Transfer zones in Mediterranean back-arc regions and tear faults

Abstract

Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat. Because the space available for retreating slabs is limited in the Mediterranean realm, slab tearing during retreat has been a major ingredient of the evolution of this region since the end of the Eocene. The association of detailed seismic tomographic models and extensive field observations makes the Mediterranean an ideal natural laboratory to study these transfer zones. We review in this paper the various structures accommodating differential retreat in the crust from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models. Simple, archetypal, crustal-scale strike-slip faults are in fact rare in these contexts above slab tears. Transfer zones are in general instead wide deformation zones, from several tens to several hundred kilometers. A partitioning of deformation is observed between the upper and the lower crust with low-angle extensional shear zones at depth and complex association of transtensional basins at the surface. In the Western Mediterranean, between the Gulf of Lion and the Valencia basin, transtensional strike-slip faults are associated with syn-rift basins and lower crustal domes elongated in the direction of retreat (a-type domes), associated with massive magmatic intrusions in the lower crust and volcanism at the surface. On the northern side of the Alboran Sea, wide E-W trending strike-slip zones show partitioned thrusting and strike-slip faulting in the external zones of the Betics, and E-W trending metamorphic core complexes in the internal zones, parallel to the main retreat direction. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. In the Aegean-Anatolian region two main tear faults with different degrees of maturity are observed. Western Anatolia (Menderes Massif) and the Eastern Aegean Sea evolved above a major left-lateral tear in the Hellenic slab. In the crust, the differential retreat was accommodated mostly by low-angle shear zones with a constant direction of stretching and the formation of a-type high-temperature domes. On the opposite side of the Aegean region, the Corinth and Volos Rift as well as the Kephalonia fault offshore, accommodate the formation of a dextral tear fault. We discuss the rare occurrence of pure strike-slip faults in these contexts and propose that the high heat flow above the retreating slabs and more especially above slab tears favors a ductile behavior with distributed deformation of the crust and the formation of low-angle shear zones and high-temperature domes. While retreat proceeds, aided by tears, true strike-slip fault system may localize and propagate toward the retreating trench, ultimately leading to the formation of new plate boundary, as shown by the example of the North Anatolian Fault.

Segmentation of the Apenninic Margin of the Tyrrhenian Back‐Arc Basin Forced by the Subduction of an Inherited Transform System

“…Obliquity between the rifted margin and the belt caused the central Apennines to enter the collisional stage earlier than the southern Apennines (Figures 10c and 11c), where faster south-east rollback and overall slab arching occurred. Opening of the Tyrrhenian back-arc basin occurred since the Miocene, with an E-ward migration of the extensional wave (e.g., Jolivet et al, 2021), and led to hyperextension, mantle exhumation (Prada et al, 2014) and, since the Messinian, oceanic crust production in the southern Tyrrhenian sub-basin. During the Pliocene, progressive rollback and slab arching in the southern Apennines led to the further widening of the southern Tyrrhenian sub-basin with respect to the northern one (Figures 10c and 11d).…”

“…On the upper plate, since the Miocene, crustal stretching led to the opening of the triangularly shaped Tyrrhenian back-arc basin (e.g., Jolivet et al, 1998;Mattei et al, 2002;Sartori et al, 2001, Figures 1a and 1b). The central portion of the basin is floored by exhumed mantle and Messinian to Pleistocene oceanic crust (e.g., Jolivet et al, 2021;Kastens & Mascle, 1990;Mascle & Rehault, 1990;Sartori et al, 2004;Savelli & Ligi, 2017;Scrocca et al, 2012), whereas the eastern margin of the basin, that is, the Apenninic margin, is mostly affected by <1 Ma to present potassic volcanism developed during extension (e.g., Acocella & Funiciello, 2006;Carminati et al, 2010;Conti et al, 2017;Serri et al, 1993) (Figure 1d). In the eastern Tyrrhenian margin, active extensional tectonics migrated E-NE-ward (e.g., Bartole, 1995;Cavinato & Celles, 1999) from late Miocene-Pliocene offshore and onshore basins developing on thinned continental crust to the west (e.g., Bellotti, et al, 1997;Bruno et al, 2000;Casciello et al, 2006;Cipollari et al, 1999;Conti et al, 2017;Cosentino et al, 2006;Milia & Torrente, 2015;Milia et al, 2009) to actively extending intramontane basins located in the axial zone of the belt (e.g., Cavinato & Celles, 1999;Cosentino et al, 2017;Galadini & Messina, 2004;Valente et al, 2019), where NE-SW oriented extension has produced earthquakes with Mw > 6 in recent years (e.g., Bernard & Zollo, 1989;Boncio & Lavecchia, 2000;Chiarabba et al, 2009;Cello et al, 1997;G...…”

“…Here, we apply the concept of structural inheritances to the large-scale interpretation of the lateral segmentation of the eastern margin of the Tyrrhenian back-arc basin (western Mediterranean Sea; Figure 1a). This basin is divided into a northern sub-basin, characterized by thinned continental crust (e.g., Jolivet et al, 1998), and a central/southern sub-basin, floored by thinned continental crust, exhumed mantle (e.g., Jolivet et al, 2021;Prada et al, 2014), and late Miocene to recent oceanic crust (e.g., Kastens & Mascle, 1990;Savelli & Ligi, 2017) (Figure 1b). Many authors propose a gradual transition between the two sub-basins, with the oceanic and exhumed mantle domains of the southern basin fading out northward into thinned continental crust (e.g., Jolivet et al, 2021).…”

“…This basin is divided into a northern sub-basin, characterized by thinned continental crust (e.g., Jolivet et al, 1998), and a central/southern sub-basin, floored by thinned continental crust, exhumed mantle (e.g., Jolivet et al, 2021;Prada et al, 2014), and late Miocene to recent oceanic crust (e.g., Kastens & Mascle, 1990;Savelli & Ligi, 2017) (Figure 1b). Many authors propose a gradual transition between the two sub-basins, with the oceanic and exhumed mantle domains of the southern basin fading out northward into thinned continental crust (e.g., Jolivet et al, 2021). Others suggest, instead, a localized transition along a major E-W transform zone placed along the 41st parallel (Spadini & Wezel, 1994) (Figure 1b), whose development is inferred to be the consequence of a lithospheric tear fault within the subducting slab, which would retrace the ocean-to-continent transition established during Mesozoic rifting (Rosenbaum & Lister, 2004).…”

Structural style of the Languedoc Pyrenean thrust belt in relation with the inherited Mesozoic structures and with the rifting of the Gulf of Lion margin, southern France

“…Such transfer zones correspond to an area of distributed deformation in the upper crust and sedimentary cover that accommodates mid-or lower crustal strike-slip faulting. The major Catalan transfer zone, south of the study area (Maillard et al, 2020;Jolivet et al, 2021), is interpreted as the response of the overriding plate above slab tears, in a retreating subduction (Romagny et al, 2020). However, it is unlikely that the minor transfer zones documented here depict multiple slab tears, instead, they probably represents partitioning of the extensional deformation in the proximal part of the overriding plate, driven by shearing at the base of the lithosphere, and leading to lower continental crust exhumation in the distal part of the margin (Jolivet et al, 2020).…”