My recent research in reflection seismology at the University of Arizona has been concentrated on three broad areas of investigation: (1) detailed analysis of extensional fault geometries and basin evolution in the western U.S., (2) complex deep crustal structure in extended and unextended regions, (3) P- wave and S-wave analysis of Poisson's ratio, crustal composition, and in situ rock properties. An exciting aspect of these investigations is that they are interrelated; progress in any area enhances the understanding of others.
Current, major projects on which my graduate students and I are working involve: (1) analysis and interpretation of a detailed grid of marine seismic reflection data from the Great Salt Lake, northern Utah; (2) a wide-angle reflection and magnetotelluric investigation of crustal structure and fluid content in NE Nevada; and (3) structural and tectonic analysis of complex structure imaged beneath the northern California Coast Range and Northwestern Great Valley. These projects are outlined below.
A new project, involving work by Helge Gonnermann, an MS student in Reflection Seismology, involves developing new computer techniques for three-dimensional tau-p (delay-time - apparent slowness) analysis of 3-D seismic data. This work holds promise for rapid, objective evaluation of structure and anisotropy in three dimensions.
An extensive grid of marine seismic reflection data from the Great Salt Lake, Utah, was provided to the University of Arizona through a cooperative research arrangement with Amoco Production Company. Included with the release of these data were well logs, cores and cuttings, borehole and surface gravity data, lithological and palynological analyses, stratigraphic correlations, radiometric analyses, drilling reports, maps, and access to other information about the project. The seismic profiles cover roughly 70% of the area of the Great Salt Lake and, as such, constitute perhaps the most detailed and areally extensive network of data from an intracontinental extensional terrane in the western U.S. These geophysical and geological data are the focus of ongoing research and have spawned two NSF-supported investigations.
We have completed seismic data processing on several key seismic reflection profiles, and have established a relationship with a commercial data processing firm that has agreed to reprocess all of the Great Salt Lake profiles at no cost. This will provide a uniform database for workstation-based seismic interpretation, and will permit us to focus on special problems in data processing and modeling.
Analysis of the seismic profiles shows that the major bounding faults beneath the eastern Great Salt Lake are listric in nature, begin at angles of ~60 degrees near the surface, and sole out at depths of 8-10 km. Although the shallow brittle-ductile transition in the eastern Great Basin probably affected the evolution of Tertiary normal-fault geometries, normal-fault reactivation of Sevier-age thrust faults, evident on some of the seismic profiles, suggests that the maximum depth of penetration and soling of listric faults may have been controlled by these older structures. This is the subject of George Petropoulos' recently completed MS thesis.
Finite-element modeling of a reactivated thrust-fault surface evident on some of the seismic profiles shows that the first- formed secondary faults will be synthetic to the dip of the thrust-fault ramp. After formation of a synthetic fault, stresses rise farther away from the thrust ramp and ultimately lead to formation of antithetic faults. These temporal and spatial relationships for secondary synthetic and antithetic normal faults are seen on the seismic profiles and provide support for the thrust-fault-reactivation model that we are proposing. This work forms an important part of Gopal Mohapatra's dissertation research. Gopal recently completed a manuscript on his modeling results that will be submitted to Tectonics.
Balanced reconstructions for various intervals of time represented on the seismic profiles indicate that extension across the Great Salt Lake since the Miocene has been 50% - 60%, consistent with some other estimates of extension for the region. Horizontal deformation and strain rate apparently were greatest during the Pliocene, but the Pleistocene-to-Recent extension rate also was high. The average horizontal deformation rate since the Miocene was about 0.55 mm/yr.
This data set also yields evidence for an earlier (Eocene) sedimentary basin now unconformably overlain by a greater volume of Miocene and younger strata. The earlier basin indicates that exten-sion in the hinterland of the Sevier foreland fold and thrust belt began almost immediately as compression waned, and provides additional evidence of regional extensional collapse of the Rocky Mountain Cordillera from southern Canada to at least Utah within about 2 Ma after thrusting ceased. This is the subject of a paper by Kurt Constenius, a PhD student in Reflection Seismology, that is in press for publication in the GSA Bulletin.
In August 1992 and June 1993, the Universities of Arizona and Wyoming collected densely sampled, reversed, multicomponent wide- angle reflection data adjacent to the Ruby Mountain metamorphic core complex to place constraints on the presence and importance of fluids in the crust, crustal anisotropy, mid-crustal detachment zones, magma at the Moho, lower crustal inflow beneath uplifted core complexes, underplating, and similarity of core complex crust to "normal" Basin-and-Range crust. To enhance the degree of resolution on interpretations, these data are being combined with complementary magnetotelluric, gravity and geological studies.
Initial analysis of the normal-incidence to wide-angle data shows that P-wave reflections from the Moho are well recorded at near- normal incidence, but that reflection amplitudes decrease as shot- to-receiver offsets become larger. This is surprising, since just the opposite behavior is more commonly observed, and earlier studies at the northeastern end of Ruby Valley showed significant Moho reflection amplitudes at offsets of greater than 20 km. These observations are being modeled to determine the abruptness of the lower crust-to-upper mantle transition. Perhaps most surprisingly, these results suggest that the nature of the Moho changes significantly over distances of only a few tens of kilometers.
Also of interest is the observed complexity of refraction arrivals which show abrupt time shifts and changes in apparent velocity at the northern end of the survey area. Analysis of several shots that were recorded into the same receiver locations shows that the complexities in the arrivals are due principally to relatively near-surface structure. These data show the effects of crossing several small- to large-scale faults at the northern end of Ruby Valley, which apparently cut through mylonitic fabrics associated with Tertiary ductile faulting.
Work completed by Carl Mitchell for his MS degree suggests that upper crustal velocities increase toward the northern end of the Ruby Mountains. Increasing velocity also suggests that density increases to the north. The northern Ruby Mountains area represents the deepest levels of exposure of mid-Tertiary rocks which have been uplifted ~12-18 km. However, determinations of crustal thickness so far indicate a surprisingly uniform crustal thickness (~31 km) throughout the region, with no significant isostatic gravity anomaly associated with high topography in the range. An intriguing problem that arises is how greater uplift of presumably higher density rocks in the northern Ruby Mountains can be reconciled with uniform crustal thickness. This is the primary focus of ongoing work by Peangta Satarugsa, a PhD student in Reflection Seismology.
Exposed in the California Coast Ranges and the Great Valley is the classic analog of a subduction-type convergent margin. The study of California convergent-margin tectonics recently has focused on Early Tertiary(?) and Pliocene-Recent Franciscan thrust-wedging above blind thrusts along the Coast Ranges-Great Valley boundary. The Great Valley homocline, an east-dipping panel of Great Valley strata that extends for nearly 600 km along the western margin of the Great Valley, has been interpreted to be a surface manifestation of the Franciscan wedge structure. In the northern Great Valley, there has been little Neogene tectonism, yet evidence of major subduction-complex deformation exists with geometrically similar outcrop patterns to those seen in the southern and central Great Valley.
Our work on this area has focused on analysis of an extensive grid of seismic reflection and deep borehole data from the Sites anticline area. These data have been used to divide the Great Valley sequence and related units into five tectonostratigraphic packages, each bounded by major structural and/or depositional discontinuities: (1) Franciscan Complex, (2) Jurassic Coast Range ophiolite, (3) Jurassic-Lower Cretaceous Great Valley strata of the Stony Creek petrofacies, (4) Lower-Upper Cretaceous Great Valley strata, and (5) Neogene Tehama Formation.
Although outcrop relations suggest the Great Valley sequence is a conformable succession of strata, subsurface relations suggest that a structurally higher, Cretaceous Great Valley package overlies a deeper, Jurassic Great Valley package along a fault contact and/or major angular unconformity; this juxtaposition provides compelling evidence for two distinct phases of deformation of the Great Valley sequence. Cretaceous-Paleocene upper Great Valley sequence rocks exhibit homoclinal eastward tilt at the surface. However, in the subsurface, these rocks display east-vergent onlap and/or fault truncation against crystalline basement. The eastward termination direction implies structural delamination or stratigraphic onlap along an original west-dipping surface and/or structural boundary, and this surface may have been exploited later as a detachment horizon above which strata experienced large-scale buckling without deforming the footwall. This project forms a major component of the dissertation research of Kurt Constenius, who organized our acquisition of the seismic data and managed its reprocessing.