Extensional reactivation of a thrust ramp and implications of deformation in the Confusion Range, west central Utah
Jean Pierre Dubé
The Conger Range fault (CRF) is exposed for 3.6 km along the west edge of the Conger Range, 20 km southwest of Conger Mountain in the Confusion Range. This fault preserves evidence of both contractional deformation during the Sevier orogeny, and extensional reactivation during early Tertiary time, possibly associated with exhumation of the Snake Range metamorphic core complex. The CRF places Permian Kaibab Limestone in the hanging wall on Silurian Laketown Dolomite in the footwall, omitting about 3500 m of section. The fault strikes northeast and dips northwest under Snake Valley at an angle of 25°, as indicated by orientations at the base of the hangingwall and along low-angle erosional surfaces. The fault zone contains chaotic fault slices of otherwise omitted upper Paleozoic units.
The structural block to the east of the CRF consists of southeast-dipping Ordovician to Devonian carbonate rocks that deformed as a rigid block creating an anomalous topographic and structural high in the Conger Range. To the west a distinctly different structural block consists of complexly folded carbonate and clastic rocks of Mississippian to Traissic age, exposed as low-lying hills and pediment outcrops.
We interpret lower Paleozoic rocks east of the CRF to be part of a ramp anticline formed in the hangingwall of an east-vergent thrust during the early Cretaceous (early Sevier) as the thrust sheet moved from a lower detachment in the Ordovician Pogonip Group to an upper detachment in the Mississippian Chainman Shale. The position of the abandoned thrust toe indicates 7 to 8 km of thrust displacment. Above the rigid thrust sheet, the upper Paleozoic units deformed in a more ductile fashion developing complex tight folds. West-vergent folding in these unites may have developed during late Cretacious backthrusting associated with deep burial of the Snake Range core complex. Early Tertiary (Oligocene?) extensional reactivation of the thrust ramp propagated upward through the thrust sheet, leaving the east limb of the earlie formed ramp anticline as an anomalous block. Stratigraphic offset suggests 7 km of west-side-down displacement on the CRF during this extensional phase. the 25° dip on the fault suggests that the CRF intersects the east-dipping Snake Range decollement at a depth of 3 to 4 km beneath Snake Valley.