Ripping and tearing the rolling-back New Hebrides slab

Abstract

The three-dimensional (3D) geometry of subducting lithospheric slabs has four-dimensional (4D) implications, i.e. in three dimensions plus time. To understand the implications of present 3D geometry, we need to consider what has gone before, what is happening now, and what will happen in the future. We illustrate this point by examining the 4D evolution of the New Hebrides slab, concluding that the Australian lithosphere tore as it began to subduct, and is still ripping in the present day. Southward motion of a north-dipping flap has been enabled as the result of westward propagation of an active rip, accompanied by southward foundering of newly created transform segments. Foundering was progressive but the rate appears to have been episodic. Additional transforms formed in the upper-plate in consequence. Subduction transform foundering is reflected by steps in the height of the subducted slab, and this needs to be taken into account so as not to significantly overestimate the area of subducted material. The principles illustrated are of general interest because foundering of subduction transforms may be a common occurrence in curved subduction zones worldwide.