09:15 〜 09:30
[SIT20-02] Oblique Convergence Recorded in Strain Patterns of the Cretaceous Accretionary Complex of SW Japan
★Invited Papers
キーワード：斜め沈み込み、三波川帯、四万十帯
During oblique plate convergence is rarely if ever entirely head on—subduction zones are commonly curved and motion vectors described by rotation about a Euler pole change with increasing distance from the pole and are likely to vary along convergent margins. The strike-slip component can be accommodated by the development of strike slip faults in the rear of an accretionary complex. Modern examples of such cases are SW Japan and the Burmese arc both of which are associated with major dextral strike-slip faults. If most or all of the strike-slip component of oblique convergence is accommodated by slip on such major faults the convergence direction in the remaining domain will be normal to the margin. This is in agreement with observations in some ancient examples of accretionary complexes that show movement directions perpendicular to the strike of the belt (e.g. Franciscan belt, W USA). However, in other cases (e.g. the Sanbagawa belt, SW Japan) movement directions are oblique to the orogen suggesting other ways in which oblique convergence is accommodated.
Cretaceous Plate reconstructions show the Izanagi plate was moving rapidly at rates of around 20 cm/yr with a large sinistral strike-slip component with respect to the contemporaneous Japanese margin. At that time two major accretionary tectonic domains, the Shimanto and Sanbagawa belts were formed in the Japanese margin at this time. The lack of any subsequent collision of large continental blocks makes this region well-suited to investigating the effects of oblique convergence on tectonic events recorded in the rock record. Both the Shimanto and Sanbagawa belts are present throughout the length of SW Japan but their orientation has been locally disrupted by collision of the Izu peninsula. The Ryoke high T/P metamorphic belt is the remnants of a Cretaceous volcanic arc and its distribution can be taken as parallel to the ancient convergent margin. We can then discuss movement directions with respect to this orientation. The Shimanto belt is oceanward of the Sanabgawa belt and the convergence-related movement direction recorded by stretching lineations in the Kyushu and Shikoku areas is dominantly orogen normal with no strike-slip component. In contrast the Sanbagawa belt, which is close to the former arc, shows lineations that lie close to the trend of the orogen. In the Kii peninsula the Sanbagawa belt thins and the Shimanto belt is closer to Ryoke arc than in Shikoku and Kyushu and here the Shimanto belt also shows stretching lineations oblique to the orogen. Therefore, the change from orogen-normal to closer to orogen parallel is not a function of the geological unit but the position within the accretionary complex. A change from orogen-normal to close to orogen parallel is expected for accretionary wedges that have a viscous rather than Coulomb-type rheology.
A top to the west sense of shear is reported throughout the Sanbagawa belt and is consistent with the predicted subduction polarity at the time. However, the orientation of the lineation is neither parallel to the orogen nor the reconstructed plate direction. A further important observation is that the movement direction becomes closer to orogen normal close to the northern boundary of the Sanbagawa belt interpreted as the former backstop to the Cretaceous accretionary complex. The orientation of the lineation can be explained by a combination of plate convergence vector and an extension direction related to ductile thinning during exhumation. The change from close to orogen-parallel to orogen normal near the backstop suggest that deformation was not partitioned into strike slip faulting in this case and there is no need for large amounts of strike slip movement on the MTL as suggested by some studies.
Cretaceous Plate reconstructions show the Izanagi plate was moving rapidly at rates of around 20 cm/yr with a large sinistral strike-slip component with respect to the contemporaneous Japanese margin. At that time two major accretionary tectonic domains, the Shimanto and Sanbagawa belts were formed in the Japanese margin at this time. The lack of any subsequent collision of large continental blocks makes this region well-suited to investigating the effects of oblique convergence on tectonic events recorded in the rock record. Both the Shimanto and Sanbagawa belts are present throughout the length of SW Japan but their orientation has been locally disrupted by collision of the Izu peninsula. The Ryoke high T/P metamorphic belt is the remnants of a Cretaceous volcanic arc and its distribution can be taken as parallel to the ancient convergent margin. We can then discuss movement directions with respect to this orientation. The Shimanto belt is oceanward of the Sanabgawa belt and the convergence-related movement direction recorded by stretching lineations in the Kyushu and Shikoku areas is dominantly orogen normal with no strike-slip component. In contrast the Sanbagawa belt, which is close to the former arc, shows lineations that lie close to the trend of the orogen. In the Kii peninsula the Sanbagawa belt thins and the Shimanto belt is closer to Ryoke arc than in Shikoku and Kyushu and here the Shimanto belt also shows stretching lineations oblique to the orogen. Therefore, the change from orogen-normal to closer to orogen parallel is not a function of the geological unit but the position within the accretionary complex. A change from orogen-normal to close to orogen parallel is expected for accretionary wedges that have a viscous rather than Coulomb-type rheology.
A top to the west sense of shear is reported throughout the Sanbagawa belt and is consistent with the predicted subduction polarity at the time. However, the orientation of the lineation is neither parallel to the orogen nor the reconstructed plate direction. A further important observation is that the movement direction becomes closer to orogen normal close to the northern boundary of the Sanbagawa belt interpreted as the former backstop to the Cretaceous accretionary complex. The orientation of the lineation can be explained by a combination of plate convergence vector and an extension direction related to ductile thinning during exhumation. The change from close to orogen-parallel to orogen normal near the backstop suggest that deformation was not partitioned into strike slip faulting in this case and there is no need for large amounts of strike slip movement on the MTL as suggested by some studies.