STRUCTURE AND RHEOLOGY OF THE SANDHILL CORNER SHEAR ZONE, NORUMBEGA FAULT SYSTEM, MAINE: A STUDY OF A FAULT FROM THE BASE OF THE SEISMOGENIC ZONE

Earthquakes that can devastate nearby human populations occur along active seismogenic faults (e.g. San Andreas Fault, California). Seismic activity on such large-displacement, strike-slip faults occurs above or within a depth interval of 10-20 km, known as the frictional-to-viscous transition (FVT). The FVT is a zone where frictional deformational mechanisms (fractures and frictional slip during an earthquake) characteristic of the upper crust transitions to viscous deformation mechanisms (temperature-dependent flow between major earthquakes) in the mid-to-lower crust.  The importance of the FVT lies in the fact that it identifies the deepest extent of frictional slip during an earthquake.  Because of this, the style of deformation at a given point within the FVT is dependent on the seismic cycle.  During an earthquake where the rupture reaches down to the FVT, transient frictional deformation that occurs over seconds to minutes is superimposed on the steady-state, viscous deformation that occurs over millions of years.  Therefore, investigation into the strength and behavior of the crust during the seismic cycle has come to focus largely on rocks that were once at FVT depths but have since been exhumed to the surface.

This study is focused on the Sandhill Corner strand of the Norumbega fault system (NFS) of Maine, an ancient, strike-slip, seismogenic fault zone much like the San Andreas Fault that exposes rocks that were at FVT depths when the NFS was seismically active.  Using samples collected from this fossil fault, this study (1) identifies evidence of transient deformation, such as frictionally-derived melt veins; (2) documents how transient structures influenced the development of the steady-state structures; (3) describes the architecture of the fault, and (4) evaluates the behavior and strength of the fault.  This research suggests that the fault had a weakened core within FVT depths that developed through the interaction between transient and steady-state processes.  Stress estimates of 60-100MPa determined from samples of deformed quartz ribbons collected from the inner part of the Sandhill Corner strand represent the maximum strength of the shear zone.   A weakened fault core within the FVT correlates well with the known weak fault core of a fault at the near surface.