Deformation processes in the Shimanto Subduction Complex, S.W. Japan
File(s)
Author(s)
Agar, Susan Margaret
Type
Thesis
Abstract
Deformation processes in the Shimanto Subduction Complex in the eastern Hata Peninsula, SW Japan have been studied at regional, mesoscopic and microscopic levels. Field mapping has been used in conjunction with a sampling program for optical and electron microscopy, XRD studies-, K-Ar dating and apatite fission track cooling age estimates. These provide a database for interpretations of deformation processes and the tectonic
evolution of the Shimanto Subduction Complex. Eocene to Cretaceous broken formations and melanges form
approximately 70% of the coastal exposures of the eastern Hata Peninsula. Their disrupted state results from progressive, accretion-related deformation which occurred during sediment burial and lithification. Structural evidence indicates that an early layer-parallel extension was overprinted by layer-parallel shearing as the accreted lithologies were dewatered. Interpretations of deformation mechanism paths during broken formation and melange^ fabric evolution suggest that microstructural processes were controlled by pore-fluid pressures, strain rates and lithologies. Small areas of Miocene to Eocene coherent strata in the complex are interpreted as slope strata which were deformed at slower strain rates than the surrounding broken formations. Early layer-parallel shearing in the broken formations was synchronous with and succeeded by more discrete normal and thrust faulting. As the fault-bound packets of broken strata were backrotated during early thrusting the early fault planes seized up and a new generation of thrust planes was initiated. The later out-of-sequence thrusts shortened the complex internally. Changing physical and chemical conditions during the evolution of fault systems are reflected in the fault rock microstructures. Shortening in the accreted broken formations affected the overlying slope strata where a similar sequence of faulting evolved diachronously. Deformation throughout the area occurred mainly under diagenetic to lower epizone conditions. An early phase of deformation in the late Cretaceous was followed by a second major phase in the late Eocene. The broken formations and melange were uplifted during the middle to late Miocene during the next phase of accretion.
evolution of the Shimanto Subduction Complex. Eocene to Cretaceous broken formations and melanges form
approximately 70% of the coastal exposures of the eastern Hata Peninsula. Their disrupted state results from progressive, accretion-related deformation which occurred during sediment burial and lithification. Structural evidence indicates that an early layer-parallel extension was overprinted by layer-parallel shearing as the accreted lithologies were dewatered. Interpretations of deformation mechanism paths during broken formation and melange^ fabric evolution suggest that microstructural processes were controlled by pore-fluid pressures, strain rates and lithologies. Small areas of Miocene to Eocene coherent strata in the complex are interpreted as slope strata which were deformed at slower strain rates than the surrounding broken formations. Early layer-parallel shearing in the broken formations was synchronous with and succeeded by more discrete normal and thrust faulting. As the fault-bound packets of broken strata were backrotated during early thrusting the early fault planes seized up and a new generation of thrust planes was initiated. The later out-of-sequence thrusts shortened the complex internally. Changing physical and chemical conditions during the evolution of fault systems are reflected in the fault rock microstructures. Shortening in the accreted broken formations affected the overlying slope strata where a similar sequence of faulting evolved diachronously. Deformation throughout the area occurred mainly under diagenetic to lower epizone conditions. An early phase of deformation in the late Cretaceous was followed by a second major phase in the late Eocene. The broken formations and melange were uplifted during the middle to late Miocene during the next phase of accretion.
Version
Open Access
Date Issued
1987-10
Date Awarded
1987-10
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Leggett, J. K.
Taira, A.
Sponsor
Shell UK Oil (Firm)
Publisher Department
Geology
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)
Author Permission
Permission granted