PRELIMINARY STRUCTURAL ANALYSIS OF THE CHRISTMAS MOUNTAINS, WEST TEXAS, TO ASSESS THE ROLE OF TRANSFER ZONE MECHANISMS IN RIFT ZONES

The Trans-Pecos region of west Texas encompasses structures with a complex history. The region has been studied by many researchers to characterize reactivated structures, transfer zone mechanisms, and the southern extent of the Rio Grande Rift. The Christmas Mountains (29°25'50.21"N/103°26'30.04"W) are located in the center of a zone bounded by the Tascotal Mesa fault to the north, the Tornillo Graben to the east, Burro Mesa fault to the south, and the Terlingua fault to the west. This is a ~100 km wide zone of E-W right-lateral strike slip faults linked to N-NW steep normal faults bounding a series of subparallel grabens. These mountains are the remnants of a ~42 Ma laccolith-caldera system, thus fracture systems may be local to the intrusions and/or collapse stage, rather than defining a regional structural grain. The geology is composed of tilted Cretaceous limestones, shales and sandstones cut by Tertiary intrusions. Some tuffs and volcaniclastic deposits are also present. The goal is to assess fracture and fault networks to understand the changes in stress fields within an extensional transfer zone.

Preliminary field mapping in the northern Christmas Mountains suggests the presence of sub-parallel <0.25 km oblique shear zones that strike between N50W and N75W with a moderate to steep dip. This is consistent with the structural grain as defined regionally by long-lived reactivated structures, some of which are interpreted to be associated with the Texas Lineament Corridor. Alternatively, because they cut Tertiary deposits, these may be more consistent with Rio Grande extension, as some workers have proposed. Unfilled Mode II fractures and various conjugate sets strike between N53E and N83E with a steep to vertical dip, suggesting a local change in the stress field that may be associated with a regional oblique shear. Multiple fractures were observed to be associated with secondary mineralization veins in the carbonate rocks. Historically, this area was mined for fluorspar, which is found in vein networks associated with fracture systems. A few fault surfaces contain evidence of multiple slip directions suggesting possible reactivation of slip. The fault networks and geometries observed here can help us assess the extension mechanism associated with a wide transfer zone in the southernmost part of the Rio Grande Rift.