Anelastic strain recovery reveals extension across SW Japan subduction zone

Abstract

Sediment dominated convergent margins typically record substantial horizontal shortening often associated with great earthquakes. The convergent margin south of Japan is arguably one of the most extensively investigated margins and previous studies have documented extensive evidence for accretion and horizontal shortening. Here, we show results from anelastic strains recovered from three partially lithified sediment samples (∼40% porosities) across the southwest Japan accretionary prism and propose that the margin is dominated by horizontal extension rather than compression. The anelastic strain results are also consistent with stress directions interpreted from two independent techniques ‐ bore hole breakout orientations and core‐scale fault data. We interpret this unexpected result to reflect geologically recent underthrusting of a thick sediment package and concomitant weakening of the decollément.