MYLONITE, CATACLASITE, AND PSEUDOTACHYLYTE IN THE JONES CORNER FAULT ZONE, SOUTH-CENTRAL MAINE: A POLYPHASE HISTORY OF SHEARING, FAULTING AND SEISMICITY

Detailed bedrock geologic mapping in south-central Maine has revealed the presence of a previously unrecognized complex zone of both ductile and brittle deformation herein referred to as the Jones Corner fault zone. This poorly exposed fault zone is located approximately 3.5 kilometers southeast of the Sandhill Corner mylonite zone – the main strand of the regionally extensive Norumbega fault system. The Jones Corner fault zone trends approximately 060^o, and while poor exposure limits the complete delineation of the zone of deformation, it is at least 3 kilometers long and up to 250 meters wide. The fault zone is hosted entirely within amphibolite facies quartzo-feldspathic gneiss and biotite schist of the Ordovician Cape Elizabeth Formation.

Rocks within the fault zone preserve an early relatively high temperature (~ 400^oC) mylonitic fabric with kinematics consistent with dextral shear. Dynamically recrystallized quartz is abundant and microstructures suggest it formed primarily by subgrain rotation recrystallization. Superimposed on this early ductile fabric are multiple generations of ultra-cataclasite and pseudotachylyte. The pseudotachylyte occurs as both fault and injection veins with thicknesses up to 5 cm. Thicker isolated irregular blobs of pseudotachylyte (melt reservoirs?) are also locally present. Spectacular mesoscopic and microscopic flow folds and microlites in the pseudotachylyte indicate a melt-origin for at least some of the veins. Comparisons of bulk rock geochemistry between pseudotachylyte veins and their host rocks reveal relatively minor differences in major and trace elements and suggest a local origin for the veins.

Early ductile fabrics in the Jones Corner fault zone are most likely associated with greenschist facies Late Devonian-Early Carboniferous regional dextral shear deformation. The overprinting pseudotachylyte veins likely formed significantly later following uplift of the present erosion surface to higher structural levels in the Late Paleozoic or Mesozoic. Numerous examples of cross-cutting cm-scale pseudotachylyte veins indicate multiple episodes high velocity slip and suggest that, when active, the Jones Corner fault zone hosted multiple high magnitude earthquakes.