10:45 〜 12:15
[SIT20-P04] A paradigm of structural evolution in retro-wedge of the Taiwan mountain belt: backthrusting? backfolding? Upward flowering? somethings else?
キーワード：Retrowedge, Deformation fabrics, Flowering, Taiwan, Yuli Belt
Based mainly on field detailed investigations and structural analyses on microscopic thin sections, we intend to delineate the overall architecture of deformation fabrics in the retro-wedge of the Taiwan mountain belt. The Taiwan mountain building has been commonly considered as a doubly vergent wedge, which is characterized by a fan-shaped dominant cleavage. However, intense folding and faulting occurred in a variety of metamorphic schists that makes it quite difficult to retro-deform or differentiate the sequence of deformation features, in particular several generations of deformation fabrics. In the past decades, scientists have been proposing a few main deformation mechanisms, in particular back-thrusting and/or back-folding, to interpret the vertical to E-vergent main cleavage distributed in the retro-wedge, that is the eastern Central Range, of the Taiwan mountain belt.
In this study, we conducted field investigations along several rivers in the eastern Central Range, which geological occupied the so-called Yuli belt. This was a Miocene turbidite sediment package, being subducted into 40-50 km depth and experiencing metamorphism and highly deformation, before exhuming to the Earth’s surface probably since 5-3 Ma. Structurally, we identify at least three sets of penetrative metamorphic fabrics, S1, S2 and S3 in the Middle and Lower Yuli Belt, which have been considered to correspond to three major tectonic events, D1, D2, and D3. However, only two major sets of fabrics, S1 and S2 have been observed in the Upper Yuli Belt, or so-called eastern Slate belt.
In more detail, S1 usually parallel to the stratigraphic depositional layers, S0, which however is largely destroyed by pressure dissolution during D1 or D2 deformation event. S2 represents regionally the most dominant fabrics, often characterizing by axial plane cleavage of folded or crenulated S0/S1 compositional layers. Careful, detailed field observations allow us to trace the S2 across the transect across the entire Yuli Belt, showing a deeply W-dipping of 50-70 degrees in the west, and dramatically change to shallowly dipping toward the east in the retro-wedge of the belt. S3 seemingly is a late-stage product developed as a sub-horizontal fabric overprinted on locally kinked or crenulated S2 fabrics.
We thus use the dominant, penetrative, and deeply developed cleavage S2 as a key horizon in the Yuli belt, although not necessarily flat lying when it formed near the maximum depth, probably in the subduction channel. As the subduction rock suite of the Yuli Belt later exhumed rapidly but gradually up to shallow surface level in the retro-wedge of the mountain belt, we suspect that upward flowering of S2 fabrics occurred in a mostly overturned fashion, due to convergence and collision between Philippine Sea plate and Eurasia. During the exhumation and flowering process, S3 sub-horizontal fabrics developed at the shallow level, together with widespread normal faulting of obliquely across-strike extension.
In this study, we conducted field investigations along several rivers in the eastern Central Range, which geological occupied the so-called Yuli belt. This was a Miocene turbidite sediment package, being subducted into 40-50 km depth and experiencing metamorphism and highly deformation, before exhuming to the Earth’s surface probably since 5-3 Ma. Structurally, we identify at least three sets of penetrative metamorphic fabrics, S1, S2 and S3 in the Middle and Lower Yuli Belt, which have been considered to correspond to three major tectonic events, D1, D2, and D3. However, only two major sets of fabrics, S1 and S2 have been observed in the Upper Yuli Belt, or so-called eastern Slate belt.
In more detail, S1 usually parallel to the stratigraphic depositional layers, S0, which however is largely destroyed by pressure dissolution during D1 or D2 deformation event. S2 represents regionally the most dominant fabrics, often characterizing by axial plane cleavage of folded or crenulated S0/S1 compositional layers. Careful, detailed field observations allow us to trace the S2 across the transect across the entire Yuli Belt, showing a deeply W-dipping of 50-70 degrees in the west, and dramatically change to shallowly dipping toward the east in the retro-wedge of the belt. S3 seemingly is a late-stage product developed as a sub-horizontal fabric overprinted on locally kinked or crenulated S2 fabrics.
We thus use the dominant, penetrative, and deeply developed cleavage S2 as a key horizon in the Yuli belt, although not necessarily flat lying when it formed near the maximum depth, probably in the subduction channel. As the subduction rock suite of the Yuli Belt later exhumed rapidly but gradually up to shallow surface level in the retro-wedge of the mountain belt, we suspect that upward flowering of S2 fabrics occurred in a mostly overturned fashion, due to convergence and collision between Philippine Sea plate and Eurasia. During the exhumation and flowering process, S3 sub-horizontal fabrics developed at the shallow level, together with widespread normal faulting of obliquely across-strike extension.