LAKE BONNEVILLE SHORELINES AS TECTONIC TILTMETERS AND DISPLACEMENT DIPSTICKS

Highstand shorelines of Late Pleistocene Lake Bonneville are in close spatial proximity to much of the Wasatch Fault of north-central Utah. Detailed analysis of Bonneville-age shorelines reveals complex kinematic and displacement behavior of the Salt Lake City segment of the fault. Accurate (+/- 20 cm vertical) LiDAR-derived digital elevation data show maximum vertical fault displacement of 11.3-15.2 m over the past 16.3-18.5 kyr. In combination with published trenching data, this means that >60-90% of net vertical displacement has taken place in the past 6 kyr, supporting the theory that the unloading of Lake Bonneville has influenced Holocene Wasatch fault behavior. Calculated vertical slip rates of 0.66-0.93 mm/yr at the southern end and significantly lower slip rates (0.15-0.34 mm/yr) in the central portion of the segment are at odds with the “half-ellipse” model of normal fault segment displacement. The spatial variability of displacement along the fault may be related to strain uptake of the West Valley and East Bench faults in the central part of the Salt Lake Valley. Results demonstrate the utility of high-resolution LiDAR data for detailed understanding of fault geometry and displacement, provided meaningful datums can be established. The analysis also illustrates some of the difficulties in separating the relative effects of isostatic rebound and fault displacement in the eastern Great Basin.