Information for prospective students in metamorphic geology & tectonics

Graduate students who work with me are typically involved in field-based research, but also use a wide range of analytical and modeling techniques for chemical and microstructural analysis of rocks and minerals. The University of Minnesota has many facilities accessible to students, so the types of research that can be done in the general fields of petrology and tectonics are greatly varied. I am particularly interested in integrating metamorphic petrology and structural geology/tectonics, and exploring new applications of electron microscopy and experimental mineral deformation to studies of metamorphic minerals and textures.     Ph.D. student Stacia Gordon in the North Cascades, summer 2007

Some current and future projects include

Gneiss Domes & Orogeny

Gneiss domes cored by migmatites occur worldwide in collisional orogens of all ages. Their origin has been debated for more than 50 years. Typically, more than one is present, they are elongate and aligned parallel to the strike of the orogen, and they are characterized by a core of migmatites and plutons surrounded by high-grade metasedimentary rocks. Some unresolved questions about gneiss domes include:

• What is the thermal and/or mechanical significance of gneiss domes in orogeny?
• What are the internal dynamics of domes?  
• How do dome rocks relate (structurally, thermally) to their overlying metamorphic rocks and structures (which in many cases are detachment faults of metamorphic core complexes)?
• Why do domes typically occur in swarms or belts, and what is the significance of their size, spacing, and distribution?
• What are the pressure-temperature-time paths of metamorphic rocks within and beyond the dome?
• What is the significance of migmatites in the cores of domes?
• Is there a characteristic timing of partial melting and deformation relative to dome formation?
• Is there a characteristic timing or rate of exhumation of the high-grade cores of domes?

This work on gneiss domes is in collaboration with Christian Teyssier and Patrice Rey (University of Sydney). Four of our current students are working on aspects of gneiss dome/core complex structures in the Cordillera (Seth Kruckenberg, Rory McFadden, Stacia Gordon, Erkan Toraman), the Aegean (Seth Kruckenberg), and Antarctica (Rory McFadden -- in collaboration with Dr. Christine Siddoway of Colorado College), but there is much work still to be done.

The chemical and physical properties of metamorphic minerals such as garnet and Al2SiO5 polymorphs (andalusite, kyanite, sillimanite) have long been central to petrologic and structural studies of orogeny because these minerals may record the pressure, temperature, strain, and fluid histories of rocks through time.

Methods and applications that my research group has been involved with include field, analytical, and experimental study of metamorphic minerals. Our goals are to understand the rates and mechanisms of high-T geological processes (heating, burial/decompression, mineral growth, deformation), the record of mineral-fluid-deformation interactions in the Earth through time, the mechanical behavior of minerals during regional metamorphism, and the expressions of these processes at various rates in metamorphic textures.

Examples of ongoing and future work include studies of garnet, Al2SiO5, and reaction textures:

• A microstructural study of garnets to determine garnet growth histories in relation to chemical zoning and mineral inclusion fabrics; this is an example of the application of electron backscattered diffraction (EBSD) to metamorphic petrology. This research is currently funded by an NSF grant.
• An investigation of the fracture mechanics of garnet and associated chemical effects of garnet fracture during metamorphism.
• A study of the relationship between shape-preferred orientation and crystallographic-preferred orientation of mineral inclusions in garnet, to document the conditions and consequences of garnet-inclusion interactions .
• A study of the conditions (rates, paths, temperatures) required to produce symplectite textures around Al2SiO5 phases and garnet (and other minerals) (with Ph.D. student Eric Goergen).
• A study of inclusions in minerals (garnet, zircon) using laser Raman spectroscopy.
• A microstructural study of calcite-aragonite textures in high-pressure marbles, in particular the development and overprinting of fibrous textures in marble.

Possible future projects involving students relate to applications of EBSD to metamorphic textures, continuation of the experimental deformation of Al2SiO5 study (including study or microstructures of Al2SiO5 in rocks), and research involving the chemistry and microstructure of mineral inclusions.

Continental arc evolution (Cascades, Washington)

I have worked in the North Cascades for many years, and have long been intrigued by the types of petrologic/tectonic questions that can be studied in this orogen owing to the spectacular exposure of rocks representing 0-40 km depth in an exhumed continental arc. This work is currently funded by an NSF grant for a collaborative project with Professors Bob Miller (San Jose State University) and Sam Bowring (MIT), although this phase of the research will be completed by summer 2009.

The migmatites and orthogneisses of the Skagit Gneiss comprise a large part of the core of the North Cascades. The chemistry and P-T history of the Skagit has been studied, but this work has never been integrated into a structural context: what is the record of crustal flow in the migmatites and orthogneiss (vertical? lateral?), and what is the overall structure of this part of the orogen? The crystalline core has been characterized as broadly antiformal -- how do the migmatites fit into this context? Some of these questions are the focus of Stacia Gordon's Ph.D. research.

The North Cascades are an excellent place to observe the interactions of tectonic and surficial processes: is there any dynamic link between the Cretaceous-Tertiary high-T history (metamorphism, magmatism, deformation/exhumation) and the later (Miocene) history of uplift? Low-T thermochronology data (Peter Reiners, Todd Ehlers et al.) provide a fascinating view of the uplift and erosional history, but is there any relationship between these events and earlier tectonic processes? One possible focus would be investigating the evolution of the Skagit River gorge. Why does the river bend from N-S to E-W across the high-grade core of the orogen? When did the river develop in relation to the tectonic history of the orogen? Is it an entirely young feature or does it have an older history? Why did the river incise such a deep gorge across the range?

On the eastern margin of the range, in the Ross Lake fault zone, there is a layered mafic intrusion with a granulite facies contact aureole. I have done some work on this mafic complex (Skymo Complex), but only on the southern (most accessible) end of it. The tectonic and petrologic relationship of the complex to the adjacent rocks (Skagit Gneiss on the west, Little Jack terrane on the east) and the Ross Lake fault zone is still open to question, as well as the geodynamic significance of the primitive mafic rocks in the evolution of the orogen. Preliminary (that is, not very good) geochronology data suggests that the complex is quite young -- middle Eocene -- and therefore one of the last petrologic events to affect the orogen. This is the same age as a nearby A-type granite, the Golden Horn batholith, which is on strike with the Skymo Complex and is also of uncertain origin in terms of the evolution of the arc. This project requires someone who likes to climb around on high mountains -- technical rock-climbing skills are not necessary, but you'd have to like hiking up steep ridges and climbing around on high rocky peaks.

Metamorphism, deformation, exhumation, and the evolution of strike-slip faults

I have been working in central and western Turkey for the past 14 years, focusing mostly on using the well-exposed rocks and structures to understand metamorphic and deformation processes in different tectonic regimes. One of the most interesting locations is in central Turkey, where metamorphic rocks are exhumed adjacent to a major strike-slip fault, the Central Anatolian fault zone (CAFZ). This structure pre-dates the more famous and seismically active North Anatolian fault, and oblique motion related to the CAFZ is recorded in exhumed mid-crustal orogenic rocks adjacent to the fault. Christian Teyssier, Paul Umhoefer (Northern Arizona University), Annia Fayon and I have worked out the history of vertical motion related to the fault over the past ~ 100 million years, but there are interesting questions unresolved about the later vertical vs. lateral motion of this and other intracontinental strike-slip faults and about the interaction of the fault system with metamorphic complexes to the north of the region we have studied -- for example, where the CAFZ bends to meet the North Anatolian fault. This project involves components of metamorphic geology, structural geology, and geomorphology.

Subduction Petrofabrics

Lawsonite eclogite and lawsonite blueschist, Sivrhisar, Turkey

The rheology of subducted crust strongly influences the geophysical and geochemical evolution of convergent plate boundaries, including the interaction of deformation, metamorphism, and fluid-rock reaction during subduction and exhumation, and the rates of processes such as exhumation of high-pressure rocks. This NSF-funded research addresses questions of the mechanisms of large-magnitude transport (exhumation) of high-pressure rocks in subduction zones by characterizing the kinematics of deformation from the map scale to the grain-scale using syn-kinematic high-pressure index minerals. Microstructural, petrologic, and stable isotope analyses are used to determine deformation mechanisms, flow laws, and fluid-rock interaction in the context of high-pressure conditions.

The focus of this investigation is the Sivrihisar Massif, Turkey: an exhumed subduction complex with pervasive, syn-kinematic high-pressure assemblages preserved in metasedimentary (marble, quartzite) and metabasaltic (eclogite, blueschist) rocks (Davis & Whitney, 2006; Whitney & Davis, 2006, 2008).

The research involves Dr. Nick Seaton (postdoc, UMN), Christian Teyssier, Erkan Toraman, and will continue through 2009, with possible future work focusing on the transition from high-pressure/low-temperature subduction fabrics and assemblages to lower-pressure/higher-temperature (Barrovian) fabrics and assemblages. A structurally continuous transition is preserved in the Sivrihisar Massif, from blueschist/eclogite to sillimanite zone rocks.

See my research pages for additional information about other projects. Please contact me if you would like to discuss these or other options for graduate research at the University of Minnesota.

*** Graduate students whom I advise do not necessarily need to work on theses related to projects I am already working on. I am always interested in discussing ideas for new projects, or new directions for ongoing research. ***
 

The application materials for the University of Minnesota are mostly online. See the department's Graduate Program webpages for links and information.
 

page last updated 06-08
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