Abstract

Stratigraphic studies show that widespread, albeit subtle, topographic uplifts have occurred throughout the U.S. Rocky Mountain region during middle and late Mesozoic time. Individual paleotopographic highs generally have wavelengths on the order of tens of kilometres and amplitudes on the scale of metres. In places, these features correspond to sites of previous or later deformation and so suggest the possibility of tectonic reactivation along pre-existing zones of structural weakness in the crust. Observations of modern features and modeling studies indicate that uplifts of small magnitude are both a common and expected result of changes in intraplate stress levels. Finite-element modeling suggests that broad-wavelength, low-amplitude topography can develop in a heterogeneous elastic plate under high-stress magnitudes along preexisting faults and under low-stress magnitudes where zones of high and low strength are juxtaposed. Although these features are small, they have sufficient topography to bring about large changes in paleocurrent patterns for low-gradient streams and significantly affect isopach patterns. Although subtle uplifts from this cause are difficult to predict, they are likely a common occurrence and may be the most dramatic features in an area that is otherwise tectonically quiescent. Thus, subtle uplift may be a background noise rather than a harbinger of regional orogenesis.

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