STRUCTURAL AND CHEMICAL EVOLUTION OF PSEUDOTACHYLYTES DURING SEISMIC EVENTS

The structural and chemical characteristics of pseudotachylytes genera ted during seismic events along a Pan-African fault zone in Kenya docu ment an evolution consisting of two principal steps. In the first stag e, crushing of the host rock during the onset of 'frictional sliding' led to preferential disruption of biotite and hornblende, due to their low fracture toughness and low shear yield strength. The products of this first stage are preserved as thin cataclasite zones along the mar gins of the pseudotachylyte veins. Melting of the crushed host rock oc curred during the second stage, due to the heat generated by 'friction al sliding', grain size reduction, and the release of water from bioti te and hornblende. The chemical and mineralogical composition of the c ataclasite and the increasing temperature during seismic slip were the main factors that controlled the composition of two chemically distin ct pseudotachylyte melts. During rapid cooling, amphibole microlites ( melt 1) and plagioclase microlites (melt 2) crystallized from the two pseudotachylyte melts.