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Authors: Ryan, Eric James* 
Papeschi, Samuele* 
Viola, Giulio* 
Musumeci, Giovanni* 
Mazzarini, Francesco* 
Torgersen, Espen* 
Sørensen, Bjørn Eske* 
Ganerød, Morgan* 
Title: Syn‐Orogenic Exhumation of High‐P Units by Upward Extrusion in an Accretionary Wedge: Insights From the Eastern Elba Nappe Stack (Northern Apennines, Italy)
Journal: Tectonics 
Series/Report no.: 5/40 (2021)
Publisher: Wiley Agu
Issue Date: May-2021
DOI: 10.1029/2020TC006348
Keywords: Syn-orogenic extrusion is proposed as a viable mechanism for the exhumation of HP-LT rock units of the Northern Apennines; • The Acquadolce Subunit of the Eastern Elba nappe stack was exhumed by upward extrusion within an actively shortening orogenic wedge; • Middle Miocene extension shaped the upper Northern Apennines orogenic wedge but did not contribute to the early exhumation of HP rocks.
Subject Classification04. Solid Earth
Abstract: The E-vergent Northern Apennines formed by Oligocene-Miocene convergence and westward subduction of Adria beneath Europe. Extension ensued in the Mid-Late Miocene reflecting lower plate roll-back and causing opening of the back-arc Northern Tyrrhenian Sea. Post-orogenic extension is commonly advocated as the main driver of the exhumation of the belt's inner domain highpressure/ low-temperature (HP-LT) rock units. The Acquadolce Subunit of the Eastern Elba nappe stack contains HP-LT rocks recording peak blueschist conditions of 1.5–1.8 GPa at 320°C–370°C loosely dated to the Oligocene-Early Miocene. It is sandwiched by two Late Miocene, out-of-sequence top-to-the E thrusts between Jurassic LP serpentinites on top and HT–LP contact metamorphosed marbles at its base. We document widespread W-verging ductile asymmetries within the Acquadolce Subunit, which correspond to top-to-the W extensional shearing for the nappe stack current orientation. This allowed for early syn-orogenic exhumation from blueschist- to greenschist-facies conditions, wherein coeval W-directed extension at the top of the exhuming units acted synchronously with E-directed thrusting at their base causing exhumation by extrusion in an overall contractional setting. The basal, E-vergent thrusting is, however, challenging to document as the wedge has since been reworked by Late Miocene, E-verging compressive tectonics, contact metamorphism, and later extension, obliterating much of the evidence supporting exhumation by extrusion during the early stages of wedge build-up. Syn-orogenic exhumation by extrusion from deep structural levels within the orogenic wedge is a viable mechanism to account for other exhumed HP-LT units in the inner part of the belt.
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