MICROSTRUCTURAL RECORD OF COSEISMIC/POSTSEISMIC CYCLES NEAR THE BASE OF THE SEISMOGENIC ZONE, SAND HILL CORNER MYLONITE ZONE, NORUMBEGA FAULT SYSTEM, MAINE

The Sand Hill Corner Mylonite Zone (SCMZ) of the Norumbega Fault System is a subvertical mylonitic shear zone that exposes rocks from frictional-to-viscous transition depths (10-15km). It marks the western contact between quartzo-feldspathic rocks and a schist member in the Cape Elizabeth Fm. (Liberty-Orrington Belt). Where the SCMZ overlaps the lithologic contact, a well-developed strain gradient is evident, with fabric intensity and abundance of friction melt and cataclasite increasing with proximity to the contact.

Friction melt and cataclasite cut the mylonite in foliation-parallel mm- to cm-thick single or paired veins, connected by a ladder network of fractures and injection veins. Friction melt, found at various stages of devitrification, appears as an ultra fine-grained mass dominated by phyllosilicates with quartz and feldspar clasts and needle-shaped ilmenite microlites. X-ray compositional maps show that the friction melt is relatively depleted in Si and enriched in Al, and the area surrounding the melt vein is locally enriched in Fe. Friction melt and cataclasite are locally deformed during continued ductile shear, during which the fine-grained mica and microlites align parallel to the mylonitic foliation and clasts are recrystalized. Deformed friction melt and cataclasite are locally commonly cut by subsequent generations of friction melt and cataclasite that are also deformed, which is consistent with microstructures expected for rocks near the base of the seismogenic zone.

A traverse across the SCMZ presents a transition from protomylonite to mylonite to ultramylonite. An abrupt transition occurs between protomylonite and mylonite that corresponds with a significant decrease in grain size and with the appearance of cataclasite and friction melt. Cross-cutting relationships, a paucity of porphyroclasts, and higher Fe content suggest that ultramylonite bands in the mylonite may have originated as friction melt-filled or cataclasite veins. The repeated occurrence and deformation of friction melt/cataclasite may have lead to the localization of strain where structures associated with fracture and frictional sliding formed. Understanding strain localization through such a process in the SCMZ may provide insight into the evolution of large-displacement, seismogenic faults at frictional-to-viscous transition depths.