Doming induced by rapid exhumed serpentinite bodies in the subduction mélange and the associated structures: a case study of the Yuli Belt in eastern Taiwan

Abstract:

Based on field investigations, microscopic observations, petrological analyses, and geochronological data, this study intends to characterize the doming structures induced by the relatively higher strength serpentinite bodies surrounded by pelitic rocks in the subduction mélange, which exhumed together to the surface. Our case study is the Yuli Belt of Taiwan. During subduction then exhumation of the Eurasian continental margin, the Philippine Sea plate has been approaching in the vicinity of Taiwan since the Miocene time. The Yuli belt is composed of continental margin turbiditic sediments with exotic blocks that contain high-pressure minerals (omphacite, glaucophane, garnet) with Miocene ages (15-10 Ma) suggesting rapid exhumation from depths of 40-50 km probably since 5-6 Ma.

We conducted detailed field surveys around the Serpentinite bodies within the Yuli belt in the Chingshui River area. We found that the km-scale-wide serpentinite bodies served as stronger competent rock bodies. The surrounding pelitic schist, behaved as weaker incompetent rocks, shows bending of the main penetrative cleavage set S2, which also is associated with folding and crenulation of the earlier cleavage set S0/S1. It is worth noting that the edge of the serpentinite bodies has often formed a series of layers of hydrothermal metasomatism products, such as chlorite schist and talc schist. These serpentinite-edge-schist developed a dominant sub-vertical cleavage, with orientations generally parallel to the edge shape of the serpentinite body. We also commonly observed breccia deformation bands sub-parallel to the main cleavage in the serpentinite edge, implying strongly mechanical shearing occurred around the edge of serpentinite in contact with the surrounding pelitic schist.

The deforming and bending of the S2 cleavage of the pelitic schist surrounding the km-wide serpentinite bodies indicates that the pelitic rocks encountered the serpentinite bodies after they have already formed the S2-related structures, probably during the exhumation process. Finally, in the regional scale, the shallow dipping S2 and S3 outside the serpentinite bodies reveal a dome-shape character, strongly suggesting an upward serpentinite bodies and unroofing process during the rapid exhumation of the eastern Central Range of Taiwan.