Introduction
My primary research interests focus on the kinematic evolution of highly extended terrains and the tectonics of exhumed high temperature and (ultra)high-pressure rocks, and I use them to address a wide variety of global geodynamic questions. The various kinematic models for the formation and evolution of these structures have profound implications for geologic processes as far reaching as the formation of sedimentary basins, melt generation, spreading center dynamics, and kinematics of syn- to post-orogenic collapse of overthickened continental crust. Furthermore, deep crustal rocks exhumed in highly extended terrains preserve phase assemblages and fabrics that provide a virtual window into the lower crust, and represent a unique opportunity to investigate questions regarding crust–mantle interactions and lower crustal flow, as well as the timing, metamorphic conditions and kinematics of continental collisions and ultrahigh-pressure tectonics. |
Projects
NORWEGIAN ULTRAHIGH-PRESSURE EXHUMATION
Pelitic schist with ~15 kbar porphyroblastic phase assemblage cut by ~10 kbar asymmetric shear fabrics associated with displacement along the Nordfjord–Sogn Detachment Zone. |
The Caledonides of western Norway represent one of the largest and perhaps best-exposed ultrahigh-pressure (UHP) terranes in the world. These terranes are thought to have been primarily exhumed along the Nordfjord–Sogn Detachment Zone (NSDZ), a multi-kilometer thick package of normal-sense asymmetric tectonites. Using field work and geologic mapping along the Hornelen segment of the NSDZ in conjunction with (micro)structural geology, thermobarometry and Sm/Nd garnet geochronology, I was able to show that asymmetric deformation along the NSDZ was responsible for the exhumation of UHP rocks from the base of the crust, but not from mantle depths (GSA Bulletin paper). My ongoing work in the Hornelen area focuses on muscovite 40Ar/39Ar thermochronology from the detachment tectonites. |
LOWER-CRUSTAL FLOW, LIVERPOOL LAND, EAST GREENLAND
| The Greenland Caledonides formed in the upper, or overriding plate of the Caledonian Orogeny, and present a unique opportunity to study deformation styles in the retro-arc of a continental collision. Our preliminary work in Liverpool Land, has identified amphibolite–eclogite/granulite-facies rocks at three distinct structural levels and orogenic events spanning >40 Myr (GSA 2007 poster). By correlating the Liverpool Land gneisses with the well-defined tectonostratigraphy farther to the west, this study will provide a first look at orogen-normal patterns in the timing of early Caledonian anatexis and melt generation, and add insight into the relationships between lower-crustal flow and crustal thickening, extension, and exhumation of weak lower–mid crustal material in contractional settings. |
 Kyanite + garnet gneiss saturated by plagioclase-rich melt. Note the deformed and split kyanite crystal immediately above the pencil.
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KINEMATICS OF DETACHMENT FAULT CORRUGATIONS
| Meter through 10’s-of-km-scale extension parallel folds, or corrugations, have been described associated with normal-sense detachment faults in a variety of tectonic settings including mid ocean ridges, core complexes, and lithospheric-scale shear zones. Of the many competing models for the formation of these enigmatic structures, a series of previously unidentified extension-parallel faults in the Northern Snake Range metamorphic core complex of east-central Nevada (MS Thesis) and simple finite-element modeling of exhumation and muscovite cooling ages in western Norway (AGU poster) suggest that along-strike variations in strain rate may play an important role in the formation of detachment fault corrugations. Ongoing field mapping and structural analysis of second-order folds in the Northern Snake Rage and in the Catalina Mountains of southern Arizona will further test this differential strain rate hypothesis. |
 Second-order fold in quartzite cascading toward the Sacramento Pass Basin and with its axis slightly oblique to the extension direction. Could this be an example of a flow perturbation fold? |
NEW APPLICATIONS IN LA-MC-ICPMS
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At the University of Arizona AZ LaserChron Center, I am involved in the development of several new geochronologic and geochemical applications for laser ablation multi-collector ICPMS (LA-MC-ICPMS). Projects that I am working on include: the development of suitable apatite and titanite standards for U/Pb geochronology, U/Pb age mapping of metamorphic/igneous zircon overgrowths on inherited cores (GSA 2007 poster), and the measurement of trace elements in zircon. Figure caption:
U/Pb age map plotted on a cathodoluminescence image shown with the corresponding concordia plot from an East Greenland paragneiss. This grain displays a low-U Archean core (2426 ± 150 Ma) with high-U Caledonian overgrowths (440 ± 25 Ma). |
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