Tectono-metamorphic processes beneath an obducted ophiolite: evidence from metamorphic soles and accreted units from western Turkey

Abstract:

The convergence between the Anatolide Tauride block and Eurasia during Cretaceous times lead to the closure of a branch of the Neotethyan ocean and to ophiolite obduction. After reconstruction, obducted ophiolite and their sub-ophiolitic units can be found along a 200 kilometre-long north to south transect in western Anatolia. If related to a single event, the dimension of this ophiolite makes it an exceptional object of interest to study obduction and early subduction dynamics. This contribution aim to: (1) (re)-appraise the metamorphic pressure-temperature (PT) conditions and evolution of the sub-ophiolitic units of western Anatolia; (2) reconstruct the Anatolian ophiolite and (3) understand the dynamics of a large-scale and long-lived obduction. Directly below the ophiolite (mostly made of mantle-derived rocks) lies a metamorphic sole. The upper part of is this sole is made of garnet ± clinopyroxene amphibolites. In the northern part the sole is characterised by an important blueschist-facies overprint destabilizing the amphibolite paragenesis whereas it is lacking in the south. PT conditions were refined at 10.5 kbar and 780°C for the south and at 11 kbar and 725°C using pseudosection modelling. Field and petrological observations recognize three principal units in the accretionary complex (from top to bottom, OC1, 2 and 3) with PT conditions ranging from incipient metamorphism to blueschist facies conditions. OC1 represents most of the outcropping unit, is found all along the section and shows pristine oceanic rocks to very low grade metamorphics rocks (lawsonite - pumpelltyite facies). Blueschist facies rocks including Fe-Mg carpholite-bearing layers were found in OC2 and attest high-pressure and low-temperature conditions (~10 kbar – 350°C). OC3 exhibit a clear blueschist facies metamorphism, but higher PT conditions (17 kbar - 450°C). Both OC2 and 3 were only found in the northern area close to the suture zone. Combining these data and recent advances in our understanding of subduction zone interface processes we disscuss (i) the formation of metamorphic sole and their extension (ii) the significance ands mechanism of the blueschist overprint in sole; (iii) exhumation and detachment of units at various depths and their influence on the rheological behaviour of subduction zone.