Thermochronologic evidence for orogen-parallel variability in wedge kinematics during extending convergent orogenesis of the northern Apennines, Italy.

Analysis of 146 new apatite (U-Th)/He ages, six new apatite fission-track ages, and 165 previously published apatite fission-track (AFT) ages from the northern Apennine extending convergent orogen reveals a significant along-strike change in post-late Miocene wedge kinematics and exhumation history. East of ∼11°30′E, age patterns and age-elevation relationships are diagnostic of ongoing frontal accretion and slab retreat consistent with a northeastward-migrating "orogenic wave." Enhanced erosion rates of ∼1 mm/yr over a period of ∼3-5 Ma are recorded on the contractional pro-side of the orogen and ∼0.3 mm/yr on the extending retro-side. West of ∼11°30′E, ongoing exhumation has been restricted to the range core since at least ca. 8 Ma at rates of ∼0.4 mm/yr increasing to ∼1 mm/yr in the Pliocene (ca. 3 Ma) accompanied by post-Pliocene tilting and associated faulting. This pattern can be attributed to either continued convergence (but a switch in the transfer of material into the wedge to a regime dominated by underplating or out-of-sequence shortening), or a slowdown or cessation of frontal accretion and slab retreat with enhanced Pliocene uplift and erosion triggered by a deeper seated process such as lithospheric delamination, complete slab detachment, or slab tear. These findings emphasize that no single model of wedge kinematics is likely appropriate to explain long-term northern Apennine orogenesis and synconvergent extension, but rather that different lithospheric geodynamic processes have acted at different times in different lateral segments of the orogen.