KINKING, PRESSURE SOLUTION, AND THE MECHANICS OF THE SUBDUCTION ZONE INTERFACE AT THE DEPTH OF TREMOR AND SLOW SLIP EVENTS (Invited Presentation)

The subduction zone interface in the seismic/aseismic transition zone, between ~40 and 70 km depth, is the source region for seismic tremor and slow slip events, where intermittent release of elastic strain occurs at rates 1 to 6 orders of magnitude more slowly than in the seismogenic zone. High fluid pressures and some form of velocity strengthening behavior are generally invoked to explain the characteristics of these events.

Under the conditions of this transitional region (300-500°C), metasedimentary rocks deform by ductile processes and form a strong planar fabric. This fabric (S), if close to the subduction shear plane, is likely to experience velocity perturbations. Deformation will be primarily by slip on S, but regions that slip more slowly than average will rotate in the direction of shear to maintain vorticity compatibility. These perturbations propagate normal to S, forming kink-bands or crenulations. The kink-band boundaries (kbbs) rapidly rotate due to slip on S, so that their angle to S equals to the accumulated angular shear strain since initiation. The angle between the foliation in the kink-band (S_i) and the kbb remains ~90 degrees.

The strong fabric requires that continuity is maintained across the kbbs. At small scales the most likely mechanism to allow this is dilation normal to S_i. The dilatant regions are filled with water initially, so that the kink-band propagates as a hydraulic fracture, but with the rate limited by the elastic and plastic properties of the mica-rich foliation. Propagation of dilatant kink-bands with wavelengths of the order if 1 mm will occur at rapid but sub-seismic rates, and could lead to moment release comparable to low-frequency earthquakes. Propagation of larger scale folds, with wavelengths from 1-1000 m, as observed in exhumed subduction channels, could occur by the coordinated development of mm-scale crenulation bands developed primarily in the short limbs of the folds. This could lead to a hierarchy of events ranging from tremor bursts to slow slip events.

Pressure solution will lead to the dilatant fractures being filled with minerals such as quartz or calcite. Addition of a pressure-solution component allows calculation of the compositional contrast between the dilating kink-bands (Q domains) and the intervening mica-rich bands (P domains), as a function of strain.