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Geology, gravity, and earthquake data are examined in a 200-km-wide zone along the Wasatch fault zone, the eastern boundary of the Basin and Range province in northern Utah. Gravity data define the geometry of crustal segmentation responsible for the major basin-and-range blocks. Locally, basin fill is more than 4 km thick, and minimum vertical offsets across the major normal fault zones vary between 3 and more than 5 km. East-west-trending transverse zones truncate individual basin-and-range blocks. Some zones are bounded by faults; along other zones the margins are gently downwarped. It has been suggested that the transverse zones delimit distinct, independent fault segments; however, at least one example of a major continuous zone of Quaternary scarps cross-cutting one of these zones has been identified. Preexisting structure apparently accounts for some of the transverse zones.

Major west-dipping normal fault zones and gently east-tilted range blocks coupled with widespread major Mesozoic thrust faults dipping gently to the west throughout the region favor a listric-fault or tilted-block model to account for the tectonic extension characterizing the area. However, simple models of extension are complicated by major east-dipping fault zones identified from the gravity data and by earthquake focal mechanisms that appear to rule out seismic slip on low-angle faults (dips < 30°). In addition, earthquake focal depths suggest that at least some faulting penetrates to depths within the Precambrian crystalline basement (depths greater than ~10 km).

Correlations between earthquakes, surface faults, and even major fault zones identified by the gravity data are poor. Well-studied earthquake sequences indicate that the major seismically active fault planes at depth trend obliquely to the surface trend of basin-range blocks and the faults that bound them.

Current extensional deformation across the region can be accounted for by an east-west to west-southwest-east-northeast least principal stress/strain axis. Analyses of slickenside data along major fault zones with young activity show a similar pattern of deformation extending back possibly 0.5 to 1.0 m.y. The east-west to west-southwest-east-northeast least principal stress/strain axis in northern Utah represents an approximately 30° counterclockwise rotation of deformational axes relative to most of the rest of the Basin and Range province.

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