Abstract

A paleoseismological investigation of flexural-slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the potential to generate earthquakes with damaging magnitude. The Carbondale collapse center, in the southern Rocky Mountains of Colorado, is a morpho-structural depression of ∼1200 km2 where Miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite-bearing evaporites. On the western margin of the collapse center, the debuttressing effect related to active evaporite dissolution drives unfolding of the steeply dipping late Laramide Grand Hogback monocline, accompanied by displacement on bedding-parallel faults. These flexural-slip faults rupture unconformable Miocene basalts and Quaternary mantled pediments, generating conspicuous half-graben depressions bounded by antislope fault scarps parallel to the underlying strata of the monocline. Two trenches dug across flexural-slip fault scarps developed in each stratigraphic marker (basalt cap, mantled pediment) revealed unexpected evidence of multiple late Quaternary faulting events (e.g., faulted colluvial wedge, sharp unconformities), with displacement-per-event values of ≥1 m. Three faulting events were inferred from the trench dug in the pediment (<32 ka, 32–28 ka, 5.6–1.5 ka), and four events from the trench sited in the basalts, all probably older than 20 ka. The probable length (∼25 km) and downdip width (∼7.5 km) of the flexural-slip faults associated with the Carbondale collapse center suggests that they might have the potential to generate damaging “unfolding earthquakes” with moment magnitude (Mw) around 6.

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