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

Abstract

Analysis of 146 new apatite (U-Th)/He ages, six new apatite fi ssion-track ages, and 165 previously published apatite fission track (AFT) ages from the northern Apennine extending convergent orogen reveals a signifi cant 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 fi ndings 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

Early exhumation of underthrust units near the toe of an ancient erosive subduction zone: A case study from the Northern Apennines of Italy

“…A regional thermochronological study by Thomson et al (2010) constrains the thermal imprint of the material cropping out in the southwesternmost sector of the Sestola-Vidiciatico tectonic unit. Data indicate that this material underthrust from the Aquitanian (early Miocene) to the Langhian (middle Miocene) and that it reached temperatures suffi cient to reset the AFT system (>120 °C).…”

“…In the Emilia sector of the Northern Apennines, which represents the frontal part of the overriding plate, the few available bedrock data (Thomson et al, 2010) suggest that the thermal imprint for the shallower Ligurian Units and the overlying Epiligurian Units was less than 70 °C. A detrital AFT study on the modern sands of the rivers fl owing toward the Po Plain (Malusà and Balestrieri, 2012) suggests that it is likely that the deeper part of the Ligurian Units, especially in the northwestern sector of the Emilia Apennines, might be totally reset with an exhumation age between 6.5 and 4.5 Ma.…”

“…All the above-mentioned units were underthrust together, but the Sestola-Vidiciatico tectonic unit and the Subligurian Units were exhumed less than 10 m.y. ago with the whole Apennine chain (Thomson et al, 2010) (Fig. 1).…”

Appraisal of active tectonics using DEM-based hypsometric integral and trend surface analysis in Emilia-Romagna Apennines, northern Italy

“…The recent tectonic activity of the Mt. Cimone area is also supported by thermochronologic age-elevation relationships for both the apatite fission track and (U-Th)/He thermochronometry (Thomson et al, 2010). Here the rocks of the Macigno Formation are little disturbed by normal faulting.…”

A linear inversion method to infer exhumation rates in space and time from thermochronometric data

“…An alternative to these computationally expensive threedimensional thermo-kinematic inverse models is to simply map spatial patterns in exhumation by converting isolated ages into time-averaged exhumation rates (e.g., Van den Haute, 1984;Omar et al, 1989;Brandon et al, 1998;Berger et al, 2008;Thomson et al, 2010). Vernon et al (2008) computed the difference in elevation of isoage surfaces, which are interpolated from the data distributed in three dimensions, to estimate variations in exhumation rate.…”