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