TECTONIC GEOMORPHOLOGY AND LATE-QUATERNARY UPLIFT IN BEAR LAKE VALLEY, UTAH AND IDAHO

Bear Lake valley forms a complex graben in the eastern Basin and Range province, a region that has apparently undergone extension due to relative movement of the North American plate and the Yellowstone hotspot. Previous workers have suggested that active normal faulting in this region has migrated outward away from the Snake River plain (southward in Bear Lake valley) during the Quaternary. The morphology of range fronts bordering Bear Lake valley was examined to determine patterns of Quaternary normal faulting on the east and west Bear Lake faults. Three segments were identified on the east Bear Lake fault and two on the west Bear Lake fault based on discontinuities in strike, height, slope angle, and sinuosity of range fronts. Increasingly straight and steep mountain fronts to the south on the east Bear Lake fault suggest that the focus of normal faulting has shifted southward with time. Slip rates on the east Bear Lake fault were estimated using ages of offset lacustrine deposits dated by amino acid and calendar-year calibrated radiocarbon geochronology. Emergent shoreline deposits have been identified 8, 11, and 25 m above modern Bear Lake and represent highstands of the lake at ca. 15, 40, and 130 ka, respectively. These deposits reside in areas of minimal or no normal faulting, and correlate to similar-age, uplifted lacustrine deposits on the footwall of the southern part of the east Bear Lake fault. Differences in the elevations of similar-age, offset lacustrine deposits yield minimum slip rates of 0.33 - 0.52 m/ka since ~130 ka, 0.43 - 1.05 m/ka since ~40 ka, and average slip rates of 0.81 - 1.22 m/ka since ~15 ka. The average slip rates since 15 ka compare favorably with the results of trenching studies of the southern east Bear Lake fault. The southward change in morphology of the east Bear Lake fault and increasing slip rates on its southern segment since ~130 ka support the theory that normal faulting has migrated from north to south in Bear Lake valley.