structural geology, propagation mechanics and - Stanford School of ...

3.3. Deformation
The Aztec also has been subjected to a varied and punctuated history of deformation
driven by tectonic activity, starting with late Mesozoic compression of the Sevier
Orogeny (e.g. Armstrong, 1968; Fleck, 1970; Brock and Engelder, 1977; Bohannon,
1983) and jumping to Basin and Range extension beginning in Miocene time and
continuing to the present (e.g. Bohannon, 1983; Bohannon et al., 1993; Campagna and
Aydin, 1994; Flodin and Aydin, 2004; Myers and Aydin, 2004)). Figure 1.3 presents a
generalized structure and tectonic map for the region, as compiled from multiple sources.
Thin-skinned, east-vergent thrust faulting of the Sevier Orogeny may have initiated
along the flanks of the expanding magmatic arc to the west of the study area during
deposition of the Aztec, and progressively encroached on the Valley of Fire into latest
Cretaceous time (Burchfiel et al., 1992). In general, Paleozoic carbonate rocks were
placed atop Mesozoic clastic rocks along a piggy-backed sequence of low angle thrust
ramps, each one accommodating up to tens of kilometers of shortening (Armstrong,
1968). The easternmost extent of Sevier tectonism, at least insofar as expressed by
exposed thrusts, reached just up to the Valley of Fire (Figure 1.3), first with the thin,
short-traveled (kilometers) Summit-Willow Tank thrust, which placed lower Aztec atop
upper Aztec and as much as 600 m of Cretaceous deposits, then with the kilometers thick,
far-traveled (tens of kilometers) Muddy Mountain thrust, which placed Paleozoic
carbonates over Aztec riding atop the Summit-Willow Tank thrust (Armstrong, 1968;
Bohannon, 1983; Brock and Engelder, 1979; Carpenter and Carpenter, 1994; Longwell,
1949). The preponderance of stratigraphic, structural and diagenetic evidence indicates
that the Aztec sandstone now exposed in the main tourist part of the Valley of Fire State
Park (the primary study area) was never buried by either thrust sheet, only by the upward-
coarsening sequence of Cretaceous clastic deposits shed from them (Bohannon, 1984;
Taylor, 1999; Eichhubl et al., 2004; Sternlof et al., 2005), which consist largely of
reworked Aztec sandstone (Bohannon, 1983).
Structures formed in the Aztec sandstone that are associated with Sevier tectonism
include the north-northwest trending, steeply east-dipping CBs that are the subject of this
paper, and low-angle shear bands that tend to parallel depositional bedding and exhibit a
top-to-the-east sense of shear which mimics the large-scale thrust faults.
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