Drainage on evolving fold‐thrust belts: a study of transverse canyons in the Apennines

Abstract

Anticlinal ridges of the actively deforming Umbria–Marche Apennines fold‐thrust belt are transected by deep gorges, accommodating a drainage pattern which almost completely ignores the presence of pronounced anticlinal mountains. Because the region was below sea level until the folds began to form, simple antecedence cannot explain these transverse canyons. In addition, the fold belt is too young for there to have been a flat‐lying cover from which the rivers could have been superposed.In 1978, Mazzanti & Trevisan proposed an explanation for these gorges which deserves wider recognition. They suggested that the Apennine fold ridges emerged from the sea in sequence, with the erosional debris from each ridge piling up against the next incipient ridge to emerge, gradually extending the coastal plain seaward. The new coastal plain adjacent to each incipient anticline provided a flat surface on which a newly elongated river could cross the fold, positioning it to cut a gorge as the fold grew. Their mechanism is thus a combination of antecedence and superposition in which folds, overlying sedimentary cover and downstream elongations of the rivers all form at the same time.A study of Apennine drainage, using the sequence of older‐to‐younger transected Apennine folds as a proxy for the historical evolution of drainage cutting through a single fold, shows that transverse drainage forms when sedimentation dominates at the advancing coastline. Longitudinal drainage forms when uplift dominates, the folds first emerge as offshore islands and the Mazzanti–Trevisan mechanism is suppressed.Complicating factors include several departures from steady‐state growth of the fold‐thrust belt, a possible case of precursory submarine drainage, early emergence of anticlinal culminations and the location of several transverse canyons at the structurally highest point along anticlinal axes.