Geosciences

The southeastern Conger Range, located in the Basin and Range province of west-central Utah, forms part of a large structural block bounded by convergently dipping regional detachment faults. The Conger Range antiform, characterized by a "pretzel-like" geometry, is interpreted to have formed by a series of backthrusting events propagating from ramped surfaces (Figure 1). Previous work suggests that the Conger Range is heavily faulted by high angle normal faults and two low angle detachment faults (Hose, 1977). The two major detachment surfaces were interpreted to have formed in a structural trough, causing relatively small blocks to form isoclinal and recumbent folds during transport.

However, the new structural models described herein provide a working interpretation that more effectively accounts for geometric constraints, and positions the Conger Range antiform into its current orientation. This interpretive model involves two deformational phases: 1) a Cretaceous compressional phase that is overprinted by 2) a later Tertiary extensional phase. Phase 1 east-vergent compression began to thrust the stratigraphy to the surface throughout much of Nevada and Utah during the Cretaceous. Prolonged compression resulted in exhumation of lower Cambrian units in the Canyon Range, hindering east-vergent thrusting. As a consequence, continued compressional stresses generated a series of west-vergent backthrusts that propagated sub-perpendicular to ramped surfaces, producing the Conger Range antiform. Evidence for at least two backthrusts in the eastern Conger Range is provided by two sub-parallel ridges of overturned stratigraphy expressing older-on-younger relationships. In addition, these backthrusts verge in opposite directions, offering an explanation of how two overturned stratigraphic sections can converge at depth (Plate 1).

Phase 1 compression is difficult to interpret because it is overprinted by Tertiary extension. This latter deformational episode caused small local blocks to break free and glide along low angle decollement faults into their current position (Figure 2). In addition, the Tertiary extensional phase is characterized by volcanic and lacustrine deposits located north and west of the study area, and rhyolitic tuffs located due north at TomÕs Knoll. These deposits were synchronous with extensional deformation and provide significant information about the timing of Basin and Range normal faulting. The major fault surfaces in the study area are not exposed, as is typical throughout the Conger Range, making it difficult to study and measure fault orientations. However, both backthrusting and decollemont surfaces are believed to be located beneath alluvium in the valley floor between the two ridges (Figure 1).

Plate 1: This cross section illustrates the complex deformational history of the southeastern Conger Range. First, Cretaceous thrusting formed a unique "pretzel-like" antiformal fold. A second period of extensional deformation overprints the antiform, dissecting the complicated structure.