El volcanismo jurásico superior de la Formación Río Damas-Tordillo (33°-35,5°S): antecedentes su sobre petrogénesis, cronología, proveniencia e implicancias tectónicas

Los depósitos continentales y volcánicos de la Formación Rio Damas-Tordillo, Jurásico Superior, representan un período restringido de sedimentación continental dentro del registro mayormente marino de la Cuenca Neuquina. Datos anteriores y los presentados en este trabajo, sugieren que el cambio a un estado de mayor acoplamiento entre placas durante el Jurásico tardío (160-140 Ma), sumado a la continua efusión de material volcánico, resultaron en una progresiva emersión del dominio de arco y ante arco, para finalmente desconectar a la cuenca de tras-arco del Océano Pacífico. Este importante cambio en la configuración del margen tuvo como resultado el desarrollo de una regresión marina y posterior sedimentación continental con aportes desde el oeste, en una cuenca de tras-arco de tipo hemigraben. Una edad máxima de depositación de 146,4±4.4 Ma obtenida en la parte superior de la secuencia sedimentaria, sugiere que los potentes depósitos de volcanismo asociado a subducción, observados en la parte superior de la unidad, fueron eruptados en un período de tiempo muy restringido, lo cual probablemente fue facilitado por la presencia de estructuras extensionales relacionadas con el desarrollo de la cuenca de tras-arco. Datos geoquímicos elementales e isotópicos, junto con modelamientos de ACF, sugieren un manto astenosférico deprimido como fuente del material ígneo, y el fraccionamiento de olivino y plagioclasa, combinado con pequeños volúmenes de asimilación de corteza inferior, como los principales procesos involucrados en la evolución de los magmas. No es posible diferenciar, en términos geoquímicos, la fuente y procesos petrogenéticos del volcanismo Jurásico reconocido en la Cordillera de la Costa y el de la Formación Río Damas-Tordillo.The uppermost Jurassic continental and volcanic deposits of the Río Damas-Tordillo Formation represent an interval of intense continental deposition within the Jurassic to Early Cretaceous dominantly marine environment of the Mendoza-Neuquén back-arc basin. Stratigraphic and geochronological data indicate that progressive emersion of the arc and forearc domain, disconnecting the back-arc region from the Pacific Ocean, occurred during occurred during the Late Jurassic and probably the Early Cretaceous (~160-140 Ma). This change in the margin configuration induced a marine regression and the subsequent deposition of continental material in the back-arc basin. The most likely source of the sediments would have been the Jurassic arc, located west of the back-arc basin. The maximum depositional age of 146.4±4.4 Ma obtained from a red sandstone immediately below volcanic rocks confirms recent Tithonian maximum depositional ages assigned to the Río Damas-Tordillo Formation, and suggests that the volcanic rocks, overlain by marine fossiliferous Tithoninan-Hauterivian sequences, should have erupted within a short time span during the Late Jurassic. Volcanism was probably facilitated by the presence of extensional structures related to the formation of the back-arc basin. Elemental and isotopic data, along with forward AFC models, suggest a depleted sub-arc asthenospheric mantle source for the volcanic rocks and the fractionation of olivine and plagioclase, along with small volumes of lower crust assimilation, as the main processes involved in the magmatic evolution. It is not possible to establish a different source and petrogenetic conditions for the Río Damas-Tordillo Formation and the magmatism in the arc domain located further west, at the present-day Coastal Cordillera.Fil: Rossel, Pablo. Universidad de Concepción; ChileFil: Oliveros, Verónica. Universidad de Concepción; ChileFil: Mescua, Jose Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Tapia, Felipe. Universidad de Chile. Facultad de Ciencias Físicas y Matemáticas. Departamento de Geología; ChileFil: Ducea, Mihai Nicolae. University of Arizona; Estados UnidosFil: Calderón, Sergio. Universidad de Chile. Facultad de Ciencias Físicas y Matemáticas. Departamento de Geología; ChileFil: Charrier González, Reynaldo. Universidad de Chile. Facultad de Ciencias Físicas y Matemáticas. Departamento de Geología; ChileFil: Hoffman, Derek. University of Arizona; Estados Unido

Velocity variations in the uppermost mantle beneath the southern Sierra Nevada and Walker Lane

We model Pn waveforms from two earthquakes in the southwestern United States (Mammoth Lakes, California, and western Nevada) to determine a velocity model of the crustal and mantle structure beneath the southern Sierra Nevada and Walker Lane. We derive a one-dimensional velocity model that includes a smooth crust-mantle transition east of Death Valley and extending south into the eastern Mojave desert. West of Death Valley and toward the Sierra Nevada a low-velocity mantle (V_p = 7.6 km/s) directly below the crust indicates the lithosphere is absent. At the base of this low-velocity structure (at 75–100 km depth) the P wave velocity jumps discontinuously to V_p 8.0 km/s. The area of low velocity is bounded by the Garlock Fault to the south and the Sierra Nevada to the west, but we cannot resolve its northern extent. However, on the basis of teleseismic travel times we postulate that the anomaly terminates at about 38°N. The presence of a low-velocity, upper mantle anomaly in this area agrees with geochemical research on xenoliths from the southern Sierras and recent studies of receiver functions, refraction profiles, tomography, and gravity. However, the velocity discontinuity at 75–100 km is a new discovery and may represent the top of the once present, now unaccounted for and possibly sunken Sierra Nevada lithosphere

Production and loss of high-density batholithic root, southern Sierra Nevada, California

Eclogites are commonly believed to be highly susceptible to delamination and sinking into the mantle from lower crustal metamorphic environments. We discuss the production of a specific class of eclogitic rocks that formed in conjunction with the production of the Sierra Nevada batholith. These high-density eclogitic rocks, however, formed by crystal-liquid equilibria and thus contrast sharply in their petrogenesis and environment of formation from eclogite facies metamorphic rocks. Experimental studies show that when hydrous mafic to intermediate composition assemblages are melted in excess of 1 GPa, the derivative liquids are typical of Cordilleran-type batholith granitoids, and garnet + clinopyroxene, which is an eclogitic mineralogy, dominate the residue assemblage. Upper mantle-lower crustal xenolith suites that were entrained in mid-Miocene volcanic centers erupted through the central Sierra Nevada batholith are dominated by such garnet clinopyroxenites, which are shown further by geochemical data to be petrogenetically related to the overlying batholith as its residue assemblage. Petrogenetic data on garnet pyroxenite and associated peridotite and granulite xenoliths, in conjunction with a southward deepening oblique crustal section and seismic data, form the basis for the synthesis of a primary lithospheric column for the Sierra Nevada batholith. Critical aspects of this column are the dominance of felsic batholithic rocks to between 35 and 40 km depths, a thick (∼35 km) underlying garnet clinopyroxenite residue sequence, and interlayered spinel and underlying garnet peridotite extending to ∼125 km depths. The peridotites appear to be the remnants of the mantle wedge from beneath the Sierran arc. The principal source for the batholith was a polygenetic hydrous mafic to intermediate composition lower crust dominated by mantle wedge-derived mafic intrusions. Genesis of the composite batholith over an ∼50 m.y. time interval entailed the complete reconstitution of the Sierran lithosphere. Sierra Nevada batholith magmatism ended by ∼80 Ma in conjunction with the onset of the Laramide orogeny, and subsequently, its underlying mantle lithosphere cooled conductively. In the southernmost Sierra-northern Mojave Desert region the subbatholith mantle lithosphere was mechanically delaminated by a shallow segment of the Laramide slab and was replaced by underthrust subduction accretion assemblages. Despite these Laramide events, the mantle lithosphere of the greater Sierra Nevada for the most part remained intact throughout much of Cenozoic time. A pronounced change in xenolith suites sampled by Pliocene-Quaternary lavas to garnet absent, spinel and plagioclase peridotites, whose thermobarometry define an asthenosphere adiabat, as well as seismic data, indicate that much of the remaining sub-Sierran lithosphere was removed in Late Miocene to Pliocene time. Such removal is suggested to have arisen from a convective instability related to high-magnitude extension in the adjacent Basin and Range province and to have worked in conjunction with the recent phase of Sierran uplift and a change in regional volcanism to more primitive compositions. In both the Mio-Pliocene and Late Cretaceous lithosphere removal events the base of the felsic batholith was the preferred locus of separation

Active foundering of a continental arc root beneath the southern Sierra Nevada in California

Seismic data provide images of crust–mantle interactions during ongoing removal of the dense batholithic root beneath the southern Sierra Nevada mountains in California. The removal appears to have initiated between 10 and 3 Myr ago with a Rayleigh–Taylor-type instability, but with a pronounced asymmetric flow into a mantle downwelling (drip) beneath the adjacent Great Valley. A nearly horizontal shear zone accommodated the detachment of the ultramafic root from its granitoid batholith. With continuing flow into the mantle drip, viscous drag at the base of the remaining ~35-km-thick crust has thickened the crust by ~7 km in a narrow welt beneath the western flank of the range. Adjacent to the welt and at the top of the drip, a V-shaped cone of crust is being dragged down tens of kilometres into the core of the mantle drip, causing the disappearance of the Moho in the seismic images. Viscous coupling between the crust and mantle is therefore apparently driving present-day surface subsidence

The Early Andean subduction system as an analogue to island arcs: evidence from across-arc geochemical variations in northern Chile

International audienceThe Upper Jurassic volcanic rocks of the Pre-Cordillera and High Andes of northern Chile (26 - 31°S) represent a back-arc magmatic chain formed during an earlier stage of Andean subduction. After the Callovian, the back-arc basin gradually changed from marine to continental conditions and was characterized by basaltic to rhyolitic rocks erupted along two belts, parallel to the coeval arc. The western belt comprises the Picudo and Algarrobal formations, whereas the eastern belt comprises the Lagunillas Formation and the Quebrada Vicuñita Beds. New major and trace elements data, along with whole rock Sr, Nd and Pb isotopes are presented for these volcanic belts and compared to the geochemical features of the Jurassic and Early Cretaceous arc magmatism. Ar-Ar and U-Pb ages constrain the back arc volcanism to have evolved between 163.9 ± 1.4 and 148.9 ± 1.2 Ma.Rocks belonging to the western belt have steep multi-element patterns and low concentrations of HREE, suggesting the presence of garnet in the source, and a more radiogenic isotopic composition than the arc magmatism. Parental magmas of these back-arc lavas would have been generated through melting of a depleted mantle, although less depleted than the sub-arc mantle, and interacted with minor amounts of Paleozoic crust. The geochemical composition of the rocks belonging to the eastern belt is more heterogeneous and suggests involvement of different magmatic sources, including depleted mantle as well as an OIB-type mantle within the wedge. In spite the fact that the Jurassic Andean arc was built over a continental plate, the architecture of the volcanic chains and geochemical variations observed among the arc and back-arc rocks in northern Chile resemble those in modern island arcs, and thus support the hypothesis that early Andean subduction developed under extensional tectonic conditions

Major Miocene exhumation by fault-propagation folding within a metamorphosed, early Paleozoic thrust belt: Northwestern Argentina

The central Andean retroarc thrust belt is characterized by a southward transition at ∼22°S in structural style (thin-skinned in Bolivia, thick-skinned in Argentina) and apparent magnitude of Cenozoic shortening (>100 km more in the north). With the aim of evaluating the abruptness and cause of this transition, we conducted a geological and geo-thermochronological study of the Cachi Range (∼24–25°S), which is a prominent topographic feature at this latitude. Our U-Pb detrital zircon results from the oldest exposed rocks (Puncoviscana Formation) constrain deposition to mainly Cambrian time, followed by major, Cambro-Ordovician shortening and ∼484 Ma magmatism. Later, Cretaceous rift faults were locally inverted during Cenozoic shortening. Coupled with previous work, our new (U-Th)/He zircon results require 8–10 km of Miocene exhumation that was likely associated with fault-propagation folding within the Cachi Range. After Miocene shortening, displacement on sinistral strike-slip faults demonstrates a change in stress state to a non-vertically orientedσ3. This change in stress state may result from an increase in gravitational potential energy in response to significant crustal thickening and/or lithospheric root removal. Our finding of localized Cenozoic shortening in the Cachi Range increases the estimate of the local magnitude of shortening, but still suggests that significantly less shortening was accommodated south of the thin-skinned Bolivian fold-thrust belt. Our results also underscore the importance of the pre-existing stratigraphic and structural architecture in orogens in influencing the style of subsequent deformation.Fil: Pearson, D. M.. University Of Arizona; Estados Unidos. University Of Idaho; Estados UnidosFil: Kapp, P.. University Of Arizona; Estados UnidosFil: Reiners, P. W.. University Of Arizona; Estados UnidosFil: Gehrels, G. E.. University Of Arizona; Estados UnidosFil: Ducea, M. N.. University Of Arizona; Estados Unidos. University of Bucharest; RumaniaFil: Pullen, A.. University Of Arizona; Estados Unidos. University of Rochester; Estados UnidosFil: Otamendi, Juan Enrique. Universidad Nacional de Rio Cuarto. Facultad de Cs.exactas Fisicoquimicas y Naturales. Departamento de Geologia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Alonso, Ricardo Narciso. Universidad Nacional de Salta; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Active megadetachment beneath the western United States

Geodetic data, interpreted in light of seismic imaging, seismicity, xenolith studies, and the late Quaternary geologic history of the northern Great Basin, suggest that a subcontinental-scale extensional detachment is localized near the Moho. To first order, seismic yielding in the upper crust at any given latitude in this region occurs via an M7 earthquake every 100 years. Here we develop the hypothesis that since 1996, the region has undergone a cycle of strain accumulation and release similar to “slow slip events” observed on subduction megathrusts, but yielding occurred on a subhorizontal surface 5–10 times larger in the slip direction, and at temperatures >800°C. Net slip was variable, ranging from 5 to 10 mm over most of the region. Strain energy with moment magnitude equivalent to an M7 earthquake was released along this “megadetachment,” primarily between 2000.0 and 2005.5. Slip initiated in late 1998 to mid-1999 in northeastern Nevada and is best expressed in late 2003 during a magma injection event at Moho depth beneath the Sierra Nevada, accompanied by more rapid eastward relative displacement across the entire region. The event ended in the east at 2004.0 and in the remainder of the network at about 2005.5. Strain energy thus appears to have been transmitted from the Cordilleran interior toward the plate boundary, from high gravitational potential to low, via yielding on the megadetachment. The size and kinematic function of the proposed structure, in light of various proxies for lithospheric thickness, imply that the subcrustal lithosphere beneath Nevada is a strong, thin plate, even though it resides in a high heat flow tectonic regime. A strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern Great Basin, deformation microstructures and low water content in dunite xenoliths in young lavas in central Nevada, and high-temperature microstructures in analog surface exposures of deformed lower crust. Large-scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere, as inferred by such proxies as low P wave velocity and mafic magmatism