Temporal distribution of strain in the active Banda orogen: a reconciliation of rival hypotheses

Abstract

Integration of geological and geophysical data from the active Banda orogen reveals important variations in structural style with time that reconcile rival hypotheses for the tectonic evolution of the Banda arc. These variations indicate the temporal distribution of strain in the collision zone by vertical and horizontal structural restorations of the collision through time. In the western part of the collision (Sumba and Savu Islands) most plate convergence occurs at a very high strain rate within 20–40 km of the deformation front. This narrow zone of frontal accretion expands eastward toward Savu where it is part of a submarine accretionary pile over 150 km wide. In West Timor the accretionary wedge is internally shortened and emergent. Uplift of the forearc upper plate during this deformational phase causes its leading edge to detach and form nappes. The remainder of the forearc is overridden along backthrusts by the orogenic wedge. Various sites of intra-wedge shortening and some strike-slip motion may account for as much as 70% of plate convergence in West Timor. The distribution of plate convergence away from the deformation front into more internal parts of the collision zone is interpreted as a function of increasing frictional resistance at the base of the orogenic wedge. Resistance to orogenic wedge advance is attributed to an abrupt increase in basal decollement slope, decrease in pore-fluid pressures and increase in strength of incoming material. The combination of continued plate convergence and increased coupling in East Timor cause the wedge to move almost entirely with the lower plate along arc-directed backthrusts and backarc thrusts. Backthrusting eventually closes the Banda forearc basin entirely. These lateral variations in structural style along the strike of the Banda orogen are attributed to the kinematics of oblique collision, and structural and stratigraphic discontinuities of the Australian lower plate. Possible relationships between different structural styles and the temporal distribution of strain in the organic zone is constrained by structural restorations of the collision. The restorations demonstrate that each of the three different models (imbricate, overthrust and upthrust) used to characterize the structure of the western Banda orogen may be applied at various times during the arc-continent collision.