INTRAPLATE EXTENSIONAL TECTONICS OF THE EASTERN BASIN-RANGE: INFERENCES ON STRUCTURAL STYLE FROM SEISMIC REFLECTION DATA, REGIONAL TECTONICS, AND THERMAL-MECHANICAL MODELS OF BRITTLE-DUCTILE DEFORMATION.

The general lack of a correlation between earthquakes and surface faulting in the interior of the western U.S. cordillera has motivated our efforts to evaluate the geometry, structural style, and mechanism of normal faulting characteristic of this region of intraplate extension. To address the problem, we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust. Rheological models of the continental crust were calculated to examine the possibility of shallow, quasi-plastic flow and its influence on faulting in the eastern Basin-Range, the western Colorado Plateau, and the Middle Rocky Mountains. Our data and interpretations have revealed the following styles of Cenozoic deformation: (1) steep- to low-angle dip, normal faulting along the Wasatch fault, (2) low-angle dip and listric normal faulting possibly associated with movement on preexisting thrusts, (3) the occurrence of asymmetric, mostly eastward tilted Tertiary basins that are bounded by low- to moderate-dipping planar and listric faults, and (4) at least three vertically stacked, en echelon low-angle reflections in the mid to upper crust that dip gently westward from ∼3 km beneath the Wasatch Plateau to over ∼15 km at the Utah-Nevada border; these reflections are interpreted as normal detachment faults. The structural style of the pervasive low- to moderate-angle dipping faults cannot be easily reconciled with classic brittle failure theory, but the interpreted termination of normal faults at or above the deeper low-angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip. An important observation, based on interpretations of seismic reflection profiles, is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits. Segment boundaries of the Wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the Precambrian Uinta aulacogen, suggesting that preexisting crustal structure partly controls the geometry of extension. To examine the influence of ductile deformation, quasi-plastic flow was modeled for appropriate geotherms of the Basin-Range and Colorado Plateau crusts. These models provide constraints on the depth to the frictional/quasi-plastic transition that occurs as shallow as ∼8 km in the eastern Basin-Range. This depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes (M 5.5) appear to nucleate at 10–15 km in or near the brittle/ductile transition. The rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region. Large-scale low-angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility.