Evolution of Continental Crust

The Origin and Evolution of Continental Crust
Bowring and his research group have been advancing our knowledge of the origin and evolution of continental crust for twenty years. Their current studies range from geological mapping of orogenic belts in Wopmay orogen of the NWT of Canada, the Snowbird zone in Saskatchewan, to the Proterozoic orogen in the southwestern U.S. A crucial part of all these studies is the integration of high-precision geochronology and thermochronology combined with field-based studies to constrain the timing of deformation, magmatism, and metamorphism. Of particular interest are quantifying the rates of geological processes within magmatic arcs and a major project is underway in the Cascades arc.

A newer aspect of our research involves using U-Pb geochronology and thermochronology of lower crustal xenoliths to better understand the thermal evolution of the lower crust and to compare it with exposures of upper crust. We have major studies completed and in progress in the Kaapvaal craton of southern Africa as well as the Four Corners region of the western U.S.

Our current studies range from constraining the temporal evolution of the classical Donegal granites ccintroof Ireland, to understanding the evolution of high P, high T metamorphism in the Snowbird zone (2.0 GPa and 900 °C), to documenting and modeling the thermal history of the Proterozoic orogenic the southwestern U.S., to xenolith studies from the Four Corners region.

The Geology of Donegal: A Geochronological Study of Granite Emplacement and Unroofing
Many mountain belts are characterized by late stage normal faults and bimodal igneous activity, both considered reflective of terminal orogenesis. Understanding the genesis of such features is fundamental to our understanding of geodynamic evolution of mountain belts. This project will address a classic example of such activity, the Caledonian Donegal granitic suite of northern Ireland, by collecting a suite of very high quality radiometric ages on igneous rocks and fabric-forming minerals in shear zones believed to be related to plutonism. These ages will allow us to: (1) investigate the synchroneity of pluton emplacement during construction of the Donegal batholith and the temporal relationship between mafic and granitic magmatism; and (2) integrate the timing of magmatism with other orogenic processes operating (e.g., mid-crustal deformation and supra-crustal basin evolution) allowing us to test geodynamic models (i.e., subduction slab detachment) proposed.

Collaborators: Emeritus Professor Wallace S. Pitcher, Department of Earth Sciences, University of Liverpool; and Professor Donny Hutton, Birmingham University, UK. Funding: National Science Foundation

Magmatic/deformational processes in the mid- to deep-crustal levels of the North Cascades.
Continental magmatic arcs, characterized by volcanic and intrusive magmatism, mountain building and rapidly changing geomorphology, are among the Earth’s most dynamic geologic settings. Much can be learned about the evolution of continental crust by examining the deep levels of an arc at the site where new continental crust is generated. In this project, we focused on evaluating rates of magmatic and deformational processes in the mid- to deep-crustal levels of an ancient continental magmatic arc, the crystalline core of the North Cascades, Washington. This area was chosen because of its superb exposure, numerous previous structural and petrological studies, and its relative youth (70-90 Ma), which permits uncertainties in calculated U-Pb dates to be at the ± 100-200 kyr level. From our work, we hope to constrain three major aspects of arc dynamics as exemplified by this arc: rates of magma chamber construction and magma fluxes, temporal and compositional variations in plutonic additions, and thermal histories and exhumation rates.

Collaborators: Robert B. Miller, San Jose State University; Scott Paterson, USC; Donna Whitney, Univ. of Minnesota; Harold Stowell, Univ. of Alabama; Sue DeBari, Western Washington Univ. Funding: National Science Foundation

The Snowbird Tectonic Zone: Growth, Assembly, and Exhumation of Deep Crust
The western Churchill Province of Canada preserves rocks formed during the growth, deep crustal evolution, exhumation, and stabilization of the Laurentian continental core. The East Athabasca segment of the Snowbird tectonic zone in northern Saskatchewan exposes a 1000’s of km2 of lower crustal granulite-facies rocks that may have resided in the deep crust for hundreds of millions of years. The region contains a number of 10-km-length -scale domains that preserve distinct rock types and tectonic (P-T-t-D) histories, including: some of the oldest eclogites on the planet, a domain of isobarically cooled opx-bearing granitoid (charnockite), a spectacular mafic dike swarm emplaced into the deep crust, and a 6-km-wide shear zone that was active during thrust-related exhumation of the high-grade region. The individual domains record, with impressive clarity, key stages in the tectonic evolution of Laurentia, and together, they provide a map view of the heterogeneity that may characterize the deep crust in general.The overall goals of this research project are to unravel the processes of lower crustal growth and assembly of the East Athabasca area, as well as to develop a tectonic model that explains the long history from Archean assembly to exhumation at 1.9 Ga. Our research at MIT is focused in two specific areas: (1) Documenting the P-T-t paths of high-pressure granulites and eclogites in the Southern Domain and understanding their tectonic implications (Baldwin, Ph.D. dissertation); and (2) Constraining the significance, duration, and distribution of the Chipman mafic dike swarm (Flowers, Ph.D., dissertation). Both projects involve an integrated approach of major and trace-element geochemistry, geochronology, geothermobarometry, thermal modeling, and isotope geochemistry. Our new scientific results for the zone are found at our Snowbird web site.

Collaborators: Michael Williams, University of Massachusetts at Amherst. Funding: National Science Foundation

The Kaapvaal Project: Formation and evolution of Archean cratons of southern Africa
Our work on the Kaapvaal Craton began in 1996 with our involvement in the Kaapvaal Craton Project, a multidisciplinary, multinational joint project directed at addressing fundamental problems of building and preserving a craton. Our contribution to this effort involved numerous geologic, geochronologic and isotopic studies with varying foci. Some of our earliest work in the Kaapvaal Craton capitalized on the abundance of Mesozoic kimberlites in southern Africa, and involved investigating lower crustal xenoliths exhumed during kimberlite eruption. Zircon and Monazite U-Pb geochronology and accessory mineral U-Pb thermochronology (sphene, rutile, and apatite) were utilized in order to document the petrologic and thermal history of both on- and off-craton continental lower crust. These rocks record both the initial formation and cooling of the Archean lower crust, as well as Proterozoic thermal reactivation due to craton-wide thermal events. This work has not only helped to establish more precisely lithospheric geotherms, but also has implications for the transient nature of cratonic geotherms, lithospheric strength, and Moho topography.Another aspect of the Kaapvaal project addresses the problem of craton stabilization by studying the thermal and tectonic history of rocks exposed at the surface today. The southeastern portion of the craton has remained cratonic, both thermally and tectonically, for 3.1 Gyr, despite its close proximity to current and paleo plate boundaries. Thus, a detailed investigation of the timing of crustal deformation (U-Pb zircon dating) and the timing and rates of exhumation and thermal stabilization (U-Pb, 40Ar/39Ar thermochronology) is being undertaken to study the effects of tectonic and thermal perturbations on this transition from orogenically active lithosphere to cratonic keel ca. 3.2-3.1 Ga.

Collaborators: M.J. de Wit, Jelsma, H.A. – University of Cape Town; Grove, T.L., Parman, S.W. – MIT. Funding: National Science Foundation

CD-ROM: Continental Dynamics Rocky Mountain Project
The Proterozoic orogenic belt of southwestern North America provides a rare opportunity to gain insight into the evolution of continental lithosphere. The rapid formation, accretion, and stabilization of Proterozoic lithosphere; the interaction of Archean and Proterozoic lithopshere; and the subsequent modification of the crust make this area an outstanding field laboratory for the processes of lithospheric development. Proterozoic provinces (the Mojave, Yavapai, and Mazatzal) accreted to each other and to the Archean Wyoming Craton between 1.8 and 1.6 billion years ago, resulting in a belt of northeast striking provinces and boundaries (see map below). Between 1.6 and 1.4 billion years ago the terranes experienced slow, near-isobaric cooling and stabilization. From 1.4 billion years ago and on, the Proteozoic belt experienced a number of reactivation events, mostly concentrated along the northeast-striking Proterozoic province boundaries, suggesting a long-term correlation between the mantle and Proterozoic structures. It is the relationship between the mantle and the lithospheric-scale Proterozoic structures of the southwest that are the primary focus of the Continental Dynamics – Rocky Mountain project, a multidisciplinary, collaborative effort of researchers from fifteen institutions.Scientitsts from 14 US universities and one from Germany are involved in this project. They put in place approximately 1,200 seismic instruments and 50 earthquake recording instruments stretching from Wyoming to New Mexico. The 1,200 seismometers recorded vibrations from mine blasts, smaller explosions, and the activities of specially designed vibrating trucks. This information was used to create profiles up to 30 miles deep, going right through the Earth’s crust.

Funding: National Science Foundation