General statement

Since the inception of plate tectonic theory four decades ago, geoscientists have developed an 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 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 that address specific problems in structural geology and tectonics.

Ultramylonite formation and strain localization in granitic rocks Top of page

My latest project will examine the processes that enabled strain localization and the formation of mylonites and a >100-m-wide ultramylonite zone in granitic rocks cut by the Kellyland shear zone in eastern Maine. This project will combine whole-rock geochemical analyses of undeformed granite, mylonites, and ultramylonites with outcrop-scale analyses of strain localization, detailed microstructural observations, and chemical analyses of individual mineral phases. These results will lead to a better understanding of how strain is localized in upper-crustal shear zones and how large ultramylonite zones form.

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 onto the southern Margin of the Archean Wyoming Province. The suture between Archean rocks and Proterozoic rocks is marked by a network of subvertical shear zones collectively known as the Cheyenne Belt. The Cheyenne belt suture zone has classically been interpreted as forming during near-orthogonal convergence. In two areas our more detailed analyses of these shear zones confirm the existing interpretations. However, we interpret the shear zones in these areas as a stretching fault system rather than a thrust system rotated into its present-day subverical orientation during late-stage folding (Sullivan et al., in press). Elsewhere, we have found evidence for significant sinistal strike-slip motion in the Cheyenne belt shear zones, and this may require revision of existing plate-tectonic models for the Medicine Bow orogeny (Sullivan et al., 2011 [PDF]).

Quartz crystallographic fabrics formed under constrictional strain Top of page

My colleague, Rachel Beane, and I analyzed quartz crystallographic fabrics in L tectonite samples from the Pigeon Point high-strain zone, Klamath Mountains, California (Sullivan and Beane, 2010 [PDF]). We concluded that these unusual asymmetrical crystallographic fabrics formed under near-constrictional conditions, and that the asymetry is a result of a small component of noncoaxial flow. Moreover, our results provide the first confirmation of the a-axis patterns predicted to form during constrictional deformation, and they demonstrate that c-axis fabric girdles formed during constriction widen with increasing temperature.

Significance of L tectonites Top of page

My Ph.D. dissertation consisted of three field-based case studies of high-strain zones that contain significant domains of L and L>S tectonites 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, 2009 [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]). The results of these case studies are with additional published data and models to provide a concise overview of L tectonites that will aid geologists in interpreting this strain phenomenon (Sullivan, in press).

Northern Great Basin metamorphic core complexes Top of page

In conjunction with my Ph.D. adviser, Art Snoke at the University of Wyoming, I created 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]). This synthis included 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]). My data shows that the WMSZ does not match any of the existing numerical models. Therefore, we proposed that the WMSZ contained stable, segregated, coeval kinematic domains of simple-shear-dominated fabrics and pure-shear-dominated fabrics that accommodate the transcurrent and contractional components of deformation separately.

Publications Top of page

Sullivan, W. A., Beane, R. J., in review, A new view of a very old suture zone: Evidence for sinistral transpression in the Cheyenne belt: submitted to the Geological Society of America Bulletin.

Sullivan, W. A., in press, L tectonites: Journal of Structural Geology.

Sullivan, W. A., Beane, R. J., Beck, E. N., Fereday, W. H., and Roberts-Pierel, A. M., 2011, Testing the transpression hypothesis in the western part of the Cheyenne belt, Medicine Bow Mountains, southeastern Wyoming: Rocky Mountain Geology, v. 46, no. 2. [PDF]

Sullivan, W. A., and Beane, R. J., 2010, Asymmetrical quartz crystallographic fabrics produced during constrictional deformation: Journal of Structural Geology, v. 32, p. 1430-1443. [PDF]

Sullivan, W. A., 2009, Kinematic significance of L tectonites in the footwall of a major terrane-bounding thrust fault, Klamath Mountains, California, USA: Journal of Structural Geology, v. 31, p. 1,197-1,211. [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]