The structure of Mediterranean arcs: New insights from the Calabrian Arc subduction system

Highlights

• We present the first modern amphibious seismic experiment conducted across Calabria.

• The section shows the forearc-to-backarc V_p structure of the subduction system.

• We infer mantle exhumation in the Marsili backarc basin, in the Tyrrhenian.

• The system is marked by spatially rapid petrological and tectonic changes.

• An analog of Tethys subduction systems formed by slab rollback is proposed.

Abstract

The formation of Cenozoic mountain belts in the Mediterranean realm was preceded by tens of millions of years of subduction, forming volcanic arcs, and frontal contractional systems. In addition, subduction usually involves slab rollback and formation of oceanic backarcs. Although such structure must have influenced the orogeny of Mediterranean mountain belts, no active analog has been mapped with modern crustal-scale seismic methods. Here, we study the entire Calabrian subduction system to map the structure resulting from Tethys lithosphere subduction and slab rollback, in a process that must be akin to that operating during a phase of the formation of the Mediterranean orogenic belts. We present a crustal-scale cross section of the entire Calabrian subduction system obtained from on- and off-shore wide-angle seismic data. The 2D P-wave velocity section shows spatially abrupt (<5 km of profile distance) structural and petrological transitions from the Ionian sedimentary wedge and Calabrian arc, to the rifted NW Calabrian margin, where the Quaternary Aeolian arc is emplaced. The margin, then, transitions northwards into the Marsili backarc region, where exhumed mantle and localized volcanism occurred during its formation. This complex structure implies rapid temporal and spatial changes between magmatic and amagmatic processes, and between compressional and extensional regimes during the evolution of this subduction system. We find that some terranes involved in the Alpine orogeny share petrological and tectonic similarities with some domains of the Calabrian subduction system. Based on the results of this study we propose the Calabrian Arc system as an analog for the subduction structuration that preceded the formation of Alpine orogenic systems.

Introduction

The Mediterranean realm includes a system of Cenozoic arcuate orogenic belts formed during the Alpine Orogeny (Dewey et al., 1973) (Fig. 1a). Mantle tomography images and plate reconstructions support that the collision events that formed these orogens were preceded by subduction of Tethys lithosphere (e.g. Dewey et al., 1973; Schettino and Turco, 2011). The evolution of the subduction systems must have often included slab retreat (e.g. Handy et al., 2010), causing the migration of volcanic arcs, extension of the overriding plate and formation of backarc basins possibly similar to the Tyrrhenian (Prada et al., 2015; Loreto et al., 2020) and the Alboran-South Balearic basins (Gomez de la Peña et al., 2018), and forearc contractional systems (e.g. Stampfli et al., 1998; Polonia et al., 2011; Marroni et al., 2017), determining thereby, the pre-collisional structuration of the lithosphere. The Gibraltar and Calabrian backarcs are no longer opening, and their domains are currently being inverted, which may indicate a current initial phase of a collision (Giaconia et al., 2015; Zitellini et al., 2020). Yet, the structure of Alpine-type belts has been associated to the imbrication of the domains formed during continental rifting (e.g. Reston and Manatschal, 2011; Mohn et al., 2014), and the major lithospheric-scale structuration created during subduction has been largely ignored, possibly because of the fragmentary geological record in mountain belt that makes reconstructions disputable, and the lack of present-day potential analogs.

Reconstructing the structure of past subduction systems from present-day orogens is a challenging task because of the tectonic and metamorphic overprinting that terranes suffer during orogeny, and the incomplete outcrop information. An alternative way to infer such structure is by exploring the present-day subduction of the Tethys lithosphere in the Mediterranean, as occurs under the Calabrian, Gibraltar and Hellenic arcs (Fig. 1a) (Spakman and Wortel, 2004; Müller et al., 2008). Possibly similar to Alpine subductions (e.g. Maffione and van Hinsbergen, 2018), the upper plate of these present-day subduction systems has been shaped by slab retreat, backarc opening, and accretionary forearc systems. Therefore, understanding of the forearc-to-backarc structure of present-day subduction systems in the Mediterranean may provide insights into the processes that shaped Alpine systems before collision.

In this work, we focus on the Calabrian subduction system, which includes the Tyrrhenian backarc basin, the Aeolian volcanic arc, the NW Calabrian margin, the Calabrian arc and the contractional Ionian wedge (Fig. 1a and 1b). Vintage (i.e. 1970) refraction studies provided 1D crustal velocity information across the subduction system, bringing first-order approximations on the crustal structure of the Calabrian arc (e.g. Cassinis et al., 2003). Regional earthquake tomography of the Calabrian region has provided insights on the structure of the slab and the petrological nature of the deep lithosphere and asthenosphere (e.g. Caló et al., 2009). However, these studies do not provide enough resolution to assess in detail the structure and petrological nature of shallow lithospheric domains (i.e. crust and uppermost mantle) across the subduction system.

To assess the forearc-to-backarc structure of this system, we present a detailed P-wave velocity (V_p) cross section of the entire subduction system using travel-time tomography from wide-angle seismic data (WAS) (Fig. 1) acquired in 2015 during the CHIANTI (Calabrian arc Hazards in IoniAN and TyrrhenIan seas) experiment (Fig. 1b). The 425 km-long model provides unprecedented information of the structure and petrology of shallow lithospheric domains across the Calabrian subduction system from the forearc to the backarc (Fig. 1b).