CONSTRAINTS ON SUBDUCTION INITIATION FROM PROTOLITH AND METAMORPHIC AGES OF METAMORPHIC SOLES

Understanding where and how subduction zones have and can initiate is important because it can provide insight into the complex interactions of crust and mantle rheology, forces acting on plates, strain, metamorphic reactions, and erosional and depositional processes at the surface. Numerical and analytical modeling indicate that subduction can only initiate at a weak zone in the lithosphere. Zones of potential weakness that have been proposed/investigated are the continent-ocean boundary, oceanic arc-oceanic crust boundary, oceanic transform faults and fracture zones, oceanic detachment faults, and active or recently extinct oceanic ridges/spreading centers. The ages of crust juxtaposed at the weak zone (i.e., the location of the future subduction thrust) at each of these settings is variable. For example, oceanic crust on either side of a fracture zone would be relatively old and of different ages, whereas oceanic crust on either side of an oceanic spreading center are both of ~zero age. Subduction initiation at each of these setting would produce a metamorphic and protolith record in the subducted and metamorphosed crust that is relatively unique to each setting. “Metamorphic soles” beneath ophiolites include high-temperature and high-pressure metamorphosed basaltic oceanic crust and sediments that were underthrust to asthenospheric mantle depths, metamorphosed, and then preserved in the hanging wall of an eventual subduction zone. The timing of metamorphism—best constrained by metamorphic zircon and closely estimated by hornblende ⁴⁰Ar/³⁹Ar ages—provides the best estimate of the time of subduction initiation. Igneous protolith zircon ages from the metamorphic sole constrain the age of the crust at the site of subduction initiation. The latter parameter is an important constraint that has been lacking in many previous geologic studies of metamorphic soles and subduction initiation. Recent work on a metamorphic sole in Palawan, Philippines, indicates that MORB-like oceanic crust less than 1 Myr old was underthrust at a nascent subduction zone, implying initiation at an oceanic spreading center. A preliminary review of available data from some other metamorphic soles (e.g., Oman) may suggest a similar tectonic setting for subduction initiation at or near an oceanic spreading center.