HR: 14:00h
AN: T23C-02 INVITED     [Abstracts]
TI: Intermediate Depth Earthquake Faulting in the Deep Continental Crust: Insights From Exhumed High-Pressure Rocks
AU: * Boundy, T M
EM: tboundy@uwm.edu
AF: University of Wisconsin Milwaukee, Department of Geosciences, Lapham Hall, Milwaukee, WI 53201 United States
AB: One of the most intriguing and surprising seismological observations in recent years is the detection of earthquakes in the deep continental crust (e.g. Jackson, 2002) suggesting a strong lower crust to depths to 70 km (or more) in some continental collision zones. How the mechanical strength of the continental crust and mantle lithosphere varies as a function of depth has widespread implications for geodynamics. Information gleaned from exhumed gabbroic and anorthositic high-pressure complexes complexes in the Bergen area and the Lofoten Islands of Norway demonstrate that deeply subducted dry rocks in the lowermost continental crust may withstand metamorphic re-equilibration for geologically significant periods of time. The persistence of metastability had a significant effect on the mechanical properties of these rocks, as evidence by the abundant pseudotachylyte faults documenting seismic failure under eclogite facies conditions at depths exceeding 60 km in the thickened continental crust. Recent field research reveals that the localized earthquake fault systems are regionally extensive throughout the exhumed high-pressure complexes (100's of km$^{2}$). Outcrops of metastable mafic granulite $>$ 9 km$^{2}$ are transected by mm-cm thick pseudotachylyte veins and associated cataclasites and fractures; the pseudotachylyte accounts for $<$$<$ 1% of the total volume. For a fault area of ~ 4 km$^{2}$ (the observable extent of the largest pseudotachylyte sheets) and coseismic slip of 1 m (based conservatively on field observations), this leads to an estimate of a minimum earthquake magnitude estimate of $\sim$6.3. The fracturing associated with the faults could have provided access for the infiltration of fluids that instigated the localized conversion of the metastable granulites to eclogite. The observations indicate that very low volatile contents may control the ability of some lower crustal rock to remain metamorphically metastable and in a brittle seismogenic state at depths of $>$60 km in continental collision zones, despite prevailing conditions of T= 600-$700\deg$C and P= 1.5-2.0 GPa. This is consistent with recent experimental studies that suggest nominal amounts of in-situ water may be key in the initiation of seismic failure in essentially dry rocks at intermediate depths (Zhang et al., 2004). Together the data are compatible with present-day earthquake focal depth distributions in some continental collision zones, such as beneath the Himalayas, and may provide an explanation for the generation of deep-crustal earthquakes in these settings.
DE: 7221 Paleoseismology
DE: 8045 Role of fluids
DE: 8102 Continental contractional orogenic belts
DE: 8159 Rheology--crust and lithosphere
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting