Thermal regime of continental subduction: The record from exhumed HP-LT terranes (New Caledonia, Oman, Corsica)

Unlike the thermal structures of present-day subduction or collision zones, the variety of possible thermal evolutions (or “thermal regimes”) accompanying the transient stage of continental subduction (i.e., the shift from oceanic subduction to continental collision) remains poorly known. We herein show that the thermal regime of continental subduction can be confidently retrieved from three well-documented fossil settings (i.e., from high-pressure low temperature continental material from Oman, New Caledonia, Corsica) that were not modified by later collision or a later metamorphic imprint. Based on recently published data, we show the striking similarity of the overall thermal and tectonic patterns of these three HP belts derived from the subduction of a continental margin. These natural examples suggest that the thermal regime of continental subduction is largely invariant through time (hence suggesting a steady-state thermal structure) and independent from the initial geodynamic setting (i.e., from the initial thermal structure of oceanic subduction, the nature of the incoming plate or of the upper plate). We propose that continental cover units subducted over a short time period (i.e., ~10 My) represent cold underplated material that buffers the subduction thermal regime, whatever the exact structure, nature, or thermal structure of incoming material. Similarities in the type, size and P–T conditions of the various tectonic units and in the overall tectonic organization point to specific accretionary-type subduction dynamics, yet to differences in long-term mechanical coupling between the three case studies. Our study thereby provides constraints on exhumation dynamics and models of continental subduction.