Shallow level incipient pulverization found with a restraining bend of the Clark segment of the San Jacinto Fault, southern California

Daniel W. Peppard, Heather N. Webb, & Gary H. Girty

Submitted August 15, 2016, SCEC Contribution #7007, 2016 SCEC Annual Meeting Poster #077 (PDF)

This poster characterizes the properties of incipient pulverization within a contractional bend along the Clark segment, San Jacinto fault. About 1.5 km SE of the study site, the Clark segment bifurcates into two branches. The NE-most splay, the topic of this study, is located within a meander bend of Alkali Wash where the Pleistocene Bautista Fm structural underlies metasedimentary rocks of the Mesozoic Burnt Valley complex. Within this bend, the fault lies at ~200 m below a Tertiary erosional surface, and is marked by an ~30 cm thick black fault core that dips ~60° SW, but further to the SE shallows to ~12-22° SW. The damage zone within the Bautista Fm is subdivided into inner and outer zones. The outer damage zone lies ~35-75 m below the over thrust Burnt Valley complex. It consists of both undamaged and damaged sandstones that give way toward the black fault core to cataclasite series rocks within the ~10-22 cm wide inner damage zone. In undamaged sandstones, matrix makes up ~17-21% while framework minerals exhibit little if any evidence for in situ cracking. In contrast, matrix within damaged sandstones makes up 16-36%, and some, but not all, quartz framework grains are fragmented. Within the inner damage zone, evidence for cataclastic flow is ubiquitous, and matrix exceeds 71% in matrix-rich banded cataclasites. In unbanded cataclasites and matrix-poor banded cataclasites, matrix ranges from ~67-70%. The results of clay mineralogical studies of the < 2 µm fractions indicates that the matrix in undamaged sandstones and in matrix-rich banded cataclasites consists of illite(I) + kaolinite(Ka) + mixed-layer illite/smectite (I/S) with ~10% I. The clay mineral assemblage in damaged sandstones is similar, but the %I in I/S varies from ~10% to 20% I. In the unbanded cataclasite series, the clay mineral assemblage is like that in the damaged sandstones but the %I in I/S varies from ~30% to 40%. In matrix-poor banded cataclasites, the clay-mineral assemblage includes either I + Ka, or I + Ka + I/S with 20% I. The clay assemblage in undamaged sandstones likely reflects the cool and wet Pleistocene climate in which the Bautista Fm evolved. The increase in I in I/S in unbanded and in matrix-poor banded cataclasites likely reflects fluid-driven K metasomatism. The clay mineralogy in the matrix-rich samples is similar to that in undamaged sandstones, and suggests that fluid flow through the damage zone was disconnected and temporally irregular.

Key Words
Fault zone architecture, In Situ pulverization, Clay Mineralogy, Thin Section Analysis

Peppard, D. W., Webb, H. N., & Girty, G. H. (2016, 08). Shallow level incipient pulverization found with a restraining bend of the Clark segment of the San Jacinto Fault, southern California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)