Pre-collisional accretionary growth of the southern Laurasian active margin, Central Pontides, Turkey

Abstract

Cretaceous subduction–accretionary complexes crop out over wide areas in the central part of the Pontides, northern Turkey. To the north, the wedge consists of a low-grade metaflysch sequence with blocks of marble, Na-amphibole-bearing metabasite (PT=7–12kbar; 400±70°C) and serpentinite. 40Ar/39Ar phengite ages from the phyllites of the metaflysch are ca. 100Ma. The metaflysch sequence is underlain by oceanic crust-derived HP/LT metabasites and micaschists along a major detachment fault. The metabasites are epidote-blueschists consisting of glaucophane, epidote, titanite, and phengite locally with garnet. Fresh lawsonite-blueschists are exposed as blocks along the detachment fault. Peak metamorphic conditions of a garnet-blueschist are constrained to 17±1kbar and 500±40°C and of a lawsonite-blueschist to 14±2kbar and 370–440°C. 40Ar/39Ar phengite dating on the micaschists constrains the HP/LT metamorphism as 101–92Ma, younging southward. Middle Jurassic (ca. 160Ma) accretionary complexes consisting of blueschist to lower greenschist facies metabasites, marble and volcanogenic metasediment intercalations are exposed at the southern part of the Cretaceous wedge. In the studied area, the North Anatolian Fault forms the contact between Cretaceous and Middle Jurassic HP/LT metamorphic rocks. Wide distribution of Cretaceous subduction–accretionary complexes implies accretionary tectonic continental growth along the Laurasian active margin. High amount of clastic sediment flux into the trench has a major effect on enlarging the wedge during the Albian. Tectonic thickening of the oceanic HP/LT metamorphic sequence, however, was possibly achieved by propagation of the décollement along the retreating slab which can create the space necessary for progressive deep level basal underplating and extension of the wedge for subsequent syn-subduction exhumation.