DAMAGE MICROSTRUCTURES IN GARNET AND FELDSPAR PRESERVE COSEISMIC CYCLES AT MID-CRUSTAL DEPTHS IN AN ANCIENT STRIKE-SLIP FAULT SYSTEM IN SOUTH-CENTRAL MAINE

Active strike-slip faults show pronounced damage zones consisting of variably fractured and pulverized rocks. Seismic experiments and numerical modeling suggest that this brittle damage tapers off with depth, and it remains an open question as to whether or not significant coseismic damage occurs at frictional-to-viscous transition (FVT) depths. The significance of this question relates to transient fluid flow and rheological changes that would occur with deeply rooted brittle damage.

Temperature of mylonitization at the FVT in continental crust typically precludes crystal plasticity in major rock-forming minerals such as garnet and feldspar, as well as accessory minerals such as tourmaline. Thus, unlike quartz in which coseismic damage is overprinted by post-seismic viscous creep, these minerals can preserve multiple generations of coseismic microfracturing. However, identifying microfracturing as coseismic can be challenging because both feldspar and garnet in non-seismogenic environments commonly contain microfractures.

To identify microfracturing and damage related to the seismic cycle, we employed scanning electron microscope cathodoluminescence (CL) and backscattered electron (BSE) images to explore healed cracks in damaged feldspar and grain size distribution in pulverized garnet, respectively. Our preliminary results show that garnet and feldspar porphyroclasts show: (1) gradients in microfracture density across the shear zone, (2) post-seismic displacement along originally tensile microfractures, and (3) quartz, feldspar, and mica precipitation in microfractures. Within the shear zone core and inner shear zone (~30 m in width), CL images of feldspar exhibit braided, healed microfractures, and garnet grains in BSE image show microfracturing and fragmentation with a wide range of fragment size (from < 1 μm to hundreds of microns). In the outer shear zone and host rock, no fragmented garnet or shattered feldspar is observed. The microstructural gradient across the shear zone from core to host rocks is consistent with coseismic loading and post-seismic stress relaxation coupled with fluid redistribution driven by dilatational strains. Rheological changes associated with the observed damage may facilitate strain localization at FVT depths.