TRANSLATIONAL STEADY-STATE OROGENY IN THE NORTHERN APENNINES: DO COUPLED FRONTS REALLY EXIST?

The Northern Apennines are an unusual fold-and-thrust belt, where both compression and extension have been going on since the late Tertiary. In 1975, Elter, Giglia, Tongiorgi, and Trevisan pointed out that since the Pliocene, an extensional front has closely followed a compressional wave moving from southwest to northeast, with thrust faults accreting material into the orogen at about the same rate extension is attenuating material at the rear. This creates an unusual kind of "steady-state" situation, augmenting the currently recognized list of 4 or 5 kinds of orogenic steady state. The coupled-front model has been widely accepted for a quarter century. However, recent work is calling this fundamental basis of Italian tectonics into question. To add precision to this model, and as a prerequisite for testing it, we introduce and quantify the concept of "translational steady state," in which an orogen remains a fixed width while moving laterally. In this framework, deviations of the Apennines from the translational steady-state model are evident. Nonparallel fronts of compression and extension lie at different places relative to each other along the length of the mountain belt, so they are not coupled in a simple way. Different depths of fold exposure and lack of wind gaps along the Apennine drainage divide provide geomorphic evidence for temporal discontinuity in the thrusting and extensional processes, leading to the central question whether the Northern Apennines are a single prograding fold-thrust belt, or are made of two different belts with different origins, one in Tuscany and the other in Umbria-Marche, as suggested by Barchi, Minelli, and Pialli in 1998. Rapid uplift since the Pliocene, out-of-sequence thrusting, and different levels of basement involvement, all presently under debate in the Italian literature, would require modification of Elter et al.’s original concept. We suggest that the coupled-front concept requires modification and that compression and extension are at least partly independent. The superposition of extension onto earlier compression could be due to mantle uplift, as suggested by D’Agostino, Jackson, Dramis and Funiciello in 2001, or to propagation of extensional rifting from the widening Tyrrhenian Sea in the south toward the Apennines in the north.