Does the Mesoproterozoic Represent a Stagnant or Mobile Lid Tectonic Regime?
- Dr. Kent Condie
Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology
Several lines of evidence have been used to implicate a stagnant lid tectonic regime in the Mesoproterozoic (1.6-1.0 Ga), including the abundance of dry magmas (i.e., A-type granites and anorthosites), and the lack of passive margins. However, A-type granites are not a distinctive feature of the Mesoproterozoic, as increasing numbers of these granites are being recognized in Phanerozoic orogens. Long belts of A-type granites occur in continental back arc settings clearly associated with convergent plate margins since the end of the Archean. A large peak in the frequency of these granites at ~1.5 Ga may reflect a sample bias weighted towards Laurentia. Although massif anorthosite provinces are widespread between 1.9-0.9 Ga, they first appear in the Neoarchean, and examples are found through the Phanerozoic. After 2.5 Ga, anorthosites are commonly associated with A-type granites in convergent margin tectonic settings. The near absence of passive margins in the Mesoproterozoic is contradicted by recent paleogeographic continental reconstructions. In the time frame of 1.6-1.2 Ga, a minimum of 12 passive margins must have come into existence as the supercontinent Nuna fragmented. Crustal growth has often been deemed to be low in the Mesoproterozoic. As shown by Nd and Hf isotope data, juvenile crust continued to be produced throughout the Mesoproterozoic in accretionary orogens on the margin of the Nuna supercontinent. An outstanding question is why these crustal provinces are less abundant than both older and younger crustal provinces? If crust of this age has been preferentially recycled into the mantle, why during this time period? Paleogeographic continental reconstructions suggest that plate tectonics rapidly propagated around the globe beginning at 2.0-1.9 Ga, and indicate significant separation of continental landmasses subsequent to Nuna formation, and prior to Rodinia formation. In summary, Earth did not revert to a stagnant lid regime during the Mesoproterozoic.
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