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.