HR: 08:30h
AN: S31B-03    [Abstracts]
TI: Internal structure of a thrust associated with subduction underplating from the Okitsu melange in the Shimanto accretionary complex, Japan
AU: * Kanaya, T
EM: tkanaya@tamu.edu
AF: Department of Geology & Geophysics, Center for Tectonophysics, Texas A&M University, TAMU 3115, College Station, TX 77843
AU: Chester, F
EM: chesterf@geo.tamu.edu
AF: Department of Geology & Geophysics, Center for Tectonophysics, Texas A&M University, TAMU 3115, College Station, TX 77843
AU: Sakaguchi, A
EM: arito@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, 3173-25 Showa-machi, Kanazawa, Yukohama, 236-0001 Japan
AB: The structure of large-displacement, plate-boundary faults in subduction zones is poorly defined relative to other tectonic settings. The thrust faults in the Okitsu melange constitute a duplex, which juxtapose oceanic pillow basalt (hanging walls) and trench-fill-sedimentary rocks (footwalls), and may represent a paleo-underplating zone of a plate-boundary subduction thrust at seismogenic depth. One well exposed duplex-fault, with displacement greater than several kilometers, is examined through structural mapping at scales of 1:100 to 1:1, mesoscale fabric analysis, and geochemical analysis. The duplex-fault zone displays a unique, asymmetric internal structure: brittle deformation dominates in the hanging wall and brittle and ductile deformation is evident in the footwall. The internal structure and surrounding host rocks reflect three accretionary processes: underthrusting, underplating, and uplifting along Out-of-Sequence-Thrusts (OOSTs).
On the basis of style of deformation and displacement field, two distinct deformation episodes are identified, which likely correspond to underthrusting and underplating. The elongation of basalt pillows and the boudinage structure of sandstone blocks in shale indicate an elongation of 1.1 to 1.5 parallel to the oceanic crust layer throughout the host rocks of both the hanging wall and footwall, consistent with uniform extension of the crust during underthrusting.
Pillow basalt and sedimentary rocks are juxtaposed by a single cataclasite layer composed of decimeter thick ultracataclasite derived from both rock types along the duplex-fault. Mesoscale implosion breccia and micro textures indicative of pressure solution are observed along the master duplex-fault and thought to be products of alternating fast- and slow-rate slip, respectively. Mesoscale fault fabric indicates a paleostress with the maximum principal compression at 15-20° to the master duplex-fault reflecting layer parallel contraction, which is consistent with the displacement field expected for underplating and a high frictional strength of the duplex-fault. The asymmetric structure may reflect dissimilar tectonic paths of the hanging wall and footwall. The basalt in the hanging wall displays only a localized, brittle structure because it is faulted only during underplating. An increase in fracture density and decrease in the size of comminuted pillows towards the fault correspond to mapped zonation of protocataclasite, cataclasite, and ultracataclasite. In contrast, the footwall consists of the ultracataclasite layer flanked by a 20-meter-thick zone of ductile shear, reflecting deformation during both underthrusting and underplating. A significant post-underplating overprint is lacking in both the duplex-fault and host rocks, consistent with translation of the entire melange along OOSTs, possibly represented by the tectonic contacts of the Okitsu melange.
DE: 8005 Folds and folding
DE: 8025 Mesoscopic fabrics
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8150 Plate boundary: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Seismology [S]
MN: Fall Meeting 2005