of the Medicine Bow orogenic belt
Deformation mechanisms near the brittle-ductile transition Top of page
Theoretically, the brittle-ductile transition is the strongest part of the Earth's crust. Hence, the mechanisms that localize and accommodate deformation in this zone are some of the most important variables controlling plate-scale processes. My students and I are working to better understand the strain-localization mechanisms at the brittle-plastic transition and the long-term evolution of crustal-scale fault zones by using the Kellyland fault zone in eastern Maine as a natural laboratory. This fault zone cuts both homogeneous granite and heterogeneous metasedimentary rocks. The granite records a three-phase history of relatively rapid strain localization including: (1) an initial high-temperature phase where the rheology was dictated by dislocation creep of quartz, (2) a transient phase of largely brittle deformation, and (3) a final long-term phase of ductile deformation where the rheology was governed by granular flow of ultra-fine-grained, poly mineralic agregates formed during the brittle phase (Sullivan et al., in review). This year we are extending our research into the metasedimentary rocks to compare their deformation and rheologic evolution with the granite.
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. This project tested existing models for the Medicine Bow orogeny using detailed kinematic analyses of the Cheyenne belt shear zones. Initially our more detailed analyses of these shear zones agreed with existing datasets. However, we interpreted the Cheyenne belt in these areas as a stretching fault system rather than a thrust system rotated into its present-day subvertical orientation during late-stage folding (Sullivan et al., 2011 [PDF]). Subsequently, we found evidence for significant sinistal strike-slip motion in the Cheyenne belt shear zones. Fabrics related to sinsitral strike-slip are largely overprinted by fabrics related to SE-side-up motion, but serveral lines of evidence indicate that this was the dominant deformation style along the belt. These data led us to propose a new model for the evolution of the Medicine Bow orogenic belt (Sullivan and Beane, 2013 [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.
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 integrated with additional published data and models to provide a concise overview of L tectonites that will aid geologists in interpreting this strain phenomenon (Sullivan, 2013 [PDF]).
Northern Great Basin metamorphic core complexes Top of page
Publications Top of page
Sullivan, W. A., Boyd, A. S., and
Monz, M. E., in review, Strain localization in
homogeneous granite near the brittle-ductile
transition: A case study of the Kellyland fault zone,
Maine: submitted to the Journal of Structural Geology.
Sullivan, W. A., 2013, L
tectonites: Journal of Structural Geology. [PDF]
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, p. 111-135. [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.
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]