Blueschists and Blue Amphiboles: How much Subduction do they Need?

Abstract

We focus on blueschist formation in evolving, nascent intra-oceanic subduction zones where cessation of subduction, e.g., by collision with continental margins, preserves the first-formed blueschist products. The Caribbean area provides a case study for a tectonic regime that has led to obducted blueschist occurrences without obvious coeval volcanism, raising the question of the minimum subduction duration and amount of convergence necessary to produce them. Systematic numerical modeling shows that for slab ages of 20-100 Ma and subduction rates of 2-14 cm/yr blueschist-facies conditions can be attained in only 0.25 to 3 m.y., with amounts of total convergence ranging between 35 and 75 km. Because of the geometrical interplay between the subducting slab "nose" and the evolving array of isotherms, younger and hotter slabs can lead to earlier blueschist formation than older, cooler slabs. There is a distinct optimum for slabs between 40 and 60 m.y. In principle, Andean-type, continental margin models yield almost identical values. Depending on the specific exhumation scenario, the time required for exhumation must in general be added to the minimum life span of the subduction zone.