Rocky Mountain Section - 72nd Annual Meeting - 2020

Paper No. 19-3
Presentation Time: 2:10 PM


DUNHAM, John B., Union Oil Company of California, Retired, Los Angeles, CA 90017

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 originally 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 shallow-water carbonate platform basically slid-under the base of the accretionary prism. The timing of emplacement of carbonate below deep-water mudstone was constrained by Mississippian fossils recovered from the Webb Formation which overlaps the toe of the thrust. Yet, continental crust is much less dense than ocean crust, so some workers questioned the possibility that continental crust could be subducted into a trench.

Modern geophysical data provide clear evidence for descent of continental crust into subduction trenches. My presentation is based on work in eastern Indonesia and Australia where this process is taking place at present. I will compare the size and shape of Recent subduction zones to the Roberts Mountains allochthon. Modern seismic sections offer compelling analogs for this interpretation. Continental crust of the western Australian plate is descending into the Banda Trench of Indonesia. Exploration wells document a thick Tertiary carbonate platform that is being subducted into the trench. A basal detachment surface marks the boundary between underlying shallow-water carbonate sediments and overlying deep-water mudstones in an imbricately thrusted accretionary prism. Everything is driven by gravity. Nothing is really “thrusted” on top of anything else.

Though this has always been considered a thrusting process with implied horizontal compression as the driving force; the real driving force is gravity. The principal stress therefore has a vertical orientation and the result is that the dominant tectonic activity affecting the descending plate was extensional.