POSTGLACIAL GEOLOGIC PROCESSES IN NORTHERN YELLOWSTONE LAKE

We use an extensive grid (~ 450 km) of high-resolution seismic-reflection profiles collected concurrently with multibeam bathymetric data to investigate postglacial geologic processes, features, and deposits in northern Yellowstone Lake. Sublacustrine geothermal features include the spectacular Mary Bay (20 x 10⁶ m³) and Elliot’s (12 x 10⁶ m³) hydrothermal explosion craters. Ejecta from Mary Bay is distributed uniformly around the crater whereas Elliot’s crater ejecta have an asymmetric distribution and form a distinctive, ~2-km-long, hummocky lobe on the lake floor. The highest concentration of hydrothermal vents and small domes is in and near explosion craters and along linear fissures. Geothermal features are nearly absent in the deep (>90 m) central lake basin. Four submerged shoreline terraces along the margins of northern Yellowstone Lake are probably younger than 7,000 yr B.P. and add to the record of postglacial lake-level fluctuations attributed to inflation and deflation of the Yellowstone magmatic body and overlying caldera.

The Lake Hotel fault cuts through northwestern Yellowstone Lake and is inferred to be part of a 25-km-long distributed extensional deformation zone. Three postglacial ruptures indicate a slip rate of about 0.27 to 0.34 mm/yr. The largest (3.0 m slip) and most recent event occurred in about the last 2100 years. High heat flow in the crust limits the rupture area of the Lake Hotel fault zone, however, future earthquakes of magnitude ~ 5.3 – 6.5 are possible. Earthquakes and hydrothermal explosions may have triggered landslides, common features around the lake margins.

Few high-resolution seismic-reflection surveys have been conducted in lakes in active geothermal areas. Our data provide important criteria and analogs for recognition of comparable geothermal features in other subaqueous volcanic and geothermal environments.