CAUSE OF TECTONIC REACTIVATION AND SUBTLE UPLIFTS IN THE ROCKY-MOUNTAIN REGION AND ITS EFFECT ON THE STRATIGRAPHIC RECORD

Stratigraphic studies show that widespread, albeit subtle, topographic uplifts have occurred throughout the U.S. Rocky Mountain region durin g middle and late Mesozoic time. Individual paleotopographic highs gen erally have wavelengths on the order of tens of kilometres and amplitu des on the scale of metres. In places, these features correspond to si tes 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 in dicate 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 pr eexisting faults and under low-stress magnitudes where zones of high a nd low strength are juxtaposed. Although these features are small, the y have sufficient topography to bring about large changes in paleocurr ent patterns for low-gradient streams and significantly affect isopach patterns. Although subtle uplifts from this cause are difficult to pr edict, they are likely a common occurrence and may be the most dramati c features in an area that is otherwise tectonically quiescent. Thus, subtle uplift may be a background noise rather than a harbinger of reg ional orogenesis.