Deep seismic reflection images of the Wharton Basin oceanic crust and uppermost mantle offshore Northern Sumatra: Relation with active and past deformation

Abstract

AbstractWe present deep seismic reflection images along two profiles collected in 2006 in the Wharton Basin offshore Northern Sumatra. The main profile is located subparallel to the Sumatran trench at a distance of 32–66 km. Faulting of the entire sedimentary section (strike‐slip deformation sometimes accompanied by a dip‐slip component) is imaged over two fracture zones of the extinct Wharton Spreading Center that prior studies have shown to be reactivated as left‐lateral faults. The western fracture zone is associated with a wide region of strong basement topography, a difference in crustal thickness of ~1.5 km, and an age offset of 9 Ma. The epicenters of the 11 April 2012 Mw 8.6 great strike‐slip earthquake, its Mw 7.2 foreshock, and Mw 8.2 aftershock align along this major structure > 100 km south of the profile intersection. Our high‐quality long‐offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of ~24 km into the oceanic mantle (~37 km below sea level). Apparent dips are mostly 25–35°, corresponding to 30–55° along either N‐S to NNE‐SSW or E‐W to WNW‐ESE directions, which encompass the directions of plate fabric and nodal planes of the Mw 8.6 event. We suggest that these enigmatic reflections arise from presently inactive dip‐slip fault planes reaching for the deepest ones to the base of the brittle layer. Possible origins include extension related to plate bending or an episode of now inactive thrust‐type deformation reactivating paleonormal faults, similar to that taking place in the Central Indian Basin.