Extension-Related Structural Traps in Fault Basins of Eastern Nevada

C. Trevor Walker, J. G. Dennis, W. W. Lumsden

Low-angle younger-over-older faults have been widely reported in eastern Nevada, although no general agreement exists on their origin. Three preferred models are (1) local gravity sliding, (2) mid-crustal ductile extension, and (3) a master detachment possibly extending into the mantle. None are fully supported by field evidence. Some low-angle faults in the White Pine, Duck Creek and Schell Creek Ranges involve ductile extension along incompetent sedimentary units and brittle extension of intercalated competent units to form lenticular stretch structures. Upper crustal extension may be an indirect response to mid-crustal ductile extension that occurred during a Tertiary heating event.

Between the Duck Creek and Schell Creek Ranges, extension attenuated the pre-Carboniferous section, creating a depression in which Carboniferous rocks were preserved. Since this structure, termed a "pseudograben," can be traced northward into a typical graben, basins may be initiated by attenuation, with rifting occurring later. In such basins, subsurface detachments should be expected. Some low-angle faults resemble megalandslides because fragmented competent units slid into depressions created by attenuation. With retreat of the geotherms, ductile extension was confined to deep crustal levels and rifting replaced low-angle faulting at higher levels.

Potential extensional traps in basins are large-scale stretch structures, porous and permeable units truncated and sealed by detachment, and tilted blocks cut by steep faults. Best prospects are likely to occur in basins subjected only to Tertiary heating because oil generated at that time would be tapped in developing extension structures.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.