Publication
Emergence of modern-style continental subduction modulated by rifted margin-subduction feedback
Publisher:
Research Square Platform LLC
Date:
05-07-2023
DOI:
10.21203/RS.3.RS-2924524/V1
Abstract: Modern-style continental subduction is the main carrier of rifted margins to mantle depths ( 90 km) where ultra-high pressure (UHP) metamorphism above coesite stability is attained[1,2]. An open question is why exhumed UHP rocks, a key feature of modern-style continental orogens, only appeared and became common late in Earth’s history. The long-standing explanation relies on the higher geothermal gradient of the ancient Earth, which would either prevent continental subduction to reach UHP conditions or prevent their exhumation[3,4]. Although different exhumation mechanisms for UHP rocks have been discussed[5], none of them integrate the recent paradigm shifts in understanding the architecture and lithology of rifted continental margins[7]. Here, we perform high-resolution thermomechanical numerical experiments to demonstrate that segments of magma-poor rifted margins that reach UHP conditions can exhume back to shallower levels, while segments of magma-rich rifted margins cannot. This is because the thick layer of basaltic rocks in magma-rich margins becomes negatively buoyant during metamorphism, preventing their exhumation. We further argue that the secular decrease in both mantle potential temperature and fertility would have favoured the formation of magma-rich rifted margins in the first half of Earth’s history, affecting the recycling and exhumation of deep subducted margins. The record shows that the conditions for development of magma-poor rifted margins may have become achievable during the Earth’s middle age (1.5-0.8 Ga), when cold and more refractory, depleted mantle limited melting and therefore magmatism. When these magma-poor rifted margins reached collision zones in the Neoproterozoic, their positive buoyancy forced the exhumation of the oldest unequivocally coesite-bearing UHP rocks at 0.6 Ga[8,9]. Since then, UHP rocks have become a key and ubiquitous feature of modern-style continental orogens.