronologic data (Erslev et al., 2004; Sanders et al., 2006) indicate that one of the nearby range-bounding thrust faults on the eastern flank of the Sangre de Cristo Mountains, the north-striking Montezuma fault, was an active, contractile, east-down structure at ca. 1.0 Ga. Thermochronologic data also suggest that prior to the beginning of differential uplift at ca. 1.0 Ga, basement rocks exposed in the southern Sangre de Cristo Mountains were at 200–350 °C, corresponding to a probable paleodepth of ∼8–12 km. Fault slip at such conditions should have a preserved record of at least local mylonitic fabrics and other evidence of deformation near the ductile-brittle transition. However, with the possible exception of the “drag” folds in layered metasedimentary rocks (Miller et al., 1963), no evidence for ductile deformation along the Picuris-Pecos fault has been found.

Contrary to the interpretation by Fankhauser and Erslev (2004) that early deformation on the Picuris-Pecos fault occurred near the brittle-ductile transition, detailed mapping at a scale of 1:6000, as well as petrographic and electron-microprobe analysis of brecciated Proterozoic granite-gneiss along the Picuris-Pecos fault at Deer Creek (∼18 km southeast of Santa Fe) by S.M. Cather, A.S. Read, and G. Rawling, has revealed no mylonitic fabrics or other evidence of ductile deformation along the fault; only evidence for brecciation and cataclasis has been observed. Indeed, except for a fault in the Joyita Hills near Socorro (Beck and Chapin, 1994) and the Fowler Pass fault (Andronicos and Carrick, 2003), no mylonitic precursor to any major Phanerozoic fault has been described in New Mexico. These relationships imply that the inception of most major Phanerozoic faults in New Mexico occurred in a brittle regime. Although the timing of cooling through the brittle-ductile transition may have varied among individual mountain ranges, the Proterozoic rocks of the Sangre de Cristo Mountains did not pass into the purely brittle regime (<300–250 °C; Passchier and Trouw, 1996; Scholz, 2002) until the Grenville episode or later (ca. 1.2–0.9 Ga; Erslev et al., 2004, their Figure 4c; Sanders et al., 2006). Thus, we infer that the inception of north-striking brittle faulting in New Mexico was unlikely prior to the Grenville orogeny.

From ca. 0.8 Ga to ca. 0.6 Ga (Lund et al., 2003), western North America underwent several episodes of rifting that ultimately led to the establishment of a passive margin (the Cordilleran miogeosyncline) in the western United States by Cambrian time. East-west extension produced north-striking normal faults in the Grand Canyon region (Timmons et al., 2001) and may have initiated much of the north-south structural grain throughout the Rocky Mountain–Colorado Plateau region (Karlstrom and Humphreys, 1998; Marshak et al., 2000; Timmons et al., 2001). Such east-west extension would not impart significant lateral slip on the north-striking faults of northern New Mexico.

Development of the Neoproterozoic Uinta aulacogen (or intracratonic rift; Condie et al., 2001) to the north of what is now the Colorado Plateau implies that north-south extension occurred in that area (Fig. 8C). If this extension caused southward relative motion of the proto–Colorado Plateau block, and if such motion were accommodated by north-striking faults in northern New Mexico, then the predicted sense of lateral slip on these faults is sinistral. The magnitude of slip needed to accommodate extension in the Uinta aulacogen probably would not have been large (probably kilometers, not tens of kilometers). Although Late Proterozoic activity along the Nacimiento, Tusas-Picuris, or Picuris-Pecos fault has yet to be demonstrated, thermochronologic data indicate that the nearby, north-striking Montezuma fault was reactivated ca. 0.7 Ga as a west-down extensional structure (Erslev et al., 2004; Sanders et al., 2006).

The southern Oklahoma aulacogen formed as a failed arm during Late Proterozoic–Early Cambrian rifting of southeastern North America. The aulacogen trends west-northwest and is a deeply subsided trough filled with a bimodal suite of extrusive and intrusive igneous rocks that accumulated ca. 580(?)–530 Ma (Bowring et al., 1982; Keller and Baldridge, 1995; Hogan and Gilbert, 1997). Sparse igneous stocks and west-northwest–striking dikes of Cambrian age may mark its intrusive underpinnings in southern Colorado and northern New Mexico (Larson et al., 1985). Alternatively, these intrusive rocks may be part of a Cambrian–Ordovician (574–427 Ma) magmatic suite that is broadly distributed throughout New Mexico and southern Colorado (i.e., the north-trending New Mexico aulacogen of McMillan and McLemore, 2004.)

The Pennsylvanian Apishapa uplift of southeastern Colorado is presently cored by Proterozoic granite and lies approximately on-trend with the southern Oklahoma aulacogen. If the bounding structures of the Apishapa uplift represent the reactivated (i.e., inverted) western part of the aulacogen (Larson et al., 1985, p. 1371), then it is possible that Cambrian rifting extended west-northwest and terminated near the Rocky Mountain front. If such crustal extension were accommodated by the relative southward motion of the Texas–eastern New Mexico block, then north-striking faults in northern New Mexico potentially slipped dextrally during the Cambrian (Fig. 9A). The magnitude of dextral slip necessary to accommodate such extension would not have been large (probably a few kilometers, not tens of kilometers).

In the late Mississippian, Pennsylvanian, and early Permian, southwestern and south-central North America was strongly deformed by the Ancestral Rocky Mountain orogeny. In Colorado and Oklahoma, the principal late Paleozoic fault fabric strikes northwest or west-northwest. Major thrust faults that dip south-southwest (Wichita uplift) or northeast (Uncompahgre uplift) have been documented (Brewer et al., 1983; Frahme and Vaughn, 1983), and substantial sinistral slip on the west-northwest–trending Wichita uplift has been proposed (e.g., Budnik, 1986). West-trending faults in the Matador arch of Texas are also interpreted to have slipped sinistrally in the Pennsylvanian (Brister et al., 2002).

In parts of New Mexico, the dominant late Paleozoic structural trends are north-south (e.g., Woodward, 1996; Baltz and Myers, 1999; Broadhead, 1997, 2001a, 2001b, 2001c). Faults of this orientation are commonly steeply dipping with reverse components of slip, although at least one clear example of a north-striking, sinistral oblique normal fault of late Pennsylvanian age has been documented (Beck and Chapin, 1994). Most other indicators of lateral slip on north-striking Pennsylvanian faults in New Mexico and southern Colorado, however, are dextral. These include the dextral, north-northwest–striking bounding faults of the Central Basin Platform in southeastern New Mexico (Yang and Dorobek, 1995), minor fault arrays and northwest-trending en echelon folds along the west flank of the Pennsylvanian Pedernal uplift near Alamogordo, New Mexico (Otte, 1959; Cather, 2000; Howell et al., 2002), contractile, north-northwest–trending en echelon basins and uplifts east of the Picuris-Pecos fault (Baltz and Myers, 1999), and northwest-trending en echelon growth-folds in Pennsylvanian–Permian deposits adjacent to the Pleasant Valley fault near Salida, Colorado (Wallace et al., 2000).

The dominance of dextral components of slip on north-striking faults, probable sinistral slip on some west- and west-northwest–striking faults, and major thrust faults that dip northeast or south-southwest favor a tectonic model that involves northeast-southwest crustal shortening. Ye et al. (1996) proposed such a model, although it should be emphasized that parts of their Laramide-style model are questionable (such as the lack of Pennsylvanian subduction-related magmatism).

If the kinematics of Ancestral Rocky Mountain deformation were dominated by northeast-southwest contraction, then north-striking faults in northern New Mexico potentially slipped in a dextral sense at that time (Fig. 9B). Stratigraphic data indicate late Paleozoic activity on the Nacimiento fault (Woodward, 1996) and the Picuris-Pecos fault (Miller et al., 1963; Casey, 1980; Soegaard, 1990; Soegaard and Caldwell, 1990). Baars and Stevenson (1984) and Baltz and Myers (1999) interpreted that Pennsylvanian dextral slip occurred on the Picuris-Pecos fault. Breccia along the Picuris-Pecos fault locally contains fracture- and fissure-fills of Mississippian carbonate (Fankhauser and Erslev, 2004) that indicate that brittle deformation on the fault occurred during sedimentation in the late Mississippian. Because of the complexity of the Ancestral Rocky Mountain orogen and overprinting by younger deformations, it is not possible to deduce the magnitude of dextral slip from regional structural-balancing considerations. Pennsylvanian dextral-slip components on north-striking faults in northern New Mexico therefore were possibly, but not demonstrably, large (cf. Woodward et al., 1999).

The role of Laramide strike-slip faulting in New Mexico has been extensively debated in the recent literature (e.g., Cather, 2004; Erslev et al., 2004), and the reader is referred to these papers for details of this ongoing controversy. Laramide deformation involved regional northeast or east-northeast crustal shortening, as shown by numerous criteria including the dominant northwest trend of contractile structures in Wyoming and on the central Colorado Plateau (Kelley, 1955; Hamilton, 1981, 1988; Chapin and Cather, 1981; Brown, 1993), minor-fault arrays in the southern Rocky Mountains (Erslev, 2001; Wawrzyniec et al., 2002; Erslev et al., 2004), and joint patterns (e.g., Lorenz and Cooper, 2003). Laramide northeast-southwest shortening is compatible with dextral components of slip on reactivated north-striking faults in northern New Mexico (Fig. 9C), and evidence for such slip has been described along the Nacimiento fault (Baltz, 1967; Woodward et al., 1992; Cather, 1999; Pollock et al., 2004).

Laramide dextral slip on the Picuris-Pecos fault has also been inferred by many workers (Chapin and Cather, 1981; Karlstrom and Daniel, 1993; Daniel et al., 1995; Bauer and Ralser, 1995; Cather, 1999) but is contested by others (e.g., Fankhauser and Erslev, 2004; Ault et al., 2005). The only definitive evidence for Laramide strike slip yet documented on a part of the Picuris-Pecos fault system (the Glorieta Mesa–Estancia Basin fault; Fig. 1) is the mismatched Upper Cretaceous Dakota Sandstone stratigraphy across strands of the fault, which requires substantial amounts (at least several kilometers) of post–Late Cretaceous, pre–27 Ma dextral strike juxtaposition (Cather and Lucas, 2004). In contrast, Fankhauser and Erslev (2004) correlated limestone beds and mapped a small fault across the Picuris-Pecos fault at Deer Creek (∼18 km southeast of Santa Fe), and argued that these features may prohibit major post-Paleozoic dip slip or strike slip on the fault. Stratigraphic analysis by A.S. Read, D. Ulmer-Scholle, and S.M. Cather (Erslev et al., 2004, p. 29–30), however, has shown that the limestone correlation depicted by Fankhauser and Erslev (2004) is a miscorrelation between Mississippian strata on the west with the middle or upper part of the Pennsylvanian Madera Group on the east; this relationship indicates that a major strand of the Picuris-Pecos fault lies beneath Deer Creek with possibly hundreds of meters of east-down stratigraphic separation. Moreover, unpublished geologic mapping by S.M. Cather and A.S. Read indicates that a northwest-striking fault (the “keel” fault of Fankhauser and Erslev [2004] and Erslev et al. [2004]) cannot be traced to the east side of Deer Creek and thus does not pin the Picuris-Pecos fault.

Most recent tectonic models for the Laramide orogeny invoke dextral components of slip along the eastern margin of the Colorado Plateau, with slip accommodated on a family of north-striking structures. Estimates of the magnitude of dextral slip on these structures vary markedly (Chapin and Cather, 1981; Karlstrom and Daniel, 1993; Woodward et al., 1997; Wawrzyniec et al. 2002; Cather, 2004). Perhaps the most robust argument that significant dextral slip of Laramide age occurred in the southern Rockies is given by the need to structurally balance large amounts of crustal shortening north of the Colorado Plateau. This balancing requirement was the basis for some of the early papers that invoked substantial dextral slip in the southern Rockies (Chapin and Cather, 1981; Hamilton, 1981) but has rarely been addressed by later critics of the concept. While we acknowledge the ongoing controversy, these balancing requirements seemingly cannot be met without invoking large (tens of kilometers) Laramide dextral slip in northern New Mexico. Dextral oblique convergence in New Mexico may have been accommodated by partitioned slip in a system of discrete strike-slip and dip-slip faults (Cather, 2004; Cather and Lucas, 2004), but likely gave way northward to a weakly partitioned system of dextral oblique faults in Colorado similar to that described by Wawrzyniec et al. (2002).

Incipient extension in the Rio Grande rift began ca. 36–35 Ma (Cather, 1990; Mack et al., 1994), but basins generally did not begin accumulating sediments until ca. 26 Ma (Chapin and Seager, 1975; Chapin and Cather, 1994; Smith, 2004). The kinematics of extension during early rifting are poorly known. Accumulations of fault-controlled Oligocene sedimentary deposits in the southern Rocky Mountain–Rio Grande rift area, however, were modest during this interval, implying that strain rates were relatively low.

The Miocene (principally the middle to late Miocene) saw the main phase of subsidence and sedimentation in the Rio Grande rift. Sinistral oblique deformation related to northeast-southwest extension occurred in the Rio Grande rift at this time as shown by several lines of evidence. (1) Rift-related basins, including the Browns Park Basin and the Split Rock Basin, formed north of the Colorado Plateau by north-south extension and accumulated Miocene continental deposits (Chapin and Cather, 1994, p. 8). These extensional basins require the plateau to have had a southward component of motion relative to cratonic North America, thus imparting sinistral components to the Rio Grande rift. (2) Although late Oligocene to late Miocene igneous dikes are variably oriented near the rift, presumably due to preexisting faults and other inhomogeneities, dikes that intrude relatively isotropic Oligocene stocks near Socorro and Questa strike northwest and are thus indicative of northeast-southwest extension (Aldrich et al., 1986, p. 6207). (3) Transfer faults that link half grabens of opposing tilt polarity, such as the northeast-striking Embudo fault north of Española (Fig. 2),²⁰², are typically subparallel to the extension direction (Faulds and Varga, 1998). (4) The presence of a northwest-trending early rift basin near Albuquerque (Hudson et al., 2001; Grauch et al., 1999) is suggestive of northeast-southwest extension. (5) Paleomagnetic data indicate that rotations of post-Laramide rocks in New Mexico were mostly counterclockwise (Brown and Golombek, 1985, 1986; Salyards et al., 1994; Hudson et al., 2004; Harlan and Geissman, 2004). These data are compatible with interpretations of sinistral components of slip in the rift (e.g., Salyards et al., 1994). In the Española Basin, however, Wawrzyniec et al. (2002) argued that counterclockwise rotations of elongate fault blocks may have occurred in response to dextral oblique extension.

Wawrzyniec et al. (2002) argued that west-northwest extension caused dextral components of slip in the Rio Grande rift, based on minor-fault arrays from six localities in Colorado. This interpretation requires northward components of relative motion of the Colorado Plateau, but this is at odds with several aspects of the regional geology. During the preceding Laramide orogeny, contractile deformation in the Uinta uplift accommodated part of the northeast-southwest convergence between the Colorado Plateau and the Wyoming craton. Following the cessation of Laramide tectonism, the inactive range-bounding thrusts of the Uinta uplift were beveled by the Gilbert Peak erosion surface and then mantled by the Bishop Conglomerate (Hansen, 1984, 1986). These thrust faults have seen little or no contractile reactivation since deposition of the Bishop Conglomerate, which contains lower Oligocene ash beds (Balls et al., 2004). These relationships suggest that northward components of relative Colorado Plateau motion have not been renewed since the end of the Laramide. We also note that several localities in Colorado have yielded minor-fault arrays in post-Laramide rocks that are compatible with sinistral oblique rifting (Finnan and Erslev, 2001; E.A. Erslev, 2001, oral commun.; C.E. Chapin and S.M. Cather, unpublished data).

Of the six minor-fault localities indicated by Wawrzyniec et al. (2002) to represent Rio Grande rift deformation, only the Cripple Creek, Colorado, locality (32–28 Ma) can reasonably be inferred to reflect rift-related deformation, and perhaps only during the early phase of rifting. Fault data recently compiled from Cripple Creek, however, show east-west extension (Rampe et al., 2005) or west-southwest extension (M.D. Melker, 2005, personal commun.), not west-northwest extension as interpreted by Wawrzyniec et al. (2002). All other purported rift-related fault localities described by Wawrzyniec et al. (2002) are from prerift (Eocene and older) rocks or are from areas outside the rift.

From these relationships, we infer that Miocene extension in the Rio Grande rift was sinistral oblique (Fig. 9D). Based on the magnitude of Miocene extension north of the Colorado Plateau, the sinistral component was not large (a few kilometers, not tens of kilometers). Parts of the Picuris-Pecos fault system were clearly active during Miocene rifting, but perhaps mainly as dip-slip structures. These include the northern extension of the Picuris-Pecos fault where it exhibits normal components of slip along the eastern boundary of the San Luis Basin (the Sangre de Cristo fault; Brister and Gries, 1994), and the Jarilla normal fault to the south in the Tularosa Basin (Fig. 1; Cather and Harrison, 2002). South of Taos, strands of the Picuris-Pecos fault bend to the northwest and exhibit dextral kinematic indicators in rocks as young as early Miocene, possibly resulting from counterclockwise rotation of fault blocks by rift-age sinistral slip on the adjacent Embudo fault (Cather, 2004; McDonald and Nielsen, 2004). South of Lamy, New Mexico, the 27 Ma Galisteo dike effectively pins most of the Picuris-Pecos fault system (Lisenbee, 2000; Cather and Lucas, 2004) and shows that no major lateral slip has occurred there since the late Oligocene. The Sand Hill fault, the southward extension of the Nacimiento fault, was also probably active in the Miocene, but lateral components of slip, if any, are poorly understood.

The sense of obliquity is not well constrained for the younger phase of rift extension (ca. 10–0 Ma). Sedimentation in extensional basins north of the Colorado Plateau ceased in the late Miocene (Chapin and Cather, 1994), which implies either a cessation of southward components of relative plateau motion or that erosional degradation began to exceed extension-related accommodation at that time. Kinematic indicators suggest dextral components of slip related to Pliocene–Pleistocene extension in the Santo Domingo Basin (Karlstrom et al., 1999, p. 163; Chamberlin, 1999). In the nearby Española Basin, however, counterclockwise paleomagnetic rotations in Pliocene volcanic rocks (Hudson et al., 2004) may imply post–2.5 Ma sinistral slip. Menges (1990) also inferred sinistral slip components for the north-striking Sangre de Cristo fault in northern New Mexico. Thus, although no clear sense of obliquity can be attributed to late rift extension, the relatively modest rates of deformation that characterize younger rifting (Chapin and Cather, 1994) imply that strike-slip components were small.

The palinspastically restored aeromagnetic map of north-central New Mexico (Fig. 4),⁴⁰² provides a geophysical image of a subhorizontal slice through the upper middle crust (10–15 km depth) at the close of the 1.4 Ga magmatic and orogenic event. Major east-northeast–trending aeromagnetic lineaments represent major ductile thrust-sense shear zones that juxtapose rocks with low aeromagnetic values (mostly metasedimentary rocks) against those associated with high aeromagnetic values (principally metaplutonic and metavolcanic rocks). These shear zones represent a broad region of progressive deformation associated with suturing of the Yavapai and Mazatzal Provinces that was variably reactivated at 1.4 Ga.

North-south fault systems of brittle character offset aeromagnetic lineaments in dextral fashion. These faults formed as brittle structures during or after the Grenville orogeny. Analysis of geologically constrained aeromagnetic patterns in north-central New Mexico indicates that net dextral separation on major north-striking faults is at least ∼55 km and may be as much as ∼90 km. These separations may be the cumulative result of numerous slip events, not all of them dextral.

Our analysis suggests that the only known deformations in northern New Mexico that could have produced large dextral separations on north-striking faults are the late Paleozoic Ancestral Rocky Mountain orogeny and the Late Cretaceous–Eocene Laramide orogeny ⁰¹. All other major tectonic events were either potentially sinistral or produced only relatively minor dextral components. If sinistral slip occurred during Proterozoic deformations, then these slip events were fully compensated by subsequent Phanerozoic dextral slip. Our analysis is consistent with the results of Cather and Harrison (2002), who interpreted ∼70–110 km of post-Devonian (Ancestral Rocky Mountain and Laramide) net dextral slip in southern New Mexico, based on dextral separations of early Paleozoic isopach contours. The relative importance of Ancestral Rocky Mountain versus Laramide slip to dextral separations on faults in northern New Mexico has not yet been determined.

Cather acknowledges field and logistical support from the New Mexico Bureau of Geology and Mineral Resources (P.A. Scholle, Director). Lynne Hemenway typed the manuscript, and Leo Gabaldon, Martha Cather, Adam Read, and Mark Mansell constructed the figures. We benefited from reviews by Eric Erslev and an anonymous reviewer, and from discussions with Chuck Chapin, Eric Erslev, Seth Fankhauser, Tien Grauch, Randy Keller, Karl Kellogg, Geoff Rawling, Adam Read, Rob Sanders, and Dana Ulmer-Scholle. Careful editing by Associate Editor Art Goldstein was much appreciated. Discussions with Marc Melker and Dave Vardiman of AngloGold Ashanti were particularly useful in furthering our understanding of the Cripple Creek area.