Evolution of the SW Laurentian Margin: Implications for Proterozoic Crustal Growth and Geodynamics

点击翻译按钮获取中文摘要

This research is directed at improving our understanding of the formation and dispersal of the Laurentian (ancestral North America) supercontinent and the relationship between these events and the episodic generation and preservation of continental crust along its southwestern margin. The southwestern margin of Laurentia is globally unique because it contains remnants of a former Archean microcontinent (Wyoming province) and two distinct Paleoproterozoic orogenic events at 1.6-1.9 billion years ago and 2.2-2.5 billion years ago, which produced new continental crust. Formation of Laurentia in the Paleoproterozoic coincides with a global period of exceptionally rapid growth of juvenile crust and aggregation of existing micrcontinents and arcs during the late Paleoproterozoic (1.6-2.0 billion years ago). This period of rapid growth was preceded, however, by a global period (2.0-2.5 billion years ago) with a pronounced paucity of preserved crust. Crust generated during this interval is less commonly preserved than crust from periods preceding and succeeding it in the global rock record. In addition, such crust is unevenly distributed among the modern continents. It is more commonly preserved in modern southern hemisphere continents, making its presence along the southwest Laurentian margin unique. To complete this research scientists from the University of Florida are conducting a detailed and integrated geochemical and structural/tectonic investigation of both the early and late Paleoproterozoic rocks preserved along a part of the southwestern margin of Laurentia. This is being accomplished by geologic mapping in two critical areas, the Highland Mountains of Montana and the Wasatch Mountains of Utah, which form the basis for sampling this crust for chemical, isotopic, and petrologic studies. This work is leading to more complete understanding of the ages and origins of crust and lithosphere generated during the Paleoproterozoic and will provide critical and unique insight into the global scale problem of low crustal production rates in the early Paleoproterozoic vs. much higher rates characteristic of the late Proterozoic and late Archean. These data are also providing new constraints on the relationship between supercontinent cycles and changes in crustal growth rates and a specific test of Proterozoic geodynamic models based on assumptions that certain features such as the Diamantina lineament of Australia or Aekit terrane in Siberia share a common geologic history with the Great Falls tectonic zone and, therefore, represent a combined feature that transects the Neoproterozoic margin of Laurentia. In addition to the core science objectives, project funds are supporting undergraduate, graduate, and post-doctoral students by providing opportunities for their participation in various aspects of the research that is enhancing their mapping, sample preparation, analytical, and interpretive skills important to their professional development and employability. The research is benefiting from an established cooperative agreement to aid in supporting new undergraduate field research program supported by both the Keck Foundation and the National Science Foundation. The research is also being coordinated with an ongoing a U.S. Geological Survey mineral resources project (Metallogenic Evolution of Mesoproterozoic Sedimentary Rocks in Idaho and Montana) that is dependent on an accurate assessment of the age and distribution on pre-existing continental crust, such as that targeted in this project. On a longer time frame, this research will provide valuable input to decisions made for the NSF-sponsored EarthScope research project. When complete, the results of the research will be integrated into an on-line map that will allow both technically oriented individuals as well as the general public to see how the results from this research related to existing data from this region and elsewhere. Results of this research also are being made available through peer-reviewed scientific journals, on-line global databases, and presentations at professional society meetings.