Color contour map showing regional variations in compiled U-Pb zircon ages from plutonic rocks of the SNB, Salinian block, and Mojave Desert draped over a DEM: Supplement 2 from "Late Cretaceous gravitational collapse of the southern Sierra Nevada batholith and adjacent areas above underplated schists, southern California" (Thesis)

Description

The greater Sierra Nevada batholith (SNB) is an ~ 600 km long NNW-trending composite arc assemblage consisting of a myriad of plutons exhibiting a distinct transverse zonation in structural, petrologic, geochronologic, and isotopic patterns. However, south of 35.5 °N: 1) the depth of exposure increases markedly; 2) primary zonation patterns swing up to 90˚ westward, taking on an east-west trend; 3) western zone rocks are truncated by eastern zone rocks along the proto-Kern Canyon fault, a Late Cretaceous oblique ductile thrust; and 4) fragments of shallow-level eastern SNB affinity rocks overlie deeper-level western zone rocks and subjacent subduction accretion assemblages (Rand, San Emigdio, and Sierra de Salinas schists) along a major Late Cretaceous detachment system. Integration of these observations with new and existing data reveals a temporal relationship between schist unroofing and upper crustal extension and rotation. I present a model whereby Late Cretaceous shallow subduction and subsequent trench-directed channelized extrusion of the schist triggered gravitational collapse of the overlying crustal column. This overarching model is based on several investigations summarized below. Thermobarometry, thermodynamic modelling and garnet diffusion modelling are presented that elucidate the tectonics of subduction and eduction of the San Emigdio Schist. I document an upsection increase in peak temperature (i.e. inverted metamorphism), from 590 to 700 °C, peak pressures ranging from 8.5 to 11.1 kbar, limited partial melting, microstructural evidence for large seismic events, rapid cooling (825–380 °C/Myr) from peak conditions and an "out and back" P–T path. Progressive cooling and tectonic underplating beneath an initially hot upper plate following the onset of shallow subduction provide a working hypothesis explaining high temperatures, inverted metamorphism, partial melting, and the observed P–T trajectory calculated from the San Emigdio body. New geologic mapping and microstructural analysis indicate that the schist was transported to the SSW during structural ascent 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. Structural, thermobarometric, U-Pb geochronologic, and geochemical data from plutonic and metamorphic framework assemblages in the southern SNB also suggest SSW-directed transport of upper plate(s) along a major Late Cretaceous detachment system. The timing and pattern of regional dispersion of crustal fragments in the southern SNB is most consistent with Late Cretaceous collapse above the underplated schist. These observations imply a high degree of coupling between the shallow and deep crust during high magnitude extension. Zircon (U-Th)/He data presented herein reveal a rapid cooling event at 77 ± 5 Ma, probably reflecting the time of large magnitude detachment faulting. A comparison of this dataset with existing apatite (U-Th)/He thermochronometry suggests that the development of modern landscape and arrangement of tectonic elements in southern California was greatly preconditioned by Late Cretaceous tectonics. Finally, detrital and metamorphic zircon of the structurally highest and earliest subducted portions of the San Emigdio Schist constrain the depositional age to between ca. 102 and 98 Ma. Zircon oxygen isotope data from both lower plate schist and upper plate batholithic assemblages reveal a δ18O shift of ~ 1.5‰ between igneous (~ 5.5‰) and metamorphic (~ 7‰) domains. These results, taken with previous zircon and whole-rock δ18O measurements, provide evidence for massive devolatilization of the San Emigdio Schist and fluid traversal of upper plate batholithic assemblages, thereby altering the isotopic composition of overlying material. Furthermore, the timing of fluid-rock interaction in the southwestern SNB is coincident with eastward arc migration and an associated pulse of voluminous magmatism. I posit that during flattening of the Farallon slab the schist was rapidly delivered to the magmatic source, where ensuing devolatilization triggered a magmatic flare-up in the southeastern SNB. This short-lived (less than 15 Myr) high-flux event was followed by the termination of arc magmatism as the shallow subduction zone approached thermal equilibrium.

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