NEW INSIGHTS INTO THE STRUCTURAL DEVELOPMENT OF THE BLOCK A AREA, NORTH SUMATRA BASIN: CONSTRAINTS FROM SUBSIDENCE ANALYSIS AND PALINSPATIC RECONSTRUCTION

摘要

Located in the hinge line between the Sundaland block extrusion and India-Eurasia Plate collision, the North Sumatra Basin (NSB) exhibits a complex basin characteristic which considerably differs from its neighboring back-arc basins. Hence, to evaluate basin structures and evolution in Block-A, a preliminary approach from subsidence analysis has been performed. The result shows that after Paleogene rifts and Lower Miocene thermal sags, the basin experienced a further subsidence marked by rapid sedimentation of hemipelagic deposits reaching up to 550 m/Ma. This phenomenon caused confusion since massive subsidence during the "post-rift" period corresponded with global relative sea level fall. Due to this issue, detailed structural analysis was then re-evaluated to understand the relationship between structural styles and development associated with accommodation space creation as well as to validate NSB tectonostratigraphic evolution. Furthermore, evidence from surface geological maps and subsurface data near Barisan Thrust-Front show the presence of southwest-dipping thrust faults with a total of 2km vertical offsets over 10km of distance. Additionally, palinspatic reconstruction, which resulted in 17 % rate of shortening, demonstrates the occurrence of uplifts probably associated with northeast-ward encroachment of Barisan orogeny. The extensive exhumation was also aggravated by westward relative-movement of Sundaland continental crust transforming previous underfilled-flysch basin into overfilled-molasses basin. In terms of accommodation space creation, the growth of Barisan Mountains has led to the development of foredeep depozone due to massive orogenic loads which are confirmed by southwest-ward thickening of deep-water Baong Fm. This paper proposed a possible flexural mechanism on the structural development of the southern NSB during Early to Middle Miocene which is significantly different compared to the previous traditional model of merely post-rift relaxed basin. More importantly, the result of this work is useful for validating parameters required for basin-scale petroleum system characterization (e.g. heatflow, structural entrapment, seal breaching and erosion, subsurface pressure and migration pathway).