W. A. Sullivan Research

Emergent projects

L Tectonites

Northern Great Basin metamorphic core complexes

White Mountain shear zone

Publications

Bill Sullivan Home

Colby Geology Home

Back to Colby Geology Faculty and Staff

Last Updated September, 2007

1. 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 to discern the causes and significance of heterogeneity within these systems such as the spatial and temporal distribution of magmatism within a volcanic arc or the diffuse partitioning of strain along transcurrent plate boundaries. Therefore, the underlying theme of my research in structural geology is understanding the causes and significance of heterogeneities within geologic structures 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.

2. Emergent projects Top of page

2.1. Evolution of the Medicine Bow orogenic belt    The Medicine Bow orogeny marks the onset of the accretion of over 1000 km of continental crust (the Colorado Province) onto the southern Margin of the Archean Wyoming Province. Timing of deformation along the suture zone (known as the Cheyenne belt) between the Wyoming and Colorado Provinces is well constrained to 1.78–1.74 Ga by cross-cutting relationships with intrusive bodies. The middle- and lower-crustal roots of this orogenic belt are well exposed in the Sierra Madre, Medicine Bow, and Laramie Mountains of southern Wyoming; and the regional geology is relatively well understood. Moreover, there has been little extensional overprinting of contractional deformation features in this area as with other deep-crustal exposures of orogenic belts. For these reasons, I feel that the Medicine Bow orogenic belt provides a unique opportunity to study the processes occurring in the middle and lower crust during a large-scale contractional (potentially transpressional) orogenic event. Potential studies in this area of both regional and global (process-oriented) importance include detailed spatial and kinematic analyses of the plastic shear zones within the Cheyenne belt in order to better understand strain partitioning in this orogenic system, and coupled metamorphic, petrogenetic, and high-temperature thermochronologic studies of rocks deformed during the Medicine Bow orogeny that will better constrain the evolution of the lower crust during contractional deformation and continental assembly.

2.2. Precambrian history of the Albion-Raft River-Grouse Creek metamorphic core complex    Metamorphic and igneous rocks exposed in the footwalls of the range-bounding low-angle detachment faults in the Albion, Raft River, and Grouse Creek Mountains, Utah and Idaho include a Precambrian supra-crustal package that is intruded by granitic, trondhjemitic, and gabbroic rocks tentatively assigned an Archean age. However, little is known about the pre-Mesozoic deformational, metamorphic, and magmatic history of what is probably the largest exposure of Archean rocks in the Cordillera west of Wyoming. Potential research projects in this region include detailed mapping and structural analyses of the Precambrian footwall rocks coupled with petrologic and in-situ geochronologic analyses in order to separate Mesozoic and Tertiary metamorphic and deformational overprinting from earlier Precambrian metamorphism and deformation.

3. Significance of L tectonites Top of page

L tectonite from the Laramie Mountains

 

Prolate stretched cobles from the Klamath Mountains

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); 2) mafic metavolcanic rocks deformed during oceanic terrane accretion exposed in the Klamath Mountains, Cal.; and 3) quartzite, schist, and granite deformed in a footwall shear zone of a metamorphic core complex exposed in the Raft River Mountains, Utah. 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.

4. Northern Great Basin metamorphic core complexes Top of page

Detachment surface (skyline) in the Raft River Mountains

 

Metamorphic and igneous infrastructure in the Ruby Mountains

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.

5. 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.

6. Publications Top of page

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

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

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