Takuya Shimajiri

Henri-Pierre Jacquot De Rouville

Valérie Heitz

Tomoyuki Akutagawa

Takanori Fukushima

Yusuke Ishigaki

Takanori Suzuki

Henri-Pierre Jacquot

Tomoyuki Aktagawa

Recognition and Reactivity by Chalcogen Bonds in Tetracyanoquinodimethanes Fused with [1,2,5]Chalcogenadiazoles

Keywords: supramolecular chemistry, host-guest systems, heterocycles, electron transfer, chalcogen bond, charge-transfer complex, overcrowded ethylene, crystal structure

Introduction

Secondary bonding interaction (SBI) has recently attracted a great deal of attention. It is of special importance in the field of crystal engineering, 1,2 which enables the prediction and regulation of crystal packing, thus contributing to the design of new solids with desired physical and chemical properties. Along with the weak hydrogen bond (WHB) which involves less acidic C-H as a hydrogen donor, 3 the chalcogen bond (ChB) 4,5 is a representative SBI that exhibits high directionality in a crystal. In an atomic array of LB ••• E-Z, where E is an electrophilic chalcogen atom and Z is an electron-withdrawing group, a Lewis base (LB) is bound to E through electron donation from n(LB) to the σ*(E-Z) orbital. Electrostatic attraction also plays an important role in ChB. Each atom has two σ-holes, which are positively charged regions that extend in the direction opposite of each E-Z bond. Thus, the strength of the ChB is influenced by several factors, including, the nature of the electron-deficient Z group attached to E, the linearity of the LB ••• E-Z atomic array, and the basicity of LB. 6 The E atom are has dramatic effects because it becomes both more polarizable and electropositive as it grows. Both of these factors lead to a deeper σ-hole, and thus to a generally stronger ChB for Se than for S, as has been demonstrated for a series of compounds. 7 [1,2,5]Chalcogenadiazoles 8 possess both an E atom and imine nitrogen atoms, and thus can form ChB via E ••• N contacts among themselves. They are versatile units for the generation of supramolecular synthons with an (E ••• N)2-square-dyad motif (Z = N, LB = N) with duplicate ChB (Scheme 1a). 9 The nitrogen atom of a cyano group is a much stronger LB than that of an imine group, and the ChB through E ••• N≡C contacts has been reported for certain inorganic materials. 10 We previously reported that short, linear E ••• N≡C contacts are quite often found in a series of tetracyanoquinodimethane (TCNQ) derivatives fused with chalcogenadiazole(s), which are characterized by the geometrical parameters shown in Scheme 1c, as in the case of WHB (Scheme 1d). In both their pristine crystals and their molecular complexes and anion radical salts, 11 the molecules are connected to each other by ChB with dual E ••• N≡C contacts as shown in Scheme 1b, which induces characteristic features and functions, such as molecular recognition or electrical conduction in the solid state. This Account describes our journey studying a family of chalcogenadiazolo-TCNQs over the decades.

Chalcogen Bonds in Crystal Structures of BTDA and its Se-analogues

Bis[1,2,5]thiadiazolotetracyanoquinodimethane (BTDA, 1) 11,12 is a π-extended analogue of TCNQ that can be prepared from the corresponding quinone 4 13 by condensation reactions with CH2(CN)2 in the presence of TiCl4 and pyridine 14,15 (Scheme 2). It undergoes reversible four-stage one-electron reduction at -0.40, -0.87, -1.59, and -2.14 V vs Fc/Fc + in MeCN. Based on the comparison of E1 red with that of the parent TCNQ (E1 red : -0.20 V), 1 is a slightly weaker acceptor, as for other TCNQ derivatives annulated with two aromatic heterocycles. 16 Thanks to the πextension, however, both singly-charged anion radical (1 -• ) and doubly-charged dianion (1 2-) were isolated as stable salts. 17 The triply-charged trianion radical (1 3-• ) could be generated electrochemically, 18 and an electron spin resonance study showed that it has a large spin density in the heterocyclic part. In contrast to other sterically deformed TCNQ derivatives fused with benzene rings, 19 1 and its anionic states adopt planar geometries.

As revealed by an X-ray analysis (250 K), a striking feature of its crystal packing is the formation of a coplanar two-dimensional (2D) sheet-like network (Figure 1), in which a molecule of 1 is surrounded by four neighbors connected by eight-fold S ••• N≡C contacts. 11,20,21 The E Its Se-analogues (TSDA, 2 and BSDA, 3), 11,22 in which one or two thiadiazole rings are replaced by selenadiazole rings, were prepared from the corresponding quinones 5 11 and 6, 13 as shown by the greater Vs,max value (+33.2 kcal mol -1 for 1 and +39.1 kcal mol -1 for 3, respectively) in their electrostatic potential maps (Figure 2). Thus stronger electrostatic attraction is expected by the Se ••• N≡C contact. Natural bond orbital analyses showed that the E ••• N≡C contact induces the donation of a lone pair of the cyano nitrogen to the antibonding E-N orbital, with a second-order perturbation stabilization of +2.05 kcal mol - 1 for 1 and +4.84 kcal mol -1 for 3, respectively.

The above X-ray analyses and DFT calculations demonstrate that bis[1,2,5]chalcogenadiazolo-TCNQs are unique electron acceptors endowed with the ability to form ChB through E ••• N≡C contacts. Despite the strong ChB in Se-analogues, further studies on the exploitation of ChB-related functions have been mainly conducted using 1, based on the higher solubility and stronger electron-accepting properties of 1 than of 2 and 3 (E1 red : -0.50 V and -0.63 V vs Fc/Fc + in MeCN, respectively).

Electronic Effects of Chalcogen Bonds in Organic Conductors of BTDA

The theoretical consideration indicated that the LUMO of BTDA (1) has the same symmetry as that of TCNQ. While there are large coefficients on cyano groups, S atoms are on the nodal plane, suggesting that stronger ChB through S ••• N≡C contact is expected for anion-radical salts due to an increase in the negative charge-density on cyano N atoms, whereas σ-holes on S atoms are not affected by one-electron reduction. Due to its adequate electron-accepting ability, 1 was readily reduced to 1 -• upon treatment with 1.5 equivalents of iodide, such as EtMe3N + I -, to give a stable crystalline salt of EtMe3N + 1 -• . 23 The X-ray analysis revealed the formation of a coplanar ribbon-like network by ChB though S ••• N≡C contacts (Figure 3a), which is just a strip-subunit of a 2D sheet-like network of pristine 1. The counter cations are incorporated between the two laterally expanded ribbon networks. The N + atom is located on the same plane as 1 -• , which maximizes the Coulombic attraction between 1 -• and the counter cation. Perpendicular to the ribbon network, a columnar π-π stack of 1 -• is formed with a short interplanar distance of 3.24 Å (Figure 3b), which would result from the orbital interaction with formation of the band structure. This salt exhibits a high electrical conductivity in the direction of π-π stacking (0.030 S cm -1 ), despite the completely filled conduction band of the salt with a cation : 1 -• molar ratio of 1 : 1 (Mott insulator). This means that the disproportionation [1 -• (mol-1) + 1 -• (mol-2) ⇌ 1 0 (mol-1) + 1 2-(mol-2); mol-1 and -2 are the neighboring molecules in the columnar stack] in the columnar stack is facilitated by the reduced on-site Coulombic repulsion in the doubly charged species due to the enlarged π-system of 1 compared to those of other TCNQ-type electron acceptors.

The anion radical salts are sometimes formed with a molar ratio other than 1:1 (e.g. 2 : 3, 1 : 2, or 2 : 5), and higher conductivities are expected for such salts of 1 -• since they have a partially filled conduction band. In those salts, the electron conduction can be induced by the formal electron exchange

[1 -• (mol-1) + 1 0 (mol- 2) ⇌ 1 0 (mol-1) + 1 -• (mol-2)].
Although such regulation of the molar ratio is usually difficult, it is easy for a series of salts of 1 -• with alkylammoniums as a counter cation, by the crystalengineering approach based on ChB through S ••• N≡C contacts. Thus, further X-ray analyses showed that above-mentioned ribbon-like networks were commonly observed in the crystals of two other salts [Et4N + 12 -• (1:2) and nBu3MeN + 12 -• (1:2)]. . Again, the counter cations are incorporated between the ribbon networks. Accordingly, for the combination of 1 -• with the larger ammonium, the π-π stacking of 1 -• could not be maintained while maintaining a molar ratio of 1 : 1. Thus, the ratio gradually changes to 2:3, 1:2, and then 2:5, with formal insertion of neutral 1 between the molecules of 1 -• in the columnar stack, upon elongation of the side chain of the alkylammonium. In this way, small ammoniums such as H4N + , Me4N + only formed 1:1 salts along with EtMe3N + , whereas Et2Me2N + and Et4N + only formed 2:3 and 1:2 salts, respectively. For the combination with a larger R4N + , only 1:2 salts were obtained for nPr4N + and nBu4N + whereas nPen4N + gave only 2:5 salt. These salts with a partially-filled conduction band are generally highly conductive [e.g., 0.32 S cm - 1 measured on a compaction pellet for nBu4N + 12 -• (1:2)]. Thus, modification of the molar ratio by ChB-based crystal engineering can be used to control the electron conduction of 1 -• salts.

It would be particularly interesting to clarify whether or not electron conduction can be induced along the ribbon network formed by ChB. Measurements performed on a single-crystalline sample of nBu3MeN + 12 -• (1:2) showed that conductivity was observed not only in the direction of π-π stacking (0.59 S cm -1 ) but also along the ribbon network expanded laterally (0.027 S cm -1 ), demonstrating the 2D electronic structure of the 1 -• salts. 23 The direction passing over the counter cation showed a resistivity 100-fold greater than that in the stacking direction. This example clearly shows that electron conduction occurs along the molecular network formed by ChB, which plays an important role in modification of the solid-state electronic structure. 24 Template for SYNLETT Thieme

Both anion radicals salt and certain solid-state charge-transfer (CT) complexes of 1, 25 such as tetraceno [5,6-cd:11,12c'd']bis [1,2]diselenole (tetraselenatetracene, TSeT) complex, exhibit high electrical conductivity. The crystal of TSeT•1 (1:1) complex is a molecular metal with partial charge-transfer of 90%, the conductivity of which increases with a decrease in temperature. X-ray structural analysis revealed the formation of segregated columnar stacks for both TSeT and 1 (interplanar distance: 3.32 Å and 3.26 Å, respectively), which are the major conduction paths in the solid. Between the columnar stacks, there are close contacts between Se atoms of TSeT and cyano groups of 1 (Figure 4a). These ChB contacts were shown to be the key factor in stabilizing the metallic state, with suppression of the metal-insulator transition (Peierls transition) even at very low temperature down to 1.5 K. 26 The Selective complexation of 1 also occurs for p-chlorotoluene among its isomers (E ox : +1.74 -+1.87 V vs Fc/Fc + in MeCN), similar to the results with xylene (+1.57 -+1.69 V). On the other hand, not only p-but also o-isomers of chloroanisoles (+1.38 -+1.48 V) form stable CT complexes with 1. For a series of methylanisoles (+1.15 -+1.23 V), all of the isomers gave crystalline CT complexes with 1. By considering that pmethylanisole•1 (1:1) and o-methylanisole•1 (1:1) have a similar crystal packing with ribbon networks via ChB through S ••• N≡C contacts (Figure 5), the enhanced CT interaction in the donoracceptor mixed columnar stack covers up the unsuitable molecular shape of o-isomer to be located in the inclusion cavity, thus concealing the recognition properties upon clathrate formation when the CT interaction between 1 and the donor increases.

Molecular Recognition by Chalcogen Bonds in Molecular Complexes of BTDA

With the use of a weaker electron acceptor 2 instead of 1, CT interaction decreases. Thus, selectivity for p-isomers was apparent for chloroanisoles and methylanisoles. On the other hand, while the much weaker acceptor 3 does not form any crystalline complexes with the disubstituted benzenes shown above, it does form such complexes with dimethoxybenzene isomers with much lower E ox values (+0.86 -+1.05 V). It was shown that molecular recognition of 1 -3 via ChB through E ••• N≡C contacts can be realized with the proper combination of electron donors to attain suitable CT interaction, because the selectivity is concealed for the donors that are too strong, whereas donors that are too weak do not form CT-type clathrates with 1 -3. Thus, while a complementary shape and size are important, in general, for host-guest-type complexation, the present clathrates are also stabilized by CT interaction, resulting in recognition not only by the shape but also by the electrondonating properties of the guest. Actually, oDV•1 (1:1) is transformed into 7oDV while maintaining its single crystallinity, 30 so that the solid-state photoinduced electron-transfer reaction could be followed by Xray analyses. The crystal structure of the as-photolyzed 7oDV is quite similar to that of oDV•1 (1:1) and exhibits ribbon networks with S ••• N≡C contacts (Figure 6b), showing that ChB can facilitate the single-crystal-to-single-crystal transformation upon topotactic transformation. Based on the fact that achiral components of 1 and oDV crystallized in a chiral space group 31 (P21), the molecular arrangement in the reacting face-to-face overlap is chiral. Although such chirality is lost upon dissolution, photoconversion resulted in a transformation of chirality of crystal packing to point chirality of the product molecule. When a certain single crystal of oDV•1 (1:1) (right-handed crystal) was photoirradiated, (R)-(+)-7oDV was obtained in 96% ee, whereas (S)-(-)-7oDV with 95% ee was obtained upon irradiation of another single crystal (left-handed crystal) which has only the mirror-imaged face-to-face overlap (Scheme 5). This photoreaction demonstrates successful absolute asymmetric synthesis, 32 in which no external chiral element is used to generate optically pure material.

Scheme 5 Absolute asymmetric synthesis of 7oDV via single-crystalline state photoinduced electron transfer reaction of oDV•1 (1:1)

Overcrowded Ethylene Composed of a BTDA Substructure

BTDA-dimer (10) 33 is a π-extended analogue with a tetracyanobiphenoquinodimethane skeleton 34 fused with four thiadiazole rings. In contrast to 1 with a planar geometry, the sterically hindered bis(tricyclic ene) structure endows 10 with unique properties as an overcrowded ethylene, 35 similar to bianthrone 11 36 which is known to be a representative thermochromic and mechanochromic material (Scheme 6). A color change of 11 upon partial isomerization from a yellow stable form with a folded geometry into a green metastable form with a twisted geometry has been postulated. However, the identity and geometrical parameters of the metastable form of 11 were not clarified for a long time since the metastable form could not be isolated until very recently by the introduction of proper substituents. 37 In contrast, the yellow folded form and violet twisted form of 10 were easily isolated and analyzed by X-ray to confirm their geometrical features, thanks to ChB through S ••• N≡C contacts.

Upon treatment of (Na + )21 2-salt with aqueous HCl, a dihydro derivative of BTDA ( 12) was obtained, 17 which was further converted to mono(dicyanomethylene) derivative ( 13) 11 (Scheme 7). Dehydrogenative dimerization using SeO2 gave 10. A solution in ClCH2CH2Cl exhibits a violet color characteristic of the twisted form while exhibiting reversible four-stage one-electron reduction (-0.30, -0.47, -1.48, and -1.63 V vs Fc/Fc + in CH2Cl2). From this solution, yellow crystals were obtained upon concentration. Crushing of these yellow crystals gave a violet color (mechanochromism), whereas a yellow color returned when the crushed violet solid was heated (thermochromism). This behavior is characteristic of an overcrowded ethylene. According to DFT calculations (M06-2X/6-31G**), the folded conformer in the gas phase is marginally more stable than the twisted form only by 2.47 kcal mol -1 .

In the crystal of the yellow folded form of 10, a 2D sheet-like network is formed by ChB through S ••• N≡C contacts (Figure 7a), which stabilizes the folded form in the pristine crystal. The observed network is similar to those in 1 and 13. For the latter, ChB through S ••• N≡C contacts were observed as well as a (S ••• N)2-square-dyad motif (Figure 8). On the other hand, the violet twisted form of 10 was isolated as a solvate upon recrystallization from CH2Cl2 or PhCN. X-ray analysis of the latter showed that the ChB network between 10 is partially broken into a 1D network to incorporate solvent molecules (Figure 7b). This is the first demonstration that a change in intermolecular interactions in a crystal allows the isolation of multiple forms of overcrowded ethylenes. 38 Scheme 6 BTDA-dimer-type overcrowded ethylene (10) and bianthrone (11) Scheme 7 Preparation of BTDA-dimer (10)

TCNQ Analogues Fused with a [1,2,5]Chalcogenadiazole

Crystal Structures of Chalcogenadiazolo-TCNQs

Tetracyanoquinodimethanes fused to only one chalcogenadiazole (14 -16) 39 were prepared from the corresponding dihalobenzochalcogenadiazoles 9g by using Pd(0)catalyzed coupling with NaCH(CN)2 (Scheme 8). These are strong electron acceptors (E1 red : -0.16, -0.26, and -0.34 V vs Fc/Fc + in MeCN for 14 -16, respectively) and form highly conductive CT complexes with some derivatives of TTF.

After we examined a series of crystal structures of BTDA (1) with ChB, which induces various functions in the solid state, we unexpectedly found that the crystal of thiadiazolo-TCNQ (15) does not exhibit ChB through S ••• N≡C contacts. The molecular arrangement is similar to that of oxadiazolo-TCNQ (14). Thus, the crystal structure is characterized by the coplanar dimeric structure (Figure 9a Cooperative control of crystal packing by ChB and WHB occurred not only for Se-containing compounds, as shown by a TCNQ derivative ( 17) 40 that fused with both thiadiazole and pyrazine ring by weakening the C-H-donating properties for WHB more than in the case of 15. Thus, 17 adopts a planar geometry and forms coplanar 2D sheet-like network, as in 1, with the formation of both a ChB-dyad and a WHB-dyad (Figure 10).

Crystal Structures of Chalcogenadiazolo-TCNNQs: E ••• N≡C Chalcogen Bond versus C-H ••• N≡C Weak Hydrogen Bond

The crystal structures shown in the previous subsection indicate that ChB through E ••• N≡C contacts and WHB through C-H ••• N≡C contacts act cooperatively to determine the crystal packing, as found in selenadiazolo-TCNQ ( 16) and in pyrazinefused derivative (17) with 2D sheet-like networks. On the other hand, WHB predominantly determines the molecular arrangement with overwhelming ChB as in the crystal of 15, suggesting that ChB and WHB could compete against each other. Based on the fact that molecular recognition by ChB 41 is still considered to be in its infancy, 4e further studies will be needed on tetracyanonaphthoquinodimethane (TCNNQ) derivatives with a non-planar structure, which may accommodate guest molecules more selectively when clathrate-type complexation occurs by ChB through E ••• N≡C contacts. Special attention has been focused on the relative importance of ChB and WHB in determining the crystal packing, and on conditions under which WHB does not interfere with, but rather cooperate with, the molecular recognition via ChB.

Thiadiazole-fused TCNNQs (18a -18c) and their Se-analogues (19a -19c) were obtained from the corresponding quinone derivatives (Scheme 9). 12,42 Regardless of the kind of fused ring, namely, benzene (a), o-xylene (b), or naphthalene (c), they all adopt a non-planar geometry due to steric repulsion between the cyano groups and the C-H groups at peri-positions. X-ray analyses were conducted to examine whether the ChB-based dyad or the WHB-based dyad is formed in 18a -18c and 19a -19c to better understand the relative importance of ChB versus WHB in these systems (Scheme 10).

Thiadiazolo-TCNNQ (18a) showed a WHB-dyad (Figure 11a), and ChB through S ••• N≡C contacts are largely suppressed. On the other hand, the corresponding Se-analogue (19a) showed only a ChB-dyad through Se ••• N≡C contacts (Figure 11b). This

Molecular Recognition by Chalcogen Bonds in TCNNQ Derivatives

Upon recrystallization of the yellow solid of selenadiazolo-TCNNQ (19a) in the presence of 2,6-dimethylnaphthalene (DMN), red crystals of 2,6-DMN•(19a)2 CT complex were obtained. 43 On the other hand, 2,7-DMN did not form the corresponding CT complex with 19a, despite the fact that 2,6-DMN and 2,7-DMN have very similar molecular shapes to form a eutectic mixture.

Since 2,6-DMN is an important material for industrial applications, 44 the observed ability of 19a to recognize DMN isomers is outstanding. 45 By a simple mixing-filtration-heating process similar to that shown in Scheme 3, pure 2,6-DMN (97.2 wt%) was obtained with very slight contamination by 2,7-DMN (1.1 wt%) by starting from a C12 liquid containing 9.7 wt% of 2,6-DMN and 9.4 wt% of 2,7-DMN. Similar experiments using the Scongener 18a gave unfruitful results. Even when we used BTDA (1) with high recognition properties toward p-xylene, pure 2,6-DMN was not obtained (hydrocarbon obtained by thermal decomposition of CT crystals: 2,6-DMN, 77.9 wt%; 2,7-DMN, 4.5%; others, 17. 6 wt%).

To better understand the higher recognition properties of 19a toward DMN, the crystal packings of pristine 19a and 2,6-DMN•(19a)2 were analyzed in detail. In the crystal of 19a, the aforementioned ChB-dyad is connected by another ChB (along the crystallographic b axis) to form a 1D dyad-ribbon network, which is further connected by WHB (along the c axis) into a 2D sheet-like network (Figure 12a). In the crystal of 2,6-DMN•(19a)2, a very similar molecular arrangement was observed. Thus, a ChB-dyad is connected by another ChB (along the crystallographic b axis) to form a dyad-ribbon network. The original WHB in pristine 19a is broken but reconnects at the shifted positions to generate a 2D sheet-like network containing inclusion cavities, in which 2,6-DMN is incorporated (Figure 12b). The 2,6-DMN in the cavity is additionally sandwiched in a π-π stacking manner with two molecules of 19a in the neighboring 2D sheet-like networks. Such a three-dimensional cavity formed by the sterically demanding non-planar electron acceptor 19a would be key for the observed outstanding molecular recognition, which also involves WHB.

Outlook

As shown above, a family of chalcogenadiazolo-TCNQs that can form ChB are promising materials for the development of novel solid-state functions. Outstanding are sorting electron-donating guest molecules upon CT complexation and photoinduced asymmetric reaction. Furthermore, two-dimensional network structure by ChB is key for the novel electric-field-controllable conductance switching of a BTDA-dimer (10) 46 and for the mixed-valence semiconducting framework of BTDA (1), 47 which have been reported recently. Both the development of new members of the BTDA family 8e,48 and a better understanding of ChB 49 should continue to expand the chemistry of this attractive class of compounds.

Funding Information

  Scheme 1 (a) (E ••• N)2-square-dyad and (b) E ••• N≡C motifs of chalcogen bond (ChB). Geometrical definition of (c) ChB and (d) WHB.

  • N contact distance [D: 3.03(1) Å, E = S] is much smaller than the sum of the van der Waals (vdW) radii of S and N (3.35 Å). The outstanding linearity of the N-S ••• N≡C atomic array is shown by the angles of N-E ••• N (φ) and E ••• N≡C (θ) [167.9(1)° and 170.7(1)°, respectively, E = S] (Scheme 1c). The laterally expanded 2D sheet-like networks are regularly stacked at the heterocyclic parts with an equal interplanar distance of 3.56 Å, which is larger than the usual π-π overlap of aromatic rings (3.40 Å). Thus, we conclude that the packing arrangement of 1 is controlled by ChB through S ••• N≡C contacts.

Figure 1

 1 Figure 1 2D sheet-like network in 1 by ChB through S ••• N≡C contacts. The distance is shown in Å.

Figure 2

 2 Figure 2 Electrostatic potentials of (a) BTDA 1 and (b) BSDA 3 calculated by DFT method (M06-2X/6-31G**) based on the crystallographic coordinates.

  as shown in Scheme 2. They adopt crystal packing isomorphous to 1, with complete positional disorder of S and Se in 2. The strongest ChB is suggested through the shorter Se ••• N≡C contacts in 3 [D: 2.94(1) Å (sum of vdW radii of Se and N: 3.45 Å), φ: 169.6(1)°, θ: °measured at 250 K]. According to DFT calculations (M06-2X/6-31G**) for 1 and 3, 21 3 has deeper σ-holes than in 1,

Figure 3

 3 Figure 3 Molecular packing in the crystal of EtMe3N + 1 -• (1:1) salt. The distances corresponding to D1 and D2 are shown in Å. (a) Top view showing the ribbon network and (b) side view showing π-π stacking. The geometrical data are as follows: D1: 3.12 Å, φ1: 149.3°, θ1: 120.2°; D2: 3.25 Å, φ2: 150.1°, θ2: 115.7°, respectively, for two sets of S ••• N≡C contacts

Figure 4

 4 Figure 4 (a) Molecular packing in the crystal of TSeT•1 (1:1) CT complex viewed along the segregating stacks. The distances corresponding to D1 and D2 are shown in Å. The geometrical data are as follows: D1: 3.08 Å; θ1: 164.7°; D2: 3.10 Å; θ2: 124.2°, respectively, for two sets of Se ••• N≡C contacts, and the separation of 3.87 Å for S ••• N≡C is too far to be considered as ChB. (b) Molecular packing in the crystal of TTF•1 (1:1) CT complex viewed along the mixed stack. The distances corresponding to D1 and D2 are shown in Å. The geometrical data are as follows: D1: 3.37 Å, φ1: 125.7°, θ1: 134.1°; D2: 3.15 Å; θ2: 104.0°, respectively, for two sets of S ••• N≡C contacts.

  respectively, for two sets of S ••• N≡C contacts). An inclusion cavities that incorporate p-xylene molecules is formed between the two ribbon networks, as similar to Figure 4b. Molecular recognition is the result of molecular shapes of o-and m-xylene that are unsuitable for inclusion in the cavity formed by ChB. Recrystallization of 1 from pure o-and m-xylene resulted in no formation of CT complexes, which demonstrates the remarkable recognition properties of 1 via ChB.

Scheme 3 Figure 6

 36 Scheme 3 Simple mixing-filtration-heating process for separation of p-xylene from the isomer mixture by using clathrate formation of BTDA (1) by ChB through S ••• N≡C contacts a)

Figure 7

 7 Figure 7 Molecular packing of BTDA-dimer (10) in the crystals of (a) the folded conformer and (b) the twisted conformer containing benzonitrile (solvate). The distances corresponding to D1 -D4 are shown in Å. The geometrical data are as follows: (a) D1: 3.30 Å, φ1: 164.4°, θ1: 165.3°; D2: 3.32 Å, φ2: 131.8°, θ2: 108.7°; D3: 3.36 Å, φ3: 126.2°, θ3: 95.3°, respectively, for three sets of S ••• N≡C contacts; (b) D1: 3.01 Å, φ1: 150.3°, θ1: 114.6°; D2: 3.08 Å, φ2: 166.8°, θ2: 119.2°, respectively, for two sets of S ••• N≡C contacts, and D3: 3.21 Å, φ3: 173.9°; D4: 3.21 Å, φ4: 171.6°, respectively, for (S ••• N)2 -square-dyad motif.

Figure 8

 8 Figure 8 2D sheet-like network in 13. The distances corresponding to D1 and D2 are shown in Å. The geometrical data are as follows: D1: 3.04 Å, φ1: 174.5°, θ1: 174.6° for the S ••• N≡C contact, and D2: 3.09 Å, φ2: 171.8° for (S ••• N)2square-dyad motif.

  ,b), which is connected by WHB through C-H ••• N≡C contacts (Scheme 1d). Such observations of the WHBdyad can be rationalized by supposing that ChB is overwhelmed by WHB in 15, especially because of the enhanced acidity of a C-H group on the quinoid sp2 carbon due to polarization. On the other hand, the dimeric structure caused by WHB is present in selenadiazolo-TCNQ (16). However, molecules are further connected by ChB through Se ••• N≡C contact as well as (Se ••• N)2-square-dyad motif to form another dimeric structure by ChB (ChB-dyad) (Figure 9c). By connecting another ChB through Se ••• N≡C contact (D: 3.02 Å, φ: 169.2°, θ: 152.4°), there is a 2D sheet-like network in 16. Thus, the stronger ChB involving a Se atom can help determine the crystal packing along with WHB.

Figure 9

 9 Scheme 8 Preparation of chalcogenadiazolo-TCNQs (14 -16) a) b)

Figure 10

 10 Figure 10 2D sheet-like network in 17. The distances corresponding to D1, D2, d1, and d2 are shown in Å. The geometrical data are as follows: D1: 3.08 Å, φ1: 171.6°, θ1: 171.6°; D2: 3.13 Å, φ2: 157.4°, θ2: 166.1°, respectively, for two sets of S ••• N≡C contacts, and d1: 2.54 Å, χ1: 127°, ω1: 112°; d2: 2.64 Å, χ2: 173°, ω2: 177°, respectively, for two sets of C-H ••• N≡C contacts.

Figure 11

 11 Scheme 9 Preparation of chalcogenadiazolo-TCNNQs(18a -18c and 19a - 19c)

Figure 12

 12 Figure 12 Crystal structures of (a) 19a and (b) 2,6-DMN•(19a)2. The distances corresponding to D, D1, D2, and d are shown in Å. The geometrical parameters are as follows: (a) D: 3.04 Å, φ: 176.6°, θ: 159.0°; d: 2.73 Å, χ: 127°, ω: 128°; (b) D1: 2.99 Å, φ1: 167.8, θ1: 159.6°; D2: 3.27 Å, φ2: 153.8, θ2: 121.2°; d: 2.72 Å, χ: 128°, ω: 146°, respectively.

Acknowledgment

This work was also supported by the Research Program of "Five-star Alliance" in "NJRC Mater. & Dev." MEXT. Y.I. and T.Sh acknowledge Toyota Riken Scholar, the Foundation for the Promotion of Ion Engineering.

Conflict of Interest

The authors declare no conflict of interest.

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

Stephane Dominguez

Graham K Westbrook

Pascal Leroy

Deep structure

Keywords: Iberia, Morocco, accretionary wedge, crustal structure, active faults

HAL is

Introduction

The Gulf of Cadiz is located offshore Southwest Iberia and Northwest Morocco just west of the Betic-Rif mountain belt and astride the Africa -Eurasia plate boundary. However, the nature and exact location of the plate boundary here is uncertain, due to a broad region of complex deformation and diffuse seismicity roughly 200 km wide, from north to south [START_REF] Sartori | Eastern segment of the Azores-Gibraltar line (central-eastern Atlantic): An oceanic plate boundary with diffuse compressional deformation[END_REF]Tortella et al., 1997;[START_REF] Jimenez-Munt | The transition from linear to diffuse plate boundary in the Azores-Gibraltar region[END_REF] (Fig. 1). Two types of geodynamic models have been proposed to explain the recent tectonics and formation of this region; models involoving delamination of continental lithosphere beneath the Betic-Rif Alboran Sea region [START_REF] Platt | Extensional collapse of thickened continental lithosphere: A working hypothesis for the Alboran Sea and Gibraltar arc[END_REF][START_REF] Calvert | Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography[END_REF], and those including subduction of oceanic lithosphere, with associated roll-back [START_REF] Lonergan | Origin of the Betic-Rif mountain belt[END_REF][START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Duggen | Magmatic evolution of the Alboran Region: The role of subduction in forming the western Mediterranean and causing the Messinian Salinity Crisis[END_REF]. Tomographic data image an east-dipping slab of cold, dense lithosphere extending continuously from the Atlantic domain of the Gulf of Cadiz to the 660 km discontinuity beneath the Alboran Sea [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Spakman | A tomographic view on Western Mediterranean Geodynamics[END_REF]. A recent study which examined the dispersion of teleseismic body waves concluded that this east dipping slab of dense lithosphere was oceanic in nature, thus supporting the subduction model and effectively ruling out the delamination model [START_REF] Bokelmann | Mantle structure under Gibraltar constrained by seismic waveform complexity[END_REF].

Seismic reflection and refraction profiles image a gently eastward dipping basement and eastward dipping decollement beneath a thick pile (>6 km) of deformed sediments which was interpreted to be an accretionary wedge, related the east-dipping subduction zone beneath the Cadiz-Gibraltar region [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF]Thiebot and Gutscher, 2006). GPS data reveal the presence of a west to southwestward moving tectonic block in the Betic-Rif-Alboran region [START_REF] Stich | Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake : Implications for the 1755 Lisbon earthquake[END_REF]Tahayt et al., 2008), which has been alternatively interpreted as being related to subduction roll-back [START_REF] Fernandes | Surface velocity field of the Ibero-Maghrebian segment of the Eurasia-Nubia plate boundary[END_REF] or as roll-back linked to delamination [START_REF] Fadil | Active tectonics of the western Mediterranean: evidence for roll-back of a delaminated subcontinental lithospheric slab beneath the Rif Mountains Morocco[END_REF].

The Gulf of Cadiz region has experienced strong instrumental earthquakes like the 1969 M7.9 Cape St. Vincent earthquake [START_REF] Fukao | Thrust faulting at a lithospheric plate boundary: The Portugal earthquake of 1969[END_REF], the 1964 M6.5 Huelva earthquake and the recent 12 Feb. 2007 M6.0 Cape St. Vincent earthquake [START_REF] Stich | Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake : Implications for the 1755 Lisbon earthquake[END_REF]). The strongest earthquake in European history struck offshore SW Iberia in 1755, the M8.7 Great Lisbon earthquake [START_REF] Martinez-Solares | Isoseismal map of the 1755 Lisbon earthquake obtained from Spanish data[END_REF][START_REF] Johnston | Seismic moment assessment of earthquakes in stable continental regions -III[END_REF], though the location of the source remains controversial [START_REF] Zitellini | Source of 1755 Lisbon earthquake and tsunami investigated[END_REF]Terrinha et al., 2003;Gracia et al., 2003a;[START_REF] Gutscher | What caused the Great Lisbon Earthquake?[END_REF]. During the past decade, many marine geophysical studies have been performed in the region in an effort to identify the source region of the 1755 earthquake and to obtain better estimates on the long-term frequency of such high magnitude events. An accurate inventory of the active faults in the area can make a significant contribution to improving long-term seismic hazard assessments for the region. The purpose of this study is to present the state of the art on the deep crustal structure of the region as well as examine the shallow morphology. These observations will be related to the recent and ongoing tectonics in the area in an effort to answer the question whether subduction is still active and poses significant seismic hazard to the region.

Data Seafloor bathymetry

As a result of numerous marine geophysical surveys performed since the year 2000, the majority of the Gulf of Cadiz region has been covered by multi-beam swath-mapping. The bathymetric compilation presented here (Fig. 2) incorporates data from the following surveys; Cadisar, Cadisar2, TV-GIB and DelSis (all using the R/V Suroit EM300 system), the Delila and MatesPro cruises (with the NRP Dom Carlos EM120 system), the GAP cruise (R/V Sonne EM120 system) and a transit performed by the R/V Marion Dufresne following the Microsystems Cruise. The Marion Dufresne data were processed by X. Morin. Additional bathymetric data were provided by D.

Masson and C. Berndt from the 2006 Hermes Cruise onboard the R/V Charles Darwin. The R/V Suroit data from the Cadisar [START_REF] Mulder | The Gulf of Cadiz: an unstable giant contouritic levee[END_REF] and Cadisar2 [START_REF] Mulder | The western part of the Gulf of Cadiz: contour currents and turbidity currents interactions[END_REF] cruises were processed with Caraïbes software (developed by Ifremer) and typically have a spatial resolution better than 30 m (in water depths < 2000 m). The MatesPro data (Terrinha et al., in press) were processed by the Portuguese Instituto Hidrografico Team. The GAP bathymetric data (Kopf et al., 2003) in the SE portion of the Gulf of Cadiz were processed using Caraïbes software and included in this compilation for areas not mapped by the Cadisar or Delila surveys.

The bathymetric map (Fig. 2A) reveals several distinct morphological provinces. These include, the flat and nearly featureless abyssal plains, bounded by generally E-W to N60E trending basement highs and by the adjacent continental margins. The southern Portuguese (Algarve) margin is steep and deeply incised by S to SW draining canyons. The SW Spanish margin is much flatter and marked by diverse sedimentary structures, (a contourite levee with accompanying channels, contourite drift, seafloor sand-dunes, etc.) formed as the Mediterranean Outflow Water locally erodes and redeposits large quantities of sediment [START_REF] Mulder | The Gulf of Cadiz: an unstable giant contouritic levee[END_REF][START_REF] Pinheiro | MV Seis: Tectonic control, deep crustal structure and fluid escape pathways in the Gulf of Cadiz Mud Volcano Field[END_REF]. The most prominent feature however, is a region of gently undulating morphology with elongate curvi-linear ridges and troughs (Fig. 2B). This gently inclined slope (1° westward dip on average) forms the surface of a sediment pile with a relatively high rugosity (compared to the abyssal plain regions). The deformed sedimentary slope is bounded by a continuous, horseshoe-shaped deformation front between 300 and 400 km in total length. The southern boundary is particularly sharp, and forms an E-W lineament off the Moroccan margin at the northern limit of the Rharb submarine valley.

Seismic images of the deformation front

Deep sounding seismic data have been acquired in the Gulf of Cadiz imaging the deformation front and internal portions of the deformed sediment slope, in particular the Sismar 360-channel seismic data acquired by the R/V Nadir in April 2001 (Gutscher et al., 2002, Thiebot andGutscher, 2006) and 24-channel seismic data acquired by the R/V Suroit in April 2005 during the DelSis cruise [START_REF] Gutscher | Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: observations from multi-beam bathymetry and seismic profiling[END_REF]. These data image the sediment slope to be an eastward thickening pile of sediment, with strong internal deformation, overlying a region of sub-horizontal to gently east dipping reflectors (separated by a thin decollement layer). West vergent ramp thrusts are observed soling out to this decollement. The seismic data clearly demonstrate that the base of the accretionary wedge (the decollement and top basement reflectors) is eastward dipping and therefore that the wedge has a primarily tectonic origin [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF] and is not a large-scale gravitational feature. At a more shallow level (<2000m water depth in the easternmost portion of the wedge) some superficial gravity sliding is occurring, likely along rheologically weak layers (highly overpressured shales or evaporates) [START_REF] Maestro | Large-scale slope failure involving Triassic and Middle Miocene salt and shale in the Gulf of Cadiz[END_REF][START_REF] Medialdea | Structure and evolution of the ''Olistostrome'' complex of the Gibraltar Arc in the Gulf of Cadiz (eastern Central Atlantic): evidence from two long seismic cross-sections[END_REF][START_REF] Gutscher | Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: observations from multi-beam bathymetry and seismic profiling[END_REF]. The majority of the accretionary wedge was constructed by offscraping of deep sea sediments during Miocene (and earlier) westward motion of the Gibraltar arc (Thiebot and Gutscher, 2006;[START_REF] Iribarren | The structure of the Atlantic-Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia -Africa plate boundary)[END_REF].

There is still ongoing debate as to the current day activity of the system. Deep seismic profiles BS1 and GC3 on the SW Spanish margin image deformation of shallow sedimentary layers to the SW of the Guadalquivir river (due W of the city of Cadiz) (Gracia et al., 2003b) at the NE lateral boundary of the accretionary wedge. Here we present some high resolution Chirp profiles acquired by the R/V Suroit during the DelSis cruise which document recent deformation in the uppermost sedimentary layers at the western and southeastern lateral boundaries of the accretionary wedge.

Chirp profile 19 crosses the NW deformation front at the eastern termination of the Horseshoe abyssal plain (Fig. 3A). The highly reflective, sub-horizontal turbiditic sediments, associated with the abyssal plain can be clearly seen to the NW. The nature of the sediments was confirmed by gravity coring at the NW end of the profile, which revealed alternating fine clay hemipelagic sediments, and turbiditic sequences grading downwards from fine to coarse grained (sandy) layers (Babbonneau, et al., 2006). There is an abrupt change in slope at the deformation front and further SE the sedimentary strata are less reflective and gently folded, rising to form an undulating slope on the sea-floor. The sediment thickness varies between the minor sub-basins in the troughs and on the crests of the anticlinal ridges, suggesting a syn-tectonic sedimentation pattern (Fig. 3A). Some of the best structural images were obtained from the shallow water Eastern Rharb Valley (<1600 m depth). Two spectacular images of surface and shallow subsurface deformation at the seafloor are shown below crossing the northern edge of the Rharb marine valley (the southern limit of the accretionary wedge) (Figs. 3 B,C). The former is in water depths of roughly 500-600 m and shows folds associated with faults which emerge at the seafloor (Fig. 3B). The latter is on the shallow NW Moroccan continental platform in water depths between 100 m and 150 m. Here subvertical faults can be seen, most of which are truncated by a major horizontal unconformity. This unconformity is at about 120 m water depth and corresponds to the sea-level lowstand during the last glacial maximum (LGM) at about 20 ka. The LGM unconformity is gently folded in places and slightly offset by a fault, indicating recent deformation (post dating 20 ka) (Fig. 3C). This provides definitive proof that active deformation occurs on the SE flank of the accretionary wedge.

A 24-channel seismic profile acquired by the R/V Suroit (DelSis cruise) in April 2005 crosses the entire Rharb submarine valley at 2800 m water depth and provides additional constraints on the timing of deformation on the southern flank of the accretionary wedge (Fig. 4). At the base a thrust anticline is imaged, which may represents the early Miocene thrust wedge identified by other workers [START_REF] Iribarren | The structure of the Atlantic-Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia -Africa plate boundary)[END_REF]. The sequence containing the anticline is overlain by a chaotic facies (the Tortonian Olistostrome) and by horizontal valley fill. The surface of the chaotic facies is deformed, eroded and then sealed by the valley fill. The wedge of the chaotic facies follows the shape of the underlying anticline, and so was either folded with the underlying strata, placing the age of the folding as Tortonian or later, or pushed as a wedge across the anticline. The former seems more likely. The chaotic facies does not pond in the synclinal trough, and so was not deposited in this location after the fold was formed. The NE portion of the chaotic facies is more strongly deformed and uplifted along a NE dipping thrust fault, marking the edge of the active accretionary wedge. This profile thus reveals a sequence of several events; 1) an earlier episode of SW vergent thrusting onto the toe of the adjacent Moroccan margin (presumably until the mid-Miocene), 2) deposition of a NE-ward thickening chaotic sedimentary unit (the Tortonian Olistostrome), 3) deformation of the chaotic facies and thickening, 4) deposition of well stratified horizontal valley fill, sealing the early Miocene wedge and the chaotic facies unit in the Rharb Valley, 5) tectonic reactivation of the chaotic facies along a SW vergent ramp thrust, causing uplift and deformation of the chaotic facies unit. This ramp thrust marks the limit of the active accretionary wedge and its expression at the seafloor coincides with the E-W oriented lineament at the northern limit of the Rharb Valley. This profile clearly shows that the lineament is tectonic in origin and not purely an erosional feature.

Structural Maps (basement, sediment, Moho and crustal maps)

All available seismic profiles (multi-channel reflection, as well as wide angle refraction) were used to construct 3-D grids of the crustal structure in the Gulf of Cadiz area (Fig. 5). The principle horizons picked were, the sediment-basement interface and the Moho, with values in two-way way travel time (TWT) sampled every 10 km along the seismic reflection profiles. The TWT to the top basement reflection was converted to depth using a mean sedimentary velocity of 3 km/ and basement thickness was likewise converted from TWT to depth using a mean velocity of 6 km/s. Depth sections were available for some seismic profiles, with ocean-bottom seismometer (OBS) data; lines SIS-4 [START_REF] Contrucci | The crustal structure of the NW-Moroccan continental margin from wideangle and reflection seismic data[END_REF], SIS-16 [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF] and/or where pre-stack depth migration was performed; SIS-22 (Thiebot and Gutscher, 2006) and offered an independent calibration of depth to basement and depth to Moho. On land, depth to basement was taken from published geological maps, which included borehole data in the Guadalquivir and Rharb Basins. All the x,y,z depth to basement points were converted into a depth-to basement grid using the GMT gridding algorithm surface (Wessel and Smith, 1991). In similar fashion all the x,y,z depth to Moho points obtained from the seismic profiles, were taken together with a few values from published Moho maps of NW Africa or the SW Iberia region, and gridded to obtain a 3-D surface to the Moho. Together with the depth to sea-floor grid (obtained from multi-beam bathymetry), three 3-D surfaces were available across the study area. The depth to basement map is shown in Fig. 5A. By taking the difference between the bathymetry and depth to basement grids, it was possible to obtain a sediment thickness map (Fig. 5B).

The depth to Moho map is also shown (Fig. 5C), and likewise, by taking the difference between the depth to basement and the depth to Moho maps, it was possible to construct a map of total crustal thickness map (Fig. 5D).

The depth to basement map (Fig. 5A) shows a roughly E-W oriented trough, in the Gulf of Cadiz. On the SW Iberian margin and the NW Moroccan margin, depth to basement is typically about 4-5 km and decreases to 0.5-2 km near the coast. Below the adjacent abyssal plains, the depth to basement ranges from about 8-10 km. Within the E-W trough, there is a gradual deepening towards the east, reaching maximum values of 13-14 km. The region of greatest basement depth terminates to the east in a crescent shaped trough, sub-parallel to the Betic-Rif mountain belt. This trough represents the flexural foreland basin caused by the construction of the orogen. It is noteworthy however, that the depth to basement only reaches values of 2-3 km onland (Guadalquivir basin in SW Spain and Rharb Basin in NW Morocco which both overly continental crust), whereas the depth to basement in the Gulf of Cadiz (likely floored by oceanic crust) reaches 12-14 km.

The total sediment thickness map shows similarities to the basement map; a thick, generally E-W oriented sedimentary basin, terminating in a crescent shaped basin in the foreland of the Betic and Rif mountain belts (Fig. 5B). The main difference however, is that while the crescent shaped foreland basin never exceeds values of 3-4 km on land, the basin centered within the Gulf of Cadiz is more circular and the maximum thickness reaches values of 12-13 km. Again this maximum thickness is located on the central to eastern side of the Gulf of Cadiz. It is possible that sediment thicknesses may increase further east, however, no seismic data are available here (between 7°W and 6°W). The trends observed here are similar to those described in a recent study of sedimentary thicknesses [START_REF] Iribarren | The structure of the Atlantic-Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia -Africa plate boundary)[END_REF] where a maximum of 12 km was reported in the central Gulf.

The regional Moho map reveals four distinct domains, the Iberian and Moroccan continental domains (with a Moho depth of 28-38 km), an Atlantic domain (depths of 12-16 km) and the Alboran domain (with depths of 8-16 km in the East Alboran Sea and 18-20 km in the West Alboran Sea) (Fig. 5C). Moho depth is greatest beneath the Betic Mountains (34-38 km), where a thick crustal root compensates the highest mountains of the Sierra Nevada (3700 m). A more modest, crescent shaped crustal root appears to underlie the western Betics, the straits of Gibraltar and the SW edge of the Rif mountains (with Moho depths of about 32 km), although these regions are not well constrained by wide-angle seismic data. These results are in good agreement with the Moho depth values reported in a recent regional lithospheric study [START_REF] Fullea | A rapid method to map the crustal and lithospheric thickness using elevation, geoid anomaly and thermal analysis. Application to the Gibraltar Arc System, Atlas Mountains and adjacent zones[END_REF].

The crustal thickness map (Fig. 5D) is similar to the Moho map, but the Gulf of Cadiz region appears much more clearly as an E-W corridor of thin crust, with values of 6-10 km. The NW corner of the African continent appears clearly as an angular promontory, with a N30°E oriented Atlantic margin to the west, and a roughly N80°E oriented "Cadiz margin" to the north. The Alboran region also changes in appearance, with a greater similarity between the western and eastern Alboran domains (crustal thicknesses of 6-10 km separated by a 16 km thick, NE-SW oriented ridge, passing through Alboran Island.

Interpretation and regional lithospheric structure

As the 3-D geometry of the lithosphere in the Southern Iberia region is rather complex, the results of the crustal maps above will now be discussed within the regional geodynamic framework. A narrow east-dipping slab of dense lithosphere has been imaged in tomographic cross-sections [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Spakman | A tomographic view on Western Mediterranean Geodynamics[END_REF] The deep geometry in horizontal however, indicates an asymmetric curved shaped body at depth, elongated parallel to the Betics sections (Wortel and Spakman, 2000;Spakmand and Wortel, 2004) (Figure 6). This high p-wave velocity body at depth, which represents a curved slab of dense lithosphere is the locus of the majority of the intermediate-depth seismicity beneath the West Alboran suggesting it is a slab of oceanic lithosphere, This is confirmed by the dispersion of teleseismic waves passing through this body [START_REF] Bokelmann | Mantle structure under Gibraltar constrained by seismic waveform complexity[END_REF]. The presence of a narrow corridor of oceanic lithosphere beneath the western and central Gulf of Cadiz is suggested by the presence of generally E-W oriented magnetic anomalies here (Verhoef et al., 1991) and by the very low crustal thickness observed in the crustal thickness map.

Additional seismic studies of the crustal types here would be necessary to test this interpretation, which if confirmed would further support the subduction model. It is also instructive to note the numerous sub-parallel arcuate features in the Gibraltar region at depth and at the surface. These include: the high p-wave velocity body (at 200 km depth beneath the Alboran Sea); the distribution of intermediate earthquakes (60-120 km depth); the sedimentary trough in the West Alboran Sea (as expressed by the negative gravity anomaly and representing the low density sedimentary fill in the arcuate back-arc basin); the surface relief of the Betic and Rif mountain chain (related to the fold and thrust belt which constructed the orogen). West of Gibraltar additional arc shaped features include the gravity minimum in the the Gulf of Cadiz (related to the great sedimentary thickness here and which also has an E-W axis of symmetry) and finally much further west the roughly horseshoe shaped deformation front of the accretionary wedge.

It seems inescapable that 6 independent sub-parallel arcuate shaped structures at varying depths and expressed in completely independent data sets must be genetically linked. The only single process, which can explain all these observations, is east-dipping subduction of a narrow slab of oceanic lithosphere (but which is curved at depth) as already suggested by earlier authors [START_REF] Lonergan | Origin of the Betic-Rif mountain belt[END_REF].

This lithospheric structure is expressed schematically by two cross-sections (Fig. 7). The longer E-W section crossing Gibraltar passes along the axis of symmetry of the curved subduction system. A 2-D density model of this section (constructed on the basis of seismic refraction p-wave velocities) can successfully predict the observed free-air gravity anomaly along this profile (Thiebot and Gutscher, 2006). The main features are; the deep sedimentary trough in the West Alboran Sea, the thick sedimentary wedge in the Gulf of Cadiz and the east-dipping basement beneath this accretionary wedge. The detailed crustal structure beneath the Rif-Betic mountain belt (near the Straits of Gibraltar) is poorly constrained and somewhat speculative. The N-S cross-section passes along an existing deep seismic sounding profile (Sismar-13) and suggests a corridor of thin crust underlying the accretionary wedge, which may be oceanic in nature. However, more detailed analysis of OBS data will be necessary before this can be conclusively determined.

Summing up, these structural data from the Gulf of Cadiz clearly reveal the presence of an eastward thickening wedge of sediments, situated in an E-W oriented trough, underlain by a thin (6-10 km) crust with a gentle eastward dip. This geometry in confirmed by other regional studies and the current consensus is that this thick accumulation of sediments is an accretionary wedge formed by the westward motion of the Gibraltar arc until at least the Miocene [START_REF] Iribarren | The structure of the Atlantic-Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia -Africa plate boundary)[END_REF]. The questions, which remain unresolved, are: is deformation continuing today in this tectonic wedge? And if so is it due to continuing subduction rollback (inducing E-W compression) or could it be caused by the overall NW-SE convergence of Africa and Eurasia? In order to address these questions analog modeling of accretionary wedge growth and subsequent collision with a rigid indenter was performed.

Analog modeling of accretion and an oblique indenter

An accretionary wedge sliding along a basal detachment (decollement) can be considered to be a critical Coulomb wedge, with material added to the front of the wedge as a series of imbricate thrust slices [START_REF] Davis | Mechanics of fold and thrust belts and accretionary wedges[END_REF]. The wedge also deforms internally (along out-of-sequence thrusts and back-thrusts) to maintain a surface profile, which is a function of the basal friction and the internal friction of the material. The sum of the basal slope and surface slope is known as the wedge taper and for regular accretion at the front, a constant wedge taper is maintained. This notion is called critical wedge theory and predicts self-similar behavior, independent of scale [START_REF] Davis | Mechanics of fold and thrust belts and accretionary wedges[END_REF]. Therefore, the mechanical behavior of thrust wedges can be modeled (at a smaller scale) using granular materials and successfully reproduces a variety of accretionary processes (frontal accretion, underthrusting and underplating) observed in accretionary wedges worldwide [START_REF] Lallemand | Coulomb theory applied to accretionary and nonaccretionary wedges: Possible causes for tectonic erosion and/or frontal accretion[END_REF]Gutscher et al., 1998a,b). The impact of subducting asperities (seamounts or linear ridges) can also be modeled using the same approach [START_REF] Dominguez | Upper plate deformation associated with seamount subduction[END_REF].

This approach was applied to study the deformation of the Cadiz accretionary wedge and the impact of an oblique ridge on the deformation front. However, since the geometry of the Gulf of Cadiz and Coral Patch Ridge are highly 3-D, the experimental apparatus selected was a 3-D table (Fig. 8), rather than the more commonly used glass-sided 2-D box. A 15mm thick layer of granular materials were deposited on a sheet of mylar film. These materials included 4 mm of glass beads at the base (to obtain the lowest possible basal friction), 10 mm of sand and 1mm of a fine colored cohesive powder to better preserve the fine structure of deformation at the surface. The mylar film and overlying granular layer was pulled beneath a rigid, curved, semi-circular backstop. The shape of the rigid experimental backstop (Fig. 8, Fig. 9 -0cm) was chosen to represent the overall curvature of the Betic and Rif mountain belts, which form the orogenic core or "backstop" of the subduction system (Fig. 1, Fig. 6). Pulling the film beneath the backstop is kinematically equivalent to pushing the backstop over the film, which represents the westward motion of the Rif-Betic block which led to the construction of the accretionary wedge [START_REF] Lonergan | Origin of the Betic-Rif mountain belt[END_REF][START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Iribarren | The structure of the Atlantic-Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia -Africa plate boundary)[END_REF].

During the initial phase of the experiment (Fig. 9 12-25 cm) frontal accretion occurs with imbrication of a series of thrust slices at a fairly regular spacing. These develop in a curved manner, in response to the backstop shape and a horseshoe-shaped deformation front is rapidly established (Fig. 6 37-50 cm). Next, the rigid oblique ridge approaches the horseshoe-shaped accretionary wedge (Fig. 6 -63 cm). As the oblique ridge begins to collide with the wedge, the propagation of frontal thrusts is blocked, and the deformation front steepens here, while it is free to continue advancing on the left (Fig. 9 -76 cm). In the final stage the deformation front has become asymmetric, with a marked embayment at the point of impact, and a steeper slope here where the indenter has caused underthrusting and some local sliding (frontal erosion) (Fig. 9

-89 cm).

There are some significant differences in the overall geometry of the modeled thrust wedge with respect to the natural example. The submarine accretionary wedge is more elongate (roughly 200 km E-W and 150 km N-S) and has a shallower surface slope and wedge taper (1° surface slope, with a 2° mean basal dip and thus 3° wedge taper) than the model wedge (N-S roughly 100 cm, E-W roughly 50 cm, surface slope 6°, basal dip = 0° and thus 6° wedge taper). This appears to be a limitation of the dry granular materials used which are stronger (have a higher coefficient of internal friction) than underconsolidated and often overpressured marine sediments. In particular fluid overpressures likely present within the decollement layer, are not reproduced adequately and the ensuing basal friction is thus too high in the model wedge. The accretionary wedge may also contain evaporates which can exhibit some of the weakest rheologies known [START_REF] Gutscher | Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: observations from multi-beam bathymetry and seismic profiling[END_REF]. The higher basal friction in the model wedge results in a higher taper and shorter length [START_REF] Davis | Mechanics of fold and thrust belts and accretionary wedges[END_REF][START_REF] Lallemand | Coulomb theory applied to accretionary and nonaccretionary wedges: Possible causes for tectonic erosion and/or frontal accretion[END_REF], Gutscher et al., 1998b). The submarine wedge is much thinner and much longer. Despite these limitations, the final stage of the experiment with the oversteepened deformation front, and the curved and deflected anticlinal thrust ridges on the accretionary wedge (even locally overriding the ridge itself) (Fig. 10A) is highly reminiscent of the 3-D geometry of the seafloor at the point of impact between Coral Patch Ridge and the deformation front (Fig. 10B). In the natural example the deformation front has clearly advanced further to the north (to the left) and has a gentler surface slope here as well. The asymmetry of the deformation front, with the embayment south of the indenter clearly indicates the relative sense of motion of the backstop with respect to the indenter (equivalent to SW to WSW for the Gulf of Cadiz).

If the primary direction of tectonic shortening pushing the accretionary wedge into the indenter were towards the NW (Africa -Iberia motion), then the embayment would appear on the northern side of the Coral Patch Ridge. The analog model thus unequivocally demonstrates a predominantly WSW compressional motion of the Rif-Betic backstop towards the seafloor in the Horseshoe and Seine abyssal plain region. This tectonic motion (of the Rif -Betic block) is also borne out by GPS data from the region [START_REF] Fernandes | Surface velocity field of the Ibero-Maghrebian segment of the Eurasia-Nubia plate boundary[END_REF][START_REF] Stich | Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake : Implications for the 1755 Lisbon earthquake[END_REF]Tahayt et al., 2008). As the seafloor morphology (imaged by multi-beam bathymetry) and shallow sediment deformation imaged by highresolution seismic profiles at the intersection of the indenter and the accretionary wedge are all very recent [START_REF] Gutscher | Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: observations from multi-beam bathymetry and seismic profiling[END_REF], this demonstrates that the subduction rollback mechanism continues to drive tectonic deformation today. Understanding the plate tectonic forces active today in the region is of primary importance for a proper assessment of the modern day seismic hazard. The first and most important step in this effort is trying to identify the source region of the strongest earthquake in European history the Great Lisbon earthquake of 1755.

Implication for the 1755 earthquake Numerous candidate sources have been proposed for the 1755 earthquake; Gorringe Bank, Marquis de Pombal and the Horseshoe fault (Fig. 1). Following the M7.9 St. Vincent earthquake in 1969, which occurred on the Horseshoe Fault in the Horseshoe abyssal plain, some authors proposed this to be the source of the 1755 earthquake as well [START_REF] Fukao | Thrust faulting at a lithospheric plate boundary: The Portugal earthquake of 1969[END_REF]. The fault plane solution indicates conjugate N60° striking, 45° dipping fault planes with a reverse sense of motion. Other authors proposed the nearby Gorringe Bank, an uplifted flake of oceanic crust (rising to a water depth of 25 m), to be the source [START_REF] Johnston | Seismic moment assessment of earthquakes in stable continental regions -III[END_REF]. Seismic profiles reveal steep scarps and deformed sediments suggestive of recent thrust deformation here [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] Hayward | A seismic reflection and GLORIA study of compressional deformation in the Gorringe Bank region, eastern North Atlantic[END_REF]. The potential dimensions of a lithospheric scale thrust fault for Gorringe Bank are about 200 km x 80 km (for a 50 km thick elastic lithosphere, and 40° fault dip). Using a very high rigidity value of 6.5 x 10 10 Pa (for a fault entirely in oceanic lithosphere), a magnitude of 8.7 was calculated for a uniform slip of 12 m [START_REF] Johnston | Seismic moment assessment of earthquakes in stable continental regions -III[END_REF]. For a more typical rigidity value of about 3 x 10 10 Pa, a uniform slip of 24 m would be required to produce an M8.7 event.

More recently, authors have focussed on the Marquis de Pombal, a basement high off the SW corner of Portugal [START_REF] Zitellini | Source of 1755 Lisbon earthquake and tsunami investigated[END_REF]Gracia et al., 2003a). Tsunami modeling suggests a source located at the Marquis de Pombal structure can provide a reasonable fit for arrival time for most stations, though discrepancies exist for the Gulf of Cadiz and NW Morroccan coast [START_REF] Baptista | Constraints on the source of the 1755 Lisbon tsunami inferred from numerical modelling of historical data on the source of the 1755 Lisbon tsunami[END_REF]. The primary objection to the Marquis de Pombal source, however, is that the dimensions of this structure (about 100 km x 70 km) are too modest to produce an earthquake of M>8.5. A calculation using a uniform slip of 15 m, yields only M=8.3 [START_REF] Zitellini | Source of 1755 Lisbon earthquake and tsunami investigated[END_REF]. An event of this magnitude would not be felt in Hamburg or the Cape Verde Islands. Indeed, an empirical relation has been established between earthquake magnitude and the rupture area of the fault on the basis of earthquakes M≤8 (Fig. 11) (Wells and Coppersmith, 1994). The relation also holds true when great subduction earthquakes of the past 100 years are added. Thus, according to this relation, the source region of an M8.7 earthquake should be over 30,000 km2 in area (roughly 150 km x 200 km) (Fig. 11). Of the known tectonic structures off SW Iberia, only the subduction fault plane (and to a lesser extent Gorringe Bank) has sufficiently large dimensions (Fig. 11).

Finally, some authors have proposed combining a series of faults in a chain, with rupture along one segment triggering rupture along the following segment, and thus attempted to explain the large seismic moment (Gracia et al., 2003;[START_REF] Baptista | New study of the 1755 earthquake source based on multi-channel seismic survey data and tsunami modeling[END_REF][START_REF] Stich | Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake : Implications for the 1755 Lisbon earthquake[END_REF]. However, even after adding two such segments together (e.g. -the Marquis de Pombal and Horseshoe faults) the surface area is still far too small to generate the appropriate seismic moment (Fig. 11). Three such faults are also insufficient. And a scenario where four (or more) small faults rupture in rapid succession becomes increasingly unlikely and untenable as a hypothesis.

Conclusions

Morpho-bathymetric data from the Gulf of Cadiz image a rugous sea-floor slope with curvilinear ridges and troughs comprising an eastward thickening wedge of tectonically deformed sediments. Deep crustal structural maps are presented which demonstrate that sediment thicknesses in the central Gulf of Cadiz reach a maximum of 12-13 km. The basement beneath the western and central Gulf is generally between 7 and 10 km thick and forms a roughly E-W oriented trough between the thicker (20-30 km) continental crust of SW Iberia and NW Morocco. The thick tectonically deformed sedimentary wedge is an accretionary wedge formed by the W to SW migration of the Rif-Betic block. The frontal (western) and lateral (south-eastern) boundaries of the accretionary wedge show signs of recent deformation. Ongoing hydrological activity of the accretionary wedge is indicated by the presence of abundant mud volcanoes [START_REF] Pinheiro | Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise[END_REF][START_REF] Pinheiro | MV Seis: Tectonic control, deep crustal structure and fluid escape pathways in the Gulf of Cadiz Mud Volcano Field[END_REF] as well as the geochemistry of fluids emanating from these mud volcanoes, which indicate an origin due to clay mineral dehydration processes, typical of accretionary wedges [START_REF] Hensen | Sources of mud volcano fluids in the Gulf of Cadiz -indications for hydrothermal input[END_REF]. Eastward diminishing heatflow as observed on the accretionary wedge also requires active underthrusting (subduction) at rates ≥5mm/yr [START_REF] Grevemeyer | Heat flow anomalies in the Gulf of Cadiz and off Cape San Vicente, Portugal[END_REF]. All of these data are consistent with ongoing subduction.

The formation and evolution of the accretionary wedge is investigated by analog modeling.

The development of a curved, horseshoe-shaped deformation front is reproduced by accretion against a curved rigid backstop (representing the orogenic core of the Rif and Betic Mountain belts). The collision of an oblique trending linear ridge with the accretionary wedge is modeled as well. The impact is an asymmetric deformation front with an embayment (to the south) marked by a steeper surface slope. These morphological features successfully reproduce the seafloor morphology imaged by the bathymetric data and provide kinematic constraints on the relative shortening between the Rif-Betic block and the oceanic abyssal plains in the Atlantic domain to the west. These demonstrate a WSW directed relative convergence direction, driven by subduction roll-back. The tectonic activity of the subduction system has a strong impact on the seismic hazard assessment for the region. Indeed, the shallow east-dipping subduction fault plane has the appropriate dimensions (180 km N-S x 210 km E-W) to generate an earthquake of magnitude 8.6 for 10 m of uniform co-seismic slip. The subduction fault plane is the only single tectonic structure in the region with sufficiently large dimensions to generate such a great earthquake. Stich, D., Serpelloni, E., Mancilla, F.d.L., Morales, J., 2007. Kinematics of the Iberia -Maghreb plate contact from seismic moment tensors and GPS observations. Tectonophysics, v. 426, 295-317, doi:10.1016/j.tecto.2006.08.004 Tahayt, A., Mourabit, T., Rigo, A., Feigl, K.L., Fadil, A., McClusky, S., Reilinger, R., Serroukh, M., Ouazzani-Touhami, A., Ben-Sari, D., and Vernant, P., 2008. Mouvements actuels des blocs tectoniques dans l'arc Bético-Rifain à partir des mesures GPS entre 1999[START_REF] Lay | The Great Sumatra-Andaman earthquake of 26 December 2004[END_REF]. Comptes Rendus Geoscience, 340, 400-413. Terrinha, P., Pinheiro, L.M., Henriet, J.-P., Matias, L., Ivanov, A.K., Monteiro, J.H., Akhmetzhanov, A., Volkonskaya, Cunha, M.R., Shaskin, P., Rovere, M., 2003. Tsunamigenic-seismogenic structures, neotectonics, sedimentary processes and slope instability on the Southwest Portuguese Margin. Marine Geology 195, 1-19. Terrinha, P., Matias, L., Vicente, J., Duarte, J., Luís, J., Pinheiro, L., Lourenço, N., Diez, S., Rosas, F., Magalhães, V., Valadares, V., Zitellini, N., Roque, C., Mendes Víctor, L., and MATESPRO Team, (in press). Strain partitioning and morpho-tectonics at the Iberia-Africa plate boundary from multibeam and seismic reflection data. Marine Geology, (in press).

Thiebot, E., and Gutscher, M.-A., 2006. The Gibraltar Arc seismogenic zone (part1): constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic data, gravity and thermal modeling. Tectonophysics Sp. Vol. "Natural laboratories on seismogenic faults ", v. 427, p. 135-152, doi:10.1016/j.tecto.2006.02.024. Torelli, L., Sartori, R., and Zitellini, N., 1997. The giant chaotic body in the Atlantic off Gibraltar: new results from a deep seismic reflection survey: Marine and Petroleum Geology, 14, 125-138. Tortella, D., Torne, M, and Perez-Estaun, A., 1997. Geodynamic evolution of the eastern segment of the Azores-Gibraltar Zone: the Gorringe Bank and Gulf of Cadiz region, Marine Geophys. Res. 19, 211-230. van der Meijde, M., van der Lee, S. and Giardini, D., 2003. Crustal structure beneath broad-band seismic stations in the Mediterranean region. Geophys. J. Int., 152, 729-739. Verhoef, J., Collette, B.J., Danobeitia, J.J., Roeser, H.A., andRoest, W.R., 1991. Magnetic anomalies off West-Africa, Mar. Geophys. Res., 13, 81-103. Wells, D., andCoppersmith, K., 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area and surface displacement. Bull. Seism. Soc. Am. 84: 974-1002. Wessel, P., andW.H.F. Smith, 1991. Free software helps map and display data, EOS 72, 441. Wortel, M.J.R. and Spakman, W., 2000, Subduction and slab detachment in the Mediterranean-Carpathian region: Science, v. 290, p. 1910Science, v. 290, p. -1917. . Working Group for Deep Seismic sounding in Spain 1974Spain -1975Spain , 1978. Crustal seismic profiles in the Alboran Sea -Preliminary results. Pure Appl. Geophys., 116, 167-180.

Figure Captions

Figure 1: General location map, seismicity and primary faults of the S. Iberian / N. Moroccan region (after Gutscher, 2004). Shaded hill relief shown here in background and in Figures 5 and6 from Gebco 1 min global grid [START_REF] Ioc | Centenary edition of the GEBCO Digital Atlas[END_REF]. Major earthquakes (M>6) are shown with date. Dashed line represents likely position of plate boundary separating Iberia (Eurasia) from Africa (Nubia) and defining a third, pie-shaped block (the Rif-Betic-Alboran domain). The green shaded faults; G = Gorringe Bank [START_REF] Johnston | Seismic moment assessment of earthquakes in stable continental regions -III[END_REF], HF = Horseshoe Fault (Gracia et al., 2003a;[START_REF] Stich | Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake : Implications for the 1755 Lisbon earthquake[END_REF], Mq = Marques de Pombal [START_REF] Zitellini | Source of 1755 Lisbon earthquake and tsunami investigated[END_REF]Gracia et al., 2003a) as well as the subduction fault plane (below the accretionary wedge) [START_REF] Gutscher | What caused the Great Lisbon Earthquake?[END_REF][START_REF] Gutscher | The Gibraltar Arc seismogenic zone (part 2): constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity[END_REF] have been proposed as being the potential source of the 1755 M8.7 Great "Lisbon" earthquake. [START_REF] Hatzfeld | Grands profils sismiques dans la région de l'arc de Gibraltar[END_REF]Working Group, 1978;Banda and Ansorge, 1980;Docherty and Banda, 1995;Seber et al., 2001;[START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Contrucci | The crustal structure of the NW-Moroccan continental margin from wideangle and reflection seismic data[END_REF]. Where no seismic data were available (NE Atlantic, Iberia or NW Africa) some values were taken from regional crustal thickness compilations of the Mediterranean region (van der Meijde et al., 2003;[START_REF] Dezes | European Map of the Mohorovicic discontinuity. 2nd EUCOR-URGENT Workshop (Upper Rhine Graben Evolution and Neotectonics)[END_REF] [START_REF] Sandwell | Marine gravity anomaly from Geosat and ERS 1 satellite altimetry[END_REF] with location of Sismar seismic profiles shown [START_REF] Gutscher | Evidence for active subduction beneath Gibraltar[END_REF][START_REF] Contrucci | The crustal structure of the NW-Moroccan continental margin from wideangle and reflection seismic data[END_REF]. (Abbreviations are the same as in Fig. 6.) Most gravity maxima are associated with basement highs (e.g. -Gorringe Bank, Coral Patch seamount, etc.) or continental margins. (Bottom) Two schematic lithospheric cross-sections, with co-located 2-D free-air gravity anomaly profiles (red dots). Locations indicated on the free-air gravity map above. Gravity minima are typically associated with thick sedimentary accumulations. Figure 8: Experimental set-up of analog model using granular materials. The curved backstop consists of a thin, rigid board overlain by a 4cm thick layer of sand. Tectonic convergence is simulated by pulling a mylar film beneath the rigid backstop. The layer of granular materials overlying the mylar consists of a 10 mm thick layer of sand and 1 mm cohesive colored powder overlying a 4mm thick glass bead layer. These granular materials are progressively scraped off and accumulate against the curved backstop.

Figure 2 :

 2 Figure 2: Seafloor morphology of the Gulf of Cadiz A: Compilation of multibeam bathymetric data acquired during the following cruises: GAP(Kopf et al., 2003); Cadisar[START_REF] Mulder | The Gulf of Cadiz: an unstable giant contouritic levee[END_REF]; Cadisar2[START_REF] Mulder | The western part of the Gulf of Cadiz: contour currents and turbidity currents interactions[END_REF];MatesPro (Terrinha et al., in press); TV-GIB(Gutscher, 2005);Hermes (D. Masson, R/V CharlesDarwin, 2006); Delila and DelSis[START_REF] Gutscher | Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: observations from multi-beam bathymetry and seismic profiling[END_REF] and Microsystems (D. Blamart, R/V Marion Dufresne, July 2008). In regions without multi-beam coverage colored contour lines are shown every 200 m from the GEBCO 1 min grid (IOC et al., 2003). Red lines show the portions of seismic profiles presented in the paper below, with the numbers indicating the figure numbers. B: 3-D perspective view looking E towards the straits of Gibraltar. Note the WNW-ESE trending basement high (Coral Patch Ridge) indenting the SW portion of the accretionary wedge.

Figure 3 :

 3 Figure 3: High resolution Chirp seismic profiles acquired by the R/V Suroit (DelSis cruise) in April 2005 A: NE-SW oriented chirp profile (SE at right) crossing the toe of the deformation front (profile is 9 km long, 1 trace = 3 m). Note highly reflective strata at adjacent abyssal plain and folded strata on the accretionary wedge (VE=30). B: N-S chirp profile (N at left) crossing northern limit of Rharb Valley fault at 6°56'W and a mean depth of 500-600m, (profile length = 15 km) note folded and eroded strata (VE=30). C: NNE-SSW chirp profile (N at right) on Moroccan shelf (crossing the prolongation of the Rharb Valley fault at 6°39'W) at 115-135m water depth, note folded strata and discordance at 120-130m depth, likely due to the sea-level lowstand during the Last Glacial Maximum (VE=40)

Figure 5 :

 5 Figure 5: Structural maps of the S. Iberian / N. Moroccan region A: depth to basement map obtained from joint interpretation of all seismic profiles shown (Thiebot and Gutscher, 2006); B: sediment thickness map obtained by combining relief (adding topography and subtracting bathymetry) with the depth to basement map; C: depth to Moho map obtained by integrating all published seismic reflection, and seismic refraction profiles[START_REF] Hatzfeld | Grands profils sismiques dans la région de l'arc de Gibraltar[END_REF] Working Group, 1978; Banda

  ) D: crustal thickness map obtained by subtracting depth to basement from depth to Moho map.

Figure 6 :

 6 Figure 6: Relief map of Southern Iberia region (Gebco 1 min grid -IOC et al., 2003), with location of arcuate structures shown in map view (see explanation in Figure). Six independent arc-shaped features can be identified at depths ranging from 60-200 km to the surface, and are aligned along a WSW-ENE axis of symmetry, along the trend of the straits of Gibraltar. Regions likely floored by oceanic crust are shown in yellow. TAP = Tagus Abyssal Plain, HAP = Horseshoe Abyssal Plain, SAP = Seine Abyssal Plain, Gor = Gorringe Bank, CP = Coral Patch seamount, GR = Guadalquivir Ridge; WAS = West Alboran Sea. (More details are given in the text.)

Figure 9 :

 9 Figure 9: Photographs at different stages of the analog modeling experiment, with the amount of convergence indicated in cm in each case. 0 cm is the initial stage. The rose colored sand on the edges also overlies the mylar film and represents the SW Iberian and NW Moroccan continental margins.

Figure 10 :

 10 Figure 10: Comparison of experiment and sea-floor morphology. A: Close-up of final stage of experiment (after 89 cm of shortening) B: Close-up 3-D bathymetric view of Coral-Patch Ridge indenting the deformation front. Note in both cases, the asymmetric shape of the deformation front, with the embayment to the right associated with a steeper frontal slope.

Figure 11 :

 11 Figure 11: Empirical relation between fault surface area and earthquake magnitude (redrawn after Wells and Coppersmith, 1994). Several large subduction earthquakes have been included like the Sumatra 2004 M9.3 and 2005 M8.7 events (Lay et al., 2005) and the Chile 1960 M9.5 and Alaska 1964 M9.2 earthquakes. The surface areas of various candidate faults for the 1755 Lisbon earthquake have been plotted (see Fig. 1 and caption to Fig. 1 for references). Note that only the subduction fault plane has sufficient dimensions (180 km x 210 km) to generate a M>8.5 earthquake.

Acknowledgments 13 Thanks to the Captain and the crew of the Portuguese R/V Dom Carlos and to the Instituto Hidrografico for acquisition and processing of the Delila bathymetric data. We also thank D. Blamart (LSCE-CEA/CNRS-UVSQ) chief scientist, H. Leau (IPEV) Chief of the operations and the Captain and crew of the R/V Marion Dufresne for acquisition of bathymetric data during the Microsystems cruise. Additional bathymetric data from the 2006 R/V Charles Darwin Hermes cruise were kindly provided by D. Masson, NOC Southampton. We gratefully acknowledge financial support through the

French GDR Marges project and the ESF Euromargins program as well as the European FP6 Project "Nearest". We thank the reviewers for constructive comments. Figures 1,2, 5, 6, 7 and 10b were drafted using GMT software (Wessel and Smith, 1991). This paper is IUEM contribution number XXXX.