Abstract

A review of selected subsurface data (mostly seismic-reflection data) from the Great Basin indicates that the basins form (1) as relatively simple sags associated with one or more major steep, planar normal faults; (2) as prisms above tilted bedrock ramps that are displaced by moderately to deeply penetrating listric normal faults; or (3) as assemblages of complexly deformed subbasins associated with sharply curving, shallow, listric faults and planar faults that sole in a detachment surface. Type 1 is interpreted to be associated with narrow zones of deep extension located directly beneath the structurally deepest part of the sags and separated from one another by 20 to 30 km. In type 2, the zones of deep extension are laterally displaced from the surface trace of the master faults by distances that depend on the radius of curvature of the major listric faults. The third type of basin forms by displacements on combinations of listic and planar faults as range-sized blocks separate above a detachment surface. There is no requirement for simultaneous extension below the detachment surface directly beneath basins and ranges of the third type. The locus of deep crustal accommodation may be laterally displaced large distances from surface fault traces, and its position depends on the dip of the detachment surface. The style of deep accommodation may be either rigid or plastic.

Historic, Holocene, or latest Pleistocene surface ruptures are associated with each type of basin. Indentification of the type of normal faulting associated with each basin, as well as with the province boundaries, is critical to accurate assessment of both earthquake hazards and resource potential.

The subsurface data indicate that, as basins mature, they grow larger by one or more of the following sedimentary or structural processes: (1) onlap of the downwarped basin-floor ramp by progressively younger strata, (2) lateral migration of the basin margin away from its axis by outward stepping to successively younger faults, (3) coalescence of early-formed subbasins by deactivation of their controlling faults, or (4) burial of transverse or longitudinal paleotopographic ridges. The data suggest that these processes can be associated with either the sagged faulted basins, the large homoclinally rotated basins, or the complex basin mosaics that overlie detachment faults.

A corollary to the aspect of basin enlargement seems to be that, as basins grow broader or coalesce, the deformation style becomes less complex. Early subbasins may display opposed stratal tilt directions either along strike or across separating horst blocks. Also, they may represent contrasting times and rates of basin formation. The complexity of early-formed parts of basins suggests equally complex extensional strain domains. These complexities are generally absent in the surface and shallow subsurface stratigraphic and structural relationships of the mature broad basins, indicating a broadening and simplifying of strain domains with time. The possibility exists that some of the proposed simplification results from sedimentary rather than structural processes. These evolutionary trends reflect not only a broadening of basins but also a broadening of the spacing between the faults that play an active role in basin development. They may be part of a pattern of cooling and strengthening of the crust with time. The possibility exists that, in some areas, extension on widely spaced, steep, deeply penetrating faults is a late-stage process that evolved from earlier (Miocene in many areas) large-magnitude extension on shallow listric faults, rotated planar faults, and/or detachment faults that developed near sites of strong thermal (magmatic) disturbance.

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