Role of extrusion of the Rand and Sierra de Salinas schists in Late Cretaceous extension and rotation of the southern Sierra Nevada and vicinity

The Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent Mojave-Salinia region along a shallow segment of the subducting Farallon plate in Late Cretaceous time. Various mechanisms, including return flow, isostatically driven uplift, upper plate normal faulting, erosion, or some combination thereof, have been proposed for the exhumation of the schist. We supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the Rand and Sierra de Salinas schists. These data indicate that the schist was transported to the SSW from deep to shallow crustal levels along a mylonitic contact (the Rand fault and Salinas shear zone) with upper plate assemblages. Crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth, suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary. The resulting flow type within the channel is that of general shear extrusion. Integration of these observations with published geochronologic, thermochronometric, thermobarometric, and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation. We present a model whereby trench-directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate

Provenance of Eocene river sediments from the central northern Sierra Nevada and implications for paleotopography

Geochronology of fluvial deposits can be used to characterize provenance, the paleotopography of sediment source regions, and the development of regional drainage systems. We present U-Pb and (U-Th)/He ages of detrital zircon grains from Eocene gravels preserved in several paleoriver systems along the western flank of the central and northern Sierra Nevada. These ages allow us to trace the sourcing of detritus in paleorivers and to constrain the evolution of the Sierra Nevada range front. U-Pb zircon age distributions are bimodal, with a dominant peak between 110 and 95 Ma and smaller but significant peaks in the Middle to Late Jurassic, matching the predominant ages of the Sierra Nevada batholith. A small fraction (<6%) of grains has pre-Mesozoic ages, which consistently match ages from prebatholithic assemblages within the northern part of the range. (U-Th)/He ages of a subset of double-dated zircons cluster between 114 and 74 Ma and are consistent with batholithic (U-Th)/He cooling ages in the northern Sierra. Our results indicate that the Eocene river systems in the central northern Sierra Nevada likely had proximal headwaters and had relatively steep axial gradients, draining smaller areas than was commonly thought. This also suggests that the northern Sierra Nevada would have had an established drainage divide and would have acted as a major topographic barrier during the early to mid-Cenozoic. The data presented here support a model of the Eocene northern Sierra Nevada characterized by a western slope with a gradient broadly similar to that of today

The distribution of radiogenic heat production as a function of depth

Abstract Geochemical analyses and geobarometric determinations have been combined to create a depth vs. radiogenic heat production database for the Sierra Nevada batholith, California. This database shows that mean heat production values first increase, then decrease, with increasing depth. Heat production is~2 AW/m 3 within the~3-km-thick volcanic pile at the top of the batholith, below which it increases to an average value of~3.5 AW/m 3 at~5.5 km depth, then decreases to~0.5-1 AW/m 3 at 15 km depth and remains at these values through the entire crust below 15 km. Below the crust, from depths of~40-125 km, the batholith&apos;s root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of~0.14 AW/m 3 . This is higher than the rates from most published xenolith studies, but reasonable given the presence of crustal components in the arc root assemblages. The pattern of radiogenic heat production interpreted from the depth vs. heat production database is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. The interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the Sierra Nevada could be generated within the~35 km thick crust. This requires a very low heat flux from the mantle, which is consistent with a model of cessation of Sierran magmatism during Laramide flat-slab subduction, followed by conductive cooling of the upper mantle for~70 m.y. The heat production variation with depth is principally due to large variations in uranium and thorium concentration; potassium is less variable in concentration within the Sierran crust, and produces relatively little of the heat in high heat production rocks. Because silica content is relatively constant through the upper~30 km of the Sierran batholith, while U, Th, and K concentrations are highly variable, radiogenic heat production does not vary directly with silica content.

Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

Low-angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio-Pleistocene cinder cones. Whole-rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer-scale compositional layering. The shallow parts are depleted in radiogenic Nd (ε_(Nd) = -13 to -6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low-angle subduction. The deeper samples are enriched in radiogenic Nd (ε_(Nd) = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25–28% low-pressure melt depletion and the former formed by refertilization of harzburgites by mid-ocean-ridge-basalt-like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension-related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid-ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low-angle subduction

In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900 °C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700 °C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab. In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes

Geochemical Evidence of A Near-Surface History for Source Rocks of the Central Coast Mountains Batholith, British Columbia

Major and trace elemental concentrations as well as Sr and Pb isotopic data, obtained for 41 plutonic samples from the Coast Mountains Batholith ranging in age from ∼108 to ∼50 Ma, indicate that the source regions for these rocks were relatively uniform and typical of Cordilleran arcs. The studied rocks are mineralogically and chemically metaluminous to weakly peraluminous and are mainly calc-alkaline. Initial whole-rock 87Sr/86Sr ratios range from 0.7035 up to 0.7053, whereas lead isotopic data range from 18.586 to 19.078 for 206Pb/204Pb, 15.545 to 15.634 for 207Pb/204Pb, and 37.115 to 38.661 for 208Pb/204Pb. In contrast to these relatively primitive isotopic data, δ 18O values for quartz separates determined for 19 of the samples range from 6.8 up to 10.0‰. These δ 18O values preclude the possibility that these melts were exclusively generated from the Mesozoic mantle wedge of this continental arc, just as the Sr and Pb data preclude significant involvement of an old (Precambrian) crustal/mantle lithospheric source. We interpret the high δ 18O component to represent materials that had a multi-stage crustal evolution. They were originally mafic rocks derived from a circum-Pacific juvenile mantle wedge that experienced a period of near-surface residence after initial crystallization. During this interval, these primitive rocks interacted with meteoric waters at low temperatures, as indicated by the high δ 18O values. Subsequently, these materials were buried to lower crustal depths where they remelted to form the high δ 18O component of the Coast Mountains Batholith. This component makes up at least 40% (mass) of the Cretaceous through Eocene batholith in the studied area. The remainder of the source materials comprising the Coast Mountains Batholith had to be new additions from the mantle wedge. A prolonged period of contractional deformation beginning with the Early Cretaceous collisional accretion of the Insular superterrane is inferred to have been responsible for underthrusting the high δ 18O component into the lower crust. We suggest that mafic rocks of the Insular superterrane (e.g. Alexander–Wrangellia) are of appropriate composition, and were accreted to and overthrust by what would become the Coast Mountains Batholith just prior to initiation of magmatism in the region

The age and origin of a thick mafic–ultramafic keel from beneath the Sierra Nevada batholith

We present evidence for a thick (∼100 km) sequence of cogenetic rocks which make up the root of the Sierra Nevada batholith of California. The Sierran magmatism produced tonalitic and granodioritic magmas which reside in the Sierra Nevada upper- to mid-crust, as well as deep eclogite facies crust/upper mantle mafic–ultramafic cumulates. Samples of the mafic–ultramafic sequence are preserved as xenoliths in Miocene volcanic rocks which erupted through the central part of the batholith. We have performed Rb-Sr and Sm-Nd mineral geochronologic analyses on seven fresh, cumulate textured, olivine-free mafic–ultramafic xenoliths with large grainsize, one garnet peridotite, and one high pressure metasedimentary rock. The garnet peridotite, which equilibrated at ∼130 km beneath the batholith, yields a Miocene (10 Ma) Nd age, indicating that in this sample, the Nd isotopes were maintained in equilibrium up to the time of entrainment. All other samples equilibrated between ∼35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 80 and 120 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The Rb-Sr ages are generally consistent with the Sm-Nd ages, but are more scattered. The ^(87)Sr/^(86)Sr and ^(143)Nd/^(144)Nd intercepts of the igneous-textured xenoliths are similar to the ratios published for rocks outcroping in the central Sierra Nevada. We interpret the mafic/ultramafic xenoliths to be magmatically related to the upper- and mid-crustal granitoids as cumulates and/or restites. This more complete view of the vertical dimension in a batholith indicates that there is a large mass of mafic–ultramafic rocks at depth which complement the granitic batholiths, as predicted by mass balance calculations and experimental studies. The Sierran magmatism was a large scale process responsible for segregating a column of ∼30 km thick granitoids from at least ∼70 km of mainly olivine free mafic–ultramafic residues/cumulates. These rocks have resided under the batholith as granulite and eclogite facies rocks for at least 70 million years. The presence of this thick mafic–ultramafic keel also calls into question the existence of a “flat” (i.e., shallowly subducted) slab at Central California latitudes during Late Cretaceous–Early Cenozoic, in contrast to the southernmost Sierra Nevada and Mojave regions