Role of Dolomite in the Development of Weak Fault Zones

Rachel Wells, Texas A&M University
Julie Newman, Texas A&M University
Steven Wojtal, Oberlin College

Rock adjacent to low-temperature (<200°C) fault zones typically are cut by veins with a range of orientations relative to the thrust surface. Two exposures of the Copper Creek thrust fault, TN(separated ~40 km along strike) contain numerous fault parallel calcite veins within a narrow (2-24 cm thick) shear zone. Both exposures contain Rome Formation (shale, siltstone, and dolomite) in the hanging wall and Moccasin Formation (shale and limestone) in the footwall; the primary difference between the two exposures is the thickness and location of a thick dolomite bed in the hanging wall. One exposure contains a 1-3 m thick dolomite bed within1-10m of the fault plane (with shale immediately adjacent to the fault plane), whereas the other exposure contains an~8 m thick dolomite bed immediately adjacent to the fault plane. Meso- and microfractures (with little displacement) are common in the dolomite rocks. Within the calcite veins at each exposure, microstructures suggest grain size reduction by plasticity-induced fracturing followed by diffusion creep of fine-grained calcite. The lack of mesoscopic fabrics and deformation microstructures in the wall rocks suggest that deformation was primarily accommodated within these calcite filled veins and not within shale adjacent to the fault.

Similar mesoscopic structures and distribution of microstructures prevail at the Town Knob fault, TN,where the numbers of veins increase within the dolomite (Rome Formation) hanging wall and the limestone (Conasauga Group) footwall towards a narrow, "vein-like" shear zone. Curiously, the veins here contain both calcite and feldspar.

Along both the Copper Creek and Town Knob faults, the veins have formed within or near dolomite units located within the Rome Formation. At the macroscale, the Copper Creek, Town Knob and other thrust faults tend to occur in comparable sections of the Rome Formation containing dolomite beds.

Both the Copper Creek and the Town Knob faults formed at contacts between dolomite and shale or limestone. We hypothesize that the contact between the stronger dolomite beds and the weaker shale/limestone beds created a stress concentration, resulting in fracturing between the dolomite and shale or limestone.These fractures created passageways for fluids, allowing the precipitation of weaker material. Continued development of veins with low flow strengths and subsequent localization of deformation along the veins are a plausible mechanism to accommodate substantial displacement along these thrust faults.