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Initial deformation in a subduction thrust system: polygonal normal faulting in the incoming sedimentary sequence of the Nankai subduction zone, southwestern Japan

By
A. S. Heffernan
A. S. Heffernan
Department of Earth Sciences, University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA (e-mail: adam_heffernan@aoageophysics.com)
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J. C. Moore
J. C. Moore
Department of Earth Sciences, University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA (e-mail: adam_heffernan@aoageophysics.com)
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N. L. Bangs
N. L. Bangs
University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, TX 78759, USA
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G. F. Moore
G. F. Moore
Department of Geology and Geophysics, 1680 East West Rd., University of Hawaii, Honolulu, HI 96822, USA
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T. H. Shipley
T. H. Shipley
University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, TX 78759, USA
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Published:
January 01, 2004

Abstract

3D seismic data from the Nankai margin provide detailed imagery documenting the onset of deformation at an active sediment-dominated accretionary prism, including a previously unmapped network of normal faults. The Nankai margin off southwest Japan is characterized by active subduction, seismogenesis, and a large accretionary prism with fold-and-thrust belt structure. Imbricate thrusting is the dominant structural style of the outer 20 km of the prism. This structural domain develops at the prism toe, where an incipient imbricate thrust displays significant along-strike variability in dip, offset, and development of hangingwall anticlines.

Compressional deformation is preceded by normal faulting that initiates seaward of the trench axis. Seismic data in this area reveal a complex, intersecting pattern of normal faults within the incoming hemipelagic sediments. Underlying the faulted section is a high-amplitude reflector interpreted as representing oceanic basement. This reflector contains elongate horsts and grabens oriented perpendicular to the margin interpreted as relict spreading centre fabric.

Analysis of the orientation of normal faults within the Shikoku basin sequence shows a correlation between fault geometry and basement structure. This faulting is notably similar to layer-bound compaction faults, documented in the North Sea and elsewhere, attributed to both hydrofracturing and volumetric contraction of fine-grained sediments. Mapped normal faults may thus be the result of a combination of differential compaction of sediments above irregular, dipping oceanic basement and compactional dewatering seaward of the toe of the accretionary prism.

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Contents

Geological Society, London, Memoirs

3D Seismic Technology: Application to the Exploration of Sedimentary Basins

Richard J. Davies
Richard J. Davies
Cardiff University, UK
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Joseph A. Cartwright
Joseph A. Cartwright
Cardiff University, UK
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Simon A. Stewart
Simon A. Stewart
BP, Azerbaijan
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Mark Lappin
Mark Lappin
ExxonMobil Exploration Company, USA
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John R. Underhill
John R. Underhill
The University of Edinburgh, UK
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Geological Society of London
Volume
29
ISBN electronic:
9781862394049
Publication date:
January 01, 2004

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