[1] The western Mediterranean subduction zone (WMSZ) extends from the northern Apennine to southern Spain and turns around forming the narrow and tight Calabrian and Gibraltar Arcs. The evolution of the WMSZ is characterized by a first phase of orogenic wedging followed, from 30 Ma on, by trench retreat and back-arc extension. Combining new and previous geological data, new tomographic images of the western Mediterranean mantle, and plate kinematics, we describe the evolution of the WMSZ during the last 35 Myr. Our reconstruction shows that the two arcs form by fragmentation of the 1500 km long WMSZ in small, narrow slabs. Once formed, these two narrow slabs retreat outward, producing back-arc extension and large scale rotation of the flanks, shaping the arcs. The Gibraltar Arc first formed during the middle Miocene, while the Calabrian Arc formed later, during the late Miocene-Pliocene. Despite the different paleogeographic settings, the mechanism of rupture and backward migration of the narrow slabs presents similarities on both sides of the western Mediterranean, suggesting that the slab deformation is also driven by lateral mantle flow that is particularly efficient in a restricted (upper mantle) style of mantle convection.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2002TC001488

Lateral slab deformation and the origin of the western Mediterranean arcs

SUMMARY We use continuously recording GPS (CGPS) and survey-mode GPS (SGPS) observations to determine Euler vectors for relative motion of the African (Nubian), Arabian and Eurasian plates. We present a well-constrained Eurasia‐Nubia Euler vector derived from 23 IGS sites in Europe and four CGPS and three SGPS sites on the Nubian Plate (−0.95 ± 4.8 ◦ N, −21.8 ± 4.3 ◦ E, 0.06 ± 0.005 ◦ Myr −1 ). We see no significant (> 1m m yr −1 ) internal deformation of the Nubian Plate. The GPS Nubian‐Eurasian Euler vector differs significantly from NUVEL-1A (21.0 ± 4.2 ◦ N, −20.6 ± 0.6 ◦ E, 0.12 ± 0.015 ◦ Myr −1 ), implying more westward motion of Africa relative to Eurasia and slower convergence in the eastern Mediterranean. The Arabia‐ Eurasia and Arabia‐Nubia GPS Euler vectors are less well determined, based on only one CGPS and three SGPS sites on the Arabian Plate. The preliminary Arabia‐Eurasia and Arabia‐ Nubia Euler vectors are 27.4 ± 1.0 ◦ N, 18.4 ± 2.5 ◦ E, 0.40 ± 0.04 ◦ Myr −1 , and 30.5 ± 1.0 ◦ N, 25.7 ± 2.3 ◦ E, 0.37 ± 0.04 ◦ Myr −1 , respectively. The GPS Arabia‐Nubia Euler vector differs significantly from NUVEL-1A (24.1 ± 1.7 ◦ N, 24.0 ± 3.5 ◦ E, 0.40 ± 0.05 ◦ Myr −1 ), but is statistically consistent at the 95 per cent confidence level with the revised Euler vector reported by Chu & Gordon based on a re-evaluation of magnetic anomalies in the Red Sea (31.5 ± 1.2 ◦ N, 23.0 ± 2.7 ◦ E, 0.40 ± 0.05 ◦ Myr −1 ). The motion implied in the Gulf of Aqaba and on the Dead Sea fault (DSF) by the new GPS Nubia‐Arabia Euler vector (i.e. ignoring possible Sinai block motion and possible internal plate deformation) grades from pure left lateral strike-slip in the Gulf and on the southern DSF with increasing compression on the central and northern DSF with relative motion increasing from 5.6 to 7.5 mm yr −1 (± 1m m yr −1 ) from south to north. Along the northern DSF (i.e. north of the Lebanon restraining bend) motion is partitioned between 6 ± 1m m yr −1 left-lateral motion parallel to the fault trace and 4 ± 1m m yr −1 faultnormal compression. Relative motions on other plate boundaries (including the Anatolian and Aegean microplates) derived from the GPS Euler vectors agree qualitatively with the sense of motion indicated by focal mechanisms for large crustal earthquakes (M > 6). Where data are available on fault-slip rates on plate bounding faults (North Anatolian fault, East Anatolian fault, Dead Sea fault, Red Sea rift), they are generally lower than, but not significantly different from, the full plate motion estimates suggesting that the majority of relative plate motion is accommodated on these structures.

/pdf/gps-constraints-on-africa-nubia-and-arabia-plate-motions-h9x6u7u4wg.pdf

GPS constraints on Africa (Nubia) and Arabia plate motions

Lithosphere tearing at STEP faults: response to edges of subduction zones

The Messinian salinity crisis—the desiccation of the Mediterranean Sea between 5.96 and 5.33 million years (Myr) ago1—was one of the most dramatic events on Earth during the Cenozoic era2. It resulted from the closure of marine gateways between the Atlantic Ocean and the Mediterranean Sea, the causes of which remain enigmatic. Here we use the age and composition of volcanic rocks to reconstruct the geodynamic evolution of the westernmost Mediterranean from the Middle Miocene epoch to the Pleistocene epoch (about 12.1–0.65 Myr ago). Our data show that a marked shift in the geochemistry of mantle-derived volcanic rocks, reflecting a change from subduction-related to intraplate-type volcanism, occurred between 6.3 and 4.8 Myr ago, largely synchronous with the Messinian salinity crisis. Using a thermomechanical model, we show that westward roll back of subducted Tethys oceanic lithosphere and associated asthenospheric upwelling provides a plausible mechanism for producing the shift in magma chemistry and the necessary uplift (approx1 km) along the African and Iberian continental margins to close the Miocene marine gateways, thereby causing the Messinian salinity crisis.

Deep roots of the Messinian salinity crisis

SUMMARY The Western Mediterranean displays a complex pattern of crustal deformation distributed along tectonically active belts developed in the framework of slow oblique plate convergence. We used earthquake and Global Positioning System (GPS) data to study the present-day kinematics and tectonics of the Africa-Eurasia plate boundary in this region. Crustal seismicity and focal mechanisms, analysed in terms of seismic moment release and seismic deformation, outline the geometry of major seismic belts and characterize their tectonics and kinematics. Continuous GPS data have been analysed to determine Euler vectors for the Nubian and Eurasian plates and to provide the global frame for a new Mediterranean GPS velocity field, obtained by merging continuous and campaign observations collected in the 1991‐2005 time span. GPS velocities and displacements predicted by the Nubia-Eurasia rotation pole provide estimates of the deformation accommodated across the tectonically active belts. The rather simple deformation occurring in the Atlantic region, characterized by extension about perpendicular to the Middle Atlantic and Terceira ridges and right-lateral motion along the Gloria transform fault, turns into a complex pattern of deformation, occurring along broader seismic belts, where continental lithosphere is involved. Our analysis reveals a more complex fragmentation of the plate boundary than previously proposed. The roughly E-W trending mainly compressive segments (i.e. southwestern Iberia, northern Algeria and southern Tyrrhenian), where plate convergence is largely accomodated across rather localized deformation zones, and partially transferred northward to the adjacent domains (i.e. the Algero-Balearic and Tyrrhenian basins), are interrupted by regions of more distributed deformation (i.e. the Rif-Alboran-Betics, Tunisia-Libya and eastern Sicily) or limited seismicity (i.e. the Strait of Sicily), which are characterized by less homogeneous tectonics regimes (mainly transcurrent to extensional). In correspondence of the observed breaks, tectonic structures with different orientation interfere, and we find belts with only limited deformation (i.e. the High and Middle Atlas, the Tunisian Atlas and the offshore Tunisia-Libya belt) that extends from the plate boundary into the Nubian plate, along pre-existing tectonic lineaments. Our analysis suggest that the Sicilian-Pelagian domain is moving independently from Nubia, according to the presence of a right-lateral and extensional decoupling zone corresponding to the Tunisia-Libya and Strait of Sicily deformation zone. Despite the space variability of active tectonic regimes, plate convergence still governs most of the seismotectonic and kinematic setting up to the central Aeolian region. In general, local complexities derive from pre-existing structural features, inherited from the tectonic evolution of the Mediterranean region. On the contrary, along Calabria and the Apennines the contribution of the subducted Ionian oceanic lithosphere and the occurrence of microplates (i.e. Adria) appear to substantially modify both tectonics and kinematics. Finally, GPS data across the Gibraltar Arc and the Tyrrhenian-Calabria domain support the hypothesis that slab rollback in these regions is mostly slowed down or stopped.

/pdf/kinematics-of-the-western-africa-eurasia-plate-boundary-from-22mvv5o50v.pdf

Kinematics of the Western Africa-Eurasia plate boundary from focal mechanisms and GPS data

We report on a marine seismic survey that images an active accretionary wedge west of Gibraltar. Ramp thrusts offset the seafloor and sole out to an east-dipping decollement, indicating ongoing westward-vergent tectonic shortening. New traveltime tomographic re- sults image a slab of oceanic lithosphere descending from the Atlantic domain of the Gulf of Cadiz, passing through intermediate-depth (60-120 km) seismicity beneath the west- ernmost Alboran Sea, and merging with a region of deep-focus earthquakes 600-660 km below Granada, Spain. Together, these new data provide compelling evidence for an active east-dipping subduction zone.

Evidence for active subduction beneath Gibraltar

We welcome the opportunity to discuss the geodynamics of the Rif-Betic region and will address the points in the order raised in the comment.

1. There is no rigid Alboran plate. Instead, a portion of the upper plate (the Gibraltar block) is pulled westward by hydrostatic suction, induced by the

/pdf/evidence-for-active-subduction-beneath-gibraltar-comment-and-58pbcqs7kp.pdf

L. Mendes-Victor

Papers

Evidence for active subduction beneath Gibraltar

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