Crustal Geophysics and Geochemistry Science Center

Geophysics of the Rio Grande Basins

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San Luis Basin Geophysics

Central San Luis Basin - Drenth Dissertation Summary

Geophysical Constraints on Rio Grande Rift Structure in the Central San Luis Basin, Colorado and New Mexico

ABSTRACT

Interpretation of gravity and aeromagnetic data reveals patterns of rifting, rift-sediment thicknesses, distribution of pre-rift volcanic and sedimentary rocks, and distribution of syn-rift volcanic rocks in the central San Luis Basin, the northernmost of the major basins that make up the Rio Grande rift. Rift-sediment thicknesses for the central San Luis Basin calculated using a three-dimensional gravity inversion indicate that syn-rift Santa Fe Group sediments have a maximum thickness of ~2 km in the Sanchez graben near the eastern margin of the basin along the central Sangre de Cristo fault zone. Under the Costilla Plains, thickness is estimated to reach ~1.3 km, although no independent thickness constraints exist, and a range of thicknesses of 600 m to 2 km are geophysically reasonable. Considerable ambiguity exists regarding what rocks may lie between the bottom of the Santa Fe Group sediments and Precambrian basement beneath the Costilla Plains, and the presence of Mesozoic and Paleozoic sedimentary rocks cannot be ruled out. The Santa Fe Group sediments also reach a thickness of nearly 1 km within the Monte Vista graben near the western basin margin along the San Juan Mountains. A narrow, north-south-trending structural high beneath San Pedro Mesa with about 2 km of positive relief with respect to the base of the Sanchez graben separates the graben from the structural depression beneath the Costilla Plains. Geophysical data provide new evidence that this high is rooted in the Precambrian basement. A structural high composed of pre-rift rocks, long inferred to extend from under the San Luis Hills to the Taos Plateau, is confirmed and found to be denser than previously believed, with little or no overlying Santa Fe Group sediments. Major faults in the study area are delineated by geophysical data and models; these faults include significant vertical offsets (> 1 km) of Precambrian rocks along the central and southern zones of the Sangre de Cristo fault system. Other faults with similarly large offsets of the Santa Fe Group include a fault bounding the western margin of San Pedro Mesa, and other faults that bound the Monte Vista graben in an area previously assumed to be a simple hinge zone at the western edge of the San Luis Basin. A major north-south-trending structure interpreted to be a down-to-the-east normal fault or fault zone occurs at the boundary between the Costilla Plains and the San Luis Hills structural high and is shown in our gravity modeling. This fault does not have much expression as a major rift fault; it is likely related to pre-rift tectonic events. Aeromagnetic anomalies are interpreted to mainly reflect variations of remanent magnetic polarity and burial depth of the 5.3-3.7 Ma Servilleta basalt of the Taos Plateau volcanic field. Magnetic-source depth estimates indicate patterns of subsidence following eruption of the basalt and show that the Sanchez graben has been the site of maximum subsidence.

INTRODUCTION

The San Luis Basin is the northernmost of the major basins that comprise the Rio Grande rift, extending for roughly 250 km north-south through south-central Colorado and northern New Mexico. The present-day geomorphic basin is bounded on the west by the San Juan Mountains and volcanic field in Colorado and the Tusas Mountains in New Mexico, and on the east by the Sangre de Cristo Mountains (Fig. A1). The central portion of the San Luis Basin is loosely defined as the region that includes an eastward-offset embayment in the Sangre de Cristo range, the Culebra reentrant (Upson, 1939; Wallace, 2004), the San Luis Hills, Costilla Plains, San Pedro Mesa, the narrow valley coincident with the Sanchez graben between San Pedro Mesa and the Culebra Range, and the region lying between the San Luis Hills and San Juan Mountains to the west (Figs. A1 & A2). The general subsurface geometry of the central portion of the basin is poorly understood, although the surface geology is well constrained by recent mapping (e.g., Thompson and Machette, 1989; Kirkham, 2004; Thompson et al., 2007a; Machette et al., 2008).

This study uses recently acquired gravity and aeromagnetic geophysical datasets, along with geologic map data and subsurface constraints derived from interpreted drill hole logs, to interpret the configuration of tectonic elements present along an east-west transect through a portion of the central San Luis Basin. The main goal of this study is to determine how the Rio Grande rift is manifested within the central San Luis Basin, including the following specific goals: to estimate the thickness of rift-fill sediments that are critical hosts of groundwater supplies (Topper et al., 2003); to determine what the rift-sediment thickness distribution indicates about basin structure and temporal pattern of rifting; to evaluate the distribution of syn-rift volcanic rocks and implications for the history of rifting; to interpret the extent and structural configuration of pre-rift rocks and their relationship to the rift; and to better delineate rift-bounding structures than surface geologic mapping is able to do.

GEOLOGIC BACKGROUND

The region occupied by the San Luis Basin has been structurally and topographically high for much of its history from the Paleozoic to the Late Eocene, spanning the Ancestral Rocky Mountain and Laramide orogenies. Subsequent extension associated with continental rifting occurred along much of the uplifted orogen (Sales, 1983; Kellogg, 1999; Kluth, 2007). Like most of the southern Rocky Mountain region, the area currently underlain by the San Luis Basin was occupied by middle-Tertiary volcanic rocks of the southern Rocky Mountain volcanic field (Steven, 1975; Lipman, 2007). Extension of the central San Luis Basin began ~28-25 Ma, following eruption of the uppermost ash-flow tuffs of the San Juan volcanic field (Lipman, 1975a, b; Lipman and Mehnert, 1975; Thompson and Dungan, 1985; Thompson and Machette, 1989; Thompson et al., 1991; Lipman, 2007). In some areas of the basin, the Santa Fe Group lies directly on Precambrian basement, whereas much of the central and western portion of the basin includes a thick section of Oligocene volcanic rocks between the Santa Fe Group and Precambrian basement rocks.

Faults generally trend along directions similar to preexisting north-northwest-trending Precambrian faults, a pattern reflected in basin-margin and intra-basin faults and structures (Tweto, 1979a, b). The general structure of the basin is commonly considered to be an east-tilted half graben, with a hinge zone along its margin with the San Juan Mountains, a major north-south-trending normal fault system bounding its eastern margin with the Sangre de Cristo Mountains, and a north-south-trending structural high through the basin center (e.g., Tweto, 1979b). This simplistic picture is more complicated in the central San Luis Basin, where deformation along the margin of the Sangre de Cristo Mountains has formed the Culebra reentrant (Fig. A1), and San Pedro Mesa represents a secondary intra-rift structural high.

The Sangre de Cristo fault system forms the eastern tectonic boundary of the entire San Luis Basin and is divided into northern, central, and southern zones (Personius and Machette, 1984; Ruleman and Machette, 2007). The southern portion of the central zone occurs in the study area along the margin between the Culebra Range and Sanchez graben (Fig. A2), and has a relatively gentle geomorphologic expression that suggests a lower slip rate compared to the northern and southern zones and/or the fact that the footwall of the fault is underlain by basin-fill sediments (Ruleman and Machette, 2007). The northern portion of the southern Sangre de Cristo fault zone forms the western boundary of San Pedro Mesa. Offset along the fault increases to the south into New Mexico, where a sharp range front fault is evident south of the village of Costilla. The down-to-west Mesita fault (Thompson et al., 2007b), synthetic to the southern Sangre de Cristo fault zone, trends north-south through the central Costilla Plains (Fig. A2), although offset is only about 13 m at the surface.

Precambrian igneous and metamorphic rocks, mainly granitic/gneissic in composition, form the western portion of the Culebra Range and are bounded on the west by the central Sangre de Cristo fault zone. A recently mapped outcrop of Precambrian rock lies near the western margin of San Pedro Mesa (Fig. A2) (Kirkham, 2006; Thompson et al., 2007a), where the basin floor was previously assumed to have little or no relief (e.g., Keller et al., 1984). Magnetotelluric soundings provided the first indication that this outcrop was rooted at depth (B. Rodriguez, personal comm., 2006), showing that significant relief of Precambrian rock is present below the mesa. A borehole in the southern Sanchez graben, the Williamson well (Fig. A2) (Kirkham et al., 2005; Thompson et al., 2007a), reached Precambrian rocks at a depth of 1.9 km, where they are directly overlain by Santa Fe Group sediments. No additional Precambrian rock crops out or has been encountered in boreholes in the study area west of San Pedro Mesa.

Because the study area has been structurally and topographically high during much of its geologic history, little or no Paleozoic or Mesozoic rocks have been thought to remain in the study area (e.g. Brister and Gries, 1994). While this is likely the case in the Sanchez graben, as demonstrated by drilling, the presence of Mesozoic and possibly Paleozoic rocks (Shirley, 1995; Watkins et al., 1995; Morel and Watkins, 1997; Hoy and Ridgway, 2002) in the northern San Luis Basin (outside the study area) suggests that these rocks may exist at depth within the study area under the Costilla Plains. Younger pre-rift sedimentary rocks, such as Eocene sediments, may also exist in significant thicknesses under the Costilla Plains but do not exceed 10 m in thickness where they crop out on the margin of San Pedro Mesa (Thompson et al., 2007a).

As much as 600 m of Oligocene volcanic rocks crop out in the San Luis Hills (Thompson and Machette, 1989; Thompson et al., 1991) and likely form much of the lower portion of the basin. These rocks are correlative with the 35-30 Ma Conejos Formation that makes up a portion of the San Juan volcanic field to the west; in the San Luis Hills these rocks include intermediate-composition lava flows and volcaniclastic sedimentary deposits as young as 29 Ma (R.A. Thompson, personal comm., 2008). The Conejos Formation is assumed to extend into the subsurface to the west, where it is a major basin-filling unit within the Monte Vista graben, a north-south-trending depocenter that extends far north of the study area and was mainly active prior to what is typically considered Rio Grande rift extension (Brister and Gries, 1994). These rocks again reach the surface immediately west of the Monte Vista graben in the San Juan Mountains (included in rocks mapped as Tv, Fig. A2). The extent of the Conejos Formation in the subsurface under the Costilla Plains is unknown, although poor-quality seismic reflection data suggest that it may be present as an eastward-thinning package below Santa Fe Group sediments under at least a portion of the Costilla Plains (Uitti, 1980). However, it does not crop out on San Pedro Mesa (Thompson et al., 2007a) and was not encountered by the Williamson well in the Sanchez graben. Ash-flow tuffs, erupted from the San Juan and Latir (south of study area) volcanic fields (Lipman et al., 1986), are likely to overlie the Conejos Formation in much of the study area, although their thicknesses are unknown. Erosional topography that formed on the Conejos Formation between ~29 and ~26 Ma in the San Luis Hills area was partially filled by basalts of the Hinsdale Formation that now remain as mesa-capping outcrops (Dungan et al., 1989; Thompson and Machette, 1989). The Hinsdale basalts also crop out on southern San Pedro Mesa above another package of Tertiary volcanic rocks (Thompson et al., 2007a). These basalts are regionally dated 27 to 6 Ma, but are dated between 27 and 25 Ma in the San Luis Hills (Lipman and Mehnert, 1975; Thompson and Machette, 1989).

Intermediate composition intrusions crop out in the San Luis Hills (stars on Fig. A2, unit Tiq of Thompson and Machette, 1989) and correlate in age with the Conejos Formation, although their extent at depth is unknown. Other Oligocene intrusions crop out in the southern Rocky Mountain region (Lipman, 2007) and are typically associated with gravity lows (Plouff and Pakiser, 1972; Lipman, 1983; Cordell et al., 1985).

The San Luis Hills are a horst that is structurally high, composed of pre-rift rocks, and flanked by structural lows filled by rift sediments around its margin. The hills are part of a 150-200 km, north-south-trending, mostly buried horst that is thought to exist underneath the San Luis Valley (e.g., Keller et al., 1984; Grauch and Keller, 2004). Locally, the northwestern tectonic margin of the hills is inferred to be a concealed down-to-the-northwest fault (Tweto, 1979a, b), informally known as the Manassa fault (Fig. A2). Other faults have been inferred along the eastern margin of the San Luis Hills, in both a north-south zone along the Rio Grande and a northeast-southwest zone along the southern margin of the northeastern San Luis Hills (Burroughs, 1972; Tweto, 1979a, b; Thompson et al., 1991), although detailed geologic mapping (Thompson and Machette, 1989) did not include these structures since they have little or no surface expression.

Santa Fe Group sediments comprise the basin fill in the central San Luis Basin. Locally, in the transect area, these deposits post-date the youngest ash-flow tuffs from the southeastern San Juan volcanic field (< ~28 Ma) and are as young as mid-Pleistocene age (Ingersoll et al., 1990; Brister and Gries, 1994). Lithologies encountered in wells are primarily weakly to strongly consolidated claystone, sandstone, and conglomerate.

Interbedded with the uppermost Santa Fe Group sediments are numerous flows of the regionally extensive 5.3-3.7 Ma Servilleta basalt of the ~6-2 Ma Taos Plateau volcanic field (Lipman and Mehnert, 1975; Lipman, 1979; Dungan et al., 1984; Dungan et al., 1989; Appelt, 1998). These flows are less than 200 m thick in the study area and generally less than 70 m thick (indicated by shallow borehole data, available from the Colorado Division of Water Resources).

Within the study area, basalt flows crop out along the northern Taos Plateau and on San Pedro Mesa. Multiple wells indicate that the basalt is shallowly buried (generally < 350 m, data available from Colorado Division of Water Resources) beneath the Costilla Plains. Total thicknesses range from approximately 200 m on the Taos Plateau, to ~140 m in the Sanchez graben (encountered in the Williamson well), to 30-65 m on San Pedro Mesa and beneath the Costilla Plains. Basalts of similar age crop out on the northeastern flank of Los Mogotes volcano (Fig. A2) (Lipman and Mehnert, 1975; Appelt, 1998). Other rocks associated with the Taos Plateau volcanic field include dacites that form large volcanic domes, such as Ute Mountain in the southern part of the study area (unit Td, Fig. A2).

GEOPHYSICAL DATA AND METHODS

Gravity Methods

Gravity anomalies reflect lateral variations of density, with gravity highs occurring over regions of relatively high densities, such as mountains composed of crystalline basement, and gravity lows occurring over large volumes of low-density materials, such as unconsolidated sediments. Large density contrasts between low-density rift-filling sediments, such as the Santa Fe Group, and older rocks make gravity data useful for defining the configuration of basins within the Rio Grande rift (Cordell, 1978; Daggett et al., 1986; Grauch et al., 2006).

Regional quality (1-5 km station spacing) gravity data were extracted from the PACES gravity database that is maintained by the University of Texas at El Paso (http://paces.geo.utep.edu/) and supplemented with the acquisition of about 130 new stations in the San Pedro Mesa-Culebra reentrant region during 2006 and 2007. The PACES database consists of data collected over decades by many previous workers and was compiled as the result of a major cooperative effort between federal agencies and universities (Keller et al., 2006). Standard techniques (e.g., Blakely, 1995) were used to process the gravity data and calculate complete Bouguer anomalies, including corrections for predicted gravitational attraction at the elevation and latitude of the observation point (theoretical and free air corrections), effects of homogeneous masses underneath (Bouguer correction), and effects of topographic masses (terrain corrections). The standard reduction density of 2670 kg/m3 (Hinze, 2003) was used for the Bouguer and terrain corrections. An additional step of computing isostatic residual anomalies was performed, in order to remove the effect of long-wavelength anomalies that correlate with regional topography. This method is only one possible way of removing a regional field and results in a gravity anomaly map (Fig. A3) that is a better representation of upper-crustal density variations than a more standard complete Bouguer anomaly map (Simpson et al., 1986; Blakely, 1995). Computation of isostatic residual anomalies requires estimates of crustal thickness and Moho density contrast, but these values do not need to be accurate to yield useful results. Estimates of these parameters resulting from a lengthy investigation for gravity data for the state of New Mexico (Heywood, 1992) were used in this study, including a Moho density contrast of 300 kg/m3 and a normal (assuming ground surface at sea level) crustal thickness of 20 km.

Previous three-dimensional gravity models of the San Luis Basin (Keller et al., 1984) facilitated mapping of major tectonic elements and thickness of the Santa Fe Group. However, this work assumed a single density contrast (350 kg/m3) between the Santa Fe Group and older rocks, and had limited data in several key areas where new data have since been acquired.

For this study we implemented a more sophisticated approach to determine basin geometry: an inverse method that attempts to separate the gravitational effect of the low-density Santa Fe Group sediments from that of older, denser rocks (Jachens and Moring, 1990; Blakely and Jachens, 1991; Blakely, 1995). Unlike other basin-depth estimation methods, this approach allows user-defined density-depth functions for the basin fill while also accommodating density variations of the pre-rift rocks. The method includes the following steps. First, an initial approximation for the gravity field caused by the pre-rift rocks is computed from only those gravity stations located on outcrops of those rocks and then is subtracted from the isostatic residual gravity anomaly field. The result is a crude approximation of the gravity field due to variations in the thickness of the Santa Fe Group alone. This residual field was inverted for sediment thickness using a density-depth function (Fig. A4) estimated from well logs in the Albuquerque and Española Basins to the south and assumed to be valid for the northern Rio Grande rift (Grauch et al., 2006). The inversion is based on a method that assumes the basin extends infinitely laterally and iteratively solves for basin depths (Bott, 1960; Blakely, 1995). Next, the calculated gravitational effect of the resulting Santa Fe Group thickness distribution was subtracted from the isostatic residual field at the pre-rift gravity stations, in order to produce an improved estimate of the gravity field of all pre-rift rocks. The entire process is iteratively repeated until the difference between observed and calculated thicknesses becomes minimal. Results include a gravity map due to all pre-Santa Fe Group (pre-rift) rocks (Fig. A5), thickness distribution of Santa Fe Group deposits (Fig. A5), and a structural elevation on the base of the Santa Fe Group computed by subtracting the thickness distribution from the surface topography (Fig. A6). Constraints on the depth to the bottom of the Santa Fe Group for the inversion came from mapped locations of older rocks (places where the Santa Fe Group thickness is zero) (Tweto, 1979a; Green, 1992; Anderson and Jones, 1994; Green and Jones, 1997; Thompson et al., 2007a), interpretations of drilling records (HRS Water Consultants, 1987; Brister, 1990; Kirkham et al., 2005), and results of seismic reflection surveys (Gries and Brister, 1989).

The presence of Servilleta basalt within the basin may lead to errors in estimates of Santa Fe Group thickness, particularly in areas where the volume of the basalt is large relative to the size of the basin (locations where basalt thicknesses exceed ~100 m, Santa Fe Group thicknesses are less than 1 km, and no borehole or seismic constraints exist). However, the relatively small volume of the basalt compared to the volume of the basin in most of the study area corresponds to a small error (< 6%) in the final computed depths. The basalts have densities of about 2700 kg/m3 (M. Anderson, Colorado College, personal comm., 2008) and displace densities of 2170 kg/m3 in the upper 1.25 km of sediments. The difference in assumed mass leads to incorrect Santa Fe Group thickness estimates where no independent thickness constraints are placed on the model. This effect may have led to an underestimation of the thickness of the Santa Fe Group beneath the Taos Plateau, where basalt thicknesses reach 200 m and few independent constraints exist on the depth to the base of the Santa Fe Group.

Magnetic Methods

Magnetic anomalies reflect spatial variations of total magnetization, the vector sum of induced and remanent magnetizations. Induced magnetization, an instantaneous property, is proportional to magnetic susceptibility and has the same direction as the present-day ambient field (inclination of 64 degrees, declination of 10 degrees in the study area). Remanent magnetization is a long-lived property, is related to a rock's formation and geologic history, and may be directed in a different direction than the induced magnetization. Volcanic rocks commonly carry large-magnitude components of remanent magnetization. In regions with near-surface volcanic rocks and locally high topographic relief, magnetic anomalies can be produced in several different ways, depending on rock magnetic properties, volume and depth of the rock unit, and relation to topography. In this study area, magnetic lows can be sourced by reversely polarized, highly magnetic rocks (generally represented by relatively large-magnitude magnetic lows), weakly magnetized rocks (commonly represented by subtle, broad lows), or terrain and/or anomaly shape effects such as topographically low areas surrounded by or adjacent to normally magnetized topography. Magnetic anomaly highs can alternatively be interpreted as caused by normally magnetized rocks, magnetic rocks with induced magnetization that is much greater than the remanent magnetization, or topographically low areas surrounded by or adjacent to reversely magnetized topography. Terrain and anomaly shape effects can be particularly well developed along edges of magnetic rock units, such as truncated basalt flows that compose topography. Linear changes in magnetic patterns may also be caused by source edges, such as basalt flows offset by faulting or linear paleotopography.

Data from three high-resolution (200 meter flightline spacing, 150 m above the ground) total-field aeromagnetic surveys acquired during 2003-2005 were draped to a surface 100 m above the ground and merged to create an aeromagnetic map of the study area (Bankey et al., 2004; Bankey et al., 2005; Bankey et al., 2006). A reduction-to-pole transformation, a standard geophysical technique to center anomalies over their sources, was applied to the aeromagnetic data using an inclination of 64 degrees and declination of 10 degrees (Fig. A7) (Baranov and Naudy, 1964; Blakely, 1995). Identification of strongly or weakly magnetized terrain and identification of correlations between outcropping rock units and aeromagnetic anomalies is best done by careful inspection of these relations in map view. An example is presented here, using a larger scale view of the southern San Luis Hills that combines reduced-to-pole aeromagnetic anomalies, terrain, and mapped geology (Fig. A8). There, Hinsdale basalt and Conejos Formation rocks form individual topographic features, and aeromagnetic anomalies are well correlated positively with outcrop boundaries, indicating that the terrain is moderately to strongly magnetized and that the magnetization directions are close to the ambient field direction.

The geophysical expression of relatively deep and/or broad geologic sources can be enhanced by filtering anomalies according to wavelength, since deep/broad sources produce longer wavelength anomalies than do surficial sources. Here, we applied a low-pass filter with a 5 km filter cutoff to the reduced-to-pole aeromagnetic anomalies in order to isolate anomalies broader than the short wavelengths observed over individual topographic features in the San Luis Hills (Fig. A9), and thereby removed anomalies clearly related to terrain and surface geology.

Quantitative estimates of depth to magnetic sources help facilitate geologic interpretation by differentiating shallow from deep sources, allowing delineation of different tectonic patterns. Unlike the low-pass filter approach to separating deep from shallow sources, these calculations provide direct estimates of source depth beneath the ground surface. Numerous depth estimation techniques exist, and for this study we chose to use the local wavenumber method, since this method has been shown to be effective for aeromagnetic datasets (such as this one) with low noise levels (Thurston and Smith, 1997; Smith et al., 1998; Phillips, 2000, 2007; Phillips et al., 2007). In this approach, spatial derivatives of the measured magnetic field are related to the depth of magnetic sources, resulting in a distribution of depth estimates (Fig. A10). These results indicate the estimated depth to the top of the shallowest magnetic sources, which may be strongly magnetized rocks that crop out (such as basalt on the surface of San Pedro Mesa) or are buried (such as basalts buried in the Sanchez graben).

GEOPHYSICAL EXPRESSION OF GEOLOGIC FEATURES

The San Luis Basin is expressed geophysically by a north-south trending gravity low that reflects the relatively low density of the Santa Fe Group sediments compared to the Precambrian basement (Cordell, 1978). Within this trend is a gravity high trending north-south through the area north of Alamosa coincident with a buried intra-rift horst (Gaca and Karig, 1966; Keller et al., 1984; Kluth and Schaftenaar, 1994). The gravity high also continues south of Alamosa over the San Luis Hills and into New Mexico, where a structural high has long been inferred (Lipman, 1979; Grauch and Keller, 2004). The following section discusses the geophysical characteristics of the main tectonic features present within the central San Luis Basin and key results of geophysical data analyses.

Culebra Range and Eastern Basin Boundary

The eastern boundary of the San Luis Basin in the study area is geologically defined by the central Sangre de Cristo fault zone, and geophysically by the gradient that separates a gravity high on the east from a gravity low on the west (Fig. A3). The gravity low west of the fault zone is caused by thick, low-density Santa Fe Group sediments in the Sanchez graben.The gravity high is caused by higher-density Precambrian rocks.

The gravity high over the Culebra Range is not as large as the high caused by pre-rift rocks within the basin to the west over the San Luis Hills (Fig. A3). The high gravity values over the Culebra Range decrease to the south, with the lowest values observed in northern New Mexico (Fig. A3). Two possible explanations exist for such low gravity values over Precambrian rocks. The first is that during the Laramide orogeny, Precambrian rocks of the modern Sangre de Cristo range were thrust over Cretaceous and Tertiary sedimentary rocks of the Raton Basin east of the study area, so that the low-density sedimentary rocks now lie under Precambrian crystalline rocks that form the high mountains (Trevino et al., 2004). This is supported by recent mapping of the eastern Culebra Range that demonstrates that the scenario is spatially/geometrically reasonable (Fridrich and Kirkham, 2007). Another possibility is that a silicic intrusion or series of intrusions within the Culebra Range are less dense than the Precambrian country rocks, causing the low gravity values. Silicic intrusions in the region are mainly Precambrian or Tertiary in age and have been interpreted to be sources for gravity lows elsewhere in the Sangre de Cristo Mountains (Cordell and Keller, 1984; Cordell et al., 1985; Grauch and Keller, 2004; Quezada et al., 2004).

San Pedro Mesa-Sanchez graben

Recent gravity data acquisition for this study revealed a ~12 mGal gravity high over the west-central portion of San Pedro Mesa (SPM on Fig. A3), with the highest anomaly amplitude located over the newly discovered outcrop of Precambrian basement rock. The southern Sangre de Cristo fault zone bounds this anomaly on the west (Ruleman and Machette, 2007; Thompson et al., 2007a). Relatively high gravity values trend south near the western edge of San Pedro Mesa into New Mexico, merging with a high over the Sangre de Cristo Mountains near Costilla where Precambrian rocks crop out (SDC on Fig. A3). North of the San Pedro Mesa basement outcrop (SPM and north of SPM on Fig. A3), gravity values decrease and merge with lower amplitude, broader gravity anomalies over the northern Sanchez graben and the town of San Luis. Gravity values smoothly decrease to a greater extent to the east of the basement outcrop on San Pedro Mesa, until a low is reached over the southern Sanchez graben (SG), where Precambrian rocks were encountered at a depth of 1.9 km in the Williamson well. These observations are consistent with the hypothesis developed from magnetotelluric soundings that the Precambrian basement outcrop on San Pedro Mesa is rooted and is not a detached fragment from a previous orogeny underlain by low-density sediments (see profile model section below).

The interpretation of rooted basement under San Pedro Mesa was built into the gravity inversion by forcing the basement to reach the surface at the outcrop location, with the result being a north-south-trending basement high along the western margin of the mesa (Fig. A6). The deepest portion of the Sanchez graben lies just north of the Colorado-New Mexico state border. A broad, relatively shallow area of the basin occurs beneath the northern Sanchez graben and the town of San Luis (SL). The inversion results also show that vertical displacement on north-south-trending faults bounding both east and west sides of San Pedro Mesa decreases to the north.

Aeromagnetic anomalies over San Pedro Mesa display a complex pattern of short-wavelength, often high-amplitude highs and lows (Fig. A7). This is a common aeromagnetic anomaly signature over basalts. Paleomagnetic measurements of oriented core samples of basalt along the mesa show the presence of both normal and reverse polarities of remanent magnetization (M. Hudson, personal comm., 2007), which explains the alternating normal-reverse anomaly signature observed along the mesa edge. Landslide blocks immediately below the western edge of the mesa include large slabs of Servilleta basalt, producing complex north-south-trending aeromagnetic highs and lows that lie adjacent to the southern Sangre de Cristo fault zone.

Aeromagnetic anomalies tend to be more subdued over the Sanchez graben and the area around San Luis, indicating deeper sources. The Williamson well places Servilleta basalt flows at a depth of about 270 m in the southern Sanchez graben. Anomaly sources throughout the remainder of the graben are interpreted as being buried basalt.

Magnetic-source depth estimates (Fig. A10) indicate shallow (< 350 m) depths over San Pedro Mesa and the basalt-capped mesa northwest of San Luis, reflecting the effect of shallowly buried and outcropping basalts. Zones of relatively deep (>450 m) solutions include most of the southern Sanchez graben, as well as two smaller zones south and north of San Luis. Some of these zones are bounded by recently mapped faults that cross the graben along a northwest-southeast trend (near label SG, Fig. A10) (Thompson et al., 2007a).

Costilla Plains

The Costilla Plains are expressed geophysically by a broad gravity low reflecting a large thickness of Santa Fe Group sediments, bounded on the west and east by gravity highs associated with the San Luis Hills and San Pedro Mesa, respectively (Fig. A3). Along the western margin of San Pedro Mesa, a gravity gradient tracks the location of the southern Sangre de Cristo fault zone, reflecting shallow basement to the east and deep basement to the west. A larger magnitude gradient trends north-south along the western margin of the low, near the Rio Grande, with gravity values increasing to the west where pre-rift rocks crop out in the San Luis Hills (see following paragraphs). The broad gravity low over the Costilla Plains reaches its lowest values along the state border and into Sunshine Valley (SV on Fig. A3).

The north-south-trending gravity gradient corresponding roughly to the location of the Rio Grande, with high values west of the river and low values to the east, indicates a fundamental difference in the composition and/or structure of pre-rift rocks from the southern San Luis Hills area to the Costilla Plains. The north-south trend of the gradient as expressed in the pre-rift gravity map (Fig. A5) is consistent with pre-rift tectonic patterns observed throughout the San Luis Basin region (Tweto, 1979b) and suggests that pre-rift tectonics may have influenced the formation of the western boundary of the Costilla Plains. An alternative hypothesis is that the gradient is reflective of a major syn-rift fault, although there is little surface evidence for such a structure (e.g., Thompson and Machette, 1989).

In order to address this question, end-member models of the possible source of the gravity gradient were tested using the gravity inversion. The thickness of Santa Fe Group sediments under the Costilla Plains has been interpreted to reach a maximum of about 2 km, on the basis of poor-quality seismic reflection data (Uitti, 1980), which would support the hypothesis that the gradient reflects a major syn-rift fault. The first test used the seismic reflection estimate to represent the maximum possible thickness of the Santa Fe Group, emphasizing the hypothesized syn-rift effect of the N-S trending gravity gradient. The second test forced the pre-rift gravity field under the Costilla Plains to be similar to the gravity field over the northeastern San Luis Hills (see following section), which had the effect of producing a minimum Santa Fe Group thickness and emphasizing the pre-rift effect of the gravity gradient. The second test resulted in a maximum thickness of only 500 m for the Santa Fe Group, contrasted to the ~2 km estimated from seismic data, which we consider to be a maximum reasonable thickness. Given the large uncertainty of the seismic interpretation and the lack of surficial evidence for a major syn-rift fault along the Rio Grande, we chose to present a compromise model that was not constrained by the seismic reflection interpretation, assigned a significant portion of the Rio Grande gravity gradient to the pre-rift field, and produced a maximum Santa Fe Group thickness of 1.3 km, with the greatest thickness under the southern Costilla Plains (Fig. A5).

Structural offset along the southern Sangre de Cristo fault zone is estimated from the inversion results to be 500 m to 1 km, with offset increasing to the south. A northwest-trending splay of this fault zone off the northern end of San Pedro Mesa (CP1 on Fig. A6) is also reflected in the inversion results, as a down-to-the-southwest normal fault with an approximate offset of 300-400 m. Another north-south-trending structural offset (CP2 on Fig. A6) about 6 km west of the western edge of San Pedro Mesa is most clearly apparent near the state border, but can be extended to a total length of ~16 km using the gravity inversion results (dashed white line, Fig. A6). This offset bounds the most depressed portion of the Costilla Plains structural low and corresponds to the location of down-to-the-west normal faults mapped near the state border (CP2). The western boundary of the structural low is bounded by the down-to-west Mesita fault (Fig. A6) that has about 13 m of displacement, an inconsistent relation that may indicate the fault has been reactivated from an earlier down-to-east normal fault.

The southwestern structural boundary of the Costilla Plains is indicated to be a broad fault zone or ramp along a north-northwest trend in the area of Ute Mountain along which the basin structure narrows to the south into Sunshine Valley, although independent constraints are lacking in this region. Ute Mountain is the site of a data artifact in the inversion results (Fig. A6), where subtraction of the thickness distribution from the surface topography failed to remove the entire effect of Ute Mountain. In other words, this should not be taken as an indication that a structural high exists beneath Ute Mountain.

The aeromagnetic map (Fig. A7) displays a similar pattern of short-wavelength anomalies over the Costilla Plains as the one over San Pedro Mesa. The anomaly patterns probably reflect Servilleta basalt buried at a shallow depth beneath sediments. Near the western edge of San Pedro Mesa, anomalies appear to follow the trends of nearby anomalies caused by alternating remanent polarities of basalt exposed on the mesa, suggesting lateral continuity of basalt flows across the southern Sangre de Cristo fault zone into the Costilla Plains subsurface. The anomaly patterns over the entire Costilla Plains may also be explained by alternating remanent polarities of basalts within the basin fill, but this interpretation is tentative due to lack of exposure. Linear anomalies in the aeromagnetic map (Fig. A7) coincide with the trace of the Mesita fault as well as faults exposed in the southeastern portion of the Costilla Plains, likely indicating where faults offset basalt. A profound change in anomaly character also occurs across the Mesita fault, with high values to the east and low values to the west (Fig. A7). This may be an expression of an important geologic boundary, such as an extension of Conejos Formation rocks into the subsurface from the west. Another possibility is that the Mesita fault has in the past been a more important structure than it is today (only showing ~13 m of offset), and that emplacement of Servilleta basalt was affected by the fault. This could have resulted in different individual flows with different magnetic properties being present on either side of the fault.

Depth estimates show that basalts beneath the Costilla Plains are generally buried at depths of less than 350 m (Fig. A10). Water well logs from the area typically show multiple stacked flows of variable thickness (and possibly different magnetic properties) separated by sediments. The tops of the uppermost basalt flows are generally less than 350 m deep, and individual flows may be as much as 10-15 m thick. Conspicuous zones of deeper solutions exist along the southwestern portion of the fault that splays northwest off the north end of San Pedro Mesa (Fig. A10), as well as along the southern portion of the Costilla Plains into Sunshine Valley. Seismic reflection data also indicate that the Servilleta basalt deepens near the latitude of Ute Mountain (Uitti, 1980).

San Luis Hills and Northern Taos Plateau

A broad, high-amplitude gravity high lies over the San Luis Hills and the northern Taos Plateau volcanic field (Fig. A3). Based on the significant exposures of pre-rift volcanic rocks in these areas, this anomaly has long been interpreted as reflecting a mid-basin structural high (Gaca and Karig, 1966; Keller et al., 1984; Grauch and Keller, 2004). The gravitational effect of pre-rift rocks (Fig. A5) suggests that the highest gravity values in the study area are over the San Luis Hills west of the Rio Grande in a broadly north-south orientation. A significant gravity low in the signature of pre-rift rocks exists over the northeastern San Luis Hills, implying a large difference in densities between the volumes of rocks that underlie the northeastern and western portions of the hills. The source of this low is unknown, but it may be due to relatively thick pre-rift sediments and/or sedimentary rocks, a low-density igneous intrusion of Oligocene age(?), or low-density rocks within the Precambrian basement. Inversion results also show that the structural high associated with the San Luis Hills extends to the south and west under the northern Taos Plateau with little or no (<200 m) Santa Fe Group present beneath Servilleta basalt (Fig. A6). Structures bounding the San Luis Hills include the fault zone or ramp bordering the Costilla Plains (discussed above), as well as a zone of deepening along the northwestern margin of the San Luis Hills that corresponds to the approximately located Manassa fault.

Aeromagnetic anomalies over the Taos Plateau have a similar short-wavelength character to those observed elsewhere over exposed and inferred Servilleta basalt. A large-amplitude aeromagnetic low over Ute Mountain is likely caused by reversely polarized volcanic rocks that form the mountain. The circular anomaly resembles similar ones over individual volcanoes and vents in the Taos Plateau south of the study area, where paleomagnetic and radiometric dating evidence supports the presence of strong remanent magnetizations of both normal and reverse polarities (Grauch and Keller, 2004).

The San Luis Hills have a different geophysical character than surrounding regions covered by Servilleta basalt, and the aeromagnetic anomalies over the hills reflect a pattern of strongly magnetized topography. Hinsdale basalt caps high ridges and produces strong (> 1000 nT) positive aeromagnetic anomalies that mimic the pattern of the topography, suggesting that the basalts are dominantly normally polarized (Figs. A7 & A8). Much lower anomaly amplitudes (up to hundreds of nT) are observed over outcrops of the Conejos Formation, and relationships between aeromagnetic anomalies and topography indicate that these rocks are mainly normally polarized in the San Hills west of the Rio Grande, and mainly reversely polarized in the northeastern hills. The areas of different polarity generally correspond to different stratigraphic levels within the Conejos Formation, with normally polarized rocks corresponding to the lower Conejos Formation west of the river, and reversely polarized rocks to the east of the river corresponding to the upper Conejos Formation. This relation implies that ages of the rocks span at least one reversal of the Earth's field.

On the western side of the San Luis Hills, anomaly patterns corresponding to areas of outcropping volcanic rocks extend well beyond those outcrops, in a complex pattern of broadly east-west-trending anomalies (Figs. A7 & A8). Because cliff exposures show that Hinsdale basalts were erupted onto a deeply incised surface cut into the Conejos rocks (Thompson and Machette, 1989), we hypothesize that the east-west-trending anomalies reflect buried paleotopography in the areas that are covered. The sources of east-west-trending lows could be channels cut into normally polarized units of the Conejos Formation that are now filled with weakly magnetized sediments or with younger, reversely polarized volcanic flows.

The anomaly patterns extending west of the San Luis Hills gradually become broader and more subdued, reflecting deeper sources. This effect is demonstrated quantitatively by predominantly greater source depth estimates in a ~10-km-wide, north-northeast-trending zone west of the San Luis Hills (Fig. A10) that corresponds to the Manassa fault as defined by the gravity inversion (Fig. A6).

A broad, sub-circular, composite aeromagnetic high >400 nT in amplitude occurs over the southern San Luis Hills (Fig. A7 & outlined on Fig. A9). At 15-20 km in diameter, this anomaly is much broader than the dominant wavelengths of anomalies related to terrain, apparent on the long-wavelength filtered map (Fig. A9). As discussed above, relatively broad anomalies reflect relatively deep and/or broad sources. Also, magnetic depth estimates include a number of deep (>450 m) solutions in this area, which may be related to the anomaly's source. Therefore, we interpret the source of this anomaly to be a buried, strongly magnetized body with the approximate dimensions of the anomaly itself, as shown on Fig. A9. This interpretation is discussed in detail below.

Monte Vista Graben and Western Basin Boundary

The Monte Vista graben in the study area lies about 50 km south of where it is well defined by seismic and borehole data northwest of Alamosa (Brister and Gries, 1994). A ~15 mGal, north-south-trending isostatic gravity low extends south into the study area, suggesting that the structures lie along a similar trend and are likely related. Thus, the name Monte Vista graben is used here.

Gravity inversion results indicate a graben with a significant thickness of Santa Fe Group sediments (Figs. A5 & A6). The western edge of the graben represents the western boundary of the San Luis Basin with the San Juan volcanic field. The abrupt structural relief imaged there suggests that this may be a fault zone, contrary to previous interpretations (e.g., Tweto, 1979b) that this is a hinge zone with unfaulted San Juan volcanic rocks dipping gently into the western San Luis Basin. The eastern boundary of the graben (MVGEB, Fig. A6) occurs along an abrupt northeast-trending margin of the structural high associated with the San Luis Hills and Taos Plateau that is similar in trend although not directly aligned with the inferred Manassa fault trend. To the south, a structural high along the state border bounds the deeper portion of the graben, although due south in New Mexico the extreme northern portion of another structural low is imaged (Fig. A6).

Aeromagnetic anomalies and depth estimates over the inferred graben are typical of outcropping or shallowly buried basalt flows, and correlate with Servilleta-like basalt flows that extend north and northeast from the Los Mogotes volcano (Figs. A2, A7, & A10). These flows overlie the inferred graben and extend into the subsurface beyond the northern edge of their outcrop, indicated by a pattern of short-wavelength aeromagnetic anomalies and shallow depth estimates.

PROFILE MODEL

In order to display the interpreted geologic relationships in a more quantitative manner, a 2.5D gravity and magnetic model along profile A-A' (Figs. A3 & A11) was constructed using the results of the gravity inversion as a starting point and well data, outcrop locations, magnetotelluric modeling (Drenth et al., in prep.), and magnetic source depth estimates as constraints (using GM-SYS software, http://www.geosoft.com). The gravity and magnetic model's location was chosen to most effectively capture the main tectonic features in the study area (Figs. A2 & A3). The following sections discuss details of the construction of the model and interpretation of the final model.

Model Construction and Geophysical Properties

Because the Precambrian rocks likely cause the largest and broadest variations in the pre-rift gravity effect (Fig. A11a) and their densities are unknown, the first step was to construct an equivalent density model for the Precambrian basement. The equivalent model serves as a proxy to compute and remove the broad basement effects and focus on other geologic units represented by the profile model. The equivalent basement model was constructed by assigning variable densities to vertically bounded zones that are 8 km thick until the gravity field was matched (Fig. A11d). The densities of other pre-rift rocks, such as the Conejos Formation, were not allowed to vary across the profile.

The second step was to develop the geometry of the base of the Santa Fe Group. The results of the gravity inversion (Fig. A6) provided a robust first approximation to the Santa Fe Group thickness (inverted from "rift basin effect" curve, Fig. A11a), and subsequent adjustments to the model resulted in only small differences (<10%) between the inversion results and the final model (Fig. A11d). The minor adjustments account for thickness/depth constraints from magnetotelluric data (Drenth et al., in prep.), outcrop locations, and wells, and provided a good fit to the observed gravity profile.

The final step was to develop a model of geologic units above the basement that honored all independent constraints on depths, unit thicknesses, and physical properties. Physical rock properties (summarized in Table 1) assigned to the specific units in the model were based on unpublished field and lab measurements, published averages based on lithology (Telford et al., 1990), and fits to the model. The Conejos Formation is an example of the latter case; density logs in San Juan volcanic field (San Juan Mountains) indicate that an overall average density for the Conejos Formation is about 2500 kg/m3 (R. Gries, written comm., 2007), yet there, the unit is mainly composed of andesite flows. In the San Luis Basin, the Conejos Formation includes a significant proportion of volcaniclastic deposits that are likely to have lower densities than andesite.In the course of profile modeling it was found that a density of 2500 kg/m3 made a proper fit difficult while still honoring well constraints and the gravity inversion results. For these reasons, a density of 2400 kg/m3 was used because it produced a better fit to the observed data.

No independent constraints exist on the depth to the bottom of the Santa Fe Group sediments or to what lies under these sediments beneath the Costilla Plains, and therefore considerable uncertainty exists. The Conejos Formation, pre-rift Tertiary sedimentary rocks, or Mesozoic or Paleozoic sedimentary rocks have similar densities and cannot be distinguished on the basis of gravity modeling. These pre-rift units have likely densities ranging from 2400-2600 kg/m3, and a possible thickness distribution for them was found using an averaged density of 2500 kg/m3 (Fig. A11d, "unknown pre-rift volcanic/sedimentary rocks").

The Santa Fe Group was modeled using the assumed density-depth function discussed above (Fig. A4), although only the upper two density zones were needed for the depths encountered along the profile. As discussed above, greater and lesser thicknesses of the Santa Fe Group were tested using the 3D inversion in order to determine geophysically reasonable end-members of thicknesses, specifically under the Costilla Plains. The model shown (Fig. A11d) is our preferred model, although different thicknesses were tested in 2D, as well as in part of the magnetotelluric modeling (Drenth et al., in prep.). No magnetic properties were assigned to these sediments. Despite measured magnetic susceptibilities on the order of 1 to 10 x 10-3 (SI units) for sediments in this and other Rio Grande rift basins (Grauch et al., 2001; Grauch and Hudson, 2007; Hudson et al., 2008), wavelengths of the aeromagnetic anomalies modeled are more regional at the profile model scale and are dominantly produced by volcanic/crystalline rocks.

Basalts are present in most of the study area, have high densities, and are strongly magnetized. Although massive basalts often have densities near 3000 kg/m3 (Telford et al., 1990), samples of the Servilleta basalt have measured densities of about 2700 kg/m3 (M. Anderson, personal comm., 2008). Densities have not been measured on the Hinsdale basalts but were also assigned a density of 2700 kg/m3. Oriented samples of Servilleta basalt were collected on San Pedro Mesa and yielded remanent intensities of 1-8 A/m in both normal and reverse polarities, with intensities varying considerably over short distances (M. Hudson, personal comm., 2007). Magnetic susceptibilities also vary considerably for these basalt samples, and a typical value of 0.015 SI units was used in the model. The Hinsdale basalt has high measured magnetic susceptibilities, although the remanences of the basalt and similar rocks are not known. However, inspection of anomalies compared to geology and topography suggests the Hinsdale basalt is normally polarized nearly everywhere in the study area and almost certainly carries some remanence. Therefore, a very high susceptibility (0.1 SI units) was used in the model to simulate the combined effect of both the high susceptibility and normal polarity remanence.

Interpretation of Gravity-Magnetic Model (A-A')

Given the setup of the model along A-A', the interpretation of the preferred model follows. The part of the model representing Precambrian rocks is an equivalent model and is required to fit low gravity values over the Culebra Range and high values over the San Luis Hills. Accordingly, the lowest densities occur at the eastern portion of the profile and, as discussed above may represent the effect of overthrusted, low-density sedimentary rocks at depth and/or a low-density intrusion in the Culebra Range. The model presented here includes low densities in the Culebra Range, and is not meant to endorse one case over the other. The highest densities of the Precambrian equivalent model occur underneath the structural high associated with the San Luis Hills and northern Taos Plateau volcanic field, although these density variations may also occur in pre-rift rocks younger than Precambrian age.

About 2 km of relief is interpreted for the Precambrian rocks between the base of the Sanchez graben and the outcrop on top of San Pedro Mesa, and is fairly well-constrained as discussed previously. This impressive, asymmetric basement ridge has an extremely steep western margin that lies roughly 1 km east of the trace of the southern Sangre de Cristo fault zone, suggesting that the fault zone is or has been wider at depth than its current surface expression. The eastern margin of the basement high is only slightly less precipitous, plunging from the surface near the western rim of San Pedro Mesa to a depth of more than 1 km beneath its eastern rim. Independent constraints on depth to Precambrian basement in the study area are unavailable west of the outcrop on San Pedro Mesa. The model as drawn (Fig. A11d) indicates basement at nearly the shallowest levels possible, constrained by deep wells in the San Luis Hills area that do not reach basement.

Maximum thicknesses of the Santa Fe Group under the entire Costilla Plains and in the Sanchez graben are interpreted to be 1.3 and 2 km, respectively. The structural high of Precambrian rocks under San Pedro Mesa forms a prominent boundary separating the two grabens. Areas where sharp changes occur in Santa Fe Group thickness correlate well with mapped fault zones, including the central and southern Sangre de Cristo fault zones and the fault bounding the eastern margin of San Pedro Mesa (Thompson et al., 2007a). The western margin of the Costilla Plains has long been thought to be controlled by a fault (Tweto, 1979a), but no direct evidence for faulting has been found on the surface. However, the model indicates that a fault zone or ramp marks the boundary between the San Luis Hills and Costilla Plains, roughly coincident with the Rio Grande. We believe the most reasonable explanation is that these inferred structures form a zone of down-to-the-east normal faults. The timing of movement on these faults, however, is difficult to determine, given the uncertainties of what lies beneath the Santa Fe Group under the Costilla Plains.

As much as 800 m of Santa Fe Group sediments are interpreted to lie within the Monte Vista graben. The large structural offsets bounding the graben can be interpreted as fault-controlled, contrary to the assumption that the San Luis Basin's western margin is a simple hinge zone with little structural relief.

The thicknesses, lateral extents, and dips of post-Precambrian pre-rift rocks shown in the model are only crude approximations at best, since they are not well constrained within the study area. These include Eocene and possibly Mesozoic and/or Paleozoic sedimentary rocks that may exist in the Monte Vista graben (Brister and Gries, 1994) and that may be present under the Conejos Formation in the western San Luis Hills as suggested by borehole data. The presence of these rocks, as well as Conejos Formation and ash-flow tuffs of the San Juan volcanic field, cannot be ruled out beneath the Costilla Plains. Units that lie above Precambrian basement and pre-date Santa Fe Group rocks and sediments do not crop out on or east of San Pedro Mesa.

Aeromagnetic anomalies along the profile are well explained by variations in magnetic properties and burial depths of volcanic rocks, primarily the Servilleta basalt in most of the study area. The approximate burial depth and thickness of the basalt within the Sanchez graben are approximately known from the Williamson well 5 km south of the profile and are consistent with magnetic depth estimates. In the Sanchez graben a normal polarity was assigned to the basalt (N1, Fig. A11d), with the magnitude varied until the observed aeromagnetic anomaly was matched (1 A/m was required). A similar approach was used for the basalt that crops out on the surface of San Pedro Mesa, although there a reverse polarity signature was interpreted (R1, 4 A/m). The burial depth of basalt beneath the Costilla Plains is well known from water well logs, but the total thickness is not definitively known. The basalt is thought to be confined to the uppermost sedimentary section there, with a total thickness of roughly 70 m; we assumed this geometry for the model across the Costilla Plains. Immediately west of San Pedro Mesa, an aeromagnetic low is interpreted to reflect dominantly reversely polarized basalt (R2, 8 A/m) that may be laterally correlative with basalt on the mesa having the same polarity signature. An area of higher aeromagnetic values to the west is interpreted to be a zone of mainly normally polarized basalt (N2, 3 A/m). Extending this alternating polarity approach west, however, is not attempted beyond assigning a reverse polarity (R3, 5 A/m) to basalt beneath the Costilla Plains, because the magnetic properties of the Sevilleta basalt are not constrained beyond San Pedro Mesa, resulting in the progressive failure of the model to match anomalies there. Andesites and dacites of San Pedro Mesa do not appear to be strongly magnetized nor do they appear to produce gravity anomalies, although the gravity station coverage over these rocks may not be adequate. Therefore, it is difficult to model their subsurface extent or to conclusively determine whether they are underlain by Santa Fe Group sediments or rest directly on Precambrian rocks. Based on the density assigned to these rocks in the profile model (2400 kg/m3, based on their lithology) and the limited available gravity data, it would appear that the andesites and dacites are underlain by Santa Fe Group sediments.

The Conejos Formation is likely to have strongly heterogeneous magnetic properties, given that it is volcanic (thus likely to be strongly magnetized) and volcaniclastic (likely to be weakly to moderately magnetized). The geometry of these variations, however, is not known, and the unit is modeled as having uniform properties. As discussed above, exposed Conejos Formation near the profile has mainly normal polarity (west of the Rio Grande). It is difficult to capture the geometry of the interpreted east-west-trending paleotopography of the San Luis Hills using an east-west profile, although the inclusion in the model of Hinsdale basalt in the shallow subsurface provides a fit to some of the short-wavelength aeromagnetic anomalies over the hills.

Two strongly magnetized, shallowly buried bodies under the San Luis Hills (labeled "Intrusions?" on Fig. A11d) were included to provide a possible interpretation for longer-wavelength anomalies over the San Luis Hills. These inferred bodies do not produce a gravity low, unlike many Oligocene intrusions in the southern Rocky Mountain region. Precambrian rocks were assigned moderate magnetic susceptibilities as needed to account for the longest-wavelength trends along the profile.

DISCUSSION

Computed thickness variations of the Santa Fe Group sediments may indicate patterns of rifting over time, provided that simplifying assumptions are made about the geology. These assumptions include: the deepest Santa Fe Group sediments in the study area correspond to areas involved in rifting since its inception (28-25 Ma), dips within the Santa Fe Group are sub-horizontal, sediment supply has been constant throughout the history of rifting, computed Santa Fe Group thickness is an effective proxy for the amount of subsidence, and the Servilleta basalt (5.3-3.7 Ma) is consistent enough in age across the study area to make it a robust time horizon. The thickness of the Santa Fe Group representing the time interval from inception of rifting to the eruption of Servilleta basalt is greater in the Sanchez graben (1.2 km) than under the Costilla Plains (up to 900 m in the preferred model), suggesting that the greatest subsidence occurred close to the margin of the Culebra Range. The same time interval accounts for the ~1 km of sedimentation/subsidence in the Monte Vista graben. Rift-related subsidence appears to have followed a similar pattern since or during emplacement of the Servilleta basalt, as indicated by a greater thickness of basalt and overlying Santa Fe Group sediments in the Sanchez graben compared to other portions of the study area. This is also expressed in the magnetic depth estimates as relatively deep solutions within the Sanchez graben (Fig. A10).

The gravity inversion and profile model provide a new perspective on faulting patterns within the central San Luis Basin (Figs. A6, A11d, & A12). The western boundary of the basin has long been assumed to be a simple hinge zone at the margin of the San Juan Mountains. However, large structural offsets are noted at the boundaries of the Monte Vista graben and are interpreted to be fault zones. The eastern boundary of the Monte Vista graben is shown to be at least partially controlled by the Manassa fault. Published maps show differing interpretations of the boundary between the San Luis Hills and Costilla Plains, with Tweto (1979 a,b) showing fault zones and Thompson and Machette (1989) omitting them due to a lack of surface expression. Results of modeling indicate that a significant fault zone or structural ramp does exist along the eastern margin of the San Luis Hills that has displaced the Costilla Plains downward. Geophysical observations and interpretations along this structure or set of structures include the following: (1) it is the location of a major gradient in the gravity field of pre-rift rocks (Fig. A5), (2) it forms the western and northwestern boundary of the Costilla Plains structural depression as shown by the gravity inversion results (Figs. A6 and A12), and (3) it may offset pre-rift rocks (Fig. A11d). Modeling further suggests that the offset along this structure may be greater for pre-rift rocks than for the Santa Fe Group (Fig. A11e), suggesting a long-lived structure possibly inherited from earlier tectonism that had significant offset prior to rifting, relatively minor offset during formation of the rift, and is no longer active today. The western boundary of the structural depression beneath the Costilla Plains is gentler, with thinner Santa Fe Group sediments, than its eastern margin (Fig. A11d) along San Pedro Mesa and the active southern Sangre de Cristo fault zone. These observations are consistent with the western depression margin being less active throughout the history of rifting than the eastern margin. Other faults under the Costilla Plains may be responsible for the thickness variations of the Santa Fe Group sediments, including faults with recent offsets such as the Mesita fault and other faults near the western margin of San Pedro Mesa (Fig. A6). Both the central and southern zones of the Sangre de Cristo fault system, as well as the fault or fault zone that bounds San Pedro Mesa on the east, are shown by gravity modeling to be major structures (Figs. A6, A11d).

A broad (15-20 km diameter) aeromagnetic high over the southern San Luis Hills is interpreted to be produced by a buried source (Fig. A12). The anomaly only partially correlates spatially with the gravity high over the San Luis Hills (Figs. A5 & A7) and occurs over a much smaller area. The source may be an Oligocene intrusion or intrusive complex related to the pre-rift volcanic rocks in the overlying hills. In this scenario, a candidate for the material that composes the intrusion is unit Tiq of Thompson and Machette (1989), which forms a number of small outcropping intrusions of intermediate composition in the southern San Luis Hills (stars, Fig. A2) and displays moderately high magnetic susceptibilities (0.005-0.030 SI units, V. Grauch, unpublished data, 2006). Unlike other Oligocene intrusions in the southern Rocky Mountains, however, this body does not produce a gravity low. Another possibility is that the source is a relatively strongly magnetized zone within heterogeneous Precambrian rocks, perhaps mafic or ultramafic rocks. In this scenario, the source rocks may be related to the source of the gravity high over the San Luis Hills (Fig. A5).

The north-south-trending Precambrian basement structural high that underlies western San Pedro Mesa is interpreted to have ~2 km of relief from the basement in the Sanchez graben, as indicated by borehole information from the Williamson well. Units that lie above Precambrian basement and pre-date Santa Fe Group sediments do not crop out on or east of San Pedro Mesa, indicating that the structural high underneath the mesa may have already been present when rifting began. Volcanic rocks older than Servilleta basalt on southeastern San Pedro Mesa, as well as those within the Sanchez graben, have not been found west of the mesa (e.g., Fig. A11d). Therefore, the structural high may have been a long-lived barrier separating the area that would become the Costilla Plains from the Sanchez graben, forming prior to rifting and remaining high throughout at least early stages of rifting.

CONCLUSIONS

Three-dimensional gravity inversion and modeling reveal new details of the spatial distribution of rift-sediment thicknesses within the central San Luis Basin, part of the northern Rio Grande rift, as summarized in Fig. A12. The structural depression beneath the Costilla Plains is interpreted to contain up to 1.3 km of Santa Fe Group sediments, although as little as 600 m and as much as 2 km are geophysically reasonable, with the greatest accumulation near the depression's eastern margin. The southern Sanchez graben contains about 2 km of Santa Fe Group sediments, and at the western margin of the San Luis Basin, the Monte Vista graben is filled with up to ~1 km of Santa Fe Group. The greatest amount of rift-related subsidence apparently has been concentrated along the eastern margin of the basin adjacent to the Culebra Range, a pattern that appears to have intensified throughout the history of rifting. Areas of relatively thin sediments indicate areas of structural highs, including a narrow north-south-trending ridge of Precambrian basement rocks nearly completely buried beneath San Pedro Mesa that may be a long-lived structural high. A structural platform with <900 m of overlying sediments occurs under the northern Sanchez graben and the town of San Luis.

Modeling results show that the horst composed of pre-rift rocks exposed at the San Luis Hills extends under at least the northern Taos Plateau (Figs. A6 & A12). Less than 200 m of Santa Fe Group sediments are inferred to exist under the northern portion of the plateau. The gravity high over the San Luis Hills (Fig. A3) was thought to be due to the structural high there, although results presented here show not only a structural high but also high-density pre-rift rocks, consistent with speculation by Brister and Gries (1994).

Aeromagnetic anomalies over the Taos Plateau, Costilla Plains, San Pedro Mesa, and Sanchez graben largely reflect the burial depth and magnetic properties of the Servilleta basalt. Strongly magnetized terrain composed of Conejos Formation and Hinsdale basalt characterizes the San Luis Hills.

A number of features highlighted in this study deserve future investigation to improve our understanding of the geologic history of the central San Luis Basin. The distribution of pre-rift volcanic and sedimentary rocks under the Costilla Plains is highly uncertain. Previous studies have implicitly or explicitly assumed that little or no Mesozoic/Paleozoic rocks exist there, but the presence of Mesozoic rocks in the northern San Luis Basin (e.g., Hoy and Ridgway, 2002) suggests that they may also exist under the Costilla Plains. The models and interpretations presented here do not rule out their presence. These rocks are currently being targeted for petroleum exploration in the northern San Luis Basin.

The origins of gravity anomalies caused by pre-rift rocks (Fig. A5) are largely unknown, other than the possibilities that the gravity low over the Culebra Range is produced by overthrust sediments and/or a low-density intrusion. The subsurface distribution of the Conejos Formation, as well as pre-rift sedimentary rocks, is not constrained. The gravity low over the northeastern San Luis Hills is noteworthy for its sub-circular extent and difference in character from the gravity high over the San Luis Hills west of the Rio Grande. The anomaly source may be a relatively thick accumulation of pre-rift sedimentary rocks, or may be magmatic in origin, such as an Oligocene intrusion.

Table 1: Geophysical characteristics of units in model A-A'
Unit Density (kg/m3) Assigned magnetic susceptibility (range) (10-3 SI units) Remanence intensity (relative where not measured)
Servilleta basalt and similar rocks 2700 15 (2-26) 1-8 A/m, normal and reverse polarities observed
Santa Fe Group 2170 and 2350 Assumed zero, possibly significant for local studies Assumed none
Hinsdale basalt and similar rocks 2700 100 (assumed) Not measured or modeled, but potentially very large (included in susceptibility value)
Andesite and dacite of San Pedro Mesa 2400 1 (0.001-6) Likely small
Ash-flow tuffs of San Juan V.F. 2200 Volumetrically insignificant Volumetrically insignificant
Conejos Formation 2400 30 (assumed) Assumed small, but potentially large locally
Buried body under San Luis Hills (labeled Intrusions?, Fig. A11d) 2700 60 (assumed) Likely small if intrusion, possibly important if Precambrian volcanic rocks
Eocene sedimentary rocks 2400 Assumed small Assumed none
Cretaceous sedimentary rocks 2400-2600 Assumed small Assumed none
Paleozoic sedimentary rocks 2600 Assumed small Assumed none
Precambrian basement Variable (arbitrary) 0-20 (assumed) Assumed none

Figure A1.
Figure A1: Physiography and geography of the San Luis basin.Gray box defines area of this study and subsequent figures. Inset map shows location of Fig. A1 in relation to the Rio Grande rift.

Figure A2.
Figure A2: Simplified geology and physiography of the study area (Burroughs, 1972; Lipman, 1975b; Tweto, 1979a; Lipman and Reed, 1989; Thompson and Machette, 1989; Green, 1992; Anderson and Jones, 1994; Thompson and Lipman, 1994; Green and Jones, 1997; Kirkham et al., 2004; Thompson et al., 2007a; Machette et al., 2008). Additional data from Machette and others, unpublished mapping; Thompson, unpublished Taos Plateau mapping.

Figure A3.
Figure A3: Isostatic residual gravity anomaly map of the study area. Selected linework from Fig. A2 included for reference. Black dots are gravity station locations. Inverted red triangles are locations of magnetotelluric (MT) soundings (Drenth et al., in prep.). Location of profile model A-A' shown. Labels: CR, Culebra Range; SDC, Sangre de Cristo Mountains; SL, San Luis; SG, Sanchez graben; SPM, San Pedro Mesa; CP, Costilla Plains; SV, Sunshine Valley; SL, San Luis Hills; TP, Taos Plateau; MVG, Monte Vista graben.

Figure A4.
Figure A4: Density-depth function for Santa Fe Group rift sediments, from Grauch et al. (2006). Compare to Bouguer reduction density of 2670 kg/m3.

Figure A5.
Figure A5: Gravitational effect of pre-rift (pre-Santa Fe Group) rocks, from 3-dimensional gravity inversion. Black squares are locations of well and seismic reflection constraints. Location of profile model A-A' shown. Red dots are gravity stations on pre-rift rocks; diagonal patterned areas are outcrops of pre-rift rocks. Contour lines show computed thickness of Santa Fe Group.

Figure A6.
Figure A6: Elevation of the base of Santa Fe Group sediments, from 3-dimensional gravity inversion. Location of profile model A-A' shown. Additional labels: CP1, fault splay off northwest edge of San Pedro Mesa; CP2, structural boundary under Costilla Plains; MF1, possible southern extension of Mesita Fault.

Figure A7.
Figure A7: Reduced-to-pole aeromagnetic anomalies. Area of Fig. A8 shown by blue box. Location of profile model A-A' shown.

Figure A8.
Figure A8: Reduced-to-pole aeromagnetic anomalies (left) and elevation (right) maps of the southern San Luis Hills. Outcrops of Conejos Formation (Tc) and Hinsdale basalt (Thb) shown.

Figure A9.
Figure A9: Filtered reduced-to-pole aeromagnetic anomalies, with wavelengths greater than 5 km retained. Stars indicate locations of Oligocene intrusions in San Luis Hills (see Fig. A2). Location of profile model A-A' shown.

Figure A10.
Figure A10: Estimates of depth to magnetic sources from the local wavenumber method (see text for references).

Figure A11.
Figure A11: Geophysical profiles and model along A-A' (Fig. A3). A: Results of 3-dimensional gravity inversion, showing estimated effects of pre-rift rocks and basin geometry. B: Isostatic residual gravity anomalies (projected from within 2 km of profile location) with gravitational effect of geologic model. C: Reduced-to-pole aeromagnetic anomalies with response of geologic model. Brown line shows calculated response of Precambrian sources alone. D: Geologic model, vertically exaggerated 4 times. Densities in kg/m3 (D) and susceptibilities in SI units (S) shown for Precambrian rocks, properties of other units shown in Table 1. E: Geologic model, no vertical exaggeration.

Figure A12.
Figure A12: Summary of selected new geologic interpretations derived from geophysical data analysis. Colors and contours indicate estimated thickness of Santa Fe Group sediments from gravity inversion (CI = 200 m). Locations of inferred basin- and graben-bounding normal faults shown by dashed lines with ball on downthrown side.

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