Seismic tomography models of the three-dimensional upper mantle velocity structure of the Mediterranean-Carpathian region provide a better understanding of the lithospheric processes governing its geodynamical evolution. Slab detachment, in particular lateral migration of this process along the plate boundary, is a key element in the lithospheric dynamics of the region during the last 20 to 30 million years. It strongly affects arc and trench migration, and causes along-strike variations in vertical motions, stress fields, and magmatism. In a terminal-stage subduction zone, involving collision and suturing, slab detachment is the natural last stage in the gravitational settling of subducted lithosphere.

http://www.mantleplumes.org/WebDocuments/Wortel2000.pdf

Subduction and Slab Detachment in the Mediterranean-Carpathian Region

[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

/pdf/reconciling-plate-tectonic-reconstructions-of-alpine-tethys-1do3i5vm2t.pdf

Reconciling plate-tectonic reconstructions of Alpine Tethys with the geological–geophysical record of spreading and subduction in the Alps

In recent years, the origin of the Betic-Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late-orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic-Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present-day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic-Rif orocline during the early to middle Miocene.

https://spiral.imperial.ac.uk/bitstream/10044/1/21686/9/tect1148.pdf

Origin of the Betic-Rif mountain belt

Retreating subduction boundaries, formed where the rate of subduction exceeds the rate of overall plate convergence, appear to be commonly developed features within regions of early or incomplete continent-continent collision. They are characterized by regional extension within the overriding plate and, at their leading edge, by thin-skinned arcuate thrust belts that are concave towards the overriding plate. As is illustrated by examples from the Mediterranean region, the formation of retreating subduction boundaries is intimately related to the process of continental collision. During the early stages of collision, retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary. Retreating plate boundaries can also form before the main collision, and the associated thrust belts emplaced as precollisional accretionary assemblages. Because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab (slab pull), subduction usually ceases when, and only when, thick buoyant continental crust enters the subduction zone. Thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted. In some cases, retreating subduction boundaries may “escape” into the open ocean, where they form nearly isolated, local tectonic systems. In these systems the rate of subduction is approximately compensated by the rate of upper plate extension, and migration of the system across the oceanic region may be very rapid. For example, the Horseshoe Seamounts, located about 800 km offshore in the eastern North Atlantic, may be the active expression of an east dipping, westwardly migrating retreating subduction boundary that has evolved from the Betic Cordillera-Rif system active in Miocene time and may now be progressing across the Atlantic at approximately 50 mm/yr. An analogous situation may be represented by the Scotia Arc system, a westward dipping retreating subduction system located between the South American and Antarctic plates, which may have “escaped” into the South Atlantic ocean from a zone of crustal shortening in the Andes and is now progressing across the Atlantic at a rate of about 80 mm/yr.

Evolution of retreating subduction boundaries formed during continental collision

Leg 107 of the Ocean Drilling Program drilled a west-northwest-east-southeast transect of seven sites across the Tyrrhenian Sea, the youngest of the sub-basins of the Mediterranean Sea. Sites 654, 653, 652, and 656 document the rifting and subsidence of the Sardinia passive continental margin. On the upper margin (Site 654), we cored a classic transgressive sequence: subaerial conglomerates, overlain by oyster-bearing sands, overlain by marine marl. Comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are Tortonian (late Miocene) to Messinian (latest Miocene) in age, whereas synrift sediments on the lower margin are Messinian to Pliocene in age. During the Messinian desiccation of the Mediterranean, Sites 654 and 653, now on the upper Sardinian margin, apparently occupied a basinal setting, where they received nannoplankton-bearing clays interbedded with laminated gypsum. Sites 656 and 652, now on the lower Sardinia margin, were apparently higher standing during the desiccation event; their Messinian facies are subaerial and lacustrine, respectively. We infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin. Such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a "rolling-back" subduction zone, or in terms of extension of continental crust, by shear along a deep "detachment fault."

Sites 655, 651, and 650 were drilled into two small basalt-floored basins of the central and eastern Tyrrhenian. Emplacement of basaltic crust in the central Tyrrhenian (Vavilov Basin) apparently began more than a million years before, emplacement of basaltic crust in the eastern Tyrrhenian (Marsili Basin). This observation is compatible with previous suggestions that the Tyrrhenian has grown southeastward in response to "rollback" of the down-going slab that currently dips northwestward under the toe of Italy. At the easternmost site, high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2,500 m. It has apparently subsequently subsided to its present depth of >4,100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid-ocean ridge.

ODP Leg 107 in the Tyrrhenian Sea: Insights into passive margin and back-arc basin evolution

Plio-Pleistocene benthic foraminiferal assemblages from Holes 652A, 653A, and 654A of ODP Leg 107 (Tyrrhenian Sea, Western Mediterranean) are examined by means of Q-mode factor analysis. As a result, seven assemblages (A, B, Ca, Cb, D, Ea, and Eb) are distinguished in four zones (I-IV). The relationship between zones and assemblages and their stratigraphic ranges are as follows: Zone I. Assemblage A; MP11 biozone in the lowest Pliocene, Zone II. Assemblage B; MP12 biozone to lower part of MP15 biozone in the Pliocene, Zone III. Assemblages Ca (Sites 652 and 653) and Cb (Site 654); middle of MP15 biozone to middle of MP16 bio­ zone in the upper Pliocene, and Zone IV. Assemblages D, Ea, and Eb; upper of MP16 (uppermost Pliocene) and the Pleistocene. These faunal changes suggest that the whole Tyrrhenian Basin has become deeper from upper epibathyal to lower mesobathyal depths since the beginning of the Pliocene. This increasing depth was the differential that become more and more with the advance of time: Site 652 was deepest and Site 654 was shallowest, consistently. This basin sank rap­ idly during late Pliocene (Zone III). The assemblages in Zone III and lower part of Zone IV indicate that the Tyrrhenian Basin has been occupied by the modern Mediterranean-type warmer deep water since the late Pliocene. At the same time, these assemblages also sug­ gest that this region was affected in some degree by cool deep water like NADW in the Atlantic during late Pliocene to early Pleistocene. Less oxygenated bottom conditions are represented by Assemblage E which prevailed in the three-site area as early as early Pleistocene time.

http://www-odp.tamu.edu/publications/107_SR/VOLUME/CHAPTERS/sr107_29.pdf

29. quantitative analysis of benthic foraminiferal assemblages from plio-pleistocene sequences in the tyrrhenian sea, odp leg 1071

High-resolution bio- and chemostratigraphy of an earliest Pliocene section from ODP Site 652 indicates that postflood paleoceanographic conditions in the Tyrrhenian Sea can be sub-divided into two discrete intervals. The first is manifested by an acme of Sphaeroidinellopsis spp., increasing carbonate contents, and a progressive decrease upsection in both the 613C and 5lsO values of the planktonic foraminifera. The lower part of the acme interval contains unusual surface-to-bottom water isotope gradients suggesting a stratification of two water masses. Normal gradients in the up­ per part of the acme interval suggest a well-mixed water body. Between the end of the acme interval and the MP11/MP12 boundary, denoted by the first occurrence (F.O.) of Globorotalia margaritae, a migrational first appearance, there was a catastrophic collapse of the gradient marking an onset of the second post-flood interval. The disintegration of habitable conditions is suggested by a sharp decrease in carbonate content and the disappearance of the benthonic assemblage, which is subsequently replaced predominantly by Uvigerinapygmea, indicative of cold, low-oxygenated bottom waters. The introduction of benthonic species denoting well-oxygenated bottom conditions occurs within the lower MP12 zone. Superimposed on these overall trends are shorter term, warm-cold cycles, which are interpreted as orbitally induced, cli­ matic fluctuations. Correlative studies of the less complete earliest Pliocene sections from ODP Holes 653B and 654A confirm these interpretations. A scenario derived from an integration of all the stratigraphic data indicates that normal paleoceanographic condi­ tions were operating in the Tyrrhenian Sea only approximately 250,000 yr after the cessation of Messinian evaporative conditions at the Miocene/Pliocene boundary. The post-flood interval is marked by an initial period of gradual infill­ ing, the Sphaeroidinellopsis spp. acme interval, followed by a disintegration of oceanographic conditions and a second recovery period. A sudden influx of cold, deep Atlantic waters into the Tyrrhenian Sea, resulting from a major tectonic break in the Gibraltar sill, may have caused this catastrophic reversal in the orderly recovery of normal paleoceano­ graphic conditions in the post-flood period.

http://www-odp.tamu.edu/publications/107_SR/VOLUME/CHAPTERS/sr107_26.pdf

Rodolfo Sprovieri

Papers

ODP Leg 107 in the Tyrrhenian Sea: Insights into passive margin and back-arc basin evolution

29. quantitative analysis of benthic foraminiferal assemblages from plio-pleistocene sequences in the tyrrhenian sea, odp leg 1071

26. paleoceanographic conditions following the earliest pliocene flooding of the tyrrhenian sea1

Plio-Pleistocene benthic foraminifera of the Tyrrhenian Sea

La campagne 107 du Joides Resolution (Ocean Drilling Program) en mer Tyrrhénienne: premiers résultats