Wed 20th March @ noon, Rm 312.222
Abstract:
At the present day, different tectonic settings exhibit variations in heat flow that are registered as contrasting metamorphic facies series in distinct terranes; how far back in time these relationships are reliable is unclear. Here I use a dataset of temperature (T), pressure (P) and thermobaric ratio (T/P at the metamorphic ‘peak’), and age of metamorphism (timing of the metamorphic ‘peak’) for 564 localities from the Cenozoic to the Eoarchean to interrogate the crustal record of metamorphism in relation to secular change and geodynamics. Based on T/P, metamorphic rocks are classified into three natural groups: high T/P type (>775 °C/GPa, arithmetic mean ~1105 °C/GPa), including common and UHT granulites, intermediate T/P type (775–375 °C/GPa, arithmetic mean ~575 °C/GPa), including HP granulites and HT/MT eclogites, and low T/P type (<375 °C/GPa, arithmetic mean ~255 °C/GPa), including blueschists, LT eclogites and UHP rocks. Plots of T, P and T/P for high or intermediate T/P metamorphism, and the PDF of age for all localities show that since c. 3.0 Ga cyclic variations in the heat budget of the crust have been superimposed on secular cooling. The cyclicity is similar to that for global glaciations, but slightly younger in timing. Stable subduction and the emergence of a sustainable network of plate boundaries became possible after the balance between heat production and heat loss changed in favor of secular cooling, possibly as early as c. 3.0 Ga in some areas, which enabled the rise of protocontinents and the accumulation of sediments at continental edges to lubricate subduction. The Proterozoic was characterized by stability from the formation of Columbia to the breakup of Rodinia, generating higher than average T and T/P of high T/P metamorphism, a lower volume of continental crust and the occurrence of massif-type anorthosites, particularly in the Mesoproterozoic. Notwithstanding several low T/P localities, collision was generally warm rather than cold due to shallow slab breakoff. Deeper slab breakoff and cold collision, to generate and preserve the widespread record low T/P metamorphism that is a characteristic of orogenesis in the modern tectonic regime, occurred once the ΔTP of ambient mantle had decreased to <100 °C after c. 1.0 Ga, possibly related to the after effects of the ‘snowball’ earth.
Short bio:
Michael Brown obtained his BA and PhD degrees from the University of Keele in the UK. He held academic appointments at the rank of Lecturer to Professor in the UK between 1972 and 1990, including eight years as a Head of Department. In 1990, he moved to the USA as Professor of Geology and Chair of Department at the University of Maryland. He was reappointed Chair four times, finishing in 2011, and in 1998–2000 was concurrently the Interim Director of the Earth System Science Interdisciplinary Center. Brown has held visiting appointments at Kingston University, Kyoto University, the Universidade do Estado do Rio de Janeiro, the Johannes Gutenberg-Universität Mainz, Curtin University (twice) and ETH Zurich.
Brown’s research has contributed to understanding the petrogenesis of migmatites and associated granites, high/ultrahigh temperature and high/ultrahigh pressure metamorphism, the tectonics of metamorphic belts and secular change in metamorphism. This work has furthered our knowledge of processes associated with reworking and differentiation of the continental crust, particularly how heat and mass are transferred, the role of crustal melting in the development of orogens, and the secular evolution of geodynamic regimes on Earth. Over the past 48 years, Brown’s research has been made available through several books, >160 peer-reviewed chapters and articles in books and journals, >70 other articles, conference proceedings, editorials, reviews and field excursion guides, and by >440 presentations at scientific meetings. He founded the Journal of Metamorphic Geology in 1982 and has contributed extensive service to several major scientific societies, most recently as President of the Mineralogical Society of America. In recognition of his accomplishments, he received the Collins Medal from The Mineralogical Society of Great Britain and Ireland for 2014 and the Major John Sacheverell A’Deane Coke Medal from The Geological Society for 2005.
