Q. 91: How do overthrusts occur and folded mountain ranges arise?

CPT Q. #91 – 101 Q&A on Catastrophic Plate Tectonics

Question: 91. Do overthrusts and folded mountain ranges represent features that conventional geology cannot explain? Walt Brown is convinced that overthrusts are not possible. I see problems with the conventional view of how mountain rock layers got to be folded. (It is claimed that the layers were once at great depth, but how and why would sedimentary rock ever be at great depth?) Please provide your view on these issues.

Response: Let me begin by discussing overthrusts. Overthrusts are real. As an example, I would point to the Keystone Thrust just west of Las Vegas, Nevada shown in the photograph below, which I am reproducing from my answer to question 30. The Keystone Thrust represents a horizontal displacement of a sheet of rock by several tens of kilometers. Most of the displacement occurred in the late Cretaceous when the Farallon slab was subducting horizontally beneath western North America.

“The Keystone thrust west of Las Vegas, Nevada, is a spectacular example of a thrust fault. The dark-gray Cambrian limestone of the Bonanza King Formation on the left moved to the right and over the pink Jurassic Aztec Sandstone in the center. Editor’s picture source, jpg”

Below is a cross-sectional view of the Keystone thrust sheet, some 13,000 feet in thickness. Along its eastern edge, its basal layer of Cambrian Bonanza King Limestone now lies on top of Jurassic Aztec sandstone, which is some 15,000 feet higher in the regional sediment sequence.

East-west cross section from just west of Las Vegas, Nevada, showing the Keystone Thrust Sheet, interpreted here as a gigantic gravity slide feature.

Below is a simplified version of this same cross section.

Cross section of the Keystone thrust, Red Rock Canyon area, west of Las Vegas, Nevada.

Below is another aerial photo showing the fault contact a bit better than the previous color photo.

Aerial photo looking toward the northwest showing the Keystone fault contact between the overlying dark Bonanza King limestone and underlying light-colored Aztec sandstone in the Red Rock Canyon area west of Las Vegas, Nevada.

A simple analysis shows that pushing such a massive sheet of rock from the side is not a viable possibility. Rock simply does not have sufficient compressive strength to support the edge forces required. The only type of force capable of accomplishing such a mechanical feat is a body force. The main two candidates are gravity and inertial deceleration. Gravity sliding in response to tectonic deformation of the earth’s surface seems to be the most plausible choice. The figure below attempts to illustrate how gravity sliding in response to tectonic uplift of the western margin of North America associated with the rapid subduction of the Farallon Plate may have produced the complex pattern of imbricated fault sheets and spectacular folding observed, for example, in the Canadian Rockies.

The violence associated with the rapid subduction of ocean plate beneath the continent likely included a temporary but dramatic uplift of the continent edge causing sections of the recently deposited sediments to collapse downslope along curved faults. Sections of the sediment sequence tens of miles wide slid downslope for tens of miles along the entire western edge of North America as shown in this cross section through the Canadian Rockies.

This is how I would account for such features that are moderately common in the world’s tectonic belts.