Structural control of Mesozoic orogens on SE Asia Basin opening

International audienceA comparative study of basement structures, basins geometry and evolution, around SE Asia, highlights an intrinsic relationship between crustal inherited structures, and deformation through time from the Late Mesozoic to the Present. The present continental margin developed alongside of a large magmatic arc active from the Permian to the middle of the Cretaceous subsequently stretched during the Cenozoic. The Permo-Triassic crustal configuration was affected by a regional compression during the middle to late Cretaceous which appears to be mostly radial to the horse-shoe shape of the magmatic belt, and is best imaged by large regional shear zones, broad folds and densely distributed shear joints. A subsequent extension perpendicular to the margin followed in the Paleogene, which is firstly marked by an early stretching characterized by block faulting, dykes, and parallel extensional joint. We observed that the main necking zones follow former sutures, and the direction of opening is interestingly also perpendicular to them. The following thinning phase initiated with only minor discrepancies in the timing of initiation from E to W of SE Asia, marked by large crustal tilted blocks underlining a first thinning of the crust. This event was dominated by the occurrence of low angle normal faults bounding basins which capture most of the clastics derived from the formerly uplifted magmatic arc. These faults are frequently continent-dipping (counter-regional). Rooted into the lower crust, they often reactivate former thrust faults and plutons boundaries on the edges of the Mesozoic magmatic arc. Therefore, a link with the relaxation of the Late Mesozoic thickened crust is invoked. In a later stage, rifting developed across the outer edges of the previously thinned crust, outside the magmatic arc and up to the final continent-ocean boundary (COB of the South China Sea in the East and the Andaman Sea to the West). It is marked by distal low angle normal faults mostly ocean dipping (regional) that do not seem to be sensitive to margin fabric but more dependant to the thermal state and boundary stress conditions. Therefore, the pattern of basin structure and development seems conditioned by the intrinsic crustal configuration of SE Asia, specifically by the Triassic and Cretaceous orogenic structures (suture, crustal faults, and crustal thickness variations). The Late Mesozoic margins were indeed characterized by thickened crust which may have undergone collapse just before the onset of rifting sensus stricto. As a consequence, the general dip direction of these major crustal boundaries was often preserved during the Cenozoic extension that affected the marginal regions. Where the crustal thinning was strong enough, the fault plane dipped oceanward as a result from the crust/mantle relative motion. It is proposed that the rifting of the Cenozoic basins was triggered and conditioned by the relaxation of the orogens that assembled Sunda Plate in the past