INSIGHTS INTO DEVELOPMENT OF A MAJOR EXTENSIONAL FAULT ZONE BASED ON TEXTURAL AND COMPOSITIONAL EVIDENCE FROM LA GRANGE FAULT, CA, ULTRACATACLASITE

Foliated ultra fine-grained fault rocks capping the exposed footwall of the La Grange fault near Weaverville, CA, contain extensive textural evidence of intense comminution and sustained cataclastic deformation accompanied by repeated fracturing and veining events. Textural and compositional observations from these rocks can be used to evaluate models for fragmentation processes operating during development of a major extensional fault zone. SEM and petrographic observations of samples collected from a 30 cm-thick layer of foliated flinty black fault rocks show thin (0.2-5 mm) ultracataclasite layers with local pinch-and-swell structure, micro-scale normal faults, and sets of tiny (5-10 micron) calcite veins. The ultracataclasite is composed of 2-15 % rounded grains (0.5-100 micron) of quartz, rutile, apatite, pyrite and sphene, in a very fine-grained (<< 0.1 mm) matrix. Less common foliation-parallel cataclasite zones contain angular to subrounded ultracataclasite (<=0.5mm) and calcite or, less commonly, barite vein fragments. The cataclasite layers record re-breakage of ultracataclasite relatively late in fault zone development, and attest to a history of repeated failure, veining, and cataclasis. The composition of La Grange ultracataclasite shows the importance of parent rock lithology in the mechanical development of fault rocks. Because of the lithologic contrast between hanging wall (qtz-mica schist, calc schist, silicic metasedimentary rocks) and footwall rocks (mafic amphibolite), individual contributions of footwall and hanging wall rocks to the ultracataclasite can be assessed. Compositional data from an energy-dispersive system on the SEM shows that ultracataclasite matrix contains Si, Al, Mg, Ca, Fe, ± K, Na, and Ti, a bulk composition like that of underlying amphibolite. In contrast, larger resistant qtz, rutile, and apatite grains were most likely derived from upper plate rocks. Preferential preservation of specific minerals as larger grains within the ultracataclasite shows the importance of contrasting mechanical properties of materials in fragmentation processes. Properties of different mineral constituents could influence fault rock development, particularly when modeling textural characteristics such as fractal dimension of particle size distributions in fault rocks.