THE CONTROLS AND STYLE OF UNDERPLATING ALONG THE SUBDUCTION PLATE INTERFACE (Invited Presentation)

The rocks involved in subduction processes provide critical constraints on the structural and rheological evolution of the subduction interface at convergent margins. Exhumed subduction complexes record syn-metamorphic deformation, stacking, and underplating of continental and oceanic slivers along the subduction interface. We present work from two exhumed subduction complexes from a continental and oceanic setting, the Nevado-Filábride Complex (NFC) in Southern Spain and the Catalina Schist in Catalina Island/Pimu, California, and discuss the processes and timing of underplating and exhumation of continental vs. oceanic subduction complexes.

The structurally lowest unit of the NFC is dominantly comprised of lithologically monotonous Paleozoic metamorphic basement rocks and show no evidence of large-scale internal duplications, suggesting it behaved as a coherent and buoyant basement during subduction. In contrast, the structurally higher levels of the NFC are characterized by the stacking of older-on-younger coherent slices with distinct metamorphic ages. These relationships document syn-subduction structural repetitions and tectonic stacking of imbricate slivers (∼100s of m) during subduction underplating. n Catalina Island, older epidote blueschist has been juxtaposed over younger lawsonite blueschist rocks. Field observations suggest that in places the contact between the two lithologies is marked by ~meter scale thick garnet amphibolite blocks. Ongoing petrologic and modeling work will reveal the pressure and temperature conditions of underplating and the sequence of thrusting that led to the exhumation of these slivers, but preliminary work suggests less imbricate stacking than in continental subduction systems such as the NFC further supporting that the underlying "basement" sequence of continental versus oceanic is the main control on the style of deformation.

This work highlights that the pre-subduction stratigraphic architecture is a key control on the style and timing of deformation and metamorphism along the subduction interface. Large-scale underplating and antiformal stacking of slivers within subduction complexes may be more prevalent than previously recognized due to modern high-precision petrochronology techniques which reveal buoyant, coherent underplated sequences that facilitate exhumation.