[S-IT20] New perspectives on accretion and exhumation processes during oblique plate convergence

[SIT20-P05] Late-stage horizontal extension and horizontal flow at the shallow level during exhumation of high-pressure rocks in Taiwan

*Gong-ruei Ho¹、Jian-Cheng Lee¹ (1.Academia Sinica, Institute of Earth Sciences)

キーワード：Deformation fabrics, Transtensional fabrics, Shallow horizontal flow, Taiwan

In the young and presently active Taiwan orogen, it provides a large database of structural, geochronological, and petrological research with two-stage exhumation mechanisms in high-pressure metamorphism. The major D2 deformation event, which generated metamorphic fabrics S2, was well-preserved along the mountain and river cross-sections and suggested oblique compression deformation with significant strike-slip component exists in the collision or early stage of exhumation at the level of the lower and middle crust depth. This penetrative, main foliation, S2, forms a flowering structure, sometimes in an overturned fashion, during exhumation. The late D3 tectonic event with often subtle metamorphic fabrics S3 is believed to be related to the tectonic extension. This presumably transtensional tectonic event D3 also exhibits several other deformation structures, including normal faults and sub-vertical fractures often filling with various mineral veins. As for the deformation fabrics S3, it generally develops in the sub-horizontal plane and is usually overprinting on the regional dominant S2 fabrics, regardless of S2 orientation.

A synthesis of available data suggests key exhumation mechanisms from D2-transpression to D3-transtension, and here we document: 1) the distribution, the D3 structures can be observed through the whole retrowedge of the orogen belt; 2) the kinematics of deformation, asymmetric structures including S-C fabrics, en-échelon veins, R-and-R’ shears, and sigmoidal core-and-mantle structures as shear sense indicators showing S3 is characterized by a top-to-NE and/or SW shear, implying significant horizontal flow; 3) the D3 syn-kinematic veins showed an Ar-Ar age of 1.1-1.5Ma.