EVIDENCE FOR ELEMENTAL AND BULK MASS REDISTRIBUTION ASSOCIATED WITH THE DEVELOPMENT OF AN ULTRACATACLASTIC FAULT CORE AND DAMAGE ZONE; CLARK STRAND, SAN JACINTO FAULT ZONE, SOUTHERN CALIFORNIA

Relationships between fault zone properties and rupture are incompletely understood. This study describes the textural and chemical attributes of the fault core and damage zone of a portion of the Clark strand of the San Jacinto fault zone. In Horse Canyon, the fault zone transects Cretaceous tonalite, and is exposed as a 42 cm wide black ultracataclasite fault core bounded to the NE by a 187 cm thick damage zone.

In the outer part of the damage zone, large tonalitic fragments (~15–30 cm) are cut by ~0.5 - 1 cm thick laterally discontinuous seams of cataclasite. With increasing proximity to the core, tonalitic fragments in the inner damage zone take on elliptical shapes and their sizes progressively decrease as the proportion of cataclasite increases. Intra-fragment fractures and nascent cataclasite seams are ubiquitous features of the damage zone as a whole. Assuming that the mass of Ti is conserved, chemical data suggest that the transition from inner to outer damage zone is marked by an ~1-2 cm thick zone in which bulk and elemental mass change is negligible. This zone of negligible mass transfer is bounded by large gains in bulk mass as measured in fractured tonalite fragments (inner: ~20%, outer: ~10%) and losses in bulk mass in cataclasite seams (inner: ~11-29%, outer: ~8%). Losses in bulk mass are accommodated through the transfer of Si, Al, Ca, Na, and to a lesser degree K elemental mass from cataclastic seams where transport function (t) values are negative, to fracture and nascent bands of cataclasite within tonalite fragments where t values are positive. In contrast, changes in the masses of Fe, Mg, and Mn are negligible in both cataclastic seams and tonalite fragments within the outer damage zone, but then systematically increase through the inner damage zone and fault core. The microcrystalline and generally aphanitic fault core is depleted in Al, Ca, Na, and to a lesser degree K elemental mass while changes in Si mass are negligible. These observations imply that elemental mass is transferred from cataclastic seams in the damage zone and fault core into tonalitic clasts via a network of fractures and nascent zones of cataclasite. We suggest that the transfer of elemental mass may have been facilitated by a fluid phase released during the breakdown of hydrated mineralogical components during dynamic rupture.