GEOCHRONOLOGIC ANALYSIS OF MULTIPLE OCCURRENCES OF FAULT-RELATED CALCITE ON A VARIABLE-SLIP-DIRECTION FAULT IN BIG BEND NATIONAL PARK, TEXAS

Fault-related calcite precipitates on and along a mapped, unnamed normal fault in the Sierra del Carmen horst system in the Big Bend region of Texas are being investigated structurally and geochronologically. This north-northwest-striking fault is in the southern segment of the Rio Grande rift, the southeastern portion of the Cenozoic extensional system of western North America that includes the Basin and Range province. It is mapped as one of many north-northwest-striking normal faults in the region. Slickenlines present on the normal fault surface express two orientations, however: strike-slip motion preserved in rakes of 08 degrees and dip-slip motion preserved in rakes of 78 to 98 degrees. These are calculated stereographically to represent north-south and east-northeast to west-southwest principal extensional strain axes, respectively. No cross-cutting relationships are apparent that would reveal the ages of the two disparate motion directions; therefore, samples were collected from each directional set of slickenlines for U-Pb geochronological dating, to reveal the relationship between the slickenline orientations and identify the initial and later directions of crustal extension. At each of the two directional sets of slickenlines, a euhedral calcite occurrence is closely associated with the calcite-lineated fault surface. Thin section analysis supports close association of the two occurrences of calcite, lineated and non-lineated, near both the strike-slip and dip-slip slickenlines. The fault-associated, non-striated euhedral crystals are also being analyzed with U-Pb geochronology to determine whether the non-striated crystals that are coincident with slickenlines were produced coincidentally; if so, fault movement could be dated using the more abundant non-striated crystals.