LAKE ABERT HALF GRABEN: ESTIMATE OF BASIN FILL FROM GRAVITY PROFILING AND GEOLOGIC MAPPING

The Northwest Basin and Range is characterized by several major north-south trending normal faults from west to east, including: Winter Rim, Abert Rim, Hart Mountain, and Steens Mountain. The geometry and amount of tectonic separation along segments of these faults where offset is the greatest is hard to determine because they are buried by sediment filled basins. To better understand the geometry and displacement along the Abert Rim, ~60 gravity measurements have been collected with an L and R model G gravimeter at a station spacing of ~ 0.2 miles along the northern shore of the Lake. Modeling of the gravity data is achieved by using a software program called GM-SYS. Geologic mapping and samples collected for determining densities will help to constrain the model. Previous gravity studies from the Summer Lake basin (west of Lake Abert) and the Alvord basin (east of Lake Abert) show that valley fill is ~2-3 km thick. These studies use a density contrast of 0.4-0.6 g/cc between the valley fill and the basement (Cleary et al., 1981; Diggles et al., 1990). A preliminary interpretation of our gravity data shows a bouguer gravity anomaly of ~11-15 mGals from west to east across the Lake Abert basin. Gravity changes of 5-8 mGals equate to ~1 km of basin fill elsewhere within the Basin and Range Province. Simple models of the Lake Abert basin transect that use a maximum density contrast of 0.6 g/cc yield a minimum valley fill thickness of ~1 km. In contrast, if a minimum density contrast of 0.3 g/cc is used, then a maximum valley fill thickness of ~ 3 km is attained. These models are consistent with the amount of fill predicted by the eastward dip of units exposed in the hanging wall and data from previous studies. The estimate of basin fill at Lake Abert is smaller than that found in areas within the center of the Basin and Range Province. The small amount of basin fill likely reflects a smaller degree of extension within the margin of the province in southern Oregon.