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Microtectonics and39Ar-40Ar dating of high pressure metamorphic rocks of the south Ryukyu Arc and their bearings on the pre-Eocene geodynamic evolution of Eastern Asia

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DOI: 10.1016/0040-1951(88)90287-9
  • Archaeology
  • Earth Science


Abstract The pre-Eocene metamorphic rocks found in the Yaeyama Islands (the southern part of the Ryukyu Arc) have disputed relationships with the metamorphic formations of Japan and Taiwan. They are divided into the Tomuru and Fusaki formations. 1. (1) The Tomuru Formation is composed of high pressure (HP) metamorphic rocks derived from oceanic sediments and the uppermost oceanic crust. The first deformation phase is characterized by a synmetamorphic NW-SE trending stretching lineation ( L 1). Kinematic analysis shows that the deformation corresponds to a subhorizontal ductile shear directed along L 1 from the northwest to the southeast. 2. (2) The Fusaki Formation or chaotic formation is an olistostrome of Triassic or Lower Jurassic radiolarite olistoliths. The age of the matrix is assumed to be late Jurassic. It is slightly schistose and only a S-vergent deformation phase is observed. It is assumed that the HP schists overthrust the Mesozoic olistostrome. 39Ar- 40Ar step-heating experiments have been carried out on phengite and Na amphibole from the Tomuru Formation. Phengite and zoned barroisite-crossite give well defined plateaus at 225 ± 4.8 Ma and 237 ± 6.3 Ma respectively. They are interpreted as the age of the ductile deformation. Crossite yields an integrated data of 104.8 ± 8.3 Ma, interpreted as the age of the thrusting of the HP schists above the chaotic formation. The lithostratigraphy, petrology, microtectonics and 39Ar- 40Ar ages of the HP schists and the chaotic formation are similar to the Permian Sangun HP schists and the Jurassic Tanba olistostrome of the Inner Belt of southwestern Japan respectively. There, the Sangun schists also overthrust the Tanba olistostrome during the Mesozoic orogeny. The same Tanba-like Mesozoic olistostrome is also found in Taiwan, but was intensely deformed and metamorphosed in Miocene times. The geodynamic collision model proposed to account for the late Permian to early Triassic orogen of southwestern Japan is extended to the South Ryukyu and Taiwan. Conversely, the Mesozoic Sanbagawa HP schists and the upper Jurassic to lower Miocene turbidites and olistostrome (Sanbosan-Shimanto zones) of southwestern Japan are neither exposed in the south Ryukyu Arc, nor in Taiwan. Hypotheses are discussed that call upon tectonic erosion or non-deposition to account for this absence.

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