Constraining the Spatial Extent of Strain Localization at the Base of the Seismogenic Zone: A Case Study from the Sandhill Corner Shear Zone, Maine, USA

Knowledge of the structure and processes of strain localization at the base of the seismogenic zone can provide constraints on the rheologic evolution of shear zones at depth and the spatial extent that seismicity influences the surrounding rock. Such knowledge is hindered by limitations of borehole measurements from such depths and a lack of structural/mineralogical preservation of original rock fabric. The Sandhill Corner Shear Zone, Maine, USA provides an opportunity to study well-preserved structures relating to strain localization along major faults exhumed from seismogenic depths. In south-central Maine, this shear zone juxtaposes two rock types: the Cape Elizabeth Formation and the Crummett Mountain Formation. This work focuses on the less-studied Crummett Mountain Formation. Measurements of strain markers (quartz vein thickness, linear fracture density and aspect ratio in garnet) delimit strain gradients reflecting portions of the strain localization history in the field area. Optical and electron microscope observations of microstructures along the gradient provide context for changing deformation conditions reflected by changes in the slope of the strain gradients. Our results show that localization occurred initially at high temperatures (400-500oC) in a 90m-wide zone. The onset of seismicity narrowed the zone of localization, producing a 7m-wide zone of high-strain, containing a ~1.7m-wide core that preserves structures related to seismicity at the frictional-to-viscous transition. The results of this study refine previous observations on the structure of the Crummett Mountain Formation, provide minimum estimates of finite shear strain and displacement along the Sandhill Corner Shear Zone, and provide insight on the role that earthquake mechanics play in strain localization at the base of the seismogenic zone