SLAB ROLLBACK: THE DRIVING FORCE BEHIND THE COLLISION OF AN OCEAN TRENCH WITH THE PALEOZOIC PASSIVE MARGIN OF WESTERN LAURENTIA, LEADING TO THE ANTLER EVENT AND EMPLACEMENT OF THE ROBERTS MOUNTAINS ALLOCHTHON

Various tectonic processes have been proposed to explain the Middle Paleozoic Antler orogenic belt of southern Idaho, Nevada, and eastern California. My talk submits that modern analogs to the Antler allochthon exist where continental crust has encountered a subduction zone. Near the present-day Roberts Mountains of central Nevada, shallow water predominately carbonate sediments of Cambrian through Devonian age accumulated on the Laurentian continental shelf. This passive margin phase ended with the emplacement of imbricately thrust deepwater sediment atop continental-shelf carbonates. I interpret that the driving force for the Antler event was the process of slab-rollback, by which oceanic trenches can migrate for thousands of kilometers across plates of ocean crust and collide with passive continental margins. I show seismic examples from Western Australia and Eastern Indonesia, where subduction trenches consumed continental shelves. The continental margin gradually travels down a ramp into the trench, where it slides beneath an accretionary prism of imbricately thrusted deepwater sediment scraped from the continental slope of the former passive margin.

The age and travel distance of sediment in the prism varies as it builds-up progressively from forward-breaking thrusts. The youngest thrust at the prism’s base lifts all the older imbricates behind it. The result is that the youngest and least-far-traveled sediment in the allochthon is at its leading edge, while the oldest, most deformed, and most-far-traveled sediment is at the trailing edge of the prism. By this interpretation, the Antler allochthon is an accretionary prism bounded north and south by transform faults. The hypothesis is that the Antler event resulted from the chance encounter of a west-dipping subduction zone with the Laurentian continental shelf.

The arc need not impact the shelf perpendicularly; the collision was likely at some oblique angle. Impact angle along the arc length depends on local headlands or embayments on the continental margin. Timing of impact at any locality varied depending on the impact angle along the length of the trench, resulting in a succession of orthogonal to oblique to strike-slip tectonic events occurring over an extended period as the trench consumed the continental margin. Gravity was the force that ultimately drove slab rollback and allochthon emplacement.