The domal metamorphic core complexes form a belt which is oriented northwest-southeast (Figure 1). Recent models predict flow of the lower crust from areas of little to zero extension to areas of great extension to explain these areas of uplift in a highly extended terrain (Gans, 1987; Buck, 1988; Block and Royden, 1990; Kruse et al., 1991, Spencer and Reynolds, 1991, Wernicke, 1992). The models involve pressure gradients that are established due to differential thinning of the crust, in which areas with less extension experience a higher pressure at depth than areas with greater extension where pressure at depth has been alleviated as a result of denudation (Block and Royden, 1990). These differential pressure gradients produce flow of lower crustal material from areas of high pressure (less extension) into areas of low pressure (great extension) (Block and Royden, 1990). References
Numerous models have been proposed to explain the smaller scale extension-parallel synforms and antiforms which are superimposed on the uplifted metamorphic core complexes (Figures 1, 2 and 3). Most models involve development of these structures during early stages of extension (John, 1987; Davis and Lister, 1988; Spencer and Reynolds, 1991) or mid- to late-stage extension (Yin, 1991; Yin and Dunn, 1992; Mancktelow and Pavlis, 1994; Dorsey and Roberts, 1996). The authors relating extension-parallel structures to early activity of the detachment fault view the structures as primary corrugations (Spencer and Reynolds, 1991), or mullions (John, 1987; Davis and Lister, 1988) that originated as irregularities in the fault surface as slip was initiated at the breakaway fault. The mechanisms for formation of extension-parallel structures during later stages of extension involve folding as a result of northwest-southeast constriction due to lateral migration of the lower crust (Wernicke, 1990). Other models explain synextensional folding by a component of horizontal constriction normal to the extension direction due to stress fields at proximal shear zones involving strike-slip faults (Mancktelow and Pavlis, 1994; Yin and Dunn, 1992). References
Extension in the western Buckskin Mountains began at approximately 24 Ma and continued until approximately 14 Ma (Spencer and Reynolds, 1989a). The amount of extension is estimated at 66+8 km in the Buckskin Mountains (Spencer and Reynolds, 1989a; 1991). The extension direction is N50°E+10°, based both on mylonitic lineations oriented N40°E to N50°E (Shackelford, 1989; Davis, 1988; Spencer and Reynolds, 1989a) and extension-parallel fold axes oriented N55°E to N60°E (Spencer and Reynolds, 1989a). References
Recent thermochronologic studies in the Colorado River extensional corridor have helped constrain rates of uplift of metamorphic core complexes, cooling histories, and the angle of dip of detachment faults (Richard et al., 1990; Foster et al., 1993; John and Foster, 1993). These studies have consistently documented a pattern of cooling ages of lower-plate rocks younging to the northeast. This records the exhumation of the lower plate along a northeast-dipping detachment fault. The rate of movement of the detachment fault in the Buckskin and Rawhide Mountains has been estimated at 8.3 mm/yr (Foster et al., 1993). Implicit in these models are assumptions about the geothermal gradient at the time of extension, which is estimated to be 30-50°C/km (Foster et al., 1993). References
The presently exposed detachment fault in the Buckskin and Rawhide Mountains truncates upper plate normal faults and mylonitic fabric (Shackelford, 1989; Spencer and Reynolds, 1989a; Scott and Lister, 1992). This is suggestive of a process known as excisement which involves the transfer of slip from a lower detachment fault to a more recent detachment fault that cuts into the upper plate (John, 1987; Davis and Lister, 1988). This results in a section of upper plate being transferred to the lower plate of the new detachment fault (John, 1987; Davis and Lister, 1988; Spencer and Reynolds, 1989a; Scott and Lister, 1992). References
Our understanding of the upper-plate rocks in the region has been greatly enhanced by the contributions of researchers in the sedimentary basins of the Colorado River extensional corridor (Yarnold and Lombard, 1989; Nielson and Beratan, 1990, 1995; Beratan, 1991; Fedo and Miller, 1992; Yin and Dunn, 1992; Yarnold, 1993a; Dorsey and Becker, 1995; Dorsey and Roberts, 1996). Stratigraphic units northwest of the study area have been described by Nielson (1986), Nielson and Beratan (1990, 1995) and Dorsey and Roberts (1996) and subsequently classified in a stratigraphic scheme consisting of sequences I through IV (Nielson, 1986; Nielson and Beratan, 1990). Characteristics of the sequences in the Parker Dam section, which is closest to the study area, are summarized in Table 1. All sequences are bounded by unconformities, and in most cases they are angular unconformities (Nielson and Beratan, 1990). In brief, sequence I mostly consists of sedimentary units, generally contains megabreccia deposits and rests unconformably on pre-Tertiary upper-plate rocks. Sequence II consists of the Peach Springs Tuff which has been dated at 18.5 + 0.2 Ma based on 40Ar/39Ar methods on sanidine (Nielson et al., 1990). Sequence III typically contains older Tertiary clasts, including characteristic clasts of Peach Springs Tuff (Nielson and Beratan, 1990). Sequence IV consists of basalt flows, rhyolite flows and interbedded conglomerate (Nielson and Beratan, 1990). References
Part of this study involves attempting to place the stratigraphic section in the context of the stratigraphic sequence scheme of Nielson and Beratan (1990). Correlation of stratigraphy observed in the field area with stratigraphy to the northwest will allow for regional interpretations regarding extension. References