General statement

Since the inception of plate tectonic theory four decades ago, geoscientists have been able to develop a first-order understanding of most of the major features found within the Earth’s crust such as transform faults or island-arc volcanic chains. Now we are trying to reach beyond our first-order understanding of these fundamental concepts in order to comprehend exactly how and why these complex systems work. Perhaps one of the most important steps in understanding how Earth systems work is discerning the causes and significance of heterogeneity within these systems. Therefore, the underlying theme of my research in structural geology is understanding the causes and significance of strain partitioning at scales ranging from the thin section to that of an entire orogenic belt. Because I find plastic deformation of the middle and lower crust to be particularly interesting, my research focuses on field-based case studies of plastic high-strain zones that address specific problems in structural geology and tectonics.

Evolution of the Medicine Bow orogenic belt Top of page

The Medicine Bow orogeny marks the onset of the accretion of over 1000 km of continental crust (the Proterozoic Colorado Province) onto the southern Margin of the Archean Wyoming Province. The suture between Archean rocks and Proterozoic rocks is marked by an eastward-branching network of NE–SW-striking, subvertical shear zones collectively known as the Cheyenne Belt. These shear zones generally record north-northwest–south-southeast-directed contraction coupled with subvertical elongation (Dubendorfer and Houston, 1987). Because of this, the Cheyenne belt suture zone has classically been interpreted as forming during north-northwest–south-southeast-directed, near-orthogonal convergence (e.g. Karlstrom and Houston, 1984; Dubendorfer and Houston, 1987). However, numerical simulations (e.g. Fossen and Tikoff, 1993; Lin et al., 1998; Jones et al., 2004) indicate that shear zones exhibiting this distortional-strain geometry may in fact record transpressional deformation, and I propose that this is the case in the Cheyenne belt shear zones. To test this transpression hypothesis my students and I are conducting a series of detailed studies across the suture zone. These localities are: 1) the northern mylonite zone near Bear Lake, 2) the northern mylonite zone in the North Mullen Creek canyon, and 3) the northern mylonite zone along the North Platte River. In each area detailed geologic mapping will be combined with semi-quantitative microstructural and crystallographic-fabric analysis techniques that were not available during the late 1970’s and early 1980’s when the Cheyenne belt shear zones were last examined in detail.

Quartz crystallographic fabrics formed under constrictional strain Top of page

Quartz-crystallographic-fabric formation and the resulting fabric geometries are sensitive to variations in deformation temperature and/or strain rate, the noncoaxiality of flow, and the resulting distortional strain geometries that develop. The initial link between distortional-strain geometry and quartz c-axis-fabric geometry was made by Lister and Hobbs (1980) using numerical simulations. A variety of fabrics from naturally and experimentally deformed samples support the results of Lister and Hobbs’ (1980) numerical simulations for plane-strain and flattening-strain deformation conditions (e.g. Tullis et al., 1973; Majoribanks, 1976; Tullis, 1977; Compton, 1980; Law et al., 1984; Price, 1985; Schmid and Casey, 1986; Law, 1986). However, natural fabrics produced under apparent constrictional-strain conditions are relatively rare, and constrictional deformation of rocks has not been reproduced in experiments. To fill this void, my colleague, Rachel Beane, and I are analyzing quartz and mica (biotite + muscovite) crystallographic fabrics in L tectonite samples from the Pigeon Point high-strain zone, Klamath Mountains, California. The quartz c-axis fabrics developed in these samples are very similar to those predicted by Lister and Hobbs’ (1980) numerical simulations except that the double girdles are distinctly asymmetrical about the elongation lineations. Currently we are evaluating a number of hypotheses that might explain this unusual fabric geometry.

Significance of L tectonites Top of page

The goal of my Ph.D. dissertation was to better understand the significance of deformation fabrics produced under apparent constrictional strain, or L tectonites. In order to accomplish this, I undertook three field-based case studies of high-strain zones that exhibit large domains of apparent constrictional strain in diverse structural, rheological, and tectonic settings. These areas include: 1) granitic rocks that suffered contractional deformation associated with continental assembly exposed in the Laramie Mountains, Wyo. (Sullivan, 2006 [PDF]); 2) mafic metavolcanic rocks deformed during oceanic terrane accretion exposed in the Klamath Mountains, Cal. (Sullivan, in press [PDF]); and 3) quartzite, schist, and granite deformed in a footwall shear zone of a metamorphic core complex exposed in the Raft River Mountains, Utah (Sullivan, 2008 [PDF]). My work in these areas was centered about detailed geologic mapping. Field data are complimented by cross-section reconstructions, microstructural analyses, petrographic analyses, crystallographic-fabric analyses, and strain analyses of deformed cobbles and pebbles. These results are integrated with data and models from the literature in order to provide a concise overview of L tectonites that will aid geologists in interpreting this strain phenomenon.

Northern Great Basin metamorphic core complexes Top of page

In conjunction with my Ph.D. adviser, Art Snoke at the University of Wyoming, I also undertook an in-depth analysis of the structural, magmatic, and metamorphic histories of the Snake Range, Ruby-East Humboldt, and Albion-Raft River-Grouse Creek metamorphic core complexes in the northern Great Basin (Sullivan and Snoke, 2007 [PDF]). The goal of this project was to integrate existing data and interpretations in order to produce a concise overview of the deformational, magmatic, and metamorphic histories recorded in each terrane. These syntheses allowed for a regional-scale along- and across-strike examination of: 1) the processes operating in the hinterland of the Sevier orogenic belt and 2) its subsequent crustal-scale collapse and the extensional exhumation of its mid-crustal roots.

Strain-path partitioning in the White Mountain shear zone Top of page

For my M.S. thesis (under Rick Law at Virginia Tech) I produced a detailed description of a dextral transpression zone, the White Mountain shear zone (WMSZ), with a range of lineation orientations and compared these natural data to numerical models that predict a change in the maximum stretching direction from subhorizontal to subvertical (Sullivan and Law, 2007 [PDF]). The WMSZ is characterized by steeply dipping foliations, with dominant shallowly plunging lineations and coeval subordinate domains of steeply plunging lineations. Within shallowly lineated domains, foliation geometry, shear-sense indicators and quartz c-axis fabrics indicate a large component of simple shear, while microstructural and quartz c-axis fabric data from steeply lineated domains indicate a large component of pure shear. Geometric relationships between foliations and lineations and quartz c-axis fabrics demonstrate that lineation orientation has remained constant during much of the deformation history. Comparison of numerical models with the data collected from WMSZ shows that the shear zone geometry and the observed strain-path partitioning do not match any of these models. Therefore, we proposed a conceptual kinematic model for the WMSZ involving stable, segregated, coeval kinematic domains of simple-shear-dominated fabrics and pure-shear-dominated fabrics that accommodate the transcurrent and contractional components of deformation respectively.

Publications Top of page

Sullivan, W.A., in press, Kinematic significance of L tectonites in the footwall of a major terrane-bounding thrust fault, Klamath Mountains, California, USA, Journal of Structural Geology. [PDF]

Sullivan, W.A., 2008, Significance of transport-parallel strain variations in part of the Raft River shear zone, Raft River Mountains, Utah, USA: Journal of Structural Geology, v. 30, p. 138–158. [PDF]

Sullivan, W.A., and Snoke, A.W., 2007, Comparative anatomy of core-complex development in the northeastern Great Basin, U.S.A.: Rocky Mountain Geology, v. 42, p. 1–29. [PDF]

Sullivan, W.A., and Law, R.D., 2007, Strain path partitioning in the transpressional White Mountain shear zone, California and Nevada: Journal of Structural Geology, v. 29, p. 583–598. [PDF]

Sullivan, W.A., 2006, Structural significance of L tectonites in the eastern-central Laramie Mountains, Wyoming: Journal of Geology, v. 114, p. 513–531. [PDF]