Structural Controls on the Formation of Bottom-simulating Reflectors Offshore Southwestern Taiwan from a Dense Seismic Reflection Survey

Abstract

A dense seismic reflection survey with a 400-m (1312-ft) line spacing has been conducted in a 14- by 16-km (9- by 10-mi) area offshore southwestern Taiwan where bottom-simulating reflectors (BSRs) are highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north-south–trending thrust faults and northeast–southwest oblique ramps exist in this region, which impacts on the distribution and seismic characteristics of the BSR. A clear BSR is observed almost continuously in the southeastern half of the survey box, whereas BSRs either appear as small patches or are absent in the northwestern half. The reflection coefficient at the BSR presents abrupt lateral variations, with an average value of 0.094 or 32% of the reflection coefficient at the sea floor. A pattern of seismic blanking beneath the ridge crests overlying a high-amplitude BSR with bright reflections below the BSR in the slope basins reveals the significance of tectonics and sedimentation of the formation of BSRs. Local heat-flow values estimated from BSR subbottom depths show different ranges across the northeast–southwest-trending Yung-An lineament (YAL), with higher heat-flow values ranging from 55 to 70 mW/m2 in the southeastern half of the survey area, whereas the heat-flow values range from 45 to 55 mW/m2 to the northwest of the YAL. In the southeastern part of the survey area, an elongated diapiric feature deforms the sedimentary layers, BSRs occur shallower, and high heat-flow values are inferred. Furthermore, local dips of BSRs and sedimentary strata that crosscut the BSR at intersections of any two seismic profiles have been computed. A dominant N308 stratigraphic updip direction characterizes the anticlinal ridges; stratal dips fan out toward N285 south of the ridges and toward N330 north of the ridges. The concentric patterns of steep stratigraphic dips constitute favorable migration paths for the upward transport of fluids with significant dissolved-gas content across the BSR, which correlate well with the high gas volumes sampled in this area.