We combine 15 new 40Ar/39Ar ages with existing age constraints of basalts to investigate the incision and denudation history of the ∼150-km-long Rio San Jose (RSJ) of west-central New Mexico (USA) over the past 4 Ma. Temporal and spatial scales of differential incision may help evaluate the relative importance of neotectonic, geomorphic and climatic forcings. The RSJ is a southeast-flowing river that orthogonally crosses the northeast-trending Jemez volcanic lineament, which is underlain by a zone of low-velocity mantle. Preserved basalt flows along the length of the river at different elevations that directly overlie river gravels are used to construct paleoprofiles of the RSJ and give insight into the differential incision history, which can test the hypothesis that epeirogenic uplift associated with the Jemez lineament influenced differential incision of the RSJ. Observations include (1) a northeast-trending graben along the central reach of the RSJ (El Malpais valley graben) which is parallel to the Jemez lineament, (2) the present-day east tilt of the originally west-flowing 3.7 Ma Mesa Lucero flow along the eastern edge of the Jemez lineament, and (3) modern profile convexities that are colocated with ca. 3 Ma paleoprofile convexities and are centered above the Jemez lineament. The arched ca. 3 Ma paleoprofile defined by the pre–Mount Taylor strath has greater convexity than younger profiles, suggesting neotectonic bowing of ∼135 m (∼50 m/Ma) in this reach over the past ∼3 Ma relative to areas off axis of the Jemez lineament, in spite of graben subsidence and aggradational fill in this reach exceeding 100 m.
Differential incision of the 184 ka Suwanee flow at the edge of the Colorado Plateau may be attributable to base-level fall in downstream reaches of the RSJ and/or headwater uplift, and more erosive climate in the past several hundred thousand years. However, these observations, when considered together, cannot be explained entirely by geomorphic or climatic forcings. Rather, they are best interpreted as resulting from surface uplift centered over the northeast-trending Jemez lineament, and our model suggests that both the faulting and broad bending may relate to mantle driven epeirogeny that caused differential river incision. Several interacting neotectonic and magmatic mechanisms may have contributed to postulated uplift. Magmatically driven geodynamic uplift forcings may include construction of the Mount Taylor stratovolcano just north of the RSJ that changed surface elevation by several kilometers at the volcanic peak itself. However, semisteady denudation and similar incision rates in other rivers in the region indicate that a regional erosional landscape was the primary driver of differential river incision over the past 5–8 Ma. Our focus on the pre–Mount Tayler RSJ paleoprofile reinforces this conclusion. Other mantle-related uplift mechanisms that may have generated mantle buoyancy include thermal buoyancy or magmatic inflation due to dike and sill networks related to the building of the Mount Taylor stratovolcano and eruption of Zuni-Bandera volcanic fields. Both could have contributed to uplift, but their relative importance is unknown. Broad epeirogenic uplift is also possible due to small-scale upper mantle convection beneath a thin elastic plate and resulting dynamic topography.
Rivers can be sensitive gauges of differential uplift, and the incision history and evolution of longitudinal river paleoprofiles can reveal aspects of the dynamic balance between the tectonic, climatic, and geomorphic forcings that shape landscapes (e.g., Merritts et al., 1994; Leland et al., 1998; Wobus et al., 2010; Kirby and Whipple, 2012; Schildgen et al., 2012; Miller et al., 2013). In addition, regional tectonic studies highlight mantle to surface connections in western U.S. intraplate locations where high elevation and high-relief landscapes may owe part of their character to mantle processes that transferred magma, volatiles, and heat across the lithosphere–asthenosphere boundary (Humphreys and Dueker, 1994; Wernicke et al., 1996; Fliedner and Ruppert, 1996; Park et al., 1996; Lowry et al., 2000; Moucha et al., 2009; Braun, 2010; Levander et al., 2011). In Colorado, the CREST (Colorado Rockies Experiment and Seismic Transects) experiment documented an association of thinnest crust, highest topography, and low-velocity mantle beneath the southern Rocky Mountains that reinforced the hypothesis that the Rocky Mountains are a rootless mountain range supported by buoyant mantle rather than thick crust (Hansen et al., 2013; MacCarthy et al., 2014). In Karlstrom et al. (2011), river profile analyses and differential incision of the greater Colorado River system and thermochronology from the Colorado Plateau–Rocky Mountain region were used to support the hypothesis that there was a component of mantle-driven uplift of the region in the past 10 Ma. In New Mexico, mantle-driven uplift above the Jemez lineament was proposed by Wisniewski and Pazzaglia (2002) for the segment of Jemez lineament on the Great Plains, where convexities in the Canadian River were linked to mantle-driven uplift. This conclusion was supported by Nereson et al. (2013), who used tilted basalt paleosurfaces and differential denudation rates and patterns that are parallel to and colocated with the Jemez mantle lineament to suggest differential uplift of the Rockies in the past 4 Ma related to young magmatism and mantle buoyancy changes.
This study is motivated by continued interest in how mantle-driven melt transfer, convection, and buoyancy changes may have influenced surface topography in the Rocky Mountain region in general (Karlstrom et al., 2011) and across the Jemez lineament in New Mexico in particular (Wisniewski and Pazzaglia, 2002; Nereson et al., 2013; Priewisch et al., 2014). The purpose of this paper is to constrain the differential incision history of an ∼150-km-long tributary river to the Rio Grande system, the Rio San Jose (RSJ), which orthogonally crosses the Jemez lineament in New Mexico. We reconstruct the paleoprofiles of the RSJ over the past 4 Ma to examine the interplay between possible neotectonic and geomorphic processes that may explain the observed river profile evolution and related denudation of this part of the western Colorado Plateau. Our objective is to explore whether surface topography in this region has been influenced by mantle-driven magmatism and differential uplift. Different scales and what we hypothesize as manifestations of mantle-driven processes include construction of the Mount Taylor volcanic edifice, broad epeirogenic uplift of the Earth’s surface by changes in crust-mantle buoyancy across the lineament, and faulting related to volcanism and/or uplift.
The Jemez lineament is a nearly 600-km-long and 100–150-km-wide zone of late Cenozoic volcanism (Fig. 1A; Mayo, 1958). The volcanic eruptive centers are spaced at ∼100–150 km and are dominantly basaltic, with major exceptions including Mount Taylor trachyandesite and alkali rhyolite domes, Jemez Volcanic Field high-silica rhyolite, and the Valles and Toledo caldera ignimbrites (Perry et al., 1990; Phillips et al., 2007). The fields follow the southeast edge of the Colorado Plateau from Springerville, Arizona, across the Rio Grande Rift onto the Great Plains (Nereson et al., 2013). Mantle tomographic images (Fig. 1B) from the EarthScope and CREST seismic experiments (MacCarthy et al., 2014) show that the surface volcanic lineament is mimicked at depth by an ∼150-km-wide zone of low-velocity mantle that extends from near the base of the crust to the mantle transition zone (Schmandt and Humphreys, 2010). This low-velocity domain sourced the asthenospheric component of the magmatism (Liviccarri and Perry, 1993) and could possibly have driven differential surface uplift. The magmatism took place over the past ∼10 Ma with an increase in asthenospheric-derived basalt component through time, indicated by Nd isotope data, and a subtle migration through time of a locus of magmatism inward from the edge of the Colorado Plateau (Crow et al., 2011), but no migration along the lineament (Bachman and Mehnert, 1978; Crumpler, 1982; Baldridge et al., 1987; Dunbar, 2005).
The RSJ is a tributary of the Rio Puerco, which is a tributary of the Rio Grande (Fig. 1). The RSJ crosses the Jemez lineament orthogonally just southwest of the 3.4 km elevation stratovolcano at Mount Taylor. Its headwaters are along a relatively low elevation (2.2 km) and rather indistinct segment of the Continental Divide and it flows into the Rio Puerco near the bounding faults that mark the boundary between the Colorado Plateau and the westernmost Rio Grande Rift (Fig. 2A). We constructed a longitudinal profile of the modern river that follows the thalweg on U.S. Geological Survey 7.5 min topographic maps (Fig. 2B). The profile is convex in its central reach and has a knickpoint at the margin of the Colorado Plateau, just west of its confluence with the Rio Puerco (as discussed in the following).
The RSJ profile evolution involved interactions with the Mount Taylor volcanic field (to the north) and Zuni-Bandera volcanic field (to the south), which have been active over the past <4 Ma and past <1.5 Ma, respectively. Prior to the ca. 4–2 Ma volcanism that built Mount Taylor, the river flowed on shallowly dipping Dakota Sandstone. On the flanks of Mount Taylor, thin but widespread lag gravels overlie Cretaceous rocks and underlie the first volcanic rocks of the Mount Taylor volcanic field (Goff et al., 2008), suggesting that early basalt flows flowed to the lowest local drainages and preserve a record of landscape evolution.
New 40Ar/39Ar ages (n = 15) were combined with compiled 40Ar/39Ar, K-Ar, and 36Cl age dates on basalts to constrain the ages of past positions of the RSJ. Paleoprofile segments are defined where elevated paleo–RSJ river gravels were found beneath basalts. Paleoland surfaces were defined where no gravels were found beneath basalts. The gravels with basalts above them (Fig. 3) form dated river incision rate points that were used to measure differential incision rates of the RSJ through time since ca. 5 Ma. Basalt flows related to the 4–2 Ma Mount Taylor volcanic field flowed into the paleo–RSJ valley from the northeast. Post–0.5 Ma basalts were sourced in the Zuni-Bandera volcanic field and flowed northeast tens of kilometers down the RSJ, and these runouts directly preserve paleo–RSJ profile segments (Fig. 3). We also dated and analyzed topographically inverted basalt-capped mesas in the Lucero volcanic field and other nearby areas within the drainage basin but outside the immediate RSJ river valley to estimate denudation in the adjacent landscape over the past ∼8 Ma.
The long time period under study encompasses the time frame of the 4–2 Ma building of Mount Taylor stratovolcano directly adjacent to the RSJ (Fig. 3), the ca. 2.6 Ma climate shift to more erosive climate leading into the Quaternary (Molnar, 2004), the integration of the Rio Grande to southern New Mexico basins in the past 5 Ma and to the Gulf of Mexico in the past 1 Ma (Mack et al., 2006), and includes late Quaternary glacial-interglacial aggradational and incisional oscillations recorded by young terraces (e.g., Connell et al., 2005). Our goal is to use the dated basalts and associated gravels to reconstruct the differential incision history along the river profile at ∼100 km spatial scale and several-million-year time scale.
40Ar/39Ar Analytical Methods
Groundmass concentrates for 16 samples were prepared using standard separation techniques. Samples were first crushed and sieved to between 300 and 450 μm, then ultrasonically cleaned in distilled water to remove adhering dust. Highly magnetic grains and phenocrysts were removed with a Frantz magnetic separator. The groundmass concentrates were then treated in 10% HCl for 20–30 min and triple rinsed in distilled water using an ultrasonic bath. Optical picking using a binocular microscope helped to identify and remove grains that contained phenocryst fragments as well as any discolored or iron-stained grains. The samples were then placed in 12 hole aluminum irradiation trays with the 28.201 Ma FC-2 flux monitor and irradiated for 1 h at the U.S. Geological Survey Triga nuclear reactor in Denver, Colorado.
Samples were dated at the New Mexico Geochronology Research Laboratory located at New Mexico Institute of Mining and Technology. Approximately six FC-2 sanidine crystals from each of the six radial positions for each tray were fused using a Photon Machines 55-W CO2 laser to calculate the neutron flux. Groundmass concentrate samples were step-heated using a defocused Photon Machines diode laser. The diode laser beam was rastered over sample wells to homogeneously step-heat the groundmass concentrate. Sample gas was cleaned in an all-metal, fully automated extraction line equipped with a GP-50 SAES getter pump and a cold-trap operated at ∼–138 °C. Isotopic ratios were determined using the high-sensitivity multicollector ARGUS VI mass spectrometer. Atmospheric air aliquots were measured before, during, and after the experiment to intercalibrate the detectors and monitor instrument drift. The in-house pyChron software was used to operate the laser, extraction line, and ARGUS VI mass spectrometer. The 40Ar/39Ar results are summarized in Table 1, and full analytical details can be found in the Supplemental File1.
Late Cenozoic basalt mesas adjacent to the modern RSJ form classic examples of inverted topography because the basalts flowed into the paleo-RSJ or local low areas but are now high in the landscape due to higher erodability of underlying sedimentary rocks relative to the basalt flows. Where basalt flowed onto river gravels they preserve segments of paleoprofiles, elsewhere they preserve past landscape positions (Fig. 3). Incision and denudation rates were calculated by dividing the strath height above local base level closest to the sample location by the age of the basalt flow. Landscape denudation rates are reported for volcanic necks, plugs, and cinder cones at distance from rivers where no RSJ gravels were found beneath the flow. RSJ incision rates are reported for basalt flows underlain by gravels (Figs. 3 and 4). Elevations of perched gravels and the base of basalt flows were determined primarily via laser range finder, global positioning system instruments, and ArcGIS (www.arcgis.com/) computer software as well as previous studies. Our elevation-age calculation is reported in units of meters of incision per million years (m/Ma) and is the average rate measured over the time interval of each incision point. Table 2 summarizes both incision and denudation rates based on all of the ages of basalt flows in the region of the RSJ.
We dated 16 groundmass concentrate samples using the 40Ar/39Ar technique. Age spectra and inverse isochrons are shown in Figures 5 and 6, respectively, and 40Ar/39Ar ages reported in Table 1. For most samples, the radiogenic yield increased during the step-heating experiment. The youngest samples (e.g., Suwanee flow; MC13-SW1; Fig. 5L) had radiogenic yields <10%–20%, whereas the radiogenic yields for the oldest samples approached 80% (e.g., Horace Mesa; MC13-HM1; Fig. 5B). In general, K/Ca values decreased from ∼1 to ∼0.1 with increasing laser power and temperature. This change in K/Ca values during the experiment reflects the degassing of compositionally different domains (Olmsted, 2000). At low laser power and low temperatures, interstitial glass is the dominant degassing reservoir. K/Ca values at moderate temperatures are consistent with degassing from feldspar. Pyroxene is likely the dominant source of argon at high temperatures.
Of the 16 dated samples, 12 yielded reliable eruption ages. Ages range from 3.148 ± 0.014 Ma to 184 ± 10 ka (Table 1). Eruption ages were determined using the age spectrum plateau or from the inverse isochron. A plateau is defined as three or more contiguous steps that constitute 50% or more of the 39Ar released, and the ages are statistically indistinguishable at the 2σ error (Fleck et al., 1977). Results of each step-heating analysis were plotted on an inverse isochron in order to determine the trapped 40Ar/36Ar component and identify excess 40Ar. The spectrum plateau age, if applicable, was used if the trapped 40Ar/36Ar value of a sample was within error of atmospheric air (295.5 ± 0.5; Nier, 1950; Steiger and Jäger, 1977). Samples that yielded a 40Ar/36Ar value greater than atmosphere indicate excess 40Ar. For these samples, the age calculated from the inverse isochron is the preferred eruption age. Furthermore, the inverse isochron age was used if the step-heating experiment yielded a discordant spectrum (e.g., Figs. 5J and 6J). Many of the age spectra display some discordance at low and high temperatures. This discordance is likely the effect of minor recoil, alteration of glass or phenocryst phases, and/or xenocrystic contamination.
Reliable ages could not be determined for four samples. MC13-SW4 (Alkali Butte flow) yielded ion beam values that exceeded detector measurement limits, thus the experiment was aborted and an age was not determined. Samples MC13-SW2, MC13-SW7 (Suwanee flow), and K13-Acoma1 (youngest buried Acoma flow in drill core) yielded numerous steps with negative apparent ages and radiogenic yields. In addition, these samples have trapped 40Ar/36Ar values that are significantly lower than atmospheric. K13-Acoma 1 (Fig. 5O) yielded a plateau age of 20 ± 30 ka that is consistent with stratigraphic relationships and could be compatible with it being the ca. 50 ka Laguna flow, which is the flow at the surface in this location. However, the inverse isochron indicates a trapped 40Ar/36Ar component of 282 and a corresponding age of 1.16 Ma (Fig. 6O). Therefore, the plateau age of K13-Acoma was not used for any geologic interpretations. At present, the negative ages of some steps and subatmospheric 40Ar/36Ar trapped components are not completely understood. Incomplete gettering of hydrocarbons or the presence of HCl would result in excess 36Ar, which leads to overestimating the amount of atmospheric 40Ar in the total 40Ar measured. Alternatively, Dalrymple (1969) suggested that excess 36Ar may be related to incorporation of primitive argon that was stored in the mantle or an addition of 36Ar during cooling and/or alteration of interstitial volcanic glass.
Because of the isolated nature of most of the basalt outcrops and incomplete stratigraphic relationships involving multiple flows, there are only a few ages where geologic relationships demonstrate the reproducibility and accuracy of the data set. Samples MC13-BM1 and MC13-BM2 are 2 different samples collected ∼10 m apart at the same elevation from Black Mesa. MC13-BM1 (Fig. 5C) yielded a plateau age of 2.7 ± 0.02 Ma, but the inverse isochron indicates excess 40Ar (Fig. 6C). The inverse isochron age for this sample of 2.644 ± 0.14 Ma is statistically indistinguishable from the plateau age of 2.645 ± 0.008 from MC13-BM2 (Fig. 5D), and this age is interpreted as the age of a single flow unit. Samples K13-Acoma3 and K13-Acoma2 are lavas that were intersected in a drill core at depths of 61 and 41 m, respectively. The preferred eruption age of K13-Acoma3 is 325 ± 43 ka based on the isochron age (Fig. 6J), whereas the preferred eruption age of K13-Acoma2 is 274 ± 21 ka based on the plateau age (Fig 5K). Although these two ages are statistically indistinguishable, the ages agree with well-established stratigraphy, providing further confidence the 40Ar/39Ar ages are reasonably accurate.
The RSJ extends from the Continental Divide (west of Gallup, New Mexico) to its confluence with the Rio Puerco, ∼48 km southwest from Albuquerque, New Mexico; it is followed by Interstate 40 nearly its entire length. The river drains ∼9850 km2 (Fig. 2A) and alternates between flowing on alluvium, Mesozoic sedimentary rocks, and basalt. The current average discharge of the RSJ is ∼4.0 cfs (∼0.11 m3/s), although recent maximum discharge has been as high as ∼1400 cfs (∼39.6 m3/s) (http://waterdata.usgs.gov). High historic flows, the likelihood of higher discharge during past glacial times, the wide river valley, observations of inverted topography, and deep bedrock gorges carved into basalts (Fig. 3) attest to its longevity and erosive power over the past ∼10 Ma.
The longitudinal profile of the RSJ (Fig. 2B) has several convexities and knickpoints that we use to split the RSJ into three sections, from downstream (east) to upstream (west). (1) A lower section extends from its confluence with the Rio Puerco to the west end of the Laguna flow. This section contains a knickpoint located near the fault-localized boundary between the Colorado Plateau and Rio Grande Rift. (2) A convex central reach extends from the Laguna Pueblo flow on the east to Black Mesa on the west and includes the area of the Mount Taylor volcanic edifice. (3) An upper generally concave-up section extends from Black Mesa to the Continental Divide (Fig. 2A). Figure 2B compares the RSJ with a range of different order streams east and west of the Continental Divide. In the Rio Grande Rift east of the Continental Divide, the Rio Chama and Rio Puerco have generally concave-up profiles, whereas the Rio Grande has knickpoints in northern New Mexico where it crosses basalts related to the Taos Plateau volcanic field (Newell et al., 2004). Similarly, tributary profiles of rivers west of the Continental Divide show pronounced basalt-related knickpoints on the Little Colorado River where it crosses basalts of the Springerville volcanic field. Some smaller rivers have generally concave-up profiles, such as the nearby Puerco (of the west) and Nutria rivers of the Colorado Plateau east of the Continental Divide (Fig. 2B). Basalts can exert strong controls on river profiles due to their resistance to erosion. However, all of the rivers discussed here have had sufficient stream power to erode through basalt flows in the ∼100 ka time scale (discussed in the following).
Another important feature of the central reach of the modern RSJ profile is an aggradational portion that is filled with both basalt and sedimentary infill that reach a thickness of ∼200 m (Fig. 3). This is also documented by nearby seismic profiles (Kelly and Reynolds, 1989) and by new drill-core data acquired by the Pueblo of Acoma. Both support the presence of a graben in the El Malpais valley. The drill hole is south of the modern RSJ in the northeastern part of the El Malpais valley and was drilled to a depth of ∼61 m, intersecting several alternating layers of red clay, gravel, and basalt flows. It was interpreted that there are three distinct buried basalt flows at this location and they were sampled at 20, 41, and 61 m depth (Fig. 7). The shallowest flow (K13-Acoma1) did not yield a reliable age for reasons explained herein. However, it is interpreted to be one of the flows that is at the surface at that location, likely the Laguna flow. At one time the Laguna flow was hypothesized to be the source of the Laguna Pueblo flow that was buried by alluvium (hence the similar names). However, analytical geochemistry as well as geochronology indicates that this is not the case (Cascadden et al., 1997). Our new data show that the middle flow erupted 274 ± 0.021 ka, and the deepest flow yielded an age of 325 ± 43 ka. We interpret the deepest flow to be correlative with the Laguna Pueblo flow because of the similarity in age. The Laguna Pueblo flow has been traced from where it is exposed at the surface toward the west as far as 10 km upstream (Risser and Lyford, 1983), where our new drill-core data indicate that it is buried by ∼49 m of RSJ alluvium plus younger basalts.
RSJ Paleoprofiles and Strath Terraces
Several of the younger (post–350 ka) basalt flows have morphologies that show they flowed into and down the path of the RSJ. These flow run-outs followed a path subparallel to the modern river floodplain, and have since been incised several to tens of meters (Figs. 3, 4A, and 4B). The 348 ka Laguna Pueblo flow vent area is unknown, but this basalt likely flowed north from the Zuni-Bandera volcanic field along the El Malpais valley, based on discovery of a similar age flow in the subsurface in the Acoma well (Fig. 3). If this flow correlation (based on overlapping ages) is correct, it flowed ∼70 km east down the paleo-RSJ (Fig. 7). The 184 ka Suwanee flow originated in vents in the Lucero uplift and flowed into, then ∼40 km down, the paleo-RSJ (Figs. 2 and 7). The 57 ka Bluewater flow (Laughlin et al., 1994) originated in a nearby vent and flowed ∼10 km down the RSJ. The youngest basalt to flow into the RSJ is the McCartys flow, which has a reported (14C) age of 3.6–3.2 ka (Laughlin et al., 1994) and a 36Cl age of 3.9 ± 1.2 ka (Dunbar and Phillips, 2004). This flow traveled >40 km down the El Malpais valley to reach the RSJ and then flowed down the RSJ 10 km.
Older (4–2 Ma) flows record the construction of the Mount Taylor stratovolcano directly adjacent to the RSJ, as shown by flows in close proximity to the RSJ (Fig. 3) that are underlain by RSJ gravels (Fig. 7). Because the river has a history of flows entering it and flowing downstream, the basalt ages can be used as an approximate constraint on the age of its underlying strath. The bedrock straths are cut into the Cretaceous Dakota Sandstone and range from 200 to 300 m in height above the river. In the central reach, the highest flow with RSJ gravels beneath it is the 2.69 Ma Horace Mesa flow (strath at 292 m), followed by the 2.64 Ma Black Mesa flow (206 m), and the 2.11 Ma Wheat Mountain flow (strath at 181 m) (Fig. 7). In the eastern reach, the 3.15 Ma Mesa Redondo strath is 285 m above the river.
Figure 7 also shows basalts (as old as ca. 8 Ma) in high landscape positions but without underlying RSJ gravels. These include volcanic vents such as Flower Mountain (1.88 Ma) and Mesa Lucero (3.7 Ma). Additional flows away from the main river (Fig. 3) form inverted topography at heights of as much as ∼560 m above present local base level as measured from the closest nearby tributary drainage (Table 2; modified from Priewisch et al., 2014). Bedrock incision amounts and average rates were calculated from the strath beneath the base of the dated basalts to the modern river. This follows standard practice for long-term river incision studies (e.g., Pederson et al., 2002; Karlstrom et al., 2007) and assumes that basalts flowed into active floodplains and into the thalweg above the existing gravel bedload. In general, we cannot rule out that basalts flowed onto and preserved elevated strath terraces, but this is also a caveat for most regional incision rate studies (Aslan et al., 2010; Karlstrom et al., 2011). Over short time intervals, this method has issues that need clarification case by case. In areas of aggradation (sediment or basalt can cause aggradation of the bed) and in fault-lowered blocks, this apparent incision value can yield negative bedrock incision numbers, especially when measured to the modern river (as opposed to the bedrock strath beneath the modern river; cf. Karlstrom et al., 2007). It also neglects the fact that basalts are generally harder to erode than adjacent sedimentary rocks; by using the strath height and age, one is averaging the presumably slower incision rates of the basalt with faster incision rates once the river has carved back through the basalt. Thus, for the younger flows, we also report the incision amount and rate from the top of the flow (Table 2). All of the young RSJ runouts partially filled the river floodplain and the river has incised variable amounts back through basalt, from near zero to several meters into the flow, to tens of meters below the flow bottom. By reporting the incision amount relative to the flow top, we recognize the extra work done by the river to incise through hard rock. As shown in Figure 7, the Laguna Pueblo flow is deeply buried by part of the aggradational fill in the west, has the river flowing on top of it in its central portion, and has been nearly completely incised by the RSJ in the east. The river has only incised as much as ∼5 m into the Bluewater flow in the past ∼57 ka. In the past 184 ka, the RSJ has incised through the Suwanee flow (Cerro Verde) differentially from 0 m of incision in the west to 53 m downstream of the knickpoint. This differential incision coincides with the boundary between the Colorado Plateau and the Rio Grande Rift.
Figure 8 and Table 2 summarize long-term incision and denudation rates of the RSJ based on available age and height constraints. Incision rates over the past 3.7 Ma of ∼90 m/Ma are similar to estimates for other regions of the western Colorado Plateau and many areas of the Rocky Mountain–Colorado Plateau region (Aslan et al., 2010). Both the spatial distribution of paleoprofile remnants along the profile and the temporal constraints at any given location are fragmentary, so paleoprofiles are not uniquely determined and could be interpreted in several ways. For example, one approach would be to assume steady-state incision over this time frame (blue dashed line in Fig. 8) with different rates in some areas attributed to tectonic movement (arrows) between different areas of the paleoprofile. Alternatively, variable rates through time could be inferred (pink and blue bands of Fig. 8), although this would require ∼1600 m/m.y. between 2.7 and 2.6 Ma, a rate higher than anywhere in the western U.S. (Karlstrom et al., 2011).
Long-term landscape denudation rates shown in Figure 8 are based on basalt mesas away from the RSJ river corridor. Over the past 8 Ma, denudation rates have averaged 40–50 m/Ma, with a possible acceleration at 4–3 Ma during the construction of the Mount Taylor stratovolcano and another increase in denudation rates in the past ∼300 ka. This suggests that semisteady incision with different apparent incision rates related to tectonism and changing incision rates through time related to volcanism need to be considered when interpreting the incision record of the RSJ.
Interpretation and Conceptual Model
Figure 9 is a schematic diagram illustrating a possible model to explain the observed differential incision in both time and space. Figure 9 at time A depicts early basaltic lava flows as they might have looked as they erupted and flowed into and down the paleoprofile of the RSJ prior to ca. 4 Ma. The concave-up shape of the pre-4 Ma RSJ profile is speculative, but it is supported by a similar shape of the Puerco River west of the Continental Divide that has carved into the same Mesozoic rocks and has a drainage basin area similar to that of the RSJ (Love and Connell, 2005).
Figure 9 at time B shows the modern RSJ profile, along with the 4–3 Ma Mount Taylor stratovolcano and a possible model for progressive mantle-driven doming of paleoprofiles above the Jemez lineament. Given the geometry of the river as it crosses the Jemez lineament, broad surface epeirogenic uplift might cause lower reaches to steepen and upper reaches to shallow on opposite sides of the ∼150-km-wide Jemez lineament, while the central reaches bow and extend. We evaluate this conceptual model in terms of several aspects of the RSJ profile that need explaining. The following discussion also considers limited erodibility of basalts relative to Mesozoic strata, likely changes in incision rates through time due to magmatism and climate change, and changes due to geomorphic processes such as base-level fall. Here we discuss and interpret features of the paleoprofiles from young to old.
The Bluewater flow is located in the upper reaches of the RSJ and 3He dating yields an age of 57 ± 6 ka (Laughlin et al., 1994). This flow has not been appreciably incised since it flowed into the RSJ. This is in contrast to the downstream Suwanee flow, parts of which have undergone marked incision in the past 184 ka. These flows are likely too young to be useful for evaluating if any postemplacement tilting has occurred, but they suggest appreciable differential incision due to base-level fall affecting the Suwanee flow near the rift margin but not the Bluewater flow in the Colorado Plateau to the west.
The eastern reach offers an opportunity to compare the modern and 184 ka profiles of the RSJ at a key structural location. The modern profile has a knickpoint in the San Andres limestone–Aqua Zarca sandstone, across which there is appreciable differential incision of the 184 ka Suwanee flow. Above the knickpoint, the river flows on top of the Suwanee flow and there has been no incision in the past 184 ka. Downstream of the knickpoint, the river has incised a gorge as much as 53 m below the base of the flow (Figs. 4A, 4B). This gives a differential incision rate variation from 0 to 288 m/Ma since eruption of the 184 ± 10 ka Suwanee flow. The modern knickpoint is not controlled by basalt but may have evolved from a now-elevated paleoknickzone (convexity) preserved in the flow geometry. If so, the knickzone is a transient feature that has migrated upstream at a rate of ∼20 km/Ma. This transient of differential incision is interpreted to be due to geomorphic processes of base-level fall due to accelerated incision of the Rio Grande system in the past several hundred thousand years, as noted in other tributaries of the Rio Grande system (Dethier and Reneau, 1995; Newell et al., 2005). This increase in incision has been generally attributed to increased erosivity due to flashier climate (Connell et al., 2005). An important aspect of the differential incision along the Suwanee flow is that it shows that the RSJ had adequate steam power at the 100 ka time scale to incise through basalt, and therefore rock strength considerations interacted with tectonic and climatic forcings. Thus, the differential bedrock incision is not controlled primarily by the presence or thickness of basalt flows at >100 ka time scales.
In the central reach of the RSJ, the modern river convexity is mimicked by a convexity in the 322 ± 11 ka Laguna Pueblo flow, which, although faulted in several places, appears to be bowed upward by tens of meters over its exposed ∼18 km length (Fig. 7; Drake et al., 1991; Love and Connell, 2005). Seismic and drill-core data indicate that these basalts flowed north into the RSJ from the Zuni-Bandera volcanic field following the El Malpais valley graben, which is filled with ∼200 m of sediment and lava flows. The amplitude of the 322 ka and modern convexities are tens of meters, and are perhaps best explained in terms of hard bedrock that entered the RSJ and armored the bed. This assumes that most of the faulting predated the flows.
An alternative interpretation is that the graben area, if active, would have lowered this reach, but such lowering has been counteracted by arching of the lava flows and the 322 ka paleoprofile. The convexities overlie the ∼15-km-wide El Malpais valley graben that is suborthogonal to the RSJ, subparallel to the Jemez lineament, and aligns with the Mount Taylor stratovolcano. This graben records extension perpendicular to the Jemez lineament in the past 0.5 Ma and is central to an ∼50-km-wide zone of aggradational fill along the RSJ. The spatial correspondence of maximum aggradational fill thickness, maximum 3–2 Ma paleoprofile convexity, approximately colocated with the modern profile convexity, and highest incision rate may argue that river profile convexity reflects surface uplift that has persisted for the past ∼3 Ma in spite of graben formation and aggradational fill.
Unfortunately, 2–1 Ma paleoprofiles are not recorded in this region. At longer time scales, 3.5–2.5 Ma flows overlying gravels are well preserved. The interesting features of these older flows that need interpretation are the markedly different heights of similar age flows and the apparent tilting of the Lucero Mesa flow from originally west flowing to the now east-dipping flow surface. There are three locations south of Mount Taylor where river gravels were found between a Dakota Sandstone strath and the earliest basalt, suggesting that an originally subhorizontal strath or erosion surface was bowed, as shown by the dashed orange line in Figure 7. The amplitude of such a 2.7 Ma convexity is ∼140 m, whereas the amplitudes of the colocated 322 ka and modern convexities are tens of meters. If the older convexity was initiated at similar amplitude to the modern ones, it would have grown in amplitude through time at a rate of ∼50 m/Ma.
The alternative interpretation of different age straths is that they record differential temporal incision, but this would require that the paleo-RSJ incised 86 m from Horace Mesa to Black Mesa in 54 ka at an incision rate of ∼1592 m/Ma (Fig. 8). This seems unlikely and uncharacteristic of the area, so we favor the hypothesis that the central reaches show convexity in the ca. 3 Ma or younger erosional strath that Mount Taylor was built upon.
In the eastern lower reaches, the Mesa Lucero basalt flowed to the north and west from the vent (Fig. 7), yet today the ∼4.6-km-long surface of the mesa dips to the east. This suggests that the mesa has been tilted since the eruption of the capping basalt by a minimum of 0.25°, as no faults offset the mesa. This is compatible with a differential uplift model that suggests an uplift axis to the west of Mesa Lucero. Thus, as shown in Figure 9, available data permit the interpretation that upper reaches have been shallowed (Black Mesa), central reaches bowed (Horace Mesa compared to Black Mesa–Wheat Mountain differential incision), and the lower reaches back-tilted (Mesa Lucero) due to arching above the Jemez lineament.
SUMMARY AND IMPLICATIONS FOR GEODYNAMIC MODELS
This study uses empirical data on the differential incision history of an ∼150-km-long subregional river to reconstruct its paleoprofiles over the past 4 Ma and examine the interplay of neotectonic and geomorphic processes that may explain the observed river evolution. Tomographic images from the EarthScope experiment show that the surface volcanic lineament is mimicked at depth by an ∼150-km-wide zone of low-velocity mantle that likely sourced the volcanism and that could possibly have driven uplift over the past few million years.
Observations compatible with epeirogenic uplift associated with the Jemez lineament include the extensional features found along the central reaches of the RSJ (El Malpais valley graben as well as a small graben in the Laguna Pueblo flow; Drake et al., 1991). These features may have formed due to bending of the elastic upper crust along the crest of the uplifted region (Fig. 9, lower right inset). This interpretation is compatible with paleoprofile and modern profile convexities centered above the Jemez lineament. At long time scales the dashed orange line in Figure 7 suggests upward bowing of the pre–Mount Taylor strath, and greater convexity of older paleoprofiles compared to younger profiles suggests neotectonic bowing of ∼135 m (∼50 m/Ma) in this reach over the past ∼2.7 Ma relative to areas off-axis of the Jemez lineament, in spite of graben subsidence and aggradational fill in this reach exceeding 100 m. Our model suggests that both the faulting and broad bending may relate to mantle-driven epeirogeny that caused differential river incision.
Temporal and spatial scales of differential incision may help evaluate the relative importance of neotectonic, geomorphic, and climatic forcings. For example, differential incision of the Suwanee flow at the edge of the Colorado Plateau may be attributable to base-level fall in downstream reaches of the Rio Grande. If so, the presence of a similar knickpoint in the now-elevated basalt paleoprofile suggests that this base-level fall initiated before 184 ka and indicates the presence of a semisteady-state knickpoint near the edge of the Colorado Plateau for the past several hundred thousand years, perhaps attributable to the change from aggradation to incision in the Rio Grande in the past 0.5–1 Ma (Mack et al., 2006). A postulated base-level fall is probably not attributable to rift-margin faulting and uplift of the footwall (Colorado Plateau side) because the basalt is unfaulted and because a fault-dampened incision model (Karlstrom et al., 2007) would suggest slower incision of the downthrown block, opposite to what is observed. It is worth considering that downstream base-level fall is difficult or impossible to distinguish from headwater uplift, so broad uplift above the Jemez lineament in combination with accelerated downstream Rio Grande incision could explain the observed Suwanee flow differential incision. The observed east tilt of the originally west-flowing but unfaulted 3.7 Ma Mesa Lucero flow is also compatible with uplift associated with the Jemez lineament or broad isostatic uplift of the Colorado Plateau relative to the Rio Grande Rift, but not fault-driven base-level fall as the driver for the observed differential incision.
Geomorphic contributors to observed profile geometry and differential incision may include basalt armoring of the river channel (but this cannot explain greater concavity of older paleoprofiles or post-Suwanee differential incision), downstream base-level fall due to increased climate erosivity and/or increased downstream incision of the Rio Grande trunk stream (although this cannot explain the tilting of the 3.7 Ma Mesa Lucero flow). We cannot think of geomorphic explanations for the observed paleoprofile and graben geometries in the central reach.
The hypothesis presented here is that young and ongoing mantle-driven uplift of the Jemez lineament region may provide the best explanation for the persistent and growing paleoprofile convexities, the modern profile convexity overlying a zone of graben subsidence and maximum aggradational fill, and patterns of measured differential incision of the RSJ over the past 3–4 Ma. However, a range of likely interacting neotectonic and magmatic mechanisms may have contributed to observed geometries, including faulting and associated rebound of footwall blocks. Slip magnitudes, timing, and fault geometry are still incompletely documented for the El Malpais graben and Colorado Plateau–Rio Grande Rift margin, but available data suggest that basalts are unfaulted or offset by small faults and that post–4 Ma faulting was minor (e.g., Ricketts and Karlstrom, 2014). Various magmatically driven geodynamic forcings are possible. The most obvious is the construction of the Mount Taylor stratovolcano just north of the RSJ that changed surface elevation by several kilometers at the volcanic peak, with accompanying increase in snowmelt discharge. However, semisteady denudation and similar incision rates in other rivers in the region indicate a regional erosional landscape within which construction of the volcano was an important but not primary driver of differential river incision over the past 5–8 Ma. Our focus on the pre–Mount Tayler RSJ paleoprofile reinforces this conclusion. Other mantle-related uplift mechanisms that may have generated mantle buoyancy include thermally driven buoyancy generation (Roy et al., 2004) and/or magmatic inflation due to dike and sill networks related to the building of the Mount Taylor stratovolcano and eruption of the Zuni-Bandera volcanic fields. Both could have contributed to uplift, but their relative importance is unknown. Broad epeirogenic uplift is also possible due to small-scale upper mantle convection beneath a thin elastic plate and resulting dynamic topography (Braun, 2010; Moucha et al., 2009). The thickness of the elastic plate in this region of New Mexico is ∼35 km, but broken plate flexure models are likely given the extensional faulting along the crest of the inferred bending elastic plate such that wavelength of the resulting uplift could be similar to wavelength of the underlying mantle forcings.
We are uncertain of the relative importance of these and other potential geodynamic epeirogenic uplift mechanisms, but the ∼150 km wavelength of the observed differential incision and similar wavelength of proposed arching of paleoprofiles and the Jemez mantle anomaly suggest that, in some combination, mantle-driven neotectonic forcings were the primary driver for magmatism, broad uplift, and differential river response above the Jemez lineament in the past 4 Ma.
We are indebted to the New Mexico Geological Society, University of New Mexico College of Arts and Sciences, and the University of New Mexico Earth and Planetary Sciences for the generous funding they provided for Channer’s undergraduate research. Research was also supported in part from NSF grants EAR-1348007, IIA-1301346 to Karlstrom and EAR-1322089 to Zimmerer and Heizler. We also thank Robert Garcia, Dominik Morningdove, Steve Juanico, and the Pueblo of Acoma for their cooperation and generous contribution of drill-core data and samples. We thank Joel Pederson and Fraser Goff for reviewing an earlier version of this manuscript.