Structural controls on the formation of BSR over a diapiric anticline from a dense MCS survey offshore southwestern Taiwan

Abstract

a b s t r a c tA dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 Â 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between northesouth trending thrust faults and northwestesoutheast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a northesouth trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 C/km, increase toward 50e60 C/km in the slope basin and 55e65 C/km along the diapiric fold, and reach maximum values of 70 C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant eastewest shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements.

Discovery of numerous pingos and comet-shaped depressions offshore southwestern Taiwan

“…The accretionary wedge is considered to be a fluid expulsion prone region and is known to host pockmarks, mud volcanoes, fluid seeps, and authigenic carbonates ( Fig. 1) (Klaucke et al 2016;Lin et al 2006;Lin et al 2009;Schnürle et al 2011). Evans and Fischer (2012) suggest that fluid may migrate along dipping permeable strata within the fold-and-thrust structures.…”

BSRs, estimated heat flow, hydrate-related gas volume and their implications for methane seepage and gas hydrate in the Dongsha region, northern South China Sea

“…A number of studies have suggested that the occurrence of this methane seepage is related to subsurface gas hydrate (Li et al, 2013;Zhang et al, 2014a;Sha et al, 2015). Previous studies demonstrated that the tectonic and sedimentary conditions in the Dongsha region are beneficial for the formation and accumulation of gas hydrate (Song et al, 2000;McDonnell et al, 2000;Wu et al, 2004;Yan et al, 2006;Wang et al, 2006;Lin et al, 2009;Schnürle et al, 2011;Li et al, 2012aLi et al, , 2013. For instance, McDonnell et al (2000) indicated that marine sediments in the Dongsha region located in passive continental margin have appropriate-thickness, methane-generating potential to host gas hydrate.…”

Imaging P- and S-wave velocity structures in hydrate bearing sediments along an OBS profile across the Yuan-An Ridge, off southwest Taiwan

“…Cheng et al (2006) found westward thrusting high velocity wedges along several east-west OBS profiles and suggested them as a series of thrust faults. Schnürle et al (2011) estimated fluid and gas migration path and intensity for the southern part of the Yuan-An ridge and found that the maximum fluid and gas transport across BSR should occur below the crest of anticlines. Lin et al (2018) reported a relatively higher fundamental frequency of about 8 -9 Hz for the more rigid material, such as mud diapir and folding axes.…”