The role of the lower plate in providing provenance during Alpine convergence inception: Insights from detrital signatures of the Western Ligurian Flysch accretionary complex

In the context of enormous crustal shortening and vast lateral displacement of oceanic units as observed in the Penninic domain of the Alpine orogeny, detrital signatures provide critical insights for appraising paleogeographic reconstructions and comprehending hinterland geology and tectonic events. Despite intensive research efforts and significant advances in the understanding of subduction and obduction processes that affected the Western Alps, the paleogeographic evolution of the Alpine Tethys represents a yet debated topic in Alpine geology, and within it, the role of the opposing continental margins (passive European margin and active Adriatic margin) as source regions for Cretaceous siliciclastic turbidites bordering the convergent system is particularly disputed. A multi-proxy provenance analysis (modal framework analysis, detrital zircon chronology, paleocurrent determination) applied to the two terrigenous superimposed units of the San Remo-Monte Saccarello Unit of the Western Ligurian Flysch complex – the Hauterivian-Santonian San Bartolomeo Fm. and the Campanian-Maastrichtian Bordighera Sandstones - is here used to solve this problem along the Ligurian Alps transect. Petrographic analyses characterize the basal complex sediments of the San Bartolomeo Fm. as quartz-rich sandstones defined by modal averages of Q69F29L2 and a mean P/K feldspar ratio of 0.48. By contrast, the conformably overlying Bordighera Sandstones represent texturally and compositionally immature first-cycle arkosic arenites (average modal composition: Q49F48L3; mean P/K-ratio: 0.57). A provenance evolution from a stable craton scenario and/or transitional continental provenance setting towards rapidly uplifted bedrock (granitoid plutons and low-grade metamorphic geobodies) can be inferred. New geochronological data (U-Pb detrital zircon ages) demonstrate that virtually the same source terranes provided the provenance for both formations. The detrital age spectra moreover display dominant peaks that are practically compatible with comprehensively documented magmatic and metamorphic pulses that affected the Southern Variscides. The strong affinity of the clastic detritus towards the Paleo-European margin underlines the importance of the lower plate (i.e. the passive margin) in providing the provenance for coarse-grained turbidite systems in pre-collisional convergent settings. Based on this observation, the arrival of a migrating flexural bulge at the continental margin of the lower plate is regarded to explain the inversion of the European margin, promoting the erosion of the stable continental shelf followed by the uplift of the continental block and rapid sedimentation without significant reworking processes as its hinge line traversed towards the hinterland.