AUV Surveys Reveal Seafloor Asymmetric Depressions and Linear Troughs Along a Fault Zone Offshore Southwest Taiwan
Date Issued
2014
Date
2014
Author(s)
Chen, Tzu-Ting
Abstract
In April 2013, a Taiwan-US collaborative recent cruise was conducted in the area offshore southwest Taiwan. Numerous (over 200) asymmetric seafloor depressions and a series of linear troughs were observed along a west-vergent fault zone on the west side of the Good Weather Ridge utilizing the mapping Autonomous Underwater Vehicle (AUV) of the Monterey Bay Aquarium Research Institute (MBARI). These asymmetric seafloor depressions are distributed over an area of about 1.03 km2 in the eastern part of the surveyed area between 1100 and 1200 m water depths. The zone of depressions occurs near the boundary between the smooth seafloor of sediment basin to the west and the outcrops of a series dipping exposed on a strata sloping seafloor to the east. The unprecedented high-resolution multibeam bathymetry (1 m lateral resolution) and chirp sub-bottom profiles (11 cm vertical resolution) reveal that the depressions are 1 to 8 m deeper than the surrounding seafloor and form comet-shaped scars of ~10 to 200 m length that widen downslope. Some circular depressions also occur, which would usually be described as pockmarks. Some asymmetric depressions have knickpoints within their scarps, which are horizontally aligned and suggested the existing of some weak planes. Chirp sub-bottom profiles also show horizontal reflectors, which are parallel with the exposed beds seen in the outcrops above, indicating that these depressions occur where the sediment cover over these beds pinches out. This study implies these asymmetric depressions relate to gas seepage, because the fluid possibly migrate along the lightly dipping horizontal strata and then blowout where seafloor cute the strata that generate these depressions. Furthermore, the side-scan sonar images found a small high amplitude anomalies area around the depressions, showing a strong reflection caused by authigenic carbonates. Seafloor where has authigenic carbonates become firm and roughness causing strong backscatter energy.
On the southern partner of the surveyed area, a series of north-south tending troughs occur within a 4.5 km long and 1.5 km wide zone in water depths 975 and 1450 m. Seismic reflection profiles running across the fault zone show that these troughs are developed on top of a small young sediment wedge at the toe of the hanging wall above the west-vergent fault. The sediment of this wedge is uplifted and folded due to the fault activities, and the observed young troughs could be the seafloor expression of the deformation. This study implies these troughs are related to the fault and block movement.
However, few areas have been surveyed at this resolution and thus we speculate seafloor asymmetric depressions and seafloor troughs like the descriptions above are not unique in this area.
Subjects
水下無人載具
超高解析度水深地形
好景海脊
逸氣構造
海床不對稱孔洞
Type
thesis
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