HR: 10:35h
AN: T41E-02    [PDF]
TI: The Upper Aseismic to Seismic Transition: A Silica Mobility Threshold
AU: * Rowe, C D
EM: crowe@es.ucsc.edu
AF: Dpt. Earth Sciences, UC Santa Cruz 1156 High St, Santa Cruz, CA 95064 United States
AU: Moore, J
EM: casey@es.ucsc.edu
AF: Dpt. Earth Sciences, UC Santa Cruz 1156 High St, Santa Cruz, CA 95064 United States
AB: The up-dip portions of accretionary subduction zone decollements slide stably and are therefore aseismic, but become seismogenetic at a depth of 5-15 km. Thermal models of modern subduction zones and accretionary wedges predict that the aseismic-seismic transition occurs at 100-$150\deg$C. This correlation between temperature and the onset of seismogenesis suggests that fault behavioral properties are modified by a diagenetic-metamorphic reaction affecting the fault zone mineralogy. The Kodiak Accretionary Complex, Alaska, is a well-exposed sediment wedge associated with Mesozoic through recent Aleutian subduction. We compare two tectonic units that were subducted, one to just above the seismogenic transition, and one to within the seismogenic zone. The Eocene rocks were subducted to approximately 2.4-3.9 km and experienced temperatures of 100-$125\deg$C before accreting into the wedge. The Paleocene rocks subducted to 10-14 km (280-320 MPa) and reached 215-$290\deg$C. Both formations host disrupted zones interpreted as paleo thrust faults by previous authors, which were probably associated with paleodecollement systems. Quartz cementation is rare in the paleo-thrust faults of the Eocene rocks but ubiquitous and extensive in the paleo-thrust faults of the Paleocene rocks, in fault-parallel and fault-crossing geometries. We suggest that the formation of a quartz network along and across fault zones may cause the onset of seismogenesis. The frictional behavior of sheet silicates is generally velocity strengthening, resulting in stable sliding behavior, while quartz exhibits velocity weakening, or stick-slip frictional behavior. Thus, the aseismic-seismic transition may be controlled by quartz mobility and the appearance of volumetrically significant quartz $\pm$ calcite precipitates filling, coating, and cementing fault surfaces, creating ``deadbolts'' across slip surfaces, establishing frictional control over surfaces whose properties were previously controlled by sheet silicates.
DE: 8010 Fractures and faults
DE: 8025 Mesoscopic fabrics
DE: 8035 Pluton emplacement
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: 2003 Fall Meeting