ANATOMY OF THE LA GRANGE DETACHMENT FAULT, KLAMATH MOUNTAINS, CALIFORNIA, FROM CALTRANS CORES

In 1996, CalTrans geotechnical crews drilled two bore holes in the hanging wall of the La Grange detachment fault at the La Grange mine, Trinity County, California, for the purpose of installing a slope inclinometer and better understanding slope stability problems near the site. Hydraulic mining in the 19^th and early 20^th centuries exposed a 1.2 km long, shallowly south-dipping section of the fault surface. This surface is capped by ~20 cm thick patches of hard, black, foliated ultracataclasite, and, at its east end, by a 0.5 km long mound of remnant hanging wall material. CalTrans drilled an 18 m. bore hole (SI-11) and a 44 m. bore hole (SI-13) here, and recovered core samples at intervals between 15 and 44 m. The boring logs and cores document a complex brittle fault zone overlying the main detachment surface.

Standard penetration tests run in the upper 15 m. of SI-13 recorded increasing blow counts with depth, from 6 at 3 m. to >50 at 15 m., reflecting a transition from landslide deposits to fault zone rocks. The boring record for SI-13 reports cobbles in silty sand, and silty clay ± angular pebbles in the upper 6 m., giving way to zones of fractured rock, pebble clay, clay gouge ± slickensided surfaces, angular pebbles and sand in foliated clay, and coarse sand in gouge with shear fabric. Nine clay gouge zones, 2-20 cm thick, were identified in SI-13 between the depths of 13 and 43 m. while the core was being drilled.

Foliated and nonfoliated chaotic breccias are the most common rocks in the 7 m. of recovered core. Protocataclasite, mesocataclasite, and veined and fractured rock compose the remaining samples.

Petrographic study of 20 thin-sections shows that cored hanging wall rocks are all metamorphic rocks: quartz-mica schist, quartz-plagioclase-staurolite schist, brecciated quartzite, and amphibolite. These metamorphic rocks overlying the fault surface contrast dramatically with the sedimentary rocks – sandstone, shale, siltstone, chert, and minor conglomerate and limestone - in the 6 x 9 km hanging wall klippe (“Oregon Mountain outlier” of Irwin, 2009) south of the mine. Rocks in SI-11 and SI-13 are lithologies found in the Central Metamorphic belt, the footwall of the La Grange fault, but include a greater range of rock types than Salmon Hornblende Schist that directly underlies the fault at the mine.

Come see core samples at the poster.