Deformation history of tectonic melange and its relationship to the underplating process and relative plate motion: An example from the deeply buriedShimanto Belt, SW Japan

Map to microscopic-scale structural analysis of the Hanazono Assemblage of the Shimanto Belt in SW Japan, an excellent example of a deeply buried accr etionary complex, indicates a detailed process from under-thrusting to unde rplating. The earliest under-thursting process is recorded in fabric of mel ange, which has deformed by shear along the decollement, characterized by t hinning strain due to extensional breakage of blocks of sandstone and exoti c materials, such as basalt acid pelagic sediments of oceanic affinities. T he deformation mechanism for melange formation for domain I is plastic flow , concordant with metamorphic grade, which is higher than that of domain II . The deformation mechanism for domain II is predominantly pressure solutio n partly with grain-scale brittle breakage for the less metamorphosed part of the Hanazono Assemblage. The posterior underplating process recorded is contraction and thickening due to thrust stacking and folding. Locations of the folds developed beneath thrusts are emphasized because they are a line of evidence of thickening. The main deformation mechanism of the second st age is plastic now for domain I and cataclasis for domain II. The cataclasi s in domain II may be a result of change in strain rate and seems to be rel ated to seismicity at the time of underplating. Kinematics deduced from fab ric analysis of the earliest deformed melange indicates a good consistency with a relative plate motion between the Eurasian and oceanic plates that i s estimated from a hot spot reference plate circuit model by Engebretson el al. [1985], while that of the subsequent thrusting is not. This fact sugge sts that strain partitioning in association with oblique subduction of ocea nic plate was minimum during subduction. Another possibility for strain par titioning is the requirement of coupling between plates that are recorded b y the deformation mechanisms.