Sergio Chávez-Pérez, John N. Louie, University of Nevada, Reno & Sathish K. Pullammanappallil, Rice University
sergio@seismo.unr.edu
http://www.seismo.unr.edu/htdocs/students/CHAVEZ/seg96/seg96.html
Presented at the
Summary The Death Valley region of southeastern California is an excellent location to test Cenozoic extension models for the southern Great Basin by way of seismic depth imaging. We use prestack depth migration to define whether normal faulting is planar or listric. Towards that end, we reprocessed seismic data from COCORP Death Valley Line 9 to attain an enhanced image of shallow faulting (down to 2.5 km). Previous workers used standard seismic processing to infer normal faults from bed truncations, displacement of horizontal reflectors and diffractions. However, an interaction between processing and interpretation is necessary to unravel the origin of the shallow reflections, assisted by the development of a detailed velocity structure. We obtained a detailed velocity model by nonlinear optimization of first-arrival times picked from shot gathers, examined the unprocessed data for fault reflections, and used a prestack depth imaging procedure to properly handle velocity variations and arbitrary dips. Our analysis reveals the listric geometry of the normal fault bordering a half-graben in the southern Death Valley basin, which further supports the concept of low-angle extension and crustal-scale plumbing in this region.
Introduction The presence of shallow-dipping normal faults is one of the most important criteria for recognizing extensional regimes (e.g., Shelton, 1984). In fact, substantial extension can only be accomplished by initially shallow faults or by faults which rapidly rotate towards shallower dips as extension proceeds. Thus, resolving the geometry of normal faults is of fundamental importance in determining how crustal extension is accommodated at depth.
This study has been motivated by the need to image fault geometries that cannot be constrained by way of conventional time sections, and the ongoing controversy of how crustal extension is accommodated at depth. This has been the case in the southern Death Valley basin. Faults on seismic sections collected in this region have been inferred from sedimentary bed truncations, and not directly imaged.
Standard time imaging techniques provide a good starting point for subsequent depth imaging processing. However, the definition of fault-plane geometries demands detailed prestack imaging including nonhyperbolic, fault plane reflections. These are of fundamental value for the correct positioning in space of the main fault planes and optimum stacking of the migrated shot gathers (Louie et al., 1988; Hole et al., 1996).
New approaches to prestack depth imaging (e.g., Rajasekaran and McMechan, 1995) require only velocity analysis and depth migration. We analyze the COCORP data by following an equivalent processing scheme. We use minimum preprocessing, a well-constrained, reliable velocity model for shallow depths, and Kirchhoff prestack depth migration.
Tectonic setting Death Valley is a region of active extension and recent volcanism. It is located in southeastern California between the Panamint Mountains on the west and the Black Mountains on the east, within the southwestern portion of the Basin and Range province (Fig. 1). Death Valley is a pull-apart basin developed in a releasing bend in a dextral strike-slip fault system. The northeastern boundary of the basin is controlled by the dextral Northern Death Valley-Furnace Creek fault zone and the southwestern boundary is controlled by the dextral southern Death Valley fault zone. The central part of the basin is characterized by normal faulting. The western slopes of the Black Mountains are essentially an exhumed west-dipping normal fault that transfers motion between the two strike-slip faults (see, e.g., Topping, 1993). Death Valley is therefore a half-graben basin.
COCORP data in this region (Fig. 1) may have imaged, perhaps for the first time, crustal-scale plumbing associated with a deep magma chamber beneath a pull-apart basin (de Voogd et al., 1986). A dipping event that extends from above a midcrustal reflection at about 6 s to near the surface, by an offset cinder cone, was suggested to be a reflection from a magma conduit or feeder dike that has now consolidated. Shallow-dipping events to 2 s on Line 9 (Fig. 2) define a half-graben that appears to be bounded by a listric fault (Serpa et al., 1988).
Figure 1. Map of the eastern California--southern Nevada region showing four of the five COCORP Death Valley Lines. The numbers along Line 9 (L9) indicate initial and final VP numbers. The shading indicates relative elevations. Solid, thin lines indicate faults. NTS is the Nevada Test Site.
Prestack depth imaging results The original processing and interpretation of Serpa et al. (1988) presents two difficulties (Figs. 2a, b). The west-dipping, listric fault they drew is not imaged in their seismic section and its surface projection, which coincides with the approximate trace of the Death Valley fault zone and an offset cinder cone (QB in Fig. 2b), may be pure coincidence, as pointed out by Smithson and Johnson (1989).
We use the well-constrained (down to 2.5 km) velocity model of Pullammanappallil et al. (1996) which was developed using a simulated-annealing optimization scheme for inversion of first-arrival times (Pullammanappallil and Louie, 1994). Data preprocessing before migration only included muting and trace equalization. The prestack Kirchhoff depth migration process we utilize is similar to that used by Louie et al. (1988) and has been discussed by Louie and Qin (1991).
Our prestack migration result did not bring out the basin stratigraphy details of conventional time or poststack depth imaging. It did not reproduce the shallow layered structure interpreted as late Cenozoic basin deposits. However, the fault plane (Fig. 2b) inferred by Serpa et al. (1988) is now clearly imaged (Fig. 2c).
This underscores the need for a combined depth imaging approach which utilizes the strengths of each method to derive a quality of image neither by itself can produce. For instance, we could also obtain an estimate of extension and depth to detachment by using poststack depth migration to image basin stratigraphy and complement the prestack results. This would allow us to determine the angular relationship between shallow basin stratigraphy and the fault plane, and to construct the fault profile using hangingwall rollover geometry.
Apart from the main fault plane geometry, Fig. 2c also shows an enhanced depth image of the basin bottom. Strong, positive reflectivity spots (black arrows) clearly depict, despite apparent multiple events, the bottom of this half-graben. Note how stronger reflections (like those of the fault plane) migrate more coherently than weaker ones. This influences the overall quality of the migrated image. Of course, one of the major problems in Kirchhoff depth migration is image resolution. Migration artifacts or false images, due to sparse receiver and source coverage and unfocused converted energy, sometimes make the results difficult to interpret. This case is no exception. Artifacts appear as elliptical trajectories that contribute to image degradation, mostly beneath the basin bottom.
Figure 2. a) Stacked section and b) time to depth conversion and interpretation of the original processing by Serpa et al. (1988). A, B, and C denote important dipping reflections. Solid black lines show the trend of the basin-fill reflectors, PC-PZ indicates the location of inferred Precambrian and Paleozoic sedimentary rocks faulted against the bottom of the basin. AL indicates the area of late Cenozoic basin deposits and QB indicates the surface position of a 690,000-yr-old basaltic cinder cone. c) Enhanced imaging of the basin bottom and the main fault plane using a detailed velocity model and prestack depth migration.
Conclusions Depth imaging of structures in southern Death Valley basin from COCORP Line 9 shows a basin-bounding listric normal fault and a well-defined half-graben bottom. The image is supportive of the concept of low-angle extension in the region and strengthens the association of listric normal faulting with a proposed magma conduit traced to the surface location of a cinder cone.
Acknowledgments The first author acknowledges financial support by CONACYT, Mexico's National Council for Science and Technology. This work was funded by NSF's grant EAR9405534. Glenn Biasi, Mark Stirling, and Serdar Özalaybey provided useful criticism and suggestions.
References de Voogd, B., L. Serpa, L. Brown, E. Hauser, S. Kaufman, J. Oliver, B.W. Troxel, J. Willemin, and L.A. Wright (1986). Death Valley bright spot: A midcrustal magma body in the southern Great Basin, California?, Geology 14, 64-67.
Hole, J.A., H. Thybo, and S.L. Klemperer (1996). Seismic reflections from the near-vertical San Andreas fault, Geophys. Res. Lett. 23, 237-240.
Louie, J.N. and J. Qin (1991). Subsurface imaging of the Garlock fault, Cantil Valley, California, J. Geophys. Res. 96, 14461-14479.
Louie, J.N., R.W. Clayton, and R.J. Le Bras (1988). 3-d imaging of steeply dipping structure near the San Andreas fault, Parkfield, California, Geophysics, 53, 176-185.
Pullammanappallil, S.K. and J.N. Louie (1994). A generalized simulated-annealing optimization for inversion of first-arrival times, Bull. Seism. Soc. Am. 84, 1397-1409.
Pullammanappallil, S.K., J.N. Louie, and W.J. Honjas (1996). Velocity models for the highly-extended crust of Death Valley, California, Geophys. Res. Lett., in review.
Rajasekaran, S. and G.A. McMechan (1995). Prestack processing of land data with complex topography, Geophysics 60, 1875-1886.
Serpa, L., B. de Voogd, L. Wright, J. Willemin, J. Oliver, E. Hauser and B. Troxel (1988). Structure of the central Death Valley pull-apart basin and vicinity from COCORP profiles in the southern Great Basin, Geol. Soc. Am. Bull. 100, 1437-1450.
Shelton, J.W. (1984). Listric normal faults: An illustrated summary, AAPG Bull. 68, 801-815.
Smithson, S.B. and R.A. Johnson (1989). Crustal structure of the western U.S. based on reflection seismology, in Pakiser, L.C. and W.D. Mooney, Geophysical framework of the continental United States, Geol. Soc. Am. Memoir 172, 577-612, Boulder, CO.
Topping, D.J. (1993). Paleogeographic reconstruction of the Death Valley extended region: Evidence from Miocene large rock-avalanche deposits in the Amargosa Chaos Basin, California, Geol. Soc. Am. Bull. 105, 1190-1213.