Frictional Behavior of Carbonate-rich Sediments in Subduction Zones
Wednesday, 24 February 2016
Hannah S Rabinowitz1, Heather M Savage1, Brett M Carpenter2 and Cristiano Collettini3, (1)Columbia University of New York, Palisades, NY, United States, (2)University of Oklahoma Norman Campus, Norman, OK, United States, (3)Sapienza University of Rome, Rome, Italy
Abstract:
Carbonate-rich layers make up a significant component of subducting sediments around the world and may impact the frictional behavior of subduction zones at depths consistent with observations of shallow slow slip events. In order to investigate the effect of carbonate subduction, we conducted biaxial deformation experiments within a pressure vessel using the Brittle Rock deformAtion Versatile Apparatus (BRAVA) at INGV. We obtained input sediments for two subduction zones, the Hikurangi trench, New Zealand (ODP Site 1124) and the Peru trench (DSDP Site 321), which have carbonate/clay contents of ~40/60 wt% and ~80/20 wt%, respectively. Samples were saturated with distilled water mixed with 35 g/l sea salt and deformed at room temperature. Experiments were conducted at σN = 1–50 MPa with sliding velocities of 1–300 μm/s and hold times of 1–1000 s. Frictional strength of Hikurangi gouge is 0.35–0.55 and Peru gouge is 0.55–0.65. Velocity-stepping tests show that the Hikurangi gouge is consistently velocity strengthening (friction rate parameter (a-b) > 0). The Peru gouge is mostly velocity strengthening but exhibits a minimum in a-b at the 3–10 μm/s velocity step (with velocity weakening behavior at 25 MPa, indicating the potential for earthquake nucleation at slip velocities associated with slow slip). Slide-hold-slide tests show that the healing rate (β) of the Hikurangi gouge is 1x10-4–1x10-3 /decade which is comparable to that of clays (β~0.002 /decade) while the healing rate of Peru gouge (β~6x10-3–7x10-3 /decade) is closer to that of carbonate gouge (β~0.01 /decade). The mechanical results are complemented by microstructural analysis. In lower stress experiments, there is no obvious shear localization. At 25 and 50 MPa, pervasive boundary-parallel shears become dominant, particularly in the Peru samples. Degree of microstructural localization appears to correspond with the trends observed in velocity-dependence. Our preliminary results indicate that carbonate/clay compositions could have a significant impact on the conditions at which slow slip events might be expected.
