Building Mountains In Old Continental Crust

September 19  “Building mountains in old continental crust: examples from the northern Rockies and the central Andes” Presented by David Pearson, Idaho State University.

The Wyoming Range is very different than the roughly comparable age mountains across the State (Wind Rivers, Big Horns, Snowy Range, etc.). Why is this? Dave Pearson’s talk will explore possible reasons for these dramatically different mountain ranges.

Earth’s largest active and ancient mountain chains occur adjacent to convergent plate boundaries. As a result of plate convergence, major mountains form within fold-thrust belts as the Earth’s crust is shortened by folding and stacking of rocks. Continental fold-thrust belts present major hazards and host prolific petroleum and mineral deposits. Most plate boundaries where mountain building occurs are prior sites where continental plates were rifted and overlain by thick accumulations of sedimentary rocks. Though most fold-thrust belts roughly mimic the first-order shapes of these older sedimentary deposits, many mountain belts are characterized by along- and across-strike changes in thrust geometry and the style of deformation. In the major fold-thrust belts of North and South American, two major types of deformation are the focus of this study: “thin-skinned” fold-thrust belts that involve primarily sedimentary rocks, and “thick-skinned” fold-thrust belts that involve deformation of basement rocks. This talk will summarize recent work in two prominent mountain ranges where these two fundamentally different types of mountains occur: 1) the northern Rocky Mountains of Utah, Wyoming, Idaho, and Montana, where the thin-skinned Sevier fold-thrust belt occurs west of and adjacent to the thick-skinned Laramide fold-thrust belt; and 2) the southern central Andes of Bolivia and northern Argentina, where the thin-skinned Andean fold-thrust belt occurs west of thick-skinned faults and folds of the Sierras Pampeanas. In both regions, many researchers have hypothesized that the changes in the style of mountain building are linked to spatial variations in the dip angle of the subducting oceanic plate. However, recent work points toward an alternate control: the distribution of mechanically weak sedimentary layers prior to mountain building. Where these weak layers were present prior to mountain growth, high-magnitude crustal shortening occurred in thin-skinned fold-thrust belts, resulting in some of the largest mountains on Earth. In contrast, in the absence of these weak sedimentary rocks prior to mountain building, older weak zones within basement rocks were instead reactivated, forming generally lower elevation and irregular mountains. Thus, this work will explore the hypothesis that many aspects of the geometry and style of modern and ancient mountain belts were controlled by geologic events that long pre-date their formation.  Video