MGPV DISTINGUISHED GEOLOGIC CAREER AWARD LECTURE: SECULAR CHANGE IN METAMORPHISM AND METAMORPHIC COOLING RATES TRACK THE EVOLVING PLATE TECTONIC REGIME ON EARTH

When disturbed, dynamic emergent systems, such as tectonics on Earth, may transition from one stable state to another if the perturbation is sufficiently large. Previous analysis of the time series of metamorphic pressure (P)–temperature (T) data has established a minimum age for the emergence of plate tectonics, and provided evidence of the secular evolution of metamorphism and plate tectonics. Here, we report statistically significant state shifts in the time series of metamorphic P–T and cooling rate data, and mantle potential temperature (T_P). State shifts yield a minimum age for a perturbation since they record the new stable state. State shifts in the metamorphic time series occur in the Paleoproterozoic (jump in mean P and drop in mean T/P) and between the late Ediacaran and early Ordovician (drop in mean T, jump in mean P, drop in mean T/P and jump in mean cooling rate). There is also drop of ~130 °C in mantle T_P at c. 2 Ga, indicating a change in how the mantle was cooled. Changes in the Paleoproterozoic likely relate to the widespread expansion of stable subduction, slab breakoff during orogenesis associated with the formation of the Nuna megacontinent, and the descent of detached slabs into the deeper mantle, consistent with the operation of a distinctive style of Proterozoic plate tectonics. By contrast, state shifts in the metamorphic time series from the late Ediacaran onward may be related to the large volume of sediments generated during multiple glaciations in the Cryogenian and again in the Palaeozoic that enabled steeper subduction, transport of lower continental plates to mantle depths during orogenesis, and faster metamorphic cooling rates. These changes reflect a transition to the modern style of plate tectonics. If time allows, we will address issues specific to the abundance and distribution of ultrahigh temperature and ultrahigh pressure metamorphism, and their relationship to secular change in plate tectonics, mantle T_P and the supercontinent cycle.

Acknowledgement: recent research has been done in collaboration with Tim Johnson (Curtin University, Australia), Ross Mitchell (Institute of Geology & Geophysics, Chinese Academy of Sciences, China), Craig O'Neill (Macquarie University, Australia) and Chris Spencer (Queen's University, Canada).