HR: 10:50h
AN: T22A-03    [Abstracts]
TI: Can Pseudotachylytes be Used to Infer Earthquake Source Parameters? Limitations in the Study of Exhumed Faults
AU: * Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Dipartimento di Geologia, Paleontologia e Geofisica, Universita' di Padova, Via Giotto 1, Padova, 35137 Italy
AU: Pennacchioni, G
EM: giorgio.pennacchioni@unipd.it
AF: Dipartimento di Geologia, Paleontologia e Geofisica, Universita' di Padova, Via Giotto 1, Padova, 35137 Italy
AU: Teza, G
EM: giordano.teza@unipd.it
AF: Dipartimento di Geologia, Paleontologia e Geofisica, Universita' di Padova, Via Giotto 1, Padova, 35137 Italy
AB: Tectonic pseudotachylytes might be used to constrain earthquake source parameters, such as dynamic shear stress resistance, average dynamic friction and slip-weakening distance. Estimation of dynamic shear stress resistance and dynamic friction from field studies is based on the assumption that the volume of melt produced during coseismic slip is proportional to the frictional work converted to heat on the fault surface. Conditions conducive to a realistic estimate of dynamic shear resistance are: i) the presence of large outcrop exposures that allow for estimation of the volume of pseudotachylyte, ii) the presence of structural markers offset by faults to relate the displacement accommodated by the fault with the volume of melt produced, iii) data that provide an estimate of the initial melt temperature and of host-rock temperature during seismic faulting. An independent indication that steady-state friction in the presence of melts might be achieved during coseismic slip arises from the dependence of the fractal dimension of the fault profile (intersection of the fault surface with the outcrop surface) with displacement. This relation could also indicate the slip-weakening distance (Hirose and Shimamoto, 2003). The above conditions are all satisfied in the case of the Gole Larghe Fault Zone, which consists of hundreds subparallel strike-slip faults that cut tonalites of the Adamello batholith (Italy). The thickness of pseudotachylyte-bearing faults increases with displacement. From displacement/thickness ratios and energy balance calculations we determined the dynamic shear resistance for several pseudotachylyte-bearing faults. In the same faults the fractal dimension of the fault profile increases from 1.0 to 1.16 with displacement. This was also observed in experiments where steady state friction in the presence of melt was achieved (Hirose and Shimamoto, 2003). However, we will show that, since pseudotachylytes often overprint preexisting cataclasites, the estimate of the earthquake source parameters from field studies is limited by the uncertainties to attribute the measured displacement to a single seismic rupture.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8010 Fractures and faults
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting