KILOMETER-SCALE ISOCLINAL FOLDING WITHIN THE SEVIER HINTERLAND IN EASTERN NEVADA: INSIGHTS FROM NEW MAPPING IN THE PANCAKE RANGE

In eastern Nevada, which lies within the hinterland region of the Jurassic-Paleogene Sevier thrust belt, upper-crustal contractional deformation has generally been interpreted as gentle and low-magnitude. However, published reconnaissance-level mapping in the central Pancake Range in east-central Nevada has provided preliminary evidence for a large-scale, NNW-trending fold, the McClure Spring syncline (MSS), with steep limb dips. In addition, Cretaceous sedimentary rocks have been mapped in the hinge zone of the MSS; however, there is disagreement in previous studies as to whether they were involved in folding. Therefore, the central Pancake Range provides an important opportunity to investigate the geometry, magnitude, and timing of contractional deformation in the Sevier hinterland. To illustrate the geometry of the MSS, and to analyze field relationships that constrain the timing of its construction, we performed 1:24,000-scale geologic mapping in the central Pancake Range. Mississippian-Permian sedimentary rocks in the limbs of the MSS are exposed for a minimum along-trend distance of ~14 km and a minimum trend-normal distance of ~4 km before becoming concealed beneath Paleogene volcanic rocks. The eastern limb dips ~45°W and the western limb is overturned and dips ~40°W on average, defining an asymmetric, east-vergent, isoclinal syncline. We estimate that construction of the MSS accommodated at least 3.4 km (44%) of east-west shortening. Gently E-dipping Cretaceous sedimentary rocks overlie the eastern limb of the MSS across an angular unconformity with ~75° of dip difference, indicating that folding pre-dated their deposition. This brackets construction of the MSS between Permian and Cretaceous. Both limbs of the MSS are unconformably overlain by Paleogene volcanic rocks that dip ~20°E on average; this tilting is attributed to regional Neogene normal faulting. Retro-deformation of this tilting defines pre-extensional dips of ~60°W for both limbs of the MSS. The large-scale isoclinal folding recorded by the MSS contrasts with the more common style of open folding observed across much of eastern Nevada. Therefore, our results highlight the variability of styles and magnitudes of Cordilleran upper-crustal contractional deformation in the Sevier hinterland.