| Abstract |
- Detailed geologic mapping in the Mormon Mountains and
new geophysical data provide significant insight into
contractional and extensional tectonics in southern Nevada,
southwest Utah, and northwest Arizona. The rocks in the
region were complexly deformed during two distinct tectonic
episodes. Numerous interrelated events occurred within each
episode. The first tectonic episode, related to the Sevier
orogeny, was characterized by east-west crustal shortening
which culminated in thin-skinned decollement style folding
and thrusting during the Cretaceous. The Virgin-Beaver Dam
Mountains anticline, a Laramide-type basement-involved
uplift, represents the only thick-skinned contractional
structure in the region. The second tectonic episode,
related to basin-range rifting, was characterized by
east-west crustal extension which was accommodated by high-angle normal faults, with dips averaging 60 degrees, in
the brittle upper crust. In this area, basin-range rifting
initiated in the Oligocene and continued to Recent time.
Relations in the North Muddy Mountains in southern
Nevada suggest that the Muddy Mountain thrust sheet advanced
and overrode the Weiser syncline during the Cenomanian and
may have continued to advance in Turonian time. In the
southern Mormon Mountains, the Cambrian Bonanza King
Formation lies in the hanging wall flat position in thrust
contact with the overturned Petrified Forest Member of the
Triassic Chinle Formation at the footwall ramp. The thrust
sheet advanced eastward more than 30 km from the place of
origin. Thrust imbrication, and probably the formation of
hanging wall horses, likely occurred as the Muddy Mountain
thrust sheet encountered and ascended up the footwall ramp
zone (composed largely of competent carbonate rocks) where
slices of the thrust sheet (hanging wall horses) splayed of f
and accreted to the footwall ramp zone. A detailed
retrodeformable (balanced) regional structure section
suggests that fold-thrust shortening at the latitude of the
Mormon Mountains is a minimum of about 26%.
Extension-related structures overprint older
fold-thrust structures in the Mormon Mountains. The
west-plunging east-trending Candy Peak syncline is one of a
family of fold structures related to basin-range rifting.
The syncline formed in pre-Miocene time in association with
the northeast-striking Reber Mountain normal fault directly north and the northeast-striking Dry Canyon right-lateral
strike-slip fault directly south. The Tortoise Flat
synform, which lies southeast of the Dry Canyon fault,
developed in Miocene and possibly Pliocene time by
right-lateral flexure of early Miocene Horse Spring beds as
a result of drag associated with the Dry Canyon fault. The
Dry Canyon fault and the Tortoise Flat synform are
interpreted to be part of the right-lateral Moapa Peak-Reber
Mountain shear zone system in the southern Mormon Mountains.
Therefore, the time of formation of the Moapa Peak-Reber
Mountain shear zone system is pre-Miocene to possibly
Pliocene. The shear zone system formed in response to
different amounts of west-directed extension-related
movement of the hanging wall block of the high-angle Virgin
Beaver Dam Mountains fault, which initiated in the
Oligocene. From this, the timing of the Moapa Peak-Reber
Mountain shear zone, system is interpreted as Oligocene to
Miocene, and possibly Pliocene.
The interpretation of 261 km of seismic reflection
sections suggests that large-displacement high-angle normal
faults, typically with 60 degrees of dip, control horst and
graben structure and accommodate extension by simple shear
in the upper brittle crust. Such faults likely extend to
depths of 15 to 18 km. Below this depth extension is
thought to be accommodated by penetrative ductile
deformation. A detailed retrodeformable (balanced) regional structure section suggests that basin-range extension at the
latitude of the Mormon Mountains is about 17%.
The Virgin-Beaver Dam Mountains high-angle normal fault
is a large-displacement master fault in the area, having
more than 8,000 in of normal vertical separation at the
latitude of the Virgin Valley basin depocenter. Miocene
doming and uplift of the Mormon Mountains occurred in
response to displacement on the Virgin-Beaver Dam Mountains
fault. The Virgin Valley basin formed as the hanging wall
block downdropped, and the Mormon Mountains dome formed by
relative uplift at the opposite end of the hanging wall
block.
Half-grabens, and tilted, folded, and faulted range
blocks characterize basin-range crustal structure.
Depositional growth relations are interpreted in basins from
fanning-upward reflector geometry, and the wedge-shape of
Oligocene to Recent syntectonic basin-fill sediments.
Non-overlapping opposing east- and west-tilted half-grabens
compose the Meadow Valley-California Wash basin.
Seismic sections, gravity data, well data, and geologic
mapping demonstrate that the Mormon Peak, Tule Springs
Hills, and Beaver Dam/Castle Cliff "detachments," which were
thought to be rooted low-angle normal faults, do not exist.
The Mormon Peak and Beaver Dam/Castle Cliff low-angle normal
faults are denudational fault planes below gravity slid
masses. The widely distributed translocated Paleozoic
blocks, which were thought to be remnant pieces of large
hanging wall sheets ("extensional allochthons"), are
disjunct rootless gravity slide blocks of minor tectonic
significance. A large number of these rootless slide blocks
lie on Pliocene and Quaternary basin-fill deposits. The
Muddy Mountain-Tule Springs thrust, of Sevier age, was not
reactivated as a crustal penetrating Tule Springs Hills
low-angle normal fault, but is affected by small-scale
gravity slide features.
Rootless gravity slide blocks, secondary features to
high-angle normal faults, commonly occur from instability as
a result of the loss of lateral support induced by block
faulting and the associated erosion of range blocks.
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