Research interests
My research interests are in metamorphic petrology, structural geology, isotope geochemistry and thermochronology as applied to problems in continental tectonics. In particular, I'm interested in the relation of metamorphism, deformation, and fluid-flow to crustal growth during convergent-margin and collisional orogenesis. I am also interested in applying what we can learn about mountain belt evolution to gaining a better understanding of paleogeographic relationships between cratonic elements in early Earth history, particularly for Rodinia and Gondwana.I do field-based studies that combine geologic mapping, structural analysis, and petrographic study, with analytical approaches involving kinematic interpretation of petrofabrics, quantitative mineral analysis, geothermobarometry, thermochronology and isotope geochemistry. I address the timing and rates of orogenic processes mainly by 40Ar/39Ar and U-Pb thermochronology, and I use isotopic systems such as Lu-Hf and Sm-Nd to help constrain crustal growth. I also use stable-isotope geochemistry and mineralogy to address questions of fluid interactions with solid phases during metamorphism. Integration of these techniques allows us to constrain the sequences of events, crustal conditions, rock displacements, and the role of fluids associated with tectonic processes.
I have worked in a variety of tectonic settings, including crustal extension, subduction, rifting, and collision, particularly in the Cordilleran and Rocky Mountain regions of North America, and the Transantarctic Mountains of Antarctica. In these areas I have studied the structural and petrologic evolution of convergent plate margins (Klamath Mountains, California), Proterozoic and Archean crustal evolution (Antarctica), the structural evolution of metamorphic core complexes (Mojave Desert of California and Arizona, and Omineca belt of eastern Washington), the effects of fluids, deformation, and rock composition on mineral stability (Proterozoic belts of northern New Mexico and Colorado), strain partitioning during continental-margin transpression (Ross Orogen, Antarctica), and the use of mineral chronometers to trace sediment transport and denudation rates (Ross Orogen, Antarctica). I am now developing new work in Early Proterozoic and Archean belts in the upper Midwest.
| Most of my work in recent years has addressed the tectonic evolution of the Ross orogenic belt in Antarctica, which spans a very interesting time in Earth history between about 1 billion to 500 million years ago. Relationships in the Ross belt are helping us to better understand the tectonic changes that took place during the transformation from the Rodinia supercontinent to Gondwanaland, as well as the relations between Antarctica, Australia, and North America. I started working in the metamorphic basement to the Ross Orogen in order to understand its role in evolution of the East Antarctic shield. Recently we have been working on the supracrustal successions in order to track the sedimentary response to tectonism and changing provenance through time. Together, addressing both the mid-crustal and supracrustal elements gives us a better picture of the orogen as a whole. Using detrital minerals in the cover succession, for example, we can better tie the two together and address the denudation rates associated with magmatism, tectonic uplift and erosion. |
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New directions involve “peeking” underneath the ice cap of Antarctica to better understand crustal evolution of the East Antarctic shield. We are trying this with a combination of aeromagnetic surveys and sampling of glacial deposits as proxies of the ice-covered areas.
Hf-isotope
composition of igneous zircons
Zircons are amazing packages of geological information, including age (by U-Pb) and process of formation (by Lu-Hf). As people develop ever better techniques for determining ages of individual zircons by SHRIMP and ICP-MS, their trace-element and isotope geochemistry provide tiny vials of petrogenetic information. I am working with Jeff Vervoort (Washington State University, and UMD alumnus!) on a project to determine the Hf-isotope compositions of both igneous and detrital zircons in order to better understand the petrogenesis of the igneous hosts and the provenance of the detrital grains in siliciclastic rocks. This work is just getting underway, but has applications in both igneous petrology, crustal evolution, and sedimentary provenance.
Glacial proxies
of the age and composition of the East Antarctic shield
“ISET:
Integrated Study of East Antarctic Tills”; National Science Foundation
(Office of Polar Programs, Antarctic Geology & Geophysics);
collaborative with Kathy Licht (IUPUI)
"Glacial proxies of East Antarctic shield basement in Wilkes Land, Antarctica"; National Science Foundation (Office of Polar Programs, Antarctic Geology & Geophysics)
A variety of methods have been
used to remotely study the ice-covered shield of East Antarctica,
including over-ice geophysics and sub-ice drilling. The ice itself,
however, provides samples of the covered basement, much like volcanoes
provide proxy samples of Earth’s mantle. In these projects, we are
determining the provenance of glacial materials through petrologic and
age analysis in order to constrain the composition and age of
ice-covered source terrains.
In the ISET project, Kathy Licht and I are studying the composition of
glacial tills accumulated along the “backside” (next to the polar
plateau) of the Transantarctic Mountains and carried by the large
outlet glaciers out to the Ross Sea. Kathy wants to know what balance
of ice moved into the Ross Sea during the LGM period from West and East
Antarctica. I want to know what’s under the main East Antarctic ice
sheet. We have just finished sampling of tills in the Nimrod and Byrd
glacier areas, and these samples will be used for petrology of large
clasts, pebble compositions, detrital zircon ages, sand petrography,
size fraction analysis, and Nd-isotope study. UMD student Devon Brecke
is starting work on these samples for her MS thesis.
I am also working on marine glaciogenic deposits along the George V and
Adélie coast regions of Wilkes Land in order to characterize the
adjacent crystalline basement provinces. Glaciogenic sediment samples,
provided by Eugene Domack (Hamilton College) and Amy Leventer (Colgate
University), were collected in 2001 from the R/V Nathaniel B. Palmer.
We will be conducting petrologic and age analysis of the larger igneous
and metamorphic clasts collected by dredge hauls, and detrital zircon
analyses of sand fractions obtained from cored diamictons. SHRIMP U-Pb
zircon and monazite age analyses will be completed in collaboration
with Mark Fanning at the Australian National University. Together,
integration of ages obtained from both sources will provide a good
representation of the EAS terrains underlying the Wilkes Land ice
sheet. We are also working on Permo-Triassic sandstones of the Beacon
Supergroup and Miocene sands drilled by DSDP in order to get a
time-integrated sample set.
Transantarctic Mountains geophysics
"REVEAL (REmote View of East Antarctic Lithosphere): Geophysical mapping sub-ice crustal provinces in East Antarctica"; National Science Foundation (Office of Polar Programs, Antarctic Geology & Geophysics)
The East Antarctic shield represents one of Earth's oldest and largest cratonic assemblies, with an extensive Archean to Proterozoic history. It is the central piece in the eastern Gondwana mosaic, and it likely played an important role in the Neoproterozoic amalgamation of the Rodinia supercontinent. Yet because of nearly complete coverage by the polar ice cap, Antarctica remains the single most unexplored continent and we know little about its composition and structure. To peer through the ice cover, airborne geophysics is the best approach to characterize broad areas of sub-ice basement. With Carol Finn (USGS) and collaborators from the BGR (Germany), we completed an airborne magnetic survey during the 2003-04 austral summer across a window into the shield where it is exposed in the Nimrod Glacier area of the central Transantarctic Mountains. Our transect is designed to run from exposed rocks of the Ross Orogen, including the only bona fide Archean-Proterozoic basement of the Transantarctic Mountains (Nimrod Group), across the adjacent polar ice cap.
Ross orogen supracrustals
" Structure and sedimentology of the Beardmore Group, Antarctica: Latest Neoproterozoic to early Paleozoic tectonic evolution of the East Antarctic margin"; National Science Foundation (Office of Polar Programs, Antarctic Geology & Geophysics)
The Neoproterozoic to early Paleozoic transition is a critical period in Earth history. Coalescence and fragmentation of the supercontinent Rodinia, and subsequent amalgamation of Gondwanaland, coincided with major orogenesis, continental denudation, faunal diversification, sea-level fluctuations, and changes in sea-water composition. Craton-margin sedimentary sequences spanning this transition, including the Beardmore Group in the Transantarctic Mountains, provide detailed records of sedimentation patterns, sea-level fluctuations, faunal distributions, and post-depositional tectonism. Our work on this project (with Paul Myrow of Colorado College, Ian Williams of Australian National University, and Mike Pope of Washington State University) was designed to provide a better understanding of the Beardmore Group, its depositional history, and its role in orogenic events shaping the outer margin of Gondwanaland. We completed field seasons in the austral summers of 1998-99 and 1999-00. The project involved detailed field-based study of stratigraphy, sedimentology, and structure, as well as biostratigraphy, chemostratigraphy, and geochronology of detrital, metamorphic and igneous components.
Two unexpected but important side projects resulted from this work: