MICROSTRUCTURES AND STRESSES AT THE BASE OF SEISMOGENIC FAULTS: COUPLING OF DEFORMATION ACROSS THE FRICTIONAL TO VISCOUS TRANSITION

The frictional to viscous transition is one of the most important geological and geophysical horizons within Earth owing to its role in transmitting stresses associated with creeping flow at depth to upper-crustal faults resulting in discontinuous seismicity. However, little is known about the stresses at the base of these faults, and how deformation is coupled across the frictional to viscous transition. To explore these issues, we conducted piezometric studies on new grains of quartz from mylonite zones within the Norumbega Fault System (NFS), northeastern Appalachians. The NFS contains abundant pseudotachylyte, providing irrefutable evidence for seismic rupture at depth. Importantly, several mylonite zones shows mutually overprinting mylonite and pseudotachylyte, so that quartz new-grain piezometry in these rocks addresses the flow stresses at the base of individual faults when they were seismically active. Our preliminary results suggest that quartzo-feldspathic mylonites carried flow stresses of approximately 100 MPa at ca. 12-15 km depth. We employ 3D numerical models of the upper 30 km of Earth, and show that the orientations and magnitudes of the principle and shear stresses throughout the seismogenic zone are sensitive to mylonite flow stresses. For example, given flow stresses of 100-200 MPa on a 500 m wide mylonite zone, our preliminary modeling suggests that maximum horizontal compression directions near the overlying fault vary by ca. 45 degrees depending on proximity to the fault and the strength of the fault. Shear stresses around the fault vary by ca. 120 MPa.