Magmatic-tectonic interaction during early Rio Grande rift extension at Questa, New Mexico (original) (raw)
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Geosphere, 2007
Recent inference that Mesozoic Cordilleran plutons grew incrementally during >10 6 yr intervals, without the presence of voluminous eruptible magma at any stage, minimizes close associations with large ignimbrite calderas. Alternatively, Tertiary ignimbrites in the Rocky Mountains and elsewhere, with volumes of 1-5 × 10 3 km 3 , record multistage histories of magma accumulation, fractionation, and solidifi cation in upper parts of large subvolcanic plutons that were sufficiently liquid to erupt. Individual calderas, up to 75 km across with 2-5 km subsidence, are direct evidence for shallow magma bodies comparable to the largest granitic plutons. As exemplifi ed by the composite Southern Rocky Mountain volcanic fi eld (here summarized comprehensively for the fi rst time), which is comparable in areal extent, magma composition, eruptive volume, and duration to continental-margin volcanism of the central Andes, nested calderas that erupted compositionally diverse tuffs document deep composite subsidence and rapid evolution in subvolcanic magma bodies. Spacing of Tertiary calderas at distances of tens to hundreds of kilometers is comparable to Mesozoic Cordilleran pluton spacing. Downwind ash in eastern Cordilleran sediments records large-scale explosive volcanism concurrent with Mesozoic batholith growth. Mineral fabrics and gradients indicate unifi ed fl owage of many pluton interiors before complete solidifi cation, and some plutons contain ring dikes or other textural evidence for roof subsidence. Geophysical data show that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas. Most ignimbrites are more evolved than associated plutons; evidence that the subcaldera chambers retained voluminous residua from fractionation. Initial incre-mental pluton growth in the upper crust was likely recorded by modest eruptions from central volcanoes; preparation for calderascale ignimbrite eruption involved recurrent magma input and homogenization high in the chamber. Some eroded calderas expose shallow granites of similar age and composition to tuffs, recording sustained postcaldera magmatism. Plutons thus provide an integrated record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages. Growth of subvolcanic batholiths involved sustained multistage opensystem processes. These commonly involved ignimbrite eruptions at times of peak power input, but assembly and consolidation processes continued at diminishing rates long after peak volcanism. Some evidence cited for early incremental pluton assembly more likely records late events during or after volcanism. Contrasts between relatively primitive arc systems dominated by andesitic compositions and small upper-crustal plutons versus more silicic volcanic fi elds and associated batholiths probably refl ect intertwined contrasts in crustal thickness and magmatic power input. Lower power input would lead to a Cascade-or Aleutian-type arc system, where intermediate-composition magma erupts directly from middle-and lowercrustal storage without development of large shallow plutons. Andean and southern Rocky Mountain-type systems begin similarly with intermediate-composition volcanism, but increasing magma production, perhaps triggered by abrupt changes in plate boundaries, leads to development of larger upper-crustal reservoirs, more silicic compositions, large ignimbrites, and batholiths. Lack of geophysical evidence for voluminous eruptible magma beneath young calderas suggests that near-solidus plutons can be rejuvenated rapidly by high-temperature mafi c recharge, potentially causing large explosive eruptions with only brief precursors.
Common traits of mantle plumes are:1) domal uplift prior to volcanism, 2) a definite age progression along a volcanic chain, 3) long mafic dikes that radiate from the volcanic core, 4) large basaltic plateaus or shield volcanoes, and 5) petrochemical indicators of anomalously high temperature melt zones in the upper mantle, such as high Ni/MgO ratios in picritic basalts (Campbell, 2001). We suggest the Socorro-Magdalena magmatic system of Oligocene age (Fig.1, eruptive volume 7100 km 3) exhibits characteristics similar to a mantle plume, but at 1/10-1/100 th the scale of a deep mantle plume, which may qualify it as a miniplume? A cluster of five overlapping ignimbrite calderas is moderately well exposed in strongly extended, tilted fault-block mountain ranges of the central Rio Grande rift southwest of Socorro NM (Fig.1). The westward younging Socorro-Magdalena caldera cluster (SMCC) is 85 km long and 20-25 km wide. It parallels the southeastern margin of the Colorado Plateau and the WSW-trending San Agustin arm of the rift. The latter produces the appearance of an incipient triple junction within the dominantly north-trending rift system. Precise 40 Ar/ 39 Ar ages of sanidines from the rhyolite ignimbrites and detailed geologic mapping demonstrate that the distended calderas become progressively younger to the west-southwest (McIntosh et al., 1991: Chamberlin et al., in press). Large volume ignimbrite eruptions occurred at 31.9, 28.7, 27.9, 27.4 and 24.3 Ma. A large satellitic caldera, which formed at 28.4 Ma, is located 20 km southwest of the main overlapping trend. A small collapse structure, which is nested in the Socorro caldera, erupted at 30.0 Ma. The total volume of densely welded ignimbrite and moat-fill lavas erupted from the SMCC is 5500 km 3. Within 40 km of the northeastern margin of the caldera cluster, the rhyolite ignimbrites are interleaved with a 400-700 m thick plateau-like belt of basaltic andesite lavas (Fig.1). These mafic lavas are assigned to the La Jara Peak Basaltic Andesite (Osburn and Chapin, 1983a). They range from slightly alkaline trachybasalt to moderately alkaline basaltic trachyandesite and sub-alkaline basaltic andesite. Sparse small phenocrysts of olivine, commonly altered to reddish brown iddingsite, are characteristic. Phenocrystic plagioclase, indicative of differentiation at depths less than 30 km (Wilson, 1989), is typically absent. Individual basaltic andesite flows are commonly 7-10 m thick. Stacked flows between ignimbrites have an aggregate thickness of as much as 330m and locally define wedge-shaped prisms formed by domino-style extension in the early Rio Grande rift (Chamberlin, 1983; Ferguson, 1991). A 32-33 Ma flow and tephra unit near La Joya was locally fed by a short NE striking basalticandesite dike that appears to radiate from the 31.9-Ma Socorro caldera (Fig1). A primitive trachybasalt in the SE moat of the Socorro caldera (~31 Ma; Chamberlin et al., in press) contains 9.3 % MgO and 170 ppm Ni; this suggests a relatively hot source zone in the mantle, compared to most subduction related basalts (Campbell, 2001). The total volume of Oligocene basaltic andesite lavas peripheral to the SMCC is 1600 km 3. The maximum rate of basaltic andesite eruption, ~1800 km 3 /Ma, was coeval with the zenith of domino-style extension and apparent caldera migration at 27.9-27.4 Ma. Moderately alkaline to sub-alkaline basaltic andesite and trachybasalt dikes of Oligocene age (31-24 Ma, K/Ar, Aldrich et al., 1986; Laughlin et al., 1983) form a large semi-continuous radial array that is broadly focused on the SMCC (Fig.1). The Magdalena radial dike swarm (MRDS) fans through
Tectonophysics, 1990
As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south -central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area and consist of three interlocking regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed SW-SE line. The reversed E-W line shows no significant dip along the Moho ('32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4-6 km with respect to the northern rift and the adjacent Basin and Range province and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province. Gas Company, Dallas, Texas. 3Department of Geosciences, Purdue University, West Lafayette, Indiana. Laramide orogenic activity [Chapin and Seager, 1975]. From late Eocene through most of the Oligocene, voluminous calcalkalic volcanism took place throughout most of the southwestern United States as a result of subduction of the Farallon plate beneath the North American plate [Atwater, 1970; Lipman et al., 1972; Coney and Reynolds, 1977]. In the rift area, this activity is evidenced by the formation of the San Juan, Datil-Mogollon, and Davis Mountains, as well as other smaller volcanic fields that flank the rift [Chapin and Seager, 1975]. By late Oligocene to early Miocene time, magmatism changed from predominantly calc-alkalic compositions to more basic compositions [Seager and Morgan, 1979; Chapin, 1979]. This change in volcanism in the rift area was contemporaneous with the transition from a subduction regime to a regime of regional extension [Lipman and Mehnert, 1975] and the early initiation of rifting. The basaltic-andesitic volcanism lasted to about 20 Ma in the northern rift and to about 26 Ma in the southern rift and was followed by a magmatic lull which lasted until 13 Ma [Chapin, 1970; Seager and Morgan, 1979]. Following this lull, basaltic-rhyolitic volcanism started with basaltic activity peaking around 5 Ma [Seager andMorgan, 1979]. Basaltic fields and individual flows dotted the floor of the Rio Grande rift from southern Colorado to the Mexican border [Seager and Morgan, 1970; Chapin and Seager, 1975]. The major volcanic activity was concentrated in the Jemez Mountains-Taos Plateau area, the Socorro Peak area, and the Magdalena Peak area where major northeast trending lineaments intersect the rift [Chapin, 1971]. About 7-4 Ma, the Southern Rocky Mountains and adjacent areas were strongly uplifted due to mantle upwelling [Eaton, 1979]. In the southern Rio Grande rift, Seager et al. [1984] interpreted the change from basaltic andesite to alkali-olvine basalt to represent two different but transitional extension regimes. In the early regime (starting around 28 Ma), extension developed in a back arc setting and resulted in the eraplacement of basaltic andesite, the formation of broad NW trending basins, and the uplift of some of the region's fault-block mountains. The later 6143 helped with the CARDEX experiment, especially L.W. Braile and Carl
Geosphere, 2017
We present a detailed example of how a subbasin develops adjacent to a transfer zone in the Rio Grande rift. The Embudo transfer zone in the Rio Grande rift is considered one of the classic examples and has been used as the inspiration for several theoretical models. Despite this attention, the history of its development into a major rift structure is poorly known along its northern extent near Taos, New Mexico. Geologic evidence for all but its young rift history is concealed under Quaternary cover. We focus on under standing the preQuaternary evidence that is in the subsurface by integrat ing diverse pieces of geologic and geophysical information. As a result, we present a substantively new understanding of the tectonic configuration and evolution of the northern extent of the Embudo fault and its adjacent subbasin. We integrate geophysical, borehole, and geologic information to interpret the subsurface configuration of the rift margins formed by the Embudo and Sangre de Cristo faults and the geometry of the subbasin within the Taos em bayment. Key features interpreted include (1) an imperfect Dshaped subbasin that slopes to the east and southeast, with the deepest point ~2 km below the valley floor located northwest of Taos at ~36° 26′N latitude and 105° 37′W longitude; (2) a concealed Embudo fault system that extends as much as 7 km wider than is mapped at the surface, wherein fault strands disrupt or truncate flows of Pliocene Servilleta Basalt and step down into the subbasin with a minimum of 1.8 km of vertical displacement; and (3) a similar, wider than ex pected (5-7 km) zone of stepped, westdown normal faults associated with the Sangre de Cristo range front fault. From the geophysical interpretations and subsurface models, we infer relations between faulting and flows of Pliocene Servilleta Basalt and older, buried basaltic rocks that, combined with geologic mapping, suggest a re vised rift history involving shifts in the locus of fault activity as the Taos sub basin developed. We speculate that faults related to northstriking grabens at the end of Laramide time formed the first westdown master faults. The Em budo fault may have initiated in early Miocene southwest of the Taos region. Normaloblique slip on these early fault strands likely transitioned in space and time to dominantly leftlateral slip as the Embudo fault propagated to the northeast. During and shortly after eruption of Servilleta Basalt, proto Embudo fault strands were active along and parallel to the modern, NEaligned Rio Pueblo de Taos, ~4-7 km basinward of the modern, mapped Embudo fault zone. Faults along the northeastern subbasin margin had northwest strikes for most of the period of subbasin formation and were located ~5-7 km basinward of the modern Sangre de Cristo fault. The locus of fault activity shifted to more northerly striking faults within 2 km of the modern range front sometime after Servilleta volcanism had ceased. The northerly faults may have linked with the northeasterly protoEmbudo faults at this time, concurrent with the de velopment of Nstriking Los Cordovas normal faults within the interior of the subbasin. By middle Pleistocene(?) time, the Los Cordovas faults had become inactive, and the linked Embudo-Sangre de Cristo fault system migrated to the south, to the modern range front.
Geochemistry, Geophysics, Geosystems, 2007
1] The northwestern margin of the Basin and Range Province is characterized by a transition from lowmagnitude ($20%) extension in northwestern Nevada to relatively unextended volcanic plateaus in northeastern California. Seismic-velocity and potential-field modeling provides new control on the Mesozoic-to-present tectonic evolution of this poorly understood portion of the U.S. Cordillera. We document 2020% crustal thinning associated with Basin and Range extension from a crustal thickness of 2037 km under northeastern California to 31kmundernorthwesternNevadathatisconsistentwiththeamountofextensionrecordedintheuppercrustinnorthwesternNevada,suggestingthecrustalresponsetoextensionwasrelativelyhomogeneousovertheentirecrustalcolumn.Ourmodelingalsoshowsawelldefined,80−km−widezoneofunusuallylowupper−crustalvelocities(31 km under northwestern Nevada that is consistent with the amount of extension recorded in the upper crust in northwestern Nevada, suggesting the crustal response to extension was relatively homogeneous over the entire crustal column. Our modeling also shows a welldefined, 80-km-wide zone of unusually low upper-crustal velocities (31kmundernorthwesternNevadathatisconsistentwiththeamountofextensionrecordedintheuppercrustinnorthwesternNevada,suggestingthecrustalresponsetoextensionwasrelativelyhomogeneousovertheentirecrustalcolumn.Ourmodelingalsoshowsawelldefined,80−km−widezoneofunusuallylowupper−crustalvelocities(5.9-6.1 km/s) that coincide with the surface location of sparse Cretaceous granites, locating the elusive northern extension of the Sierra Nevada batholith through northwestern Nevada for the first time in the subsurface. Combining geological and geophysical data, we reconstruct the late Cretaceous-to-present crustal evolution of this region, documenting an interplay between magmatic addition to the crust, erosional exhumation, sedimentation, and extension that has reversed the direction of crustal thinning from a west-facing continental margin to an east-facing interior basin margin over this time interval. Finally, we find no evidence in northwestern Nevada for unusually thick crust (>40 km) prior to Basin and Range extension.
Journal of Geophysical Research, 1995
The 15.7 Ma Aztec Wash pluton is located in the central Eldorado Mountains of the Colorado River extensional corridor in southern Nevada, immediately south of the well-known imbricated volcanic sequence that has been widely cited in studies of extensional tectonism (e.g., Anderson, 1971). It is a shallow level • 5 km), essentially bimodal complex, primarily made up of granite (~ 72 wt % SiO 2) and diabase and diorite (~ 54 wt % SiO2), with minor amounts of more mafic, intermediate, and highly evolved rocks. The mafic and felsic magmas mingled extensively but mixed only to a limited extent. Late synplutonic mafic and felsic dikes represent continuing injection of the same bimodal magmas. The mafic rocks have high incompatible element concentrations (e.g., K20 ~ 3 wt %, Ba ~ 1600 ppm, light rare earth elements 350 x chondrite) and enriched isotopic compositions (eNd -7.5, 87Sr/86Sr 0.708); generation in ancient, enriched mantle lithosphere with limited subsequent crustal contamination is inferred. The granite is more potassic (~ 5 wt %) than the mafic rocks, but it has comparable or lower concentrations of most incompatible elements; its isotopic composition (eNd-10, 87Sr/S6Sr 0.710) is intermediate between those of the mafic rocks and local ancient crust. The granites thus indicate hybridization of the crust by mafic magma, but it is unclear whether this hybridization occurred at deeper levels in this magmatic system, or during an earlier mid-Tertiary or Mesozoic magmatic event. Emplacement of the Aztec Wash pluton preceded peak east west extension in the northern Eldorado Mountains (~ 15.2 Ma (Gans et al., 1994)), but it coincided with at least modest extension as indicated by the uniform NS orientation of the late dikes and the mafic injections into the magma chamber. Total extension and tilting of the pluton after crystallization was minor, in contrast to the east tilted area to the north and west tilted area to the south.
Tectonophysics, 1991
The region of the present Rio Grande rift and southeastern Colorado Plateau underwent iithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and erupted to the surface. Study of the igneous rocks erupted or intruded during this deformation yields insights into processes of magmatism associated with extension of continental lithosphere. Magmatic rocks associated with an early (late Oligocene-early Miocene) ductile phase of extension are dominantly basaltic andesites and related, talc-alkaline intermediate to silicic derivative rocks. Mafic magmas were probably derived from isotopically "enriched" lithospheric mantle. Igneous rocks associated with a later (middle Miocene-Holocene), more brittle phase of extension include widespread basaltic rocks and localixed central volcanoes of intermediite to silicic composition. Isotopic compositions of mafic rocks, which include both tholeiitic and alkalic basalts, correlate strongly with tectonic setting and lithospheric structure. Basalts erupted in areas of greatest crustal extension, such as the central and southern rift and Basin and Range province, were derived from isotopically "depleted" (correlated with "asthenospheric") mantle. Also, isotopic compositions of Pliocene to Holocene basalts are shghtly more depleted than those of Miocene basalta, suggesting that subcrustal lithospheric mantle was thinned during late Miocene extension. intermediate rocks of the central voIcanoes formed by a complex combination of processes, probably dominated by fractional crystallization and by assimilation of upper and lower crust in isolated, small magma chambers. The petrologic, geochemical, and isotopic data are compatible with a model, derived first from geophysical data, whereby lithosphere is thinned beneath the central rift and southeastern Colorado Plateau, with greatest thinning centered beneath the axis of the rift. A lithospheric model involving uniform-sense simple shear does not appear compatible with the data as presently understood. In-Magmatism is an integral feature of continental rifting. Some rifts, such as the East African rift, are associated with huge volumes of vobnic rocks (Williams, 1982). Others, such as the Rio Grande rift, have relatively small volumes of magmatic rocks exposed at the surface (e.g., Olsen et al., NEW MEXICO COLOT(ADO PLATEAU 36"-CENOZOIC MAGMATISM OF THE SOUTHEASTERN COLORADO PLATEAU AND CENTRAL RIO GRANDE RIFT MAGMATISM OF THE SOUTHEASTERN COLORADO PLATEAU AND CENTRAL RIO GRANDE RIFI-331 field, which consists of numerous overlapping Pecos magmatic province of west Texas (e.g., calderas and probably overlies a large composite Barker, 1979; Price et al., 1986). The early (late pluton (e.g., Elston et al., 1976), and the Trans-Oligocene to early Miocene) phase of magmatism TABLE 1 New whole-rock K-Ar dates, sample descriptions, localities, and analytical data for middle to late Cenozoic magmatic rocks of the southeastern Colorado Plateau transition zone Sample no. Description, location, comments K (S) Radiogenic Ar 4o Atmospheric Arm (X 1012 mole/g) (W)