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The Geometry and Mechanics of a Hanging Wall Anticline, Grand Canyon, Az

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The Geometry and Mechanics of a Hanging Wall Anticline, Grand Canyon, Az THE GEOMETRY AND MECHANICS OF A HANGING WALL ANTICLINE, GRAND CANYON, AZ. Phil Resor, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 e-mail: [email protected] Abstract anticipate this will provide the foundation for a new The western Grand Canyon provides a rare and more complete mechanical investigation into the opportunity to directly observe normal faults and formation of hanging wall structures associated with associated structures over large vertical cross continental normal faults. sections with nearly complete exposure. Detailed Geophysical observations of subsurface normal mapping and mechanical modeling of these faults suggest that deformation associated with these structures can improve our understanding of faults is localized in the hanging wall while the deformation associated with normal faults and footwall block remains relatively undeformed. This thereby improve our ability to infer hydrocarbon asymmetric distribution of deformation (Fig. 1) is migration pathways and to predict the occurrence of commonly interpreted from seismic reflection hydrocarbon traps in regions of continental profiles from extending sedimentary basins (e.g. the extension. GPS measurements of deformed Gulf of Mexico (Shelton, 1984)) and inferred from sandstones within the upper Esplanade fm. are used aftershock distributions associated with major normal to evaluate both kinematic and mechanical models. fault earthquakes (Ellsworth, personal Inclined shear, a widely used kinematic model, can communication). Most explanations for the reproduce the observed hanging wall fold shape with development of these structures, associated with fault geometries that are permitted by field normal faults, are purely kinematic and include the a observations. This model, however fails to predict priori assumption that only the hanging wall deforms. the observed footwall deformation. Simple elastic Thus, the mechanics of deformation associated with dislocation models can fit both footwall and hanging normal faults, and the asymmetry of resulting wall shapes and are consistent with field structures is still poorly understood. observations that suggest nearly planar high-angle faults to depths of ~0.5 km. 3D fold geometry and field studies of secondary deformation patterns (joints and small faults) may help to further evaluate these models by constraining the relationship between slip magnitude and fold shape and strain magnitudes and orientations, respectively. Finally, more complex mechanical models need to be evaluated to understand how elastic strains may relax, leading to an apparently “elastic” fold shape preserved over geologic time. Introduction Knowledge of the geometry of individual normal Figure 1. Examples of hanging wall faults, normal fault systems and secondary structures deformation. a) Cross section view of aftershocks is important for assessing migration pathways and (gray circles -scaled relative to magnitude) and structural trapping of hydrocarbons in areas of crustal fault interpretation from the 1995 Kozani-Grevena extension. The development of rollover (reverse- earthquake (M 6.5), Greece. Aftershocks are drag) anticlines is particularly important for generally concentrated in the hanging wall even development of large-scale traps. It is generally thought the fault appears to be relatively straight assumed that rollover is associated with concave over the depth range of the aftershocks upward (listric) faults. This relationship is well- (unpublished figure). b) Line drawing from documented in areas of “thin-skinned” extension seismic reflection profile, Gulf of Mexico. associated with prograding deltas (e.g. Gulf of Hanging wall deformation over a listric normal fault. Observed structures include antithetic and Mexico, Niger Delta), but is still controversial in synthetic faults and a hanging wall anticline. areas of continental extension. This paper describes (After Diegel et al., 1995) our effort in progress to quantify fault and rollover geometry associated with a single well-exposed fault system exposed in the western Grand Canyon. We Stanford Rock Fracture Project Vol. 13, 2002 PJ-1 Hamblin (1965) used faults of the western Grand Canyon region to illustrate a phenomenon that he termed “reverse drag” – the development of a monoclinal fold in the hanging wall with beds dipping into the fault plane. Hamblin proposed a simple kinematic model for “reverse drag”, suggesting that the folding occurred in response to a decrease in fault dip with depth. Scattered exposures along the Hurricane fault from St. George Utah to the Grand Canyon suggest that the fault dip may decrease with depth, however the outcrops are spaced over a large horizontal distance (~200 km) and may reflect variation in fault dip along strike rather than with depth. This paper presents preliminary results of an effort to map the fault surface and both hanging wall and footwall structures associated with the Froggy Fault exposed in Lone Mountain and Whitmore Point using modern quantitative field and remote sensing methods. This area affords the opportunity to map a fault with ~200 m of offset over a vertical section of more than 1,000 m, exposed over a horizontal length of less than 10,000 m. The resulting 3D structural database provides a means for evaluating both kinematic and mechanical models of hanging wall deformation. Geologic Background The Colorado River cuts through the transition between the Colorado Plateau and the Basin and Range tectonic provinces in the western Grand Canyon (Fig. 2). This transition is manifested in a series of sub-parallel normal faults of moderate offset that are well exposed in the canyon walls of the Colorado River drainage system. Fault scarps in quaternary basalts and alluvium as well as recent Figure 2. Location map and aerial photo of earthquakes (e.g. the 1992 St. George, Utah field area. The study area is highlighted by the earthquake, Mw 5.6) indicate that the area is rectangle. Through going faults in the western tectonically active (e.g. Stewart et al., 1997 and canyon are associated with the transition from the references therein). Both hanging wall and foot wall relatively undeformed Colorado Plateau to the rocks of these normal faults are well-exposed, and highly extended Basin and Range province. The the canyon contains relatively simple pre-existing study area is located along the Froggy fault, one of a series of generally north striking normal structure. The study area thus presents an opportunity faults that crosses the Colorado River in the to investigate processes of continental extension in western Grand Canyon. Oblique aerial photo of both the hanging and footwall without the large the field area shows the well-developed hanging rotations and cross-cutting fault patterns typical of wall (roll-over) anticline and a synthetic fault. The much of the Basin and Range province (e.g. Proffett, footwall is relatively undeformed with a slight 1977). Huntoon and Billingsley (1981) have mapped upwarp toward the fault and minor secondary the area at a scale of 1:48,000, but no detailed studies faulting. The well-exposed bedding surface is of the fault exposures in the western Grand Canyon comprised of the upper sands of the Permian exist. The canyon exposes more than 1,000-m of Esplanade fm. Photo from Hamblin (1965). vertical section through a variety of lithologies and therefore provides an excellent opportunity to study fault geometry and secondary structures in a setting where evidence of deformation mechanisms can be directly observed. Stanford Rock Fracture Project Vol. 13, 2002 PJ-2 slickenside orientations have been measured wherever possible. The faults are consistently steep with an average dip of 74 degrees (Fig 5). Although fault architecture varies with lithology, the overall dip of fault planes does not appear to change significantly with rock type or depth. Figure 4. Stratigraphy of the Upper Esplande fm. a) Simplified Stratigraphic column of the Figure 3. Simplified geologic map of the upper Esplanade fm. as measured in Camp study area. The Froggy fault is the largest normal Canyon. Arrows indicate surface mapped using fault with ~200-m of down to the southwest offset. differential GPS. b) Lone Mountain anticline The Lone mountain monocline is localized in the looking west from the intervening fault block. hanging wall of the Froggy fault. Parashant and Well-exposed pavement surfaces are sandstones the Grand Canyons provide 2D cross-sections from a. c) Example of a pavement surface in the through the regional structure. Modified from upper Esplanade being mapped with GPS. Note Huntoon and Billingsley (1981). hummocky erosion on exposed surface. Structural Mapping Modern mapping and GIS methods allow us to create 3D geologic databases to describe the geometry of map-scale geologic structures with spatial resolution (data density) of less than 10-m and precisions on the order of 1-m, thus providing important constraints for understanding the development of these structures through numerical models (e.g. Maerten et al., 2001). By integrating detailed geologic mapping, GPS surveying, and a high-resolution DEM we are in the process of creating a 3D structural model of the Froggy Fault and its associated secondary structures. Ongoing geologic field work has focused on Figure 5. Fault dip data from the Froggy fault mapping the trace and geometry of the Froggy and system. There is no apparent systematic trend in other major faults in the area, mapping the extent
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