A Geomorphic Framework for Interpreting Continental Interior Mountain Belt Exhumation: The Laramide Example

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

ABSTRACT A Geomorphic Framework for Interpreting Continental Interior Mountain Belt Exhumation: The Laramide Example Robert Anderson and Elizabeth Safran EAR-0003604 Between about 80 and 50 Myr ago, the Laramide orogeny uplifted a series of high, thrust fault-bounded mountain ranges that form some of the most dramatic mountain landscapes of the continental interior. These ranges and the intervening sedimentary basins stretch across much of Wyoming, Colorado, Montana, Utah, and South Dakota. Interpretations of the tectonic and climatic history of the entire western U. S. hinge on assumptions about Laramide landscape evolution, and yet this evolution remains ill-understood. The debate centers on the significance, timing, and cause of two geomorphic episodes, evidence for which is recorded in a suite of distinctive Laramide landforms. Widespread, high-elevation surfaces of low local relief capped by narrow bedrock spines record a period of declining local relief in most of the Laramide ranges. A subsequent period of regional erosion produced canyons in the ranges and broad, deep depressions in the basin fills, punctuated by numerous river terraces. Previous researchers have tried to correlate the development of these landforms with regional episodes of tectonic or climatic forcing. However, the Laramide landforms apparently do not reflect external forcing in a simple way, because limited dating suggests that they vary in age. This finding has hindered the construction of a regionally coherent geologic history, and to date no process-based model has been put forth to explain it. This study will address geomorphic mechanisms by which erosional activity might have been triggered at different times throughout this interior mountain landscape. PIs hypothesize that even a simple forcing event would produce complex erosional patterns, given a set of interlocking ranges and basins linked by an integrated drainage network. They will quantitatively address this hypothesis, focusing on the exhumational episode that shaped the Laramide landscape during the late Cenozoic. Their approach will involve: 1) improving constraints on the timing of Laramide basin exhumation by using cosmogenic radionuclides to date landforms and deposits of fluvial origin within the basins; 2) improving constraints on pre-exhumation range morphology by characterizing the geomorphology of the Granite Mountains, which they view as a modern analogue to the ancient condition of other Laramide ranges; and 3) developing numerical models of both regional and mountain range-scale landscape evolution to provide a mechanistic framework for data interpretation.