Perliminary thermal metamorphic constraints on tectonic evolution in the eastern margin of Tibetan Plateau: lessons from the slate belt of Taiwan?

<p>      The Tibetan Plateau, resulting from the active Eurasian-India collision, presents a major scientific challenge in understanding its growth and propagation. One key region is the Longmen Shan mountain belt in western Sichuan, which forms the steepest margin of the plateau and has been active as demonstrated by the Mw 7.9 Wenchuan (2008) and Mw 6.6 Lushan (2013) earthquakes. Tectonic history of the Longmen Shan belt and the neighboring Songpan-Garze terrane, however, began in the Triassic Indosinian orogenesis, which complicates the geologic records. But the major thickening of Tibet was formed in Himalayan orogenesis. Therefore, quantitative constraints on the pre-Tertiary tectonic evolution of the region are crucial in delineating Himalayan geodynamics. In this study, the raman spectroscopy of carbonaceous material (RSCM) geothermometer is applied to the metasediments of the Longmen Shan and Sonpan-Garze terrane to obtain their peak metamorphic states. Combining existing metamorphic, geochronologic and thermochronologic data, better rock thermal histories may be reconstructed, providing insights to the structure and development of the orogenic system.</p><p>      In this study, 50 samples were collected in eastern margin of Tibetan Plateau along several transects in NW-SE direction, perpendicular to the structural grain of the Longmen Shan and into the Songpan-Garze terrane. Together with existing data, distribution of the peak temperatures from RSCM analyses is not correlated to later igneous intrusions, ruling out significant contact metamorphism overprint. Along the WenChuan Fault, the Songpan-Garze terrane is of higher grade than the Longmen Shan, indicating it is a major reverse shear zone. The rather high RSCM temperatures (over 500 °C) acquired from Songpan-Garze metasediments are inconsistent with past models as remnants of a classical accretionary prism; the complex wedge kinematics involving significant basal accretion observed in the slate belt of Taiwan orogen may give clues in reconstructing the structure and evolution of eastern Tibetan Plateau.</p><p>Keywords: Tibetan Plateau; Longmen Shan; RSCM geothermometer</p>

Yuli Belt in the eastern Taiwan orogen: a part of suture zone separating Eurasian and Philippine Sea plates

<p>The Taiwan orogen formed as a consequence of the oblique subduction of the Eurasian continental margin below the Luzon volcanic arc of the Philippine Sea Plate since the late Miocene. The Yuli Belt of the eastern Taiwan Central Range, exposed in the retro-wedge of the fold-and-thrust belt, hosts slivers of a heterogeneous unit of blueschist-facies rocks that are among the youngest blueschist units worldwide. However, the palaeogeographic provenance of this unit is still debated. This is due to the fact that numerous structural aspects, including the kinematics of the Yuli Belt’s tectonic contacts with adjacent units, are improperly understood.</p><p>Our studies form part of an ongoing reinvestigation of the tectonic evolution of the Yuli Belt. A revised geological map of the Yuli Belt was generated, incorporating own structural data from several river transects. Fieldwork and microstructural analyses suggest that the Yuli Belt was polyphasely deformed. Based on newly constructed cross sections we suspect that the blueschist-facies units were tectonically emplaced along thrusts on top of a mostly greenschist-facies metasedimentary unit that locally exhibits characteristics of a mélange. Later, both blueschist-facies and metasedimentary units were tightly folded, likely during the emplacement of the Yuli Belt onto the westerly adjacent Eurasia-derived Tailuko Belt along the so-called Shoufeng Fault. Lithological and fabric transitions across this fault are gradual, suggesting that the juxtaposition of Yuli and Tailuko Belts occurred during an early W-directed transport direction before becoming refolded during later E-vergent backfolding. Peak metamorphic temperatures in the greenschist-facies metasediments, estimated by Raman spectroscopic analyses of carbonaceous material (RSCM), reveal systematic spatial variations across the Yuli belt, supporting the idea of an allochthonous nature of the blueschist units on top of the lower grade metasedimentary unit.</p><p>The incorporation of published geochronological and whole-rock geochemical data and their combination with own paleogeographic reconstructions led us to fundamentally reinterpret the structural position of the Yuli Belt. We suggest that the blueschist-facies unit most likely represents a mid-Miocene fragment of oceanic crust and mantle issued in the South China Sea before having been subducted, exhumed and ‘sandwiched’ between the (Eurasia-derived) Tailuko Belt and the easterly adjacent Coastal Ranges derived from the Philippine Sea plate. The Yuli Belt should hence be considered to contain the suture between the Eurasian and the Philippine Sea plates.</p>