RHEOLOGICAL HETEROGENEITY AND THE STATE OF STRESS IN THE ROOTS OF A LARGE-DISPLACEMENT, STRIKE-SLIP, SEISMOGENIC FAULT: THE NORUMBEGA FAULT SYSTEM IN MAINE

There are currently no direct data on the state of stress 15-20 km below vertical, large-displacement, strike-slip, seismogenic faults like the San Andreas Fault in California. Yet, it is clear from theoretical and numerical studies that the state of stress on such faults is significantly influenced by the state of stress in underlying mylonitic shear zones at depths corresponding to the frictional-to-viscous transition (FVT). Although we must reach a better understanding of the state of stress at the FVT below these faults, the difficulty lies in locating a suitable exhumed fault system. To provide relatively tight constraints on the stress tensor, the chosen field locality should contain irrefutable evidence for coseismic rupture (pseudotachylyte), and be characterized by: (a) tightly constrained kinematic boundary conditions, (b) microstructures amenable to estimates of differential stress and mean kinematic vorticity, and (c) lithological (rheological) heterogeneity providing natural variability in the stress and vorticity estimates. In such a system, 3D numerical experiments can arrive at best-fit solutions for the field-derived stress and vorticity estimates, and in doing so solve for the principal and Cartesian stresses.

One of the very few field occurrences that meet the above criteria is the Norumbega Fault System (NFS), northeastern Appalachians. The NFS represents the roots of a long-lived, Paleozoic, right-lateral, large-displacement, subvertical, strike-slip fault system. The Sandhill Corner mylonite zone (SCMZ) is the largest, most continuous mylonitic strand of the NFS, forming an impressive zone of mutually overprinting mylonite and pseudotachylyte up to 300 m wide. A portion of the SCMZ developed along a lithologic contact between quartzofeldspathic and mica-rock metasedimentary rocks. Owing to the unique exposures, we are able to measure differential stress and mean kinematic vorticity number using both optical and electron-beam techniques. These field-derived data, combined with the approximately monoclinic strain symmetry of the SCMZ, are used to constrain 3D numerical experiments that solve for the principal and Cartesian stresses. These results at depth are used as input for 3D numerical models of the upper 30 km of Earth, allowing us to explore the effects of kinematic and dynamic states at depth on active seismogenic faults above.