The Okitsu mélange in the Shimanto accretionary complex, the onshore extension of
the modern Nankai accretionary prism, consists of a kilometer-size duplex of oceanic
basalt and trench-fill sedimentary rocks, and is thought to represent rocks underplated to
the prism along the subduction plate-boundary at seismogenic depth. An internal, horsebounding
thrust of the duplex, referred to as the Suzume fault, juxtaposes basalt in the
hanging wall and sedimentary rocks in the footwall. Structure and fabric of the fault was
characterized at the mesoscale to investigate the processes and structural evolution along
a plate-boundary décollement. The fault zone in the hanging wall consists of decimeterthick
ultracataclasite bounded by a several m thick zone of fractured basalt, and likely
records 2+ km displacement along the thrust. The footwall consists of decimeter-thick
ultracataclasite bounded by a 20-m-thick zone of ductile shear in flattened sedimentary
host rock, and likely records 30+ km of displacement. The asymmetric structure across
the Suzume fault, as well as inferred displacement fields and timing relations, are
consistent with a tectonic model in which the footwall records early ductile, compactive
deformation of poorly consolidated sediments during underthrusting at the prism toe
region, followed by extremely localized cataclasis at the underplating depth. In contrast,
the hanging wall is deformed by intense cataclasis, and only during underplating.
Deformation style and strain state in the footwall of the Suzume fault is qualitatively
similar to the modern Costa Rica underthrust section at the toe region. Similarity in the
structure and fabric of the hanging wall between the Suzume fault and modern
décollement zones sampled through scientific drilling suggests that intense cataclasis
under horizontal contraction likely is a common feature for the hanging wall of the décollement zone throughout the toe to underplating regions. Structures in the Suzume
fault that are not in common with the modern décollements imply progressive
consolidation during underthrusting from the toe to underplating depths may be
responsible for the localization of shear in the footwall. At several kilometers depth,
displacement along the plate boundary is likely accommodated within an extremely
narrow zone as recorded in the ultracataclasite of the Suzume fault.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/4758 |
| Date | 25 April 2007 |
| Creators | Kanaya, Takamasa |
| Contributors | Chester, Frederick M. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | 39519760 bytes, electronic, application/pdf, born digital |
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