Stratigraphic structural and geochemical data on the Durkan Complex (Makran Accretionary Prism, SE Iran): constraints for its interpretation as a Late Cretaceous tectonically disrupted seamount chain.

The Makran Accretionary Prism (SE of Iran) is part of the Alpine-Himalayan orogenic system, and it is still associated with active subduction. The Makran results from the Cretaceous to resent-day convergence between the Arabian and Eurasian plates that was accommodated by the northward subduction of the Neo-Tethys Ocean below the southern margin of Eurasia. he North Makran is the innermost and uppermost structural domain of the accretionary wedge. It consists of distinct complexes and tectonic units representing remnants of the Cretaceous-Paleocene accretionary-subduction complex. The Durkan Complex occur in this domain, and it has been interpreted as the sedimentary cover of the Bajgan Complex that, in turn, is egarded as an early Paleozoic or older continental basement (McCall & Kidd, 1982, Hunziker et al., 2015). Thus, the so far proposed geodynamic reconstructions of the Makran area are trongly influenced by this interpretation of the Bajgan and Durkan Complexes (McCall & Kidd, 1982; Burg, 2018). Our recent multidisciplinary studies of the Durkan Complex in the western part of the North Makran provided data suggesting a different interpretation for the Durkan Complex (Barbero et al., 2021a, b). Consequently, a critical revaluation of the geodynamic econstructions so far proposed is necessary. We present here a summary of the results of geological, stratigraphic, structural, biostratigraphic studies, as well as geochemical and etrological investigations of the volcanic rocks forming the Durkan Complex. The stratigraphic and biostratigraphic data indicate that this complex is composed by distinct tectonic slices showing deformed and slightly metamorphosed successions recording volcanic activity and sedimentation in a seamount setting. Three types of successions can be recognized. Type-I is composed by a Coniacian – early Campanian pelagic succession with ntercalation of pillow lavas and minor volcaniclastic rocks recording the deep-water stages of growth of a seamount. Type-II succession includes a volcanic sequence passing to a volcano-sedimentary sequence with Cenomanian pelagic limestones, followed by a hemipelagic sequence. This succession is characterized by abundant mass-transport deposits. Type-III uccession includes volcanic and volcano-sedimentary sequences, which are stratigraphically covered by a Cenomanian platform succession. Type-II and Type-III successions record volcanism and deposition along the flank and the summit of an emerged seamount. Basaltic and metabasaltic rocks of the Durkan Complex are characterized by two main geochemical groups according to new whole rock and clinopyroxene chemical data. Group 1 shows transitional chemical affinity and compositions resembling those of plume-type mid-oceanic ridge basalts. Group 2 includes rocks with within-plate oceanic island basalt (OIB) compositions showing a clear alkaline affinity. Based on whole rock REE contents and clinopyroxene chemistry, alkaline rocks can be further subdivided in two sub-groups, namely, the Group 2a and 2b. Compared to Group 2a, Group 2b rocks show a more pronounced alkaline nature. Trace element and REE petrogenetic models show that the Durkan basaltic rocks were generated from the partial melting of depleted sub-oceanic mantle source that was metasomatized by OIB-type chemical components in a within-plate oceanic setting. The observed chemical differences are related to different combinations of partial melting degree, depths of melting, and various extent of enrichment of the mantle sources by OIB-type chemical components. These petrogenetic data confirm the stratigraphic data indicating that the Durkan Complex likely formed in a seamount setting and it was possibly related to a Late Cretaceous mantle plume activity in the northern Neo-Tethys realm. Finally, structural data show that the Durkan Complex is characterized by a polyphase deformation achieved within the Makran Accretionary Prism during the Late Cretaceous – Paleocene ?. The first deformative stage D1 is characterized by sub-isoclinal to close and W-verging folds associated to an axial plane foliation. This stage records the accretion of fragments of the seamount at shallow to intermediate levels of the Makran accretionary prism. The subsequent D2 deformation stage is characterized by open to close folds with sub-horizontal axial plane that likely developed during the progressive exhumation to the surface of previously accreted seamount fragments. The incorporation of Durkan seamounts in the frontal prism likely caused a shortening of the whole convergent margin and possibly controlled the deformation style in the Makran Accretionary Prism during Late Cretaceous–Paleocene times. Concluding, the results of our multidisciplinary studies indicate that the Durkan Complex represents a Late Cretaceous tectonically dismembered seamount or, eventually part of seamounts chain rather than the sedimentary cover of a microcontinental blocks as previously suggested in literature. These results rise serious doubts about the effective occurrence of remnants of a microcontinental blocks in the Makran. Therefore, these results imply that the time is come for a critical review of the geodynamic evolutions of the Makran Prism and the paleogeographic setting of the norther part of the Neo-Tethys during the Late Cretaceous – Paleocene.