Copyright (c) 2010 All rights reserved. http://works.bepress.com/presor Recent documents in Phillip G Resor en-us Thu, 15 Apr 2010 01:31:12 PDT 3600 http://works.bepress.com/presor/37 http://works.bepress.com/presor/37 Tue, 13 Apr 2010 08:05:03 PDT Phillip G. Resor http://works.bepress.com/presor/36 http://works.bepress.com/presor/36 Tue, 28 Apr 2009 07:52:44 PDT Phillip G. Resor Online Teaching Resources http://works.bepress.com/presor/35 http://works.bepress.com/presor/35 Tue, 28 Apr 2009 07:46:36 PDT We integrate high-precision aftershock locations with geodetic inverse modeling to create a more complete kinematic model for the Kozani-Grevena earthquake sequence. Using the double-difference algorithm, we have improved relative hypocentral locations by a factor of ∼7 and thus imaged the details of the fault network associated with the seismic sequence. The interpreted fault network consists of multiple segments including (1) a master normal fault that strikes nearly due west and dips toward the north at 43°, extending from 6 to 15 km depth; (2) an upper segment that connects the top of the seismicity to the observed surface ruptures and dips 70°; (3) hanging wall antithetic faults; (4) a more steeply dipping southwest striking linking structure at the southwest end of the rupture; and (5) a separate south dipping segment at the southwestern end of the aftershock cluster. The imaged fault segment dimensions, orientations, and geometric relationships are consistent with regional fault patterns. Using slip inversion on triangular dislocation patches, we calculate variable slip on the imaged three-dimensional fault network that best fits the surface displacements observed by satellite interferometric synthetic aperture radar (InSAR). In our preferred model we find that the majority of slip occurred at depth on the west and southwest striking segments. By comparing these results to a planar fault model derived solely from the InSAR data using nonlinear inversion methods we demonstrate that the three-dimensional model improves the fit to the geodetic data while incorporating the observations of surface rupturing and aftershock distributions. Phillip G. Resor Abstracts http://works.bepress.com/presor/34 http://works.bepress.com/presor/34 Tue, 28 Apr 2009 07:43:03 PDT Phillip G. Resor Invited Talks http://works.bepress.com/presor/33 http://works.bepress.com/presor/33 Tue, 28 Apr 2009 07:41:52 PDT Phillip G. Resor Invited Talks http://works.bepress.com/presor/32 http://works.bepress.com/presor/32 Tue, 28 Apr 2009 07:34:47 PDT In this paper, we show that deformation can be dated by combining mesoscopic and microscopic structural observations with an understanding of metamorphic mineral reactions and U-Pb ages of newly grown sphene (titanite). This approach can be used on a variety of rock types that have been deformed at a wide range of metamorphic conditions. In an example from the Proterozoic Laramie Peak shear zone of southeastern Wyoming, a single period of syntectonic sphene growth in sheared mafic dikes is documented both by a strong spatial relationship between deformation and metamorphism and by sphene microtextures. U-Pb analyses of sphene separates give overlapping concordant or nearly concordant ages with a weighted mean of the 207Pb/206Pb ages of 1763 6 7 Ma. We interpret this direct age of deformation as evidence that the Laramie Peak shear zone records the cratonic response to the 1.78- 1.74 Ga Cheyenne belt collisional event. Phillip G. Resor Abstracts http://works.bepress.com/presor/31 http://works.bepress.com/presor/31 Tue, 28 Apr 2009 07:32:07 PDT Phillip G. Resor Abstracts http://works.bepress.com/presor/30 http://works.bepress.com/presor/30 Tue, 28 Apr 2009 07:23:52 PDT Phillip G. Resor Abstracts http://works.bepress.com/presor/29 http://works.bepress.com/presor/29 Tue, 28 Apr 2009 07:22:21 PDT Phillip G. Resor Abstracts http://works.bepress.com/presor/28 http://works.bepress.com/presor/28 Thu, 23 Apr 2009 13:34:45 PDT Phillip G. Resor Abstracts http://works.bepress.com/presor/27 http://works.bepress.com/presor/27 Thu, 23 Apr 2009 13:32:01 PDT The May 13, 1995 Kozani-Grevena earthquake (Mw=6.5) is a natural laboratory for studying crustal normal fault systems. The event and its aftershocks have been well observed geodetically, seismically, and geologically, providing an opportunity to integrate data sets to create a detailed subsurface fault model and investigate triggering and deformation associated with a large normal fault earthquake. Previous modeling of the earthquake has focused primarily on single geodetic data sets (e.g. inSAR - Meyer et al, 1996, GPS - Clarke et al., 1997) and has led to conflicting subsurface fault interpretations. In order to better model the subsurface fault geometry we have relocated aftershocks and use the interpretation of multiple complementary data sets to constrain a 3D boundary-element model of the earthquake sequence. Using the Double-Difference earthquake location algorithm (Ellsworth and Wauldhauser, 2000) we have reduced the hypocentral location error by a factor of ~10, obtaining high-precision aftershock locations for 650 events recorded by a local network (Hatzfeld et al., 1997). Relocated aftershocks cluster into a system of planar structures that reveal the "fine" structure of faults that were active during the earthquake sequence. The master normal fault dips 45° north from 6-14 km depth and extends over a length of ~12 km, consistent with the Harvard CMT solution. Two south-dipping antithetic faults extend from the western half of the master fault, one located at the up-dip tip extending from 4-6 km depth, and the second located at approximately the mid-point of the master fault in cross section from 6-9 km depth. These antithetic faults dip 45° and 35° respectively. At the western end of the rupture is a system of strike-slip faults in an orientation consistent with slip-transfer or segment linking structures. Fault patterns interpreted from the aftershock distribution form the basis of a 3D boundary element model using Poly3D. This code uses a mesh of contiguous triangular dislocation elements to capture the essential features of complex fault geometry in an idealized elastic half space. The mechanical model allows us to test the consistency of fault interpretations based on aftershock locations against surface geodetic and geologic data as well as to understand the mechanics of triggering and strain accommodation associated with the earthquake sequence. An improved fault model for the Kozani-Grevena earthquake is important for assessing seismic hazard and for understanding the mechanics of normal fault earthquakes and normal fault systems. Phillip G. Resor Abstracts http://works.bepress.com/presor/26 http://works.bepress.com/presor/26 Thu, 23 Apr 2009 13:30:26 PDT Poly3D, a fast 3D boundary element numerical code and Poly3DGUI a graphical user interface, facilitate the forward modeling of multiple mechanically interacting faults with complex 3D tiplines and irregular surface geometries, limited only by data precision and computing power and memory. Poly3D is based on the analytical solution for an angular dislocation in a half space composed of a homogeneous and isotropic linear-elastic material (Comninou & Dunders, 1975). Six angular dislocations are superimposed to define triangular dislocation elements that are combined to model complex 3D fault shapes without gaps or overlaps. Boundary conditions on these elements are either a uniform displacement discontinuity or the traction vector at the element center. Tectonic deformation can be simulated using remote strain boundary conditions. The GUI runs under the Windows operating system on a PC using OpenGL and Open Inventor technologies. The power of the C++ language combined with fast PC graphics cards and gigahertz CPUs enable real-time 3D simulations of the faulting process and stunning visualizations of deformed horizons, slip distributions, and displacement vector and stress tensor fields. Forward modeling results from three recent studies use a variety of geologic and geophysical data sets to constrain fault geometries and tectonic histories. 1) GPS data on faults and deformed limestone beds within the Carmel formation are used to investigate fault slip distributions and fault interaction for four sets of intersecting normal faults at Chimney Rock, Utah. 2) GPS measurements and a high resolution DEM are used to analyze the 3D geometry of deformed sandstone beds within the upper Esplanade formation in both the hanging and foot wall of a crustal-scale normal fault in the western Grand Canyon, Arizona. 3) Digital orthoquad photographs, digital topographic maps, and GPS field data on sandstone beds of the Frontier formation within the Emigrant Gap anticline, Wyoming, are used to investigate the relationship between fold shape and the underlying thrust fault geometry and slip distribution. In each study the geometric flexibility of Poly3D and the visualization capabilities of Poly3DGUI have led to new insights into the processes of faulting, fault interaction, and fault-related folding. Phillip G. Resor Abstracts http://works.bepress.com/presor/25 http://works.bepress.com/presor/25 Thu, 23 Apr 2009 13:28:14 PDT In the western Grand Canyon, the Colorado River cuts through a series of moderate offset normal faults that mark the transition between the Colorado Plateau and the Basin and Range tectonic provinces. Both hanging wall and footwall rocks are exposed over > 1,000 m of vertical section, and contain relatively simple pre-existing structures. The area thus presents an opportunity to investigate processes of continental extension without cross-cutting fault patterns and large rotations typical of much of the Basin and Range. We have mapped 8 km of the Froggy Fault, including ~ 600 m of vertical section, and deformed strata of the Esplanade fm. over a 32 km2 area by integrating field observations, GPS surveying, and a high-resolution DEM. The 3D structure of the site is documented over well-exposed areas with no more than 10 m between data points and meter-scale precisions. In cross-section the hanging wall rolls into the fault from a regional dip of 2° to a maximum dip of 25°. Along strike, steeper dips are associated with synthetic faulting and lower dips with antithetic faulting. The footwall also is folded with a maximum dip of 12°, away from the fault, near the eroded scarp. Throw on the Froggy Fault ranges from 50 to 230 m across the field area, and fault dips are consistently steep, typically > 70°. We fit the hanging wall fold shape with both kinematic and mechanical models, however only the mechanical models predict the footwall deformation. Kinematic models (variably inclined shear) require a smoothly curving fault to generate the observed hanging wall structure, with the depth to detachment dependent upon the shear angle. Steep fault dips and the geometric assumptions of this method limit permissible shear plane dips to between 75° in the opposite direction and 70° in the same direction as the fault. Alternatively, the observed folding may be fit by a planar fault in an elastic half space with slip over an interval from 0 to 1.2 km depth. This model predicts both footwall and hanging wall deformation, while honoring the observed near-planar fault geometry. Predicted strain gradients are high, suggesting that, although elastic stresses appear to play an important role in the development of the fold shape, a time-dependent (e.g. viscoelastic) rheology may provide better models of this structure. Phillip G. Resor Abstracts http://works.bepress.com/presor/24 http://works.bepress.com/presor/24 Thu, 23 Apr 2009 13:26:37 PDT We present a new 3D slip-inversion method based on the analytical solution of an angular dislocation in a linear-elastic, homogeneous, isotropic, whole- or half-space. The approach uses a boundary element method (BEM) that employs planar triangular elements of constant displacement to model fault surfaces. Discretization of surfaces into triangular boundary elements allows the construction of complex 3D fault surfaces with irregular tipline and no overlaps or gaps. A damped least squares method is used to minimize the functional &parallel.b-d∥2+ɛ^{2}∥ bigtriangledown .b∥ ^{2},where b represents the slip distribution on the faults, φ the influence coefficient matrix and d the observed deformation data. bigtriangledown is a discrete Laplacian operator for triangulated 2-manifolds, which serves as the measure of the roughness of the slip distribution, and ɛ represents the smoothing parameter. We have tested the method on synthetic forward elastic models using complex 3D fault geometry. Only one component of the computed displacement field (Ux, Uy, or Uz) was needed to constrain the inversion. Slip inversion results were used to refine initially simple models, developing more complex models that approached the fault geometry of the original forward model. We have also used the method to invert for fault slip on several natural examples employing a variety of observational data including: (i) field measurements of deformed stratigraphic layers, (ii) GPS and (iii) inSAR measurements of coseismic displacements. Phillip G. Resor Abstracts http://works.bepress.com/presor/23 http://works.bepress.com/presor/23 Thu, 23 Apr 2009 08:42:53 PDT Many studies of earthquake triggering and fault interaction have relied on highly-idealized fault geometries and slip distributions. Geological and geophysical observations, however, reveal that faults typically are not single planar surfaces with uniform slip bounded by rectangular tiplines, but are composed of multiple curved surfaces with curved tiplines and heterogeneous slip distributions. The segments typically are organized into echelon, conjugate, and intersecting patterns. The discontinuities, bends, intersections, and slip heterogeneities generate stress concentrations that may promote or inhibit slip on nearby faults and thus play an important role in the mechanics of fault systems. It is therefore important to incorporate both realistic fault geometry and slip distributions when evaluating models of fault mechanics. We have developed a new three-dimensional slip-inversion method based on the analytical solution for an angular dislocation in a linear-elastic, homogeneous, isotropic, half-space. The approach uses the boundary element code Poly3D that employs a set of planar triangular elements of constant displacement discontinuity to model fault surfaces. The use of triangulated surfaces as discontinuities permits construction of fault models that better approximate curved three-dimensional surfaces with no overlaps or gaps, bounded by curved tiplines. Slip inversion on three-dimensional surfaces therefore allows investigations of fault models that incorporate more realistic geometry and heterogeneous slip. We have applied the method to invert for coseismic slip associated with the 1999 Hector Mine and 1995 Kozani-Grevena earthquakes, using InSAR and GPS observations of surface displacements. Three dimensional fault models were constructed by integrating available data sets including mapped surface ruptures, relocated aftershocks, and previous inversions for subsurface geometry. The resulting models improve the fit to the near-field geodetic data and more faithfully honor observations of fault rupture geometry. Models such as these form the starting point for more complete evaluations of fault mechanics and failure criteria. Phillip G. Resor Abstracts http://works.bepress.com/presor/22 http://works.bepress.com/presor/22 Thu, 23 Apr 2009 08:41:24 PDT Models are useful for teaching about the scientific process and the complex phenomena we investigate. Stimac et al. (this session) introduce the Modeling Structural Processes resource collection, initiated at the Teaching Structural Geology in the 21st Century (TSG21) summer workshop. The TSG21 Modeling resource collection will catalog analog and mathematical models useful for teaching. From a topical standpoint, such models are used to illustrate 1) geometries of structures, 2) properties of rocks, and 3) deformation processes. Models also address a number of other learning objectives. In this contribution, we describe some other learning goals that may be addressed using models in the classroom, and we showcase modeling exercises from the TSG21 resource collection that address those learning goals. Models can be used to improve student learning by engaging students with the material; building students' intuition about geometry, properties or process; and addressing multiple learning styles. Models are useful for guiding students to make careful observations, in preparation for "noisier" field experiences. Models may be used to develop students' quantitative literacy, especially when analog and mathematical models are paired. Quantitative literacy may be honed as students perform data collection and analysis in a modeling exercise. Projects involving hypothesis generation and testing are readily accomplished with analog and mathematical models. Ultimately, students may learn to critically evaluate models with respect to the structural geologic process they are meant to represent. Teaching with models engages students in the practice of science, in which the development, application and evaluation of models is central. Phillip G. Resor Abstracts http://works.bepress.com/presor/21 http://works.bepress.com/presor/21 Thu, 23 Apr 2009 08:36:48 PDT The Modeling Structural Processes Working Group of the recent NAGT Workshop: "Teaching Structural Geology in the 21st Century" (TSG21) presents a catalog of models used in the teaching of structural geology to undergraduate students. Structural geology models are simplified constructs of complex earth system processes that have long been known to engage students and enhance their intuition of the deformation of earth materials. We recognize four classes of models: conceptual models, data representation models (including maps, stereonets, three-dimensional renderings, and statistical descriptions), analog models (including experiments with rock and non-rock materials), and mathematical models (including analytical expressions and deterministic or stochastic numerical models). We have limited this teaching catalog to analog and mathematical (including analytical, numeric, and statistical). Crider et al. (this session) address the educational goals of using models to address different learning objectives. The premise behind modeling in structural geology is to understand how earth materials deform. We have divided the analog and mathematical modeling resources by subject matter and activity length (in-class demonstration to long-term projects). All presented models share a number of characteristics: they are accelerated relative to natural phenomena; they are descriptive; they can handle large variations in problem type; they allow the selection of appropriate variables and alternatives for comparison back to known parameters, and they show the complexity of natural systems. Goals for the Modeling Structural Processes resource collection include a searchable catalog of analog and mathematical models useful for teaching, with photos, reviews, and a discussion of their best use in the classroom. Entries will include limits and assumptions made using the models and a list of "expert" contacts concerning the various models. We hope that this catalog of models will serve as a teaching resource for structural geology instructors, but success relies on the completeness of the catalog. We encourage our colleagues to consider contributions to this site by contacting either Michele Cooke (cooke@geo.umass.edu) or Basil Tikoff (basil@geology.wisc.edu). Phillip G. Resor Abstracts http://works.bepress.com/presor/20 http://works.bepress.com/presor/20 Thu, 23 Apr 2009 08:27:58 PDT Recent advances in geologic mapping, aftershock location, and reflection seismology allow geoscientists to image surface and subsurface structures with greater precision. These images demonstrate that earthquake ruptures typically occur along faults or fault systems that display complex 3D geometries. Poly3D, a 3D boundary element code and user interface, enables the integration of these varied data sets to constrain fault geometry and accurately models the complex geometries, limited only by data precision and computing power. Poly3D is based on the analytical solution for the elastic boundary value problem of an angular dislocation in a half space composed of a homogeneous and isotropic linear-elastic material (Comninou & Dunders, 1975). One of the major advantages that Poly3D has over other commonly used dislocation models (e.g. based on Okada, 1985) is the use of a triangular rather than rectangular uniform dislocation patch. The triangular shape enables one to model complex 3D shapes without gaps or overlaps. A further advantage of Poly3D is the possibility of using remote strain boundary conditions to simulate tectonic deformation and traction boundary conditions to simulate stress drop on fault segments. Poly3D has been applied to numerous problems of fault interaction and earthquake deformation over the last 5 years. We present results from three recent studies that have focused on fault interaction and earthquake triggering using a variety of geologic and geophysical data sets to constrain fault geometries and deformation. 1) GPS field mapping of faults and deformed strata is used to investigate fault slip and fault interaction around a set of normal faults at Chimney Rock, Utah. 2) Large-scale geologic mapping and measured slip distributions are integrated with published geophysical data to study the interaction between the 1967 Mudurnu Valley and 1999 Kocaeli earthquakes in Turkey. 3) Aftershock triggering and the development of normal fault systems are investigated by integrating high-precision aftershock locations and published geological and geodetic data sets from the 1995 Kozani- Grevena earthquake in Greece. In each of these studies the geometric flexibility of Poly3D and the ability to integrate available data sets has led to new insights into the processes of faulting, fault interaction, and earthquake triggering. Phillip G. Resor Abstracts http://works.bepress.com/presor/19 http://works.bepress.com/presor/19 Thu, 23 Apr 2009 08:25:57 PDT Phillip G. Resor Abstracts http://works.bepress.com/presor/18 http://works.bepress.com/presor/18 Thu, 23 Apr 2009 08:21:31 PDT The western Grand Canyon is a natural laboratory for investigating processes of continental extension due to the great vertical exposure (> 1 km) and the relatively simple pre-extensional structure. We have undertaken a detailed field study of joint frequency in Parashant Canyon, a natural cross-section through the normal fault related Lone Mountain monocline, in order to better understand the role that joints play in accommodating extensional folding. The Lone Mountain monocline is made up of two half-monoclinal flexures: a hanging wall fold in which dips gradually increase toward the fault from a regional dip of 2° to a maximum dip of 25° toward the fault over a distance of approximately 1.5 km and a footwall fold in which beds dip away from the fault with maximum dips of 12° near the eroded fault scarp that gradually return to a regional dip of 2° over a distance of approximately 0.5 km away from the fault. The fold is most pronounced southeast of Parashant Canyon where the fault system is made up of a master normal fault and smaller synthetic fault and less well developed to the northwest where the fault system is comprised of two parallel half-grabens. Fracture frequency across the fold has been quantified using scanline surveys of a ~6-m thick sandstone from within the Manakacha Formation and panoramic photo interpretation of a ~30-m thick amalgamated sandstone within the Esplanade Formation. In both layers southeast of Parashant Canyon a near fault parallel joint set is well-developed in the hanging wall and nearly absent in the footwall. The frequency of this joint set increases from ~0.4/m at distances > 1 km from fault to ~1.6/m at ~150 m within the Manakacha sandstone and from 0.05/m to 0.2/m over distances from 1400 m to 700 m from the fault in the Esplanade sandstone. Joint frequency patterns are not as clear along the northwest side of the canyon where the fault pattern is more complex and folding less pronounced. We propose that the increased fracture density may be acting to accommodate or localize folding associated with slip on the Froggy fault, a hypothesis that we plan to investigate through numerical modeling. Phillip G. Resor Abstracts