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Paper No. 89-2
Presentation Time: 9:00 AM-5:30 PM
THE COMPOSITION AND STRUCTURE FROM DRILL CORE OF THE SHALLOW PORTION OF THE SAN ANDREAS FAULT, LAKE ELIZABETH TUNNEL, SOUTHERN CALIFORNIA
Newly acquired geotechnical core across the San Andreas Fault (SAF) near the Elizabeth Tunnel of the Los Angeles aqueduct crossing, northern Sierra Pelona Mountains, CA provides insight into the composition and structure of the upper part of SAF zone. Seven 55°- 65° northeast plunging boreholes acquired a total of ~ 750 m of rock core to a maximum vertical depth of 140 m, across the ~150 m wide fault zone. Unconsolidated deposits here are up to 40 m thick, core recovery was ~95%, and the core samples consist of a range of indurated fault-related rocks including zones of foliated cataclasite and a few local intervals of clay-rich gouge. The cataclasite zones have an estimated local thickness up to 2 - 3 m and are developed within granodiorite to granitoid gneiss. Outside of the primary cataclasite/gouge zones, the damage zones are defined by more discrete deformation such as cataclastic shear bands, typically in orientations similar to those observed in foliated cataclasites. Cataclasite and gouge zones are easily identified by prominent resistivity lows observed in wireline logs. A tentative correlation of slip surfaces observed across the boreholes indicates that the principal fault surfaces flatten near the upper parts of the holes and may connect to the interpreted active surface trace of the SAF. Fault-related rocks throughout the boreholes exhibit evidence for fluid-fault interactions in the form of variably developed clay-rich shear bands/zones and hydrothermal alteration products. The degree of induration and alteration indicate that the fault zone here consists of numerous slip surfaces in a hydrothermally altered and “cemented” sequence. Future analyses of core samples will provide insight into the combined mechanical and chemical processes responsible for fault-zone development in this upper part of the SAF, with a particular focus on how these processes operate throughout the seismic cycle.