THE THICKNESS OF PLATE BOUNDARY THRUST FAULTS: IMPLICATIONS FOR DEFORMATION MECHANISM AND THE ROCK RECORD OF SUBDUCTION

The thickness of the actively deforming zone in a subduction thrust plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of plate boundary faults. Here we summarize observations of plate boundary faults at different depths from ocean drilling, seismic reflection studies, and structural measurements of exhumed ancient faults. These compiled measurements show that although cumulative deformation of subducting sediments may result in the development of sheared zones and mélanges hundreds of meters thick, that the active thickness of a plate boundary fault is likely much thinner (<50 m, usually 10-35 m). Anastomosing fault surfaces may be simultaneously active or alternating between earthquake cycles, for a total width of the geologically instantaneous plate boundary on order 100-400 meters. Sharp, smooth faults which are certain or possible earthquake rupture surfaces are found within or along the boundary of these ~10 m thick fault strands. Therefore, the effective deforming width of the plate boundary fault is at a minimum during earthquakes (<1-20 cm) but during afterslip and interseismic creep, the effective width is 2-3 orders of magnitude thicker. This compilation has implications for understanding the subduction and transference of material between tectonic plates and may be applied to constrain models which suggest material backflow within the plate interface.