GRAVITY ANOMALIES AND ALLUVIAL FAN AREAS OF THE LOST RIVER VALLEY AND IMPLICATIONS FOR BASIN ARCHITECTURE AND THE DIP OF THE LOST RIVER FAULT

This study reports new gravity data and models that provide a three-dimensional perspective on the basin geometry of the Lost River Valley. Our primary goals are to use gravity data and models to constrain the dip angles of basin-bounding Lost River Fault and to constrain the thickness of basin sediment fill. The complete Bouguer gravity anomaly shows tremendous variation from SE, on the Eastern Snake River Plain (ESRP), to NW adjacent to the Borah Peak surface rupture. While some of this variation is due to crustal-scale density anomalies of the ESRP, here we highlight gravity anomalies due to basin structure and fill. We find that fault dip varies moderately from one fault segment to the next along the length of the Lost River Fault (LRF), from 50° SW in the Thousand Springs fault segment to the Arco fault segment with a dip of 22° W. Significant variations in basin sediment thickness also occur along valley fault segments, with the thickest sediment fill (on order 2500-3000m) in the Thousand Springs segment and sediment thickness shallowing near the Mackay reservoir and near the town of Arco. Some of this change in sediment thickness may be due to variations in fault displacement and some variation may be due to the presence of the Eocene Challis volcanics present between basin sediments and the Paleozoic basement. The results of a study of alluvial fan areas suggest that half graben basins at the southern tips of the Lost River and Lemhi ranges are among the fastest subsiding parts of these two systems. Conversely, fans of southern Beaverhead Range suggest slower subsidence than all segments to the north. We argue that the faster rates in the southern Lost River and Lemhi systems reflect an addition component of ongoing flexural subsidence related to isostatic adjustments caused by the presence of a dense mid-crustal sill in the adjacent ESRP.