USING TDR, HELIUM DENSITY, DIGITAL GRAIN SIZE ANALYSIS, AND CHEMICAL DATA TO CHARACTERIZE THE PHYSICAL PROPERTIES OF NEAR SURFACE FAULT ZONES: A PROOF OF CONCEPT APPROACH

We have mapped a conjugate set of strike-slip faults consisting of over 35 individual faults that are probably younger than ~0.5 – 0.6 million years in the San Felipe Hills, SE, California. Hinge lines of early formed folds serve as piercing points for determining amounts of offset for many of the faults, and indicate that displacements mostly vary from 1-3 meters to as great as 250+ meters. The faults cut poorly consolidated mudstones and sandstones of the Pliocene-Pleistocene Borrego Formation and sandstones and minor gravels of the overlying Pleistocene Ocotillo Conglomerate (~1.1 to ~0.5 Ma).

Fault zones in the San Felipe Hills vary from ~10 to 615 cm in thickness, and fault zone thickness (fzt) correlates with fault displacement (fd) (fzt = 0.003*fd + 0.222; R2 = 0.67). From the initial set of 35 faults, we selected two faults for detailed study, one recording dextral, and the other sinistral slip. Eleven samples were collected beginning ~9 m outside the dextral fault zone, the last 2 samples being within the fault core. Samples were spaced ~1.5 m apart in sandy siltstone just below and above the contact separating the Ocotillo Conglomerate and Borrego Formation. For the sinistral fault beginning ~9 m outside the fault zone 5 samples were collected at a spacing of ~ 2 m within laminated siltstones of the lower Ocotillo Conglomerate. At each sample site dry bulk density was measured using time domain reflectometry (TDR), while collected samples yielded helium density (grain density), digital grain size distributions, and chemical data. From these data porosity and volume strains are calculated. Preliminary data from one traverse show that porosity systematically decreases from ~42% outside the fault zone to ~14% within the fault zone. This change in porosity translates into an ~47% decrease in volume. Thin sections of the damaged zone show that granulation and fragmentation did not accommodate observed volume strains or porosity reduction. Though we are currently completing our analyses of data collected from other traverses, we suggest based on the above observations, that when faults transect sandy to silty intervals, the majority of slip occurs along a single slip surface (the principal slip surface), while packing and reduction of pore space is a major process operating within a narrow ~1 m zone adjacent to the fault zone.