SLAB ROLLBACK INDUCED THE COLLISION OF AN INTRA-OCEANIC ISLAND ARC WITH THE PALEOZOIC PASSIVE MARGIN OF WESTERN LAURENTIA, LEADING TO THE ANTLER EVENT AND EMPLACEMENT OF THE ROBERTS MOUNTAINS ALLOCHTHON

Western Laurentia existed as a passive margin through to the Late Devonian. At 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 sediments atop continental-shelf carbonates. This talk aims to illustrate how slab rollback could explain this event. Gravity is the force that powers slab-rollback, by which intra-oceanic island arcs can migrate for thousands of kilometers across plates of ocean crust and ultimately collide with passive continental margins. I show seismic examples from Western Australia and Eastern Indonesia, where subduction trenches consumed continental shelves. The continental margin progressively travels down a ramp into the trench, where it slips beneath an accretionary prism of imbricately thrusted deepwater sediment scraped from the slope of the former passive margin. Slab rollback is real; recent GPS measurements prove that it proceeds at plate-tectonic rates of centimeters per year. The hypothesis is that the Antler event may have resulted from the chance encounter of a west-dipping intra-oceanic subduction zone with the Laurentian continental shelf. Modern intra-oceanic arcs are on the order of 1000 km long. Impact timing and approach angle may vary along this trench length depending on the geography of local capes or bights along the continental margin. The result was a series of tectonic events developed over multiple time periods, seemingly from different directions, as the Laurentian continental margin gradually fell westward into the trench.