SHALLOW SLIP LOCALIZATION ALONG MEGATHRUSTS: INVESTIGATING THE ROLE OF SCALY FABRIC

Abstract:

Scaly fabric is classically interpreted as a low strain-rate structure, resulting from progressive shearing with episodic formation and destruction of oriented and flattened clay layers and aggregates.

Scientific Ocean drilling of the Japan Trench in response to the 2011 Tohoku-Oki EQ sampled the active plate-boundary décollement zone in a place of known, large, and very recent displacement. The visual inspection of core material from the plate boundary décollement reveals a clay layer with scaly fabric, cut by a sharp discontinuity that may be the record of co-seismic slip (Chester et al., 2013). This result brought to the need to re-evaluate the role and the characteristics of scaly fabric.

Scaly fabric is the typical meso/microstructure marking the location of slip concentration in all the active décollements cored near the trench (i.e. Barbados, Nankai, Costa Rica, Japan Trench) and in analogue fossil examples cropping out onland. Scaly fabric tends to form self-similar patterns, and usually areas with smaller phacoids are interpreted as more deformed. We know that scaliness develops in the early stages of deformation, that the slip surfaces defining the phacoids are sharp and they do not occur randomly, that they grow and coalesce forming a progressively finer anastomosing network, and that eventually the anastomoising slip surfaces are “enhanced” or “cut” by straight slip surfaces. Advances in identifying detailed evolutionary history of slip localization from scaly fabric to discrete surfaces have been paralleled by laboratory experiments. Here we try to summarize direct and indirect information on physical properties of clay layers deformed at shallow depth and, possibly, their links to the seismic cycle. These challenges include future work on the role of scaly fabric on earthquake deformation along faults.