Abstract
Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat in back-arc regions. 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 in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between high-angle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts. 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 in the brittle field 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 with a transition in time from ductile to brittle deformation. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. Deep structures are not known there. 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 exhumed from the middle and lower crust. These low-angle shear zones evolve through time from ductile to brittle. 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. Here, only the brittle crust can be observed, but seismological data suggest low-angle shear zones at depth below the rifts. 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.
Highlights
Slab tears, or STEP-faults (Subduction-Transform-Edge Propagator, from Govers and Wortel, 2005; Wortel et al, 2009), are among the first-order drivers of retreating subduction zones dynamics and of deformation in overriding plates
We review in this paper the various structures in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between highangle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts
A similar situation can be deduced for TZ#10 and #11 on either side of the Gibraltar torn slab. The longest of these transfer zones is the CBTSZ (#7), which results from the progressive tearing of the future Apennine slab and which separated the future Apennine subduction from the Algerian subduction. All these transfer zones have distinctive characteristics, they are more or less wide, they have brought to the surface deep crustal tectonic units or, on the opposite, show mostly brittle structures, but they show some common features that are due to their position above a slab tear in a back-arc region
Summary
STEP-faults (Subduction-Transform-Edge Propagator, from Govers and Wortel, 2005; Wortel et al, 2009), are among the first-order drivers of retreating subduction zones dynamics and of deformation in overriding plates. A systematic description of structures at different scales in the back-arc region related to slab tearing and their evolution is not yet available These structures form within zones of variable width transferring deformation between domains of different amounts of retreat and magnitude of stretching in the overriding plate. They accommodate lateral differences in extension rates or differences in extensional structures geometry, such as different sense of block tilting such as in the Suez Rift (Colletta et al, 1987) or in the Rhine Graben (Brun et al, 1991, 1992) They are either strictly parallel to the regional stretching directions, in which case simple geometrical rules link the steep normal faults, the detachments and the strike-slip faults (Gibbs, 1990), or oblique if they are reactivated structures (Colletta et al, 1987). We show how the deformation may progressively localize until the eventual formation of a new plate boundary
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