L. Torelli

Eastern segment of the Azores-Gibraltar line (central-eastern Atlantic) : An oceanic plate boundary with diffuse compressional deformation

New seismic-reflection images across the eastern segment of the Azores-Gibraltar line west of the collisional area between the African and Iberian plates have revealed a complex pattern of compressional deformation involving the Mesozoic oceanic lithosphere. The compressional deformation developed in a region of slow plate convergence and is diffused, at different lithospheric levels, across an area spanning ∼200 km from the Gorringe Ridge to the Seine Plain. The convergence between the African and Iberian plates has been active since Tertiary time, and our results indicate that no subduction zone exists across this part of the plate boundary.

The Calabrian Arc subduction complex in the Ionian Sea: Regional architecture, active deformation, and seismic hazard

We analyzed the structure and evolution of the external Calabrian Arc (CA) subduction complex through an integrated geophysical approach involving multichannel and single‐channel seismic data at different scales. Pre‐stack depth migrated crustal‐scale seismic profiles have been used to reconstruct the overall geometry of the subduction complex, i.e., depth of the basal detachment, geometry and structural style of different tectonic domains, and location and geometry of major faults. High‐resolution multichannel seismic (MCS) and sub‐bottom CHIRP profiles acquired in key areas during a recent cruise, as well as multibeam data, integrate deep data and constrain the fine structure of the accretionary wedge as well as the activity of individual fault strands. We identified four main morpho‐structural domains in the subduction complex: 1) the post‐Messinian accretionary wedge; 2) a slope terrace; 3) the pre‐Messinian accretionary wedge and 4) the inner plateau. Variation of structural style and seafloor morphology in these domains are related to different tectonic processes, such as frontal accretion, out‐of-sequence thrusting, underplating and complex faulting. The CA subduction complex is segmented longitudinally into two different lobes characterized by different structural style, deformation rates and basal detachment depths. They are delimited by a NW/SE deformation zone that accommodates differential movements of the Calabrian and the Peloritan portions of CA and represent a recent phase of plate re‐organization in the central Mediterranean. Although shallow thrust‐type seismicity along the CA is lacking, we identified active deformation of the shallowest sedimentary units at the wedge front and in the inner portions of the subduction complex. This implies that subduction could be active but aseismic or with a locked fault plane. On the other hand, if underthrusting of the African plate has stopped recently, active shortening may be accommodated through more distributed deformation. Our findings have consequences on seismic hazard, since we identified tectonic structures likely to have caused large earthquakes in the past and to be the source regions for future events.

Plio–Quaternary tectonic evolution and structure of the Catania foredeep, the northern Hyblean Plateau and the Ionian shelf (SE Sicily)

Abstract Available multi- and single-channel seismic reflection profiles, calibrated by onshore borehole data, have been used for defining the structural styles in the shelf and slope of the Ionian Sea between Catania and Augusta (SE Sicily). The geological and geophysical data suggest that this area represents a segment of the foredeep–foreland system which collapsed after Late Pliocene times. The foundering was controlled by normal faults trending NE–SW, which flank the southern margin of the Catania foredeep. Onland, in outcrop, these faults appear largely to be post-dated by Lower Pleistocene sediments, nearshore carbonates passing laterally into basinal clays, which lie unconformably upon older substrata. Offshore, close to the southern edge of the foredeep, seismic lines allow recognition of two distinct units: a syn-rift wedge (Upper Pliocene submarine tholeiites and sediments), and a post-rift sequence which can be correlated with Lower Pleistocene carbonates, sands and clays recognisable on land, both in outcrop and by borehole data. The true frontal part of the thrust belt, as detected by the seismic lines, occupies the inner part of the area investigated and is buried by Upper Pliocene and Lower Pleistocene sediments. However, the compressive deformation seems to propagate toward the south-southeast by means of growing detachment levels developing at depth within Pleistocene marine clays, for a length of about 10 km, ahead of the present-day thrust front. Offshore, the faults trending NE–SW are dissected towards the east by faults trending NNW–SSE, subparallel to the Malta Escarpment, which flank the edge of the submerged Messina Rise. These faults, originating in a steep scarp which drops eastwards to the deep Ionian basin, have triggered submarine slides and affected the present-day seafloor sediments. As shown by seismic lines and as stressed by the modern seismicity of the area, some of the faults along the Malta Escarpment could be still active.

Extensional collapse related to compressional uplift in the alpine chain off northern Tunisia (Central Mediterranean)

Abstract Geophysical and geological marine data recently collected allow to outline the structure of the basement-involved fold-thrust belt, developed during the Late Cenozoic between Sardinia and Tunisia, along the Africa-Europe plate boundary. By integrating these with inland data, it is possible to document, step by step, the progression of crustal thickening from north to south, and the collapse of the first uplifted northern units, while collision was still going on. The geodynamic setting suggests that coupled extensional collapse and fold-thrust propagation were driven by island arc drifting above a southward-retreating subduction zone.

Neogene evolution of the southwestern Tyrrhenian Sea (Sardinia Basin and western Bathyal plain)

Abstract The reprocessing of MCS reflection lines and sampling data across the SW Tyrrhenian Basin, from the Sardinia–Tunisia Strait (STS) to the Orosei Canyon Line (OCL), show that the acoustic basement of the area is made by Variscan crystalline-metamorphic rocks covered by a thick sequence of sediments, including the Messinian interval. The pre-Messinian sequence can be subdivided into three seismic intervals or subunits, largely calibrated by sampling data. The oldest pre-Messinian subunit is strongly deformed by compressional and/or transpressional structures and the recovered samples are deep-water arkosic sediments, Aquitanian in age; however, the whole interval may encompass a wider time span, probably from Late Oligocene to Langhian. This subunit was laid down in a fore-arc basin located between a volcanic arc (Sardinia) and an accretionary wedge (Calabrian–Peloritanian–Kabilide or CK units) prior to the collision of the latter elements with the continental margin of Africa in Sicily and Tunisia along the so-called Drepano Thrust Front (DTF). Coeval to collision, the generation of the Main Sardinia Thrust (MST) occurred, carrying the Sardinia basement rocks over the CK units, and deforming the fore-arc basin sediments. The intermediate pre-Messinian sedimentary subunit spans from part of Serravallian to part of Tortonian. It is post-tectonic with respect to the underlying subunit, but it represents a pre-rift succession, if related to the overlying one. It consists of terrigenous deposits prograding from Sardinia to the ENE into a basin some hundreds of meters deep, which was left in the area after collision. The youngest pre-evaporitic subunit (late Tortonian–early Messinian in age) displays wedge-shaped reflectors in the Sardinia Basin, and forms the lower portion of a syn-rift complex including the Messinian and part of the Pliocene sediments. This succession records the onset of extensional tectonics in the area, as also testified by the Tortonian age of exumation of the basement. While in the Tyrrhenian Basin, north of the OCL, intra-Tortonian to intra-Pliocene rifting was strong but discontinuous and migrating with time to the east, in the SW Tyrrhenian Basin, in contrast, rifting was less severe but continuous during the same time interval. This emphasizes the role played by the OCL, a inherited Mesozoic discontinuity reactivated as a transfer zone during the Neogene. These new data allow correlation of the main Miocene deformational events observed in the study area to those of the adjacent emerged areas, namely the Peloritani and Maghrebian units in Sicily, prior to the Pliocene–Quaternary full development of the southern Tyrrhenian Sea.

The Eastern Ross Sea continental shelf during the Cenozoic: implications for the West Antarctic ice sheet development

Abstract The present-day bathymetric profile in the Ross Sea, as in other regions around the Antarctic margin, is deepening landward and shows unusually high water-depths: up to 1000 m in the inner shelf. These two features are the product of multiple ice sheet advances and retreats on the continental shelf. In this paper, we present a reconstruction of paleo-bathymetric profiles of the Eastern Ross Sea throughout the Cenozoic. The evolution of the sea-floor morphology from shallow and seaward dipping to the present-day configuration gives new insights into the understanding of the West Antarctic Ice Sheet (WAIS) history in this sector. Paleo-bathymetric profiles have been calculated by applying a reverse post-rift modelling, starting from a cross-section derived from multichannel seismic data. The post-rift reverse modelling includes: sediment decompaction, isostatic compensation after removing and recovering sediments of the post-rift thermal subsidence. The major uncertainty in our model is due to the paucity of stratigraphic constraints for the late Miocene and Pliocene sequences that prevents precise values of paleowater-depth being estimated. Nevertheless, major changes in the shape of the continental shelf and slope throughout the Cenozoic can be recognised, and mark some critical steps in the Ross Sea evolution. (1) Pre-Miocene: the Eastern Ross Sea was a deep structural basin bordered to the west by areas (e.g., the Central High) outcropping the sea level and hosting valley glaciers or small ice caps. A continental shelf edge was not clearly developed yet, the eastern flank of the Central High appeared as an inclined ramp, dipping towards the ocean. (2) Early to middle Miocene: tectonic subsidence gradually produced a marine flooding over most of the pre-Miocene sub-aerial areas. A continental shelf, slope and rise are gradually delineated. The shelf profile was seaward dipping and not yet overdeepened. The geometry of the depositional sequences is mainly determined by eustacy, tectonic and sediment supply. (3) Starting from Late Miocene (likely from 10 Ma to at least 4 Ma) the bathymetric profile evolved progressively from seaward to landward dipping and reached an overdeepened configuration, very similar to the present-day profile. Depositional and erosional processes over the continental shelf were largely controlled by ice streams. Outcropping of large parts of the continental shelf during the early Cenozoic has important implications on the volume of the West Antarctic Ice Sheet. At that time, the WAIS contribution to the eustatic fluctuations was most likely much larger than today.

Active fault kinematics and crustal stresses along the Ionian margin of southeastern Sicily

Since the late Cretaceous onset of plate convergence between Africa and Europe, the Malta Escarpment has been converted from a Mesozoic passive margin into a mega-hinge fault system with an additional sinistral strike-slip component. The modern tectonic stress regime with NW–SE-directed maximum horizontal stresses has been established since Late Messinian times. Since the Pleistocene, sinistral strike-slip deformation and contemporaneous normal faulting along the Malta Escarpment have induced the opening of oblique trending onshore grabens at the eastern margin of the Hyblean Plateau. In this study, we focus on the kinematics, the controlling state of stress, and the temporal variation of the neotectonic to active strike-slip and normal fault structures. The stress-tensor calculations reveals that the widespread map-scaled to meso-scaled normal fault structures are governed by the long-term extensional state of stress during the Quaternary. This long-term stress tensor is predominantly controlled by gravitational induced stresses due to vertical load (σ1=SV) and lateral extension due to the topographic gradient of the Malta Escarpment (σ3=Sh=NE–SW). In this case, the average tectonic stresses (σ2=SH=NW–SE) transmitted by the regional to plate-tectonic stress field are significantly smaller than the gravitational induced stresses. In contrast, the clear localization of conjugate sets of meso-scaled strike-slip fault structures and shear zones without accompanying normal fault structures give strong indications for episodic seismotectonic strike-slip faulting under critical stress conditions. In this state, tectonically induced maximum horizontal stresses are successively increased by ongoing plate convergence from low-level stress magnitudes (σ1=SV, σ2=SH=NW–SE) up to critical stress magnitudes (σ1=SH=NW–SE, σ2=SV), which are significantly larger than gravitational stresses. At the critical state, seismotectonic stress release occurs by active strike-slip faulting, as recently indicated in mid-crustal levels by the moderate 1990 Augusta earthquake, and re-establishes the long-term extensional state of stress. Because the strike-slip faults and shear zones in the study area were formed as surface ruptures, they additionally give indications of episodic large earthquakes in this region with magnitudes greater than 6.0, as reported by several large earthquakes devastating E and SE Sicily in historical times.

Structural variability at the active continental marginoff southernmost Chile

Abstract Newly collected seismic reflection data offshore Southernmost Chile, between 54°Sand 57°S, reveal the complex tectonic setting of this active continental margin. This region is stillpoorly known, because frequent bad weather conditions among these latitudes have prevented theacquisition of geological and geophysical data. Three main tectonic domains are clearly imaged south of the strait of Magellan: 1. (1) The oceanic area of the Antarctic plate, where a 2 km thick and largely undeformedsedimentary section, rests on the oceanic basalts; 2. (2) The subduction complex, formed by a relatively narrow accretionary wedge ofhighly deformed sediments and a thick forearc basin; 3. (3) The seaward dipping continental backstop formed partly by the PatagonianBatholith and partly by the Paleo-Mesozoic subduction complex. The accretionary complex is related to the subduction of the Antarctic plate beneath Scotiaplate which resumed after the collision between the Chile Trench and the Chile Ridge (10 14 Ma).The structural style of the subduction complex, such as structural vergence, width of theaccretionary wedge, taper angle and deformation in the forearc basin, varies along the margin.Large taper values are related to narrow wedges and seaward vergent structures. Low tapersoccur where deformation at the toe of the accretionary complex is spread over wide areas and isrelated both to landward and seaward vergent thrust faults. The parameters which control thesestructural variations are: 1. • thickness of the offscraped sedimentary section (variations in the depth of the decollement level); 2. • presence of overpressured fluids (the BSR is extraordinarily continuous where thelandward vergence structures are well developed); 3. • configuration of the continental margin after the consumption of the Chilean midoceanic ridge (14 Ma) and its related phase of tectonic erosion. 4. presence of strike slip faults belonging to the South America-Scotia plate boundarysystem which could bound crustal blocks with different mechanical behaviour (i.e. Strait ofMagellan-Lago Fagnano andBeagle channel faults)

Premières données thermo-chronologiques sur les socles sarde et kabylo-péloritain submergés dans le canal de Sardaigne (Méditerranée occidentale)

Abstract Granite and gneiss sampled from the submarine fault scarps of the Sardinia Channel were dated using the apatite fission-track method. One sample provides an age of 22.8± 1.3 Ma, which is in the range of the cooling ages of the Calabrian-Peloritan basement, where cooling is due to erosion. Three other samples have ages around 10 Ma, probably due to a tectonic denudation during the Tortonian extension in the Sardinia Channel.

Antarctic/Scotia plate convergence off southernmost Chile

The southern tip of South America off Chile has suffered a long phase of ocean-continent convergence which has shaped the continental margin through different phases of accretion and tectonic erosion. The present accretionary wedge is a discontinuous geological record of plate convergence and records only part of the accretionary processes resumed after Chile ridge consumption (14 Ma). The structural style of the subduction complex, such as rates of sediment accretion and tectonic erosion, structural vergence, width of the accretionary wedge, taper angle and deformation in the forearc basin, varies along the margin. Large taper values are related to narrow wedges and seaward vergent structures. Low tapers occur where deformation at the toe of the accretionary complex is spread over wide areas and is related both to landward and seaward vergent thrust faults. Seismic data interpretation contributes to define more accurately frontal wedge morphology and geometry of subduction and suggests that different modes of accretion together with tectonic erosion may be active concurrently along the trench at different locations. In areas of subduction driven accretionary processes the majority of trench sediments are involved in accretionary processes and sediments are uplifted and piled up in the form of imbricate thrust sheets. In areas where the wedge is non-accretionary the continental margin shows steeper continental slopes associated with narrow accretionary wedges, more intense sediment disruption and very shallow decollement levels. Variation in structural style and in the geometry of the forearc region setting off Southernmost Chile, has been interpreted as related to the existence of different structural domains: the nature of their boundaries is still unclear mainly for the lack of high resolution bathymetric data. They have been tentatively related to tectonic lineaments belonging to the Magellan Fault system and/or to the character and morphology of the converging plates (lateral heterogeneities, sea-mounts and fracture zones), which produce a segmentation of the margin.