Dynamic Development and Reactivation of a Newly Discovered Frictional-Plastic Deformation System Joseph L. Allen, Department of Physical Sciences, Concord University, Athens, WV Understanding the origin and early geologic evolution of faults is important to petroleum exploration and reservoir development because they serve as zones of weakness in the crust that are preferentially reactivated and rejuvenated through time. The reactivation of old zones of weakness can subsequently serve as a path for the migration of fluids. Reactivation can also influence the distribution, thickness, and 3-D shape of overlying sedimentary strata that form petroleum traps. The goal of this research is to define controls on the location, geometry, and dynamics of fault rupture in the middle part of the Earth’s crust (approximately 10-12 km deep), and investigate how these geologic structures localized shallow fault reactivation that affected sedimentary strata 1.4 billion years later. We have mapped the geometry of fault-related rupture in the middle crust (see photo above left of frictionally melted fault rock from the study area). These fault rocks are the fossil record of ancient earthquakes in the middle crust. The fault zone core preserves overprinting brittle and plastic fabrics within a mid-crustal thrust fault (see figure below left). Early fabric development and cyclic brittle–plastic faulting controlled brittle reactivation in the shallow crust (<10 km deep) 1.4 billion years later (see photo below right showing northeast-oriented fold in younger sedimentary strata).