Significance of the Volcanogenic Nilüfer Unit and Related Components of the Triassic Karakaya Complex for Tethyan Subduction/Accretion Processes in NW Turkey

Authors: ELIZABETH A. PICKETT, ALASTAIR H. F. ROBERTSON

Abstract: The Tethyan Karakaya Complex extends east-west across northern Turkey from the Aegean coast to Iran (c. 1100 km), and is interpreted as a Mid-Late Triassic subduction/accretion complex. It comprises strongly deformed fragments of Triassic oceanic seamounts, mid-ocean ridge-type oceanic crust, trench-type sedimentary rocks (Ortaoba Unit) and Permian-Triassic continental fragments (Çal Unit). We focus on the Triassic Nilüfer Unit, which comprises altered basic volcanic and volcaniclastic rocks, volcanogenic sedimentary rocks, heterogeneous debris flow deposits, calciturbidites, siliceous pelagic sedimentary rocks and, in places, Triassic shallow-water limestones. The predominant lithologies are massive basalt and reworked basic pyroclastic deposits. Additional detrital lithologies include volcanogenic sandstone/siltstone/mudstone and volcanogenic breccia. These lithologies document the construction of one, or several, volcanic seamounts within the Triassic Tethyan ocean, removed from a supply of terrigenous detrital sediment. Whole-rock geochemical analysis of basalt and electron-microprobe analysis of clinopyroxene phenocrysts confirm a within-plate, "enriched" composition, without a detectable subduction influence. The Nilüfer Unit generally exhibits greenschist-facies metamorphism and locally also HP/LT (up to eclogitic) facies metamorphism. The structurally overlying Ortaoba (Hodul) Unit records the accretion of mid-ocean-ridge-type basalts, radiolarian chert and minor serpentinised ultramafic rocks in a trench-type setting where mainly arkosic sediments rich in altered silicic volcanic grains accumulated. Above this, the unmetamorphosed Çal Unit is dominated by blocks of Upper Permian neritic limestone associated with within-plate-type basalt. The bases of locally intact successions are commonly depositionally associated with micaceous shale of terrigenous origin suggesting the former presence of a continental substratum that was probably removed by subduction. One or more of such continental fragments were possibly rifted from the southern margin (Anatolides) and drifted northward across the Triassic Tethys before being accreted to the Eurasian margin. The direction of subduction related to initial tectonic accretion is assumed to have been northward although definitive structural evidence is lacking. The Karakaya Complex was finally emplaced northward over the leading edge of the Eurasian margin (Sakarya basement), possibly in response to the collision of the Çal continental fragment(s) with the subduction zone, prior to covering by shelf carbonate in middle Jurassic time.

Keywords: Nilüfer Unit, NW Turkey, Karakaya Complex, subduction/accretion, Triassic Tethys

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