STRAIN ANALYSIS AND GEOCHRONOLOGY OF A SHEAR ZONE AND SLICKENLINES ALONG A SECTION OF THE BOQUILLAS FAULT IN BIG BEND NATIONAL PARK, TEXAS

Strain analysis of a mesoscale shear zone outcropping adjacent to the Boquillas fault was undertaken to relate this fault-proximal deformation zone with sense of motion recorded on the fault. The NNW-striking Boquillas fault is mapped as a normal fault within the Sierra del Carmen horst system, part of the late Cenozoic extensional province of the Big Bend region of western Texas and northern Mexico, but in this small study area it records multiple directions of movement in calcite slickenlines exposed on the fault surface. Within an area of approximately five meters on the exposed fault surface and adjacent to the analyzed shear zone, four distinct directions of slickenlines are measurable with dominant ones closer to dip slip and strike slip: they range between 04 degrees and 110 degrees of rake. The shear zone adjacent to the fault is exposed in cross section and was mapped in four separate, homogenous zones. Thin sections from among these zones were also analyzed for sense of motion. All four geologic map sections show a top-to-the-right sense of motion which most closely corresponds to the strike-slip lineation. This may suggest this fault motion is the most recent of the variable recorded directions. Because no cross-cutting relationships among the varied slickenlines orientations are visible, relative age of these different motion directions cannot be determined. Therefore, samples of the calcite slickenlines were collected and submitted for U-Pb geochronological analysis. Geochronology data may reveal a pattern of change in strain directions along this fault and correlation between the strain directions documented in the adjacent shear zone. Samples are currently being analyzed at the University of Texas at El Paso.