João C Duarte

Filipe M Rosas

Pedro Terrinha

Marc-André M-A Gutscher

Jacques Malavielle

Sonia Silva

Luis Matias

Jacques Malavieille

Sónia Silva

Thrust-wrench interference tectonics in the Gulf of Cadiz (Africa-Iberia plate boundary in the North-East Atlantic): Insights from analog models

Keywords: SWIM wrench system, Gulf of Cadiz Accretionary Wedge (GCAW), Thrust -wrench interference, Analog modeling, Tethyan-related plate boundary

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Introduction

The Gulf of Cadiz is situated in the North-East Atlantic Ocean, west of the Gibraltar Straights, offshore SW Iberia and NW Morocco (Fig. 1). This zone marks the transition between the Mediterranean Alpine Collision Belt and the Atlantic Azores -Gibraltar Fracture Zone (AGFZ; see Fig. 1) and corresponds to a segment of the Africa-Eurasia plate boundary previously described as tectonically diffuse (e.g. [START_REF] Sartori | Eastern segment of the Azores-Gibraltar line (central-eastern Atlantic): an oceanic plate boundary with diffuse compressional deformation[END_REF][START_REF] Medialdea | Structure and evolution of the ''Olistostrome'' complex of the Gibraltar Arc in the Gulf of Cádiz (eastern Central Atlantic): evidence from two long seismic cross-sections[END_REF]. Accordingly, a variety of tectonic structures with different orientations, corresponding mostly to W-NW directed thrusts and WNW-ESE dextral strike-slip faults (Fig. 1B), are thought to accommodate a WNW-ESE present convergence between Eurasia (Iberia sub-plate) and Africa (Nubia sub-plate) at a rate of ca. 4-5 mm/year [START_REF] Argus | Closure of the Africa-Eurasia-North America plate motion circuit and tectonics of the Gloria Fault[END_REF][START_REF] Demets | Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions[END_REF][START_REF] Sella | REVEL; a model for recent plate velocities from space geodesy[END_REF][START_REF] Calais | Evidence for a post-3.16-Ma change in Nubia-Eurasia-North America plate motions?[END_REF][START_REF] Fernandes | The relative motion between Africa and Eurasia as derived from ITRF 2000 and GPS data[END_REF][START_REF] Fernandes | Present-day kinematics at the Azores-Gibraltar plate boundary as derived from GPS observations[END_REF][START_REF] Nocquet | Geodetic measurements of crustal deformation in the western Mediterranean and Europe[END_REF]Fernandes et al., 2007).

The seismicity of this domain has been characterized as moderate, although several high magnitude historical and instrumental earthquakes are known (e.g. Ms=7.9 28/02/1969 and Mw=6.0,12/02/2007 earthquakes respectively, Fukao, 1973and Stich et al., 2006[START_REF] Withjack | Scaled Experimental Models of Extension: Dry Sand vs.Wet Clay[END_REF]. Among these, the 1755 Great Lisbon Earthquake (estimated magnitude of 8.5 to 8.7; [START_REF] Abe | Size of Great Earthquakes of 1837-1974 Inferred from Tsunami Data[END_REF][START_REF] Johnston | Seismic moment assessment of earthquakes in stable continental regions: III[END_REF][START_REF] Martinez-Solares | The great historical 1755 earthquake: effects and damage in Spain[END_REF] triggered a devastating tsunami and destroyed the Portuguese capital [START_REF] Baptista | The 1755 Lisbon tsunami; evaluation of the tsunami parameters[END_REF][START_REF] Zitellini | Source of 1755 Lisbon Earthquake and Tsunami Investigated[END_REF][START_REF] Martinez-Solares | The great historical 1755 earthquake: effects and damage in Spain[END_REF]. Despite recent mapping updates of the main tectonic structures of this region based on the interpretation of a great variety of newly acquired data (e.g. multi-beam swath bathymetry, reflection and refraction seismics, geodetic; [START_REF] Johnston | Seismic moment assessment of earthquakes in stable continental regions: III[END_REF], Gonzalez et al., 1996;[START_REF] Zitellini | Source of 1755 Lisbon Earthquake and Tsunami Investigated[END_REF][START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF]Gutscher et al., , 2009a,b;,b;[START_REF] Baptista | Newstudyof the 1755 earthquake source based on multi-channel seismic survey data and tsunami modeling[END_REF]Gracia et al., 2003a,b;[START_REF] Mulder | The Gulf of Cadiz: an unstable giant contouritic levee[END_REF][START_REF] Zitellini | Neogene through quaternary tectonic reactivation of SW Iberian passive margin[END_REF][START_REF] Rosas | Morphotectonic characterization of major bathymetric lineaments in NW Gulf of Cadiz (Africa-Iberia plate boundary): insights from analogue modelling experiments[END_REF][START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF][START_REF] Zitellini | The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar[END_REF], the precise location of the seismogenic/tsunamigenic source of 1755 major event is still the subject of ongoing debate (e.g. [START_REF] Baptista | The 1755 Lisbon tsunami; evaluation of the tsunami parameters[END_REF][START_REF] Buforn | Seismicity, source mechanisms and tectonics of the Azores-Gibraltar plate boundary[END_REF][START_REF] Zitellini | Source of 1755 Lisbon Earthquake and Tsunami Investigated[END_REF]Gracia et al., 2003a,b;[START_REF] Terrinha | Tsunamigenic-seismogenic structures, neotectonics, sedimentary processes and slope instability on the southwest Portuguese Margin[END_REF][START_REF] Gutscher | What caused the Great Lisbon earthquake?[END_REF][START_REF] Thiebot | The Gibraltar Arc seismogenic zone (part 1): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic data, gravity and thermal modeling[END_REF][START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF].

In the tectonic map of figure 1B three main sets of structures are immediately recognized: a) several NE-SW striking, westwards directed thrust faults (e.g. Gorringe, Horseshoe and Marquês de Pombal faults); b) major WNW-ESE striking dextral strike-slip faults (the SWIM faults) and c) a major thrust bounding the so called Gulf of Cadiz Accretionary Wedge (GCAW frontal thrust). Structures of the first set have previously been described as active and, individually (e.g. Gorringe fault), or together (Marques de Pombal and Horseshoe faults), successively considered and dismissed as possible seismogenic/tsunamigenic sources of the 1755 Great Lisbon Earhquake (e.g. [START_REF] Baptista | The 1755 Lisbon tsunami; evaluation of the tsunami parameters[END_REF][START_REF] Buforn | Seismicity, source mechanisms and tectonics of the Azores-Gibraltar plate boundary[END_REF][START_REF] Zitellini | Source of 1755 Lisbon Earthquake and Tsunami Investigated[END_REF][START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF]. The other two sets of tectonic structures support the following two fundamental ideas for the interpretation of the Gulf of Cadiz tectonic framework [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF]Zitellini et al., 2009):

- [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF], building on previously reported similar ideas (e.g. [START_REF] Royden | Evolution of retreating subduction boundaries formed during continental collision[END_REF][START_REF] Lonergan | Origin of the Betic-Rif mountain belt: Tectonics[END_REF][START_REF] Lonergan | Origin of the Betic-Rif mountain belt: Tectonics[END_REF], considered the tectonic evolution of this plate boundary as being dominated by active (roll back) subduction of a retreating east-dipping lithospheric slab, presently positioned beneath the Gibraltar Arc (Fig. 1B). Accordingly, associated synthetic accretion of sediments is also reported to occur, represented by the Gulf of Cadiz Accretionary Wedge (GCAW, Fig. 1B), in which several imbricated west-directed active thrusts accommodate on-going shortening.

- [START_REF] Zitellini | The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar[END_REF] argued for the existence of a broad transpressive deformation band, comprising a set of WNW-ESE striking, subvertical, dextral strike-slip faults (SWIM faults in Fig. 1B), which as a whole extend for more than 600 km, from the eastern part of the Gulf of Cadiz to the southern limit of the Gorringe bank. These faults are interpreted by the cited authors as lithospheric faults, and in view of that the SWIM fault system was proposed to mark a newly formed plate boundary connecting the AGFZ to the Rif Mountain belt in northern Morocco.

It should be noted that [START_REF] Zitellini | The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar[END_REF] considered the dextral transcurrent SWIM faults in the Gulf of Cadiz as presently active, and cutting the GCAW thrusts, whose activity is considered to be negligible since late Miocene times. Conversely, [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF] considered the GCAW thrusts as still active, and as referred above related with present on-going subduction beneath the Gibraltar arc.

In the present work, we test the above assumptions using analog modeling sandbox experiments to model the interference between the two major tectonic systems in the Gulf of Cadiz, i.e. SWIM dextral wrenching and GCAW thrusting, assuming three main different possibilities:

1) The GCAW is presently active and the SWIM faults are thought to correspond to inactive basement faults, inherited from a previous tectonic evolution;

2) The SWIM faults are thought to correspond to major, presently active, dextral strike-slips, cutting a previously formed inactive GCAW;

3) Both tectonic systems are presently active.

Thorough comparison of the obtained results with the observed natural morphotectonic pattern is carried out for each of the above cases. Accordingly, resulting tectonic implications for the local and whole scale evolution of this segment of the Eurasia-Africa plate boundary are evaluated and explored.

Morphotectonic characterization of the study area

The morphology of the Gulf of Cadiz is largely controlled by the main tectonic processes in the area: the thrusting accommodating the GCAW accretion and the wrenching associated to the SWIM fault system. The high-resolution bathymetry of the GCAW shows a west dipping U-shaped body (Figs. 1B and2A) that extends for more than 250 km from about longitude 7ºW to 9º30'W, with depths ranging from 200 to 4300 meters. It narrows slightly to the west with a width varying from 160 to 140 km (see Fig. 1B and2A). Its wrinkled surface morphology is shaped not only by thrustrelated slope breaks and associated folds, which define a large scale stepping morphology, but is also the result of the different combined manifestations of gravitational and fluid escape processes (e.g. "raft-tectonics" type features, sub-circular collapse depressions, mud volcanoes, salt diapirs; [START_REF] Mulder | The Gulf of Cadiz: an unstable giant contouritic levee[END_REF][START_REF] Pinheiro | Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise[END_REF]Gutscher et al., 2009b;[START_REF] Zitellini | The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar[END_REF][START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF]. From a structural point of view, the accretionary wedge corresponds to an eastward thickening pile of westward thrust sediments (Fig. 2B andC), reaching a maximum thickness of ca.

15 km near the Gibraltar Straits [START_REF] Thiebot | The Gibraltar Arc seismogenic zone (part 1): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic data, gravity and thermal modeling[END_REF]Gutscher et al., 2009a).

The thrusts root in a common sub-horizontal to gently east dipping decollement layer, exhibiting an overall geometry complying with on-going eastwards subduction beneath the Gibraltar Arc [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF]. During the last 5 Ma, the E-W convergence rate implied in such a subduction is thought to have diminished from ca. 2 cm/year to 0.5 cm/year (Gutscher et al., 2009a), with a consequent decrease in the activity of the wedge thrusts during this time span. Accordingly, the same authors argue that the present internal deformation is preferentially accommodated by small increments of local reactivation of inherited blind thrusts, being more homogeneously distributed over the entire wedge, rather than concentrated on newly formed major frontal thrusts.

The morphologic expression of the SWIM faults corresponds to a continuous alignment of seafloor crests and troughs, sometimes exhibiting an en-échelon geometrical disposition, and commonly punctuated by active mud volcanoes within the domain of the accretionary wedge (see Fig. 1B; [START_REF] Duarte | Anatomy and tectonic significance of WNW-ESE and NE-SW lineaments at a transpressive plate boundary (Nubia-Iberia)[END_REF][START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF][START_REF] Zitellini | The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar[END_REF]. This overall linear morphology is more prominent both close to the northern part of the GCAW front and in the Horseshoe Valley, where lineaments can be followed almost continuously for more than 200 kilometers (e.g. SWIM 1 in Fig. 1B and3A). The available reflection seismic dataset in the study area shows that SWIM faults correspond to aligned arrays of deep-rooted faults, often breaching out through the present seafloor sediments and showing extensive evidence for associated fluid migration (e.g. seismic blanking along fluid extrusion paths in Fig. 3B andC; [START_REF] Rosas | Morphotectonic characterization of major bathymetric lineaments in NW Gulf of Cadiz (Africa-Iberia plate boundary): insights from analogue modelling experiments[END_REF][START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF]. From the inspection of the IAM 4 and 3 seismic profiles in figure 3, it is apparent that the SWIM faults are aligned along basement pre-existent (Mesozoic) faults (dashed black lines in Fig. 3; see also Duarte et al., 2009 and[START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF]. Analog modeling of sets of en-échelon folds formed in soft-cover sediments overlying some of the SWIM basement dextral strike-slips [START_REF] Rosas | Morphotectonic characterization of major bathymetric lineaments in NW Gulf of Cadiz (Africa-Iberia plate boundary): insights from analogue modelling experiments[END_REF] yielded an age for their activity of ca. 1.8 Ma.

The SWIM-GCAW interference area

The critical area to understand the interference between the SWIM strike-slips and the GCAW is close to its deformation front (Fig. 4A). Along this front, from north to south, three interference sub-areas were thoroughly analyzed (B to D in Fig. 4). The SWIM 2 fault intersects a northern segment of the GCAW front (Fig. 4B) in the vicinity of the Sagres Valley, at an angle of about 40º. Within the thrust wedge to the east, the morphological expression of the SWIM 2 is well marked by a slightly arched trough that splays in the same direction. Conversely, in the Sagres Valley it is almost indiscernible due to the widespread presence of scours and slumps, and it is only detectable by the presence of a gentle slope break. In this area no offset of the wedge deformation front is observed, instead only a small wrinkled WNW-ESE elongate bulge can be detected in the bathymetry of the frontal GCAW (see Fig. 4B). The SWIM 1 fault exhibits a clear morphological expression in the Horseshoe Valley, intersecting the wedge deformation front just to the north of the Coral Patch Ridge, at an angle of about 70º (Fig. 4C). Its continuation to the east is less visible, although punctuated by mud volcanoes. Similarly to what was described for the SWIM 2 fault, there is no bathymetric evidence for the offset of the wedge front by the SWIM 1 fault. The SWIM 3 fault cuts along the southern flank of the Coral Patch Ridge with a strong morphological imprint (Fig. 4D), corresponding to a lineament marked by several elongated WNW-ESE ridges, troughs and slope breaks. To the east, this lineament crosses a small portion of the Seine Abyssal Plain and intersects the accretionary wedge deformation front, at an angle of about 80º. In this location the wedge front is marked by a slight embayment. On the wedge surface to the north of the fault and parallel to it, an elongated protuberance is also unmistakably observed, limited by two descending slope breaks and vanishing progressively towards the east. As in the two cases described above, the observed morphology does not account for any kind of fault offset overprint relationships.

Analog Modeling

Our objective is to test several simple chronological possibilities of mechanical interference between a strike-slip fault and a thrust wedge front, under model conditions comparable to the ones governing the SWIM-GCAW tectonic interference. Three main experiments were carried out to investigate the different deformation patterns resulting from: a) A thrust wedge developed on top of an inactive basement (strike-slip) fault; b) An active dextral strike-slip fault affecting an inactive thrust wedge; c) The simultaneous alternate activity of a thrust wedge front and a dextral strike-slip fault.

Experiments were performed to respect the general, simplified, geometry, kinematics and rheology ascribed to the natural strike-slip (SWIM) system and to the (GCAW) thrust wedge. However, analog modeling with dry granular materials cannot reproduce all the complex processes (e.g. fluid overpressure and expulsion, sedimentary deposition, and local gravitational instabilities) which occur in submarine environments.

Nevertheless, the regional kinematic and relative timing caused by the local tectonic driving forces and the ensuing structural evolution can be well investigated.

Experimental method

Material properties and scaling

The material used as an analog of the GCAW sedimentary rocks was dry quartz sand, whose properties are summarized in Table 1. Sand is considered a Coulomb material deforming in a brittle way according to the Coulomb fracture criterion (e.g. [START_REF] Hubbert | Theory of scale models as applied to the study of geologic structures[END_REF][START_REF] Hubbert | Mechanical basis for certain familiar geologic structures[END_REF]Davis et al., 1983, Appendix A), and it has been extensively used in scaled model experiments simulating similar brittle deformation in the upper crust (e.g. [START_REF] Mandl | Shear zones in granular material; an experimental study of their structure and mechanical genesis[END_REF][START_REF] Mulugeta | Modelling the geometry of Coulomb thrust wedges[END_REF][START_REF] Casas | Analogue models of transpressive systems[END_REF][START_REF] Marques | Topography as a major factor in the development of arcuate thrust belts: insights from sandbox experiments[END_REF].

The present models were properly scaled according to the scale model theory of [START_REF] Hubbert | Theory of scale models as applied to the study of geologic structures[END_REF]. The assumed model -prototype ratios are presented in Table 1, and the detailed procedure of scaling is specified in Appendix A

Apparatus and initial stage

Experiments were done using a rectangular 100 cm × 60 cm Perspex deformation rig, comprising two laterally juxtaposed basal plates and a moving backstop (Fig. 5A). In the initial stage of the experiments, 1 cm thick layered sand cake was built on top of the basal rigid plates by pouring batches of differently colored sand from a moving elongated funnel, guaranteeing the leveling of its top surface. The basal plates in the model move laterally relatively to each other, and account for the natural basement beneath the thrust wedge decollement (see Fig. 2C). The vertical contact plane between these plates complies with the dominant WNW-ESE orientation of the SWIM 1 fault system (see Fig. 1 and4). The model sand cake corresponds to the overlying cover sediments, in which accretion is simulated by pushing against it a backstop that slides on top of both basal plates. The backstop lacks any kind of correspondence with any natural feature, and was exclusively used to produce a classical model thrust wedge.

Likewise the (~0.2 cm thick) layering in the sand cake has also no correspondence with any natural structures, and was used merely as a 3D passive strain marker. In all experiments the dimensions of the deformation rig were sufficiently large to guarantee that the bulk of the model was not affected by boundary conditions. The experiments were repeated several times to ensure the reproducibility of the obtained results. Top view photographs were taken at regular time intervals. The final stage of the model was humidified and serially sectioned for three-dimensional analysis.

Procedure

Three main experiments were carried out to study three basic possibilities of mechanical interference between a strike-slip basement fault and a thrust wedge:

1) Active thrust wedge and inactive basement fault: the backstop was initially pushed against the sand-cake, on top of the two immobile basal plates.

2) Active basement fault and inactive thrust wedge: after a thrust wedge was previously formed in the sand, the basal plates were dextrally moved relatively to each other.

3) Active basement fault and active thrust wedge: the reactivation of a previously formed thrust wedge was successively alternated with the right-lateral movement between the basal plates.

Experimental results

Experiment 1: active thrust wedge and inactive basement fault

Up until ~30% of shortening a thrust wedge was classically obtained, through the forward propagation of regularly spaced thrusts, as a result of moving the backstop to the left (see Fig. 5B and Fig. 6A). For a shortening of 31 % (Fig. 6A) the front of the accretionary wedge developed an embayment coinciding with the direction of the inactive basement fault (yellow dashed line in Fig. 6A). For a shortening of 38 % a new thrust (n+1 in Fig. 6B) was formed exclusively to the north of that basement discontinuity. Only when the shortening reached 39 %, did the equivalent new thrust form to the south, resulting in a misleading left-lateral offset geometry (yellow arrows in Fig. 6C), in spite of the total absence of relative movement between the two basal rigid plates. For a shortening of 48 % this false offset in front of the newer thrust disappeared (Fig. 6D). In the wedge lower area a linear southeastward splaying, slightly anastomosing, deformation pattern (white lines in Fig. 6D) was progressively developed as shortening accumulated and new outward thrusts successively formed.

Experiment 2: active basement fault and inactive thrust wedge

After 48% of shortening the basement strike-slip fault was activated with dextral movement (Fig. 7) When the horizontal displacement between the basal plates reached only 0.2 cm, a thin elongated bulge formed coinciding with the basement strike-slip direction (Fig. 7A). At this stage no offset of the thrust wedge was observed. A further right-lateral displacement of 0.6 cm between the basal plates clearly offset the thrust wedge, preferably affecting its frontal outer thrust (Fig. 7B). For a displacement of 1.3 cm the strike-slip fault seemingly propagated inwards, but only across the following three thrusts (Fig. 7C-D). The resultant model deformation pattern always showed the preferential offset of the frontal thin part of the accretionary wedge, relative to its innermost domains that remained unaffected.

Experiment 3: active basement fault and active thrust wedge

After the formation of a thrust wedge due to 47% of shortening, a right-lateral displacement of 1cm was applied to the basal plates (Fig. 8A). As a result, a linear bulge formed in the sand foreland, along the strike-slip direction, cutting across the thrust wedge and offsetting its front. Similarly to experiment 2, the amount of right-lateral displacement diminished towards the thicker inner part of the wedge. Subsequently, another 0.4% of incremental shortening was applied to the model (Fig. 8B). As a result, a new frontal thrust formed, but only to the north of the strike-slip fault. The resultant geometry mimics a false left-lateral offset of the outer frontal thrust (apparent as n+1 in Fig. 8B), although careful consideration of the propagation chronology of the thrusts revealed a true dextral offset (affecting thrust n in Fig. 8B), complying with the underlying basement fault kinematics. Further 1% of applied shortening (Fig. 8C) produced the new thrust (n+1) also to the south of the strike-slip fault, erasing the previously formed false offset. Another 3.5 cm of dextral strike-slip displacement was additionally applied to the basal plates (Fig. 8D), and as a consequence, the wedge front was once more kinematically truly dextrally offset. Riedel faults formed on the wedge surface displaying a clear en-échelon spatial disposition, and interfering with the preexistent stepping morphology associated with the thrust stacking. Finally, after another 2.6% of shortening (Fig. 9), reaching a total accumulated amount of 51%, the frontal wedge offset was once again almost completely attenuated, although the total accumulated basal strike-slip displacement was of 4.5 cm (corresponding to 22.5 km). It should be noted that the previously formed linear en-échelon pattern was preserved in the wedge surface, including in its innermost domain.

Discussion

Experiment 1 shows that an active thrust wedge forming in cover sediments above an inactive basement fault records the resultant mechanical interference in the form of a linear, inner splaying, perturbation of the wedge morphological surface (white lines in Fig. 6C-D). This formed as the result of the successive thrust propagation across the basement anisotropy, which behaved as a mechanical obstacle for the lateral propagation of the frontal thrust. Since this basement anisotropy remains stationary relatively to an external reference frame, and the thrust front propagates forward (to the left), the resultant interference area between both these features migrates from the lower periphery of the thrust front to its central domain (compare the position of the linear perturbation pattern in Figs. 6A to D). Comparison with the natural example shows that the observed inner splaying lineaments in the GCAW (see SWIM 1 in Fig. 1B and SWIM 2 in Fig. 4B), could have originated simply as a consequence of preexisting inactive basement faults. This agrees with the fact that these lineaments are observed in the inner thicker part of the natural thrust wedge, in the complete absence of offset of its thinner frontal part.

Experiment 2 shows that an active basement strike-slip fault affecting an overlying preexistent thrust-wedge, must produce a clear offset of its thinner front, even for minor increments of strike-slip basal displacement (1.3 cm corresponding to 6.5 km). This offset vanishes rapidly towards the inner thicker parts of the wedge, affecting almost exclusively the outer frontal thrusts. Comparison with the natural example shows that if the GCAW is presently inactive, and cut by active dextral strike-slip (SWIM) faults, then unambiguous offset of the GCAW front has to exist, which is clearly not the case (see Figs. 1B, 2 and4).

Experiment 3 shows that if both the basal strike-slip fault and the thrust wedge are active, with alternating incremental fault slips, then the resultant interference deformation pattern can be the cyclic repetition of the following three possibilities: a) true right-lateral offset of the thrust wedge front (see Fig. 8A andD); b) false left-lateral offset of the thrust wedge front (see Fig. 8B, similar to experiment 1 see Fig. 6C); c) no offset of the thrust wedge front (see Fig. 8C and9). It should be noted that the absence of thrust wedge offset does not imply an absence of basement strike-slip displacement, which quite on the contrary is continually increasing. However, the alternation with the incremental shortening that drives the successive forward thrust propagation cyclically erases the offset of the frontal thrust wedge. Differently, the resultant linear en-échelon interference pattern tends to be increasingly well marked, including in the innermost domains of the thrust wedge (compare the lineament in the Figs. 8A and9). Similarly to experiment 1, the intermediate experimental stage in which a false left-lateral offset originates (see Fig. 8B), is here interpreted as the result of a delayed propagation of the newer outer thrust across the strike-slip fault, which in accordance seemingly behaves as a mechanical obstacle to such propagation.

In view of these results, and considering the fact that in the natural example the GCAW front is not offset across any of the mapped SWIM faults, one of the two following tectonic scenarios is possible: a) either the GCAW is active, and forming over a basement anisotropy (inactive SWIM fault?); or b) both the GCAW and the SWIM faults are active. The possibility of an inactive GCAW being cut by active dextral strike-slip SWIM faults is clearly ruled out by the present experimental results.

Tectonic implications

The west Mediterranean tectonic evolution comprised the Mesozoic opening of the Tethys Ocean (Fig. 10A; e.g. [START_REF] Maldonado | The Betic orogen and the Iberian-African boundary in the Gulf of Cadiz: geological evolution (central North Atlantic)[END_REF][START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF]Gràcia et al., 2003a;[START_REF] Stampfli | Western Alps geological constraints on western Tethyan reconstructions[END_REF][START_REF] Jiménez-Munt | The lithospheric structure of the Gorringe Bank: insights into its origin and tectonic evolution[END_REF]. This would account for a basement tectonic anisotropy, hypothetically, either consisting in previous transform faults, or in rift-related normal faults (Fig. 10A andB). In view of the presented experimental results, the accretionary wedge (GCAW) is here interpreted to have formed on top of such basement faults (present day SWIM faults) as a consequence of roll-back subduction beneath Gibraltar. During this period (Middle Miocene to ~1.8Ma?, Figs. 10C to E) these faults were probably inactive, although capable of originating a linear-like perturbation in the GCAW surface morphology (experiment 1).

At same time, the main regional convergence direction between Iberia and Africa gradually suffered a counterclockwise rotation, shifting from N-S (Fig. 10 B) to near WNW-ESE (Fig. 10E). Such reorientation is interpreted to have triggered (since at least ~1.8Ma) a dextral strike-slip reactivation of the basement (SWIM) faults (Fig. 10F), during a period in which the subduction driving the GCAW growth was still active (experiment 3), although slowing down (Gutscher, 2009a). It should be noted that this rotation of the main convergence direction also agrees with the general strain partitioning tectonic scenario previously proposed by [START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF], according to which besides dextral strike slip faulting along near E-W orientated faults, northwest directed thrusts also occur along NE-SW orientated tectonic structures (e.g. Horseshoe, Marquês de Pombal and Gorringe Faults in Fig. 1B).

Other indirect evidence supporting present simultaneous activity of SWIM and GCAW includes the fact that not only all known mud volcanoes in the Gulf of Cadiz are located on top of the accretionary wedge, but also the circumstance that most of the deep mud volcanoes are symptomatically aligned and coincident with the SWIM faults (Fig. 1A). This suggests that the fluid migration and escape may be simultaneously controlled by the activity of both structures. If that is in fact the case, then the mud volcanoes could be preferentially located at the intersection between the SWIM strike-slip and the GCAW thrust faults, with such loci providing good pathways for the fluid to ascend [START_REF] Pinheiro | Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise[END_REF][START_REF] Duarte | Interpretation of recent sedimentary and tectonic structures off SW Iberia from multibeam bathymetry, seismic reflection and experimental modelling[END_REF][START_REF] Duarte | Interpretation of recent sedimentary and tectonic structures off SW Iberia from multibeam bathymetry, seismic reflection and experimental modelling[END_REF][START_REF] Rosas | Morphotectonic characterization of major bathymetric lineaments in NW Gulf of Cadiz (Africa-Iberia plate boundary): insights from analogue modelling experiments[END_REF]Zitellini et al., 2009;[START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF].

Conclusions

1. In view of the presented experimental results, the observed morphotectonic pattern of the frontal GCAW area is compatible with:

i) The development of the GCAW on top of inactive, previously formed, basement faults (present day SWIM fault system).

ii) The simultaneously (alternating) activity of the GCAW thrusting with the activity of the SWIM-related dextral strike-slip faults.

Conversely, the comparison of the same experimental results with the natural example unambiguously excludes the possibility of active strike-slip faulting (SWIM system) affecting an inactive preexistent GCAW, implying that if the SWIM fault system is active then the GCAW must also be presently active. [START_REF] Duarte | Anatomy and tectonic significance of WNW-ESE and NE-SW lineaments at a transpressive plate boundary (Nubia-Iberia)[END_REF], Terrinha et al., 2009, Zitellini et al., 2009[START_REF] Duarte | Crescent-shaped morphotectonic features in the Gulf of Cadiz (offshore SW Iberia)[END_REF]; bathymetry from the SWIM compilation; [START_REF] Zitellini | The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar[END_REF]. Gulf of Cadiz Accretionary Wedge (GCAW) -dark grey outline; SWIM wrench system according to Zitellini et al. ( 2009 [START_REF] Rosenbaum | Reconstruction of the tectonic evolution of the western Mediterranean since the Oligocene[END_REF]2004;and complemented with Maldonado et al., 1999;[START_REF] Michard | How does the Alpine belt end between Spain and Moroco?[END_REF]Gràcia et al., 2003a and[START_REF] Terrinha | Morphotectonics and Strain Partitioning at the Iberia-Africa plate boundary from multibeam and seismic reflection data[END_REF]. Ab -Alboran; Cb -Calabria; Cs -Corsica; Sd -Sardinia. See detailed explanation in the text. Note: --Scaled fundamental units are in bold --A mean cohesion of Co=40 MPa was assumed from the natural prototype (e.g. Hoshino et al., 1972;Weijermars et al., 1993)

Parameters and material properties

2.

  Figure captions

Fig. 1 .

 1 Fig. 1. (A) Location of the Gulf of Cadiz area in the general tectonic setting of the

  )white lines. Black dots correspond to the location of known mud volcanoes (e.g. Hensen et al., 2007).

Fig. 2 .

 2 Fig. 2. (A) Perspective view (from southwest) of the Gulf of Cadiz Accretionary Wedge

Fig. 3 .

 3 Fig. 3. (A) Perspective view (from southwest) of the SWIM lineaments; (B)

Fig. 4 .

 4 Fig. 4. General (A) and detailed (B, C and D) bathymetric imaging and respective

Fig. 5 .

 5 Fig. 5. (A) -Sketch of experimental apparatus and model set-up at initial stage. (B)

Fig. 7 .

 7 Fig. 7..Results of Experiment 2: active basement fault and inactive thrust wedge. A,B

Fig. 8 .

 8 Fig. 8. Results of Experiment 3: active basement fault and active thrust wedge. A,B, C

Fig. 9 .

 9 Fig. 9. Final stage of Experiment 3: active basement fault and active thrust wedge

Fig. 10 .

 10 Fig. 10. Tectonic model: summarized, schematic representation of the main chronologic

Figure 1 Figure 2

 12 Figure 1

Figure 3

 3 Figure 3

Figure 4 Figure 5

 45 Figure 4

Figure 6

 6 Figure 6

Figure 7 Figure 8

 78 Figure 7

Figure 9

 9 Figure 9

  Figure 10

Table 1 -

 1 Parameters and material properties

Acknowledgments

This work was sponsored by ALMOND (Multiscale modeling of deformation in the Gulf of Cadiz, PTDC/CTE-GIN/71862/2006 ), EUROMARGINS SWIM (Earthquake and Tsunami hazards of active faults at the South-West Iberian Margin: deep structure, high-resolution imaging and paleo-seismic signature, REN2002-11234-EMAR, 01-LEC-EMA09F), TOPOEUROPE/0001/2007-TOPOMED (Plate reorganization in the western Mediterranean: lithospheric causes and topographic consequences) and NEAREST (Integrated observations from NEAR shore sourcES of Tsunamis: towards an early warning system, ESF EuroMargins Program, contract n. 01-LEC-EMA09F and from EU Specific Programme "Integrating and Strengthening the European Research Area", Sub-Priority 1.1.6.3, "Global Change and Ecosystems", contract n. 037110) projects. Experiments were performed in the Analog Modeling Laboratory of IDL. J. Duarte and S. Silva thank FCT for the PhD grants SFRH/BD/31188/2006 and SFRH/BD/46227/2008, respectively. The support by Landmark Graphics Corporation via the Landmark University Grant Program and NASA WorldWind is also acknowledged. We specially thank Stephane Dominguez (University of Montpellier), Sandy Cruden and David Boutelier (Monash University) and Vasco Valadares (LNEG) for insightful discussions. Ruth Keppler, Ana Costa, André Blanco and Liliana D'Almeida are acknowledged for their help in the lab.

Appendix A

The material used as an analog of the upper crust sedimentary rocks was dry quartz sand, which is considered a Coulomb material deforming in a brittle way according to the Coulomb fracture criterion (e.g. [START_REF] Hubbert | Theory of scale models as applied to the study of geologic structures[END_REF][START_REF] Hubbert | Mechanical basis for certain familiar geologic structures[END_REF]Davis et al., 1983):

where 𝜏 !! is the shear stress, 𝜇 ! is the coefficient of internal friction (𝜇 ! = tan 𝜙, and 𝜙 = internal friction angle), 𝜎 ! is the normal stress, and 𝑐 ! is the cohesion of the material. According to the scale model theory [START_REF] Hubbert | Theory of scale models as applied to the study of geologic structures[END_REF], proper scaling is achieved when the ratios between model and natural prototype are independently established for the three fundamental units of length (λ), time (τ) and mass (µ):

where 𝐿 = length, 𝑇 =time and 𝑀 =mass, and (𝑚) stands for model and (𝑝) for natural prototype. The Coulomb fracture criterion governs time independent deformation of brittle materials like sedimentary upper crustal rocks, since yield stress is insensitive to the rate of deformation provided that the inertial forces are negligible, as in the present case. This means that 𝜏 ratio is not needed for scaling in this situation. Length ratio (𝜆)

was chosen given the maximum dimensions of the deformation apparatus used in the experiments (see section 3), and was conveniently established as 𝜆 = 2×10 -6 . In the present case, of the two relevant material properties, coefficient of internal friction (𝜇 ! )

and cohesion (𝑐 ! ), the first is dimensionless, and approximately the same in both model and prototype, whereas the second has dimension of stress and thus must be scaled accordingly [START_REF] Hubbert | Theory of scale models as applied to the study of geologic structures[END_REF]:

where and 𝛾 are the model/prototype ratio for stress and for acceleration respectively.

Since inertial forces are negligible when compared with gravity,

where 𝛾 ! is the model/prototype gravity acceleration ratio. Thus, substituting 𝛾 = 1 in equation ( 3) allows the following simplification:

where 𝛿 corresponds to the model/prototype density ratio. Substituting 𝛿 and 𝜆 in equation ( 5) by the respective values in Table 1, immediately allows the determination of the implied mass ratio = 4×10 !!" . It should also be noted that since 𝛿 is generally close to one (between 0.5 and 0.7, e.g. [START_REF] Withjack | Scaled Experimental Models of Extension: Dry Sand vs.Wet Clay[END_REF] the strength of the materials expressed by Σ is scaled with the lengh (𝜆). Given the fact that in the present case 𝜆 = 2×10 -6 and since cohesion for upper crustal rocks is clearly typically less than 50 MPa, it becomes immediately evident the utility of model materials with very low cohesion (<100Pa), such as dry quartz sand, as analogs of upper crustal rocks.


B Nagaraju

Michèle Ude

Stanislaw Trzesien

Bernard Dussardier

Ravi K Varshney

Gérard Monnom

Wilfried Blanc

Bishnu P Pal
email: bppal@physics.iitd

S Trzésien

Design and fabrication of an asymmetric twin-core fiber directional coupler for gain-flattened EDFA

Keywords: Erbium doped fiber amplifiers, Gain flattened EDFA, Modified chemical vapor deposition technique

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Erbium doped fiber amplifiers (EDFA) find wide scale applications in various medium to long-haul optical communication networks including metro networks because of their broad gain, low noise and high-speed response. Due to broad gain spectrum these find extensive applications in dense wavelength division multiplexing (DWDM) systems. However, the non-uniform gain spectrum in conjunction with the saturation effects of EDFAs could cause degradation of the optical signal-to-noise ratio (OSNR) to unacceptable values in systems consisting of cascaded chains of EDFAs. These features could limit the usable bandwidth of EDFAs and hence the data transmission capacity of the system. Accordingly various schemes of gain equalizing filters have evolved in the literature. From this perspective, an intrinsically gain flattened EDFA would cut down the cost on the gain equalization filter head. Such inherently gain equalized EDFAs are attractive for metro-centric networks because low cost components and less number of components (to cut down the network maintenance cost) have been a major driver in a metro network design. Exploitation of a wavelength filtering mechanism inherent in a co-axial dual-core fiber design scheme to realize a gain flattened EDFA was reported by us earlier [START_REF] Nagaraju | Design and fabrication of an intrinsically gain flattened Erbium doped fiber amplifier[END_REF]. Laming et al were possibly first to propose in 1993 a scheme for gain equalization utilizing a twin core amplifier [START_REF] Laming | Twin-core erbium doped fiber amplifier with passive spectral gain equalization[END_REF]. In their design, the equalized gain bandwidth was quite narrow centered on the gain peak wavelength. Later, other authors [START_REF] Wu | A twin-core erbium-doped fiber amplifier[END_REF] proposed modified scheme and logistic for better and efficient use of a twincore fiber directional coupler concept to achieve flat gain with a gain excursion of 0.7 dB over 30 nm (1525 -1555 nm) in the C-band. It consisted of a twin-core Erbium doped fiber (EDF) in which only one of the cores (core 1) is doped with Er 3+ and 980/1550 nm power through a WDM coupler was launched and retrieved from the core 1 only. The excess power at wavelength around 1530 nm due to non-uniform gain characteristic of typical EDFAs was coupled strongly to the undoped core 2. However, a drawback with this idea was that the absolute gain levels showed significant variation with change in input pump power and hence such a configuration would be quite susceptible to transient changes in the pump and signal powers, and thus would require some type of complex gain-clamping arrangement. Moreover, no theoretical analysis is given regarding the optimization of the twin core fiber, which is important for fabricating such fiber. In this paper, we propose a new asymmetric twin-core (ATC) gain flattened EDF design that is based on filtering action induced by resonant power coupling from doped core to un-doped core.

FIBER DESIGN

We consider a highly asymmetric twin core (ATC) fiber, in which the Er 3+ -doped core is relatively large with a smaller core-cladding refractive index contrast and the un-doped core, of a larger index contrast, is relatively narrow as shown in Fig. 1.

The fiber parameters were carefully chosen such that the fundamental modes corresponding to the isolated fibers are phase-matched at a wavelength of nearly 1530 nm (λ p ). It is well known that in an asymmetric directional coupler, the supermodes of the coupler correspond to almost the individual fiber modes away from λ p . Below λ p , effective mode index (n eff ) of the fundamental mode of the composite structure would lie very close to that of core 1 while above λ p , its n eff lies close to that of core 2 and as one crosses λ p , n eff of the two normal modes vary very rapidly. The variation of effective modal index (n eff ) of two normal modes in combination with the doped and un-doped core modes with wavelength is shown in Fig. 2. This asymmetric directional coupler structure essentially acts like a wavelength filter with dip in the transmittance versus λ curve around λ p . With these physical features in mind, at the design stage we optimized the fiber parameters, doping level and signal and pump powers in order to minimize the gain excursion in the gain spectrum of the EDFA.

THEORY AND DESIGN OPTIMIZATION METHODOLOGY

The gain characteristics of this ATC EDF were modeled through the standard three-level rate equation model [START_REF] Pedersen | Small-signal erbium-doped fiber amplifiers pumped at 980 nm: a design study[END_REF]. We have assumed forward pumping at the wavelength of 980 nm. The nonlinearly coupled differential equations, which govern the propagation of ASE, signal and pump power along the length of the fiber, were solved using fourth-order Runge-Kutta method under the adaptive step size approach. Modal intensities of supermodes required in the numerical simulation were obtained using Coupled Mode Theory [START_REF] Snyder | Optical Fiber Couplers-Optimum Solution for Unequal Cores[END_REF]. When the individual cores are weakly coupled and single moded in isolation from each other, electric field ± (x,y) of the normal modes can be approximated as-

( , ) ( , ) ( , ) x y

x y a x y

± ± Ψ =Ψ + Ψ ( 1 )
Plus and minus correspond to two supermodes, namely first and second supermodes of the combined structure. 1 (x,y) and 2 (x,y) represent the transverse mode field pattern of fiber-1 and 2, respectively in absence of other. We have considered the excitation of both supermodes in our modeling. Variation of excitation efficiency of the fundamental and first higher-order modes with wavelength is shown in Fig. 3. As can be seen from this figure, the excitation efficiency of two modes varies rapidly around λ p . For optimization of fiber parameters, as a first step, we had chosen first fiber core's parameters in such a way that it supports single-mode. Next we calculated the parameters of second core in such a way that it is also single moded and the propagation constants of the two cores match at the λ p ~ 1530 nm. However, for achieving gain flattening, λ p as well as slopes of two curves for n eff vs λ corresponding to the individual core modes needs to be optimized. After obtaining this initial set of parameters, separation between the two cores is set. Then the gain characteristics were studied using the model described in [START_REF] Pedersen | Small-signal erbium-doped fiber amplifiers pumped at 980 nm: a design study[END_REF] through iteration till the absolute gain and gain fluctuations could be obtained close to the target values.

RESULTS AND DISCUSSIONS

With the optimized parameters obtained through the procedure outlined in section 2, a sample of calculated gain spectrum of the proposed ATC fiber for one set of design parameters is shown in Fig. 4. As can be seen from this figure, the gain peak around 1530 nm goes down because of the filtering action of the fiber at selective wavelengths around 1530 nm. The figure shows a median gain of ~ 38 dB with a gain ripple within ± 1 dB within the C-band. Corresponding noise figure variation with wavelength is shown in Fig. 5. It can be seen from this figure that the noise figure improves at longer wavelengths. This is due to enhanced overlap of pump and signal powers in the doped region at longer wavelengths. We have also made a detailed tolerance study as to different fiber parameters from the point of view of any detrimental effect on the flattened gain spectrum.

Our study revealed that control of the un-doped core parameters is very critical. With so obtained simulation studies and taking in to account limitations of the MCVD fabrication facility available with us, we redesigned the ATC for realizing an EDFA with medium gain of ~ 20 dB with gain excursions below ± 1.5 dB. With the so designed nominal parameters for the ATC as initial parameters to define the fabrication recipe, two independent fiber preforms were fabricated; core 1 was solution-doped with Er 3+ ions and core 2 was a relatively highly doped with germania. These were then selectively polished for achieving a target separation between the two cores at the fiber stage and assembled. The assembled polished fiber preforms were then drawn into a twin core fiber purported to function as an ATC fiber directional coupler. The optical micrograph and the refractive index profile along the axis going through centres of both cores of the fabricated ATC fiber is shown in Fig. 6. The core diameter (and core-cladding index difference) of the Er-doped and undoped core are 8.5 µm (Δn = 0.005) and 2.1 µm (Δn = 0.021), respectively. This fabricated ATC fiber has a core separation equal to 11 µm. A preliminary proof of light coupling from one core to the other is shown in Fig. 7. The amplified spontaneous emission (ASE) generated in the Er-doped core by 980-nm pumping is collected from both cores at the output end. This figure clearly illustrates a wavelength-sensitive share of the ASE. It can also be seen that the typical 1530 nm emission peak is filtered out to the undoped core output. Detailed experimental study of gain characteristics of the fabricated fiber is underway and will be reported elsewhere.

CONCLUSIONS

We have proposed an inherently gain flattened fiber amplifier using asymmetric twin core fiber in which one of the cores is doped with Er 3+ while other core is un-doped. The proposed design has a theoretical gain spectrum flattened within ~ ± 1 dB over 32 nm of bandwidth (from 1530 nm to 1562 nm). By keeping the tolerances of MCVD fabrication process in mind, we then targeted for a design having 20 dB gain with gain fluctuations within ± 1.5 dB. We then fabricated the proposed asymmetric twin core fiber by polishing and assembling two independent preforms and drawing the assembly. Preliminary characterization results show the ASE peak filtering through power coupling from Er-doped core to the undoped core. Such an intrinsically gain flattened EDFA should be attractive for reduced cost-driven transparent metro networks as it would cut down the cost on gain equalization filter head of EDFAs required for such networks.

Fig. 2 .

 2 Fig.2.Variation of effective refractive indices of individual core modes, and the super modes of the ATC fiber.

Fig. 1

 1 Fig. 1 Schematic diagram of (a) cross sectional view and (b) corresponding RIP of the twin core fiber.

Fig. 3 :

 3 Fig.3: Variation of excitation efficiencies of the fundamental and first higher-order modes when the EDF is spliced with a SMF-28 fiber.

Fig. 5 :Fig. 4 :Fig. 6 :

 546 Fig. 5: Noise figure of proposed ATC based EDFA. Noise figure (dB) 5.8 6.0

work was partially supported at the India-end by a UK-India Education and Research Initiative (UKIERI) project. BPP acknowledges support by CNRS for hosting his short visit to LPMC.