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 during development of the supercontinents Nuna, 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.
My main areas of research have centered both on the orogenic
development of the Ross Orogen in Antarctica (~500 Ma) and crustal
evolution of East Antarctica. Fortunately, there are excellent
exposures of both domains in the Transantarctic Mountains. In addition
to what we learn about Antarctic geotectonic development, these domains
also inform us about assembly and breakup of the supercontinents
Gondwana, Rodinia and Nuna.
The Ross orogenic belt in Antarctica 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. Perhaps the best region to study the Ross Orogen comprehensively is in the central TAM, where we find ancient reactivated basement, rift-margin successions, active-orogen sedimentary-volcanic successions, and a prolific calc-alkaline magmatic arc, all involved in Late Cambrian-Ordovician deformation. There is no other area where we can study all of these tectonic elements of the orogen. 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.
Unfortunately, the area of Archean-Proterozoic basement exposure is
limited, so 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 various indirect means to study the
vast ice-covered interior of the East Antarctic craton. These include
detrital zircon analysis of sedimentary strata and offshore marine
sediment; age and isotopic analysis of crystalline rock clasts
recovered from glacial moraines; and aeromagnetic surveys to image the
subglacial geology. From these types of materials, we focus more and
more on zircon because of its use both as a chronometer and isotopic
reservoir. What’s exciting is that we are discovering entirely new
chapters in the history of the East Antarctic craton, yet none of these
“proxy” methods provides us with direct sampling of the deep interior
geology. For that I am currently developing an entirely new type of
drilling system that will have the ability to fully penetrate the ice
sheets of Antarctica and take core samples at depth, described briefly
Rapid Access Ice Drill (RAID)
“Development of a rapid access ice drilling (RAID) platform for research in Antarctica"; National Science Foundation (Division of Polar Programs, Antarctic Instrumentation & Support)
I am the lead PI on an NSF-funded project that started in 2012 to develop a new mobile drilling platform that we plan to deploy in East Antarctica from the direction of McMurdo and South Pole. The Rapid Access Ice Drill (RAID) will penetrate the Antarctic ice sheets in order to core through deep ice, the glacial bed, and into bedrock below. This new technology will provide a critical first look at the interface between major ice caps and their subglacial geology. Currently in construction, RAID is a mobile drilling system capable of making several long boreholes in a single field season in Antarctica. RAID is interdisciplinary and will allow access to polar paleoclimate records in ice >1 million years old, direct observation at the base of the ice sheets, and recovery of rock cores from the ice-covered East Antarctic craton. Near the bottom of the ice sheet, we will use a wireline bottom-hole assembly to take cores of ice, the glacial bed, and bedrock below. Once complete, boreholes will be kept open with a stabilizing fluid, capped, and made available for future down-hole measurement of thermal gradient, heat flow, ice chronology, and ice deformation. RAID will also sample for extremophile microorganisms. RAID is designed to penetrate up to 3,300 meters of ice and take sample cores in less than 200 hours. Its rapid turn-around will allow us to complete a borehole in about 10 days before moving to the next drilling site. RAID is unique because it can provide fast borehole access through thick ice; take short ice cores for paleoclimate study; sample the glacial bed to determine ice-flow conditions; take cores of subglacial bedrock for age dating and crustal history; and create boreholes for use as an observatory in the ice sheets. Together, the rapid drilling capability and mobility of the drilling system, along with ice-penetrating imaging methods, will provide a unique 3D picture of the interior Antarctic ice sheets. It’s pretty exciting and we are currently on track for eventual science deployment in 2017!
Hf- and O-isotope composition of zircons
“Age and composition of the East Antarctic shield by isotopic analysis of granite and glacial till"; National Science Foundation (Office of Polar Programs, Antarctic Geology & Geophysics)
Zircons are amazing packages of geological information, giving clues to age (by U-Pb) and process of formation (by Lu-Hf and O-isotopes). As people develop ever better techniques for determining ages of individual zircon components by SHRIMP and MC-ICPMS, their trace-element and isotope geochemistry provide tiny vials of petrogenetic information. Jeff Vervoort (Washington State University, and UMD alumnus!) and I recently completed a project to test the provenance of detrital zircons in rift-margin successions in Antarctica. These siliciclastic rocks contain significant proportions of Mesoproterozoic zircons (~1.4 Ga), whose source might be in basement akin to North America. First, we needed to document the Hf-isotope compositions of Laurentian 1.4 Ga granites [link image], and then we were able to compare Antarctic detrital zircons to this distinctive North American igneous province [link image].
With Jeff and Mark Fanning (Australian National University), we are continuing to study the composition and history of the ice-covered East Antarctic shield with a new project to study zircon compositions in glacial clasts and Ross Orogen granites along the length of the Transantarctic Mountains. These two proxy approaches can help to peer into the composition and age of the ancient shield buried beneath the ice cap.
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 continental cratons, with a rich Archean to Proterozoic history. It is the central piece in eastern Gondwanaland, and it played an important role in the Mesoproterozoic amalgamation of the Rodinia supercontinent (~1.3-1.1 Ga). Yet because of nearly complete coverage by the polar ice cap, Antarctica remains the single most unexplored continent and we know little about the composition and structure of the interior. 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 was designed to run from exposed rocks of the Ross Orogen, including the only bona fide Archean-Proterozoic basement of the Transantarctic Mountains (Nimrod Group), and across the adjacent polar ice cap. Survey results [link image] highlight the importance of igneous rocks in the adjacent shield area and help to establish the Neoproterozoic rift margin.
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:
- tectonic relationship between rift-margin and syn-tectonic siliciclastic deposits
- denudation rates of the Ross belt from detrital mineral ages
Transantarctic Mountains SHRIMP geochronology
"SHRIMP U-Pb geochronology of Transantarctic Mountains basement"; National Science Foundation (Office of Polar Programs, Antarctic Geology & Geophysics)
As a follow-up to field-based study of the Nimrod and Beardmore groups, we carried out a detailed U-Pb geochronology investigation of basement and supracrustal rock units of the central TAM. Crystalline basement exposures of the East Antarctic shield (Nimrod Group), juxtaposed against Neoproterozoic and lower Paleozoic marginal-basin sedimentary assemblages (Beardmore and Byrd groups), record Archean crustal growth, Proterozoic crustal modifications, sediment deposition across a Neoproterozoic rift margin, and early Paleozoic plate convergence. Lab work on this project was completed during a research leave to Australian National University in 2000, in collaboration with Ian Williams and Mark Fanning. We now have a detailed age dataset obtained by SHRIMP from samples of igneous, metamorphic and sedimentary rocks. These age data constrain crustal growth of the East Antarctic shield, provide better age constraints on Ross magmatism, and constrain the depositional ages and provenance of Proterozoic and lower Paleozoic siliciclastic rocks of the Ross margin.
Supracrustal rocks of the Ross Orogen
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. This project with Paul Myrow (Colorado College), Ian Williams (Australian National University), and Mike Pope (now at Texas A&M University) was designed to better understand the depositional history of the Beardmore Group, and its role in orogenic events shaping the outer margin of Gondwanaland. Field seasons in the austral summers of 1998-99 and 1999-00 involved field-based study of stratigraphy, sedimentology, and structure, as well as biostratigraphy, chemostratigraphy, and geochronology of detrital, metamorphic and igneous components. In addition to refining the rift history represented by the Beardmore Group, we soon came to recognize that there is much less of this Neoproterozoic assemblage that first thought! As a result, other unexpected but important side projects resulted from this work:
• recognition of a major syn-orogenic (Ross-age) molasse basin
• defining the tectonic relationship between rift-margin and syn-tectonic siliciclastic deposits
• determining denudation rates of the Ross belt from detrital mineral ages
Ross Orogen basement
"Comparative petrologic, structural and geochronometric study of high-grade metamorphic rocks in the Transantarctic Mountains"; National Science Foundation (Office of Polar Programs, Antarctic Geology & Geophysics)
"Petrogenesis and crustal structure of metamorphic rocks in the central Transantarctic Mountains: an integrated petrologic, structural and geochronologic study"; National Science Foundation (Polar Earth Sciences)
I got my start in the geology of the Transantarctic Mountains with study of
high-grade basement rocks that represent the hinterland basement of the Ross
Orogen. Good exposure in the Nimrod Glacier area of the TAM and in the Lanterman
Range provided good opportunity for field-based petrotectonic study of these
high-grade metamorphic rocks. These rocks represent the crystalline roots of
the Ross orogenic belt, and they give us an opportunity to study both crustal
evolution of the East Antarctic shield as well as the role of basement in the
Ross Orogen. Our integrated studies of deformation, petrogenesis, and geochronometry
involved detailed field geologic mapping; structural, microstructural, kinematic
and fabric analysis; metamorphic petrology and quantitative geothermobarometry;
and 40Ar/39Ar and U-Pb thermochronology. The projects were initiated in 1989
and undertaken with David Dallmeyer (University of Georgia), Vicki Hansen (Southern
Methodist University), Simon Peacock (Arizona State University), Brad Smith
(Arizona State University), and Nick Walker (University of Texas). We completed
field seasons in 1989, 1990 and 1993, along with a follow-up visit in 1999.
Pursuing evolution of the Nimrod Group led to an initial SHRIMP U-Pb study with Mark Fanning and Vickie Bennett at ANU. One of the most intriguing things we found was evidence for 2.5 billion years of punctuated Earth history recorded in a single sample of layered gneiss, which was even picked up by BBC News. These old gneisses (~3.0 Ga) were reworked during deep-crustal metamorphism and magmatism during the Nimrod Orogeny at ~1.7 Ga, and then again at about 500 Ma during the Ross Orogeny.
Early Mesozoic accretion-related metamorphism in the Klamath Mountains, northern California
- P-T evolution of coherent and melange-type accretionary belts along the early Mesozoic convergent margin of North America
- thermochronology of olistolithic materials in forearc basins and paleogeographic ties of the accretionary belts to the North American margin
Petrogenetic relationships within pelitic metamorphic rocks, Picuris Range, New Mexico (with Mike Holdaway, SMU)
- rock-buffered fluid evolution determined from mineralogical and stable isotope evidence
- rock pressure vs. fluid pressure as a controlling influence on mineral stability
Leucoxene fish as a micro-kinematic indicator in quartz-rich rocks