USING THERMOBAROMETRY TO DETECT FAULT RAMP FOLDING OF ACADIAN ISOTHERMAL AND ISOBARIC SURFACES: AN EXAMPLE FROM THE ALABAMA EASTERN BLUE RIDGE BELT

The southern Appalachian eastern Blue Ridge (EBR) belt is composed predominantly of Neoproterozoic metaclastic and metavolcanic rocks metamorphosed by an Acadian dynamothermal event. This Acadian metamorphic event recrystallized rocks of the EBR to lower to uppermost amphibolite facies, and perhaps granulite facies. Late Paleozoic Alleghenian orogenic activity affected the EBR in Alabama primarily by thrusting and folding the terrane to the present structural position.

This study focuses on the EBR lithologies east of the Hollins Line (HL) thrust system, a large-displacement thrust fault that juxtaposes high-grade EBR hanging wall lithologies against the low-grade Talladega Belt (TB) footwall block. A footwall duplex exists within the TB immediately northwest of the EBR study area, and the geometry of the duplex indicates a significant component of dextral strike slip during HL thrusting.

Metapelitic, metagreywacke, and amphibolite lithologies from the study area were analyzed with the electron microprobe for the coexisting assemblages: Ga+Bi+Pl+Mu, Ga+Hbl+Pl, and Ga+Cpx+Pl. EBR samples yielded a mean geothermal gradient of 21 deg. C per km. However, field and petrographic evidence indicate both prograde and retrograde subsets that define a PT-time clockwise "loop" that we believe represents an uplift (decompression) event. In addition, the temperature and pressure estimates from the higher grade samples approach granulite facies conditions (775 C, 8.6 kb). These conditions appear to be confined to a narrow zone sub-parallel and east of the regional strike of the HL. We speculate that folding of metamorphic isograd surfaces above a footwall ramp produced this trend. Additionally, the geometry of isothermal and isobaric contours calculated from thermobarometry is compatible with dextral slip as indicated by the footwall duplex in the TB. A fault ramp somewhat oblique to the strike of the HL would account for the structural and thermobarometry relationships.