[1] We present the interpretation of newly acquired high-quality industry-standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the Mentawai Fault Zone (MFZ). The MFZ lies along the boundary between the accretionary wedge and the proposed continental backstop. This zone exhibits arcuate ridges on the seafloor, convex toward the east. Beneath these ridges the structures developed as landward-vergent imbricated backthrusts in the inner part of the accretionary wedge and higher-angle backthrusts that deformed the forearc basin sediments. In the forearc high, anticlines were developed due to the seaward-vergent forearc high thrusts originating in the accretionary wedge. The imbricated backthrusts may have initiated during the Early-Middle Miocene contemporaneously with the slide and back-rotation of forearc high thrusts. In the Late Miocene, the higher-angle backthrusts were initiated. Continuous contraction induced the frontal higher-angle backthrusts and formed a fold-thrust belt toward the east during the Pliocene. The folds and thrusts were disturbed by diapirs and mud volcanoes. Backthrusting and fold-thrust belts developed in the MFZ may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern Sumatra margin. The backthrusts along the MFZ are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small.
The Mentawai segment of the Sumatra subduction zone is locked and likely to produce a large earthquake in the near future. A part of this locked zone ruptured on 12 September 2007 producing twin earthquakes of Mw = 8.5 and 7.9. Recently, a third earthquake of Mw = 7.8 occurred on the 25th October 2010, SW of Pagai Island, Sumatra. The earthquake generated an unexpected very large tsunami on Pagai Islands with run‐up height of up to 8 m. Here we present seismic reflection and bathymetry images from the 2010 epicentral region acquired before the earthquake. We find that the frontal thrust is the main active fault in this region and might have ruptured up to the seafloor at 6 km water depth uplifting the water column and producing a large tsunami. Furthermore, finite fault models indicate that this earthquake ruptured the frontal section of the subduction zone, which is generally believed to slip aseismically and be incapable of producing large earthquakes. The presence of aftershocks near the subduction front further confirms that the frontal section of the subduction zone is not aseismic. If the rest of the Mentawai locked zone ruptures the frontal section of the subduction zone during a megathrust, then the resulting tsunami in the Indian Ocean could be devastating.
The microstructures of turbiditic and hemipelagic muds and mudstones were investigated using a scanning electron microscope to determine whether there are microstructural features that can differentiate turbiditic from hemipelagic sedimentary processes. Both types of muddy deposits are, in general, characterized by randomly-oriented clay particles. However, turbiditic muds and mudstones also characteristically contain aggregates of 'edge to face' contacts between clay particles with long-axis lengths of up to 30 µm. Based on observations of the clay fabric of the experimentally-formed muds settled from previously agitated muddy fluids, these types of aggregates, hereafter referred to as 'aggregates of clay particles', are interpreted to have been formed by the collision of component flocs in turbulent fluids. Furthermore, some aggregates of clay particles have 'face to face' contacts between clay particles; this is similar to face to face aggregates characteristically developed in fluid-mud deposits that are commonly recognized only in turbiditic mudstones, indicating the possibility of a final stage of deposition under highly-dense conditions, such as temporary fluid muds. In conjunction with earlier proposed lithofacies-based and ichnofacies-based criteria, aggregates of clay particles should be useful for the differentiation of turbiditic and hemipelagic muddy deposits, particularly with limited volumes of non-oriented samples from deep-water successions.This article is protected by copyright. All rights reserved.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.