T91 Deformation Localization throughout the Crust

Monday, 20 October 2014
8:00 AM - 12:00 PM
118

STRUCTURE OF THE PLATE BOUNDARY FAULT IN THE JAPAN TRENCH, AND IMPLICATIONS FOR SLIP BEHAVIOUR

Paper number: T91-2
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed one year after the Tohoku-oki earthquake and drilled into the plate boundary ∼7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary décollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ∼5 – 15 m thick.

Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished and lineated surfaces with two predominant orientations. Scaly fabrics have been described in other subduction thrusts, landslides, and sub-glacial tills, but there is no constitutive understanding of the behaviour of anastomosing networks during shear deformation. We present analog models using lentils to simulate scaly fabrics, and describe the distribution and localization of shear during deformation.

The scaly fabric is crosscut in several places by discrete faults across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. A very thin (~5 micron) amorphous layer has been discovered in transmission electron microscope observations of the main discrete fault cutting the scaly fabrics. The mechanism of amorphization is unknown, but may be related to shear heating on a micro-localized seismic slip surface. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the décollement suggests that rupture to the trench may be characteristic of this margin.