MINIMUM FINITE SHEAR STRAIN ESTIMATES AND IMPLICATIONS FOR STRAIN LOCALIZATION AT THE BASE OF THE SEISMOGENIC ZONE: A CASE STUDY FROM THE SANDHILL CORNER SHEAR ZONE, MAINE, USA

Most studies concerning the evolution of large-displacement strike-slip faults use near-surface observations of modern and ancient faults. These studies couple fault offset with microstructural evolution to interpret processes of strain localization and dynamic changes in fault strength. However, the evolution of faults from the base of the seismogenic zone remains poorly understood due to deformational and metamorphic overprinting of fault zones during exhumation. The Sandhill Corner Shear Zone (SCSZ), a major mylonitic strand of the Norumbega Fault System (NFS) in south-central Maine, represents a preserved seismogenic strike-slip fault exhumed from depths corresponding to the frictional-to-viscous transition (FVT) zone. This shear zone separates two rock types: The Cape Elizabeth Formation (CEF: interlayered quartzofeldspathic granofels and pelite) and the Crummett Mountain Member (CMM: pelitic schist). The tectonic significance of this fault system in accommodating dextral shearing during the Acadian orogeny has been a focus of study over the past few decades. To date, robust estimates of shear strain along strands of the NFS are lacking. An asymmetrically-distributed strain gradient along the SCSZ represents deformation at the FVT, (~50m wide vs ~5m wide in the CEF and CMM, respectively). A minimum estimate of finite shear strain was calculated from sheared quartz veins ubiquitously distributed within the CMM and values of the kinematic vorticity number from previous study. This strain estimate, coupled with microstructural observations, reveals the timing and processes leading to strain localization at the base of the seismogenic zone. These results provide insight to the rheological evolution of fault zones, and the asymmetrically-distributed strain gradient may reflect preferred rupture directivity along the SCSZ.