A story of Devonian ocean plate stratigraphy hosted by the Ulaanbaatar accretionary complex, northern Mongolia: implications from geological, structural and U–Pb detrital zircon data

Accretionary complexes and their hosted Ocean Plate Stratigraphy (OPS) units record a full story of ancient oceans from their birth at mid-oceanic ridges to their death at subduction zones. However, their reconstruction is difficult because accretionary complexes possess very complicated structures and therefore require careful and detailed in-field geological and structural studies and up-to-date analytical data. In this paper, we present results of detailed geological and structural studies of OPS sediments and new U–Pb detrital zircon data from sandstones of the Gorki Formation, a major part of the Ulaanbaatar accretionary prism/complex. The structure of the Gorki Fm. consists of five units separated from each other by parallel NE-striking regional faults and deformed by intra-formational detachments. It formed by offscraping and accretion of OPS sediments and subsequent thrusting and duplexing. Each unit is characterized by a specific association of OPS rocks, type and degree of deformation, style of folding and bedding geometry suggesting dychronous deposition and accretion of marine sediments. The ages and lithologies of the five units of the Gorki Fm. allowed us to trace the whole story of OPS sedimentation and further deformation—from oceanic floor (pre-accretion), to young and growing accretionary prism (syn-accretion) to accretionary prism stabilization (post-accretion). The dominantly southward vergence of the Gorki OPS thrust sheets suggests a generally northward subduction polarity. The greywacke sandstones of Unit I yielded U–Pb zircon ages ranging from 389 to 317 Ma with two main peaks at 346 and 367 Ma suggesting a provenance dominated by Famennian to Visean igneous complexes. The absence of the Famennian peak in the U–Pb detrital zircon spectra of sandstones from younger Unit II suggests tectonic erosion of a late Devonian arc. In general, the repeated layer-parallel thrusts inside lithologically uniform sediment beds indicate a regime of strong compression along the Benioff plane during subduction at a Pacific-type convergent plate boundary.