Permian–Triassic back-arc basin development in response to Paleo-Tethys subduction, Sa Kaeo–Chanthaburi area in Southeastern Thailand

Abstract

The Sa Kaeo Back-arc Basin in Southeastern Thailand developed behind the Sukhothai Arc along the western margin of the Indochina Block during the Permian to Triassic in response to the subduction of the Paleo-Tethys oceanic plate. To clarify its tectonic evolution of spreading, basin-infilling, and closure, we have investigated the provenance of its clastic sediments, the U-Pb ages of detrital zircons, the geochemistry of its basaltic rocks, and the structures in mélange. The rocks in the Sa Kaeo Back-arc Basin include two contrasting lithological units: the Thung Kabin Mélange, composed mainly of basaltic rocks, chert, limestone, and sandstone, along with mélange; and the Pong Nam Ron Formation, a thick succession of clastic sediments. Our observations suggest that the clastic sediments of the Pong Nam Ron Formation were deposited widely over the oceanic rocks (basaltic rocks, chert, and limestone) of the Thung Kabin Mélange. Geochemical analyses indicate that sandstones in both units (the Thung Kabin Mélange and the Pong Nam Ron Formation) are characterized by a provenance of mostly basaltic rocks. The U-Pb ages of detrital zircons from both units present similar age distributions: the youngest single-grain zircon (YSG) is early Late Triassic in age, the youngest cluster of zircons (YC1σ) is latest Permian to earliest Triassic in age, and there is a range of other Paleozoic and Proterozoic zircons. Based on these geochemical analyses and the U-Pb dating, the sandstones of both units had the same provenance. In addition, the basaltic rocks of the Thung Kabin Mélange are originated from back-arc basin basalts. These new observations have allowed us to reconstruct the tectonic evolution of the basin. In the Early Permian, spreading took place in the basin accompanied by basaltic magmatism. Subsequently, corresponding to an inactive period of the Sukhothai Arc into the Middle Triassic, the Sa Kaeo Back-arc Basin subsided, and a deep-sea environment prevailed with the formation of Permian cherts, Permian limestones, and Middle Triassic cherts. During the Triassic, the basin was filled by clastic sediments and eventually closed. The clastic sediments were sourced directly from the back-arc basin basalts, as well as from the felsic volcanic rocks of the active Sukhothai Arc (which yield Permian to Triassic zircons), and from the continental materials of the Indochina Block (Paleozoic and Proterozoic zircons). After the closure of the Sa Kaeo Back-arc Basin, the discussed area was affected by the termination fault-fan related to the Mae Ping Fault Zone.