MORPHOLOGY OF THE GRANITE MOUNTAINS, WYOMING, AS AN ANALOGUE FOR PRE-EXHUMATION LARAMIDE RANGES

Our understanding of the Cenozoic tectonic and climatic history of the western U.S. hinge on interpretations of two phases of Laramide landscape evolution: 1) a period of declining local relief (basin-filling) that produced subsummit surfaces in the ranges; and 2) a subsequent period of exhumation in the basins and canyon cutting in the ranges. Numerical models of this second phase are hampered by the lack of direct data of pre-exhumational range morphology for initial model conditions. We posit that the morphology of the Granite Mountains in central Wyoming may be viewed as a modern analogue to Laramide range morphology prior to exhumation. Unique among Laramide ranges, the Granite Mountains remain partly buried by upper Cenozoic sedimentary deposits (Love, 1970) because the North Platte drainage system has little stream power to drive fluvial erosion in this region. The topography of the range – gentle sloping surfaces of regolith interrupted by well-rounded granitic knobs of order 100 m – is dramatically different from that of exhumed ranges, which have only sparse regolith cover and have high relief between peaks and bedrock canyons.

We use gravity measurements and borehole data to explore the morphology of the bedrock surface, specifically whether a buried subsummit surface exists and how much relief it displays. These data indicate that within 10 km north and south of the range crest, sedimentary deposits above granite bedrock progressively thicken away from the crest but are always less than 150 m thick. This bedrock topography is consistent with the presence of a buried subsummit surface that formed during basin filling, indicating that the morphology of exhumed Laramide ranges may have formed during or shortly after the Laramide orogeny. We infer that similar broad, low relief surfaces in exhumed ranges may represent the minimum level of fill of adjacent basins, and may underestimate the total basin fill level by on order 100 m. If the analogue holds, the modern ~100 m deep canyons in Laramide ranges were carved into crystalline basement over the last few million years, rather than during orogenic uplift. Based on the morphology of the Granite Mountains, we suggest that numerical models of late Cenozoic Laramide range evolution should incorporate an initial condition of a range with a low-relief subsummit bedrock surface without backfilled canyons.