Rheological investigation of deep crustal mylonites, San Gabriel Mountains, California

A critical new research objective in the SCEC5 program is the development of the Community Rheologic Model (CRM), whereby key research priorities include building a provisional rheology model around a geologic framework for Southern California. The geologic framework of the CRM is referred to as the Lithotectonic Block Model, where distinct blocks of crust are enclosed between significant faults of the San Andreas Fault (SAF) system. This provisional rheology involves accurate modeling of the thermo-mechanical properties of each block of crust to characterize earthquake risks and hazards, but the properties of deep crustal rocks are difficult to assess without widespread surface exposures of these rocks. However, the San Gabriel block has been uplifted along the ?Big Bend? in the SAF, exposing the deepest levels of crust within Southern California. These deep crustal exposures are sheared and deformed, and are critical to the development of the CRM because they are the only exposures of lower crustal rocks that are analogous to the inaccessible shear zone rocks at the base of the SAF. We propose a field-and lab-based study of deep crustal shear zone rocks exposed in the San Gabriel lithotectonic block to place geologic constraints on the rheology of deep-crustal shear zones for direct incorporation into SCEC?s Community Rheological Model. We will use field sampling and observation, microstructural analysis, and Zr-in-titanite thermometry to date the timing of deformation fabric development to place temporal constraints on shear zone fabric development.