UPLIFT AND EXHUMATION OF A MOUNTAIN BELT: A STABLE ISOTOPES AND THERMOCHRONOLOGY APPROACH TO THE CENTRAL APENNINES OF ITALY AND ITS GEODYNAMICAL IMPLICATIONS

Abstract

Estimates of surface uplift and exhumation are necessary to reconstruct the evolution of mountain belts and quantify the contributions of shallow and deep processes to the underlying geodynamics. We focus on the Central Apennines, which formed as an accretionary wedge before undergoing post-orogenic extension. Since ~2 Ma, it overlies an area of local slab detachment. We aim to better relate surface observations to possible geodynamic processes. To do so, we combine low-temperature thermochronology to date exhumation and stable isotope paleoaltimetry to reconstruct surface topography. Samples were collected from ten intermontane basins in the central Apennines that record both tectonic and climatic events. We present 25 new apatite (U-Th)/He ages and 10 new apatite fission track ages. from Miocene sandstones range from 8 Ma on the Thyrrenean coast to 1.62 Ma in the Central-Eastern Apennines. This pattern reflects 1) the onset of extension in the Tyrrhenian back-arc basin during the Tortonian, 2) the progressive eastwards migration of the orogenic thrust front, and 3) a regional scale uplift signal around 2 Ma, with local exhumation rates >0.5 mm/yr of 2-4 km of crust. However, exhumation history does not necessarily translate into uplift history. To constrain paleoelevations, we also analyze 1488 lacustrine and paleosol carbonate δ18O measurements collected from various elevations along the mountain range. Since the Pliocene, the gradient in δ18O between basins near sea-level today and those at high-elevation today has continuously increased. We attribute this 5 shift to increasing orographic rainout, thereby causing progressively lower δ18O in high-elevation basins as they are uplifted. Using this data and the modern meteoric water δ 18O lapse rate, we estimate that there has been more than 1 km of surface uplift since late Pliocene. Both our isotopic data and exhumation ages record an uplift and exhumation signal in the high topography area of the central Apennines at 2Ma. This event matches the suggested timing and expected amplitude of slab break-off, and supports the hypothesis that the opening of the Adriatic slab window caused substantial uplift in the central Apennines since Late Pliocene.