PALEOGENE RIFTING MECHANISM OF CENTRAL SUMATRA BASIN

• Main Author: HADIANA (NIM : 32008301); Tim Pembimbing : Ir. Benyamin Sapiie, Ph.D.; Dr. Agus Handoyo Harsol, MELI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/18903
• Institution: Institut Teknologi Bandung

Tectonis at Paleogene rifting in Central Sumatra Basin has been an interesting issue, due to western part of Sumatra is always located at compressional regime since Cenozoic. Several authors had different opinions about this issue. Some though that the rifting mechanism is a back arc tenssional proccess caused by mantle upwelling due to subduction. The others argue as the result of <br /> transtenssional fault possibility caused by the differences of plate movements since Pre-Tertiary to Eocene. The regional observasion result shows that the rift basin geometry mostly half graben with varying shapes and sizes, trending N-S and NW-SE, as well as systematic and interrelated pattens. Based on these <br /> conditions, it will very difficult to explain the rifting mechanism of Central Sumatra Basin, with the term of back arc tenssional process. Strike slip fault <br /> system can be explained such condiotions, but there must be supported special condition that controlled the above conditions. Analogue Sandbox Modeling used in this research, is one of the most effective method in understanding the mechanism of deformation process. In structural geology, modeling study with sandbox analogue has been used widely. That method can be used to modeled and investigated the structural features of the rocks that formed during the deformation, and also the 3D kinematic descriptions of stuctural evolution. Well documentations of the initial state before deformation <br /> can be compared to the state after deformation. Analogue sandbox modeling results is depend on the geological boundary condition. Subsurface geological mapping and restoration is implemented in this research to get the proper boundary condition. <br /> In order to subsurface geological mapping, 142 lines of 2D seismic line has been interpreted and 39 well data used to support the interpretation. This research, <br /> focused on North Balam, South Balam, North Aman and Rangau Sub Basin of Central Sumatra Basin. The subsurface geological mapping concluded that rocks <br /> grouped into pre-rift, syn-rift and post-rift tectonostratigraphic sequences. Pre-rift sequence consist of Pre-Tertiary rocks. Syn-rift sequence filled with Pematang Group consist of Lower Red Bed Formation, Brown Shale Formation and Upper Red Bed Formation. Whereas at post-rift sequence deposited Sihapas Group that consist of dari Menggala Formation, Bangko Formation, Bekasap Formation, Duri Formation and Telisa Formation. Structural map from seismic interpretation show the N-S and NW-SE structural pattern. Those pattern directions is consistent with pre-existing structural pattern of basement as the result of saturation of several continental plate. Restoration is one of techniques in structural geology that used to validate <br /> structural interpretation and determine its evolution. A restorable structure can be returned to its original, pre-deformation geometry with a perfect or near-perfect fit of all the segments in their correct pre-deformation order. Eleven regional 2D seismic lines has choosed in restoration. Oblique simple shear kinematics model is used to restore normal fault, whereas flexural slip kinematics model to restore inversion. <br /> Simultaneous extension model in dextral strike slip fault is applied in this analogue sandbox modeling setting. Based on that model, there are two major <br /> parameters involved in deformation which are NW-SE dextral strike slip fault and pre existing fractures at basement. Modeling result that rifting is southeast in <br /> direction generate NW-SE sub basin pattern, that corresponding to the result of subsurface geological mapping and its restoration. Those direction was controlled by uncontinuous boundary of basement. The modeling is also result another similarity, which is the presence of shape and size variations of formed rift basins, both in morphological and also vertical cross section. It is proved that based on analogue sandbox modeling, Central Sumatra Basin Paleogene rifting is result of Sumatran dextral strike slip fault that mainly controlled by pre-existing fractures or faults at basement. Based on analogue sandbox modeling strain analysis sustained 7.5% extension at the end of Lower Red Bed Formation deposition, 13% for Brown Shale Formation and 15% fpr Upper Red Bed Formation. Kinematics analysis at its morphology indicate there are rotation, translation and distortion during experiment. Another indication is there are varying changes in type displacement during (rotation/translation/distortion) during rift formation. Numerical modeling has conducted in this research to complement and enhance the credibility of analogue sandbox modeling results. Phase2software used based on finite element method. Generally, those two modeling (analogue sandbox and <br /> numerical modeling) result the similar basin evolution. The basin subsidence occur as the result of material displacement due to decolement as boundary of different physical property of material. The differences of those two modeling that are numerical modeling cannot ge form discontinuous plane at the same physical property of material.