6 Structural Geology: Recent Advances in Structural Geology

Sunday, 9 October 2011
8:00 AM - 12:00 PM
Room 200A-C

SINISTRAL MESOZOIC INVERSION OF THE DEXTRAL ALLEGHANIAN TOWALIGA FAULT

Paper number: 6-6
Program: Technical Sessions
Time: 9:05 AM – 9:20 AM

Authors

Abstract

The Alleghanian Towaliga fault (TF) is one of the largest faults in the southern Appalachian Inner Piedmont (IP), framing the NW side of the Pine Mountain window (PMW) at ~070, continuing through the eastern IP NE of the PMW at ~030 possibly to the GA/SC border. The TF contains both plastic and brittle fault rocks, with ductile fault rocks (Alleghanian) occurring as steeply NW-dipping, upper greenschist to lower amphibolite-facies proto- to ultra-mylonites with dominantly dextral shear-sense indicators and shallow-plunging mineral stretching lineations. High-dilation, ~2 x 0.3 km rhombohedral pods of silicified breccia and cataclasite occur along the fault zone, and locally truncate ductile fabrics. The distinct rhombohedral shape and geometry of silicified cataclasite ridges, along with dilational rock textures and conspicuous evidence of repeated brecciation, indicate these sites acted as dilational step-overs (fault-fracture meshses) during sinistral strike-slip faulting. Intense hydrothermal (agillic) alteration occurs close to many cataclasite pods, which acted as loci for fluid flow during rupture. Alternating crosscutting relationships between diabase dikes and the TF indicate brittle faulting occurred ~200 Ma, and opposite shear sense of the two fault fabrics supports a temporal disparity between brittle and ductile faulting episodes.

The TF truncates a suite of IP granitoids (424-299 Ma) at the NE end of the PMW, representing significant displacement along the TF during the Alleghanian orogeny. Relatively low displacement during the Mesozoic is indicated by the size of the step-overs and offset of diabase dikes. Numerous Mesozoic brittle faults with similar characteristics occur in the area (e.g., the Oxford and Shiloh faults), suggesting shortening during the rift-drift transition may have been accommodated by a number of low-displacement sinistral strike-slip faults in this portion of the IP. We see no evidence of normal slip along the main trace of the TF.