Assessing off-fault damage during development of a dismembered flower structure

Emma J. Vierra, Bonnie A. Flynn, Mario S. Bermudez, Heather N. Webb, Gary H. Girty, & Thomas K. Rockwell

Published August 15, 2018, SCEC Contribution #8645, 2018 SCEC Annual Meeting Poster #158

As part of our ongoing study of off-fault damage along the San Jacinto fault (SJF), southern California, we here document the physical and chemical properties controlling damage intensity NW of Hog Lake and compare these results to those published from the Wellman Ranch locality, ~19 km to the SE. The two sites represent portions of a dismembered flower structure that is offset along the Clark fault segment. At both sites, damage in sandstones of the Pleistocene Bautista Fm is defined by intragranular fractured quartz grains with jig-saw puzzle texture, representing pulverization at the grain scale.

At the site NW of Hog Lake, the number of quartz grains containing >10 fractures/grain was greatest in sandstones within 5 meters of the fault core. Also, within this 5 m interval, porosity, in general, decreases, and damage intensity varies as a function of the proportion of matrix. In addition, textural analysis of damaged samples at Hog Lake reveal two crudely developed directions of foliation that are not apparent in samples outside the damage zone and may record NE-SW contraction associated with thrusting. Mass balance calculations derived from chemical analysis of Hog Lake samples indicate a statistically significant loss of 31% (+30/-21) Si and 29% (+23/-17) K mass, and an overall loss of ~23% (+/- 20) bulk mass and ~33% (+/- 18) volume. In contrast, at Wellman Ranch, the number of quartz grains containing >10 fractures/grain were greatest in sandstones with <18% matrix, and damage is heterogeneously developed at distances of at least 25 m. In addition, changes in bulk mass and volumetric strain are not statistically significant.

Based on data from both locations, the site NW of Hog Lake has a more well defined ~5 m thick damage zone where pulverization falls within the limits imposed by published experimental and theoretical experiments. Our data imply that pulverization within the damage zone at this site likely occurred during the development of the flower structure and over-thrusting along the boundary thrust fault. Moreover, there is evidence that highly acidic solutions migrated through the damage zone. Hence, there may have been cycles of fragmentation followed by focused fluid flow within damaged zones during the overall development of the flower structure.

Citation
Vierra, E. J., Flynn, B. A., Bermudez, M. S., Webb, H. N., Girty, G. H., & Rockwell, T. K. (2018, 08). Assessing off-fault damage during development of a dismembered flower structure. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)