| dc.description.abstract | Taiwan orogenic belt is formed by the oblique arc-continent collision between the Eurasian and Philippine Sea plates, triggering the Penglai orogeny since the late Miocene and leading to the southward propagation of the orogen. The southern part of Backbone Range slate belt (BS), located at the transition from subduction to collision, is composed by Miocene passive margin to pelagic cover series and experiences different degrees of tectonic deformation and metamorphic histories, which make it as an exceptional place to study the early tectonic history of the Taiwan orogeny. However, previous mountain building models proposed to explain this structural deformation of the southern Taiwan range lacked thermal-metamorphic constraints, thus devoid of the role of metamorphism in the orogenic processes.
As pelitic photolith is rich in carbonaceous material, the abundant slates are chosen in this study to determine their peak metamorphic temperatures through a geothermometer (Raman spectroscopy of carbonaceous material, RSCM). Together with results of macroscopic and microscopic structural observations and analyses, an attempt is made to decipher the primary thermos-tectonic trajectory of these rocks concerning the relative timing and environment of the deformation stages within the early orogenic wedge during the arc-continent collision.
In addition to the results of north-south sampling along the Taimali-Jinlun coastline by Hsu (2008) which also help confirm that both aseismic shear strain and incident angle of laser beam relative to the C-axes of carbonaceous material have an insignificant effect on CM crystallinity, new sampling is extended to the east-west Nantien and Taimali industrial roads, resulting in 103 RSCM peak temperature data in total. According to analyses of RSCM-T and macroscopic structural observation, the Miocene Lushan Formation in the southeastern BS is found to have undergone synorogenic metamorphism, and was folded with limbs tilted at ~30˚ during the peak state as suggested by the field RSCM-T isograde. Overprinting relationship among the overturned folding, main S2 foliation, and RSCM-T isograde is established, and north-vergent deformation is observed to have commenced prior to both peak metamorphism and the end of east-vergent backfolding, as evidenced in the gentler dips of RSCM-T isograde and backfolding-associated S2 foliation.
Inferred from the RSCM result, a tectonic evolution model of the Taiwan orogenic wedge growing in the early stage of arc-continent collision is proposed. Following the subduction of the Miocene Lushan Formation deep beneath the accretionary wedge, east-vergent backthrusting started to affect the rocks resulting in overturned strata within recumbent backfolds. Backfolding and formation of associated S2 foliation continued with prograde subduction and was joined with north-vergent folding before peak state metamorphism, when the rocks were incorporated into the base of the wedge through basal accretion. During the peak state at a depth of ~10 km, the overturned bedding and foliation planes observed across the north-vergent folds are deduced to be tilted at 30° and 15°. Afterwards, the slate was exhumed to the southeastern edge of the mountain range with further north-vergent folding and resulted in RSCM-T isograde folding exhibiting ~30℃ difference at the same elevation. | en_US |