Abstract

Fractures, faults, stylolites, and strain determined from an analysis of twinning in calcite in the Mississippian Newman Limestone at the base of the Pine Mountain block show that deformation was accomplished primarily by a combination of bedding-plane slip and layer-parallel shortening. Poles to joints are either parallel to strike of the thrust sheet or are in the transport plane. Small slip surfaces with syntaxial calcite fibers lie either parallel to bedding or parallel to the strike of the Pine Mountain block and dip steeply. Movement along the former is in the direction of transport of the block, whereas movement on the latter is approximately along strike. Tectonic stylolites are rare but appear to be controlled by subtle changes in lithology. Wackestones contain more stylolites than do packstones. Strains in calcite from closely spaced samples show the converse relationship. They are highest in units without stylolites (maximum shortening strain, e1 = −10.4%) and lowest in units with stylolites or small faults (e1 = −2.0%). The maximum principal shortening-strain directions, however, are identical within the error and are parallel to both bedding and the transport direction. A sequence of events in this portion of the Pine Mountain block is (1) layer-parallel shortening producing tectonic stylolites and a strong preferred orientation of compression axes; (2) development of fractures parallel to the strike of the block on the lower hinge of the ramp to accommodate stretching parallel to the transport direction (these strike- parallel fractures can also be interpreted as having occurred quite late); (3) during (2) above and continuing, the start of bedding-plane slip marked by syntaxial calcite fibers which trend in the direction of transport; and (4) complications, such as shear parallel to the strike of the Pine Mountain fault, as a result of motion of the entire block between bounding tear faults. Events (2) through (4) most likely overlapped in time. These data suggest that the acquisition of dip of the rocks on the ramp was primarily accomplished by bedding-plane slip on the slip surfaces which experienced a shear stress of < 100 bars. Bending strains in units between slip surfaces were too small to alter the pattern of layer-parallel shortening preserved in the Newman Limestone.

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