SEISMIC REFLECTION CONSTRAINTS ON STRUCTURE AND TECTONIC EVOLUTION OF THE GREAT SALT LAKE BASIN, UTAH

Marine seismic reflection, borehole, and gravity data from the Great Salt Lake, Utah have been used to interpret the three-dimensional structure of the basin and the geometry its basin-bounding faults. The basin consists of two broadly asymmetric depocenters oriented NNW-SSE, with depths reaching as much as 4.8 km, that are bounded by two major Tertiary listric normal faults (the East Lake and Carrington faults). Depth-converted seismic sections show that the dips of the faults decrease from about 60º at the surface to about 10º-20º at depths between 4 and 7 km. Seismic interpretations further suggest that the Sevier-age Willard thrust may form the master detachment into which the Carrington fault (between Carrington Is. and Promontory Point) soles. Similarly, the East Lake normal fault (west of Antelope Island) soles into a footwall imbricate of the Willard thrust. Stratigraphic relations evident from seismic profiles, and radiometric dates from volcanic tuffs penetrated by exploration boreholes, yield evidence for a late Eocene sedimentary wedge unconformably overlain by a greater volume of Miocene and younger strata. These relations show evidence for at least two distinct phases of Cenozoic extension within the Great Salt Lake basin and that extension was well established by the late Eocene. Seismic and regional geologic evidence suggest there was at most a very short hiatus between the cessation of Sevier-age thrusting and onset of Tertiary extension. Palinspastic reconstruction of depth-converted seismic sections implies approximately 83% of Tertiary extension across the lake, compatible with extension amounts obtained from different parts of the Basin and Range Province. High fault slip rates in the Pliocene (1.1 mm/yr) and Quaternary (0.7 mm/yr) inferred from the seismic profiles, along with lake-floor bathymetric scarps and other data, imply that the East Lake and Carrington faults remain tectonically active.