Wells, Rachel Kristen (2010-12). Microstructures and Rheology of a Limestone-Shale Thrust Fault. Master's Thesis. Thesis uri icon

abstract

  • The Copper Creek thrust fault in the southern Appalachians places Cambrian over Ordovician sedimentary strata. The fault accommodated displacement of 15-20 km at 100-180 ?C. Along the hanging wall-footwall contact, microstructures within a ~2 cm thick calcite and shale shear zone suggest that calcite, not shale, controlled the rheology of the shear zone rocks. While shale deformed brittley, plasticity-induced fracturing in calcite resulted in ultrafine-grained (<1.0 ?m) fault rocks that deformed by grain boundary sliding (GBS) accommodated primarily by diffusion creep, suggesting low flow stresses. Optical and electron microscopy of samples from a transect across the footwall shale into the shear zone, shows the evolution of rheology within the shear zone. Sedimentary laminations 1 cm below the shear zone are cut by minor faults, stylolites, and fault-parallel and perpendicular calcite veins. At vein intersections, calcite grain size is reduced (to ~0.3 ?m), and microstructures include inter-and-intragranular fractures, four-grain junctions, and interpenetrating boundaries. Porosity rises to 6 percent from <1 percent in coarse (25 ?m) areas of calcite veins. In coarse-grained calcite, trails of voids follow twin boundaries, and voids occur at twin-twin and twin-grain boundary intersections. At the shear zone-footwall contact, a 350 ?m thick calcite band contains coarseand ultrafine-grained layers. Ultrafine-grained (~0.34 ?m) layers contain microstructures similar to those at vein intersections in the footwall and display no lattice-preferred orientation (LPO). Coarse-grained layers cross-cut grain-boundary alignments in the ultrafine-grained layers; coarse grains display twins and a strong LPO. Within the shear zone, ultrafine-grained calcite-aggregate clasts and shale clasts (5-350 ?m) lie within an ultrafine-grained calcite (<0.31 ?m) and shale matrix. Ultrafinegrained calcite (<0.31 ?m) forms an interconnected network around the matrix shale. Calcite vein microstructures suggest veins continued to form during deformation. Fractures at twin-twin and twin-grain boundary intersections suggest grain size reduction by plasticity-induced fracturing, resulting in <1 ?m grains. Interpenetrating boundaries, four-grain junctions, and no LPO indicate the ultrafine-grained calcite deformed by viscous grain boundary sliding. The evolution of the ultrafine-grain shear zone rocks by a combination of plastic and brittle processes and the deformation of the interconnected network of ultrafine-grained calcite by viscous GBS enabled a large displacement along a narrow fault zone.
  • The Copper Creek thrust fault in the southern Appalachians places Cambrian

    over Ordovician sedimentary strata. The fault accommodated displacement of 15-20 km

    at 100-180 ?C. Along the hanging wall-footwall contact, microstructures within a ~2 cm

    thick calcite and shale shear zone suggest that calcite, not shale, controlled the rheology

    of the shear zone rocks. While shale deformed brittley, plasticity-induced fracturing in

    calcite resulted in ultrafine-grained (<1.0 ?m) fault rocks that deformed by grain

    boundary sliding (GBS) accommodated primarily by diffusion creep, suggesting low

    flow stresses.

    Optical and electron microscopy of samples from a transect across the footwall

    shale into the shear zone, shows the evolution of rheology within the shear zone.

    Sedimentary laminations 1 cm below the shear zone are cut by minor faults, stylolites,

    and fault-parallel and perpendicular calcite veins. At vein intersections, calcite grain

    size is reduced (to ~0.3 ?m), and microstructures include inter-and-intragranular

    fractures, four-grain junctions, and interpenetrating boundaries. Porosity rises to 6 percent

    from <1 percent in coarse (25 ?m) areas of calcite veins. In coarse-grained calcite, trails of voids follow twin boundaries, and voids occur at twin-twin and twin-grain boundary

    intersections.

    At the shear zone-footwall contact, a 350 ?m thick calcite band contains coarseand

    ultrafine-grained layers. Ultrafine-grained (~0.34 ?m) layers contain

    microstructures similar to those at vein intersections in the footwall and display no

    lattice-preferred orientation (LPO). Coarse-grained layers cross-cut grain-boundary

    alignments in the ultrafine-grained layers; coarse grains display twins and a strong LPO.

    Within the shear zone, ultrafine-grained calcite-aggregate clasts and shale clasts (5-350

    ?m) lie within an ultrafine-grained calcite (<0.31 ?m) and shale matrix. Ultrafinegrained

    calcite (<0.31 ?m) forms an interconnected network around the matrix shale.

    Calcite vein microstructures suggest veins continued to form during deformation.

    Fractures at twin-twin and twin-grain boundary intersections suggest grain size reduction

    by plasticity-induced fracturing, resulting in <1 ?m grains. Interpenetrating boundaries,

    four-grain junctions, and no LPO indicate the ultrafine-grained calcite deformed by

    viscous grain boundary sliding. The evolution of the ultrafine-grain shear zone rocks by

    a combination of plastic and brittle processes and the deformation of the interconnected

    network of ultrafine-grained calcite by viscous GBS enabled a large displacement along

    a narrow fault zone.

publication date

  • December 2010