Tracing the Oligocene-Miocene Evolution of the Western Alps Drainage Divide with Pebble Petrology, Geochemistry, and Raman Spectroscopy of Foreland Basin Deposits

Abstract

The formation of the western Alps topography is the result of continental collision between the Apulian and European plates. In this study, we trace the Early Oligocene to Early Miocene development of topography and the position of the drainage divide in the southern western Alps by analyzing the erosion products preserved in the pro-side (Montmaur and Barrême) and retro-side (Torino hills) foreland basins of this orogen. Using petrologic and geochemical analyses of basalt pebbles and Raman spectroscopy of serpentine sand grains and pebbles, we identify source lithologies, which are not easily detected with more commonly used detrital thermochronology. Lower Oligocene sediments of the pro-side foreland basin contain numerous basalt pebbles that share strong geochemical similarities with the Chenaillet obducted ophiolite (Montgenèvre massif). Other ophiolite-suite derived clasts, e.g., radiolarite or serpentinite appear widely in pro- and retro-side foreland sediments since about 30 Ma. This suggests a wider distribution of Chenaillet-type obducted ophiolite rocks in the western Alps during the Oligocene, but the exact locations, except for the Chenaillet, are unknown. Raman analysis on serpentine grains and pebbles from the retro-side foreland basin deposits documents a systematic trend from antigorite (high-grade metamorphic source rocks) to lizardite (low-grade metamorphic source rock) from the Early Oligocene to the Early Miocene. This trend is attributed to a westward growth of the paleo-Dora Riparia drainage basin in the southern western Alps. Ophiolite erosion and drainage divide shift were caused by the topographic evolution of the western Alps, which we suggest to be linked to the shift in convergence direction between the Apulian and Eurasian plates from N–S to E–W and the presence of the so-called Ivrea body mantle splinter acting as a vertical indenter beneath the western Alps at that time. The drainage patterns of the paleo-Durance and paleo-Doria Riparia Rivers seem to have remained stable since the Early Miocene. In comparison to the central Alps, the drainage divide shift in the southern western Alps occurred earlier than in the central Alps, but in both locations the trend from an internal to a more external position is the same.