Earthquake dynamics from pseudotachylyte microstructure

• Microstructural analysis of wall rock damage around pseudotachylyte-bearing fault. • Systematic orientation and density difference of microfractures in the wall rocks. • Microfractures fit with the stress field caused by earthquake rupture propagation. • Fracture surface energy is lower than frictional heat and seismic fracture energy. • Seismic fracture energy is mainly composed by heat. During an earthquake, most of the dissipation of the stored elastic strain energy occurs by fracturing processes and frictional heat along and adjacent to the seismic fault. Information on earthquake energy partitioning and dynamics can be retrieved from the analysis of exhumed faults containing pseudotachylytes (solidified frictional melts produced during seismic faulting). Here, microstructural analysis is carried out on an east-west striking pseudotachylyte-bearing fault of the dextral strike-slip Gole Larghe Fault Zone within the Adamello granitoid pluton (Italian Alps), exhumed from 8–11 km depth. FESEM cathodoluminescence analysis reveals a strong fragmentation of the wall rocks, invisible with other techniques, which decays in the first centimetres from the pseudotachylyte fault-parallel vein. In the northern block the microfracture density is on average low (7448 mm -2 ) and the microfractures strike preferentially E-W. In contrast, in the southern block microfracture density is on average high (12,120 mm -2 ), and the microfractures strike preferentially N-S. This asymmetric wall rock damage provides evidence that the microfractures developed as result of the dynamic stress field associated with earthquake rupture propagation. The measured surface area associated with wall rock fracturing and the volume of the pseudotachylyte allow the estimate of the energy dissipated in fracturing processes U S (0.015–1.88 MJ m -2 ) and frictional heat Q (32 MJ m -2 ), respectively. The comparison between U S and Q implies that frictional heat is the major energy sink during rupture propagation in these intracontinental earthquakes.