Reconstructing the ancestral (Plio-Pleistocene) Rio Grande in its active tectonic setting, southern Rio Grande rift, New Mexico, USA

Deposits of the ancestral Rio Grande (aRG) belonging to the Camp Rice Formation are preserved and exposed in the uplifted southern portion of the Robledo Mountains horst of the southern Rio Grande rift. The sediments are dated palaeomagnetically to the Gauss chron (upper Pliocene). The lower part of the succession lies in a newly discovered palaeocanyon cut into underlying Eocene rocks whose margins are progressively onlapped by the upper part. Detailed sedimentological studies reveal the presence of numerous river channel and floodplain lithofacies, indicative of varied deposition in channel bar complexes of low‐sinuosity, pebbly sandbed channels that traversed generally dryland floodplains and shifted in and out of the study area five times over the 1 Myr or so recorded by the succession. Notable discoveries in the deposits are: (1) complexes of initial avulsion breakout channels at the base of major sandstone storeys; (2) common low‐angle bedsets ascribed to deposition over low‐angle dunes in active channels; (3) palaeocanyon floodplain environments with evidence of fluctuating near‐surface water tables. Sand‐body architecture is generally multistorey, with palaeocurrents indicative of funnelling of initial avulsive and main fluvial discharge from the neighbouring Mesilla basin through a narrow topographic gap into the palaeocanyon and out over the study area. An avulsion node was evidently located at the stationary southern tip to the East Robledo fault during Gauss times, with aRG channels to the north flowing close to the fault and preventing fan progradation. Subsequent Matuyama growth of the fault caused (1) deposition to cease as the whole succession was uplifted in its footwall, (2) development of a thick petrocalcic horizon, and (3) fan progradation into the Mesilla basin. Parameters for the whole aRG fluvial system are estimated as: active single channels 2 m deep and 25 m wide; valley slope 0·24–0·065°; maximum mean aggradation rate 0·05 mm year^–1; major channel belt avulsion interval 200 ky; individual channel recurrence interval 100 ky; minimum bankfull mean flow velocity 1·54 m s^–1, minimum single‐channel discharge 77 m³ s^–1, bed shear stress 22·3 N m^–2; and stream power 34·3 W m^–2.     

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.     

A test of models of fluvial architecture and palaeosol development: Camp Rice Formation (Upper Pliocene-Lower Pleistocene), southern Rio Grande rift, New Mexico, USA

Numerical, experimental and theoretical models of fluvial architecture and palaeosol development are tested with outcrops of Upper Pliocene-Lower Pleistocene sediment in the southern Rio Grande rift, New Mexico. The sediment was deposited and subsequently exhumed in the Jornada del Muerto basin, a westward-tilted half graben whose footwall corresponds to the Rincon Hills and San Diego Mountain fault blocks. The axial river, the ancestral Rio Grande, shared time between the Jornada del Muerto basin and the adjacent Corralitos basin. The ancestral Rio Grande entered the Jornada del Muerto basin via a gap between the footwall blocks, periodically flowing southward towards San Diego Mountain, or making a broad northward sweep into the northern fluvial salient towards the Rincon Hills fault block and unfaulted northern edge of the basin. Ten logged sections up to 35 m thick are correlated using the top of the formation (La Mesa surface), a 1·59 Ma pumice conglomerate, and a ground-water carbonate/opal bed. Additionally, one of the sections is dated by reversal magnetostratigraphy. Consistent with the model of Bridge & Leeder (1979 ) and Bridge & Mackey (1993a ), differential tilting of the Jornada del Muerto half graben resulted in sections directly adjacent to the faults that consist almost exclusively of multistorey channel sands/sandstones, whereas more distal sections contain a greater proportion of crevasse-splay fine sand and overbank mudstone and calcic palaeosols. Along the axis of the northern fluvial salient, a northward decrease in channel/floodplain ratio, a decrease in channel recurrence interval from 171 kyr to 685 kyr, and an increase in the maturity of calcic palaeosols are consistent with southward tilt of the unfaulted northern edge of the basin. An upsection decrease in sediment accumulation rate in the northern fluvial salient from 0·036 mm/ yr to 0·017 mm/ yr corresponds to an increase in the ratio of channel/floodplain facies and in the number of multistorey channel sands/sandstones, and is consistent with the model of Bridge & Leeder (1979 ) in which avulsion frequency is independent of sediment accumulation rate. Stage II and III calcic palaeosols indicate 10³−10⁵ year of landscape stability and soil formation between periods of floodplain deposition in response not only to basin tilting but also because the ancestral Rio Grande had multiple paths within the Jornada del Muerto basin and shared time between the Corralitos and Jornada del Muerto basins.     

Late Quaternary activity of the Pajarito fault, Rio Grande rift of northern New Mexico, USA

The Pajarito fault forms the western margin of the Rio Grande rift in north-central New Mexico, and lies adjacent to Los Alamos National Laboratory, a major Federal research facility. Vertical displacement on this normal fault over the past 1.2 Ma has created a 50- to 120-m-high fault scarp on Bandelier Tuff (1.2 Ma), yielding a long-term average slip rate of ca. 0.1 mm/yr. In support of a Laboratory-wide seismic hazards assessment, we excavated 14 trenches in the Pajarito fault zone to determine the age of the most recent displacement event, the recurrence interval between events, the displacement per event, and the variability in slip rate and recurrence through time. The large number of trenches was required by the large height of the fault scarp and the complexity of the fault zone. Only about half the trenches contained significant thicknesses of Holocene deposits, but in those trenches there was clear evidence for an early-to-mid-Holocene displacement event. The previous event was at least 20–40 ka, and the average recurrence interval over the past ca. 300 ka was about 20–40 kyr. We infer that much of the structural relief across this fault developed soon after eruption of the Bandelier Tuff between 1.0 and 1.2 Ma, and that slip rate slowed considerably after that time.     

Geochemical indicators of interbasin groundwater flow within the southern Rio Grande Valley, southwestern USA

Increased groundwater withdrawals for the growing population in the Rio Grande Valley and likely alteration of recharge to local aquifers with climate change necessitates an understanding of the groundwater connection between the Jornada del Muerto Basin and the adjoining and more heavily used aquifer in the Mesilla Basin. Separating the Jornada and Mesilla aquifers is a buried bedrock high from Tertiary intrusions. This bedrock high or divide restricts and/or retards interbasin flow from the Jornada aquifer into the Mesilla aquifer. The potentiometric surface of the southern Jornada aquifer near part of the bedrock high indicates a flow direction away from the divide because of a previously identified damming effect, but a groundwater outlet from the southern Jornada aquifer is necessary to balance inputs from the overall Jornada aquifer. Differences in geochemical constituents (major ions, δD, δ¹⁸O, δ³⁴S, and ⁸⁷Sr/⁸⁶Sr) indicate a deeper connection between the two aquifers through the Tertiary intrusions where Jornada water is geochemically altered because of a geothermal influence. Jornada groundwater likely is migrating through the bedrock high in deeper pathways formed by faults of the Jornada Fault Zone, in addition to Jornada water that overtops the bedrock high as previously identified as the only connection between the two aquifers. Increased groundwater withdrawals and lowering of the potentiometric surface of the Jornada aquifer may alter this contribution ratio with less overtopping of the bedrock high and a continued deeper flowpath contribution that could potentially increase salinity values in the Mesilla Basin near the divide.     

Depleted and enriched mantle processes under the Rio Grande rift: spinel peridotite xenoliths

Upper mantle xenoliths from the southern Rio Grande rift axis (Potrillo and Elephant Butte) and flank (Adam’s Diggings) have been investigated to determine chemical depletion and enrichment processes. The variation of modal, whole rock, and mineral compositions reflect melt extraction. Fractional melting is the likely process. Fractional melting calculations show that most spinel peridotites from rift axis locations have undergone <5% melting versus 7–14% melting for xenoliths from the rift shoulder, although the total range of fractional melting overlaps at all three locations. In the rift axis, deformed (equigranular and porphyroclastic texture) spinel peridotites are generally characterized by significantly less fractional melting (2–5%) than undeformed (protogranular) xenoliths (up to 16%). This difference may reflect undeformed xenoliths being derived from greater depths and higher temperatures than deformed rocks. Spinel peridotites from the axis and shoulder of the Rio Grande rift have undergone mantle metasomatism subsequent to melt extraction. Under the rift shoulder spinel peridotites have undergone both cryptic and patent (modal) metasomatism, possibly during separate events, whereas the upper mantle under the rift axis has undergone only cryptic metasomatism by alkali basaltic magma.     

A composite geologic and seismic profile beneath the southern Rio Grande rift, New Mexico, based on xenolith mineralogy, temperature, and pressure

A complete understanding of the processes of crustal growth and recycling in the earth remains elusive, in part because data on rock composition at depth is scarce. Seismic velocities can provide additional information about lithospheric composition and structure, however, the relationship between velocity and rock type is not unique. The diverse xenolith suite from the Potrillo volcanic field in the southern Rio Grande rift, together with velocity models derived from reflection and refraction data in the area, offers an opportunity to place constraints on the composition of the crust and upper mantle from the surface to depths of  60 km. In this work, we calculate seismic velocities of crustal and mantle xenoliths using modal mineralogy, mineral compositions, pressure and temperature estimates, and elasticity data. The pressure, temperature, and velocity estimates from xenoliths are then combined with sonic logs and stratigraphy estimated from drill cores and surface geology to produce a geologic and velocity profile through the crust and upper mantle. Lower crustal xenoliths include garnet ± sillimanite granulite, two-pyroxene granulite, charnokite, and anorthosite. Metagabbro and amphibolite account for only a small fraction of the lower crustal xenoliths, suggesting that a basaltic underplate at the crust–mantle boundary is not present beneath the southern Rio Grande rift. Abundant mid-crustal felsic to mafic igneous xenoliths, however, suggest that plutonic rocks are common in the middle crust and were intraplated rather than underplated during the Cenozoic. Calculated velocities for garnet granulite are between  6.9 and 8.0 km/s, depending on garnet content. Granulites are strongly foliated and lineated and should be seismically anisotropic. These results suggest that velocities > 7.0 km/s and a layered structure, which are often attributed to underplated mafic rocks, can also be characteristic of alternating garnet-rich and garnet-poor metasedimentary rocks. Because the lower crust appears to be composed largely of metasedimentary granulite, which requires deep burial of upper crustal materials, we suggest the initial construction of the continental crust beneath the Potrillo volcanic field occurred by thickening of supracrustal material in the absence of large scale magmatic accretion. Mantle xenoliths include spinel lherzolite and harzburgite, dunite, and clinopyroxenite. Calculated P-wave velocities for peridotites range from 7.75 km/s to 7.89 km/s, with an average of 7.82 km/s. This velocity is in good agreement with refraction and reflection studies that report P_n velocities of 7.6–7.8 km/s throughout most of the Rio Grande rift. These calculations suggest that the low P_n velocities compared to average uppermost mantle are the result of relatively high temperatures and low pressures due to thin crust, as well as a fertile, Fe-rich, bulk upper mantle composition. Partial melt or metasomatic hydration of the mantle lithosphere are not needed to produce the observed P_n velocities.     

Hydrochemical tracers in the middle Rio Grande Basin,USA: 1. Conceptualization of groundwater flow

Chemical and isotopic data for groundwater from throughout the Middle Rio Grande Basin, central New Mexico, USA, were used to identify and map groundwater flow from 12 sources of water to the basin, evaluate radiocarbon ages, and refine the conceptual model of the Santa Fe Group aquifer system.Hydrochemical zones, representing groundwater flow over thousands to tens of thousands of years, can be traced over large distances through the primarily siliciclastic aquifer system. The locations of the hydrochemical zones mostly reflect the modern predevelopment hydraulic-head distribution, but are inconsistent with a trough in predevelopment water levels in the west-central part of the basin, indicating that this trough is a transient rather than a long-term feature of the aquifer system. Radiocarbon ages adjusted for geochemical reactions, mixing, and evapotranspiration/dilution processes in the aquifer system were nearly identical to the unadjusted radiocarbon ages, and ranged from modern to more than 30 ka. Age gradients from piezometer nests ranged from 0.1 to 2 year cm^–1 and indicate a recharge rate of about 3 cm year^–1 for recharge along the eastern mountain front and infiltration from the Rio Grande near Albuquerque. There has been appreciably less recharge along the eastern mountain front north and south of Albuquerque.

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Resumen Se utilizaron datos químicos e isotópicos de agua subterránea a lo largo de la cuenca central del río Grande, Nuevo México, EEUU, para identificar y mapear el flujo de agua subterránea de 12 fuentes de agua a la cuenca para evaluar edades por medio de radio carbon y para refinar el modelo conceptual del sistema acuífero del Grupo Santa Fé. Se puede establecer zonas hidrotérmicas que representan el flujo de agua subterránea a lo largo de miles a miles de decenas de años en grandes distancias a través del sistema acuífero principalmente siliclástico. Las ubicaciones de las zonas hidroquímicas mayormente reflejan la distribucion de la cabeza hidráulica pre-desarollo moderna pero son inconsistentes con una depresión en los niveles de agua pre-desarollo en la zona central oeste de la cuenca. Esto indica que esta depresión es un rasgo transitorio y no un rasgo de largo plazo del sistema acuífero. Las edades de radio carbon ajustadas para los procesos de reaciones geoquímicas, de mezclado y de evapotranspiración-dilución son casi idénticas a los edades de radio carbon no ajustadas oscilan en un rango desde la modernidad a 30 mil años. Las gradientes de edad de nidos de piezometros van de 0.1 a 2 años cm^–1 e indican un sitio de recarga de aproximadamente 3 cm/yr para la recarga a lo largo del frente montañoso oriental e infiltración del río Grande cerca de Albuquerque. Se aprecia una recarga menor a lo largo del frente oriental de montañas al norte y al sur de Albuquerque.

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Résumé Des données sur les éléments chimiques et les isotopes présents dans leau souterraine prélevée à divers endroits dans le bassin moyen du Rio Grande, au centre du Nouveau-Mexique (É-U), ont permis de déterminer lexistence et létendue de douze sources deau régionales dans le bassin, dévaluer les âges radiocarbones et de raffiner le modèle conceptuel du système aquifère du groupe de Santa Fe. Des zones hydro-chimiques qui représentent lécoulement de leau souterraine depuis des dizaines de milliers dannées peuvent être suivies sur de longues distances à travers laquifère principalement siliclastique. La position des zones hydro-chimiques reflète principalement la distribution moderne des charges hydrauliques mais est incohérente avec une dépression dans le niveau deau dans la partie centre-ouest du bassin, ce qui indique que cette dépression est un élément transitoire du système aquifère plutôt quun élément à long terme. Les âges radiocarbones ajustés aux réactions géochimiques et aux processus de mélange et dévapotranspiration/dilution qui ont lieu dans laquifère sont presque identiques aux âges non ajustés et varient de la période moderne jusquà 30 ka. Les gradients dâge établis à partir des nids de piézomètres sétendent de 0.1 à 2 a cm^–1 et suggèrent un taux de recharge denviron 3 cm a^–1 le long du front des montagnes à lest et pour linfiltration provenant du Rio Grande près dAlbuquerque. Il y a eu substantiellement moins de recharge le long du front des montagnes à lest, au nord et au sud dAlbuquerque.

     

Metals and grain size distributions in soil of the middle Rio Grande basin, Texas USA

 This paper deals with the problem of increased heavy metal constituents in agricultural soils due to the expanded use of fertilizers and elevated atmospheric deposition. It discusses the extent of contamination in soil and establishes an environmental monitoring program in the chosen area of concern in the southern coastal region of Texas. Grain size, pH, and metals (Cu, Cd, Zn, Pb, Ni, Ba, As, Cr, Mn, and Fe) were determined in soils of the middle Rio Grande basin. The soils were mainly of sand texture and alkaline in character. Fine sand constituted the major proportion of the soil, and clay and silt ranged from 8–30% of the soil. Correlations of metal concentrations to grain size and iron contents were performed. Metals, except Cd and Pb, gave positive to negative relationships with decreases in grain size. Silt gave no relationship with metal content while clay and silt had a positive relationship. All these metals had a positive correlation with iron in the soil. The results indicate metals are associated with coarse sand, clay, and iron hydroxides surfaces of the soil. The comparison of metal content in soil of the middle Rio Grande basin with metals from other areas of the world suggests that it is relatively uncontaminated. Received: 14 December 1998 · Accepted: 19 Jaunuary 1999     

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field,northern Rio Grande rift,New Mexico

Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO₂ rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO₂) to quartz latite (67% SiO₂). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO₂ peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km³ high-SiO₂ peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.     

Hydrochemical tracers in the middle Rio Grande Basin,USA: 2. Calibration of a groundwater-flow model

The calibration of a groundwater model with the aid of hydrochemical data has demonstrated that low recharge rates in the Middle Rio Grande Basin may be responsible for a groundwater trough in the center of the basin and for a substantial amount of Rio Grande water in the regional flow system. Earlier models of the basin had difficulty reproducing these features without any hydrochemical data to constrain the rates and distribution of recharge. The objective of this study was to use the large quantity of available hydrochemical data to help calibrate the model parameters, including the recharge rates. The model was constructed using the US Geological Surveys software MODFLOW, MODPATH, and UCODE, and calibrated using ¹⁴C activities and the positions of certain flow zones defined by the hydrochemical data. Parameter estimation was performed using a combination of nonlinear regression techniques and a manual search for the minimum difference between field and simulated observations. The calibrated recharge values were substantially smaller than those used in previous models. Results from a 30,000-year transient simulation suggest that recharge was at a maximum about 20,000 years ago and at a minimum about 10,000 years ago.

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Resumen La calibración de un modelo de aguas subterráneas con el apoyo de datos hidroquímicos ha demostrado que la recarga relativamente baja en la cuenca media del Río Grande es probablemente responsable de una depresión de aguas subterráneas en el centro de la cuenca y de la presencia de una cantidad considerable de agua del Río Grande en el acuífero del Grupo Santa Fe. Los modelos propuestos con anterioridad para la cuenca tenían dificultades para reproducir estas características ya que no tenían datos hidroquímicos que permitieran delimitar los ritmos y distribución de recarga. El objetivo del presente estudio consistió en utilizar una gran cantidad de datos hidroquímicos disponibles para ayudar a calibrar los parámetros del modelo, incluyendo los ritmos de recarga. El modelo se construyó utilizando los modelos MODFLOW, MODPATH, y UCODE del USGS, mientras que la calibración se realizó en base a concentraciones de ¹⁴C y a la posición de ciertas zonas definidas con los datos hidroquímicos. La estimación de parámetros se realizó en base a una combinación de técnicas de regresiones no lineares y a una búsqueda a viva fuerza del error mínimo entre los datos observados y los simulados. Los valores de recarga calibrados fueron significativamente más bajos que los estimados en los modelos anteriores. Los resultados de una simulación transitoria de 30,000 años sugieren que la recarga durante la última glacial máxima (LGM) fue diez veces el ritmo moderno, pero que la recarga que ocurrió inmediatamente después de la LGM fue más baja que el ritmo moderno.

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Résumé Le calibrage dun modèle hydrogéologique avec laide de données hydrochimiques a démontré que la recharge relativement faible dans le Grand Bassin du Middle Rio est vraisemblablement responsable dune dépression des eaux souterraines dans le centre du bassin et de la présence dune quantité substantielle deau du Rio Grande dans laquifère du Groupe de Santa Fe. Les modèles antérieurs avaient des difficultés à reproduire ses conclusions sans laide de données hydrochimiques pour contraindre les taux et la distribution de la recharge. Lobjectif de cette étude était dutiliser une grande quantité de données hydrochimiques permettant de calibrer les paramètres du modèle, et notamment les taux de recharge. Le modèle a été construit avec les logiciels MODFLOW, MODPATH et UCODE, et calibré en utilisant les concentrations en ¹⁴C et la position de certaines zones définies par les données hydrochimiques. Lestimation de certains paramètres a été réalisée en utilisant une combinaison de techniques de régression non linéaire et une méthode de recherche exhaustive (Brute Force Search) de lerreur minimum entre les résultats des observations et les simulations. Les valeurs de la recharge calibrée sont substantiellement plus basses que celles estimées dans les modèles antérieurs. Les résultats dune simulation en régime transitoire sur 30.000 ans suggèrent que la recharge au maximum de la dernière glaciation (last glacial maximum, LGM) était 10 fois supérieure au taux actuel, mais que la recharge qui a suivit la LGM était plus bas que la recharge actuelle.

     

Oxygen isotope evidence for subduction and rift-related mantle metasomatism beneath the Colorado Plateau–Rio Grande rift transition

Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine δ¹⁸O values are high (+5.5‰), whereas δ¹⁸O values for pyroxenes are low (cpx=+5.1‰; opx=+5.4‰) compared to average mantle values. Pyroxenite δ¹⁸O values (cpx=+5.0‰; opx=+5.3‰) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with δ¹⁸O values of +21‰. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low δ¹⁸O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high δ¹⁸O calcite is interpreted to have crystallized from a high δ¹⁸O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt.     

西藏南部冈底斯大陆弧早白垩纪弧前伸展作用

在现今活动的俯冲带系统中,俯冲板片的后撤或回卷都可能导致弧前伸展作用,但弧前伸展作用是否局限于浅部构造层次,还是涉及整个岩石圈,是理解岛弧构造演化和地球化学效应的重要课题。在西藏冈底斯岩基南缘,保留着世界上典型的弧前盆地——日喀则弧前盆地。在该盆地的北缘,除了发育中新世闪长玢岩脉外,还发育一系列近东西向展布的辉绿岩脉,形成时代为106.6±0.8Ma(锆石U-Pb年龄)。基性岩脉在岩石学、矿物学和地球化学特征上较均一,具体表现为:(1)富集LREE,亏损HREE,但HREE平直;(2)强烈亏损Rb、Ba、Nb和Ta,微弱亏损Ti,但Zr和Hf不具亏损现象;(3)锆石Hf同位素组成(εHf(t)=+11.7~+15.4)较高;和(4)包含捕获的早侏罗世岩浆锆石。上述数据表明:在日喀则弧前盆地形成过程中,伴随着强烈的早白垩纪弧前伸展作用,诱发较亏损地幔发生部分熔融,形成不同于典型岛弧岩浆岩的基性岩,可能是在新特提斯洋北向俯冲过程中,海沟后撤,在弧前形成强烈的地幔角流的结果。同时,日喀则弧前盆地可能构建于早侏罗纪结晶岩系之上。     

Geochemistry and geochronology of the Sierra de Gomez Limestone-hosted U deposit,Chihuahua: Implications for distribution of Rio Grande rift mineral deposits in northern Mexico

Uranium deposits form in a variety of settings. They are partially controlled by the secular evolution of Earth processes, including deposits in extension-related settings such as the intra-cratonic Rio Grande rift. Plio-Quaternary volcanism, mineral deposits, and hydrothermal spots occur along the Chihuahua Central Graben. The age of the Sierra de Gomez U-deposit is 1.8 Ma (based on LA-MC-ICP-MS dating on a uranophane monocrystal), which is contemporaneous with the late mineralization event of the Peña Blanca U-deposit, as well as Rio Grande Rift (RGR)-type deposits in Chihuahua and intraplate volcanism. Studies of fluid inclusions in fluorite and late calcite indicate the presence of hydrocarbons and CH₄-rich brine. Homogenization temperatures range from 87 to 112 °C, and the mean composition (2.0 mol NaCl and 0.3 mol CaCl with CH₄) is comparable to mineralizing brines in MVT deposits and carbonated hydrocarbon reservoirs. Evolution of C and O stable isotopic values for the calcite cement in the Sierra de Gomez Limestone-hosted U deposit illustrates that two separate calcite precipitation events occurred: (1) travertine filling karst structures in the presence of meteoric water and (2) U mineralization during deep hydrothermal fluid circulation that included interactions with a heat source and basement leaching. In a regional context, a metallogenic model suggests that the Chihuahua Trough area is deep enough to generate fluid migration by hydrothermal and/or compaction processes through RGR extensional faults until a favorable trapping horizon is reached. This causes uranium precipitation because water/rock interaction processes generate a local redox barrier.     

Holocene landscape evolution and geoarcheology of low-order streams in the Rio Grande basin,San Juan Mountains,Colorado, USA

This geoarcheological study investigates soil stratigraphy and geochronology of alluvial deposits to determine Holocene landscape evolution within the Hot Creek, La Jara Creek, and Alamosa River drainage basins in the San Juan Mountains of Colorado. Geomorphic mapping and radiocarbon dating indicate synchronicity in patterns of erosion, deposition, and stability between drainage basins. In all three basins, the maximum age of mapped alluvial terraces and fans is ~ 3300 cal yr BP. A depositional period seen at both Hot Creek and the Alamosa River begins ~ 3300 to 3200 cal yr BP. Based on soil development, short periods of stability followed by alluvial fan aggradation occur in the Alamosa River basin ~ 2200 cal yr BP. A period of landscape stability at Hot Creek before ~ 1100 cal yr BP is followed by a period of rapid aggradation within all three drainages between ~ 1100 and 850 cal yr BP. A final aggradation event occurred between ~ 630 and 520 cal yr BP at La Jara Creek. These patterns of landscape evolution over the past ~ 3300 yr provide the framework for an archeological model that predicts the potential for buried and surficial cultural materials in the research area.     

Unconventional distribution of facies in a continental rift basin: the Pliocene–Pleistocene Mangas Basin, south-western New Mexico, USA

The Pliocene–Early Pleistocene Mangas Basin in SW New Mexico, USA, was a N–NW-trending full graben that changed southward to an eastward-tilted half graben. Unlike the facies distribution predicted in existing models, the half-graben part of the Mangas Basin was characterized by broad alluvial fans derived from the footwall scarp, smaller hangingwall-derived alluvial fans, and a shallow, closed lake (Lake Buckhorn) that locally lapped onto the hangingwall hills. The distribution of facies within the full-graben part of the Mangas Basin was also unlike that predicted in current models, primarily because of a broad belt of alluvial-fan sediment derived from the eastern footwall scarp and a narrow belt of axial-fluvial sediment adjacent to the western footwall scarp. The distribution of facies in the Mangas Basin does not appear to have been controlled by the eastward tilt of the floor of the half graben or ‘see-saw’ motion of the floor of the full graben, as predicted by existing models, but rather by the large size of the alluvial fans on the eastern side of the basin. These fans were derived from large, high-relief catchments on the footwall scarp of the Mogollon Mountains, the uplift of which began during Early Miocene. This example illustrates how earlier uplift and drainage development in a mountain range may influence facies distribution in a younger extensional basin.     

From continental margin extension to collision orogen: structural development and tectonic rotation of the Hengchun peninsula, southern Taiwan

As a result of oblique collision, the Taiwan orogen propagates southward. The Hengchun peninsula in the southern tip of the Taiwan Central Range, preserving the youngest, the least deformed and the most complete accretionary prism sequences, allows therefore better understanding of the tectonic evolution of Taiwan orogen. On the Hengchun peninsula, four main stages of paleostress can be recognized by the analysis of brittle tectonics. After recording the first two stages of paleostress, rocks of the Hengchun peninsula (the Hengchun block) have undergone both tilting and counterclockwise rotation of about 90°. The structural boundaries of this rotated Hengchun block are: the Kenting Mélange zone in the southwest, the Fongkang Fault in the north, and a submarine backthrust in the east. The angle of this rotation is principally calculated by the paleomagnetic analysis data and a physical model experiment. Through a systematic back-tilting and back-rotating restoration, the original orientations of the four paleostress stages of Hengchun peninsula are recognized. They are, from the ancient to the recent, a NW–SE extension, a combination of NW–SE transtension and NE–SW transpression, a NE–SW compression, and finally a combination of NE–SW transtension and NW–SE transpression. This result can be explained by a phenomenon of stress axes permutation, instead of a complex polyphase tectonism. This stress axes permutation is caused by the horizontal compression increase accompanying the propagation of the accretionary prism. Combining the tectonic and paleomagnetic data with paleocurrent and stratigraphic data enables us to reconstruct the tectonic evolution of the Hengchun peninsula. This reconstruction corresponds to the deformation history of a continental margin basin, from its opening to its intense deformation in the accretionary prism.     

A model for the three-dimensional evolution of continental rift basins,north-east Africa

Large areas of north-east Africa were dominated by regional extension in the Late Phanerozoic. Widespread rifting occurred in the Late Jurassic, with regional extension culminating in the Cretaceous and resulting in the greatest areal extent and degree of interconnection of the west, central and north African rift systems. Basin reactivation continued in the Paleocene and Eocene and new rifts probably formed in the Red Sea and western Kenya. In the Oligocene and Early Miocene, rifts in Kenya, Ethiopia and the Red Sea linked and expanded to form the new east African rift system.This complex history of rifting resulted in failed rift basins with low to high strain geometries, a range of associated volcanism and varying degrees of interaction with older structures. One system, the Red Sea rift, has partially attained active seafloor spreading. From a comparison of these basins, a general model of three-dimensional rift evolution is proposed. Asymmetrical crustal geometries dominated the early phases of these basins, accompanied by low angle normal faulting that has been observed at least locally in outcrop. As rifting progressed, the original fault and basin forms were modified to produce larger, more through-going structures. Some basins were abandoned, others experienced reversals in regional dip and, in general, extension and subsidence became focused along narrower zones near the rift axes. The final transition to oceanic spreading was accomplished in the Red Sea by a change to high angle, planar normal faulting and diffuse dike injection, followed by the organization of an axial magma chamber.     

Diverse mantle and crustal components in lavas of the NW Cerros del Rio volcanic field,Rio Grande Rift,New Mexico

Products of Pliocene (2–4 Ma) mafic to intermediate volcanism in the northwestern Cerros del Rio, a dominantly mafic volcanic field in the Española Basin of the Rio Grande Rift (RGR), range from 49% to 63% SiO₂ and exhibit diversity in silica saturation, trace-element patterns, and isotopic compositions. Tholeiites, which are largely confined to west of the Rio Grande, have trace-element abundances that resemble those of oceanic basalts, but with mild depletions in Nb and Ta, and high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd, and high δ¹⁸O compared to typical OIB. They are regarded as asthenospherically-derived magmas contaminated with continental crust. Alkali basalts and hawaiites erupted from vents east of the Rio Grande are geochemically distinct, having generally higher overall incompatible-element abundances, but with pronounced depletions in K, Rb, Nb and Ta with respect to Th and LREE. Spatially-associated benmoreites, mugearites and latites (collectively termed “evolved” lavas) have similar trace-element characteristics to the mafic mildly-alkaline compositions, but are typically not as depleted in K. Hawaiites and evolved lavas exhibit a good negative correlation of ¹⁴³Nd/¹⁴⁴Nd with SiO₂, due to interaction with lower continental crust. The most silicic “evolved” lavas carry the highest proportions of crustal material, and consequently have higher K/Th than the related hawaiites. Several (mostly mafic) lavas contain abundant crustally-derived resorbed quartz xenocrysts in O-isotope disequilibrium with the host magma. The δ¹⁸O values of xenocrystic quartz range over 4‰, indicating a variety of quartz-bearing crustal contaminants beneath the Española Basin. The hawaiites, with their unusual combination of trace-element enrichments and depletions, cannot be generated by any process of fractionation or crustal contamination superposed on a common mantle source type (oceanic or arc-source). It is a regional mantle source type, inasmuch as it was also present beneath NW Colorado during the mid-late Cenozoic. We argue that the hawaiite source must have originally existed as arc-source mantle enriched in LILE, generated during Mesozoic to early Cenozoic subduction at the western margin of North America. This arc-source mantle lost K, Rb and Ba, but not Th or LREE, prior to magmagenesis. Selective element loss may have occurred during lithospheric thinning and uprise of hydrated phlogopitebearing peridotite-possibly as a thermal boundary layer between lithosphere and asthenosphere — to shallow mantle depths, with consequent conversion of phlogopite to amphibole (an inferior host for K, Rb and Ba). We suggest that this occurred during the early extensional phase of the northern RGR. Further extension was accompanied by partial melting and release of magma from this source and the underlying asthenosphere, which by the Pliocene was of oceanic type. The hawaiite source mantle is the product of a long history of subduction succeeded by lithospheric extension of the formerly overriding plate. Similar chemical signatures may have developed in the mantle beneath other regions with comparable histories.     

Sources of basaltoid magmas in rift settings of an active continental margin: Example from the bimodal association of the Noen and Tost ranges of the Late Paleozoic Gobi-Tien Shan rift zone, southern Mongolia

The bimodal volcanoplutonic (basalt-peralkaline rhyolite with peralkaline granites) association of the Noen and Tost ranges was formed 318 Ma ago in the Gobi-Tien Shan rift zone of the Late Paleozoic-Early Mesozoic central Asian rift system, the development of which was related to the movement of the continental lithosphere over a mantle hot spot. A specific feature of the Late Paleozoic rifting was that it occurred within the Middle-Late Paleozoic active continental margin of the northern Asian paleocontinent. Continental margin magmatism was followed after a short time delay by the magmatism of the Gobi-Tien Shan rift zone, which was located directly in the margin of the paleocontinent. Such a geodynamic setting of the rift zone was reflected in the geochemical characteristics of rift-related rocks. The distribution of major elements and compatible trace elements in the rift-related basic and intermediate rocks corresponds to a crystallization differentiation series. The distribution of incompatible trace elements suggests contributions from several sources. This is also supported by the heterogeneity of Sr and Nd isotopic compositions of the rift-related basaltoids: ε_Nd(T) ranges from 4.4 to 6.7, and (⁸⁷Sr/⁸⁶Sr)₀, from 0.70360 to 0.70427. The geochemical characteristics of the rift-related basaltoids of the Noen and Tost ranges are not typical of rift settings (negative anomalies in Nb and Ta and positive anomalies in K and Pb) and suggest a significant role of the rocks of a metasomatized mantle wedge in their source. In addition, there are high-titanium rocks among the rift-related basaltoids, whose geochemical characteristics approach those of the basalts of mid-ocean ridges and ocean islands. This allowed us to conclude that the compositional variations of the rift-related basaltoids of the Noen and Tost ranges were controlled by three magma sources: the enriched mantle, depleted mantle (high-titanium basaltoids), and metasomatized mantle wedge (medium-Ti basaltoids). The medium-titanium basaltoids were formed in equilibrium with spinel peridotites, whereas the high-titanium magmas were formed at deeper levels both in the spinel and garnet zones. It terms of geodynamics, the occurrence of three sources of the rift-related basaltoids of the Noen and Tost ranges was related to the ascent of a mantle plume with enriched geochemical characteristics beneath a continental margin, where its influence caused melting in the overlying depleted mantle and the metasomatized mantle wedge. The formation of rift-related andesites in the Noen and Tost ranges was explained by the contamination of mantle-derived basaltoid melts with sialic (mainly sedimentary) continental crustal materials or the assimilation of anatectic granitoid melts.