Chapter 8 The Eastern (Back-arc) Basin of Central Myanmar: Basement rocks, lithostratigraphic units, palaeocurrents, provenance and developmental history

Abstract

The Eastern Basin (Back-arc Trough) was initiated on the West Myanmar Block (WMB) and occupies the eastern half of the Central Myanmar Basin (CMB), between the Central Volcanic Line (CVL) and the Sagaing Fault. Sub-basins of the CMB share a similar origin, related to the collision between India and Asia. Field evidence shows that Upper Palaeozoic carbonates and Mesozoic ophiolitic sediments and their metamorphosed derivatives form the basement rocks in the eastern and northern parts of Eastern Basin (EB), while igneous rocks of the Wuntho-Salingyi Mesozoic Arc form the basement in the western portion of the basin. The basal Tertiary unit of the EB is the Lower Eocene Tonkyauk Chaung Conglomerate, overlying the eroded surface of the pre-Tertiary rocks in the Shwebo Basin (SBB). With the exception of the SBB, Oligocene strata have not been recorded in the Eastern Basin province. The ages of the Tertiary units in the Bago Yoma and Shwebo basins, assigned by Myint Thein (1966) and Myint Thein et al. (1982), are now supported by detrital zircon ages and palaeobotanical findings. More than 3000 measurements of cross-bedding azimuths reveal that the siliciclastic sediments were brought into the basin by southwards-directed longitudinal currents, causing the retreat of the sea. Recent research on U–Pb and Hf isotopic data of detrital zircons from the CMB has demonstrated that the Gangdese Batholith of the Lhasa Terrane in South Tibet was the dominant source during the Palaeogene. By the Early Miocene, terrains of the eastern Transhimalayan and intra-Myanmar Dianxi-Burma batholiths became important areas of provenance while the western section of the Sibumasu Block, with its S-type batholiths, played a secondary role and the Wuntho-Salingyi Magmatic Arc provided a relatively minor provenance. The Tertiary sequence of the CMB contains a record of filling of the foreland (orogen-parallel) basin by river-borne sediments; …