The thickness of subduction plate boundary faults from the seafloor into the seismogenic zone

Abstract

The thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior. We have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and field studies in accretionary wedges. We describe a general geometric model for subduction thrust décollements, which includes multiple simultaneously active, anastomosing fault strands tens of meters thick. The total thickness encompassing all simultaneously active strands increases to ~100-350 m at ~1-2 km below seafloor, and this thickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters-thick fault strands. Although flattening, primary inherited chaotic fabrics, and fault migration through subducting sediments or the frontal prism may build mélange sections that are much thicker (to several kilometers), this thickness does not describe the active fault at any depth. These observations suggest that models should treat the subduction thrust plate boundary fault as <1-20 cm thick during earthquakes, with a concentration of postseismic and interseismic creep in single to several strands 5-35 m thick, with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands. © 2013 Geological Society of America

The damage is done: Low fault friction recorded in the damage zone of the shallow Japan Trench décollement

“…The 1-D geometry of the core does not capture along-fault spatial variability, so we cannot confirm whether the JFAST site is representative of the entire shallow Japan Trench décollement. However, the total fault zone thickness estimates from our data collected in the JFAST core are within the range of values for similar subduction faults at similar depths [Rowe et al, 2013], suggesting that the data may be generally representative.…”

“…Geophysical measurements and core observations show the décollement is localized onto a layer of smectite-rich fault rock that is surrounded by a complex zone of deformation [Chester et al, 2013a[Chester et al, , 2013bKirkpatrick et al, 2015], typical of subduction faults generally [Rowe et al, 2013;Ujiie and Kimura, 2014]. Temperature measurements in the fault zone [Fulton et al, 2013], laboratory friction experiments on core samples from the décollement Sawai et al, 2014;Ikari et al, 2015;Remitti et al, 2015], and measurements of the anisotropy of magnetic susceptibility all suggest that the décollement at the JFAST study site is frictionally weak.…”

Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms

“…It is likely that the heterogeneity of mélange affects aseismic slip and earthquake nucleation . Recently, Rowe et al (2013) compiled data on the thicknesses of subduction thrusts at depths of <1 to 15 km. Although mélanges can be several hundreds of meters thick, a simultaneously active part of a subduction plate boundary consists of one or multiple fault strands with a thickness of 5 to 35 m, which are sharply cut by discrete slip surfaces with thicknesses of <1 to 20 cm.…”

Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan

“…The thickness of the mélange zone in the Shimanto complex is typically less than 1 km based on field observations for the mappable distribution of mélange zones (e.g., Taira et al, 1988). Rowe et al (2013) determined the thickness of a shear zone along a subduction plate interface and found it to be up to a few hundred meters in thickness. Within the mélange zone, the same shear zones may be stacked up by duplex structures to form underplated accretionary complexes (e.g., Hashimoto and Kimura, 1999).…”

“…The evolution of deformation features and deformation mechanisms along subduction zones has been revealed from on-land geology in exhumed accretionary complexes, not only in the Shimanto Belt (e.g., summarized in Kimura et al, 2007) but also in other accretionary complexes (e.g., Fisher and Byrne, 1987;Fagereng and Toy, 2011;Rowe et al, 2013). By reviewing many previous studies, Fagereng and Toy (2011) pointed out that ductile flow by diffusion-precipitation creep and cataclastic deformation coexist as the major deformation mechanisms in seismogenic crust.…”