Northern Rio Grande Rift Research Articles

Perspectives on Continental Rifting Processes From Spatiotemporal Patterns of Faulting and Magmatism in the Rio Grande Rift, USA

AbstractAnalysis of spatiotemporal patterns of faulting and magmatism in the Rio Grande rift (RGR) in New Mexico and Colorado, USA, yields insights into continental rift processes, extension accommodation mechanisms, and rift evolution models. We combine new apatite (U‐Th‐Sm)/He and zircon (U‐Th)/He thermochronometric data with previously published thermochronometric data to assess the timing of fault initiation, magnitudes of fault exhumation, and growth and linkage patterns of rift faults. Thermal history modeling of these data reveals contemporaneous rift initiation at ca. 25 Ma in both the northern and southern RGR with continued fault initiation, growth, and linkage progressing from ca. 25 to ca. 15 Ma. The central RGR, however, shows no evidence of Cenozoic fault‐related exhumation as observed with thermochronometry and instead reveals extension accommodated through Late Cenozoic magmatic injection. Furthermore, faulting in the northern and southern RGR occurs along an approximately north‐south strike, whereas magmatism in the central RGR occurs along the northeast to southwest trending Jemez lineament. Differences in deformation orientation and rift accommodation along strike appear to be related to crustal and lithospheric properties, suggesting that rift structure and geometry are at least partly controlled by inherited lithospheric‐scale architecture. We propose an evolutionary model for the RGR that involves initiation of fault‐accommodated extension by oblique strain followed by block rotation of the Colorado Plateau, where extension in the RGR is accommodated by faulting (southern and northern RGR) and magmatism (central RGR). This study highlights different processes related to initiation, geometry, extension accommodation, and overall development of continental rifts.

Progressive opening of the northern Rio Grande rift based on fault structure and kinematics of the Tusas-Abiquiu segment in north-central New Mexico, U.S.

Extension orientation is a key parameter in rift tectonics. While a clockwise rotation of the extension orientation is widely accepted for the southern Rio Grande rift and the Basin and Range province in the western U.S., in which direction(s) the northern Rio Grande rift opened has been controversial. Moreover, slip re-orientation, a phenomenon caused by local stress rotation on weak faults under a uni-directional regional extension, imposes a serious impact on kinematic interpretations. We investigate the presently oblique Tusas-Abiquiu segment of the Rio Grande rift in north-central New Mexico to assess the rifting kinematics with the presence of pre-existing crustal weaknesses. Fault-slip data show an overall NE-SW extension on the NW-trending Tusas border faults, and WNW-ESE extension on the NNE-trending Abiquiu border and internal faults. The slip vectors on the border faults show a wide variety of oblique-slip component, whereas those on the internal faults are predominantly pure dip-slip. The majority of our field measurements can be reproduced by the slip re-orientation model, although this model's setting does not satisfy regional geological records. Clockwise rotation of the extension axes from NE-SW to WNW-ESE can also explain our fault-slip data, as well as other structures in the rift. Tectonic reactivation during the extension also influenced the landscape evolution of the early rift. The Rio Grande rift shared similar extension orientation with the southern Basin and Range during the late Oligocene. The extensional strain field of the southwestern North American plate shows a progressive, time-transgressive rotational pattern, implying that plate boundary processes, instead of asthenospheric upwelling, governed the initiation and early evolution of this intracontinental rift.

A shallow rift basin segmented in space and time: The southern San Luis Basin, Rio Grande rift, northern New Mexico, U.S.A.

ABSTRACTInterpretation of gravity, magnetotelluric, and aeromagnetic data in conjunction with geologic constraints reveals details of basin geometry, thickness, and spatiotemporal evolution of the southern San Luis Basin, one of the major basins of the northern Rio Grande rift. Spatial variations of low-density basin-fill thickness are estimated primarily using a 3D gravity inversion method that improves on previous modeling efforts by separating the effects of the low-density basin fill from the effects of pre-rift rocks. The basin is found to be significantly narrower—and more complex in the subsurface—than indicated or implied by previous modeling efforts. The basin is also estimated to be significantly shallower than previously estimated. Five distinct subbasins are recognized within the broader southern San Luis Basin. The oldest and shallowest subbasin is the Las Mesitas graben along the northwestern basin margin, formed during the Oligocene transition from Southern Rocky Mountain volcanic field magmatism to rifting. In this subbasin, sediments are estimated to reach a maximum thickness of ~400 m within a north–south elongated structural depression. Other subbasins that likely initially developed during the Miocene are the dominant tectonic features in the southern San Luis Basin. This includes the Tres Orejas subbasin, which formed in the southwestern portion of the basin by the Embudo fault zone and a hypothesized fault zone along its western margin. This subbasin reaches a maximum thickness of ~2 km, as indicated by magnetotelluric and gravity modeling. The Sunshine Valley, Questa, and Taos subbasins occupy the eastern part of the southern San Luis Basin. The southern Sangre de Cristo fault zone is the dominant tectonic feature that controlled their development after ~20 Ma. The east-down Gorge fault zone controlled the western margins of significant parts of these eastern subbasins, although much of the Taos subbasin may be superimposed on the Tres Orejas subbasin. Maximum low-density basin-fill thicknesses are estimated to be 1.2 km for the Sunshine Valley subbasin, 800 m for the Questa subbasin, and 1.8 km for the Taos subbasin. Subbasin-forming tectonic activity along the Gorge fault zone and within the Tres Orejas subbasin ceased by the end of the development of the largely Pliocene Taos Plateau volcanic field. After that, rift-related subsidence became more narrowly centered on the eastern margin of the basin, controlled mainly by the linked Embudo and southern Sangre de Cristo fault zones.

Effects of melting, subduction-related metasomatism, and sub-solidus equilibration on the distribution of water contents in the mantle beneath the Rio Grande Rift

The distribution of water in the upper mantle plays a crucial role in the Earth's deep water cycle, magmatism, and plate tectonics. To better constrain how these large-scale geochemical systems operate, peridotite and pyroxenite mantle xenoliths from Kilbourne Hole (KH) and Rio Puerco (RP) along the Rio Grande Rift (NM, USA) were analyzed for water, and major and trace element contents. These xenoliths sample a lithosphere whose composition was influenced by subduction and rifting, and can be used to examine the effects of melting, metasomatism, and sub-solidus equilibration on the behavior of water. The first result is that in KH xenoliths, olivines underwent negligible H loss during xenolith ascent, i.e. preserved their mantle water contents. These olivine water contents are used to calculate mantle viscosities of 0.5–184 · 1021 Pa·s. These viscosity values are more than 40 times higher than those of the asthenosphere and show that KH peridotites represent samples from the lithosphere. The preservation of olivine water contents is exceptional for off-cratonic xenoliths, and the KH peridotites provide the first estimate of the average concentration of water in Phanerozoic continental mantle lithosphere at 81 ± 30 ppm H2O. The mantle lithosphere beneath the Rio Grande rift is nevertheless heterogeneous with water contents ranging from <0.5 to 120 ppm H2O in peridotites and from 227 to 400 ppm H2O in pyroxenites. A composite KH xenolith of a harzburgite cross-cut by a clinopyroxenite vein shows this heterogeneity at the cm scale. The second contribution of this study stems from the majority of the KH peridotites and two of the RP peridotites having major and trace elements that can be explained by partial melting without any need to invoke metasomatic processes. This allows to show that, prior to modelling the water content variation of each peridotite mineral during melting, a correction for sub-solidus equilibration has to be applied to the water contents of the minerals. Sub-solidus equilibration also provides an explanation for the discrepancy between the clinopyroxene/orthopyroxene ratio of water contents in natural peridotites worldwide and in laboratory experiments on water partitioning in peridotite minerals. Finally, the cryptically metasomatized peridotites, rare at KH and abundant at RP, as well as the pyroxenites, permit to decipher the origin and water contents of the metasomatic melts that affected the continental lithosphere beneath the Rio Grande Rift. Trace element modelling of the metasomatized KH and RP peridotites are consistent with metasomatism via melts that are of subduction origin. Melts in equilibrium with peridotites contain more water at RP (∼1 wt.% H2O) than at KH (∼0.5 wt.% H2O), although this did not result in a more water-rich mantle lithosphere at RP. Rio Puerco lies within the northern Rio Grande rift, proposed to have been affected by a flat slab subduction, which may explain the more hydrous and extensive metasomatism compared to the south, where KH is located.

Low-temperature thermochronometric constraints on fault initiation and growth in the northern Rio Grande rift, upper Arkansas River valley, Colorado, USA

Low-temperature thermochronometric constraints on fault initiation and growth in the northern Rio Grande rift, upper Arkansas River valley, Colorado, USA

Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado

Great Sand Dunes National Park and Preserve (GRSA, Colorado) lies along the eastern margin of the San Luis Basin and the tectonically active Sangre de Cristo fault system that are part of the northern Rio Grande rift. GRSA lies within a prominent embayment in the range front where two separate sections of the Sangre de Cristo fault system intersect. Fault scarps are observed along both intersecting fault zones within older basin-fill alluvium, but have been obscured by the actively migrating dunefield. The dune sand is also strongly magnetic, locally limiting the usefulness of aeromagnetic methods for mapping concealed structure. This study uses airborne geophysical methods, primarily airborne gravity gradient data, along with constraints from geologic mapping and limited subsurface data and groundwater modeling, to interpret the subsurface basin geometry and range-front structure of the embayment. Using forward modeling of the gravity gradient data and locations of faults inferred from gravity gradient and aeromagnetic lineaments, several previously unrecognized tectonic elements are interpreted adjacent to the range front. Some of the largest rift-­related fault offsets are demonstrated to be basinward of the normal fault zones mapped at the surface along the range front of the Sangre de Cristo Mountains, along faults concealed under the dunefield and subparallel to the two fault sections. A fault-bounded structural bench, likely composed of Proterozoic rocks, underlies most of the high dunefield at depths of 500 m to 1 km. The bench is truncated on its southwest margin by a northwest-trending, southwest-dipping normal fault. A northeast-trending, northwest-dipping normal fault with ∼600 m of estimated relief lies under the southern margin of the dunefield and bounds a structurally higher bench of Proterozoic rocks concealed at <400 m depth near the range front. The northwest- and northeast-trending geophysical lineaments generally correspond well with the trends of faults mapped at the surface, and with both pre- and syn-rift structures in the Sangre de Cristo Mountains. Aeromagnetic anomalies are explained by variations in the magnetization of pre-rift rocks, and the strongly magnetic dune sand.

Seismic tomography of the Colorado Rocky Mountains upper mantle from CREST: Lithosphere–asthenosphere interactions and mantle support of topography

The CREST experiment (Colorado Rocky Mountains Experiment and Seismic Transects) integrated the EarthScope USArray Transportable Array with portable and permanent stations to provide detailed seismic imaging of crust and mantle properties beneath the highest topography region of the Rocky Mountains. Inverting approximately 14,600 P- and 3600 S-wave arrival times recorded at 160 stations for upper mantle Vp and Vs structure, we find that large Vp perturbations relative to AK135 of 7% and Vs variations of 8% take place over very short (approaching tens of kilometers) lateral distances. Highest heterogeneity is observed in the upper 300 km of the mantle, but well resolved low velocity features extend to the top of the transition zone in portions of these images. The previously noted low velocity upper mantle Aspen Anomaly is resolved into multiple features. The lowest Vp and Vs velocities in the region are found beneath the San Juan Mountains, which is clearly distinguished from other low velocity features of the northern Rio Grande Rift, Taos/Latir region, Aspen region, and below the Never Summer Mountains. We suggest that the San Juan anomaly, and a similar feature below the Taos/Latir region of northern New Mexico, are related to delamination and remnant heat (and melt) beneath these sites of extraordinarily voluminous middle-Cenozoic volcanism. We interpret a northeast–southwest grain in velocity structure that parallels the Colorado Mineral belt to depths near 150 km as being reflective of control by uppermost mantle Proterozoic accretionary lithospheric architecture. Further to the north and west, the Wyoming province and northern Colorado Plateau show high velocity features indicative of thick (∼150 km) preserved Archean and Proterozoic lithosphere, respectively. Overall, we interpret the highly heterogeneous uppermost mantle velocity structure beneath the southern Rocky Mountains as reflecting interfingered chemical Proterozoic lithosphere that has been, is currently being, replaced and modified by upwelling asthenosphere. Low velocity features resolved here indicate that this process may be sourced as deeply as the top of the mantle transition zone at 410 km. One driving mechanism for this is upper mantle interaction between upwelling hydration-induced partial melt and destabilized downwelling lithosphere in the deeper upper mantle. Tomographic imaging of mantle seismic velocity and crustal thickness results and modeling from the CREST experiment indicate that the highest elevations of the Colorado Rocky Mountains are substantially supported by the mantle, and strong correlations between low velocity mantle and thin crust/high topography are noted across the region. This, along with rich upper mantle seismic heterogeneity, suggests that mantle buoyancy and dynamics are central to present day topographic support and recent geomorphic evolution of the region.

40Ar/39Ar geochronology, isotope geochemistry (Sr, Nd, Pb), and petrology of alkaline lavas near Yampa, Colorado: Migration of alkaline volcanism and evolution of the northern Rio Grande rift

Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fields along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic field were investigated to assess correlations with late Cenozoic extension and Rio Grande rifting. In this paper we report major and trace element rock and mineral compositions and Ar, Sr, Nd, and Pb isotope data for these volcanic rocks. High-precision 40 Ar/ 39 Ar geochronology indicates westward migration of volcanism within the Yampa volcanic field between 6 and 4.5 Ma, and the Sr, Nd, and Pb isotope values are consistent with a primary source in the Proterozoic subcontinental lithospheric mantle. Relict olivine phenocrysts have Mg- and Ni-rich cores, whereas unmelted clinopyroxene cores are Na and Si enriched with finely banded Ca-, Mg-, Al-, and Ti-enriched rims, thus tracing their crystallization history from a lithospheric mantle source region to one in contact with melt prior to eruption. A regional synthesis of Neogene and younger volcanism within the Rio Grande rift corridor, from northern New Mexico to southern Wyoming, supports a systematic overall southwest migration of alkaline volcanism. We interpret this Neogene to Quaternary migration of volcanism toward the northeast margin of the Colorado Plateau to record passage of melt through subvertical zones within the lithosphere weakened by late Cenozoic extension. If the locus of Quaternary alkaline magmatism defines the current location of the Rio Grande rift, it includes the Leucite Hills, Wyoming. We suggest that alkaline volcanism in the incipient northern Rio Grande rift, north of Leadville, Colorado, represents melting of the subcontinental lithospheric mantle in response to transient infiltration of asthenospheric mantle into deep, subvertical zones of dilational crustal weakness developed during late Cenozoic extension that have been migrating toward, and subparallel to, the northeast margin of the Colorado Plateau since the middle Miocene. Quaternary volcanism within this northern Rio Grande rift corridor is evidence that the rift is continuing to evolve.

Identifying Buried Segments of Active Faults in the Northern Rio Grande Rift Using Aeromagnetic, LiDAR, and Gravity Data, South-Central Colorado, USA

Combined interpretation of aeromagnetic and LiDAR data builds on the strength of the aeromagnetic method to locate normal faults with significant offset under cover and the strength of LiDAR interpretation to identify the age and sense of motion of faults. Each data set helps resolve ambiguities in interpreting the other. In addition, gravity data can be used to infer the sense of motion for totally buried faults inferred solely from aeromagnetic data. Combined interpretation to identify active faults at the northern end of the San Luis Basin of the northern Rio Grande rift has confirmed general aspects of previous geologic mapping but has also provided significant improvements. The interpretation revises and extends mapped fault traces, confirms tectonic versus fluvial origins of steep stream banks, and gains additional information on the nature of active and potentially active partially and totally buried faults. Detailed morphology of surfaces mapped from the LiDAR data helps constrain ages of the faults that displace the deposits. The aeromagnetic data provide additional information about their extents in between discontinuous scarps and suggest that several totally buried, potentially active faults are present on both sides of the valley.

(U-Th)/He thermochronologic constraints on the evolution of the northern Rio Grande Rift, Gore Range, Colorado, and implications for rift propagation models

The Rio Grande Rift system tapers northward into the center of the southern Rocky Mountains. How rift initiation and evolution are related to development of the Rocky Mountains is not well understood. The Gore Range and adjacent Blue River Valley of central Colorado are the northernmost significant fault-related manifestations of the rift. New apatite (U-Th)/He data from the range record only middle and late Cenozoic exhumation phases, unlike neighboring Rocky Mountain uplifts that record a single phase of Late Cretaceous–early Tertiary unroofing. Middle Tertiary dates require that Gore Range basement remained covered by sedimentary rocks until at least the Middle Eocene. Normal faulting initiated during the Oligocene, inducing exhumation of basement to the surface and deposition of synrift fill in the Blue River Valley. Major cooling and unroofing in the Miocene were restricted to the southern Gore Range and continued until at least 7 Ma, with ∼2.3 km of total displacement along the Blue River normal fault. This middle to late Tertiary unroofing history is strikingly similar to that inferred for other rift flank uplifts to the south, and suggests broad synchroneity in the middle Tertiary onset and subsequent evolution of >700 km of the Rio Grande Rift. The results highlight that the notion of a northward-propagating rift, as suggested by its northward-tapering geometry, is incorrect, and preclude models invoking rift propagation to explain late Cenozoic elevation gain of the Rocky Mountains.

Paleomagnetism of Tertiary intrusive and volcaniclastic rocks of the Cerrillos Hills and surrounding region, Española Basin, New Mexico, U.S.A.: Assessment and implications of vertical-axis rotations associated with extension of the Rio Grande rift

Paleomagnetic data from parts of the northern Rio Grande rift provide evidence for clockwise (CW) and counterclockwise (CCW) vertical-axis block rotations associated with strike-slip deformation along basin-bounding faults during rift evolution. Despite the spatial consistency of the results, the quality and statistical significance of data sets are difficult to evaluate because of small sample size and potential failure to average secular variation. To understand the extent, importance, and origin of such rotations, we report paleomagnetic data from Tertiary intrusive and volcaniclastic rocks in the Cerrillos Hills and surrounding areas in the Espanola Basin. Paleomagnetic data from in situ Tertiary intrusions and tilt-corrected volcaniclastic strata of the Oligocene Espinaso Formation sampled at four localities yield a grand mean of declination = 342.9° and inclination = 58.3° (α 95 = 3.5°; N = 32 sites of normal polarity/21 reverse). Correction for minor postemplacement tilt of the Cerrillos Hills and La Cienega data sets yields a grand mean (declination = 349.5°, inclination = 55.3°, α 95 = 3.4°) that is indistinguishable from the 30 Ma reference direction for the study area, and there is no evidence of rotation (R = 1.8° ± 6.4°). However, if an alternative reference direction is used, minor CCW rotation (−6.6° ± 5.8°) is possible. Our data suggest that the magnitude of rotation in the Espanola Basin is significantly less than previously estimated and may be negligible. Regardless, paleomagnetic data from elsewhere in the basin suggest that CCW rotations may be an important component of recent rift extension and deformation.

Interaction of Asthenospheric and Lithospheric Mantle: The Genesis of Calc-alkaline Volcanism at Erciyes Volcano, Central Anatolia, Turkey

The quaternary Erciyes stratovolcano and its associated monogenetic cones exhibit tholeiitic, alkaline, and calc-alkaline suites, with the calc-alkaline series dominating. Calc-alkaline products are andesitic, dacitic, and rhyodacitic, showing moderate to low MgO and high K2O, plus LIL and HFS element contents. The andesites display trace-element patterns similar to those of the Central Mexican Volcanic belt, the Colville igneous complex (northeastern Washington, USA) and the Taos Plateau volcanic field (in north-central New Mexico, associated with northern Rio Grande Rift). Dacites from Erciyes also resemble these igneous complexes and give patterns similar to those of the upper crust. Furthermore, the high Ba/Nb ratio (> 28) is typical of arc volcanism (Fitton et al., 1988) ranging from 27 to 47 for andesites, and 27 to 50 for dacites at Erciyes. Calc-alkaline suites also show high Rb/Nb ratios (4-7.5). Moreover, 87Sr/86Sr and 143Nd/144Nd ratios range from 0.703434 to 0.705143, and 0.51294 to 0.5126 for andesites and dacites, respectively. These incompatible element and isotopic ratios suggest two possible mechanisms for the genesis of calc-alkaline suites: subduction enrichment or crustal involvement. Isotopic similarities between tholeiitic and andesitic rocks, LIL and HFS element compositions, and spider plots all indicate that various source components have been involved in the genesis of these products. The calc-alkaline products of Erciyes volcano were generated by the mixing of partial melts derived from depleted (tholeiitic) and enriched asthenospheric sources. In addition, the calc-alkaline suite was derived from an initially tholeiitic basalt end-member via a 25-30% assimilation of crustal material. Thus, post-collisional extension led to development of the Erciyes volcanism in central Anatolia. Calc-alkaline products were widely generated by the combined effects of mixing of depleted and enriched asthenospheric sources, together with a progressive AFC process that started from an initially tholeiitic component, without direct melting of the pre-subducting slab.

Assessing Roles of Volcanism and Basin Subsudence in Causing Oligocene-Lower Miocene Sedimentation in the Northern Rio Grande Rift, New Mexico, U.S.A.

Oligocene, mostly volcaniclastic, fluvial sedimentation in the Rio Grande rift of northern New Mexico has been ascribed to either aggradation in response to explosive volcanism or initial subsi- dence of extensional basins. These competing interpretations are tested by numerical simulations and by stratigraphic and petrographic study of the Oligocene-lower Miocene Abiquiu Formation. The upper 450 m of the . 600-m-thick Abiquiu Formation is tuffaceous volcaniclastic sediment that accumulated during explosive volcanic activity in the Latir volcanic field, about 150 km to the northeast. The formation extends westward beyond the most conspicuous rift-bounding faults. Simple numerical simulations suggest that two volcanism-related pro- cesses may have contributed to sedimentation. First, stream longitu- dinal profiles adjust upward as a result of (a) increasing headwater elevation in volcanic constructional topography and (b) increased sed- iment load. These combined effects may produce as much as 100 m of deposition at distances in excess of 150 km from the volcanic source area. The second volcanic mechanism is flexural subsidence resulting from the load of volcanic fields and flanking volcaniclastic aprons, and may account for 300 m or more preservation space at a distance of 150 km. Correlation of stratigraphic intervals of the Abiquiu Forma- tion defined by lithofacies geometry and sandstone composition reveals thickening of strata into the rift across rift-bounding and intra-rift faults, indicating subsidence-driven sedimentation. Petrographic and 40 Ar/ 39 Ar geochronologic data reveal that sediment was derived mostly or wholly from ignimbrites erupted in the Latir volcanic field, includ- ing widespread deposition of pumiceous debris-flow deposits following emplacement of the 25.1 Ma Amalia Tuff. Deposition of the Abiquiu Formation was therefore most likely caused by a combination of rift- basin subsidence and volcanism-related isostatic subsidence and ad- justments of stream profiles.

ABSTRACT: Structural Style of Detachment Faults Along the East Side of the Northern Rio Grande Rift in the San Luis Basin, Colorado

ABSTRACT: Structural Style of Detachment Faults Along the East Side of the Northern Rio Grande Rift in the San Luis Basin, Colorado

Active breaching of a geometric segment boundary in the Sawatch Range normal fault, Colorado, USA

The northwest-trending Sawatch Range normal fault system is the principal range-bounding fault of the northern Rio Grande Rift of central Colorado. It displays a geometric segment boundary in the form of a prominent en échelon step at the Chalk Cliffs, a zone of highly fractured and hydrothermally altered quartz monzonite. There, the modern fault geometry suggests that the Sawatch Range fault is presently breaching the boundary. Immediately north of the Chalk Cliffs, the range front fault is active whereas immediately south, the range front fault is inactive. Instead, it is replaced by a north-trending break that transects much of the boundary. The Chalk Cliffs also display a four-stage history of faulting and alteration/mineralization that appears to track the boundary's evolution. First-stage chlorite-filled faults and second-stage laumontite-filled faults affect the entire boundary and likely pre-date the breach. They record a change of fluid chemistry during oblique extension associated with the boundary's early history. Third stage, calcite-filled fractures are generally restricted to the eastern half of the boundary, between the breaching fault's projection into the Chalk Cliffs and the active range front fault. These fractures probably coincide with initiation of the breach. Modern (fourth stage) hydrothermal activity occurs at, and slightly beyond, the eastern edge. This coincidence of hydrothermal events with changes in large-scale fault geometry illustrates how normal fault zones, through breaching, can simplify their geometries and localize associated deformation and hydrothermal activity.

Neogene basins of the northern Rio Grande rift: partitioning and asymmetry inherited from Laramide and older uplifts

Three asymmetric Neogene basins in the northern Rio Grande rift of New Mexico and Colorado — the San Luis basin, the upper Arkansas River graben, and the Blue River graben — are tilted toward large flanking normal faults and lie astride the similarly asymmetric Late Cretaceous–early Tertiary (Laramide) San Juan–San Luis, Sawatch, and Front Range–Gore Range uplifts, respectively. The steep, thrust-faulted side of each uplift is on the same side as the down-rotated side of each of the Neogene basins. In addition, the direction of stratal tilt changes northward across the Villa Grove accommodation zone from east in the San Luis basin to west in the upper Arkansas River graben. This accommodation zone coincides approximately with the northward change from the east-directed San Juan–San Luis uplift to the west-directed Sawatch uplift. These observations, supported by seismic-reflection studies across the San Luis basin and studies of several other superimposed pairs of rift basins and Laramide uplifts, suggest that the basin-bounding normal faults are listric and merge at depth with the older thrusts, which are also listric and root into the crust at about 15–16 km. The Blue River graben is complicated by lack of basin fill and a thrust history along the west side of the Gore Range that is at least as old as late Paleozoic. Nonetheless, the Neogene valley is demonstrably tilted west and lies astride an overall west-directed thrust system, similar to other thrust-and-basin relationships in the northern Rio Grande rift.

Deep structure of the northern Rio Grande rift beneath the San Luis basin (Colorado) from a seismic reflection survey: implications for rift evolution

A seismic reflection survey by Chevron across the San Luis basin (northern Rio Grande rift) and San Juan volcanic field of southern Colorado is reprocessed with extended correlation to search for basement structure. The trace of the main bounding fault of the basin, a high-angle normal fault against the Sangre de Cristo Range, can be correlated to a wide zone of dipping reflection fabric and soles out at lower crustal depths (26–28 km). The deeper reflection fabric represent either broad extensional strain or pre-existing structure, such as a Laramide thrust system. The Sangre de Cristo bounding fault in San Luis basin does not sole out at mid-crustal depths but continues into the lower crust with a shallower dip. The basin architecture in the northern Rio Grande rift (San Luis basin) provides little if any evidence that the Sangre de Cristo bounding fault should flatten in a shallow listric fashion. This fault geometry is quite similar to the high-angle bounding fault in the Espanola basin but contrasts with less deeply-rooted faults in the Albuquerque basin in the central Rio Grande rift. Deeper soling out of the Sangre de Cristo bounding fault could be due to less extension in the northern Rio Grande rift and/or greater strength of the lithosphere compared to the central Rio Grande rift. Unequivocal Moho reflections beneath the San Luis basin cannot be identified, probably due to limited signal penetration or a gradational nature of the Moho. The majority of rift-related movement observed on the Sangre de Cristo bounding fault is post-Eocene. Either the western margin of the basin is marked by a tight monocline or a low-angle normal fault.

Tectonic stability of the San Luis Hills, Northern Rio Grande Rift, Colorado: Evidence from paleomagnetic measurements

The San Luis Basin is the largest of four tilted, fault bounded Neogene basins of the northern Rio Grande rift in New Mexico and Colorado. It formed during the second of two episodes of extension beginning in middle Oligocene and extending to the present. The San Luis Hills are an intrarift horst of Oligocene intermediate and basalt rocks preserved in the middle of the San Luis Basin. Previous paleomagnetic and structural studies in the Espanola Basin to the south indicate counterclockwise rotation of that region during the most recent extension. Paleomagnetic samples from the two exposed formations in the San Luis Hills yield a mean direction of inclination=55.6° and declination=347.2° (α95=6.8°, N=23) that is coincident with the expected Oligocene direction for North America. These rocks show no indication of rotation or severe tilting during rift extension.

Shear wave splitting across the Rocky Mountain Front

We search for evidence of seismic anisotropy by determining shear‐wave splitting parameters using teleseismic phases from stations placed across the Rocky Mountain Front. Three features are striking: 1) fast polarization orientations are consistent within small geographic regions, but vary rapidly across the network, 2) a large number of high quality “null” measurements indicate that little transverse anisotropy with horizontal symmetry axis is present, and 3) stations with well‐constrained but inconsistent parameters for rays from different sources imply that a single layer of anisotropic material with a horizontal symmetry axis is an inadequate parameterization. The general pattern suggests fast axes pointing toward (or away from) a central region of little anisotropy. A model of asthenospheric flow converging on or diverging from the central uplifted region is postulated. Within the southern Rocky Mountains, parameters are similar to those in the nearby northern Rio Grande Rift, suggesting that a similar mechanism causes the anisotropy in both regions.

Simultaneous Inversion of Reflection and Refraction Seismic Data and Application to Field Data From the Northern Rio Grande Rift

SUMMARY An inversion scheme that determines a velocity model simultaneously from normalincidence reflection and refraction/wide-angle reflection seismic data has been developed, tested, and applied to real seismic data. Simultaneous inversion minimizes the non-uniqueness of inversion results. A common midpoint (CMP) stacked reflection section provides a detailed image of the structural configuration in time. Refraction or wide-angle reflection data can be used to derive a velocity model. If both refraction data and normal-incidence reflection data are available along coincident profiles, simultaneous inversion of the data sets can produce an improved model. With the extra control of the interface depth provided by reflection data, it is also possible to avoid ambiguity of phase identification. The commonly used damped-least-squares (DLS) inversion method often results in significant inversion artefacts. We provide effective methods for reducing inversion artefacts using both damping and smoothing. From tests using synthetic data, we conclude that the simultaneous inversion scheme can improve resolution and can be applied to velocity models with a reasonably complex structure. The simultaneous inversion scheme has been successfully applied to coincident seismic reflection and refraction data recorded by the SAGE program in 1990-1991 in the western boundary area of the Rio Grande rift near Abiquiu, New Mexico. In this example, the simultaneous inversion derived a velocity model in which stratigraphic units and extensional fault blocks can be resolved.