Polyphase Deformation in a Fore-Arc/Back-Arc Basin, Salin Subbasin, Myanmar (Burma)

Abstract

The Salin subbasin of Myanmar (part of the larger Central basin) is a fore-arc/back-arc basin couplet situated between an oblique subduction zone to the west and a major right-lateral strike-slip fault to the east. Surface and subsurface expressions of folds and faults suggest that the basin experienced north-northwest-directed extensional deformation in the Miocene followed by east-northeast-directed Pliocene-Pleistocene transpressional deformation, resulting in a variety of structural styles, including thrust faults, oblique-reverse faults, strike-slip faults, and normal faults. Fault-propagation folds cored by west-dipping thrust faults in the basin center are located above steps in the top-of-basement surface (most likely fault controlled). Unconformities at the base of and within the Pliocene-Pleistocene synorogenic section indicate that the thrust faults were active during the Pliocene-Pleistocene. The southeastern region contains broad, north-northwest- trending uplifts and east-northeast-striking normal faults associated with thickened Miocene synorogenic deposits. Pliocene-Pleistocene sedimentary rocks lie unconformably above the Miocene section and are folded above the uplifts, reflecting Pliocene-Pleistocene compression. In the southern part of the basin, Miocene sedimentary rocks thicken dramatically over the 20°N uplift and the Yedwet uplift, suggesting that the north-northwest-striking faults that bound them represent Pliocene- Pleistocene inversion of Miocene normal faults. During the Miocene, the Burma plate acted as a fore-arc sliver coupled with the India plate, subducting obliquely underneath it, and moved northward relative to Asia along the Sagaing fault. Normal faulting and local basin formation took place at this time. When the northern part of the Burma plate collided with eastern Asia in the Pliocene, transpressional deformation predominated, creating thrust and reverse faults, positive flower structures, and inverted basins.