Recent sedimentary processes and neotectonics in the frontal orogenic wedge,off southwest Taiwan.

Wei-Zhi Liao1

Abstract 

The frontal orogenic wedge off southwest Taiwan is characterized by rapid sedimentation, erosion, and tectonic uplift. A large amount of geophysical data indicates the prevalent existence of gas-hydrate beneath the seafloor. The interactions between the processes of sedimentation, erosion, and tectonic uplift achieve a dynamic equilibrium system in which gas hydrates are preserved in the strata. In order to understand this system, we use chirp sonar and reflection seismic data to interpret the processes of sedimentation, tectonic activities, and BSR distributions off southwest Taiwan.

Seismic data reveals that the frontal orogenic wedge is dominated by a series of folds cored by blind thrusts in the west and an array of emergent thrusts in the east with the first emergent thrust, R5.1 structure, separating these two structural domains.

Chirp sonar data provide high-resolution stratigraphic configurations (i.e., echo types) of up to ~50 m beneath the seafloor. Through a joint analysis of seafloor topography and echo types of chirp sonar data, we recognized six echo types in the study area. They are distinct type, indistinct type, steep-slope type, slide-slump type, ridge type, and hyperbola type, each corresponding to various sedimentary and erosional processes. These echo types indicate that major erosional processes occur in the drainage of the Penghu canyon as well as at the hangingwalls of emergent thrusts, whereas deposition takes place in rest of the areas.

There is a correlation between subsurface structures, topography and the areal distribution of bottom simulating reflectors (BSRs) in the study area. High concentration of BSRs is found along the bathymetric ridges formed by folding and thrusting. BSRs are more prevalent in the east domain dominated by emergent thrusts, while the west domain with folds cored by blind thrusts only found scarce BSR distribution especially along bathymetric ridges.

We have computed the magnitude of heatflows using subsea depths of BSRs. Our analysis indicate that higher heatflow of ~60mW/m2 exists in east domain. By contrast, lower heatflow of 40mW/m2 is found in the west domain. There is an area in the west domain characterized by anomalously low heatflow of ~30mW/m2, which we interpret to result from the sediment blanking effect due to rapid sedimentation. Our BSR-derived heatflow pattern also correlated well with published direct measurements using heat probes.

Our analysis shows that the existence of gas hydrates in the sediments of orogenic wedge is in a state of dynamic equilibrium between sedimentation/erosion and tectonic uplift.

 

Reference 

Liu, C.S., Schnurle, P, Wang, Y.S., Chung, S.H., Chen, S.C., and Hsiuan, T.H, 2006, Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan. Terr. Atmos. Ocean. Sci., 17(4), 615-644.

(Abstract) (Full text)

Lee, S.H., Chough, S.K., Back, G.G., Kim, Y.B., 2002, Chirp(2-7 kHz) echo characters of the South Korea Plateau, East Sea: styles of mass movement and sediment gravity flow. Marine Geology, 184, 227-247.

(Abstract) (Full text)