The West Andaman Fault: A complex strain‐partitioning boundary at the seaward edge of the Aceh Basin, offshore Sumatra

Abstract

Along the Sumatran subduction zone, oblique strain is partitioned between the Sunda megathrust and the strike-slip Sumatran Fault located near the volcanic arc. Additional strike-slip motion may localize adjacent to the forearc basin system, forming the Mentawai and West Andaman Fault Zones. An alternative interpretation of the West Andaman Fault, as primarily a backthrust, has also been proposed. We combine a 2-D multichannel seismic survey (SUMUT) acquired aboard the R/V Sonne in 2008 with previous bathymetry and seismic surveys to characterize the West Andaman Fault adjacent to the Aceh forearc Basin. Two-dimensional Kirchoff prestack time migration of SUMUT profiles provides the clearest images to date of this portion of the West Andaman Fault. In these images we interpret inward dipping fault segments along the West Andaman Fault indicative of a transpressional flower structure. A reflector at the base of the Aceh Basin sediments (believed to be base Neogene in age) continues trenchward of the fault zone for >15 km and has geometry inconsistent with the geometry expected if a major backthrust were to daylight at the edge of the basin. Therefore, we conclude that the West Andaman Fault is predominantly strike-slip in nature. As such, the West Andaman is likely part of a system of faults including the Sumatran Fault that accommodate the significant shear component of strain in this oblique subduction zone. Additionally, the location of the West Andaman Fault within the forearc may be controlled by the rheologic contrast between the marginal plateau and the forearc basin.

What 2‐D multichannel seismic and multibeam bathymetric data tell us about the North Sumatra wedge structure and coseismic response

“…We processed the MCS data through poststack time migration, with targeted areas reprocessed through prestack time migration. Further processing detail is presented in Gulick et al [] and Martin et al []. The BGR MCS data have been presented by Berglar et al [], where each seismic profile is processed through poststack Kirchhoff time migration.…”

“…We interpret the West Andaman Fault along the western/seaward edge of the Aceh Basin (Figure a) [ Sibuet et al ., ; Martin et al ., ] . Within the Aceh Basin itself, blind faults appear to crosscut an acoustically strong reflector at ~6 s. Berglar et al [] and Martin et al [] interpret this strong reflector as the acoustic basement of the Aceh Basin.…”

“…Piggyback basins (Figure a: CDPs 37,700; 26,000; 24,500; and 19,000 and Figure b: CDPs 24,500; 21,500; and 14,000) are observed scattered across the plateau; in all cases, fanning of infilling sediment suggests syntectonic deposition and continuing compression of fill within these basins [ Sibuet et al ., ]. We interpret the West Andaman Fault along the western/seaward edge of the Aceh Basin (Figure a) [ Sibuet et al ., ; Martin et al ., ] . Within the Aceh Basin itself, blind faults appear to crosscut an acoustically strong reflector at ~6 s. Berglar et al [] and Martin et al [] interpret this strong reflector as the acoustic basement of the Aceh Basin.…”

“…Profile BGR06‐112 crosses SUMUT‐01 at CDP 15,600 (Figure c). Figure exhibits seafloor depths ranging from ~4500 m near the trench (Figure a: CDP 10,000; Figure b: CDP 10,000; and Figure c: CDP 1) to ~600 m (Figure a: CDP 25,400 and Figure b: 34,750) within the prism and locally only ~270 m (Figure c: CDP 26,300) near the Tuba Ridge [ Malod et al ., ; Curray , ; Mosher et al ., ; Martin et al ., ]. In Figure (profiles BGR06‐105, BGR06‐112, and SUMUT‐01), we interpret the predécollement to be at depths ranging from 8.2 to 9 s [ Dean et al ., ]; near the trench, we interpret “blind” normal faults extending down to the predécollement.…”

Evidence for slip partitioning and bimodal slip behavior on a single fault: Surface slip characteristics of the 2013 Mw7.7 Balochistan, Pakistan earthquake

“…We also explore the long-term behavior of the Hoshab fault recorded by geomorphic indicators, drawing on previous observations of fault zones worldwide that find normal, reverse, and strike-slip fault systems produce distinctive geomorphic signatures in the landscape (e.g., McCalpin et al, 1996;Burbank and Anderson, 2001). Specifically, we use the high-resolution pre-and postearthquake optical imagery to map geomorphic features that indicate past strike-slip (offset stream channels, beheaded and laterally offset alluvial surfaces, linear/axial ridges and valleys, linear fault scarps, shutter ridges, sidehill benches) and reverse-slip (uplifted and abandoned surfaces in the hanging wall, scalloped/corrugated fault scarps and traces, basin-stepping fault traces, basin-facing surface warps and scarps) motions and for their spatial distribution along the Hoshab fault.…”