The Wetar Back Arc Thrust Belt, eastern Indonesia: The effect of accretion against an irregularly shaped arc

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Abstract
The 350‐km‐long, east‐west trending Wetar back arc thrust belt in eastern Indonesia is a result of the collision of Australia with the Indonesian island arc. Along the northwest margin of Wetar Island a short (50 km) section of the thrust belt trends northeast, coincident with the offset of the Indonesian arc by the newly discovered left‐lateral Wetar‐Atauro fault, which runs along the shelf region and trends northeast. The Wetar‐Atauro fault may be viewed as a large‐scale lateral ramp or wrench fault separating the eastern Wetar thrust belt from the western Wetar thrust belt. The interaction of strike‐slip faulting and back arc thrust faulting creates several arc “blocks” whose geometry strongly affects the structure of the deformed wedge of sediment accreting in the Wetar back arc thrust belt. The varying orientations of the arc “backstop” make the Wetar back arc thrust belt a perfect laboratory for the study of oblique convergence. Along most of its length the Wetar thrust belt parallels the arc slope. However, along the northeast trending offset in the thrust belt, SeaMARC II side‐scan sonar plus seismic reflection data show that the trend of the thrust belt is not parallel to the trend of the arc slope; rather, it is intermediate between the trend of the arc slope and the perpendicular to the expected convergence direction. The side‐scan images allow us to map the geometry of four main thrust faults: the Wetar, Liran, Atauro, and Alor faults. The Wetar, Liran, and Atauro faults trend northeast, parallel to the thrust front, and do not have an en echelon relationship. The westernmost fault in the survey area, the Alor fault, trends almost east‐west. The orientation of the Wetar‐Atauro fault is consistent with a maximum principal stress within the arc oriented between 8° and 23° west of north, depending on the internal coefficient of friction. This agrees well with the P‐axes of nearby earthquake focal mechanisms which trend consistently west of north, suggesting that the regional principal tectonic compression is oriented west of north. Because the thrust belt is oblique to the arc slope, we infer the structural directions are influenced by both the arc backstop and by the regional tectonic stress. The tectonic compression due to arc‐continent collision in this region may be modified by the arc geometry in several ways: (1) the stress necessary to support the arc topography may be significant close to the arc, (2) the change in thickness of the elastic part of the lithosphere may cause a concentration of tectonic stress, and (3) the difference in material properties between the relatively rigid arc “bulldozer” and the weak basin sedimentary fill may be an important factor in producing a thrust belt that conforms roughly to the shape of the arc. The structure of the Wetar back arc thrust zone demonstrates that the development of small rigid blocks within a major collision zone may produce complex structural patterns and local directions of shortening that are highly oblique to the main direction of plate convergence.