| dc.description.abstract | Offshore southwestern Taiwan lies in the northern end of Manila Subduction System (MSS) where normal subduction progressively evolves into initial arc-continent collision. The tectonic configuration offers the area with challenges pertaining to the comprehension of subsurface imaging, deepwater stratigraphic development, gas hydrates systems, and associated thermal signatures. Two legacy 2D multichannel seismic data (MGL0905-10 and MGL0905-27), crossing the deepwater rifted Chinese continental margin to submarine Taiwan accretionary wedge, were re-processed to image the depth-domain subsurface. Together with age-controlled deep drilling, seismic stratigraphy and seismic facies analyses were utilized to redefine the stratigraphic development in the northeastern South China Sea (SCS). Additional 2D MCS time-domain data (MGL0905-05 and MGL0905-20) were included to better constraint the key stratigraphic correlation from the rifted continental margin. Six key horizons and eleven seismic facies were identified. A new chronostratigraphic column was established to explain the stratigraphic development in the study area. Paleogene fault-bounded half graben systems underlie the break-up unconformity, followed by late Oligocene shallow-marine sediments. Localized buried seamounts and sills developed during the early post-rift in late Oligocene to early Miocene. Deepwater canyons extensively developed in late Miocene, followed by the inception of arc-continent collision. Mass transport deposits (MTDs) and sediment waves deposited during the Pleistocene. In the distal rifted margin, arc-continent collision commenced in late Miocene, promoting the extensive development of fold-and-thrust belts and slope basins. Basal foreland unconformity (BFU) marks the transition from the underlying post-rift sequences to the overlying foreland sequences. As opposed to the rifted-margin domain, modern stratigraphy in the accreted slope basins of Taiwan accretionary wedge suggested isolated, hemipelagic-dominant sediments, and less of channel development. Splay fault cutting through the seafloor and the fault lying at the toe of steep slope with significant slope break, indicating that the splay fault is most likely an active fault. Decollement in the lower accretionary wedge is estimated at depths of 3 – 12 km beneath the seafloor with average angles between 4 – 6 degrees. Bottom simulating reflectors (BSRs) at depth around 300 – 600 m from the seafloor suggests active fluid expulsion controlling the gas-hydrate accumulations in the rifted margin and the offshore Taiwan fold-and-thrust belts. The convergence also promotes the development of mud diapirs and uplifted bedrocks in the Kenting Plateau of the upper accretionary wedge.
Regional seismic interpretation suggests dispersed distribution of BSRs and mud diapir in the upper slope domain, inferring pervasive gas-hydrate systems and associated free-gas distribution, including in the Quaternary Lower Fangliao Basin (LFB), a semi-enclosed slope basin in the upper wedge slope of Taiwan. BSRs are present to cross-cut both inclined stratigraphy and intruding diapirs as hinted from 3D MCS937 and 2D MGL0908-TST seismic data. The BSRs are interpreted to represent the basal phase boundary of the gas hydrate stability zone (GHSZ). 3D seismic attributes suggest that free gas and gas hydrates may be located above BSRs, and free gas below BSRs. Mud diapirs and associated faults may act as pathways along which thermogenic methane from a deep and as-yet unidentified source may migrate up into the GHSZ. First order volumes of free gas and gas-hydrates in place were estimated on the basis of geobody extraction, geophysical approximations, and Monte Carlo simulation and suggest 2,048 Bcf of total gas volume over a study area of 60 km2.
As the depth of BSRs are generally shallowing towards the accretionary wedge, the depth of BSRs in the LFB suggests anomalously deeper BSRs depth compared to other region in the upper wedge slope domain. The low geothermal gradient and heat-flow estimation, 33 oC.km-1 and 41 mW.m-2, respectively, suggest the role of thermal blanketing in controlling the present-day basin temperature. Closely spaced thermal probes and infrared imaging from piston cores revealed average values for geothermal gradients and heat flows of 55 °C.km−1 and 62 mW.m−2, respectively. Discrepancies between both measurements are related to the sensitivity of direct thermal measurements over shallow fluid flux, where shallow geothermal gradients increase locally as the fluid migrates upward. An array of data, including high-resolution seafloor bathymetry, seismic facies interpretation (from 3D MCS937, 2D MW9006-01, and 2D MW9006-02), and distribution of thermal anomaly, reveal that a paleo-Gaoping canyon had flowed through the LFB and deposited a stacked series of turbidite sands. Rapid deposition and sediment burial in offshore southwestern Taiwan had caused insufficient dewatering process in the paleo-Gaoping canyon sediments, leaving high water saturation within pore spaces and overpressured sediments. These, together, lead to lower heat flows and thermal gradients and contribute to deepen the base of GHSZ. Further mud diapiric intrusions and uplifting of seafloors had blocked the course of paleo-Gaoping canyon. The LFB was abandoned following the channel course shifted to the south along the present-day Gaoping Canyon course. | en_US |