Seismogenesis in granite of brittle-ductile transition regime

Abstract:

Brittle-ductile transition zone of the continental crust has the greatest strength with most of earthquakes nucleating within it. In this transition zone, it has been known that deformation is accommodated mainly by combination of brittle and plastic processes or low-temperature plasticity during interseismic periods and by brittle processes during seismic events. Although mechanical behavior and microstructural development in brittle-ductile transition zone have been studied much experimentally, seismogenesis in natural shear zones of this regime is still somewhat unclear. The Yecheon shear zone is an NE- to NNE-striking right-lateral shear zone cross-cutting Mesozoic granitoid belt in South Korea. The deformation temperature of the main shear zone was estimated to be about 350 ℃. In the southeastern margin of the shear zone, protomylonites change gradually into mylonites and then abruptly into ultramylonites toward southeast. Quartz and feldspar both of protomylonite and mylonite deform by dislocation creep and brittle fracturing, respectively. Greenish ultramylonite consists of quartz-, feldspar-, muscovite- and epidote-rich layers within matrix of quartz, muscovite and epidote. The deflecting S-foliation of mylonite toward ultramylonite is sharply truncated by the boundary between mylonite and ultramylonite. Thin (several mm to several cm) greenish layers occur in protomylonite subparallel to mylonitic foliation or cross-cutting the foliation at a low angle. Also, the greenish layers develop along shear bands in protomylonite. The greenish layer is composed of fragments of protomylonite and matrix of very fine-grained quartz, feldspar, muscovite and epidote. Epidote grains of ultramylonite and greenish layers are very fine (a few micrometers or below in size) and euhedral in shape. They show graphic intergrowth with quartz inclusions and embayed texture, indicating growth from melt. Thus, we interpret the greenish layers are pseudotachylyte generated during seismic event. Once pseudotachylyte layers develop, deformation might be localized along the layers in interseismic periods, and the greenish layers evolve into ultramylonite.