Abstract The history of fault initiation and reactivation in the southern Rocky Mountains remains highly debated, as does the region’s exhumation history. Nowhere has the evidence been more contested than in the southern Sangre de Cristo Mountains, where major, 30+ km dextral separations of basement rocks and their aeromagnetic anomalies have been attributed to Proterozoic, Ancestral Rocky Mountain and Laramide orogenies. Since the sum of these dextral separations is in the range of 100 km, unambiguous determination of the age(s) of faulting would have major implications to Rocky Mountain tectonics. Likewise, the history of exhumation and stabilization of the western North American craton provides an important example of continental lithospheric evolution. This field trip will start by visiting excellent exposures of spectacularly brecciated yet indurated basement rocks and flanking Paleozoic sedimentary rocks along the Picuris-Pecos fault system, which has 37 km of dextral separation of Proterozoic contacts. Hypotheses for the age(s) of slip will be examined in light of stratigraphic and fault relationships, thin section petrography and isotopic analyses. The region’s history of fault reactivation and associated K-metasomatism will be discussed by combining thermochronology, largely based on new 40Ar/39Ar K-feldspar analyses, with recent seismic data across the Laramide front of the Sangre de Cristo Mountains. The regional tectonic implications of new geologic mapping, fault analyses, 40Ar/39Ar thermochronology and seismic studies will be discussed on the outcrop, with a full examination of all hypotheses.

The axial basins of the northern and central Rio Grande rift evolved since late Oligocene as a chain of half grabens between the Colorado Plateau on the west and the interior of the craton on the east. The basins range from 80 to 240 km in length and from 5 to 95 km in width, with an average width of approximately 50 km. Basin-fill sedimentary deposits range up to 5 to 6 km in thickness. The oldest dated volcanic rocks interbedded with syn-rift sediments are about 26 Ma. Extension in the rift was left oblique and increases southward from 8 to 12% in the San Luis Basin, to about 17% in the northern Albuquerque Basin, to at least 28% in the southern Albuquerque Basin, to an average of 50% in the Socorro area. The most rapid phase of regional extension was middle to late Miocene. Sedimentary deposits of this age are volumetrically dominant within the rift basins and on the rift flanks (Ogallala and Bidahochi Formations). Basin asymmetry shifts back and forth from east-tilted to west-tilted across complex northeast-trending accommodation zones that developed along preexisting transverse structural lineaments. These accommodation zones lie on small circles about the Miocene Euler pole of rotation for the Colorado Plateau relative to the stable craton. Rotation about this Euler pole in northeastern Utah caused synchronous episodes of Miocene extension and sedimentation in the Rio Grande rift and in the central Wyoming-northern Colorado area, largely by the collapse of Laramide uplifts. Late Cenozoic rotation of the Colorado Plateau was 1.0 to 1.5° clockwise. Structural subdivision of basin floors within the rift by longitudinal master faults increased with increasing extension until, at some point in excess of 28% extension, one or more longitudinal fault blocks was uplifted isostatically through the basin surface to form an intrabasin tilted-block mountain range that subdivided the basin. Faulting and subsidence tended to migrate towards the basin axis as newer master faults cut off or merged downdip with, earlier faults. This deactivated the outer longitudinal blocks and left them stranded as shallow suballuvial benches covered by relatively thin sections of the older syn-rift sediments. Recent paleobotanical studies indicate that elevations prior to rifting were similar to present elevations. This suggests that crustal thickening during Laramide compression resulted in regional uplift during the early Cenozoic. Exhumation of the escarpment along the Rocky Mountains-High Plains boundary by erosion of approximately 400 to 600 m of Cenozoic and Cretaceous strata may be a consequence late Cenozoic climate change.

The deepest part of the northern Rio Grande rift is just northwest of the Great Sand Dunes, on the eastern side of the San Luis Valley, in south-central Colorado. Approximately 150 km (95 m) of high-quality, 20-fold Common Depth Point seismic data indicate that the basin is filled with approximately 6.4 km (21,000 ft) of Tertiary sediments, mostly Oligocene or younger in age. These data, combined with published data, provide a basis for interpreting the structural geometry of the rift. The internal structural geometry of this part of the rift is surprisingly uncomplicated. Sympathetic and antithetic faulting is not widespread, or major, within the deep part of the basin. The internal geometry of the sedimentary packages, however, indicates a complicated movement history on the faults that are present within the rift. The bounding fault zone between the rift and the Sangre de Cristo Mountains has approximately 9.2 km (30,000 ft) of vertical separation and may be very complicated. Although we were not able to resolve the bounding fault zone with our seismic data, the data constrains the minimum angle of the fault zone to be approximately 45°. Our modeling suggests that a 60° angle is the most likely orientation of the fault zone. The influence of earlier, low-angle extension recognized nearby and elsewhere in the Rio Grande rift is not directly evident in our data. Assuming 60° antithetic shear in the hanging wall, which is supported by the seismic data, a depth of detachment (flattening) is estimated to be approximately 16 km, in the brittle-ductile transition zone estimated from heat-flow data. Cross sections drawn using the constraints of the new seismic data, require only 8 to 12% extension across the rift at this latitude.

Analysis of borehole and reflection seismic data from the Alamosa basin (northern San Luis Basin, Rio Grande rift) reveals tectonic development in response to three Tertiary events, each with an associated package of rocks distinguished by mineralogy and petrology. Eocene redbeds of the Blanco Basin Formation (0 to 696 m thick) are micaceous, sandy mudstone and coarse arkosic sandstone units containing lithic pebbles derived from granitic basement rock. They were deposited in a late Laramide basin formed during wrench-fault-related segmentation of the early Laramide San Luis-Brazos uplift. The western half of the younger, rift-related Alamosa basin is superposed over this late Laramide basin. Initiation of Oligocene volcanism is marked by andesitic lava flows and volcaniclastic rocks of the Conejos Formation (0 to 2,300 m thick), also limited in extent to the western half of the Alamosa basin. Ash-flow tuffs (380 to 580 m thick) correlative to 29 to 27 Ma tuffs of the San Juan volcanic field cap the Conejos Formation in the western half of the basin and rest directly on denuded Precambrian basement in the eastern half of the basin. These tuffs exist in deep wells across the Alamosa basin and together represent a basinwide time marker. Extension related to the Rio Grande rift resulted in eastward-tilting of the entire basin area following emplacement of the ash-flow tuffs. Filling the resulting half graben is the upper Oligocene-middle Pleistocene Santa Fe Group (as much as 5.6 km thick) composed of variegated mudstones and coarse lithic sandstones and conglomerates. Lithic fragments in the Santa Fe Group represent two sources: variable-composition volcanic rocks from the San Juan volcanic field to the west (majority) and plutonic-metamorphic-sedimentary, basement-derived rocks from the Sangre de Cristo Range to the east (minority). An angular unconformity within the Santa Fe Group documents strong early tilting due to movement on the Sangre de Cristo fault zone during an early phase of rifting (late Oligocene-early Miocene). The rift-related geometry of the crust beneath the Alamosa basin is that of two east-tilted crustal blocks creating two second-order half grabens within the basin.

Previous paleomagnetic work in the north-central Rio Grande rift (RGR) of northern New Mexico has demonstrated the presence of counterclockwise paleomagnetic rotations, possibly due to left-lateral shearing across the rift. To improve the spatial coverage and resolution of the intrarift rotation, we analyzed the late Tertiary sediments within the exposed Española Basin for paleomagnetic rotations. From five sites broadly distributed across the rift, the amount of rotation varies from little on the eastern edge to over 20° counterclockwise in the central and western rift. If interpreted as tectonic net rotation axes, the amount of rotation varies from 10 to 24°. Gently northeast plunging net rotation axis orientations in the eastern part of the rift indicate dominantly extension-related deformation. Steeper axis inclinations near the center and western part of the rift and Embudo accommodation zone indicate the presence of increased horizontal shearing in these regions. The paleomagnetic data show that the north-central Rio Grande rift is not rotating as a unit but rather consists of a number of smaller independently, counterclockwise-rotating blocks.

The Albuquerque Basin is one of the largest and deepest basins of the Rio Grande rift. The latest Oligocene to middle Pleistocene Santa Fe Group is the major basin fill unit. Paleogene deposits underlie the Santa Fe and indicate that a depositional center predated the Albuquerque Basin. Pre-Santa Fe Tertiary deposits are subdivided into a lower unit that is at least partly correlative with the Eocene Galisteo and Baca Formations and an upper unit, the unit of Isleta #2, that is equivalent to the Datil Group and the overlying sequence of Oligocene volcanic rocks. Thickness of the Santa Fe Group ranges from 1,000 to 2,000 m along the basin margins to as much as 4,407 m in the basin center. Galisteo and Baca thicknesses are as much as 454 m; the unit of Isleta #2 is up to 2,185 m in the Shell West Mesa well. Galisteo-Baca sediments were deposited in basins that predated the Albuquerque Basin. These depocenters continued to receive sediments of the unit of Isleta #2 during early to middle Oligocene time. Lower Santa Fe sediments (30 to 5 Ma) were deposited into two internally drained basins. After 10 Ma, the Santa Fe had filled the basins to the point where a single, internally drained Albuquerque Basin was formed. At about 5 Ma, the basin drainage became through-flowing with the development of the ancestral Rio Grande. The first major episode of Rio Grande Valley entrenchment at about 1.0 Ma ended Santa Fe deposition. Sedimentation rates ranged from 20 to 30 m/m.y. during early and late phases of Santa Fe deposition to as much as 600 m/m.y. during middle Miocene deposition.

The Albuquerque Basin, located in the central portion of the Rio Grande rift, is filled by 7,350 m of Tertiary clastic sediments deposited on a “basement” of Mesozoic and Paleozoic sedimentary rocks and Precambrian crystalline rocks. The basin has been a center of Cenozoic volcanic activity over the last 37 m.y. Detailed examination of regional seismic reflection data, supplemented by well control and field work, has demonstrated that the Albuquerque Basin is asymmetric and structurally complex, consisting of two subbasins downdropped along low-angle to listric-normal faults of opposing structural polarity, some of which flatten at depths of about 10 km. The northern subbasin has been downdropped along a major west-dipping, listric-normal fault system whereas the southern subbasin is bordered by a system of major east-dipping, low-angle normal faults. Palinspastic restorations show that the amount of extension in the Albuquerque Basin ranges from 17% in the north basin to at least 28 to 30% in the south basin. The differential amount and polarity of extension between the two subbasins is taken up across a complex midbasin transverse structural zone/accommodation zone that seismically displays characteristics of a transtensional, negative-flower structure. General agreement on overall structural style exists relative to COCORP interpretations of the structure of the south basin. However, an alternative interpretation of the COCORP data is presented that advocates fold geometries on major basin-bounding normal faults in this portion of the basin. Seismic and outcrop evidence suggest that preexisting structures in the Precambrian basement rocks may in part have controlled the geometry of Tertiary structures in the region.

The Santa Fe Group is a succession of Neogene sandstone, siltstone, conglomerate, and intercalated volcanics that fills the basins of the central Rio Grande rift. Data from twelve deep wells and recently released seismic profiles in the Albuquerque rift basin document a dramatic basinward thickening of the Santa Fe Group across the major basinal faults. Abbreviated thicknesses of less than 1,220 m (4,000 ft) occur on the structurally higher, outer benches, but the section thickens to more than 4,270 m (14,000 ft) in the basin center, on the downthrown sides of the basin’s seismically defined master normal faults. Thickness of the Santa Fe mimics the basin’s structural asymmetry. In the north, it forms an eastward-thickening clastic wedge in response to eastward tilting of the north part of the basin toward the westward-flattening master faults (Rio Grande, Sandia, and San Francisco-Placitas faults). In the south, it thickens westward toward the Santa Fe-Coyote master faults along the southwestern margin of the basin. As a result, depositional rates of the Santa Fe were rapid on the structurally deep, master-fault sides of the basin in the northeast and southwest and relatively slow on the basin’s hinge sides in the northwest and southeast. At the northeast-trending Tijeras accommodation zone, which divides the north and south halves of the basin, the section thickens abruptly to the north in the basin center, indicating a major dip-slip as well as probable strike-slip component of motion between the two subbasins. Much of the thickening in the Santa Fe Group seems to occur in the upper Miocene part of the section, possibly implying a phase of accelerated crustal extension at that time.

A systematic relationship exists between seismically defined, master normal faults of the Albuquerque Basin and the structural style of rift shoulders on the basin margins. Rift shoulders occupying the footwalls of these master faults have at least three characteristics distinguishing them from the other shoulders of the basin: 1. They are the source areas for the basin’s major fanglomerates in the syn-rift Santa Fe Group. 2. They display distinctly Neogene apatite fission-track ages. 3. They have undergone the greatest amount of rift-related uplift in the Neogene. These relationships imply that uplift of these rift shoulders may be in part, the result of isostatic rebound of the lithosphere, accompanying tectonic unloading by the seismically defined master faults of the basin.