Pseudotachylyte-bearing strike-slip faults in mylonitic host rocks, fort foster brittle zone, Kittery, Maine

Abstract

Hundreds of small-scale, dextral strike-slip faults of late Paleozoic-age are developed parallel to the mylonitic layering of sheared gneisses, schists and amphibolites of the Rye Complex of southernmost Maine. Pseudotachylyte is found along all layer-parallel slip zones as fault veins up to ~2 cm thick, and stored in off-fault reservoirs such as splay fault intersections and injection veins, up to 34 cm long. The layer-parallel faults are concentrated along the NW Boundary Zone of a prominent ultramylonite. Oblique-to-layer splay faults cut across an Interior Zone to transfer displacement to other layer-parallel faults in a 15 m wide extensional stepover. Interaction between pairs of layer-parallel slip zones contributed to the outcrop scale generation zone pattern as a form of strike-slip duplexing between long overlapping en echelon slip zones. Fault-parallel extension as well as fault-parallel shortening was controlled by stepping geometries for active slip segments within the stronglylayered host rock. Coupled extensional and contractional linkages tied to flanking, layer-parallel slip zones also created distinctive asymmetric, plano-convex sidewall ripouts attributed to adhesive wear during coseismic slip. Steeply-dipping mylonitic layering favorably-oriented for reactivation in brittle strike-slip resulted in large initial areas of contact between sliding surfaces and an increase in frictional heat during sliding to generate abundant pseudotachylyte. Displacements on the major layer-parallel fault veins are estimated from average fault vein thickness and marker layer offsets on oblique-to-layer faults and suggest coseismic slip increments on the meter-to-submeter scale with cumulative displacements on narrow multi-strand slip zones up to ~3-5 m.