Thermal structure of low-grade accreted Lower Cretaceous distal turbidites, the Central Pontides, Turkey: insights for tectonic thickening of an accretionary wedge

Abstract

Albian-Turonian subduction-accretionary complexes are exposed widely in the Central Pontides. A major portion of the accretionary complexes is made up of a metaflysch sequence consisting of slate/phyllite and metasandstone intercalation with blocks of marble, Na-amphibole bearing metabasite, and serpentinite. The metaflysch sequence represents distal parts of a large Lower Cretaceous submarine turbidite fan deposited on the Laurasian active continental margin that was subsequently accreted and metamorphosed during the Albian. Raman spectra of carbonaceous material of the metapelitic rocks revealed that the metaflysch consists of metamorphic packets with distinct peak metamorphic temperatures. The majority of the metapelites are low-temperature (ca. 330°C) slates characterized by lack of differentiation of the graphite (G) and D2 defect bands. They possibly represent offscraped distal turbidites along the toe of the Albian accretionary wedge. Other phyllites are characterized by a slightly pronounced G band with a D2 defect band occurring on its shoulder. Peak metamorphic temperatures of these phyllites are constrained to 370-385 °C. The phyllites are associated with a strip of incipient blueschist facies metabasites and are found as a sliver within the offscraped distal turbidites. We interpret the phyllites as underplated continental sediments together with oceanic crustal basalt along the basal décollement. Tectonic emplacement of the underplated rocks into the offscraped distal turbidites was possibly achieved by out-of-sequence thrusting causing tectonic thickening and uplift of the wedge.

The Intra-Pontide suture zone in the Tosya-Kastamonu area, Northern Turkey

“…The DDU (partially corresponding to the Esenler Unit of Aygül et al, 2015) is a ∼400-500 m thick tectonic unit represented by a metasedimentary sequence (fine-grained micaschists, paragneisses, quartzites, metaradiolarites -DDms in the Main Map -and fine-grained marbles -DDma in the Main Map) and by bodies of fine-grained actinolite-bearing schists (DDmb in the Main Map; Figure 5(d)) that display E-MORB-, OIB-, BABB-and IAT-type signatures (Sayit et al, 2016). It has been affected by greenschist facies metamorphism overprinting an earlier Sayit et al (2016).…”

“…The DZU, corresponding to the Domuzdağ Complex of Okay et al (2006Okay et al ( , 2013 and Aygül et al (2015) is a ∼800-1200 m thick assemblage of micaschists and impure quartzites (DDms in the Main Map; Figure 6 (a)), minor bodies of coarse-grained marbles (DZma in the Main Map) and mafic rocks (DDam in the Main Map) preserving relicts of blueschist and amphibolite facies metamorphism. Several kilometres east of the study area, metaserpentinites with boudins of eclogites retrogressed to blueschist facies have been reported by Okay et al (2006Okay et al ( , 2013.…”

“…Coarsegrained amphibolites are associated with retrograded zones of albite + epidote + chlorite + actinolite schists with relict Na-amphibole and boudins of poorly reequilibrated eclogites and garnet + rutile + Na-amphibole-bearing schists (Figure 6(d)). HT-LP metamorphism relicts have been dated at c. 112-100 Ma (Aygül et al, 2015;Okay et al, 2013). Impure marbles show a mineral assemblage of calcite + muscovite + biotite ± epidote ± rutile ± apatite ± titanite.…”

“…The AKM (AKme in the Main Map; corresponding to the Kirazbaşı Complex of Aygül et al, 2015) is a late Santonian chaotic sedimentary deposit consisting of a succession where slide-blocks of different sizes and lithologies are enclosed in a sedimentary matrix consisting of shales, coarse-grained sandstones, pebbly mudstones and pebbly sandstones and references therein; Figure 6(f)). The slideblocks, from several metres to hectometres in size, are represented by metamorphic rocks (mainly micaschists and gneisses), by ophiolites (peridotites, gabbros, basalts and cherts) and by sedimentary rocks (cherts, neritic and pelagic limestones, marly limestones and ophiolite-bearing turbidites).…”

“…More recently the age of metamorphism and metamorphic conditions recorded in this sector of the belt have been subject of several papers (e.g. Aygül, Okay, Oberhansli, & Ziemann, 2015;Okay et al, 2013), however no detailed geological map of the area has been published. To partly fill this gap, we present a new 1:50,000 scale geological map of the IPS zone cropping out in the Tosya-Kastamonu area.…”