The Great Basin Province of the western United States is generally characterized by mountain blocks that trend northerly and are bounded by normal faults. The mountain blocks may be characterized according to whether they tilt to the east or west. Three structural zones that separate domains of east- or west-tilted blocks trend easterly across much of the Great Basin Province (Stewart, 1980a). These structural zones must accommodate contrasts in the direction, magnitude, or rate of strain due to the differences in regional structural vergence across the zones. For that reason, we define the structural zones as “extensional accommodation zones.” We interpret the distribution of Quaternary faults to show that the accommodation zones may play a controlling role in limiting the lengths of belts of range-bounding Quaternary faulting. We infer further that the accommodation zones are not singular structural features in themselves but, rather, are a series of rupture barriers generally aligned with the regional extension direction. The structural development of a barrier through repeated earthquake rupture requires decreased tectonic throw on ruptures within the barrier relative to tectonic throw on strike-contiguous faults bounding mountain blocks. This concept is consistent with patterns of Quaternary scarp heights in the Central Nevada Seismic Zone and requires that the barriers have persisted throughout a long period of geologic time. Parts of the accommodation zones generally coincide with boundaries between fault domains that exhibit homogeneous faulting histories within the coarse temporal resolution of the Quaternary fault data base. Along other parts of the zones, the accommodation of contrasts in extensional strain may occur on times scales beyond the temporal resolution of the regional fault data base.

In the western part of the Province, future earthquake rupture in a 40-km-long gap bounded on the south by the Tobin Range scarps of the 1915 Pleasant Valley earthquake and on the north by the northernmost of the three Great Basin accommodation zones would complete a belt of historical fault rupture between the central and northern accommodation zones. We define this area as the Sonoma Range Seismic Gap.

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