MSA PRESIDENTIAL ADDRESS: METAMORPHISM, SECULAR CHANGE AND GEODYNAMICS

Metamorphism of crustal rocks is due to heat generated by radioactive decay and viscous dissipation, and heat flux from the mantle. Earth’s radiogenic heat production has declined by >75% since the Hadean and mantle T_P by ~200–300 °C since c. 3.0 Ga. 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 thermal gradient (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 dT/dP type (>775 °C/GPa, arithmetic mean ~1105 °C/GPa), including common and UHT granulites, intermediate dT/dP type (775–375 °C/GPa, arithmetic mean ~575 °C/GPa), including HP granulites and eclogites, and low dT/dP type (<375 °C/GPa, arithmetic mean ~255 °C/GPa), including blueschists, low-T eclogites and UHP rocks. Plots of T and T/P for high dT/dP 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. Stable subduction and the emergence of a sustainable network of plate boundaries in a mobile-lid tectonic regime 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. When this became a globally linked system of plate boundaries and whether subduction was reduced or paused during the Proterozoic are unknown. 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 dT/dP metamorphism. In the Proterozoic tectonic regime, 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 dT/dP metamorphism that is a characteristic of orogenesis in the modern tectonic regime, became possible once the ΔT_P of ambient mantle had decreased to <100 °C after c. 1.0 Ga.