Detrital signatures of impending collision: The deep-water record of the Upper Cretaceous Bordighera Sandstone and its basal complex (Ligurian Alps, Italy)

Despite intensive research efforts and significant advances in the understanding of subduction and obduction
processes that affected several units which at the present day compose the Western Alps, the paleogeographic
evolution of the Alpine Tethys represents a debated topic in Alpine geology. 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 remains disputed. To address this question along the Ligurian Alps
transect, a multi-proxy provenance analysis is applied to the two terrigenous superimposed units
(Hauterivian-Campanian San Bartolomeo Fm. and Campanian-Maastrichtian Bordighera Sandstone) of the San
Remo-Monte Saccarello Unit of the Western Ligurian Flysch complex. Petrographic analyses characterize the
basal San Bartolomeo Fm. as quartz-rich mature sandstones. By contrast, the overlying Bordighera Sandstone
represents texturally and compositionally immature first-cycle arkosic arenites. This change records the evolution
of the sediment provenance froma stable craton into a continental basement uplift setting, reflecting erosion
of granitoid plutons and the low-grade metamorphic basement. Geochronological data (U\\Pb detrital zircon
ages) indicate that virtually the same source terranes provided the source for both formations. The detrital age
spectra display age peaks are compatiblewithwell-documented magmatic andmetamorphic pulses that affected
the Southern Variscides in the Paleo-European margin. The strong affinity of clastic detritus with the Paleo-
European margin basement rocks underlines the importance of the lower plate passive continental margin in
supplying sand-rich turbidite systems prior to the arrival of the passive margin in the subduction zone.