Description: We investigate the crustal structure of the SW Iberian margin along a 340 km-long refraction and wide-angle reflection seismic profile crossing from the central Gulf of Cadiz to the Variscan continental margin in the Algarve, Southern Portugal. The seismic velocity and crustal geometry model obtained by joint refraction and reflection travel-time inversion reveal three distinct crustal domains: the 28–30 km-thick Variscan crust in the north, a 60 km-wide transition zone offshore, where the crust abruptly thins ~ 20 km, and finally a ~ 7 km-thick and ~ 150 km-wide crustal section that appears to be oceanic in nature. The oceanic crust is overlain by a 1–3 km-thick section of Mesozoic to Eocene sediments, with an additional 3–4 km of low-velocity, unconsolidated sediments on top belonging to the Miocene age, Gulf of Cadiz imbricated wedge. The sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the Early Jurassic that followed the continental break-up in the Central Atlantic. The narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between Iberia and Africa that started shortly thereafter (Bajocian) and lasted up to the initiation of oceanic spreading in the North Atlantic at the Tithonian (late Jurassic-earliest Cretaceous). The velocity model displays four wide, prominent, south-dipping low-velocity anomalies, which seem to be related with the presence of crustal-scale faults previously identified in the area, some of which could well be extensional faults generated during this rifting episode. We propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the Alpine-Tethys with the Atlantic ocean, so it is, in turn, one of the oldest oceanic crustal fragments currently preserved on Earth. The presence of oceanic crust in the central Gulf of Cadiz is consistent with geodynamic models suggesting the existence of a narrow, westward retreating oceanic slab beneath the Gibraltar arc-Alboran basin system.

Description: The Gorringe Bank is a gigantic seamount that separates the Horseshoe and Tagus abyssal plains offshore SW Iberia, in a zone that hosts the convergent boundary between the Africa and Eurasia plates. Although the region has been the focus of numerous investigations since the early 1970s, the lack of appropriate geophysical data makes the nature of the basement, and thus the origin of the structures, still debated. In this work, we present combined P-wave seismic velocity and gravity models along a transect that crosses the Gorringe Bank from the Tagus to the Horseshoe abyssal plains. The P-wave velocity structure of the basement is similar in the Tagus and Horseshoe plains. It shows a 2.5–3.0 km-thick top layer with a velocity gradient twice stronger than oceanic Layer 2 and an abrupt change to an underlying layer with a five-fold weaker gradient. Velocity and density is lower beneath the Gorringe Bank probably due to enhanced fracturing, that have led to rock disaggregation in the sediment-starved northern flank. In contrast to previous velocity models of this region, there is no evidence of a sharp crust–mantle boundary in any of the record sections. The modelling results indicate that the sediment overlays directly serpentinite rock, exhumed from the mantle with a degree of serpentinization decreasing from a maximum of 70–80% under the top of Gorringe Bank to less than 5% at a depth of ∼20 km. We propose that the three domains were originally part of a single serpentine rock band, of nature and possibly origin similar to the Iberia Abyssal Plain ocean–continent transition, which was probably generated during the earliest phase of the North Atlantic opening that followed continental crust breakup (Early Cretaceous). During the Miocene, the NW–SE trending Eurasia–Africa convergence resulted in thrusting of the southeastern segment of the exhumed serpentinite band over the northwestern one, forming the Gorringe Bank. The local deformation associated to plate convergence and uplift could have promoted pervasive rock fracturing of the overriding plate, leading eventually to rock disaggregation in the northern flank of the GB, which could be now a potential source of rock avalanches and tsunamis.

Description: New structural images of the Calabrian accretionary wedge are presented from depth‐migrated multichannel seismic data. A combined interpretation swath‐mapping bathymetry allows us to identify five morphological domains on the basis of their tectonic style. (1) Beneath the undeformed Abyssal Plain, a set of deep, NW vergent reverse faults cuts the Ionian oceanic crust and thick pre-Messinian sediments. (2) Towards the NW, the low-taper post-Messinian wedge overlying a shallow NW dipping décollement, at the base of the Messinian evaporites and exhibiting strong tectonic thickening. (3) Beneath the flat Central Transition Zone, a backthrust marks the contact between the post- and pre-Messinian wedges. Here, the décollement dip increases (〉 3°) cutting through deeper sediments to reach 10 km depth. (4) Beneath the pre-Messinian Calabrian wedge, some steep landward dipping features are imaged where underplating may occur. (5) Beneath the inner plateau, a fore‐arc basin lies above the top of the Calabrian continental basement imaged at 8 km depth. The architecture of the Calabrian accretionary complex is very similar to the Mediterranean Ridge. Both systems consist of (a) an external low-taper post-Messinian wedge overlying a thick undeformed section of underthrust Mesozoic sediments and (b) an internal pre-Messinian wedge where the décollement steps down and where the underthrust section is presumably underplated. We perform area balancing and show that since the Messinian, the Calabrian accretionary wedge has undergone extremely rapid outward growth at an average rate of 30 km/Ma, which makes it the fastest growing accretionary wedge over the past 5 Ma. Highlights: ► Relation between the pre- and post-Messinian juxtaposed wedges. ► Contact between these wedges along a major backthrust. ► The 130 km long outer Calabrian prism: a huge salt-bearing body. ► One of the fastest growing wedges (2.0 cm/year) in recent Earth history. ►~ 170 km amount of post-Messinian subduction (subduction rate 3–4 cm/year).

Description: The rollback of a segmented slab of oceanic lithosphere is typically accompanied by vertical lithospheric tear fault(s) along the lateral slab edge(s) and by strike slip movement in the upper plate, defined as a STEP fault (Subduction Tear Edge Propagator). The Neogene evolution of the Central Mediterranean is dominated by the interaction between the slow Africa-Eurasia convergence and the SE-ward rollback of the Ionian slab, that leads to the back-arc opening of the Tyrrhenian Sea. Here, we present post-stack time migrated and pre-stack depth migrated Archimede (1997) multichannel seismic lines, that were acquired offshore eastern Sicily, at the foot of the Malta escarpment. First, we identify the recent deformation along the lateral ramp of the Calabrian accretionary wedge. Towards the east, the Calabrian wedge is formed by the accretion of the post-evaporitic sediments, above a decollement at the base of the Messinian evaporites. At the latitude of Syracuse, 50 km east of the Malta escarpment, a major N150 degrees E trending crustal scale and vertical fault slices through the entire accretionary wedge. This fault cuts by several kilometers, through the pre-evaporitic Messinian sediments and into the basement. The vertical offset along this vertical fault decreases from north to south, and the fault is no longer observed on the seismic lines, 50 km SE of the Alfeo seamount. A previously published Moho depth isocontour map, offshore Sicily and the recent GPS data, combined with the presence of strike slip movements NE onshore Sicily, allow us to identify this 200 km long crustal-scale fault as the surface expression of a STEP fault. The presence of syntectonic Pleistocene sediments on top this crustal-scale fault suggests a recent lithospheric vertical movement of the STEP fault, in response to the rollback of the Ionian slab and to the SE-ward advance of the Calabria-Peloritan block.

Description: In the Gulf of Cadiz key segment of the Africa-Iberia plate boundary (North-East Atlantic ocean), three main different modes of tectonic interference between a recently identified wrench system (SWIM) and the Gulf of Cadiz Accretionary Wedge (GCAW) were tested through analog sand-box modeling: a) An active accretionary wedge on top of a pre-existent inactive basement fault; b) An active strike-slip fault cutting a previously formed, inactive, accretionary wedge; and c) Simultaneous activity of both the accretionary wedge and the strike-slip fault. The results we obtained and the comparison with the natural deformation pattern favor a tectonic evolution comprising two main steps: i) the formation of the Gulf of Cadiz Accretionary Wedge on top of inactive, Tethyan-related, basement faults (Middle Miocene to similar to 1.8 Ma); ii) subsequent reactivation of these basement faults with dextral strike-slip motion (similar to 1.8 Ma to present) simultaneously with continued tectonic accretion in the GCAW. These results exclude the possibility of ongoing active SWIM wrench system cross-cutting an inactive GCAW structure. Our results also support a new interpretation of the SWIM wrench system as fundamentally resulting from strike-slip reactivation of an old (Tethyan-related) plate boundary

Description: Subduction of the Ionian Sea lithosphere beneath Calabria and E Sicily has shaped the evolution of Southern Italy since 5 Ma. We present reprocessed 96-channel seismic reflection data acquired during the PRISMED survey (1993), and ARCHIMEDE survey (1997), both onboard R/V Le Nadir crossing the Ionian Abyssal Plain and the deep offshore portions of the Calabrian accretionary wedge (“External Calabrian arc”). Our seismic data provide clear images of the the toe of the accretionary wedge (from the Ionian abyssal plain across the deformation front). Here, the base of the Messinian evaporites serves as a weak detachment above which the entire Messinian salt and the overlying Plio-Quaternary (PQ) sediments are accreted. Repeated imbricated thrusting and some backthrusting within the Calabrian wedge allows a doubling of the thickness of the salt layer within 30 km of the deformation front. Pre-stack depth migration of seismic line PM01 allows deeper imaging of the entire frontal wedge (post-Messinian) as well as the boundary to the older internal Calabrian wedge (pre-Messinian in age). To the east of the Malta Escarpment (East Sicily), another major tectonic structure is imaged by the Archimede profiles. The structure offsets the top of the Pre-Messinian deposits and the underlying units by 0.5 - 1 sTWT increasing from S to N. This N150°E oriented lithospheric scale fault is interpreted as a tear fault (“STEP” fault) which has allowed the roll-back of the Ionian slab. Syn-tectonic sedimentation of Plio-Quaternary units above this fault suggest ongoing deformation related to this tear fault. Together with folding of Plio-Quaternary strata at the toe of the wedge, including Augias mega-turbidite dated at 3500 BP, this suggests ongoing activity of the subduction system. Available GPS data suggest slow eastward motion (3-4mm/yr) of Calabrian stations in a Nubia fixed reference frame, consistent with slab roll-back and capable of driving E vergent deformation in the accretionary wedge. Slip along the shallow NW dipping subduction fault plane offers a possible explanation for the 1693 Catania earthquake and tsunami. The seismogenic potential of the subduction zone and a better understanding of typical earthquake magnitudes and recurrence intervals bear strongly on hazard assessment for Southern Italy.

Description: It is widely accepted that the Central and Eastern Mediterranean are remnants of the Neo-Tethys. However, the orientation and timing of spreading of this domain remain controversial. Here, we present time migrated and pre-stack depth migrated NW-SE oriented Archimede (1997) lines together with the PrisMed01 (1993) profile to constrain the evolution of the Ionian basin. Our interpretation allows us to identify a large-scale set of SW-NE striking reverse faults beneath the Ionian Abyssal Plain. These primarily NW vergent faults are characterized by a spacing comprised between 10 to 20 km and a dip ranging from 60 to 65{degree sign}. Following very recent paleogeographic reconstructions, we propose that the set of N{degree sign}55 features initially formed as normal faults during the NW-SE trending seafloor spreading of the Ionian basin after its late Triassic-early Jurassic rifting. Based on geometric comparisons with the intraplate deformation observed beneath the Central Indian Ocean, we show that the inherited oceanic normal faults were reactivated under compression as reverse faults. Well-developed Tortonian syntectonic basins developed NW of the major faults and the base of the Messinian evaporites (Mobile Unit) is slightly folded by the activity of the faults. We show that 3-4 km of total shortening occurs over a 80 km wide area beneath the Ionian Abyssal Plain, resulting in a bulk shortening of 3.5-5 %. We propose a link between the Tortonian-early Messinian inversion of the fault pattern and a plate tectonic reorganization prior to the main phase of back-arc opening of the Tyrrhenian domain.