Testing Alternative Tectono-Stratigraphic Interpretations of the Late Palaeozoic−Early Mesozoic Karakaya Complex in NW Turkey: Support for an Accretionary Origin Related to Northward Subduction of Palaeotethys

Abstract

Lower Carboniferous-Upper Triassic rocks of the Karakaya Complex exposed E-W across Turkey are critical to reconstructions of Palaeotethys in the Eastern Mediterranean region. Despite decades of research, the origin and emplacement of the Karakaya Complex remains controversial because it is mapped either as an overall stratigraphic succession of sedimentary olistostromes or as a stack of thrust sheets and mélange. Tectonic models include a continental rift, a back-arc rift, a marginal oceanic basin, and an accretionary prism formed by subduction of a wide ocean. Subduction is seen as either northwards or southwards. To test the alternatives, the various litho-tectonic units and their contact relations were studied in nine outcrops across northwestern Turkey. Our field evidence indicates that the Karakaya Complex was assembled by regional-scale thrust faulting without evidence of stratigraphical contacts or even of deformed sedimentary contacts between the main units. The structurally lower levels of the Karakaya Complex of Triassic age (~lower Karakaya assemblage) are dominated by an imbricated, mainly volcaniclastic sequence (~Nilüfer Unit) that was metamorphosed under high pressure-low temperature conditions and rapidly exhumed. Structurally higher, lower-grade rocks (~upper Karakaya assemblage) are characterised by several coherent lithotectonic units, including the Upper Permian-Lower Triassic Çal Unit, dominated by alkaline volcanics and shelf to redeposited carbonates, a contrasting mainly Upper Permian unit including terrigenous sediments, and the Triassic Ortaoba Unit, dominated by mid-ocean ridge-type basalts, radiolarian sediments and sandstone turbidites. Two associated composite units (Hodul and Orhanlar units) are interpreted as accretionary mélanges (rather than olistostromes) that were tectonically assembled and emplaced during Late Triassic time. Pre-Karakaya-age meta-siliciclastic sedimentary rocks (~Kalabak unit) are intruded by Devonian and Lower Carboniferous granites in several areas. Arkosic cover sediments (Halılar Formation) above the Kalabak unit accumulated during Late Triassic (Norian) time prior to final emplacement of the Karakya Complex. The 'basement units' are interpreted as thrust slices that were emplaced to a high structural level during final emplacement of the Karakaya Complex in latest Triassic time. Transgression by shelf sediments followed from the Early Jurassic onwards following regional uplift and erosion. In our proposed tectonic model, Palaeotethyan oceanic crust (~Triassic Ortaoba Unit) subducted northwards beneath the Sakarya Continent. Seamounts capped with carbonate build-ups formed near the southern margin of Palaeotethys (~Çal Unit). The Upper Permian neritic carbonates associated with terrigenous clastics (unnamed unit) probably rifted from the Tauride continent to the south. Large oceanic seamounts erupted within the Triassic ocean (~Nilüfer Unit). The seamounts