Constraints from Field Geology for Numerical Modeling of the Crustal Overturn Processes During the Cretaceous High-Magma-Flux Episode in the Central and Southern Sierra Nevada, USA

Building on prior studies, recent fieldwork combined with geochronology, thermobarometry and geochemistry studies in the Cretaceous Sierra Nevada arc reveal the following arc-scale features: 1) The Middle to Late Cretaceous Sierra Nevada arc has a 30-35 km thick granodioritic to tonalitic upper-middle crust and may have had up to 30-35 km of mafic to ultramafic lower crust, including dehydrated amphibolitic residues. 2) Plutons emplaced during the ~20 myr long High-Magma-Flux Episode (HMFE, 105-85 Ma) include large batholiths (~1000 km2 at exposure level) with growth histories occurring over millions of years (e.g. ~9 myr for Tuolumne Batholith). Magma pulses creating such large intrusions could vary from up to 103 km3 in dimension depending on different growth models. 3) In the central Sierra Nevada, emplacement depths of the granitoid plutons during the HMFE are 7-15 km with shallow emplaced plutons’ solidi at usually ~700 -760 °C. 4) Plutons intruding only slightly older volcanic host rocks in the central and southern Sierra Nevada indicate that host rocks’ downward displacement of ~7-25 km depths occurred within 1-3 myr. This process is accompanied with the long-lived arc exhumation since at least middle Jurassic. 5) Steep syn-emplacement subsolidus lineations, rim monoclines, and plastic shear strain in pluton aureoles suggest ductile deformations of host rock materials. 6) Partial melting occurred along the margins of plutons and in the middle-lower crust, as represented in the more deeply exposed southern Sierra (30-45 km). 7) Magmatic to subsolidus foliations in plutons and ductile shear zones in host rocks indicate NW-trending transpressional tectonics during the HMFE. 8) Isotopic oxygen data and mass balance calculation indicate that crustal components provides more than 50% of the entire arc’s mass. Intra-crustal magma sources of the HMFE are sustained possibly by thickened crust due to contractional tectonics. These observations in the central and southern Sierra Nevada allow us to apply the MILAMIN_VEP, a thermo-mechanical marker-in-cell visco-elasto-plastic finite element code, to simulate more realistic scenarios of arc-scale material exchange processes. The code deals with continuous changes of density, water content, and partial melting conditions of lithosphere rocks based on calculated thermodynamic phase diagrams of differential rock types (using Perple_X). The model also takes the central Andes as a possible modern analogy for the Cretaceous Sierra Nevada. Seismic lithospherical structures, geothermal gradient, and other geological constraints are considered in the model. Aiming to yield geologically and geophysically testable results, the simulations test the hypothesis of host rock downward flow or crustal overturn processes during the HMFE, transpressional tectonics and exhumation, and to shed light on the mechanisms and controlling factors of the downward flow processes.