Marc Fournier
email: marc.fournier@lgs.jussieu.fr

Philippe Huchon

Khaled Khanbari

Sylvie Leroy

Segmentation and along-strike asymmetry of the passive margin in Socotra, eastern Gulf of Aden: Are they controlled

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Introduction

The processes of continental rifting and break-up, which precede the emplacement of an oceanic spreading ridge, lead to the formation of a pair of conjugate margins on either side of the nascent oceanic basin. The so-called passive margins are remnants of the continental lithosphere that was stretched during rifting. Like continental rifts, passive margins have been investigated to determine which mechanical properties of the lithosphere influence the style of extension. In particular, many studies have tackled the question of whether extension is symmetric or asymmetric at the lithospheric scale [START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF]Wernicke and Burchfiel, 1982;[START_REF] Wernicke | Uniform-sense normal simple shear of the continental lithosphere[END_REF][START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF][START_REF] Lister | Detachment models for the formation of passive continental margins[END_REF][START_REF] Brun | Mantle Exhumation at Passive Margins[END_REF][START_REF] Huismans | Symmetric and asymmetric lithospheric extension: relative effects of frictional-plastic and viscous strain softening[END_REF][START_REF] Nagel | Symmetric alternative to asymmetric rifting models[END_REF].

Several continental rifts, including the Gulf of Suez [START_REF] Coletta | Longitudinal evolution of the Suez rift structure (Egypt)[END_REF][START_REF] Patton | Tectonic evolution and structural setting of the Suez Rift[END_REF], Rhine Graben [START_REF] Brun | Crustal-scale structure of the southern Rhinegraben from ECORS-DEKORP seismic reflection data[END_REF][START_REF] Wenzel | A deep reflection seismic line across the Northern Rhine Graben[END_REF], Corinth Rift [START_REF] Rigo | A Microseismic Study in the Western Part of the Gulf of Corinth (Greece): Implications for Large-Scale Normal Faulting Mechanisms[END_REF][START_REF] Sorel | A Pleistocene and still active detachment fault and the origin of Corinth Patras rift, Greece[END_REF][START_REF] Sorel | A Pleistocene and still active detachment fault and the origin of Corinth Patras rift, Greece[END_REF], Baikal Rift [START_REF] Hutchinson | Depositional and tectonic framework of the rift basins of Lake Baikal from multichannel seismic data[END_REF]Petit and Deverchère, 2006), and several segments of the East African rift system [START_REF] Morley | Variable extension in lake Tanganyika[END_REF]Ebinger, 1989a;[START_REF] Rosendahl | Comparison of the Tanganyika, Malawi, Rukwa and Turkana rift zones from analyses of seismic reflection data[END_REF], display an asymmetric structure at the crustal scale. The asymmetry is expressed in present-day topography and sediment thickness, i.e., uplift-subsidence pattern, structural style, and distribution of volcanics. The asymmetry is generally related to the existence of a bounding master fault on one side of the rift, which accommodates a large amount of extension. The sense of asymmetry of continental rifts may even change along strike across transfer (or accommodation) zones (Bosworth, 1985;[START_REF] Ebinger | Geometric and kinematic development of border faults and accommodation zones, Kivu-Rusizi rift[END_REF][START_REF] Scott | Oblique-slip deformation in extensional terrains: a case study in the lakes Tanganyika and Malawi rift zones[END_REF][START_REF] Brun | Deep crustal structure of the Rhine Graben from DEKORP-ECORS seismic reflection data: a summary[END_REF][START_REF] Mcclay | Analogue modelling of orthogonal and oblique rifting[END_REF][START_REF] Hayward | Variations in the along-axis segmentation of the Afar Rift system[END_REF].

On passive margins, symmetry sometimes prevails, as for example in the northern Red Sea where the continental margins display symmetric sets of fault blocks stepping down to an axial depression (e.g., [START_REF] Bosworth | Structure, sedimentation, and basin dynamics during rifting of the Gulf of Suez and northwestern Red Sea, in Sedimentation and Tectonics in Rift Basins: Red Sea-Gulf of Aden[END_REF]. In this area, a large geophysical data set, including heat flow (Buck et al., 1988) and gravity [START_REF] Cochran | Crustal structure and the mechanism of extension in the northern Red Sea: Constraints from gravity anomalies[END_REF] data, is satisfactorily explained with a symmetric overall geometry of the rift [START_REF] Cochran | Northern Red Sea: Nucleation of an oceanic spreading center within a continental rift[END_REF]. In the case of the conjugate margins of the North Atlantic Ocean, the Iberian and Canadian (Newfoundland) margins [START_REF] Boillot | Ocean-continent boundary off the Iberian margin: a serpentinite diapir west of the Galicia bank[END_REF][START_REF] Keen | Conjugate margins of Canada and Europe: Results from deep reflection profiling[END_REF][START_REF] Beslier | Ocean-continent boundary in the Iberia Abyssal Plain from multichannel seismic data[END_REF][START_REF] Manatschal | The role of detachment faulting in the formation of an ocean-continent transition: insights from the Iberia Abyssal Plain[END_REF][START_REF] Whitmarsh | Evolution of magma-poor continental margins from rifting to seafloor spreading[END_REF][START_REF] Wilson | Rifting along nonvolcanic passive margins: stratigraphic and seismic evidence from the Mesozoic successions of the Alps and western Iberia[END_REF][START_REF] Manatschal | New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps[END_REF][START_REF] Reston | Polyphase faulting during the development of the west Galicia rifted margin[END_REF] and the Labrador Sea margins [START_REF] Chian | Evolution of non volcanic rifted margins: New results from the conjugate margins of the Labrador Sea[END_REF][START_REF] Chalmers | Non-Volcanic Rifting of Continental Margins: A comparison of Evidence from Land and Sea[END_REF], available data point to an overall asymmetry of the margins [START_REF] Sibuet | Formation of non-volcanic passive margins: A composite model applied to the conjugate Galicia and southeastern Flemish Cap margins[END_REF][START_REF] Boillot | Seismic image of undercrusted serpentine beneath a rifted margin[END_REF][START_REF] Louden | The deep structure of non-volcanic rifted continental margins[END_REF]. Recent multichannel seismic reflection results on the Canadian margin have demonstrated the first-order asymmetry of one pair of conjugate margin segments with the absence of a detachment fault analogous to the S reflector on the Galician margin [START_REF] Funck | Crustal structure of the continent-ocean transition at Flemish Cap: Seismic refraction results[END_REF][START_REF] Hopper | Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin[END_REF]. However, analogue and numeric models have shown that the observation of crustal-scale asymmetric features on passive margins may be consistent with a symmetric extension process at a lithospheric scale (e.g., [START_REF] Brun | Mantle Exhumation at Passive Margins[END_REF][START_REF] Nagel | Symmetric alternative to asymmetric rifting models[END_REF].

In conceptual models of asymmetric extension, the asymmetry is often related to the presence of one or more detachment faults cutting through the crust or the lithosphere and separating a collapsing hanging wall from an uplifted and denuded footwall (e.g., [START_REF] Wernicke | Uniform-sense normal simple shear of the continental lithosphere[END_REF][START_REF] Davis | Detachment faulting in continental extension; perspectives from the southwestern U.S. Cordillera[END_REF][START_REF] Lister | The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A[END_REF]. Detachment faulting has been recognized to play an important role in continental extensional tectonics [START_REF] Wernicke | Low angle normal faults in the Basin and Range province: nappe tectonics in an extending orogen[END_REF] and in the formation of passive margins [START_REF] Boillot | Tectonic denudation of the upper mantle along passive margins: a model based on drilling results (ODP leg 103, western Galicia margin, Spain)[END_REF][START_REF] Lemoine | Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): In search of a genetic model[END_REF]. On passive margins, evidence for detachment faults comes from seismic reflection profiles where strong sub-horizontal reflectors are interpreted as shallow-dipping shear zones (Krawczyk et al., 1996;[START_REF] Reston | The S reflector west of Galicia (Spain): evidence from prestack depth migration for detachment faulting during continental breakup[END_REF][START_REF] Reston | The S reflector west of Galicia: the seismic signature of a detachment fault[END_REF][START_REF] Barker | Rift propagation, detachment faulting, and associated magmatism in Bransfield Strait, Antarctic Peninsula[END_REF]Maillard et al., 2005).

Rift-related detachment faults have also been recognized onland on the Red Sea margin [START_REF] Talbot | Red Sea detachment and basement core complexes in Eritrea[END_REF] and in the Alps where detachment faults from the ancient Tethyan margins are exposed [START_REF] Froitzheim | Extensional detachment faulting in the evolution of a Tethys passive continental margin, eastern Alps, Switzerland[END_REF][START_REF] Froitzheim | Kinematics of Jurassic rifting, mantle exhumation, and passive-margin formation in the Austroalpine and Penninic nappes (eastern Switzerland)[END_REF]Manatschal and Bernouilli, 1999). In addition, seismogenic moderate to shallow-dipping normal faults, interpreted as detachment faults, are currently active in the Woodlark and Corinth rifts [START_REF] Abers | Shallow dips of normal faults during rapid extension: Earthquakes in the Woodlark-D'Entrecasteaux rift system, Papua New Guinea[END_REF][START_REF] Bernard | Aigion earthquake (Greece): evidence for low angle normal faulting[END_REF][START_REF] Abers | Evidence for seismogenic normal faults at shallow dips in continental rifts[END_REF].

In contrast with the mature submarine margins of the Atlantic Ocean, the young margins of the Gulf of Aden offer the opportunity to study on land the deformation and to take into account the along-margin 3D evolution of rifting. Results obtained onland on the northern margin have demonstrated the along-strike variability of the structure of the margin segments, in particular on either side of the Socotra fracture zone (Figure!1;[START_REF] Fournier | Oblique rifting and segmentation of the NE Gulf of Aden passive margin[END_REF]. Here, we combine the results of a field survey conducted on the southern margin in Socotra, with the analysis of Landsat images, SRTM (Shuttle Radar Topography Mission) elevation data (sample spacing of 3 arc-seconds, approximately 92!m at the latitude of Socotra; [START_REF] Farr | Shuttle Radar Topography Mission produces a wealth of data[END_REF], and offshore seismic profiles acquired during the Encens-Sheba cruise/MD 117 on board the R/V Marion Dufresne [START_REF] Leroy | From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin[END_REF], to describe the structure of the southern margin and compare it to the conjugate Oman margin. From these observations, we infer information about the deep structure of the margins.

Tectonic framework: oblique opening and segmentation of the Gulf of Aden

Socotra is located on the southern continental margin of the Gulf of Aden, offshore from the Horn of Africa (Figure!1). Separation of the Arabia and Somalia plates was achieved by rifting of continental lithosphere in the Gulf of Aden during the Oligocene and early Miocene [START_REF] Roger | Données nouvelles sur la stratigraphie et l'histoire géologique du Dhofar (Sultanat d'Oman)[END_REF][START_REF] Hughes | Evidence for middle Oligocene rifting of the Gulf of Aden and for late Oligocene rifting of the southern Red Sea[END_REF][START_REF] Bott | The tectonic framework and regional hydrocarbon prospectivity of the Gulf of Aden[END_REF][START_REF] Watchorn | Rift-related sedimentation and stratigraphy, southern Yemen (Gulf of Aden)[END_REF][START_REF] Bosworth | The Red Sea and Gulf of Aden basins,![END_REF], followed by seafloor spreading along the Sheba Ridge, which was initiated 18!Ma ago (early Miocene) in the eastern part of the gulf (magnetic anomaly 5d; [START_REF] Sahota | Geophysical investigations of the Gulf of Aden Continental Margins: Geodynamic implications for the Development of the Afro-Arabian Rift System[END_REF][START_REF] Leroy | From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin[END_REF]. The current full spreading rate at the longitude of Socotra is 22!mm/yr along N25°E [START_REF] Jestin | The Somalia plate and the East African rift system: Present-day kinematics[END_REF][START_REF] Fournier | Reappraisal of the Arabia-India-Somalia triple junction kinematics[END_REF]. The direction of opening of the Gulf of Aden is thus ~40° from orthogonal to its overall N75°E trend [START_REF] Laughton | The evolution of the Gulf of Aden[END_REF][START_REF] Cochran | The Gulf of Aden: structure and evolution of a young ocean basin and continental margin[END_REF][START_REF] Fournier | Oblique rifting at oceanic ridges: Relationship between spreading and stretching directions from earthquake focal mechanisms[END_REF]. The opening obliquity is accommodated by segmentation of the ridge axis by transform faults, including the major Alula-Fartak and Socotra transform faults (Figure!1;[START_REF] Laughton | The Gulf of Aden[END_REF][START_REF] Tamsett | Structure of the Alula-Fartak fracture zone, Gulf of Aden[END_REF][START_REF] Manighetti | Propagation of rifting along the Arabia-Soamalia plate boundary: The Gulfs of Aden and Tadjoura[END_REF]. The en échelon pattern of the ridge segments is mirrored in the stepped shape of the continental margins of Arabia and Somalia. On the basis of field observations on the northern margin of the eastern Gulf of Aden, [START_REF] Fournier | Oblique rifting and segmentation of the NE Gulf of Aden passive margin[END_REF] showed that the segmentation of the Sheba Ridge by the Socotra transform fault coincided with, and was likely inherited from, the prior segmentation of the continental margin.

The first-order segment of the Gulf of Aden located between the Alula-Fartak and Socotra transform faults has been recently studied onshore [START_REF] Fournier | Oblique rifting and segmentation of the NE Gulf of Aden passive margin[END_REF]Bellahsen et al., 2006) and offshore (d'Acremont et al., 2005[START_REF] Bellahsen | Fault reactivation and rift localization: The northeastern Gulf of Aden margin[END_REF]. The main faults of this segment are reported in Figure!1. The northern margin is dominated onshore and offshore by a succession of horsts and graben. The faults of the upper part of the margin (onshore) generally have a sigmoidal shape in plan view with an overall trend parallel to the Gulf of Aden (N75°E), whereas the faults of the lower part of the margin (offshore) appear more linear and consistently strike N110°E-N120°E. The lower part of the margin seems to be segmented by second-order transfer faults, which are not observed in the upper part of the margin. The comparison of the conjugate margins points to an overall asymmetry, with the northern margin displaying horsts and graben, and the southern margin being dominated by one major normal fault, which limits the continental shelf, and a deep basin at the toe of the margin.

Seismic data however lack the penetration necessary to address the deep structure of the margins.

South and southwest of Socotra, oil industry seismic profiles across the Socotran shelf do not reveal any significant extension in the post-Cretaceous series [START_REF] Richardson | Offshore Socotra, Republic of Yemen!: Potential for a new hydrocarbon province!?[END_REF][START_REF] Richardson | A new hychocambon 'Play' area offshore Socotra Island, Republic of Yemen[END_REF][START_REF] Birse | The Mesozoic and Tertiary tectonic evolution of the Socotra area, eastern Gulf of Aden, Yemen[END_REF]. E-W normal faults of Neogene age are restricted to the northern edge of the Socotran Platform along the margin of Gulf of Aden [START_REF] Richardson | Offshore Socotra, Republic of Yemen!: Potential for a new hydrocarbon province!?[END_REF].

Along-strike evolution of the structure of the southern margin on Socotra

A field survey conducted on Socotra reveals that the NE-SW-striking Hadibo fault system divides the island into two parts exhibiting distinct stratigraphic and structural features

(Figure!2).

Stratigraphy of Socotra

Socotra is covered by a carbonate-dominated Cretaceous and Tertiary succession unconformably overlying a Proterozoic and Paleozoic basement (Figures!2a and 2b;[START_REF] Beydoun | The Geology of Socotra Island, Gulf of Aden[END_REF][START_REF] Bott | Remote Socotra and Ancillary Islands: recent field investigations of Paleozoic and Mesozoic outcrops and their relationship to speculative petroleum systems[END_REF]. Triassic and Jurassic deposits are locally preserved in a fault bounded area at the eastern end of the island [START_REF] Samuel | Field observations from the Socotran Platform: their interpretation and correlation to Southern Oman[END_REF].

The basement mainly includes Panafrican granites, which make up the 1500-m-high Haggier massif. At a regional scale, Triassic and Jurassic deposits are only found in localised grabens on the Socotran platform [START_REF] Morrison | The Cretaceous sequence stratigraphy of the Socotran platform, the Republic of Yemen[END_REF]. Like in southern Oman, where upper Permian to Jurassic deposits are generally absent [START_REF] Béchennec | Explanatory Notes to the Geological map of the Sultanate of Oman, scale 1/1,000,000[END_REF][START_REF] Le Métour | Geology and Mineral Wealth of the Sultanate of Oman. Muscat, Oman![END_REF], the region was largely emergent from the late Permian to the early Cretaceous and shallow-marine sedimentation resumed during the Barremian-Aptian transgression with the deposition of the Qishn Formation [START_REF] Roger | Données nouvelles sur la stratigraphie et l'histoire géologique du Dhofar (Sultanat d'Oman)[END_REF]Platel et al., 1992b;[START_REF] Morrison | The Cretaceous sequence stratigraphy of the Socotran platform, the Republic of Yemen[END_REF][START_REF] Samuel | Field observations from the Socotran Platform: their interpretation and correlation to Southern Oman[END_REF]. The Cretaceous sequence is characterized by an alternation of clastic and carbonate strata of variable thickness (<!300!m). A gentle angular unconformity of the Tertiary deposits with the underlying Cretaceous deposits has been described (Samuel et al., 1997), and locally, lower Tertiary deposits rest directly on the basement. The presence of Cretaceous strata all around the Haggier massif however suggests that there was little erosion prior to the deposition of the Paleocene sediments.

The Tertiary series is dominated by upper Paleocene to Eocene continental shelf carbonates, up to 600!m thick, including the massive cliff-forming limestone succession of the Umm Er Radhuma Formation, also recognized on the Yemeni, Omani, and Somali coasts of the Gulf of Aden [START_REF] Beydoun | The stratigraphy and structure of the Eastern Aden Protectorate[END_REF][START_REF] Beydoun | Southern Arabia and Northern Somalia: comparative geology[END_REF][START_REF] Roger | Données nouvelles sur la stratigraphie et l'histoire géologique du Dhofar (Sultanat d'Oman)[END_REF][START_REF] Fantozzi | Tectonic and sedimentary evolution of the eastern Gulf of Aden continental margins: new structural and stratigraphic data from Somalia and Yemen[END_REF].

The platform deposits are overlain by calciturbidites of the Shihr Group of late Oligocene to early Miocene age, which are restricted to localised basins in western Socotra. The Shihr Group succession consists of carbonate conglomerates at the base, overlain by slope deposits that testify to rapid deepening in environments of deposition. These deposits display a variety of chaotic facies, likely produced by seismic activity and steep slopes developed in the localised deep-marine basins [START_REF] Samuel | Field observations from the Socotran Platform: their interpretation and correlation to Southern Oman[END_REF]. On the basis of facies and biostratigraphic analyses, these deposits are correlated with those of Mughsayl Formation of southern Oman [START_REF] Samuel | Field observations from the Socotran Platform: their interpretation and correlation to Southern Oman[END_REF] and interpreted as syn-rift deposits coeval with the rifting of the Gulf of Aden [START_REF] Roger | Données nouvelles sur la stratigraphie et l'histoire géologique du Dhofar (Sultanat d'Oman)[END_REF][START_REF] Platel | Evolution géodynamique du Dhofar (Sultanat d'Oman) pendant le Crétacé et le Tertiaire en relation avec l'ouverture du golfe d'Aden[END_REF].

Significant uplift of the passive margin occurred after the rifting episode, leading to the emergence of the syn-rift basins, and continues at present as evidenced by uplifted Quaternary terraces [START_REF] Beydoun | The Geology of Socotra Island, Gulf of Aden[END_REF]. Post-rift uplift reported on passive margins is generally attributed to a combination of erosional processes and flexural response of the lithosphere to the corresponding redistribution of mass across the landscape (e.g., [START_REF] Japsen | Elevated, passive continental margins: Long-term highs or Neogene uplifts? New evidence from West Greenland[END_REF]Petit et al., 2006).

Asymmetric marginal anticline of eastern Socotra

In eastern Socotra, the Haggier basement massif is flanked to the south and the east by Cretaceous and Paleocene-Eocene strata gently dipping to the south (dip angle!<!10°; [START_REF] Bott | Remote Socotra and Ancillary Islands: recent field investigations of Paleozoic and Mesozoic outcrops and their relationship to speculative petroleum systems[END_REF]. However, as noticed by [START_REF] Beydoun | The Geology of Socotra Island, Gulf of Aden[END_REF], "the abundance of normal tensional faults [in Socotra], often with considerable throw, […] and the lack of any evidence of compressional forces" (p. 434) do not favour this hypothesis. Moreover, with the exception of the rifting of the Gulf of Aden, no major tectonic event occurred in this part of Africa since the early Tertiary. We therefore consider that the large-scale folding in eastern Socotra is related to the syn-and post-rift evolution of the margin. sites where normal and strike-slip fractures were observed, two subsets were distinguished and identified with A and B suffixes (sites 6 and 7). Conjugate normal faults predominantly strike between N80°E and N120°E (sites 3, 4A, 5, 6B, and 7B). These directions are representative of the strike of the major normal faults bounding the tilted blocks. The orientation of the principal stress axes determined by inversion of fault slip data sets (Table !1; [START_REF] Angelier | Tectonic analysis of fault slip data sets[END_REF] indicates a direction of extension between N and N30°E (Figure!2; sites 1, 3, 4A, 5, 6B, and 7B). In one site, an oblique N145°E direction of extension has also been documented (site 4B). The two approximately N20°E and N150°E directions of extension are characteristic of the rifting of Gulf of Aden and were identified all along the northern margin of the Gulf of Aden in Oman [START_REF] Lepvrier | Cenozoic extension in coastal Dhofar (southern Oman): Implications on the oblique rifting of the Gulf of Aden[END_REF][START_REF] Fournier | Oblique rifting and segmentation of the NE Gulf of Aden passive margin[END_REF][START_REF] Bellahsen | Fault reactivation and rift localization: The northeastern Gulf of Aden margin[END_REF] and Yemen [START_REF] Huchon | Extensional deformations in Yemen since Oligocene and the Afar triple junction[END_REF][START_REF] Huchon | Rotation of the syn-rift stress field of the northern Gulf of Aden margin, Yemen[END_REF], and further north in the Huqf area and the Oman mountains [START_REF] Fournier | Comment on "Aptian faulting in the Haushi-Huqf (Oman) and the tectonic evolution of the southeast Arabian platformmargin[END_REF](Fournier et al., , 2006)).

Array of tilted fault blocks of western Socotra

Hadibo transfer zone

The Hadibo fault system transects Socotra and separates the Haggier massif to the east from the tilted blocks of western Socotra (Figures!2b and In summary, the southern continental margin of the Gulf of Aden on Socotra is made up of two segments separated by the Hadibo transfer zone. In N-S cross-section, each margin segment displays an overall structure which is asymmetric. The western segment consists of a series of consistently southward-tilted blocks bounded by northward-dipping normal faults, whereas the eastern segment consists in an E-W-trending anticline with a steep northern limb and a gently dipping southern limb. Thus, the margin structure changes dramatically across the Hadibo transfer zone. The origin of these along-strike changes in structure will be discussed in section 4.

Offshore seismic data and comparison with the conjugate Oman margin

Two single-channel seismic profiles (ES18 and ES20) have been shot during the Encens-Sheba cruise [START_REF] Leroy | From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin[END_REF] that the margin offshore Socotra is steep, narrow, and dominated by a major, northwarddipping normal fault with a moderate dip and a minimum displacement of 2!km (d'Acremont, 2002). A large graben occurs at the toe of the normal fault, containing about 2!km of flatlying sediments including 500-900!m of the syn-rift series (d'Acremont, 2002). In map view, shows that the Oman margin is relatively broad and gentle. It is bounded to the north, on the edge of the continental shelf, by a major southward-dipping normal fault (SP!001), and it displays widely spaced, conjugate, normal faults bounding horsts and graben. Syn-and postrift sediments were deposited in these N100°-110°E-trending grabens (d'Acremont, 2002). A synthetic cross-section from margin to margin, including the seismic profiles ES18 and ES20 and the cross-section of western Socotra ( The conjugate margin of western Socotra in Oman, to the east of the Socotra transform fault, is entirely submerged with the exception of the Al Hallaniyah islands, which form a lowelevated rift shoulder (~400!m) made up of Proterozoic basement unconformably overlain by Eocene deposits. No major normal fault has been observed onland on this margin segment and no flat-lying fault with mylonite between the basement and the sedimentary cover has been described on the Al Hallaniyah islands. Moreover, the Hadibo transfer zone does not seem to extend in the northern continental margin. To the east of the Al Hallaniyah islands, the Oman margin that is conjugate to eastern Socotra is entirely submerged. No offshore data are currently available for this margin. Onshore, the margin consists of a monotonous plateau ca. 200 m above sea level, capped by Oligo-Miocene platform carbonates [START_REF] Béchennec | Explanatory Notes to the Geological map of the Sultanate of Oman, scale 1/1,000,000[END_REF][START_REF] Le Métour | Geology and Mineral Wealth of the Sultanate of Oman. Muscat, Oman![END_REF], and devoid of extensional tectonic structures. In the absence of seismic profile across this margin, the structure of the conjugate margins cannot be accurately compared on the eastern Socotra transect.

Thus, the overall structure of the conjugate margins differs significantly. The different styles of normal faulting suggest different mechanisms of deformation from one margin to the other. As will be discussed in the next section, one margin could be the footwall and the other the hanging wall of an asymmetric rift basin.

Discussion : Is asymmetry controlled by detachment faults?

The continental margin on Socotra consists of two juxtaposed rift segments displaying an asymmetric structure. They testify to the deformation regime prevailing during the rifting. [START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF][START_REF] Lister | Detachment models for the formation of passive continental margins[END_REF] presented detachment models for the formation of passive continental margins, in which they predicted that the structure of passive margins should change along strike across transfer zones and that juxtaposed asymmetric rift segments should be observed, as in Socotra (insert in Figure !6). They introduced the concept of upper-plate and lower-plate conjugate margins on either side of a detachment fault. Lower-plate margins are characterized by numerous rotated tilt blocks and half-graben, and the presence of a detachment fault in the crust, dipping towards the ocean. Upper-plate margins are characterized by widely spaced faulting and generally by a so-called "marginal anticline".

Marginal anticlines correspond to flexures of the edge of the upper plate as the result of thermal buoyancy related to rise of the asthenosphere, and/or igneous underplating due to partial melting of the mantle, and/or the presence of listric detachment faults progressively flattening towards the continent (rollover structure).

The models of [START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF][START_REF] Lister | Detachment models for the formation of passive continental margins[END_REF] account satisfactorily for the surface structures observed on Socotra, in the hypothesis where western Socotra would correspond to a lower-plate margin and eastern Socotra margin to an upper-plate margin (Figure 6).

Considering that (1) asymmetric structures are indicative of a simple shear deformation regime, and (2) asymmetric structures in the brittle upper crust are often associated with synthetic shear zones at deeper ductile levels, we further explore the hypothesis that the shallow asymmetric structures of Socotra may be associated at depth with detachment faults.

Western Socotra displays an array of tilted blocks bounded by consistently oceanwarddipping normal faults. Repetitive and asymmetrical normal faulting in the upper crust may be associated with a synthetic ductile shear zone at depth, which controls the sense of tilt of crustal blocks (e.g., [START_REF] Wernicke | Low angle normal faults in the Basin and Range province: nappe tectonics in an extending orogen[END_REF][START_REF] Faugère | Modélisation expérimentale de la distension continentale[END_REF][START_REF] Mcclay | Geometries of extensional fault systems developed in model experiments[END_REF][START_REF] Brun | Analogue modeling of detachment fault systems[END_REF]. This suggests that the normal faults of western Socotra could be connected at depth to a ductile shear zone with a top-to-the-north sense of shear. They could form a series of northward-flattening, listric normal faults merging into the detachment surface underneath the tilted blocks of western Socotra, as illustrated in Figure!6. Alternatively, the shallow highangle normal faults could sole into master normal faults with moderate dips (>!25-30°), which in turn could sole into a subhorizontal ductile shear zone. The northward-increasing tilt of the fault blocks would be indicative of a northward-increasing shear along the ductile shear zone.

In response to tectonic unloading, an upward flexure of the detachment surface would be expected to the north (Figure!7a and b). Further north offshore, the normal fault with moderate dip observed on seismic data could truncate the detachment fault of western Socotra and correspond to a new detachment fault propagated from the broad culmination of the older one, as proposed in Figure !7b andc. This interpretation would be in agreement with a migration of the deformation towards the rift axis during rifting, as often observed (e.g., [START_REF] Cowie | Spatio-temporal evolution of strain accumulation derived from multi-scale observations of Late Jurassic rifting in the northern North Sea: A critical test of models for lithospheric extension[END_REF]. In this hypothesis, synrift sediments would lie directly above the firststage detachment surface (Figure!7c), as observed in the distal parts of the Iberian margin and the Tethys margin in the Alps (Manatschal and Bernouilli, 1999). There is consequently the possibility of the presence of exhumed mantle in the transition zone (between shot points 2400 and 2700 in Figure 5a), although this remains to be better imaged.

Eastern Socotra consists of a broad E-W-trending asymmetric anticline related to the rifting of the Gulf of Aden. The uplift and erosion of the Haggier massif could be attributed to post-rift erosion and flexural rebound [START_REF] Gilchrist | Differential denudation and flexural isostasy in formation of rifted-margin upwarps[END_REF], as proposed by Petit et al. (2006) for the Marbat escarpment on the Oman margin (Figure!1). However, eastern Socotra, with the rounded shape in map view of the erosional window of basement rocks, does not resemble typically rectilinear retreating rift escarpments. Alternatively, the high topography of the Haggier Mountains and the tilted strata on their flanks could be explained by upwarping from rift-related plutonism or underplating. However, the complete absence of volcanism on the continental margins of the eastern Gulf of Aden (in contrast with the western Gulf of Aden) does not favor this hypothesis. An alternative way to exhume the basement rocks exposed in eastern Socotra may be found in the detachment models of [START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF][START_REF] Lister | Detachment models for the formation of passive continental margins[END_REF] with the formation of marginal anticlines. In these models, detachment faulting causes horizontal translation (beneath the detachment fault) of relatively cool and dense lithosphere toward the developing oceanic basin, thus exposing the base of the continental margin to warmer, and hence relatively less dense, rising asthenosphere. The result is a broad flexure of the margin and the formation of a marginal anticline. Marginal anticlines may display a steeper limb towards the ocean due to sediment loading at the toe of the margin or due to a listric detachment fault progressively flattening at depth (rollover structure). This model requires the existence of a southward-dipping detachment fault with a top-to-the-south sense of shear beneath eastern Socotra, as shown in Figure 6.

In this hypothesis, Socotra would be made up of two crustal-scale asymmetric structures juxtaposed on either side of the Hadibo transfer zone and controlled by two detachment faults with opposite dips and senses of shear. Of course, the extrapolation from observed shallow crustal features to the crustal scale remains speculative and has to be tested: seismic information would be needed to elucidate the deep structure of the two margin segments.

The comparison with the models of [START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF][START_REF] Lister | Detachment models for the formation of passive continental margins[END_REF] can be extended to the conjugate Oman margin. A reconstitution of the eastern Gulf of Aden rift before the onset of seafloor spreading, encompassing the western Socotra margin and its conjugate Oman margin, is presented in Figure!8. The conjugate margins are asymmetric: the southern margin is dominated by tilted fault blocks above a detachment fault dipping towards the oceanic basin, whereas the northern margin is characterized by widely spaced conjugate normal faults.

On this reconstruction, the locus of the breakup of the continental lithosphere does not coincide with the middle of the paleo-rift, but is offset towards the Socotra margin. The overall structure corresponds fairly well to the detachment models of [START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF][START_REF] Lister | Detachment models for the formation of passive continental margins[END_REF] where the Oman margin would represent the upper-plate margin and the western Socotra margin to the lower-plate margin, even though the Oman margin does not display a marginal anticline.

Seismic data are presently non sufficient to provide a cross-section of the eastern Socotra transect comparable to Figure!8 and discuss the possibility for the margin conjugate to eastern Socotra of being the footwall of a basin-forming detachment.

Conclusion

The eastern Gulf of Aden provides a unique example of an asymmetric passive margin with the sense of asymmetry reversing along strike. In Socotra, the passive margin alternates Topogragraphy and bathymetry compiled from SRTM data onland [START_REF] Farr | Shuttle Radar Topography Mission produces a wealth of data[END_REF] and multibeam soundings bathymetry from Encens-Sheba cruise [START_REF] Leroy | From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin[END_REF] superimposed on the world bathymetric map of [START_REF] Sandwell | Marine gravity anomaly from Geosat and ERS-1 satellite altimetry[END_REF] in the oceanic domain. Black arrows show plate relative motions [START_REF] Fournier | Reappraisal of the Arabia-India-Somalia triple junction kinematics[END_REF]. The dashed line shows location of the margin to margin cross section of Figure 5, and solid portions correspond to seismic reflection profiles ES18 and ES20 acquired during the Encens-Sheba cruise. Soc is Socotra. TF is transform fault.

Figure

  Figure!2b). This sub-horizontal platform is devoid of major structures and affected only by a

  Western Socotra is dominated by a series of tilted blocks bounded by WNW-ESEstriking normal faults consistently dipping toward the north (Figures!2b, 2d, and 3). The faults display vertical throws between 200 and 1000!m down to the north (Figure!2d). They postdate the deposition of Eocene formations and controlled the formation of southward-dipping half graben, in which the Oligo-Miocene deposits of the Shihr Group were trapped. Proterozoic basement crops out in the uplifted footwall of the normal faults in Ra's Shu'ab, Qalansiyah, and Ra's Kadarma (Figure!2b). Bathymetric data indicate that the shelf is very narrow to the north of Socotra and likely limited by a normal fault synthetic to the normal faults onshore (Figure!1). The width of the tilted blocks thus decrease northwards from 13-11!km to 7!km and to ~3-4!km for the northernmost block (Jabal Kadarma; Figures!2d and 3), while their dip increases up to 50° to the north in Jabal Kadarma (Figures!2d and 2f). The decreasing spacing of normal faults toward the Gulf of Aden and the increasing block rotation is probably indicative of higher crustal thinning toward the rift axis. Meso-scale striated fault planes were examined in several localities of western Socotra in pre-and syn-rift formations (Figure!2). All fault planes are of normal or strike-slip type. At

  3). It is marked in the topography by a series of down-to-the-northwest escarpments. The fault zone is made up of several rectilinear faults striking N50°E and steeply dipping toward the northwest(Figure!2b). Broad flexures of the Eocene strata in the fault zone attest to normal motion along the main faults. The Hadibo fault system terminates to the south with a horsetail geometry as it connects with the E-W-striking normal fault that bounds the southernmost tilted block of western Socotra (Figure!2d). The fault zone was probably inherited from linear NE-SW fracture zones in the basement, which is at shallow depth throughout the area[START_REF] Beydoun | The Geology of Socotra Island, Gulf of Aden[END_REF].A study of minor faults in the vicinity of the Hadibo fault system helps to clarify its motion. The only site located within the fault zone (site!2) displays conjugate strike-slip faults. The N50°E-to N70°E-striking faults parallel to the Hadibo fault system accommodate right-lateral motions. Two other sites display conjugate sets of strike-slip faults (sites 6A and 7A). The right-lateral faults strike N70°E to N90°E and the left-lateral faults strike N120°E to N140°E. Stress inversion of the strike-slip fault data sets gives ! Hmax !=!! 1 trending N90°E to N120°E and ! Hmin !=!! 3 trending N to N30°E. Since the strike-slip and normal paleostress tensors have similar directions of extension, they likely pertain to the same regional stress field and reflect a unique transtensional deformation regime. The stereodiagrams of Figure 2 even suggest that the direction of extension tends to rotate counterclockwise toward the Hadibo fault zone, from N30°E away from fault zone (sites 7A, 7B, and 3) to N close to it (sites!1 and 2). These observations are consistent with a right-lateral component of displacement along the Hadibo fault system. With respect to the stable southern Socotran platform, the dextral motion accounts for the relative motion between the western part of Socotra stretched by normal faulting and the eastern part shortened by large-scale folding. The Hadibo fault system thus appears as a transfer zone, or accommodation zone, between two rift segments, as illustrated in Figures!2e and 3. The direction of the maximum horizontal principal stress ! Hmax rotates counterclockwise toward the dextral transfer zone, and ! Hmax switches from ! 2 to ! 1 within the transfer zone, in a way similar to the transform faults in the oceanic domain (Figure!4). As can be observed in Figure!1, the Hadibo transfer zone does not appear to pass laterally offshore into an oceanic transform fault.

  across the conjugate margins, immediately to the east of the Socotra transform fault (Figure!5; location in Figure!1). The southern profile (ES20; Figure!5a) cuts across the western Socotra margin segment. The seismic line shows

  the main normal fault of profile ES20 is located more or less in line with the northernmost normal fault of western Socotra (Figure!1). It could correspond to the eastward continuation of this fault, which however displays a much smaller vertical throw (Figure!2d). More likely, the normal fault runs eastward along the northern coast of Socotra and bounds to the north the tilted block of Jabal Kadarma (Figure!2d). The northern profile (ES18; Figure!5b), shot across the conjugate margin segment,

  Figure!2d), highlights the contrasted structures of the opposite margins (Figure!5c; location in Figure!1). Differences between the conjugate margins are also observed in map view (Figure!1).

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. (a) Landsat image and (b) structural map of Socotra (after Landsat imagery and

Figure 3 .

 3 Figure 3. Segmentation of the continental margin by the Hadibo transfer zone illustrated by

Figure 4 .

 4 Figure 4. Comparison of maximum horizontal stress trajectories (! Hmax ) for a right-lateral

Figure 5 .

 5 Figure 5. ES18 and ES20 seismic profiles off Oman and Socotra, respectively, from the

Figure 6 .

 6 Figure 6. Along-strike change of sense of asymmetry of the southern margin of the Gulf of

Figure 7 .

 7 Figure 7. Conceptual model for the development of detachment faults onshore and offshore

Figure 8 .

 8 Figure 8. Prebreakup reconstruction of the eastern Gulf of Aden rift showing the

  Fig. 5aFig.5b

  Fig. 5aFig.5b

Table 1 .

 1 Trend and Dip of Principal Stress Axes Computed From Fault Slip Data a Here ! 1 , ! 2 , and ! 3 are maximum, intermediate, and minimum principal stress axes. " is the ratio (! 2 -! 3 )/(! 1 -! 3 ).

	Site Latitude	Longitude Altitude Number	Formation	Age	! 1	! 2	! 3	"
			m	of Faults			Strike, Dip, Strike, Dip, Strike, Dip,
							deg	deg	deg
	1 12°32.485' 053°51.499'	457	10	Umm Er Radhuma -Jeza Late Paleocene -Eocene	153, 72	266, 07	358, 16	0.64
	2 12°31.696' 053°52.602'	476	18	Umm Er Radhuma -Jeza Late Paleocene -Eocene	266, 04	142, 83	357, 06	0.48
	3 12°33.456' 053°49.975'	277	27	Umm Er Radhuma -Jeza Late Paleocene -Eocene	134, 80	290, 09	021, 04	0.43
	4A 12°37.239' 053°37.464'	222	12	Shihr Group	Oligocene -Early Miocene	295, 79	119, 11	029, 01	0.48
	4B 12°37.239' 053°37.464'	222	4	Shihr Group	Oligocene -Early Miocene	206, 72	056, 16	324, 09	0.22
	5 12°36.009' 053°41.138'	96	1, 12 joints Shihr Group	Oligocene -Early Miocene
	6A 12°32.726' 053°51.214'	425	7	Umm Er Radhuma -Jeza Late Paleocene -Eocene	284, 10	060, 77	193, 09	0.43
	6B 12°32.726' 053°51.214'	425	5	Umm Er Radhuma -Jeza Late Paleocene -Eocene	135, 63	308, 27	039, 03	0.80
	7A 12°33.650' 053°49.622'	195	7	Umm Er Radhuma -Jeza Late Paleocene -Eocene	299, 18	131, 71	030, 04	0.38
	7B 12°33.650' 053°49.622'	195	10	Umm Er Radhuma -Jeza Late Paleocene -Eocene	279, 86	126, 04	036, 02	0.39

a

Acknowledgments. We are grateful to the Associate Editor James Gaherty, and to Donna Shillington and Byrdie Renik for their thorough reviews of the manuscript. We also thank Gianreto Manatschal for his constructive comments on an earlier version of the manuscript and Carole Petit for fruitful discussions. We are indebted to our colleagues from the University of Sana'a for their help, and especially to Abdulkarim A. Al-Subbary. Special thanks go to Salam A. Al-Deri, our driver in Socotra. Seismic profiles were acquired during the Encens-Sheba cruise with the support of Institut Polaire Paul-Emile Victor. The authors acknowledge GDR Marges for sponsoring the expedition to Socotra. Volcanic Rifting of Continental Margins: Evidence from Land and Sea. Geol Soc (London), Spec Publ, 187,

from east to west from marginal anticline to rotated tilt blocks and would pass laterally across the Hadibo transfer zone from an upper-plate margin to a lower-plate margin, according to the detachment models of passive margins of [START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF]1991). The detachment faults, which are supposed to be associated with the asymmetric surface structures, would change sense of shear across the transfer zone (top-to-the-north to the west, top-to-the-south to the east). This example shows that the asymmetry of the rifting process, which is commonly tested by comparing conjugate margins, may also be revealed by the along-strike 3D evolution of the structure of margins segmented by transfer zones. In the eastern Gulf of Aden, crustal asymmetry appears as a major feature of passive margin development.


Rémi Flamary

Xavier Anguera

Nuria Oliver

Spoken WordCloud: Clustering Recurrent Patterns in Speech

The automatic summarization of speech recordings is typically carried out as a two step process: the speech is first decoded using an automatic speech recognition system and the resulting text transcripts are processed to create the summary. However, this approach might not be suitable with adverse acoustic conditions or languages with limited training resources. In order to address these limitations, we propose in this paper an automatic speech summarization method that is based on the automatic discovery of patterns in the speech: recurrent acoustic patterns are first extracted from the audio and then are clustered and ranked according to the number of repetitions in the recording. This approach allows us to build what we call a "Spoken WordCloud" because of its similarity with text-based word-clouds. We present an algorithm that achieves a cluster purity of up to 90% and an inverse purity of 71% in preliminary experiments using a small dataset of connected spoken words.

Introduction

Automatic speech recognition (ASR) systems have traditionally been the cornerstone modules to extract information from audio content. The transcript is first extracted and then analyzed to obtain information (e.g. its topic, particular keywords, etc.) or even generate a summary. Extensive research efforts have been devoted to improve ASR over the years. However, speech recognition results are still challenged by adverse acoustic conditions or when dealing with languages with limited training resources. Recent research addresses these limitations by analyzing the acoustic signal itself to find acoustic patterns that appear multiple times along one or multiple recordings. This approach was first proposed for speech by [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF][START_REF] Park | Unsupervised pattern discovery in speech[END_REF] to augment ASR transcripts, and has also been used as a new computational model to avoid acoustic modeling by [START_REF] Bosch | A computational model for unsupervised word discovery[END_REF] or as a way to summarize the most important information in the audio by [START_REF] Jansen | Towards spoken term discovery at scale with zero resources[END_REF][START_REF] Muscarielo | Audio keyword extraction by unsupervised word discovery[END_REF].

In this paper we aim at automatically summarizing acoustic data by finding the most representative and recurrent sequences in the input speech. Our goal is to obtain, as a result of this process, the set of most often recurring short acoustic words 1 which represent the most prevalent terms in the content. This could be used, for example, to automatically classify the audio or to group it with similar content. In previous similar research, Jansen et al. [START_REF] Jansen | Towards spoken term discovery at scale with zero resources[END_REF] consider that relevant recurrent information appears in long (1 second) sequences which are considered relevant even when appearing only twice in the audio. This assumption limits the representative segments only to long repetitions, which diverges from the WordCloud concept proposed here. Similar to our approach, they pay special attention to a fast implementation of the algorithm. Muscarielo et al. [START_REF] Muscarielo | Audio keyword extraction by unsupervised word discovery[END_REF] incrementally perform a recurrent sequence discovery (which they call "motifs") by locally searching for repetitions in the audio around defined time intervals. Aside from local matches, they also keep a global library of long-term motifs. Like us, they aim at detecting short segments (> 0.6 seconds), but these are only stored as long-term library motifs if they occur at least twice within the local interval, thus being prone to missing segments that might still be relevant by occurring consistently over a long audio document but never frequently enough within a particular local segment. The approach proposed in this paper is inspired by the algorithm proposed in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF][START_REF] Park | Unsupervised pattern discovery in speech[END_REF], where all recurrent short (∼ 0.5 second) sequences are first found in the acoustic data, and then clustered to form homogeneous groups. As mentioned above, the main goal in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF] is to improve speech recognition results by correcting the transcription of homogeneous segments clustered together. Therefore their effort goes into maximizing cluster purity, without paying special attention to inverse purity (i.e. how many clusters contain homogeneous segments). However, our goal is to obtain a set of the most representative spoken words in the acoustic data by building a list of the most recurrent acoustic segments, ranked by the number of times they occur. The proposed algorithm first extracts recurrent acoustic patterns from the speech by using an extension of the Unbounded-DTW algorithm proposed in [START_REF] Anguera | Partial sequence matching using an unbounded dynamic time warping algorithm[END_REF]. Next, all found patterns are clustered to form what we refer to as a "spoken Word-Cloud" (given its similarity to text-based word-clouds). A comparison with the algorithm in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF] shows that the proposed approach is faster and more accurate when tested on this task using a database composed of concatenated words spoken in isolation.

Segmental-DTW-Based Clustering

The segmental-DTW-based clustering in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF][START_REF] Park | Unsupervised pattern discovery in speech[END_REF] was shown to be able to find repeating sequences within an audio document that could be used to augment the speech recognition transcripts towards an information retrieval task. In this section we briefly review the main key points of their algorithm as we take is as the baseline for our system. In a first step, unsupervised pattern discovery in speech is performed using the so-called segmental-DTW algorithm. Segmental-DTW works as follows: A distance matrix is first constructed by storing the Euclidean distance between all acoustic feature vectors of the two sequences to be compared (these can be the same sequence in case we are looking for repetitions in the same acoustic document). The choice of acoustic feature vectors varies between standard MFCC's in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF][START_REF] Park | Unsupervised pattern discovery in speech[END_REF] to phone posteriorgrams in more recent work by the same authors. Next, local optimum paths are discovered by applying standard DTW within each of several overlapping diagonal bands along the similarity matrix by uniformly setting start-end points along the matrix axes, and constraining the possible paths to a maximum deviation from the diagonal between these start-end points. Finally, for each band, the most similar subsequence -longer than a predefined minimum length -is returned.

In a second step, repeating sequences are clustered as follows: a similarity profile is first constructed by accumulating the resulting scores from the matching sequences with respect to time, and by then selecting the most recurrent sequences at the peaks of such profile. In their implementation they do not delimit a start-end times for the selected recurrent sequences. Finally in a third step, these sequences are set to be the nodes of a graph where its paths (and matching scores) correspond to the edges between nodes. This graph can also be represented as a sparse similarity matrix as later explained in section 3.3. The graph is clustered using the algorithm proposed in [START_REF] Newman | Fast algorithm for detecting community structure[END_REF], which is fast to compute and has been shown to work well for community clustering.

Although the algorithm proposed in this paper closely resembles the general architecture proposed by [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF][START_REF] Park | Unsupervised pattern discovery in speech[END_REF], their proposal favors the creation of many clusters with few sequences in each, therefore with a high purity but low inverse purity (refer to Section 4.1 for a definition). This makes their algorithm not applicable to the creation of a "spoken WordCloud", where we are interested in a one-to-one relationship between the clusters and the actual number of unique repeating terms in the recordings.

Spoken WordCloud Algorithm

In this section we describe each of the building blocks that conform the proposed "spoken WordCloud" algorithm. As seen in Figure 1, the algorithm is composed of 3 main modules: (1) First we perform an automatic pattern discovery to detect audio segments longer than 0.5 seconds that repeat over time. We do this by using the previously proposed Unbounded Dynamic Time Warping (U-DTW) algorithm [START_REF] Anguera | Partial sequence matching using an unbounded dynamic time warping algorithm[END_REF] and extend it to process long audio recordings;

(2) Next, a similarity profile (see section 3.2) is created and the most repeated segments (which we will refer as acoustic words from now on) are obtained by finding the maxima of the profile and extracting their start-end points; (3) Finally, the three outputs of the algorithm are created: a "spoken WordCloud" which is generated by clustering the repeated words, a ranked list of the most recurrent words in the audio, and the segments that appear in each of the clusters.

Note that the proposed approach differs from [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF] in three aspects: the pattern discovery algorithm, the process to obtain the start-end positions of selected acoustic words and the similarity metric between the different acoustic words in the clustering step (and the clustering algorithm itself). In this section we describe in detail each of the aforementioned differences.

Unbounded DTW (U-DTW) for Long Sequences

The first necessary step for unsupervised pattern discovery entails finding the acoustic sequences that repetitively appear in the acoustic document over time. In [START_REF] Anguera | Partial sequence matching using an unbounded dynamic time warping algorithm[END_REF] we proposed the U-DTW algorithm that is able to find all repeated sequences in an acoustic signal longer than a given minimum duration. This algorithm differs from previous proposals [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF][START_REF] Muller | Path-constrained partial music synchronization[END_REF] in that it avoids the evaluation of the complete similarity matrix between the two sequences, hence significantly reducing its computational cost. Unbounded DTW leverages the minimum segment matching length set by design in order to avoid computing similarities between acoustic vectors which do not belong to a matching sequence. It computes an initial set of K similarities between both sequences. These locations need to be chosen to ensure that the path along the matrix of any pair of matching sequences will at least go through one of them. By carefully defining the locations where to compute these similarities (which we call synchronization points) we can achieve that K << N • M , where N and M are the sizes of both compared sequences. Then, for every synchronization point that we suspect belongs to a matching segment (i.e. with a high similarity value) we perform a forward and backward path search until the entire matching path is found or the sequence is discarded because of being too short. Note that this approach ensures that the similarity between pairs other than the synchronization points is computed only when necessary. For more information on the algorithm, please refer to [START_REF] Anguera | Partial sequence matching using an unbounded dynamic time warping algorithm[END_REF]. This algorithm automatically finds matching sequences in two acoustic segments but does not perform any clustering of the results, which we address in the rest of the paper.

A general problem of previous algorithms [START_REF] Anguera | Partial sequence matching using an unbounded dynamic time warping algorithm[END_REF][START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF][START_REF] Muller | Path-constrained partial music synchronization[END_REF] is the quadratic memory requirements of the similarity matrices, which limit their efficiency and speed when processing very long recordings (longer than 1 hour). In order to overcome this problem without constraining the possible patterns to be found, we automatically cut the sequence into manageable adjacent chunks and perform U-DTW on each of them with all the rest. As seen in Figure 2(a), an extra margin around each chunk is added to allow for any possible paths to be found even if crossing the chunk's borders. This way we ensure that no matching sequences will be lost when performing this modification to the original U-DTW algorithm.

Similarity Profile and Word Extraction

Given a list of matching paths and their scores, we wish to obtain the start-end times of the sequences in the speech segments that are matched. These will become the acoustic words that will be clustered and used to build the "Spoken WordCloud". We first compute a similarity profile, such as in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF], for all detected matching paths by adding their scores along time. This similarity profile contains, at any given time, the sum of matching similarities for all match- ing sequences that go through that instant. Therefore, the higher the values in the profile, the more recurrent the patterns are. Acoustic words are detected as each of the maxima in the similarity profile plot whose value is higher than a predefined minimum threshold. Note that we choose the maxima because we are using a similarity matrix, instead of choosing the minima if we used a distance matrix.

In order to determine the start-end times of the found acoustic words, we gather all matching paths that cross such maxima and find the median values of all their starting and ending times. Although simple, this method turns out to be more robust than, for example, finding the local minima at either side of the maxima or taking the average (probably because the selected time corresponds to existing paths starting and ending times). In order to further constrain the acoustic words, we perform a simple energy-based speech activity detection on the input data and slightly adjust the start-end times or cut acoustic words in two when they interfere with a silence region.

Similarity Matrix Between Acoustic Words

All found acoustic words are clustered in order to find the most recurrent words. The simplest strategy would be to cluster together those acoustic words that share the most common matching segments. However, this would not return optimum results for our WordCloud as similar acoustic words are usually composed of an overlapping (but quite different) set of matching sequences. In this section we define an appropriate similarity measure between acoustic words and in the next section we explain possible clustering strategies that lead to more desirable WordClouds.

In [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF], the authors use the similarity obtained between matching segments in the pattern discovery step, setting to 0 the similarity of all unmatched pairs. We shall denote this similarity K p . Note that by ignoring the similarity between pairs that have not been matched during the discovery step, some important information might get lost. Hence and in addition to testing K p , we propose to build a full similarity matrix K U (s 1 , s 2 ) between any pair of acoustic words s 1 and s 2 by using the metric in Eq. 1.

K U (s 1 , s 2 ) = e 1 -b 1 |s 1 | • e 2 -b 2 |s 2 | • udtw(s 1 , s 2 ) (1)
where |s * | is the size (in frames) of sequence s * , b * and e * correspond to the optimum starting and ending points of the best U-DTW path computed between the two sequences, and udtw(s 1 , s 2 ) is the total U-DTW score similarity, averaged by the path length (See Figure 2(b)). Note that within U-DTW we use the dot product, therefore udtw and the final similarity always fall within the [0, 1] interval.

Clustering

In this paper, we evaluate two different clustering techniques: graph clustering and spectral clustering. The graph clustering technique was used in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF]. We have implemented it by means of the Normalized Cuts algorithm [START_REF] Dhillon | Kernel k-means: spectral clustering and normalized cuts[END_REF]. Note that this is different from the implementation in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF] because they require setting the value of a threshold (which leads to an unknown a priori number of final clusters) while in Normalized Cuts the final number of clusters is defined a priori. In general, graph clustering is commonly used for large scale clustering such as pixels in images. Its main interest with respect to standard spectral clustering techniques is that it is less computationally demanding as it does not require to perform a prior factorization of the similarity matrix.

Conversely, spectral clustering performs the classification of the segments in a smaller meaningful linear subspace [START_REF] Ng | On spectral clustering: Analysis and an algorithm[END_REF]. In other words, a singular value decomposition of the similarity matrix is first performed in order to get its eigenvectors and eigenvalues. Then the matrix is projected on a small number of eigenvectors having the biggest eigenvalues in order to get the Principal Components of the matrix (PCA). Finally a standard clustering technique such as kmeans can be applied on the projected segments. The possible limitation of this approach is an intractable dimensionality reduction when too many segments have to be clustered. In such cases one can use the large scale extension of this method as proposed in [START_REF] Fowlkes | Spectral grouping using the Nyström method[END_REF].

Note that the dimensionality reduction used in spectral clustering is useful to regularize the acoustic words by frequency. For instance, words that appear a lot will correspond to a principal direction, whereas unique or rare words are neglected by the dimensionality reduction. Such behavior is interesting when clustering a long speech segment if we want to extract only the most recurrent acoustic words. This dimensionality reduction can also be seen as a smooth-ing of the similarity matrix which helps in separating the acoustic words.

Experiments

Experimental Setup

In our experiments we used the MAMI dataset (see [START_REF] Anguera | MAMI: Multimodal annotations on a mobile phone[END_REF]), which we are making publicly available to the community2 . It consists of recordings from 23 different speakers, each one uttering 47 different words, 5 times each, using a mobile phone in a noisy environment. Results using this database should be considered preliminary, given that they do not take into account the co-articulation effects of pronouncing several words together in natural speech. On the contrary, the background conditions in which each word is recorded are not ensured to be constant, unlike in other datasets like lecture recordings. We are planning on evaluating the proposed approach on larger datasets in our future work.

For each speaker we create a single long feature stream by concatenating the 235 spoken words, and apply a simple energy-based speech/silence detector to filter out silence. The presence of silence is a big problem for unsupervised word discovery as all silence regions look very much alike. Note that no bias is introduced by the concatenation of words as the silence detector separates the words and the paths cannot cross the silences. For each file we extract 20 dimensional Mel-Frequency Cepstral Coefficients (MFCC) features every 20ms. As MFCC are speaker dependent we treat all speakers independently in our experiment, leaving the multi-speaker case to future work.

The performance measures used for the evaluation of the clustering are purity, inverse purity and F-score as explained in [START_REF] Amigó | A comparison of extrinsic clustering evaluation metrics based on formal constraints[END_REF]. The purity focuses on the frequency of the most common category in each cluster. This is important as we want to prioritize having only one class of acoustic words in every given cluster. On the contrary, purity does not take into account how spread the acoustic words from a given class are in the clustering. For instance having one cluster per acoustic word would lead to a purity value of 1 but this is not desirable for a WordCloud application. Therefore, we also compute the inverse purity, which measures how accurate is the grouping of the acoustic words into the real classes. Finally, the F-score takes into account both the purity and the grouping of the true classes together.

One application of a spoken WordCloud is a summary of the spoken content with only the most representative/recurring acoustic words. Hence, it is of interest that these few acoustic words be as pure as possible. For this reason we also compute these metrics only on the n biggest clusters. In clusters, which is slightly less than the real number of different words in the MAMI database for each speaker. Note that all results correspond to the average over the 23 subjects in the database.

Segmental vs. Unbounded DTW

In this section we compare Segmental-DTW and Unbounded-DTW for unsupervised pattern discovery and posterior clustering. We use a pipeline similar to the one used in [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF]. All the parameters are exactly the same for both algorithms except for the path detection threshold that has been set to 0.7 for U-DTW and 0.6 for Segmental-DTW in order to obtain a similar number of paths in the pattern discovery step. The minimum path size is set to 0.4 seconds for both, and the minimum similarity for a maximum in the similarity profile to be considered as an acoustic word is set to 1. This means that at least 2 paths crossing the sequence are necessary to consider a sequence to be a word. An N cuts graph clustering is used with 60 clusters in both cases.

Results are shown in Table 1. The purity, the inverse purity and the F-score are all better with U-DTW (∼ 4% absolute improvement). Furthermore, the last column of the table shows the execution time -in minutes -for the pattern recognition on all 23 subjects, showing that the U-DTW is 40% quicker than Segmental-DTW. Here the time needed for word detection and clustering is neglected due to the small number of words (≈ 1 min per subject). All these results show the advantage of using U-DTW for pattern detection and posterior clustering over the Segmental-DTW algorithm for the WordCloud task.

Similarity and Clustering

In this section we investigate the use of the proposed similarity measures and two different clustering strategies (graph and spectral) using the U-DTW algorithm. From the obtained acoustic words we first compute the standard similarity matrix K P proposed by [START_REF] Park | Towards unsupervised pattern discovery in speech[END_REF] and the U-DTW similarity matrix K U proposed in section 3.3. We then compare the clusters using each similarity matrix and also using the average of the two. Both the graph and spectral clustering algorithms are set to find 60 clusters.

Results are shown on Table 2. First, we observe that on this dataset, the spectral clustering works better than the graph clustering. Concerning the similarity measure, the U-DTW measure alone is not as good as the standard one, but combining the two leads to a purity improvement of 2% absolute. Note that the three measures used (purity, inverse purity and F-score) show similar improvements for the different clustering approaches and similarity measures. This shows that we are not promoting one aspect of the clustering over others, but rather evaluating the global efficiency.

Previous results correspond to a fixed number of overall clusters, but in a real life application, the total number of spoken words is usually not known a priori. Figure 3 shows results for different numbers of clusters when using the best method (K U + K P for similarity and spectral clustering). The dotted lines correspond to the results of keeping only n = 40 biggest clusters. We can see that when the number of clusters increases the purity increases and the inverse purity decreases. But when keeping only the 40 biggest clusters, the purity remains more constant while the inverse purity still increases, leading to an overall better F-score. This shows that regardless of the total number of clusters we use in the clustering of a speech recording, our final WordCloud (containing only the n-best clusters) will usually be a good summarization of that recording.

Conclusions and Future Work

Automatic speech recognition (ASR) systems have traditionally been the cornerstone modules to extract information from audio content. However, ASR may be very challenging under adverse acoustic conditions or for languages with limited resources. Recent research address the problem by looking directly at the signal and finding repetitive content that can be later used, for example, to help improve ASR results or to summarize the acoustic content with representative acoustic snippets. In this paper we are interested in obtaining a representation of the most prevalent spoken words in the acoustic input by building a list of the most recurrent acoustic segments, ranked by the number of times they occur. We refer to the output of our system as "Spoken WordClouds" given its similarity with text-based word-clouds. We have proposed important modifications to a well known pattern discovery and clustering algorithm, originally used to improve ASR, to make it more suitable for this application. First, we have used U-DTW for unsupervised pattern discovery with good results in terms of computational time and clustering performances. Next we have investigated the use of a new similarity measure between acoustic words and the use of two different clustering algorithm. We have evaluated the proposed approach in a small database and obtained up to 90% cluster purity and 71% inverse purity. Future work will entail testing the algorithm on recorded lectures and meetings, working on a speaker-independent version and evaluating the meaningfulness of the obtained WordClouds as a summary of the content.

Figure 1 .

 1 Figure 1. Spoken WorlClouds algorithm blocks diagram

Figure 2 .

 2 Figure 2. (a) U-DTW for large scale: paths are allowed to cross the borders of the chunk; (b) U-DTW similarity measure example between segment s 1 and s 2 .

Figure 3 .

 3 Figure 3. Purity, inverse Purity and F-score for different number of clusters with K U + K P and spectral clustering.

Table 1 . Comparison between Segmental DTW and Unbounded DTW for Unsupervised Pattern Discovery.

 1 our experiments, we keep the n = 40 biggest

	Method	Pur.	IPur.	Fsc.	time (m)
	Segmental DTW	0.757	0.556	0.645	110
	Unbounded DTW	0.802	0.587	0.675	70

Table 2 . Results obtained for different similarity measures and clustering (60 clusters).

 2

				Graph					Spectral
	Sim. Meas.	Pur.	Pur. 40	IPur.	IPur. 40	Fsc.	Pur.	Pur. 40	IPur.	IPur. 40	Fsc.
	K P	0.80	0.78	0.59	0.67	0.68	0.88	0.86	0.69	0.80	0.74
	K U	0.82	0.81	0.61	0.71	0.69	0.86	0.84	0.68	0.78	0.73
	K P + K U	0.84	0.84	0.62	0.72	0.71	0.90	0.89	0.71	0.82	0.77

Although we refer to spoken words throughout the paper, as we do not use any knowledge about the language being processed, we are in fact detecting commonly repeated sequences which we hypothesize could correspond to actual words in that language

http://mm2.tid.es/mamidb/mamidb.tar.gz

Aknlowledgement

This work was done while R. Flamary was visiting Telefonica Research. During the development of this work X. Anguera was partially funded by the Torres Quevedo Spanish program