Wed 2nd November 2022 @ 12:00 nn, 312.222 and online via Webex (meeting #: 2650 711 1383 and password: 2dpBV3Xe2T3)

Abstract:

Based on the crustal record of magmatism and metamorphism, Earth likely evolved from a single-lid (sluggish or squishy) to a mobile-lid tectonic mode during the late-Archean–early-Proterozoic. Time-constrained data show a strong temporal clustering of metamorphism associated with periods of supercontinentality and a close spatial relationship with inferred plate margins since the Archean. Statistical analysis shows that bimodality in T/P—a characteristic feature of convergent plate boundaries since 200 Ma—developed during the Paleoproterozoic and became increasingly distinct thereafter; late Archean metamorphism was unimodal. Thus, a strong case can be made for plate tectonics since the early Paleoproterozoic; a single-lid episode in the Mesoproterozoic seems unlikely. However, a peak in metamorphic T/P, dominant anorthosite and rapakivi suite magmatism, and longevity of passive margins in the Mesoproterozoic suggest a plate slowdown, as verified by geophysical modeling. Statistical analysis of time series of T/P and cooling rate identifies change points in the Paleoproterozoic and early Paleozoic, recording two state shifts: one following stabilization of subduction after the emergence of continents and the Siderian glaciations, which led to an enhanced sediment supply along continental margins and a period of increased orogenic activity throughout the amalgamation of Nuna, and another following the Cryogenian glaciations, which led to an increased sediment load in trenches that kick-started modern-style plate tectonics characterized by low T/P and UHPM. In the Archean, unstable subduction at various locations created conflicting signals in the geological record, and there is evidence of late-Archean regional plate-like behavior, but the shift to global plate tectonics cannot be confirmed before the early Paleoproterozoic. Lastly, the changing abundances of komatiites, anorthosites and kimberlites through time are consistent with an evolution from early bottom-up asymmetric, to symmetric, to modern top-down asymmetric mantle geodynamics.

Short bio:

Mike is an internationally recognized leader in the field of metamorphic geology, with an emphasis on crustal melting, the petrogenesis of granulites (including UHT metamorphism) and eclogites (including UHP metamorphism), and the relationship between metamorphism and tectonics. His research uses observations and laboratory data in petrology and structural geology, integrated with those from geochemistry and geochronology, and results from numerical modeling, to investigate orogenic processes at all scales, providing original contributions to and insight about heat and mass transfer and secular change.