CONTINENTAL SUBDUCTION AS A MECHANISM FOR EMPLACEMENT OF THE ROBERTS MOUNTAINS THRUST IN NEVADA

The Antler orogeny was a Paleozoic tectonic event in Nevada that emplaced deep-water mudstones on top of shallow-water carbonate-platform sediments. The contact between the dissimilar rock masses, called the Roberts Mountains Thrust, was initially described in 1942, before the era of plate tectonics.

Later in 1981, J. Johnson of Oregon State U. hypothesized that the Antler orogeny involved subduction of continental crust at an oceanic trench. Johnson interpreted that the deep-water assemblage was an accretionary wedge while the carbonate assemblage was a continental-shelf carbonate platform of a passive continental margin. As the former passive-margin descended into the trench, the top of the shallow-water carbonate platform slid-under the accretionary prism. Mississippian fossils recovered from the Webb Formation established the age of emplacement of shallow-water carbonate beneath deep-water mudstone. Yet, continental crust is much less dense than ocean crust, so some workers questioned whether continental crust could become subducted into a trench.

Modern seismic imaging technology provides clear evidence for the descent of continental crust into subduction trenches. I compare the size and shape of the Roberts Mountains allochthon to Recent examples from eastern Indonesia and western Australia, where continental crust of the Australian plate is descending into the Banda Trench of Indonesia. Exploration wells demonstrate that a thick Tertiary carbonate platform is being subducted into the trench. A detachment surface marks the contact between underlying shallow-water carbonate sediments and overlying deep-water mudstones of an imbricated accretionary prism. Gravity drives everything. The carbonate platform is falling into the trench; nothing is actually “thrusted” on top of anything else. The greatest-principal-stress orientation is vertical, with the result that the descending plate undergoes normal faulting in an extensional domain.