Flora Gallais

David Graindorge

Marc-André Gutscher
email: gutscher@univ-brest.fr

Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)

Keywords: tear fault, rollback, accretion, multi-channel seismic reflection

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 the 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°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 through the crustal level. 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. Previously published Moho depth isocontours map, offshore Sicily and the recent GPS data, combined with the presence of strike slip movements NE onshore Sicily, allow us to identify that 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 the rollback of the Ionian slab and advance of Calabria towards SE.

Introduction

Since 80 Ma, the Mediterranean region has been affected by the very slow convergence of Africa towards stable Eurasia (1-2 cm/yr) [START_REF] Dercourt | Geological evolution of the Tethys belt from the Atlantic to the Pamirs since the Lias[END_REF][START_REF] Dewey | Kinematics of the western Mediterranean[END_REF]. During the last 30 Ma, the Western and Central Mediterranean were shaped by the trench rollback of the African subducting slab [START_REF] Rehault | The western Mediterranean Basin geological evolution[END_REF][START_REF] Malinverno | Extension in the Tyrrhenian Sea and shortening in the Apennines as result of arc migration driven by sinking of the lithosphere[END_REF]Faccenna et al., 2001a;[START_REF] Faccenna | Lateral slab deformation and the origin of the western Mediterranean arcs[END_REF]. This leads to the formation of the highly arcuate orogenic belts surrounding by young oceanic basins [START_REF] Faccenna | Lateral slab deformation and the origin of the western Mediterranean arcs[END_REF]. In the Central Mediterranean, the Ionian portion of the segmented African slab and the arched belt of Calabria are known to attain their current positions mostly between the late Miocene -Pleistocene, as a consequence of the slab rollback (Faccenna et al., 2001b;[START_REF] Rosenbaum | Neogene and Quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides[END_REF][START_REF] Rosenbaum | Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism[END_REF]. However, the lateral movements between the Calabrian 'retreating' block and its "fixed" Apulian and Hyblean-Sicilian edges, as well as the location of the exact limits of the intervening boundaries are all topics of heated debate, and thus require further investigation. The aim of this paper is to study the deep to superficial morphological expression(s) of these lateral movements, in particular offshore eastern Sicily.

To address this problematic, we present newly processed seismic lines from the Archimede survey (1997, R/V Le Nadir). The results obtained are combined with the onland studies and other geophysical data (e.g. wide-angle seismic, tomographic images) to propose a structural interpretation of the superficial to deep features present onshore NE Sicily and offshore Sicily.

2. Kinematics of vertical slab tearing: propagation of a STEP fault (Subduction Tear Edge Propagator) When subduction rollback occurs, it can be associated with a segmentation of the subducting slab [START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF][START_REF] Rosenbaum | Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism[END_REF]. Then one or more sub-vertical, lithospheric-scale tear faults can develop between the adjacent portions of the subducting plate (Figure 1) [START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF]. Such lithospheric tear faults and the trace they leave in the upper plate were defined as STEP (Subduction Tear Edge Propagator) faults [START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF]. In general, STEP faults can be identified by two characteristic types of deformation:

(1) strike-slip motion in the upper plate(s),

(2) and vertical motion between the more steeply subducting (oceanic) segment of the downgoing plate and the adjacent portion of the plate (Figure 1). STEP faults can develop either in purely oceanic domains, as along both boundaries of the South Sandwich trench or along the northern boundary of the Tonga subduction zone [START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF][START_REF] Baes | Subduction initiation along the inherited weakness zone at the edge of a slab: Insights from numerical models[END_REF]. However, they commonly develop along the Continent-Ocean Boundaries (COB) of the same lithospheric plate, as in the South America -Caribbean plate boundary example (Clark et al., 2008a).

Such large-scale lithospheric faults can often form strike-slip plate boundaries on the upper plate(s) allowing differential motion of two distinct blocks: one advances, in response to the rollback of the subducting slab, while the adjacent plate remains relatively stationary (Figure 1). The existence of lithospheric tear faults had been proposed to explain the opening of backarc basins, caused by slab rollback, and the associated motion of independent continental blocks in the Mediterranean region, in particular the Tyrrhenian Sea -Calabria -Ionian Sea subduction system (Gvirtzman and Nur, 1999b;[START_REF] Rosenbaum | Neogene and Quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides[END_REF].

Geological setting of the Calabrian subduction zone

The Calabrian subducting slab is marked by a narrow 250 km wide and 500 km deep Wadati--Benioff zone, dipping at about 70° toward the northwest [START_REF] Giardini | The deep seismicity of the Tyrrhenian Sea[END_REF][START_REF] Selvaggi | Seismicity and P-wave velocity image of the Southern Tyrrhenian subduction zone[END_REF][START_REF] Chiarabba | A new view of Italian seismicity using 20 years of instrumental recording[END_REF]. Since the Oligocene, the Calabrian trench has rolled back towards the E-SE by more than 800 km at an average rate of 2-3 cm/yr, triggered by the lateral tearing of the Calabrian slab [START_REF] Malinverno | Extension in the Tyrrhenian Sea and shortening in the Apennines as result of arc migration driven by sinking of the lithosphere[END_REF]Faccenna et al., 2001a;[START_REF] Faccenna | Lateral slab deformation and the origin of the western Mediterranean arcs[END_REF]. This leads to the opening of the Liguro--Provençal basin between 30-16 Ma and after a period of tectonic quiescence, of the Tyrrhenian basin from 12-10 Ma to the present ( (Faccenna et al., 2001a;[START_REF] Faccenna | Lateral slab deformation and the origin of the western Mediterranean arcs[END_REF] and references cited in these papers). Spreading in the Tyrrhenian back-arc basin occurred during two distinct episodes: first within the Vavilov basin (4.3-2.6 Ma) [START_REF] Kastens | The geological evolution of the Tyrrhenian Sea: An introduction to the scientific results of ODP Leg 107[END_REF] and second within the Marsili basin ( 2-1 Ma) (Figure 2) [START_REF] Nicolosi | Ultrafast oceanic spreading of the Marsili Basin, southern Tyrrhenian Sea: Evidence from magnetic anomaly analysis[END_REF][START_REF] Guillaume | Spreading pulse of the Tyrrhenian Sea during the narrowing of the Calabrian slab[END_REF].

The Marsili basin opened at a rapid rate, probably during the fast 15°-25° clockwise rotation of the Calabrian Arc (Figure 2) [START_REF] Speranza | Timing and magnitude of rotations in the frontal thrust systems of southwestern Sicily[END_REF][START_REF] Speranza | Pattern of orogenic rotations in centraleastern Sicily: implications for the timing of spreading in the Tyrrhenian Sea[END_REF][START_REF] Cifelli | Tectonic evolution of arcuate mountain belts on top of a retreating subduction slab: The example of the Calabrian Arc[END_REF][START_REF] Cifelli | Calabrian Arc oroclinal bending: The role of subduction[END_REF].

At depth, the subduction history of the Calabrian subduction zone is indicated by vertical tomographic imaging of the African -Ionian Tethyan age oceanic lithosphere subducting beneath the Southern Tyrrhenian Sea [START_REF] Lucente | Tomographic constraints on the geodynamic evolution of the Italian region[END_REF][START_REF] Wortel | Subduction and Slab Detachment in the Mediterranean-Carpathian Region[END_REF][START_REF] Piromallo | P wave tomography of the mantle under the Alpine-Mediterranean area[END_REF]. The horizontal seismic tomography images reveal two slab windows in southern Italy: one beneath the southern Apennines (~150 km width) and the other one beneath north-eastern Sicily [START_REF] Lucente | Tomographic constraints on the geodynamic evolution of the Italian region[END_REF][START_REF] Wortel | Subduction and Slab Detachment in the Mediterranean-Carpathian Region[END_REF][START_REF] Piromallo | P wave tomography of the mantle under the Alpine-Mediterranean area[END_REF][START_REF] Chiarabba | The southern Tyrrhenian subduction zone: Deep geometry, magmatism and Plio-Pleistocene evolution[END_REF][START_REF] Neri | Subduction Beneath Southern Italy Close the Ending: Results from Seismic Tomography[END_REF]. These asthenospheric windows were associated with the occurrence of vertical lithospheric tears (STEP fault) [START_REF] Guarnieri | Plio-Quaternary segmentation of the south Tyrrhenian forearc basin[END_REF][START_REF] Rosenbaum | Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism[END_REF].

Nowadays, in the southern Tyrrhenian region, the slow rate Africa/Eurasia convergence leads the formation of an active compressional tectonic ridge, located offshore North of Sicily [START_REF] Billi | Seismotectonics of the Nubia plate compressive margin in the south Tyrrhenian region, Italy: Clues for subduction inception[END_REF][START_REF] Billi | Recent tectonic reorganization of the Nubia-Eurasia convergent boundary heading for the closure of the western Mediterranean[END_REF]. Onshore NE of Sicily, a set of geological features suggest the presence at depth of a STEP type lithospheric tear-fault. The dextral Taormina Line [START_REF] Amodio-Morelli | L'arco Calabro-Peloritano nell'orogene Appenninico-Maghrebide[END_REF][START_REF] Rosenbaum | Neogene and Quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides[END_REF][START_REF] Rosenbaum | Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism[END_REF] and the Tindari-Lipari (TL) line [START_REF] Guarnieri | Structural evidence for deformation by block rotation in the context of transpressive tectonics, northwestern Sicily (Italy)[END_REF][START_REF] Billi | Tectonics and seismicity of the Tindari Fault System, southern Italy: Crustal deformation at the transition between ongoing contractional and extensional domains located above the edge of a subducting slab[END_REF][START_REF] Guarnieri | Plio-Quaternary segmentation of the south Tyrrhenian forearc basin[END_REF][START_REF] Bortoluzzi | Interactions between volcanism and tectonics in the western Aeolian sector, southern Tyrrhenian Sea[END_REF] have been proposed to be the crustal expression in upper plate of a STEP fault (Figure 2), that accommodate the remaining active portion of the slab beneath Calabria [START_REF] Billi | Tectonics and seismicity of the Tindari Fault System, southern Italy: Crustal deformation at the transition between ongoing contractional and extensional domains located above the edge of a subducting slab[END_REF]. In fact, recent GPS measurements show that a small amount of east-west extension still occurs in the Tyrrhenian basin, but only in its south-eastern most sector [START_REF] Serpelloni | Crustal velocity and strain-rate fields in Italy and surrounding regions: new results from the analysis of permanent and non-permanent GPS networks[END_REF][START_REF] Serpelloni | Kinematics of the Western Africa-Eurasia plate boundary from focal mechanisms and GPS data[END_REF]. This suggests that the SE-ward rollback of the Ionian slab is still active, but at very slow rates, as attested by the 3-4 mm/yr movement of the GPS stations in Calabria with respect to a fixed Hyblean-Malta Plateau (Figure 2) [START_REF] D'agostino | Forearc extension and slow rollback of the Calabrian Arc from GPS measurements[END_REF][START_REF] Devoti | Evidence of large scale deformation pattern from GPS in the Italian subduction boundary[END_REF][START_REF] Palano | GPS velocity and strain fields in Sicily and southern Calabria, Italy: Updated geodetic constraints on tectonic block interaction in the central Mediterranean[END_REF]. Furthermore, the formation of Mount Etna has been interpreted as a consequence of asthenospheric flow from Africa toward the Tyrrhenian, causing mantle flow through an asthenospheric window (Gvirtzman and Nur, 1999a;[START_REF] Faccenna | Topography of the Calabria subduction zone (southern Italy): Clues for the origin of Mt[END_REF]. Indeed, such flow was recently invoked to explain the uplift of the Calabrian region since 700 000 yr [START_REF] Faccenna | Topography of the Calabria subduction zone (southern Italy): Clues for the origin of Mt[END_REF].

Offshore Sicily, in the Ionian Sea, the evidence of vertical motion between the more steeply Ionian oceanic subducting segment of the African downgoing plate and the adjacent Sicilian continental portion of the African plate was also reported [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Hirn | Roots of Etna volcano in faults of great earthquakes[END_REF][START_REF] Argnani | The Southern Tyrrhenian subduction system: recent evolution and neotectonic implications[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF][START_REF] Argnani | Malta Escarpment fault zone offshore eastern Sicily: Plio-Quaternary tectonic evolution based on new multi-channel seismic data[END_REF][START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF][START_REF] Argnani | Evolution of the Tyrrhenian slab tear and active tectonics along the western edge of the Tyrrhenian subducted slab[END_REF][START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. Two distinct interpretations are still proposed concerning the position of the STEP fault beneath the Ionian Sea (Figure 2): first along the NNW-SSE oriented Malta escarpment [START_REF] Argnani | The Southern Tyrrhenian subduction system: recent evolution and neotectonic implications[END_REF][START_REF] Argnani | Malta Escarpment fault zone offshore eastern Sicily: Plio-Quaternary tectonic evolution based on new multi-channel seismic data[END_REF][START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF][START_REF] Argnani | Evolution of the Tyrrhenian slab tear and active tectonics along the western edge of the Tyrrhenian subducted slab[END_REF], second along a NW-SE crustal fault 50 km further to the east of this escarpment (Figure 2) [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Hirn | Roots of Etna volcano in faults of great earthquakes[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF][START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. Below, we offer new constraints that favour one of these interpretations and then discuss the geodynamic implications of the presence of a STEP fault (lithospheric tear fault) at the edge of the Ionian slab.

4. The 96-channel Archimede survey and processing of the data The Archimede survey was conducted during 9th to 2 4th April 1997 onboard R/V Le Nadir.

The objectives were to image the deep structure of the Calabrian accretionary wedge and adjacent region. Data were recorded by a 96-channel 2.4 km long streamer. The shot spacing of 50 meters and the 25 meters between each hydrophone provide a 24-fold coverage, with a Common Mid Point (CMP) spacing of 12.5 m. The seismic data were acquired with a 4 ms sampling rate (250 Hz). The source was an airgun array consisting of ten airguns for a total volume of 1220 cu.in, operating in a single-bubble mode [START_REF] Avedik | Single-bubble" marine source offers new perspectives for lithospheric exploration[END_REF] and placed at a depth of 20 meters. In contrast to tuned arrays, this kind of system exploits the strong energy contained in the first bubble pulse delaying the firing of guns as a function of their volume in order to synchronize the output of the guns with respect to this first pulse [START_REF] Avedik | Single-bubble" marine source offers new perspectives for lithospheric exploration[END_REF]. The single-bubble method generates a powerful low frequency signal (10 to 15 Hz), which allows a better penetration of waves to image the deep structures [START_REF] Avedik | Single-bubble" marine source offers new perspectives for lithospheric exploration[END_REF][START_REF] Bartolome | Using the OBS wide-angle reflection/refraction velocities to perform a pre-stack depth migration image of the "single bubble" multichannel seismic: example of the Moroccan margin[END_REF].

The post-stack time migration of the profile Archimede 02 (Figure 2) was performed with the GeoVecteur Software© developed by CGG (Compagnie Générale de Géophysique), particular attention was paid to improve the continuity of the deep reflectors. Table 1 lists the different processing steps which were applied to the Archimede 02 seismic sections discussed in this paper. A 6/8/55/65 pass-band and 49/50/51 notch filters, and a spherical divergence correction were applied after a spectral analysis of the small offset traces. Before stacking, a predictive deconvolution was applied to the signal with a typical window of 5 s TWT. This deconvolution attempts to predict and remove repetitions in the recorded seismograms, it also acts like an anti-multiple on many profiles [START_REF] Yilmaz | Seismic data processing[END_REF]. A velocity analysis based on a semblance computation was performed each 300 Common Mid Point (CMP) (equal to 3.75 km) to constrain the lateral velocity variations on the Archimede 02 seismic line. The velocity libraries obtained after interactive picking of these semblances were used to compute a Normal Move Out (NMO) correction. The NMO corrected CMP gather were stacked in order to increase the signal/noise ratio. After numerous tests of Kirchhoff migration using different constant and library of velocities, we finally conclude that the diffractions in particular due to the presence of salt are well corrected with a 2900 m/s constant velocity Kirchhoff migration [START_REF] Hubral | Time Migration-Some ray theorical aspects[END_REF] (see Table 1 for a summary of this processing sequence).

The Archimede 16 and 17 profiles (Figure 2) were processed using iterative Pre-Stack Depth Migration (PSDM) to obtain a depth section. In the pre-processing prior to the depth migration special attention was paid to the multiple removal (see summary of the processing sequence listed in Table 2). The PSDM is a top to bottom approach with several iterations. In the first iteration, only an approximate water velocity needs to be assumed. During the migration focusing panels and Common Image Point (CIP) gathers (migrated CMP-gathers) were calculated. By the analysis of the focusing panels and the residual move-out of the CIPgather the correct water velocity can be determined and an estimation of the velocity (interval) of the first sediment layer is added into the macro model. With this new model (velocity of water and first sediment layer) the data are migrated again and new focusing panels were calculated (2nd iteration). For both profiles, Archimede 16 and 17, six iterations were needed to build the complete macro model. The PSDM not only provides a correct image in depth for these two lines, but also yields an internal velocity model for the interpretation. Based on these two detailed and geologically meaningful images, we can more precisely determine the extent of the salt-bearing Calabrian accretionary wedge and the correct geometry of the structure, in particular of the STEP fault.

Results

East of the major lineament, it was only possible to recognize stratigraphic units locally, where there are no complex tectonic disruptions above, and thus the sub-horizontal layered reflectors could be imaged (Figures 4 and5). Based on facies comparison, the stratigraphic correlation was made following the interpretation given by [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF] (Figure 3a) and [START_REF] Gallais | A Miocene tectonic inversion in the Ionian Sea (Central Mediterranean): evidence from multi-channel seismic data[END_REF]. It allows identification of eight sedimentary layers deposited above the basement of the Ionian basin, that was interpreted as oceanic (Figure 3) [START_REF] De Voogd | Two-ship deep seismic soundings in the basins of the Eastern Mediterranean Sea (Pasiphae cruise)[END_REF]. The Plio-Quaternary sediments overlay a chaotic body, that was interpreted as a lower Pliocene olistostrome by [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. On our dataset, this chaotic unit is present on the Archimede 16 and 02 lines beneath the Ionian foreland (Figures 5 and6), its base is marked by the high amplitude A-reflector (Figure 3b) [START_REF] Finetti | Structure, stratigraphy and evolution of Central Mediterranean[END_REF]. Below, the Messinian sequence is composed of three sub-layers: the Upper Unit, the Mobile Unit and the Lower Unit [START_REF] Lofi | Seismic Atlas of The "Messinian Salinity Crisis" markers in the Mediterranean and Black Seas, Commission de la carte géologique du monde et société géologique de France[END_REF] (Figure 3b) [START_REF] Finetti | Structure, stratigraphy and evolution of Central Mediterranean[END_REF]. The underlying Tortonian sediments were deposited over the undifferentiated Tertiary sediments and the Mesozoic sediments (Figure 3b).

The major structure identified on the Archimede seismic lines, that were shot at the western boundary of the Calabrian accretionary wedge is a steeply E-dipping, crustal scale fault (Figures 4 and5). The pre-stack depth migrated E-W trending profile Archimede 17 (see position Figure 2) images this vertical crustal scale fault (sp 1300, Figure 4). The crustal scale fault sharply cuts the base of the Messinian evaporitic sediments (B-reflector) and the underlying group of pre-Messinian stratified and continuous reflectors, that is 2 to 2.5 km thick (Figure 4). The base of the Messinian evaporitic sediments is offset by 645 m along the crustal scale fault (base of Messinian Mobile Unit: 6700m -6977m) (Figure 4). A half-graben basin develops on top of this fault (Figure 4). It is characterized by a westward dipping base and filled by a ~ 645 m thick accumulation of syntectonic sediments (Figure 4). These syntectonic deposits were interpreted as Pleistocene in age [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Hirn | Roots of Etna volcano in faults of great earthquakes[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF].

The pre-stack depth migrated E-W trending profile Archimede 16, shot south of the profile Archimede 17, also images the vertical crustal scale fault (sp 1000, Figure 5). However, the more southerly position of this profile Archimede 16 allows us to image the lateral ramp of the Calabrian accretionary wedge, located at the foot of the Malta escarpment (sp 230, Figure 5). The lateral ramp of the prism is a transparent fault zone, that corresponds to the deformation front of the Calabrian post-Messinian accretionary wedge. It is marked by undulations of the seafloor (Figure 5). This fault zone is associated with the activity of thrust and backthrust (sp 230, Figure 5). Some authors have proposed that this zone accommodates strike-slip movements [START_REF] Minelli | Evolution of the Calabrian Accretionary wedge (Central Mediterranean)[END_REF], which is in agreement with the interpretation of a lateral ramp. This lateral ramp separates the undeformed foreland, at the foot of the Malta escarpment to the west, from the rugous frontal slope of the Calabrian accretionary wedge, to the east (Figure 5). Towards the east, the Calabrian accretionary wedge thickens progressively above the decollement from 1 km to 3 km 40 km up to a distance of 40 km from the lateral ramp (sp 230 to 1050, Figure 5). This supports the notion that here, the base of the Messinian Mobile Unit marks the main decollement of the wedge [START_REF] Ryan | Geological evidence concerning compressional tectonics in the eastern mediterranean[END_REF][START_REF] Chamot-Rooke | DOTMED -Deep Offshore Tectonics of the Mediterranean: A synthesis of deep marine data in eastern Mediterranean[END_REF][START_REF] Minelli | Evolution of the Calabrian Accretionary wedge (Central Mediterranean)[END_REF][START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. Further east, the crustal scale fault sharply cuts the decollement and the underlying stratified pre-evaporitic reflectors down to the top of the crust at ~ 9 km depth, as interpreted on other seismic lines (sp 1050, Figure 5) [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. The offset is 275 m (base of Messinian Mobile Unit: 6795m -7442m) (Figure 5) As northward, a half-graben basin, filled by Pleistocene deposits develops on top of this fault [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. Through correlation of this syntectonic half-graben basin with the ION-1 [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF], CROP-M3 [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF] and Archimede 17 profiles and a set of profiles acquired further towards the NE [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF] (see position on the Figure 3, inset map), this crustal fault could be followed over a distance of 200 km. This E-dipping crustal vertical scale defines a N°150 trending feature, that cuts through the western Ionian Sea and strongly affects the post-Calabrian accretionary wedge.

Numerous previous seismic studies proposed that the post-Messinian wedge is limited eastward by this vertical scale fault [START_REF] Doglioni | Why Mt Etna?[END_REF][START_REF] Chamot-Rooke | DOTMED -Deep Offshore Tectonics of the Mediterranean: A synthesis of deep marine data in eastern Mediterranean[END_REF][START_REF] Minelli | Evolution of the Calabrian Accretionary wedge (Central Mediterranean)[END_REF]. However, very recently other suggested that the contact between the post-and pre-Messinian wedges occurs further east [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. Based on our seismic images and correlations with very recent results [START_REF] Gallais | Two-stage growth of the Calabrian accretionary wedge in the Ionian Sea (Central Mediterranean): Constraints from depth migrated multi-channel seismic data[END_REF], we also propose that the boundary between the post-and pre-Messinian wedge occurs 40 km east of the fault, along a W-dipping backthrust of the Calabrian accretionary wedge (sp 750, Figure 4 and sp 1700, Figure 5). On the line Archimede 17 (Figure 4), this interpretation is supported by the downstepping of the decollement observed below the backthrust. The decollement cuts through deeper sediments to reach 8 km depth, a feature also observed on the Archimede line 18 (see position inset, Figure 3) [START_REF] Gallais | Two-stage growth of the Calabrian accretionary wedge in the Ionian Sea (Central Mediterranean): Constraints from depth migrated multi-channel seismic data[END_REF]. On the Archimede 16 profile (Figure 5), the W-ward dipping major backthrust that delineates the boundary between the post-and pre-Messinian wedges is still present. It is still associated with a downstepping of the decollement from 6.5 to 8 km depth (Figure 5). On this profile, the velocities within the post-Messinian wedge clearly decreases abruptly on both sides of the E-dipping crustal scale fault. This might be linked with the activity of the fault that was active during the emplacement of the Calabrian accretionary wedge and may have facilitate the formation of a mélange body on this E-facing side. East of the backthrust, the pre-Messinian wedge is highly deformed quasi transparent body, marred by disrupted reflectors (Figures 4 and5).

On the time migrated SW-NE trending profile Archimede 02 (Figure 6), shot south of the profile Archimede 16, no large vertical throw is observed (Figure 6A). However, this profile allows us to better describe the architecture of the Calabrian accretionary wedge. Below the foreland of the wedge, at the foot of the Malta escarpment, a ~ 10 km broad incipient deformation zone develops (Figure 6B). Here, the base of the Plio-Quaternary, including the chaotic body (A-reflector) (Figure 3b) becomes gently folded approaching the lateral ramp of the Calabrian accretionary wedge (sp 2100, Figure 6). This corresponds to the proto-thrust domain of the Calabrian accretionary wedge (Figure 6B). Directly NE of the proto-thrust domain, the lateral ramp of the wedge is marked by a change from the flat foreland to the rugous frontal slope of the Calabrian accretionary wedge (Figure 6). Below the frontal slope, some SE verging thrusts are imaged within the Plio-Quaternary and the Messinian sediments (Figure 6). The post-evaporitic sediments (Messinian Upper and Mobile Units) are tectonically thickened by activity of these thrusts above the decollement (Figure 6). The decollement at the base of the Messinian evaporites dips gently towards the NE (Figure 6).

Below this post-Messinian Calabrian wedge, the pre-evaporitic sediments correspond to a stratified and highly reflective group of continuous reflectors, that are 2 s TWT thick (Figure 6). We interpret the top of the Ionian oceanic crust to be at the base of this reflective sequence at 8 s TWT (Figure 6), in agreement with seismic reflection profiles and OBS data [START_REF] Makris | A seismic study in the Western Ionian Sea[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF][START_REF] Catalano | On the Mesozoic Ionian Basin[END_REF]. Here, no expression of a crustal scale fault and a major backthrust are still visible. This analysis reveals that the post-Messinian Calabrian wedge is cut by a E-dipping vertical and crustal scale fault, that offsets the base of the Messinian evaporitic sediments and the underlying pre-Messinian to Mesozoic sediments. The vertical throw along this N150° trending fault decreases towards the south (Figures 4 and5). Within the domain comprised in between the lines Archimede 16 and 02, the crustal faults terminates. Following these interpretations, we will now discuss the tectonic significance of this crustal scale fault, which slices through the entire post-Messinian Calabrian prism.

Discussion

The discovery of a N°150 trending crustal scale fault is not the novelty of this paper, since it was already mapped and described in previous studies (Figure 7) [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Hirn | Roots of Etna volcano in faults of great earthquakes[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF][START_REF] Chamot-Rooke | DOTMED -Deep Offshore Tectonics of the Mediterranean: A synthesis of deep marine data in eastern Mediterranean[END_REF][START_REF] Minelli | Evolution of the Calabrian Accretionary wedge (Central Mediterranean)[END_REF][START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. This work offers detailed structural constraints of the movement along this fault as well as a new hypothesis regarding its origin. Some authors have proposed that its vertical geometry could be inherited from a former strike-slip fault [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. They further suggested that this fault was later reactivated as a normal fault, with down-to-the-east movement during the Pleistocene, as attested by the presence of a subsiding half-graben basin (Figure 7) [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. Other authors have recently proposed that this crustal scale fault could correspond to a STEP fault [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF].These interpretations will now be discussed based on the recent data acquired in the Central Mediterranean and on the concept of a STEP fault [START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF] will be investigated.

Numerous structural studies and some focal mechanisms in NE Sicily (Figure 7) showed that dextral strike slip movement occurs between the Sicily -Hyblean block and the Calabrian -Peloritan (NE Sicily) block (Figure 7) [START_REF] Guarnieri | Structural evidence for deformation by block rotation in the context of transpressive tectonics, northwestern Sicily (Italy)[END_REF][START_REF] Guarnieri | Plio-Quaternary segmentation of the south Tyrrhenian forearc basin[END_REF]. This is consistent with the very recent GPS measurements, that show that Calabria and the region NE Sicily still advances at significant rate of ~ 2-3 mm/yr towards the SE, in response to the SE-ward rollback of the Ionian slab (Figure 2) [START_REF] D'agostino | Forearc extension and slow rollback of the Calabrian Arc from GPS measurements[END_REF][START_REF] Devoti | Evidence of large scale deformation pattern from GPS in the Italian subduction boundary[END_REF][START_REF] Palano | GPS velocity and strain fields in Sicily and southern Calabria, Italy: Updated geodetic constraints on tectonic block interaction in the central Mediterranean[END_REF]. Moreover, the tomographic images reveal a negative velocity anomaly at the SW edge of the subducting Ionian lithosphere, which was interpreted as lithospheric tear fault [START_REF] Lucente | Tomographic constraints on the geodynamic evolution of the Italian region[END_REF][START_REF] Wortel | Subduction and Slab Detachment in the Mediterranean-Carpathian Region[END_REF][START_REF] Piromallo | P wave tomography of the mantle under the Alpine-Mediterranean area[END_REF][START_REF] Chiarabba | The southern Tyrrhenian subduction zone: Deep geometry, magmatism and Plio-Pleistocene evolution[END_REF][START_REF] Neri | Subduction Beneath Southern Italy Close the Ending: Results from Seismic Tomography[END_REF]. Recent SKS splitting results reveal toroidal mantle flow beneath NE Sicily [START_REF] Civello | Toroidal mantle flow around the Calabrian slab (Italy) from SKS splitting[END_REF][START_REF] Baccheschi | Seismic anisotropy reveals focused mantle flow around the Calabrian slab (Southern Italy)[END_REF][START_REF] Baccheschi | Anisotropy patterns in the subducting lithosphere and in the mantle wedge: A case study-The southern Italy subduction system[END_REF], and confirm the presence of an asthenospheric window in this region. The uplift and faulting in Calabria [START_REF] Westaway | Quaternary uplift of Southern Italy[END_REF] were recently proposed to be sustained dynamically by toroidal mantle flow [START_REF] Gvirtzman | Residual topography, lithospheric structure and sunken slabs in the central Mediterranean[END_REF][START_REF] Faccenna | Topography of the Calabria subduction zone (southern Italy): Clues for the origin of Mt[END_REF]. This process was associated with the retreat of the narrow Ionian slab and proposed to provide favorable conditions for decompression melting, leading to the Mt Etna volcanism (0.5 Ma to the present) (Gvirtzman and Nur, 1999a;[START_REF] Schellart | Mount Etna-Iblean volcanism caused by rollback-induced upper mantle upwelling around the Ionian slab edge: An alternative to the plume model[END_REF]. This field of results indicates that beneath the Ionian Sea, a lithospheric vertical tear must be present between the retreating Ionian oceanic lithosphere and the stable continental Sicily -Hyblean plateau to allow the rollback to continue.

In the Ionian Sea, the position of the N150° trending fault, identified in this study correlates with a large vertical offset of the Moho (Figure 7) [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. Through correlation with other geophysical data, we propose that this N°150 trending vertical fault is the surface expression of a vertical lithospheric tear, a STEP fault, that occurs at greater depth. In this framework, we further suggest that the motion along this fault may be a purely vertical movement. Thus, the Pleistocene syntectonic basin emplaced in response to this movement between the more steeply subducting Ionian slab and the adjacent "Sicilian" portion of the plate, without any normal component as previously proposed [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. This leads us to propose that the south-western boundary of the Tyrrhenian -Calabrian -Ionian subduction system is a good example of a STEP fault, with: This lithospheric tear is consistent with the presence of an asthenospheric window, beneath NE Sicily and the SW lateral edge of the Ionian slab [START_REF] Lucente | Tomographic constraints on the geodynamic evolution of the Italian region[END_REF][START_REF] Wortel | Subduction and Slab Detachment in the Mediterranean-Carpathian Region[END_REF][START_REF] Piromallo | P wave tomography of the mantle under the Alpine-Mediterranean area[END_REF][START_REF] Chiarabba | The southern Tyrrhenian subduction zone: Deep geometry, magmatism and Plio-Pleistocene evolution[END_REF][START_REF] Neri | Subduction Beneath Southern Italy Close the Ending: Results from Seismic Tomography[END_REF].

Toroidal mantle flow, due to this windows [START_REF] Civello | Toroidal mantle flow around the Calabrian slab (Italy) from SKS splitting[END_REF][START_REF] Baccheschi | Seismic anisotropy reveals focused mantle flow around the Calabrian slab (Southern Italy)[END_REF][START_REF] Baccheschi | Anisotropy patterns in the subducting lithosphere and in the mantle wedge: A case study-The southern Italy subduction system[END_REF] successfully explains the position of the Mt Etna and its geochemical signature (oceanic basalt affinity) (Gvirtzman and Nur, 1999a;[START_REF] Schellart | Mount Etna-Iblean volcanism caused by rollback-induced upper mantle upwelling around the Ionian slab edge: An alternative to the plume model[END_REF].

This study also reveals that in the Ionian Basin, the STEP fault is clearly distinct from the Malta escarpment (Figure 8), as previously proposed [START_REF] Argnani | The Southern Tyrrhenian subduction system: recent evolution and neotectonic implications[END_REF][START_REF] Argnani | Malta Escarpment fault zone offshore eastern Sicily: Plio-Quaternary tectonic evolution based on new multi-channel seismic data[END_REF][START_REF] Govers | Lithosphere tearing at STEP faults: Reponse to edges of subduction zones[END_REF][START_REF] Argnani | Evolution of the Tyrrhenian slab tear and active tectonics along the western edge of the Tyrrhenian subducted slab[END_REF]. The N150° trending lithospheric STEP fault is located ~ 50 km further east, at the latitude of Syracuse (Figure 7) [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF][START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF]. It was proposed that the COB (Continent-Ocean Boundary) is marked by this N°150 trending vertical fault, that propagates directly east of the sigmoidal Alfeo seamount (Figure 7). In fact, the rocks exposed along the Malta escarpment, which forms the continental slope of Sicily are of continental type and these continental fragments extend as far eastward as the Alfeo seamount (Figure 7) [START_REF] Scandone | Mesozoic and Cenozoic rocks from the Malta Escarpment (Central Mediterranean)[END_REF]. The multichannel seismic line ETNA5, which was recorded at the station VEN in the Hyblean plateau (see position on Figure 7), confirms that the COB (Continent-Ocean Boundary) across this margin occurs within a domain, lying in between the Malta escarpment and a vertical structure, located 30 km further to the east (Figure 8) [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF][START_REF] Chamot-Rooke | DOTMED -Deep Offshore Tectonics of the Mediterranean: A synthesis of deep marine data in eastern Mediterranean[END_REF]. Following the ETNA-5 line, it seems that the position of the N°150 trending fault could correspond to the abrupt transition between "thinned continental crust" and "oceanic crust" (see at distance 58 km on Figure 4 in [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. Thus, this suggests that the fault may have propagated preferentially at the transition "thinned continental crust/oceanic crust" rather than at the transition "continental crust/thinned continental crust", lying at depth beneath the Malta escarpment (Figure 8) [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. Unfortunately, the quality of the OBS data [START_REF] Makris | A seismic study in the Western Ionian Sea[END_REF] in this sector and the profile ETNA5, do not offer the sufficiently detailed evidence to confirm this hypothesis. At this stage, it seems that the STEP fault propagates through the inherited weakness zone, that corresponds to the Mesozoic margin of the Ionian Basin, East of Sicily (Frizon de [START_REF] Frizon De Lamotte | The southernmost margin of the Tethys realm during the Mesozoic and Cenozoic: Initial geometry and timing of the inversion processes[END_REF][START_REF] Gallais | A Miocene tectonic inversion in the Ionian Sea (Central Mediterranean): evidence from multi-channel seismic data[END_REF], as in South America -Caribbean (Clark et al., 2008b).

Concluding remarks

The Archimede multichannel seismic lines acquired on the western corner of the post-Messinian Calabrian accretionary wedge allow us to better describe its architecture, that interacts with a crustal scale fault. We show that the lateral ramp of the wedge separates an undeformed foreland at the foot of the Malta escarpment, from the deformed Calabrian wedge. The prism progressively grows by accretion above the decollement, lying at the base of the Messinian evaporites. Further to the east, the wedge is dissected by a crustal scale and vertical fault, that offsets the crustal unit and the Mesozoic to the pre-evaporitic sediments. from Geosciences Marine from Ifremer (Brest) onboard "Pourquoi Pas?" that give her support and a good atmosphere to finish the first revision (in particular Romain Pellen, Caesar Rigoti, Yohann Kergoat, the 0-4 team and Philippe Schnürle). Group Bathymetry compilation [START_REF] Loubrieu | Morpho-Bathymetry of the Mediterranean Sea[END_REF]. Black lines show major fault systems, modified from the Geodynamic Map of the Mediterranean http://ccgm.free.fr. GPS vectors with respect to a fixed Hyblean-Malta block, as defined by the sites shown in orange [START_REF] Devoti | Evidence of large scale deformation pattern from GPS in the Italian subduction boundary[END_REF]. Thick curved blue lines: the SE movement of the Ionian portion of the segmented African slab according to [START_REF] Faccenna | Topography of the Calabria subduction zone (southern Italy): Clues for the origin of Mt[END_REF]. Green circle arrows: rotations observed in southern Apennines and Sicily [START_REF] Speranza | Timing and magnitude of rotations in the frontal thrust systems of southwestern Sicily[END_REF][START_REF] Speranza | Pattern of orogenic rotations in centraleastern Sicily: implications for the timing of spreading in the Tyrrhenian Sea[END_REF][START_REF] Mattei | The evolution of the Calabrian Arc: Evidence from paleomagnetic and GPS observations[END_REF]. MB: Marsili Basin; TL Line: Tindari-Lipari Line; Hy: Hyblean plateau;

Pe: Peloritan mountains. M3 in [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF], , ION-1 in [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF], Arc-18 and Arc-19 [START_REF] Gallais | Two-stage growth of the Calabrian accretionary wedge in the Ionian Sea (Central Mediterranean): Constraints from depth migrated multi-channel seismic data[END_REF]). (http://www.bo.ingv.it/RCMT/Italydataset.html) [START_REF] Pondrelli | The Italian CMT dataset from 1977 to the present[END_REF]. In red focal mechanism indicate strike slip movement NE Sicily. Structural interpretations onshore Sicily are taken from [START_REF] Guarnieri | Plio-Quaternary segmentation of the south Tyrrhenian forearc basin[END_REF]. Thin black contours: isobaths in kilometers of the Moho [START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF]. et al., 1986). The detailed shallow crustal structure (sedimentary basins, basement) is imaged by four multichannel seismic reflection profiles; two published previously ION-10 and ION-1 in time [START_REF] Cernobori | Crustal image of the Ionian basin and its Calabrian margins[END_REF][START_REF] Nicolich | Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution[END_REF] and CROP-M3 in depth [START_REF] Polonia | The Calabrian arc subduction complex in the Ionian Sea: regional architecture, active deformation and seismic hazard[END_REF] and the two depth migrated profiles Arc-16 and Arc-17 shown here. See position of these data on the Figure 7. CAW: Calabrian Accretionary Wedge.

-

  strike-slip movement onshore NE Sicily, that accommodates in the upper plate the advance towards the SE of the Peloritan block attached to Calabria, relative to the stable Sicily -Hyblean plateau, -lithospheric vertical movement beneath the Ionian Basin, that is transmitted at a crustal scale along the N150° trending vertical fault.

  Activity of this faults leads to the formation, of a Pleistocene syntectonic basin. Through correlations with other seismic studies, we show that a N°150 trending fault cuts more than 200 km into the Ionian Basin and that its vertical throw decreases towards the south. The southernmost termination of the N150° oriented fault can be followed as far south as the latitude 36.4°N, but no further. A set of data obtained from geodetic, tomographic, geochemical, structural and seismic studies allows us to propose that the vertical movement observed along this crustal scale fault is the surface expression of a STEP fault. This STEP fault accompanies both (a) the advanced of Peloritan -Calabrian towards the SE and (b) the SE-ward rollback of the Ionian slab, with: (a) crustal dextral strike slip movement in NE Sicily between the stable Sicily -Hyblean plateau and the Peloritan -Calabria block, as confirmed by earthquake focal mechanisms, (b) lithospheric vertical movement beneath the Ionian western Sea, that propagates upward through a N150° trending crustal scale fault. The whole dataset that was recently interpreted NE Sicily and in the western Ionian Sea, is consistent and shows the presence of a STEP fault at the SW edge of the Ionian Basin. We further suggest that this lithospheric tear propagates in the Ionian basin during the late Neogene, through the inherited Continent-Ocean Boundary, acquired during the formation of the Mesozoic passive margin of the basin.

Figure 1 :

 1 Figure 1: Schematic 3-D block diagram showing the plate and block motions related to a

Figure 2 :

 2 Figure 2: Structural map of the Central Mediterranean showing the Archimede seismic lines

Figure 3 :

 3 Figure 3: Seismic stratigraphy of the sedimentary section, identified on the CROP-M3

Figure 4 :

 4 Figure 4: (top) Pre-stack depth migrated Archimede 17 profile (Vertical exaggeration 2) (see

Figure 5 :

 5 Figure 5: (top) Pre-stack depth migrated Archimede 16 profile (Vertical exaggeration 2) (see

Figure 6 :

 6 Figure 6: (A) Architecture of the western corner of the Calabrian accretionary wedge, a SW-

Figure 7 :

 7 Figure 7: Map of the vertical crustal scale fault in the Ionian Sea (thick red line), correlated

  Note that the vertical and crustal scale fault together with the strike slip Taormina fault define the STEP, that bounds SW-ward the Ionian slab. See the position of the backthrust that is the boundary between the pre-and post-Messinian wedges. The position of schematic composite cross-section shown in Figure 8 is indicated in orange.

Figure 8 :

 8 Figure 8: Schematic composite structural cross-section (No vertical exaggeration). The overall

Table 1 :

 1 Post-stack time migration

Table 2 :

 2 Pre-stack time migration

Acknowledgments

We gratefully acknowledge Marc Schaming, Angélique Leprêtre and Laure Schenini their help for reading in reading these data with the GeoVecteur Software. We are grateful to Andrea Billi and to anonymous reviewers for their helpful comments and suggestions that improved the manuscript. The maps were drafted using the Generic Mapping Tools (GMT) software (Wessel and Smith, 1991). The first author gratefully acknowledges her colleagues