Information for prospective graduate 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. See below for information about ongoing projects.           Jennifer Wright, PhD student, doing field work in the Gorges Columbiere, Montagne Noire dome, southern France

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 questions that we have been working are:

• 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) and current PhD student Jennifer Wright. Past work on domes has involved former graduate students Stacia Gordon (PhD 2009 - now Assistant Professor, Univ of Nevada - Reno), Seth Kruckenberg (PhD 2009 - Research scientist, Univ of Wisconsin-Madison), and Rory McFadden (PhD 2009 - now Assistant Professor, Salem State Univ). We have worked on and/or are working on domes in western North America, Antarctica, the Aegean region (Greece), Turkey, France, and Australia.

Titanium in deforming quartz and the thermomechanics of extensional detachment and thrust systems

from the NSF grant abstract:

This research is an investigation of naturally and experimentally deformed quartz-rich samples to study the role of deformation in the distribution of Ti and other trace elements in quartz over a wide range of temperature and deformation/recrystallization processes. The mobility of Ti in deforming quartz is addressed directly by a set of high-pressure/high-temperature deformation experiments performed in the SiO2 -TiO2 system. Experiments are designed to test the role of dynamic recrystallization, and in particular subgrain rotation and grain boundary migration, on the efficacy of Ti migration in Qz. In parallel, this research targets a wide range of temperature conditions in nature (zones of high metamorphic gradients) as well as zones of diverse microstructures. Two groups of tectonic settings are investigated for this purpose:

(1) Thrust systems , in which rocks typically deform at low differential stress and strain rates during prograde metamorphism; microstructures equilibrate in a relatively narrow range of temperatures (300-450°C) but display a wide range of dislocation creep regimes involving various types of recrystallization dominated by bulging of grain boundaries and subgrain rotation at high stress, and grain boundary migration at lower stress.

(2) Extensional detachments that bound metamorphic core complexes; deformation fabrics form at high differential stress and strain rates in a wide temperature range (300 to 700°C), leading to subgrain rotation-dominated recrystallization.

In experiments as well as in natural examples from exhuming (detachment) and prograde (thrust) systems, our research focuses on how inherited Ti is redistributed as a function of deformation and recrystallization as well as T conditions.

Field-based research takes advantage of existing collections (e.g. Heavitree Quartzite in the Ruby Gap Duplex, central Australia) and ongoing research on extensional detachments in the western US that include the Columbia River (BC), Bitterroot (MT), and Raft River (UT) detachments. Additional fieldwork targets the Siviez-Mischabel nappe (W Alps) and the Kettle (WA) and Snake Range (NV) detachments.

Research methods include ion microprobe analysis of Ti in quartz in experimental and natural samples; petrographic and electron backscatter diffraction characterization of quartz microstructure, types of recrystallization, and crystallographic preferred orientation; cathodoluminescence imaging of trace element zoning in quartz; and electron microprobe analysis of major elements in coexisting minerals. Fe-Mg exchange thermometry and stable isotope thermometry results can be compared with Ti-in-quartz thermometry to determine whether different thermometers record different events if deformation continues after the thermal peak of metamorphism.

This research involves PhD student Will Nachlas, research scientist Nick Seaton, colleague Greg Hirth (Brown Univ), and UMN faculty Donna Whitney and Christian Teyssier.

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.
• 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).
• A study of inclusions in minerals (garnet, zircon) using laser Raman spectroscopy.

Garnet X-ray maps showing concentric zoninc of Mn and Ca (higher abundance of each element in garnet core, lower at rim), with a line (white/dashed) marked to show the location of a high-angle grain boundary detected by electron backscattered diffraction (EBSD) analysis. From Whitney & Seaton (2010).

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.

This research has involved former graduate student Eric Goergen (PhD 2009 - now a postdoc at Brown University) and research scientist Nick Seaton.

Evolution of continental lithosphere from collision to escape: the record in the rocks and landscape of Central Anatolia

View WNW from Nigde Massif to the Aladag Mountains

I have been working in central and western Turkey for the past 17 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 and East Anatolian faults, and oblique motion related to the CAFZ is recorded in exhumed mid-crustal orogenic rocks adjacent to the fault. Our research group has 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.

Large-magnitude lateral and vertical displacement of the lithosphere occurs in all orogens and is accommodated by motion of the mantle, crust, and surface of the Earth. The partitioning of lateral flow (distributed vs. localized) and the relative amounts of vertical and lateral displacement in time and space is controlled by the physical and thermal state of the lithosphere and changes in far-field plate motions. In the Cenozoic, the collision of India and Arabia with Eurasia resulted in orogenic plateaux behind the continental indenters and zones of tectonic escape between zones of collision and extension.

In the Arabia-Eurasia collision zone, a transition from collision to escape tectonics is recorded by the Anatolian plate. Anatolia is part of an orogenic belt extending from the Pyrenees to SE Asia. All regions of the orogen experienced some degree of large-scale oblique displacement, but Anatolia records a transition from distributed to localized lateral motion (escape tectonics) that resulted in major lateral and vertical motion, voluminous volcanism, and landscape reorganization. The Anatolian plate today appears to be moving as a rigid body between 2 strike-slip faults away from a collision zone and towards an extensional domain associated with a retreating subduction zone (Aegean), but the geologic record of Anatolia reveals a > 60 million year history of deformation.

Subduction Petrofabrics

lawsonite blueschist, Sivrihisar, Turkey. Field of view ~ 3 mm.

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 (Teyssier et al., 2010 - Geology). 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). This area is particularly interesting because it is one of the few places in the world where lawsonite eclogite has been exhumed to the Earth's surface (although lawsonite eclogite is predicted from phase equilibria and experimental studies to be common in all but the hottest subduction zones). In addition, the massif contains a range of mineral assemblages and structures that record the subduction-to-collision transition: from pristine high-pressure rocks and (micro)structures, a transitional zone of partial overprint, and a complete transition to lower-pressure/higher-temperature metamorphism (Whitney et al., 2011).

This research involves Dr. Nick Seaton (postdoc, UMN), Christian Teyssier, and PhD students Erkan Toraman and Katherine Fornash. Previous work was done by former graduate student Peter Davis (PhD 2008 - now Assistant Professor, Pacific Lutheran University).

Continental Subduction and Deep Crustal Melting

Continental crust can subduct to, and return from, mantle depths. The records of this deep subduction (and return) are in ultrahigh-pressure (UHP) metamorphic complexes, some of which seem to have been exhumed very rapidly. Our group is investigating migmatites (containing crystallized melt) in the Western Gneiss Region to determine their pressure-temperature-time history relative to the pods and lenses of UHP rocks that occur within the migmatite. We are interested in determining whether (and if so, how) partially molten crust is important in the metamorphic and tectonic history of subducted continental crust, and in evaluating the conditions of deformation in mylonite zones that host some of the (U)HP rocks.

Roxanne Renedo, Christian Teyssier, Haakon Fossen on the outcrops in Norway

This research involves current PhD student Roxanne Renedo, former PhD student Stacia Gordon (PhD - 2009), colleague Haakon Fossen (Univ of Bergen), and UMN faculty Christian Teyssier and Donna Whitney.

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 Nov 2011
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