The flowering in the retrowedge of a mountain belt: a case study of the exhumation deformation on the Miocene subducted mélange/turbidite of the Yuli belt and its sedimentary covers in eastern Central Range of Taiwan

Abstract:

The fan-shaped cleavage across a mountain range, in particular the flowering in the retrowedge, have been widely discussed for better understanding the mountain building processes. The high-pressure Yuli belt and its slate cover in the eastern Central Range of Taiwan show a dome-like geometry (flowering) for the dominant penetrative cleavage. In this study, we conduct field investigations and microscopic structural observations on rock outcrops along several rivers. Combining newly acquired RSCM metamorphic peak temperature data and zircon U/Pb data, we provide structural constraints to characterize the deformation evolution of this flowering of S2 cleavage.

Our field observations revealed a lithological related doming cleavage distribution, from west to east: 1) a penetrative S2 cleavage is steeply dipping to west (40°-70°), associated with down-to-west vergent folds in the quartz-mica schist of the Yuli belt, with the exception around some large high pressure mafic bodies; 2) a dominant S2 cleavage forming an asymmetric anticlinal or doming structure in meta-sandstone and phyllite of Chulai Formation, from steeply dipping to NW (40°-60°) becoming gently dipping (20°-40°) to NE. S2 in the Chulai Formation exhibits top-to-west vergent folds; 3) a penetrative S2 cleavage is dipping gently (20°-40°) to the east in the slate of Xingao Formation, the youngest formation in this Miocene subducted mélange/turbidite sequence. It is worth noting that a sub-horizontal crenulation cleavage S3 was commonly developed, albeit much less penetrative, in all three formations in the eastern Central Range. We tentatively conclude: 1) the normal depositional sequence and not-overturned S2, which likely developed at the RSCM peak temperature of 300-375°C (depth of 10-12 km) in the Chulai and Xingao Formations; 2) the Yuli belt shows an overturned S2, which probably developed at RSCM peak metamorphic temperature of 450-500°C at the greater depth (15-20 km) before exhumation of the eastern Central Range. The flowering in the eastern Central Range (retrowedge) of Taiwan mountain belt thus is characterized by a rapidly exhuming subducted mélange of the Yuli belt with an overturned main cleavage S2, which was likely thrusting (a backthrust) over the younger Chulai and Xingao Formations with non-overturned sediments and S2.