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Recognition of a dismembered positive flower structure along the San Jacinto fault: Stratigraphic and structural implications

Emma J. Vierra, Heather N. Webb, Kristen Dennis, Daniel W. Peppard, Gary H. Girty, & Thomas K. Rockwell

Published August 15, 2017, SCEC Contribution #7709, 2017 SCEC Annual Meeting Poster #220

The San Jacinto fault zone has accumulated up to ~24 km of dextral displacement. Over much of its mapped length the fault is relatively straight, varying from N50° to 60°W. Surface exposures along straight segments of the fault indicate generally steep SW to vertical dips (~77° - 90°). However, at two locations separated by ~23 km, either (1) 12°- 54° SW dipping, NE verging (site south of Wellman Ranch: ~33.52764° N, 116.57467° W), or (2) 5°- 67° NE dipping, SW verging (site north of Hog Lake: ~33.61491° N, 116.71007° W) thrust faults join the main trace. As the thrust faults join the main trace they appear to steepen rapidly to vertical dips either at the surface or with depth. Though separated by ~23 km, at both locations thrust faults place garnet amphibolite facies gneiss over alluvial sediments of the Pleistocene Bautista Formation. After removal of ~23 km of displacement, and realignment of the Thomas Mt. sill, a prominent off set igneous component of the Peninsular Ranges batholith, the oppositely verging thrusts are aligned and appear to merge from the main strand of the fault. Reverse or positive flower structures commonly exhibit such characteristics, and we therefore interpret the two locations characterized by oppositely verging thrusts segments as a dismembered flower structure.

Given the 12.1 (+3.4/-2.6) mm/yr slip rate derived from previous studies, and an ~23 km offset since its formation, the dismemberment of the flower structure probably began in the early Pleistocene ~1.7 to ~2.4 Ma. If this interpretation is correct, then the Bautista Formation within the study area is likely older than the Middle Pleistocene, which according to the Geological Society of America time scale extends from 0.78 to 0.13 Ma, and may in fact be either Calabrian (0.78 – 1.8 Ma), or possibly even Gelasian (1.8 to 2.6 Ma). In short, the reconstructed flower structure appears to be the record of vertical telescoping within an area of high contractional strain driven by high fault-normal compression within a restraining bend that was dissected and dismembered by ~23 km of dextral motion on the San Jacinto fault. Such a scenario is consistent with the idea that the evolution of faults within restraining bends, including the abandonment of old and development of new strands, allows the fault system to more efficiently accommodate strike-slip strain.

Vierra, E. J., Webb, H. N., Dennis, K., Peppard, D. W., Girty, G. H., & Rockwell, T. K. (2017, 08). Recognition of a dismembered positive flower structure along the San Jacinto fault: Stratigraphic and structural implications. Poster Presentation at 2017 SCEC Annual Meeting.

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