Extensional basins in the tectonically bimodal central Apennines fold-thrust belt, Italy: response to corner flow above a subducting slab in retrograde motion.

The Apennines fold-thrust belt has developed over the past ~20 m.y. in response to southeastward retrograde migration of the Adriatic trench in a region of ongoing subduction but little plate convergence. The orogenic belt is characterized by a gently sloping southwestern flank in a state of regional extensional collapse, and a steeply sloping northeastern flank in regional contraction. In any given area in the central Apennines, the chronologies of thrusting and subsequent extension provide an estimate of the time required for attainment of high elevation (2.5-3.0 km). The time between initial extension and eruption of local ultramafic lavas provides an estimate of the time required to collapse by regional crustal thinning. These estimates suggest that the northeastern, contractional flank of the range is rising at least twice as fast as the southwestern flank is collapsing. We propose that the bimodal state of stress in the Apennines is maintained by corner flow in the mantle wedge beneath the crest of the range.