The Oligocene Alps: Domal unroofing and drainage development during early orogenic growth

The source of huge volumes of quartzofeldspathic detritus feeding Oligo-Miocene foredeep turbidites of the Apennines has long remained an unsolved geological enigma. In this article we reinterpret the available sandstone petrography, fission-track, stratigraphic, and geomorphological evidence to demonstrate that the Adriatic foredeep was dominantly sourced by a “Himalayan-type tectonic aneurysm” growing in the Central Alps since the late Oligocene: the Lepontine Dome. The Lepontine Dome is the envelope of distinct subdomes that formed stepwise from east to west during northwestward indentation of Adria beneath the axial Alps. Focused erosion of domal culminations cut by antecedent drainage started suddenly in the late Oligocene, with unroofing of recently-emplaced plutons and young amphibolite-facies gneisses. Exhumation was tectonically driven, and possibly accelerated by positive feedback between erosion and uplift of hot weak rocks from depth. The huge volumes of resulting detritus (Gonfolite–Macigno clastic wedge) were transported southward for hundreds of kilometers along the rapidly subsiding foredeep, formed since middle Eocene times in front of the Alpine retrobelt and nascent Apennine forebelt. Such massive sediment influx, following 15 ± 2 Myr of starved-foredeep sedimentation, testifies to rapid formation of mountain relief in the Central Alps, synchronous to dextral activity along the Insubric Fault. The Lepontine Dome remains today as the only deeply-dissected region in the Alps, where medium-grade metamorphic rocks of the axial belt are extensively exposed. The relationships between geological structures and drainage patterns still mirror those formed originally at Oligocene times. River headwaters have not been significantly displaced since then, and their thalwegs constrain finite lateral displacement relative to the Southalpine retrowedge to a maximum of 10–20 km. Our reconstruction has wide implications, not only for a better understanding of the Tertiary evolution of the Alps–Apennines orogenic couple, but also for general relationships among tectonic activity, relief formation, drainage development, erosion distribution, and long-distance sediment transfer during continental collision and orogenic growth.