Southern Rio Grande Rift Research Articles

Chlorine isotope evidence for Farallon-derived metasomatism of the North American lithospheric mantle

Metasomatism of the sub-continental lithospheric mantle (SCLM) can result in enrichment in volatile elements introduced via subduction of seawater-altered oceanic lithosphere. Subduction-related metasomatism can introduce chlorine isotope heterogeneity to the SCLM inherited from the subducted source, which can be used to characterize the source of metasomatism. This study presents Cl isotope compositions (δ37Cl) of variably metasomatized continental lithospheric mantle xenoliths from the Colorado Plateau and southern Rio Grande Rift (southwestern United States). Fertile mantle xenoliths that represent asthenosphere that has recently been emplaced at the base of the North American lithosphere have δ37Cl values of 0.0 ‰ to +0.1 ‰, suggesting that the convecting mantle has an average δ37Cl value similar to seawater and chondrites. In contrast, metasomatized xenoliths span a range of δ37Cl values from +0.1 ‰ to +1.9 ‰, extending above the average convecting mantle value. The range in δ37Cl values can be explained by mixing between a subduction-related fluid with δ37Cl values between +1.0 ‰ and +1.9 ‰ and a shallow crustal fluid with a δ37Cl value less than +0.3 ‰. The most likely source of the high δ37Cl subduction-related signature is the lower crust in the subducted Farallon plate. Lower crustal Cl likely entered the SCLM either through direct dehydration of lawsonite ± phengite in the deeply subducted Farallon plate or from a mixed serpentinite-oceanic crust source in the subducted Farallon. Regardless of the mechanism, this study provides evidence for subduction of crustal Cl into the mantle, which can introduce isotopic heterogeneity into the SCLM and has implications for the volatile cycling in the HIMU mantle and SCLM.

Metasediments from the lower crust reveal the history of the Picuris orogeny, southwest USA

Petrologic and geochronologic data for metapelitic lower crustal xenoliths from New Mexico (USA) and Chihuahua (Mexico) states provide evidence for both a magmatic and collisional component to the enigmatic Mesoproterozoic Picuris orogeny. These garnet-sillimanite-bearing metapelites are found within the southern Rio Grande rift at Kilbourne Hole and Potrillo Maar in southern New Mexico and northern Chihuahua. Geothermobarometry and rutile with Quaternary U-Pb dates indicate equilibration in the local lower crust, which is actively undergoing ultra-high temperature (UHT) metamorphism (Cipar et al., 2020). The samples contain older detrital zircons dating back to the Paleoproterozoic, marking their deposition at the surface. Coupled zircon U-Pb dates and trace-element ratios (e.g., Gd/Yb) show a clear transition from oscillatory-zoned, low-Gd/Yb detrital magmatic zircon to featureless, high-Gd/Yb metamorphic zircon between 1500 and 1400 Ma, marking the transition from subduction to collision during this period. Metamorphic zircon and monazite grew in two major intervals. The first, between ca. 1450 and 1350 Ma, documents the journey of the sediments to depth within the orogen and provides evidence of extended Mesoproterozoic metamorphism in the region. The second corresponds with UHT metamorphism that commenced at ca. 32 Ma and is associated with the Rio Grande rift. Whereas nearly all garnets are homogeneous in both major and trace elements, a single garnet from one sample has a core defined by abundant quartz and acicular sillimanite inclusions. The core and rim of this garnet is homogeneous in major and most trace elements, but the rim is enriched in the slowest diffusing elements, Zr and Hf, which likely indicates rim growth at higher temperatures. We interpret the garnet core to have grown at the time of emplacement of the sediments into the lower crust. Because this occurred in the sillimanite stability field and because the metamorphic zircon and monazite all have negative Eu anomalies, indicating their equilibration with feldspar (stable at depths of <45 km), we conclude that the sediments were not emplaced via subduction and/or relamination of forearc sediments, but were instead metamorphosed under warmer, shallower conditions in an orogenic setting. Collectively, the data point to a collisional orogen during the inferred timing of the Picuris orogeny. These samples may therefore define the location of the Picuris suture zone, a key feature of this orogenic event.

Halogen enrichment in the North American lithospheric mantle from the dehydration of the Farallon plate

A greater quantity of halogens (F, Cl, Br, and I) appear to be subducted globally than are erupted at magmatic arcs, requiring either an increase in the mantle halogen budget through time or an additional output pathway or reservoir. The sub-continental lithospheric mantle (SCLM) is one such reservoir. SCLM can be enriched in volatile elements after metasomatism by fluids or melts from subducting oceanic lithosphere. We analyzed the bulk halogen content (F, Cl, Br, and I) of 28 variably metasomatized spinel and garnet peridotite xenoliths from the Navajo Volcanic Field (NVF; Central Colorado Plateau) to constrain the effects of the subducting Farallon plate-derived metasomatism on the halogen content of the western North American SCLM. We also analyzed 33 anhydrous spinel peridotites from various localities on and around the Colorado Plateau, as well as xenoliths from Oahu, Hawaii, and Eifel, Germany to compare them to the modally hydrated NVF xenoliths. Cl, Br, and I are enriched in NVF xenoliths relative to the depleted mantle, correlating with olivine oxygen isotope values and indices of metasomatism. F is less enriched, indicating preferential retention in the slab residue. Br/Cl and I/Cl are similar to partially dehydrated serpentinite, indicating that the source of these fluids is consistent with serpentinized oceanic lithosphere of the Farallon plate. Anhydrous xenoliths from the southwestern United States have more depleted mantle-like Cl and F contents but have highly enriched Br contents similar to the NVF xenoliths. Similar to NVF xenoliths, anhydrous xenolith Br/Cl and I/Cl suggest a Farallon derived serpentinite source of halogen enrichment. These results indicate that the Farallon slab sequestered halogens in the North American SCLM of the central and south Colorado Plateau and the southern Rio Grande Rift. Therefore, halogen sequestration in the SCLM may be a critical component in the global halogen cycle.

Thermochronological transect across the Basin and Range/Rio Grande rift transition: Contrasting cooling histories in contiguous extensional provinces

AbstractThe Basin and Range and Rio Grande rift (RGR) are regions of crustal extension in southwestern North America that developed after Laramide-age shortening, but it has not been clear whether onset and duration of extension in these contiguous extensional provinces were the same. We conducted a study of exhumation of fault blocks along a transect from the southeastern Basin and Range to across the RGR in southern New Mexico. A suite of 128 apatite and 63 zircon (U-Th)/He dates (AHe and ZHe), as well as 27 apatite fission-track (AFT) dates, was collected to investigate the cooling and exhumation histories of this region. Collectively, AHe dates range from 3 to 46 Ma, ZHe dates range from 2 to 288 Ma, and AFT dates range from 10 to 34 Ma with average track lengths of 10.8–14.1 µm. First-order spatiotemporal trends in the combined data set suggest that Basin and Range extension was either contemporaneous with Eocene–Oligocene Mogollon-Datil volcanism or occurred before volcanism ended ca. 28 Ma, as shown by trends in ZHe data that suggest reheating to above 240 °C at that time. AHe and ZHe dates from the southern RGR represent a wider range in dates that suggest the main phase of cooling occurred after 25 Ma, and these blocks were not reheated after exhumation. Time-temperature models created by combining AHe, AFT, and ZHe data in the modeling software HeFTy were used to interpret patterns in cooling rate across the study area and further constrain magmatic and/or volcanic versus faulting related cooling. The Chiricahua Mountains and Burro Mountains have an onset of rapid extension, defined as cooling rates in excess of >15 °C/m.y., at ca. 29–17 Ma. In the Cookes Range, a period of rapid extension occurred at ca. 19–7 Ma. In the San Andres Mountains, Franklin Mountains, Caballo Mountains, and Fra Cristobal range, rapid extension occurred from ca. 23 to 9 Ma. Measured average track lengths are longer in Rio Grande rift samples, and ZHe dates of >40 Ma are mostly present east of the Cookes Range, suggesting different levels of exhumation for the zircon partial retention zone and the AFT partial annealing zone. The main phase of fault-block uplift in the southern RGR occurred ca. 25–7 Ma, similar to what has been documented in the northern and central sections of the rift. Although rapid cooling occurred throughout southern New Mexico, thermochronological data from this study with magmatic and volcanic ages suggest rapid cooling was coeval with magmatism in the Basin and Range, whereas in the Rio Grande rift cooling occurred during an amagmatic gap. These observations support a model where an early phase of extension was facilitated by widespread ignimbrite magmatism in the southeastern Basin and Range, whereas in the southern Rio Grande rift, extension started later and continues today and may have occurred between local episodes of basaltic magmatism. These differences in cooling history make the Rio Grande rift tectonically distinct from the Basin and Range. We infer based on geologic and thermochronological evidence that the onset of extension in the southern Rio Grande rift occurred at ca. 27–25 Ma, significantly later than earlier estimates of ca. 35 Ma.

Tectonic geomorphology and paleoseismology of the northern East Franklin Mountains fault, El Paso, Texas, USA

The 45 km-long, north-trending East Franklin Mountains Fault lies in the southern Rio Grande rift (Texas). Late Quaternary normal fault scarps up to 20 m high traverse the eastern range front, but also bulge up to 2 km out onto the piedmont in lobes as much as 3.5 km wide. In the lobe areas the range-front fault strand displays no fault scarps, as if displacement has shifted to the piedmont strand since ca. 400 ka. We divide the faulted piedmont alluvial fans into seven age classes (from Middle Pleistocene to Holocene), based on erosional dissection and soil profile development. Piedmont scarps in Qf1 deposits (250–400 ka) range from 18 to 20 m high; in Qf2-3 deposits (65–100 ka), 9–10 m high; and in Qf3-4 deposits (20 ka), 2.7–4 m high (the latest displacement event). Scarp heights underestimate true vertical displacement of the footwall surface, which lies buried beneath younger fan deposits of unknown thickness on the hanging wall. Hanging-wall aggradation also causes variable scarp heights along strike on single-age landforms, with burial thickness a second-degree polynomial function (parabolic) of distance to the nearest active stream. Our paleoseismic trench on a 9.8 m-high scarp exposed evidence for five displacement events in the past 44 ka, the latest three of which had vertical displacements of 3–4 m. Such displacements imply surface ruptures longer than the 26.5 km fault length previously mapped in the USA, indicating rupture extended onto adjacent faults. The minimum long-term slip rate (>11.2 m in 44 ka) is 0.25 mm/yr; over last full seismic cycle (ca. 11–29 ka) it is 0.20–0.21 mm/yr. Optically-stimulated luminescence (OSL) dating shows that fine sand-silt grains in colluvial deposits were not all zeroed during deposition. Single-aliquot dating of quartz grains yielded multiple OSL age peaks, the youngest peaks of which yield a chronology compatible with stratigraphy and soil development in the colluvial wedge sequence.

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.

Pliocene–Holocene deformation in the southern Rio Grande rift as inferred from topography and uplifted terraces of the Franklin Mountains, southern New Mexico and western Texas

Pliocene–Holocene deformation in the southern Rio Grande rift as inferred from topography and uplifted terraces of the Franklin Mountains, southern New Mexico and western Texas

Geophysical Constraints on the Crustal Structure of the Southern Rio Grande Rift

Abstract Evidence of geologic activity still occurring in the Rio Grande rift (RGR) includes quaternary faulting, seismicity, and widening at a small rate. We map the crustal thickness and seismic velocity ratio to generate constrained crustal model cross sections that highlight the regional extension of the southern RGR (SRGR). Specifically, we computed receiver functions and receiver function stacks for 147 stations from EarthScope USArray Transportable Array and previously collected data in the southwestern United States, and interpolated the crustal and velocity results using a kriging interpolation scheme. We include a new optimized gravity inversion approach using the receiver function results as a constraint for inverted density profiles. We produce constrained crustal models that characterize the SRGR as showing a shallower Moho (30 km) in the region of the SRGR, which is thicker than previously suggested. The crust appears to be delaminated west of the RGR in the Basin and Range Province and thicker east of the RGR toward the Great Plains with a denser lower crustal layer. We also find that the Delaware basin shows a significant geophysical anomaly, likely due to thick saturated sediments. Given that there is no deep mantle anomaly determined from other studies in this region, we suggest that rifting could be the result of small‐scale convection of the uppermost mantle, and we conclude that the RGR is at a minimum the boundary between the Basin and Range Province and the Great Plains, perhaps reflecting the terminus of flat slab subduction.

Paleostress and Geometric Analysis of the Southern Rio Grande Rift in the Northern Chihuahua Trough: Franklin and Potrillos Mountains as examples

Paleostress and Geometric Analysis of the Southern Rio Grande Rift in the Northern Chihuahua Trough: Franklin and Potrillos Mountains as examples

Using geophysical techniques to trace active faults in the urbanized northern Hueco Bolson, West Texas, USA, and northern Chihuahua, Mexico

The Hueco Bolson serves as one of the primary groundwater sources for the El Paso–Ciudad Juarez metropolitan region of over 2.5 million residents. The bolson lies at the point where the strike of the southern Rio Grande rift changes from north-south to northwest-southeast, likely due to its interaction with preexisting Mesozoic and Paleozoic structures. The region is tectonically active with recent (

Synchronous opening of the Rio Grande rift along its entire length at 25–10 Ma supported by apatite (U-Th)/He and fission-track thermochronology, and evaluation of possible driving mechanisms

One-hundred and forty-seven new apatite (U-Th)/He (AHe) ages are presented from 32 sample locations along the flanks of the Rio Grande rift in New Mexico and Colorado. These data are combined with apatite fission-track (AFT) analyses of the same rocks and modeled together to create well-constrained cooling histories for Rio Grande rift flank shoulders. The data indicate rapid cooling due to extension from ca. 28 to 5 Ma in the Sawatch Range, ca. 28 Ma to Quaternary in the Sangre de Cristo Mountains, ca. 25 to 5 Ma in the Albuquerque Basin, and ca. 25 to 10 Ma in the southern Rio Grande rift in southern New Mexico. Rapid cooling of rift flanks followed the Oligocene ignimbrite flare-up, and the northern section of the Rio Grande rift in Colorado exhibits semicontinuous cooling since the Oligocene. Overall, however, rift flank cooling along the length of the rift was out of phase with high-volume magmatism and hence is inferred to have been driven mainly by exhumation due to faulting. Although each location preserves a unique cooling history, when combined with existing AHe data from the Gore Range in northern Colorado and the Sandia Mountains in New Mexico, together these data indicate that extension and exhumation of rift shoulders were synchronous along >850 km of the length of the Rio Grande rift from 25 to 10 Ma. These time-space constraints provide an important new data set with which to develop geodynamic models for initiation and evolution of continental rifting. Models involving northward propagation of rifting and Colorado Plateau rotation are not favored as primary mechanisms driving extension. Instead, a geodynamic model is proposed that involves upper-mantle dynamics during multistage foundering and rollback of a segment of the Farallon plate near the Laramide hinge region that extended between the Wyoming and SE New Mexico high-velocity mantle domains. The first stage of flat slab removal accompanied ca. 40–20 Ma volcanism in the San Juan and Mogollon-Datil ignimbrite centers, which initiated asthenospheric upwelling and circulation. A second stage involved a ca. 30–25 Ma detachment of remaining fragments of the Farallon slab, intensifying asthenospheric upwelling and focusing it along a N-S trend beneath Colorado and New Mexico. By 25 Ma, the North American lithosphere had become weakened critically along this narrow zone, so that extension was accelerated, resulting in the observed 25–10 Ma cooling indicated by the thermochronologic data. This developed a central graben with increased fault-related high strain rates and resulted in maximum sediment accumulation in the Rio Grande rift. Our geodynamic model thus involves Oligocene removal of parts of the Farallon slab beneath the ignimbrite centers followed by a major Oligocene–Miocene slab break that instigated the discrete N-S Rio Grande rift through focused upper-mantle convection beneath the southern Rocky Mountain–Rio Grande rift region.

Deep fluid circulation within crystalline basement rocks and the role of hydrologic windows in the formation of the Truth or Consequences, New Mexico low‐temperature geothermal system

AbstractHot springs can occur in amagmatic settings, but the mechanisms of heating are often obscure. We have investigated the origin of the Truth or Consequences, New Mexico low‐temperature (approximately 41°C) hot springs in the southern Rio Grande Rift. We tested two hypotheses that could account for this amagmatic geothermal anomaly: lateral forced convection in a gently dipping carbonate aquifer and circulation through high‐permeability crystalline basement rocks to depths of 8 km that is then focused through an overlying faulted hydrologic window. These hypotheses were tested using a regional two‐dimensional hydrothermal model. Model parameters were constrained by calibrating to measured temperatures, specific discharge rates, and groundwater residence times. We collected 16 temperature profiles, 11 geochemistry samples, and 6 carbon‐14 samples within the study area. The geothermal waters are Na+/Cl−‐dominated and have apparent groundwater ages ranging from 5500 to 11 500 years. Hot‐spring geochemistry is consistent with water/rock interaction in a silicate geothermal reservoir, rather than a carbonate system. Peclet number analysis of temperature profiles suggests that specific discharge rates beneath Truth or Consequences range from 2 to 4 m year−1. Geothermometry indicates maximum reservoir temperatures are around 170°C. Observed measurements were reasonably reproduced using the deep circulation permeable‐basement modeling scenario (10−12 m2) but not the lateral forced‐convection carbonate‐aquifer scenario. Focused geothermal discharge is the result of localized faulting, which has created a hydrologic window through a regional confining unit. In tectonically active areas, such as the Rio Grande Rift, deep groundwater circulation within fractured crystalline basement may play a more prominent role in the formation of geothermal systems than has generally been acknowledged.

Contrasting lithospheric signatures across the western United States revealed by Sp receiver functions

To explore how lithospheric structure varies between tectonic and magmatic terranes of the western United States, we use observations of shear-to-compressional wave conversions across lithospheric velocity interfaces. With a newly developed automatic method designed to take advantage of the EarthScope Transportable Array and other broadband seismic arrays deployed across the region, and an optimized deconvolution technique based on the extended-time multi-taper method, we map variations in the depth to the seismic Moho, lithosphere–asthenosphere boundary (LAB) and mid-lithospheric discontinuities (MLDs). Beneath areas that have undergone substantial extension in the Neogene—including the eastern Great Basin, the southern Basin and Range, and the southern Rio Grande Rift—we detect strong Sp conversions across a relatively shallow (60–80 km) seismically-defined LAB. We detect Sp conversions across an 80–90 km depth discontinuity beneath the High Rockies, which we interpret as the seismically-defined LAB. Beneath areas that have remained relatively undeformed—including the Wyoming craton and the Great Plains craton—we detect Sp conversions in the 90–140 km depth range that are substantially weaker and more distributed in depth than those beneath less stable regions. Beneath the Colorado Plateau, Sp CCP stacks show that lithospheric thinning has accompanied magmatic encroachment and the LAB beneath the margins of the Plateau is indistinguishable from the LAB of the surrounding extensional provinces. In contrast, beneath central and northern Colorado Plateau, weaker Sp conversions are observed in the 90–140 km depth range. The observed variations in lithospheric structure across structural blocks with different magmatic and deformational histories imply that: (1) both mechanical thinning and magmatic and thermal erosion have likely contributed to the thin lithosphere observed in the Basin and Range and southern Rio Grande Rift; (2) at the margins of the Colorado Plateau and beneath the High Rockies, where extension has been relatively minor, strong and shallow seismic LABs indicate magmatic alteration of the lithosphere; (3) MLDs seen in comparatively thick, stable lithosphere produce weaker Sp conversions and are often more distributed in depth than seismically-observed LABs in regions of thin lithosphere; (4) MLDs and weak, deep phases consistent with a seismic LAB are sometimes observed together.

Mixed Geothermal and Shallow Meteoric Origin of Opal and Calcite Beds In Pliocene-Lower Pleistocene Axial-Fluvial Strata, Southern Rio Grande Rift, Rincon Hills, New Mexico, U.S.A

Abstract Exposed in upper Pliocene fluvial strata in the eastern Rincon Hills and adjacent Jornada del Muerto Basin, southern Rio Grande rift, New Mexico, USA, are five laterally continuous beds 0.2 to 1.0 m thick of opal-CT, which straddle the East Rincon Hills normal fault. Four of these opal beds change basinward over ~ 1 km or less to white, friable, lithofeldspathic sand with opal glaebules, and then to microcrystalline calcite. The opal and calcite generally replaced fine sand at shallow depths, although two beds locally precipitated from standing water. 87Sr/86Sr values (0.71301 to 0.71724) of the opal beds are consistent with precipitation from an advecting, high-temperature geothermal fluid that flowed through bedrock and/or from a high-temperature geothermal fluid derived from a deep-crustal source. Carbon and oxygen isotopic values (δ13CPDB = −7.1 to −0.6; δ18OPDB = −9.1 to −5.6) of the calcite beds lie between and overlap those of Paleozoic carbonate and Pliocene paleosol calcite, suggesting precipitation from a mixture of a high-temperature geothermal fluid that interacted with Paleozoic carbonates and shallow, meteoric groundwater, for which paleosol calcite is a proxy. The geothermal fluids are interpreted to have upwelled along the East Rincon Hills fault, flowed laterally at or just below the surface, and precipitated opal in response to rapid cooling and evaporation. Downflow precipitation of opal glaebules occurred as a result of decreasing silica concentrations and perhaps by mixing with shallow, meteoric groundwater, from which calcite precipitated, probably by CO2 degassing.

On the origin of low angle normal faulting in the Southern Rio Grande Rift

La parte sur del Rift Rio Grande está definida por una serie de fallas normales de alto ángulo que separan bloques elevados de grandes cuencas de depósitos del Cenozoico. Una serie de fallas normales de bajo ángulo se encuentran en los bloques elevados y sus relaciones con la cuenca y con las estructuras montañosas es controversial. Estudios preliminares concluyeron que las fallas de bajo ángulo se formaron en una fase previa de extensión y después fueron inclinadas sobre la orientación actual de fallas más recientes. Otros estudios concluyeron que estas fallas son el resultado de un fallamiento reciente y son un factor clave en el desarrollo de las cuncas actuales y de la topografía montañosa. Por esta razón, en esta parte de la dorsal de Rift Rio Grande el análisis de esfuerzos es una tarea difícil. Fechar la edad geológica de las estructuras y el basculamiento extensivo del área son las principales fuentes de error. En este artículo reconstruimos el régimen de esfuerzos usando técnicas numéricas basadas en estadística para dos montañas del Rift del Rio Grande: East Potrillo Mountains Y Franklin Mountains. El campo de esfuerzos y la historia tectónica también se discuten. Las direcciones de las venas extensionales fueron comparadas con el campo general de esfuerzos. Se encontró que la mayoría de las fallas fueron reactivadas en la primera etapa de extensión de Rio Grande, además con la ocurrencia de un basculamiento extensivo con rotaciones. En Potrillo Mountains se requiere un proceso de corrección angular por basculamiento de W25 SE y W45 SE orientado N30W para obtener dos campos de esfuerzos homogéneos. El plano de falla corregido muestra un componente oblicuo en estos dos campos de esfuerzos; el primero corresponde a la reactivación de las fallas con el σ1 y el σ3 orientados N78E y N69E respectivamente mientras el segundo campo de esfuerzos es relacionado con las fallas más actuales y es orientado N64W y N61E respectivamente. En Franklin Mountains el basculamiento es menos importante sin embargo dos campos de esfuerzos con orientaciones similares pero valores de phi diferentes fueron detectados (reconocidos). La historia de esfuerzos post - Laramidica de la región es controlada de diferentes eventos extensionales que influenciaron las estructucturas pre - existentes y también generaron un sistema de falla nueva.

Oligocene Laramide deformation in southern New Mexico and its implications for Farallon plate geodynamics

The Silver City Range in southwest New Mexico contains Proterozoic basement rocks that are overlain by a sequence of Paleozoic, Mesozoic, and Paleogene strata. These rocks are folded in a broad, NW-SE–trending, east-facing monocline that lies structurally above an east-directed thrust fault. The youngest rocks folded in the Silver City monocline are similar to other late Eocene and early Oligocene volcanic rocks of the Mogollon-Datil volcanic field; an ash-flow tuff near the bottom of the volcanic sequence gives an 40 Ar/ 39 Ar age on sanidine of 34.9 ± 0.4 Ma (2σ), and another tuff near the top of the section contains zircons that yield a weighted 206 Pb/ 238 U age of 34.6 ± 0.6 Ma (2σ). We interpret similar structures in the Little Burro Mountains, Lone Mountain, and Bayard area, immediately east and west of the Silver City monocline, to all be genetically related to a system of basement-involved thrust faults. Modeling of these structures from the Mangas Valley in the southwest to the Mimbres Valley in the northeast, suggests ∼17% total shortening. We conclude that Laramide shortening was active in southwest New Mexico generally, and the Silver City region in particular, from the Cretaceous until the earliest phase of Mogollon-Datil volcanism beginning at ∼37 Ma, during which time the earliest extension in the southern Rio Grande rift was initiated. The final stage of Laramide shortening, recorded in the Silver City monocline, took place during a lull of volcanism (and extension) from ∼31.5 to ∼29.3 Ma. We explain the contemporaneity of shortening, significant ignimbrite eruptions, and crustal extension as the consequence of intermittent slab breakoff and renewed underthrusting of the downgoing Farallon plate.

Tectonic and climatic controls on Holocene channel migration, incision and terrace formation by the Rio Grande in the Palomas half graben, southern Rio Grande rift, USA

In order to evaluate potential effects of tectonics and climate change on the behaviour of the axial Rio Grande in the Rio Grande rift, a 16·5 km stretch of modern floodplain and Holocene terraces were mapped in the tectonically active Palomas half graben, south-central New Mexico, USA. In addition, 51 cores and natural exposures were logged and 20 radiocarbon dates were obtained from charcoal, bulk organic matter, mollusc shells and pedogenic calcite. The Holocene alluvium comprises four terraces above the modern floodplain, each of which formed by a period of river incision followed by stability and renewed floodplain construction to a level below that of the previous terraces. Estimated times of incision between Terraces I and II, II and III, and III and IV are after 12 400, 8040 to 5310, and 760 to 550 yr bp, respectively, whereas the incision between Terrace IV and the modern floodplain occurred within the last 260 years. Although there is some evidence for tectonic control on river behaviour in the southern part of the basin, terrace formation is interpreted as being related to climate change, with periods of incision corresponding to times of increased aridity and low sediment/water discharge ratio in the Rio Grande. This process may have resulted from a reduction in intensity and magnitude of summer storms which supply sediment to the axial river, coupled with an increase in spring discharge peak caused by snowmelt in upstream mountain catchments.

New sites of 3.1-Ma pumice beds in axial fluvial strata of the Camp Rice and Palomas formations, southern Rio Grande rift

New sites of 3.1-Ma pumice beds in axial fluvial strata of the Camp Rice and Palomas formations, southern Rio Grande rift

Modern Flood Deposition, Erosion, and Fan-Channel Avulsion on the Semiarid Red Canyon and Palomas Canyon Alluvial Fans in the Southern Rio Grande Rift, New Mexico, U.S.A.

Abstract In the summer of 2006, large floods occurred on the Red Canyon (footwall-derived) and Palomas Canyon (hanging-wall-derived) alluvial fans, which are located on opposite sides of the Rio Grande floodplain in the Palomas half graben of the southern Rio Grande rift. The initial phase of the Red Canyon flood took place on the active central fan lobe, but subsequent avulsion of the main channel shifted the flood to the south, where it eroded all but two small relicts of the former inactive, toe-cut lobe before depositing up to 2 m of coarse gravel. Deposition of 0.5 m of gravel-size detritus by the Palomas Canyon flood was largely confined to the active, incised channel and to gravel bars that built into the river, although floodwater spilled over the southern bank of the channel near its mouth, depositing a thin (< 15 cm) veneer of fine sand on the floodplain. Measurements of mean and maximum grain size of the flood deposits, channel-bed slopes, and flood depth based on vegetative high-water marks allow estimates of mean flood velocity (Red Canyon = 8.6 m s−1; Palomas Canyon = 7.4 m s−1), floodwater discharge (Red Canyon = 971 m3 s−1; Palomas Canyon = 179 m3 s−1), and volume of flood deposits (Red Canyon = 3.19 × 105 m3; Palomas Canyon = 1.72 × 104 m3). Mapping of the modern floodplain and outcrop analysis of the Palomas Formation (Pliocene–lower Pleistocene) in the Palomas basin provide insight into facies architecture in half grabens. Footwall–derived alluvial-fan detritus consists principally of shallow, multistory gravel–conglomerate channels produced by channel avulsion across the surface of the small fans. Asymmetrical basin tilting causes toe-cutting of the footwall-derived fans by the axial river and incision of hanging-wall-derived alluvial-fan channels, the latter resulting in ribbon-shaped gravel–conglomerate channels interbedded with finer-grained, distal-fan sediment. The Red Canyon and Palomas Canyon floods changed the location, width, and depth of the Rio Grande, causing flooding of the western floodplain for ~ 1 km upstream of the constriction. Because the Rio Grande has been dammed since 1916, it no longer has the stream power to effectively redistribute large volumes of fan detritus and must be dredged in order to avoid upstream flooding by the river.

Geographic Patterns of Ground-dwelling Arthropods Across an Ecoregional Transition in the North American Southwest

Abstract We examined the biogeographic patterns of ground-dwelling arthropod communities across a heterogeneous semiarid region of the Southern Rio Grande Rift Valley of New Mexico. Our 3 sites included portions of 5 ecoregions, with the middle site a transition area where all ecoregions converged. We addressed the following 3 questions: (1) Do the species assemblage patterns for ground arthropods across habitats and sites conform to recognized ecoregions? (2) Are arthropod assemblages in distinct vegetation-defined habitats within an ecoregion more similar to each other or to assemblages in similar vegetation-defined habitats in other ecoregions? (3) Is there a detectable edge effect with increased arthropod diversity in the area of converging ecoregions? We encountered 442 target arthropod species from pitfall traps operating continuously for 7 years over a series of different habitats at each of the 3 sites. We examined geographic distributions of spider and cricket/grasshopper species in detail, and t...