Anelastic strain recovery reveals extension across SW Japan subduction zone

Abstract

[1] 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. Citation: Byrne, T. B.,

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

“…Wireline log Formation MicroImager (FMI) resistivity images and caliper data were used to identify the orientation of borehole enlargement, indicating the minimum horizontal stress orientation ("borehole breakouts"). These results allowed the in situ stress orientation to be determined (Lin et al, 2010), complementing measurements of in situ stress magnitude from other tools (see below) and in situ stress measurements at other sites across the margin (McNeill et al, 2004;Ienaga et al, 2006;Byrne et al, 2009). Minimum horizontal stress consistently trends NE-SW downhole (~700-1600 mbsf); therefore, the maximum horizontal stress trends NW-SE (Fig.…”

Site C0022

“…Analyses indicate a mean breakout azimuth of 49.5°a nd, therefore, an azimuth of 139.5° for the maximum horizontal stress (S HMAX ). This northwestsoutheast trend is roughly parallel to the convergent direction of the Philippine Sea plate and to the dominant S HMAX determined at previous IODP Nankai Trough Seismogenic Zone Experiment (NanTro-SEIZE) Sites C0001, C0004, C0006, and C0009 (Chang et al, 2010;Lin et al, 2010;Byrne et al, 2009;Kinoshita et al, 2008) and Ocean Drilling Program Site 808 (Ienaga et al, 2006;McNeill et al, 2004).…”

Untitled

“…Measurements of current stress (i.e., postearthquake slip) also can be used to explore different models to explain how dynamic slip occurred and the degree to which stress was released. The in situ stress will also be determined through the use of LWD image logs to characterize borehole deformation (e.g., Zoback et al, 2003) and measurements of anelastic strain recovery (ASR) of core samples (e.g., Byrne et al, 2009). Several samples were collected for ASR, and these time-sensitive measurements were begun during this expedition.…”