The Andaman Basin Central Fault Zone, Andaman Sea: Characteristics of a major deepwater strike-slip fault system in a polyphase rift

Abstract

2D and 3D seismic reflection data from Thailand waters in the Andaman Sea have revealed a major NNE-SSW trending strike-slip fault (Andaman Basin Central Fault Zone, ABCFZ). This fault developed following a change from E-W to ENE-WSW extension during the Oligocene-earliest Miocene, to NNW-SSE transtension. The ABCFZ is interpreted to have formed subsequent to an extensional detachment translating the upper crust, including the extensional rift basins, to the west, leaving the ABCFZ to be developed on thinned, lower plate, lower continental crust. The ABCFZ was active during the Early and early Middle Miocene. A very extensive belt of ENE-WSW R′ shears up to 60 km wide, hundreds of kilometres in length (NNE-SSW direction), with faults spaced about every kilometer bounds both sides of the ABCFZ. R-shears occur more infrequently and predominantly occur close to the ABCFZ. The distinctive and unusual structural style of regionally dominant R′ shears is interpreted to be a result of NNW-SSE extension related to the northwards movement of the Indian Plate relative to SE Asia. The ABCFZ displays large-scale uplift (~100 km wavelength) on its eastern margin. If the uplift occurred at a restraining bend, then the nearest one requires at least 50 km of dextral displacement. Canyons mapped traversing this high provided pathways for gravity driven sediments to enter the deepwater area. The ABCFZ and the Sagaing Fault to the north are both associated with large basins whose depocentres lie parallel to the fault trace. They represent a different type of strike-slip basin from classic pull-apart basins, and are more laterally extensive than typical pull-apart basins. The East Andaman Basin margin shows regional variations in crustal thickness which have N–S trending contours related to Oligocene-age extension. The ABCFZ follows the location and trend of the necking zone within continental crust, suggesting this was a mechanically favourable location for development of the fault. The ABCFZ illustrates a type of strike-slip deformation and sedimentation pattern that can be expected on other hyper extended continental margins that have evolved from extension to highly oblique transtension.