QUARTZ GRAIN SIZE PIEZOMETRY AND C-AXIS FABRIC THERMOMETRY IN THE SANDHILL CORNER MYLONITE ZONE: IMPLICATIONS FOR SHEAR ZONE EVOLUTION

The Norumbega fault zone in Maine is widely recognized to have experienced large-scale localization from a ~30 km wide zone of ductile dextral deformation (c. 370 Ma) to <1 km wide zones of ultramylonite late in its evolution (320-290 Ma). This localization occurred during regional cooling and exhumation following the Acadian orogeny. If this broader deformation zone was continually overprinted by lower temperature microstructures and increasingly localized deformation, a transect across the shear zone should record the evolution of stresses, deformation temperatures and shear zone width. This study focuses on describing such a record in the Sandhill Corner mylonite zone in south-central Maine, which is a strand of the larger Norumbega fault system. The core of this shear zone is a 0.1 km wide ultramylonite zone within host rocks of the dextrally deformed Casco Bay Group. We analyze quartz microstructures from samples collected along a 2.5 km transect from the ultramylonites, SE toward the outer margin of the Sandhill Corner mylonite zone.

Preliminary average grain sizes in deformed monomineralic quartz veins from several samples are ~14μm in mylonites and are as large as ~200μm furthest from the shear zone core. Corresponding differential stress estimates based on recrystallized grain size piezometry are 60 to 7 MPa. Quartz c-axis fabric thermometry (measured by EBSD) has been used to estimate the evolving deformation temperature, which ranges from 400±50 to 520±50°C at 0.1-1.1 km away from the shear zone center. These preliminary data are consistent with progressive narrowing of the shear zone during cooling and exhumation and yet lower temperatures may be recorded close to the shear zone center.

This microstructural record can be used to estimate the total offset on the Sandhill Corner mylonite zone if the evolving strain rate and duration of deformation are known. We estimate strain rates based on published quartz flow laws and the above-mentioned stresses and temperatures. The previously well-established regional cooling history can be linked to deformation temperatures to constrain the timing of deformation. These constraints suggest a microstructure-based total displacement as great as 150 km on the Sandhill Corner mylonite zone. Work is in progress to reconcile the geologic and microstructural records.