Southern Rio Grande Rift Research Articles

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.

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.

Grenville-age A-type and related magmatism in southern Laurentia, Texas and New Mexico, U.S.A.

Large Grenville-age plutons are known to be present in the 1340–1370 Ma basement in Texas and eastern New Mexico, but their distribution and compositional ranges are poorly known. New U–Pb zircon dates (SHRIMP-RG, Stanford University) on subsurface samples have identified widespread presence of compositionally diverse Grenville-age (1070–1110 Ma) plutonic rocks. In the Texas Panhandle, core from a > 175 m thick, sill-like gabbroic intrusion yielded a date of 1081 ± 8.3 Ma. In situ differentiation of this tholeiitic magma led to a ∼ 7-fold increase in incompatible element concentrations as REE patterns remained essentially flat. Trace element data suggest an E-MORB-like source. To the south, alkali–feldspar granite with A-type affinities was recovered from drill core from the Abilene gravity minimum near Albany, Texas. It yielded a date of 1078 ± 23 Ma; similar to undeformed (post-orogenic) granites in the Llano uplift. Further west, an anorthosite xenolith from the Eocene Three Sisters intrusion in El Paso, Texas yielded a date of 1068 ± 30 Ma which is within error of the 1110 ± 19 Ma age determined for the main stage of the nearby A-type Red Bluff granitic suite. Xenoliths from Potrillo maar volcano in the center of the southern Rio Grande rift include monzonitic xenoliths with granulitic texture which yielded dates of ∼ 1072 Ma. In contrast, Potrillo maar xenoliths with igneous textures yielded U–Pb dates of ∼ 27 Ma. Development of the granulitic texture is interpreted to reflect granulite facies metamorphism during Tertiary time (26–27 Ma) on the basis of age zoning and U concentrations in the zircon crystals. Our results show that Grenville-age magmatism in the Texas and New Mexico subsurface was widespread and was coeval with syn- and post-deformation granites in the Llano uplift of central Texas. The compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were “A-type”. These magmatic affinities are thought to be associated with crustal extension rather than with subduction. Moreover, the presence of anorthosite xenoliths in the El Paso-Kilbourne Hole region, the broadly tholeiitic character of mafic rocks, and the A-type nature of the granitic rocks are broadly similar to AMCG (Anorthosite–Mangerite–Charnockite–Granite) suites present in the Grenville province of eastern Laurentia.

Pliocene and Quaternary history of the Rio Grande, the axial river of the southern Rio Grande rift, New Mexico, USA

The history of the Rio Grande, the axial river of the southern Rio Grande rift, New Mexico, may be divided into an aggradational phase (∼ 5 to ∼ 0.8 Ma), during which time about 100 m of fluvial sediment was deposited in eight basins, and a degradational phase (∼ 0.8 Ma to present), when the river alternately incised and partially backfilled its basins. Reversal magnetostratigraphy, dated volcanic rocks, and vertebrate biostratigraphy allow correlation of sediment of the aggradational phase with the Gilbert, Gauss, and Matuyama Chrons, while the degradational phase primarily encompasses the Brunhes Chron. Tectonics was the primary control on river behaviour during the aggradational phase. In half grabens, multistorey channel sands were deposited in narrow belts near the footwall uplifts, in response to tilt of the basin floor towards the border fault and smaller footwall catchments and their alluvial fans compared to those of the hanging-wall mountains. In contrast, the river traversed almost the entire width of full grabens, depositing interbedded channel and floodplain sediment with moderately mature palaeosols. Condensed sections dominated by floodplain sediment with very mature palaeosols developed in basins largely abandoned by the river due to preferential tilting of the basin floor, lateral growth or renewal of fault activity, or alternating flow of the river between adjacent basins. The degradational phase was dominated by net basin erosion related to glacial-climate cycles, despite continued activity on many of the basin-bounding faults.

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.

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

AbstractNumerical, 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 &amp; Leeder (1979) and Bridge &amp; 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 &amp; 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.

“Sour gas” hydrothermal jarosite: ancient to modern acid-sulfate mineralization in the southern Rio Grande Rift

As many as 29 mining districts along the Rio Grande Rift in southern New Mexico contain Rio Grande Rift-type (RGR) deposits consisting of fluorite–barite±sulfide–jarosite, and additional RGR deposits occur to the south in the Basin and Range province near Chihuahua, Mexico. Jarosite occurs in many of these deposits as a late-stage hydrothermal mineral coprecipitated with fluorite, or in veinlets that crosscut barite. In these deposits, many of which are limestone-hosted, jarosite is followed by natrojarosite and is nested within silicified or argillized wallrock and a sequence of fluorite–barite±sulfide and late hematite–gypsum. These deposits range in age from ∼10 to 0.4 Ma on the basis of 40Ar/ 39Ar dating of jarosite. There is a crude north–south distribution of ages, with older deposits concentrated toward the south. Recent deposits also occur in the south, but are confined to the central axis of the rift and are associated with modern geothermal systems. The duration of hydrothermal jarosite mineralization in one of the deposits was approximately 1.0 my. Most Δ 18O SO 4–OH values indicate that jarosite precipitated between 80 and 240 °C, which is consistent with the range of filling temperatures of fluid inclusions in late fluorite throughout the rift, and in jarosite (180 °C) from Peña Blanca, Chihuahua, Mexico. These temperatures, along with mineral occurrence, require that the jarosite have had a hydrothermal origin in a shallow steam-heated environment wherein the low pH necessary for the precipitation of jarosite was achieved by the oxidation of H 2S derived from deeper hydrothermal fluids. The jarosite also has high trace-element contents (notably As and F), and the jarosite parental fluids have calculated isotopic signatures similar to those of modern geothermal waters along the southern rift; isotopic values range from those typical of meteoric water to those of deep brine that has been shown to form from the dissolution of Permian evaporite by deeply circulating meteoric water. Jarosite δ 34S values range from −24‰ to 5‰, overlapping the values for barite and gypsum at the high end of the range and for sulfides at the low end. Most δ 34S values for barite are 10.6‰ to 13.1‰, and many δ 34S values for gypsum range from 13.1‰ to 13.9‰ indicating that a component of aqueous sulfate was derived from Permian evaporites ( δ 34S=12±2‰). The requisite H 2SO 4 for jarosite formation was derived from oxidation of H 2S which was likely largely sour gas derived from the thermochemical reduction of Permian sulfate. The low δ 34S values for the precursor H 2S probably resulted from exchange deeper in the basin with the more abundant Permian SO 4 2− at ∼150 to 200 °C. Jarosite formed at shallow levels after the pH buffering capacity of the host rock (typically limestone) was neutralized by precipitation of earlier minerals. Some limestone-hosted deposits contain caves that may have been caused by the low pH of the deep basin fluids due to the addition of deep-seated HF and other magmatic gases during periods of renewed rifting. Caves in other deposits may be due to sulfuric acid speleogenesis as a result of H 2S incursion into oxygenated groundwaters. The isotopic data in these “sour gas” jarosite occurrences encode a record of episodic tectonic or hydrologic processes that have operated in the rift over the last 10 my.

Synclinal‐horst basins: examples from the southern Rio Grande rift and southern transition zone of southwestern New Mexico, USA

AbstractIn areas of broadly distributed extensional strain, the back‐tilted edges of a wider than normal horst block may create a synclinal‐horst basin. Three Neogene synclinal‐horst basins are described from the southern Rio Grande rift and southern Transition Zone of southwestern New Mexico, USA. The late Miocene–Quaternary Uvas Valley basin developed between two fault blocks that dip 6–8° toward one another. Containing a maximum of 200 m of sediment, the Uvas Valley basin has a nearly symmetrical distribution of sediment thickness and appears to have been hydrologically closed throughout its history. The Miocene Gila Wilderness synclinal‐horst basin is bordered on three sides by gently tilted (10°, 15°, 20°) fault blocks. Despite evidence of an axial drainage that may have exited the northern edge of the basin, 200–300 m of sediment accumulated in the basin, probably as a result of high sediment yields from the large, high‐relief catchments. The Jornada del Muerto synclinal‐horst basin is positioned between the east‐tilted Caballo and west‐tilted San Andres fault blocks. Despite uplift and probable tilting of the adjacent fault blocks in the latest Oligocene and Miocene time, sediment was transported off the horst and deposited in an adjacent basin to the south. Sediment only began to accumulate in the Jornada del Muerto basin in Pliocene and Quaternary time, when an east‐dipping normal fault along the axis of the syncline created a small half graben. Overall, synclinal‐horst basins are rare, because horsts wide enough to develop broad synclines are uncommon in extensional terrains. Synclinal‐horst basins may be most common along the margins of extensional terrains, where thicker, colder crust results in wider fault spacing.

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.

The distribution and discrimination of shallow, authigenic carbonate in the Pliocene-Pleistocene Palomas Basin, southern Rio Grande rift

Nine types of authigenic carbonate are present in the Pliocene–Pleistocene alluvial-fan and axial-fluvial sediment of the Palomas half graben in the southern Rio Grande rift. Pedogenic and other vadose carbonate includes (1) pedogenic carbonate of stage II and stage III morphology underlying Bw and Bt horizons, (2) mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone, (3) pedogenic calcic nodules and tubules in eolian sand, and (4) gully-bed cement of proximal-fan conglomerates formed by infiltration and evaporation of surface runoff. Shallow groundwater carbonate exists as (5) thin (30–50 cm), massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe, (6) thick (1.5–3 m), massive beds deposited by lateral flow of groundwater or at springs, and (7) thin (30 cm), calcified root mats associated with near-surface, water-saturated sediment or springs. Phreatic spar cements (8) occupy the interstices of conglomerates and sandstones and locally exist as (9) oriented concretions. Groundwater carbonates are best developed near the toes of the large, hanging-wall–derived alluvial fans, whereas phreatic cement preferentially exists in footwall-derived, alluvial-fan conglomerates. Pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons, a predominantly vertical arrangement of root traces, peds, desiccation cracks, and calcic tubules, and by gradational contacts. The δ 13 C and δ 18 O values are similar among pedogenic and shallow groundwater carbonate, although locally nodules in eolian sand and gully-bed cement have higher δ 18 O values, perhaps due to the effects of evaporation. Some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of δ 13 C and δ 18 O. Authigenic carbonate in footwall-derived, alluvial-fan sediment has consistently higher values of δ 13 C than that in hanging-wall–derived sediment, which may reflect differences in vegetative type and/or density on either side of the basin.

The distribution and discrimination of shallow, authigenic carbonate in the Pliocene–Pleistocene Palomas Basin, southern Rio Grande rift

The distribution and discrimination of shallow, authigenic carbonate in the Pliocene–Pleistocene Palomas Basin, southern Rio Grande rift

Evolution of magma source regions in the Rio Grande rift, southern New Mexico

Early magmatism of the southern Rio Grande rift was strongly controlled by the thermal history of the lithosphere. The compositions of igneous rocks in southern New Mexico record a general shift in magma source regions through the Cenozoic from lithosphere to asthenosphere and from upper crust to lower crust and, finally, to little crustal involvement. Extension began as early as 36 Ma, coincident with the onset of bimodal volcanism of the middle Tertiary ignimbrite flare-up and production of half grabens after a short episode of post-Laramide subduction-related volcanism. Oligocene mafic magmas have incompatible trace element patterns similar to modern continental-arc basalts (low Nb, Ta, and Ti; high Rb/Nb, Ba/Nb, and La/Nb). The most mafic magma has 87Sr/86Sr and ϵNd values near bulk Earth (0.704771 and +0.2, respectively). The Oligocene mafic to intermediate-composition suites evolved toward slightly higher 87Sr/86Sr, lower ϵNd, and nonradiogenic Pb isotopic compositions (87Sr/86Sr = 0.70440–0.70785; ϵNd = −2.2 to −4.8; 206Pb/204Pb = 17.039–18.084, 207Pb/204Pb = 15.387–15.498; 208Pb/204Pb = 37.094–38.130) and are interpreted as partial melts of slightly hydrated lithosphere that were contaminated in the lower crust. Contemporaneous rhyolitic magmas have more radiogenic Sr and Pb isotopic compositions (87Sr/86Sr = 0.7111; ϵNd = −4.5; 206Pb/204Pb = 18.435; 207Pb/204Pb = 15.538; 208Pb/204Pb = 38.607) and record the involvement of an upper-crustal component. Silicic volcanism ceased abruptly at 28.5 Ma, but mafic to intermediate-composition lithospheric magmatism persisted until 24 Ma. Although extension continued, forming half grabens with sedimentary fill, the period between 24 and 10 Ma was amagmatic. This lack of volcanism and the end of lithosphere-dominated magma genesis reflect the effective scavenging of the hydrated parts of the lithospheric mantle by middle Tertiary magmatism, which produced a refractory, infertile, subcontinental lithosphere. Volcanism resumed at 10 Ma with sporadic eruption of tholeiitic and alkalic basalts having trace element patterns similar to oceanic-island basalts (high Nb, Ta, and Ti; low Ba/Nb and La/Nb), depleted Sr and Nd isotopic compositions (87Sr/86Sr = 0.70297–0.70396; ϵNd = +4.7 to +7.3), and more-radiogenic Pb isotopic compositions (206Pb/204Pb = 18.460–19.698; 207Pb/204Pb = 15.461–15.684; 208Pb/204Pb = 38.091–39.411) than the lithosphere-derived suites. The composition of these late Cenozoic basalts records a dramatic shift in source region to upwelling asthenosphere that melted by adiabatic decompression.

Climatic and tectonic controls on alluvial-fan and axial-fluvial sedimentation in the Plio-Pleistocene Palomas half graben, southern Rio Grande Rift

ABSTRACT Six steep (0.6-4.3°), short (radial length 1.5-5 km), deeply incised Pliocene to early Pleistocene paleofans and associated axial-fluvial strata are preserved adjacent to the footwall of the Palomas half graben in the southern Rio Grande rift. Two different scales of interbedding of lithofacies are evident in the exposed basin-fill sediment. The footwall-derived alluvial-fan strata are arranged into 3-10 m-thick cyclothems representing three events: (1) development of calcic paleosols that were contemporaneously or subsequently truncated by channels up to 4 m deep, (2) infilling of channels by turbulent-flow, horizontally bedded and cross-bedded conglomerates, and (3) deposition of hyperconcentrated-flow pebbly sandstones. These cyclothems probably resulted from paleoclimatic changes on the scale of 150 kyr. On a larger geographic and temporal scale, the axial river (ancestral Rio Grande), which primarily deposited cross-bedded pebbly sand, experienced kilometer-scale lateral movement toward the footwall block, resulting in one thick (80 m) axial-fluvial lithosome in the northern part of the basin and two or three thinner (8-25 m) axial-fluvial lithosomes in the south. A protracted period of fault movement and consequent basin tilting was the probable driving force behind lateral movement of the river, and toe cutting of the fans by the river was the dominant process.

Hydrogeothermal Studies in New Mexico and Implications for Ground-Water Resources

Subsurface temperatures and heat-flow data provide valuable information on both regional and local ground-water flow patterns. In the San Juan Basin of northwestern New Mexico and southwestern Colorado, heat-flow data are used to estimate basin-scale horizontal flow and predict hydraulic conductivities much higher than values measured in pumping tests or laboratory studies. Along the central Rio Grande rift in the Albuquerque Basin, hot water at depth may be present at a number of locations; however, the hydrogeology is such that warm springs are absent. In south-central New Mexico, along the southern Rio Grande rift, a number of warm springs result from tectonic activity and erosion exposing aquifers which allow deeply-circulating ground water to come near surface. In the Roswell Basin of southeastern New Mexico, recharge to the Pecos River deep aquifer is predicted from depths of ~1,000–1,500 m; in addition a ground-water divide appears present near the Mescalero ridge. On the Llano Estacado downward ground-water movement from the surface is suggested to depths of ~1,000–1,500 m. Along the Rio Grande at Bosque del Apache and Canutillo, and along the Pecos River at Artesia, there appear to be dual-flow systems with river water moving downward to depths of ~50–100 m, while upward flow of ground water occurs from much greater depths. This implies separation of the ground-water systems at these locales, an important ramification regarding the recharge of different zones. The deep pumping at Canutillo appears to have exceeded the natural recharge, and this may well be the case at other locales where pumping depths are below river recharge. Shallow pumping at Canutillo appears to have caused a much greater recharge from the river than was the prepumping case.

Channel shifting of the Rio Grande, southern Rio Grande rift: implications for alluvial stratigraphic models

Presented here is the history of channel course changes of the Rio Grande, based on Bureau of Reclamation and other maps, from prior to 1844 (exact date unknown) to 1912 over an 80 km stretch of the semi-arid Mesilla and upper Hueco half grabens of the southern Rio Grande rift. The historical Rio Grande was a pebbly sand bedload stream, occupying a relatively narrow (≤ 8 km) floodplain incised about 100 m into Pliocene and early Pleistocene alluvium. Prior to completion of Elephant Butte Dam in 1916, the river experienced late spring floods due to snowmelt near its headwaters. Throughout the 68+ year history recorded here the river channel had variable width (1300-100 m) and sinuosity (1.9-1.2), and displayed kilometres of lateral migration of both straight and sinuous stretches, neck cutoff of meander loops, and large- and small-scale avulsions. Three major avulsions occurred in different parts of the Mesilla basin: (1) between 1844 and 1852 in the north-central part of the basin, (2) in 1865 in the northern part of the basin, and (3) between 1903 and 1912 in the southern part of the basin. The length scale of the avulsions (distance from node to reoccupation of previous channel) ranged from 17 to 30 km, and the 1865 avulsion is documented to have occurred in one flood season. The historical Rio Grande is unique among other rivers described in the literature because of the high frequency of channel migration and avulsion. It sheds doubt on the applicability of sedimentary models requiring time-dependant (levee growth) avulsion functions.

Spillover models for axial rivers in regions of continental extension: the Rio Mimbres and Rio Grande in the southern Rio Grande rift, USA

ABSTRACTThe Pliocene‐early Pleistocene history of the ancestral Rio Grande and Quaternary history of the Rio Mimbres in the southern Rio Grande rift, New Mexico, illustrate how axial rivers may alternately spill into and subsequently abandon extensional basins. Three types of spillover basins are recognized, based on the angle at which the axial river enters the basin and whether it descends the hanging wall dip slope or footwall scarp to reach the basin floor. In the Mimbres basin type, the axial river enters and flows through the spillover basin nearly parallel to the footwall scarp, resulting in a narrow belt of basin‐axis‐parallel channel sand bodies located near the footwall scarp. In contrast, an axial river may enter a spillover basin at a high angle to its axis, either descending the hanging wall dip slope (Columbus basin type) or footwall scarp (Tularosa basin type), and construct a fluvial fan, consisting of radiating distributary channels orientated nearly perpendicular to the basin axis. Faulting exerts significant control on river spillover by creating the topographic gaps through which the axial river moves and by terminating spillover by subsequently uplifting or tilting the gap. Spillover may also be autocyclic in origin as a result of aggradation to the level of a pre‐existing gap, headward erosion creating and/or intersecting a gap, or simple river avulsion upstream of a gap. Predicting facies architecture in the three types of spillover basins is critical to successful subsurface exploration for hydrocarbon reservoirs, groundwater aquifers or placer mineral deposits.

Cooling histories of mountain ranges in the southern Rio Grande rift based on apatite fission-track analysis--A reconnaissance survey

Cooling histories of mountain ranges in the southern Rio Grande rift based on apatite fission-track analysis--A reconnaissance survey

Plio-pleistocene pumice floods in the ancestral Rio Grande, southern Rio Grande rift, USA

At least four times during the late Pliocene and early Pleistocene pyroclastic eruptions in the Jemez volcanic field, northern Rio Grande rift, flooded the ancestral Rio Grande with gravel-sized pumice. Following as much as 400 km of fluvial transport, the pumice was deposited in beds 0.2 to 2.0 m thick in the Camp Rice Formation of the southern Rio Grande rift. A combination of reversal magnetostratigraphy and single-crystal sanidine 40Ar/39Ar dating constrains the ages of pumice-clast conglomerates at 3.1, ∼2.0, 1.6, and 1.3 Ma. The coarsest pumice beds (cobbles, boulders) were deposited as antidune-like bedforms in a fluvial channel and as a crevasse-splay sheet. Granule and pebble-sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain. The abundance of pumice clasts in the gravel fraction (60–100%) suggests very rapid transport downriver, probably in a few days or weeks. The two older pumice-clast conglomerates correlate with the Puye Formation in the Jemez volcanic field, whereas the younger two are coeval to the Lower Bandelier Tuff and Cerro Toledo Rhyolite.

First quantitative test of alluvial stratigraphic models: Southern Rio Grande rift, New Mexico

Since 1978 the results of computational architectural models have been widely used to aid interpretation of ancient alluvial successions: here we present the first quantitative test of such models. We parameterize variables from field and magnetostratigraphic data collected from the well-exposed Pliocene-Pleistocene Camp Rice and Palomas Formations of the Rio Grande rift in south-central New Mexico. Computational runs establish that the LAB (Leeder, Allen, and Bridge) model correctly predicts the gross architectural patterns of ancestral axial Rio Grande half grabens and full grabens. Convergence of tectonic subsidence rate and mean sedimentation rate over the studied interval suggests that the dynamic basis of the models is correct; i.e., it is the tectonic “drawdown” of axially supplied sediment that controls the net preservation potential of alluvial successions.

Transfer zones in the southern Rio Grande rift

Despite recent interest in transfer zones in regions of continental extension, their detailed geometry and kinematics remain unclear. Four late Palaeogene and Neogene transfer zones within the southern Rio Grande rift, USA, are described, based on geological mapping at the scale of 1:24 000 and analysis of synrift stratigraphy. The Cutter Sag transfer zone is located between the overlapping tips of north-trending, west-dipping faults, and is characterized by a north-dipping structural ramp segmented by closely spaced, NE-trending normal faults. Similarly, the West Potrillo transfer zone occupies a position in the zone of overlap of north-trending, west-dipping faults, but the structural details of the transfer zone are largely hidden beneath Quaternary lava flows. However, isolated bedrock exposures outside the lava field, gravity data, and the alignment of cinder cones suggest that the West Potrillo transfer zone is a NW-dipping synclinal ramp cut by NE-trending fractures and/or faults. Positioned between north-trending, overlapping border faults that dip toward each other, the Cedar Hills transfer zone is a broad, north-trending arch whose summit collapsed into a graben along a series of north-trending normal faults. The Rincon transfer zone separates border faults that do not overlap and has a long history of faulting and sedimentation beginning in latest Oligocene and extending into the Quaternary. With the possible exception of reactivated early Tertiary compressional structures in the Rincon transfer zone, the transfer zones described here do not appear to be influenced by pre-rift zones of weakness. However, the Cutter Sag and West Potrillo transfer zones are preferred sites of basaltic volcanism, and the Cutter Sag.- Rincon, and Cedar Hill transfer zones influenced the location of the Plio-Pleistocene ancestral Rio Grande.