THE UPPER ASEISMIC TO SEISMIC TRANSITION: A SILICA CEMENTATION THRESHOLD

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°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, unaltered sediment wedge associated with Mesozoic through Eocene 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 Sitkalidak Fm. was subducted to approximately 2.4-3.9 km and experienced temperatures of 100-125°C before accreting into the wedge. The Paleocene Ghost Rocks Fm. subducted to 10-14 km (280-320 MPa) and reached 215-290°C. Both formations host black, shiny shear surfaces associated with disrupted zones interpreted as paleodecollements by previous authors.

X-ray diffraction and microprobe results suggest that the mineralogy of the shear surfaces in both formations contain chlorite and variable amounts of other sheet silicates, i.e. illite/muscovite, smectite, and kaolinite, which may be detrital and/or diagenetic/metamorphic. Quartz cementation is rare in the Sitkalidak Formation but ubiquitous in the Ghost Rocks Formation, in fault-parallel and fault-crossing geometries. We suggest that the formation of a quartz network across fault zones may trigger the onset of seismogenesis. The frictional behavior of sheet silicates is generally velocity strengthening, resulting in stable sliding behavior, while quartz is velocity weakening, showing stick-slip frictional behavior. Thus, the aseismic-seismic transition may be controlled by quartz mobility and the appearance of volumetrically significant quartz +/- calcite precipitates filling, coating, and cementing fault surfaces, creating “deadbolts” across slip surfaces previously dominated by sheet silicates.