Atypical syn-collisional wedges at thrust fronts: Their tectonic and gravity-driven development, and interpretation-related controversies

Abstract

Examples of atypical syn-collisional wedges from offshore Sabah and Palawan, Indo-Burma Ranges, Gibraltar Arc, Banda Arc resemble accretionary prisms, but formed over subducted and/or underthrust lower plate continental crust in orogens and display changes in lithology, provenance, and structural development from the preceding accretionary prisms. The distal folds and thrusts of these wedges mark the thrust front of the orogen. These syn-collision wedges can be in the order of several hundred kilometers wide (in the transport direction), and > 8 km thick. The wedge is predominantly composed of sediments ‘accreted’ from the deepwater continental margin together with sediments deposited on top of the wedge. Initial wedge development is similar to the frontal regions of accretionary prisms, and dominated by closely spaced imbricate thrusts. The mud-prone (and occasional salt-prone), overpressured wedges flow in response to proximal loading by sedimentation to create synformal and/or growth-fault controlled shale withdrawal basins. The older syn-wedge deposits are incorporated into the wedge by burial, thrusting and folding. As tectonic shortening declines, later stage deformation by gravitational instability of the wedge becomes more important. Mud volcanism is an important consequence of structural activity and sedimentation. Wedges with this strong gravity-driven effect are referred to as being ‘sediment loaded’. Such syn-collisional wedges are interpreted here as representing late-stage orogenic shortening of a weak, overpressured stratigraphy deposited (mostly) on a thinned continental margin, subject to rapid loading by sediments primarily derived from the orogenic belt. The duration and preservation of this type of syn-orogenic wedge varies considerably within orogens, controlled by such factors as the width, nature of the thinned, lower plate rifted continental margin, the sedimentary thickness and distribution along the margin, the convergence rate, and most importantly the tectonic configuration of the orogen. The presence of loaded syn-collisional wedges indicates that the conditions for a classic foreland fold and thrust belt have not been met because: 1) the mechanical stratigraphy of the continental margin, or the continental margin geometry is not appropriate (e.g. Gibraltar Arc, Sabah-Palawan), 2) the region stopped deforming before that stage was reached (also the Sabah-Palawan), 3) the region is still deforming, but has not reached (and may never reach) that stage (e.g. Indo-Burma Ranges), 4) the presence of an unusual plate configuration (Gibraltar Arc). Sediment loaded syn-collision wedges provide important insights into upper crustal responses to tectonic settings, development of continental relief, and production of sediment at convergent boundaries.