2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 25
Presentation Time: 8:00 AM-12:00 PM


LINDEMANN, Christie D., Dept. of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051 and RIGGS, Eric M., Dept. of Earth and Atmospheric Sciences and CRESME, Purdue University, West Lafayette, IN 47907-2051, clindemann@purdue.edu

Outcrop-scale fracture system characteristics have been frequently explored as paleo-stress indicators to provide insight into the structural, tectonic, and stratigraphic frameworks of complex regions. Our field investigations reveal systematic, clast-scale fracture patterns within conglomerates of Eocene-Miocene synextensional sedimentary strata of the Titus Canyon Fm. in the Grapevine and Funeral Mountains of SW Nevada and SE California. Approximately 1000 fracture orientations in clasts of well-cemented conglomerates were observed at over 30 locations along 30 km of exposures within Titus Canyon, Monarch Canyon, and the NE flank of the Funeral Mountains near the trace of the Boundary Canyon detachment fault. Field sampling was conducted within identified units of the Titus Canyon Fm. with locations distributed evenly throughout the field area.

The most prominent grouping of fracture measurements cluster around a mean strike of 206° and a mean dip of 60° with variations from the general trend attributable to younger local fault block rotation or folding. Clasts are of varying lithologies, but are dominantly quartzite and other metasediments. Clast damage was segregated by size, with 86% of clasts larger than 3 cm fractured, but only 54% of small clasts (0.5 - 3 cm) displaying one or more fractures. Higher fracture densities appear to correlate with areas close to regional fault structures. Fractures were observed to propagate into the matrix, suggesting that deformation was post-depositional and post-lithification. Many basal boulder conglomerate clasts exhibit shear zones with clear kinematic slip indicators, and show the pervasive fractures to be Mode I fractures that coalesce locally into the observed Mode II shear zones.

Based on this data, we estimate a maximum stress direction of 215°, 50°SW. Despite numerous minor fault structures mapped in the field area, the remarkably consistent fracture orientations and the interpreted maximum stress direction imply a similar or single source of stress potentially associated with the through-going regional detachment fault structures, such as the Boundary Canyon detachment fault and the Monarch Spring fault. Further fracture analysis may lead to a better understanding of the source, timing, and mechanisms associated with these deformational features.