Foreland flexural extension and salt diapir reactivation in oblique extensional systems.

摘要

The Timor foreland basin, as a young, under-filled foreland basin seated on the NW margin of Australia, formed in the Neogene as a result of the subduction-collision between Northwest Australia and the Banda Arc. It provides us a great opportunity to investigate flexural extensional structures in the early-stage foreland development. Based on detailed seismic interpretation and fault analysis using 2D/3D seismic data, this thesis shows that the timing of flexural normal faulting on the foredeep part of the basin is from late Miocene to present, which is generally consistent with the development of Timor orogenic belt. Using a new normal fault classification scheme proposed in this study, fault throw profiles indicate that the Neogene normal faults contain three basic components: newly formed fault, Jurassic fault reactivation, and growth fault. Results show that the Neogene faults usually initiate in the Miocene carbonate sections, while the shaly section in Eocene and Oligocene strata act as a propagation barrier zone. Three key factors are identified for controlling the distribution, propagation pattern, and linkage pattern of Neogene normal faults: 1) pre-existing Jurassic faults, 2) mechanical stratigraphy of post-rift strata, and 3) extension obliquity. These three factors can also be used to better understand the hourglass structures observed in the basin. This study also provides important insights on the flexural stresses, tectonic districting scheme, and 4D concept of foreland basin.;Within the foredeep part of the Timor foreland, I documented a geological phenomenon from Vulcan Sub-basin in NW Australia of a salt diapir reactivated in an oblique extensional system. This is arguably the first well-documented natural example that combines salt diapir reactivation and oblique extension. With a detailed analysis of the normal fault system, I characterized the oblique extensional system and investigated how the pre-existing structural fabrics and salt diapir controlled deformation and interacted under oblique extension. After comparison with forward modelling results and using constraints from geological evidence, our fault strike analysis indicates that Neogene flexural extension orientation in the oblique extensional system is around 347°, revealing a perpendicular relationship between extension direction and fault strikes from the inner deformation zone. The salt diapir, reactivated during Neogene extension, strongly influences local structure in the oblique extensional system by altering fault strikes, stepping patterns, fault zone width and fault density, leaving two less-deformed areas near the salt diapir as structural highs. These structures indicate a "stress/strain concentration" effect of salt diapir in extension due to its extremely low strength.;To further investigate the kinematics and mechanisms behind the deformation observed in nature, I also used scaled physical modeling techniques to simulate the evolution of salt diapir reactivation in an oblique extensional system. Eight models were divided into two groups based on the different diapir setups: 1) isolated diapir; 2) diapir with base source layer. Research results show that the diapirs in both cases, i.e. with or without source layer, all reveal deformation concentration effect during extension, including strain accumulation along near-diapir faults, fault strike deflection, and deformation zone width narrowing, along with less-deformed structural highs near the diapir. Different diapir roof subsidence histories from the two cases demonstrate the presence of source layer plays a major factor in controlling the diapir's subsidence behavior during the diapir reactivation under extension. As I observe in the natural example, that the diapir roof strata are higher than the surrounding graben areas, our modeling results demonstrate that the Swan salt diapir in Vulcan Sub-basin had an open feeder connected to a deeper salt layer in Neogene time, instead of being an isolated diapir during the extension.;The results of this study provide geoscientists or explorationists another perspective to explore new territories or revisit existing petroleum-bearing foreland basins. This study also has important implications and ramifications for the salt diapirisms, near-salt structures and oblique extensional systems. These are crucial for petroleum exploration and development activities in the Northwest Shelf of Australia and other sedimentary basins worldwide.