Guangrong Li

Stanislas Sizaret
email: stanislas.sizaret@univ-orlenas.fr

Yannick Branquet

Luc Barbanson

Yan Chen

Bo Wang

Changzhi Wu

Lianxing Gu

Liangshu S Shu

Initial geometry and paleoflow reconstruction of the Yamansu skarn-related iron deposit of eastern Tianshan (China) from paleomagnetic and magnetic fabrics investigations

Keywords: anisotropy of magnetic susceptibility, paleomagnetism, eastern Tianshan, Yamansu, skarn-related iron deposit, ore genesis, Late Paleozoic, Permian strike-slip

published or not. The documents may come

Initial geometry and paleoflow reconstruction of the Yamansu skarn-related iron deposit of eastern Tianshan

(China) from paleomagnetic and magnetic fabrics investigations

Yamansu deposit at 323 Ma, and uses this deposit to define a model of Submarine Volcanogenic Iron Oxide (SVIO) deposits (Hou et al., 2014a, b). In the proposed model the Yamansu stratoïd ore bodies formed horizontally within the central venting part of a large submarine composite volcano in a back-arc environment. However, at Yamansu the ore bodies are tilted vertically by the large scale dextral strike-slip faulting during the Permian. The Carboniferous positions are thus overprinted and unknown. Understanding the initial geometry and structures of the Yamansu deposit is then necessary to test and/or improve the proposed SVIO model. Field observations evidence an emplacement of the basalt as sill and their alteration into skarn. Magnetic mineralogy results show that the magnetic remanence and susceptibility are carried by magnetite. Paleomagnetic measurements on massive garnet skarn and basaltic sill distinguish two groups of samples. The higher coercivity samples give a mean remanence direction that is compatible with regional Late Carboniferous directions after bedding correction. This result indicates that both the basaltic sill and the massive garnet skarn were horizontal when formed. The AMS results agree with the paleomagnetic data because the magnetic foliations after bedding correction are horizontal and the well-defined magnetic lineation suggests the existence of a horizontal fluid flow during metasomatism. These results indicate that the basaltic sill horizontally intruded the Carboniferous limestones intercalated within the volcanic pile. Metasomatic iron-rich fluid probably ascended along a pre-existing fault and was injected horizontally into the limestone and sill, and then caused extensive

Introduction

As an important metallogenic province, the eastern Tianshan can be divided into four units (Fig. 1): the Dananhu arc belt, Aqishan-Yamansu belt, Middle Tianshan terrane and South Tianshan terrane. These units have been formed by the accretion-collision of arcs and microplates from the Ordovician to Carboniferous [START_REF] Charvet | Paleozoic structural and geodynamic evolution of eastern Tianshan (NW China). welding of the Tarim and Junggar plates[END_REF][START_REF] Charvet | Palaeozoic tectonic evolution of the Tianshan belt, NW China[END_REF]; [START_REF] Wang | Understanding and study perspectives on tectonic evolution and crustal structure of the Paleozoic Chinese Tianshan[END_REF]. By the end of the Carboniferous all of the units were welded and deformed. Major dextral shearing occurred during the Permian due to the relative displacement of the Jungar and Yili blocks, below Tarim and Siberian [START_REF] Wang | Primary Carboniferous and Permian paleomagnetic results from the Yili Block (NW China) and their implications on the geodynamic evolution of Chinese Tianshan Belt[END_REF][START_REF] Wang | Late paleozoic pre-and syn-kinematic plutons of the Kanguer-Huangshan shear zone inference on the tectonic evolution of the eastern chinese north Tianshan[END_REF]. In spite of compressive Cenozoic re-activation and subsequent uplift, those different tectonic units are still delineated by large Permian strike-slip faults which accommodated the dextral movement between the Tarim and Siberian blocks.

The Carboniferous and Permian were productive periods for mineralization in the eastern Tianshan. Most mineralization correspond to skarn-related iron deposits, porphyry Cu deposits, orthomagmatic Cu-Ni and V-Ti deposits with associated mafic/ultramafic complexes, and orogenic and epithermal Au deposits. Three main metallogenic stages have been identified (Zhang et al., 2008 and references therein):

(1) An early subduction-island arc stage within the Dananhu arc leading to the formation of the porphyry Cu deposits such as the Yangdong and Tuwu porphyry Cu deposits [START_REF] Zhang | The relationship between ore[END_REF][START_REF] Han | Major Types and Characteristics of Late Paleozoic Copper Deposits in North Xinjiang, Northwest China[END_REF] and base metal volcanic massive sulfide deposits. In the Late Carboniferous, the magmatic front migrated southward to form the Aqishan-Yamansu arc during the closure of the ancient Tianshan ocean [START_REF] Ma | Tectonic evolution and mineralization of Eastern Tianshan[END_REF][START_REF] Hou | Geochemical characteristics of the Late Paleozoic Volcanics in Jueluotage tectonic belt. eastern Tianshan and its implication[END_REF][START_REF] Wu | Geochronology , geochemistry and tectonic significances of the Hongyuntan granitoids in the Qoaag area Eastern Tianshan[END_REF]Zou et al., 2006;[START_REF] Hou | Study on the mineralogy of volcanics of the Yamansu group in the Jueluotage tectonic belt , East Tianshan[END_REF].

The 320-280 Ma time span corresponds to the collisional-accretionary tectonic stage between arcs and micro-plates. During this tectonic phase major thrusts, folds and co-genetic foliations developed. Except for the main 290-282 Ma ore-forming stages of the Kanggur orogenic-type gold deposit (Zhang et al., 2003), this Late Carboniferous-Early Permian period is nearly devoid of mineralizing events throughout the eastern Tianshan.

(2) A post-collisional stage (280-245Ma) is marked by shearing along the major right-lateral Permian strike-slip faults. This stage corresponds to the formation of world-class orthomagmatic Cu-Ni-Ti-V deposits associated with mafic-ultramafic complexes (Wang et al., 2008 a ; 2008 b ; Wang et al., 2009; Mao et al., 2011). Some of the ore-bearing intrusions are typical sheeted syntectonic intrusions in the Permian dextral shear zones [START_REF] Branquet | Synkinematic mafic/ultramafic sheeted intrusions. emplacement mechanism and strain restoration of the Permian Huangshan Ni-Cu ore belt (Eastern Tianshan, NW China)[END_REF]. This period also marks the peak of gold mineralization throughout eastern Tianshan. The gold deposits are either lodes of the shear-zone-type in the Permian strike-slip zones or veins of the epithermal/magmatic hydrothermal type related to the Permian volcanism and intrusions [START_REF] Pirajno | Gold Deposits in the Eastern Tian Shan, Northwestern China[END_REF][START_REF] Zhang | Geochemical characteristics and source of two-type oreforming fluids in Kanggultage gold ore belt, east Tianshan[END_REF][START_REF] Wang | Strike-slip fault controls on mineralization in the Kanggurtag gold belt in the Eastern Tianshan, Xinjiang, NW China[END_REF][START_REF] Zhang | Multiple mineralization events in the eastern Tianshan district, NW China. Isotopic geochronology and geological significance[END_REF].

(3) A final Triassic stage (240-220 Ma) during which Fe-V-Ti oxide ores formed in mafic/ultramafic intrusions such as the Weiya deposit (Zhang et al., 2005).

In the eastern Tianshan, many deposits of iron, gold or silver are reported to be of the "skarn-type" [START_REF] Mao | A tectono-genetic model for porphyry-skarn-stratabound Cu-Au-Mo-Fe and magnetite-apatite deposits along the Middle-Lower Yangtze River Valley, Eastern China[END_REF][START_REF] Zhang | Multiple mineralization events in the eastern Tianshan district, NW China. Isotopic geochronology and geological significance[END_REF]. As explained below and to avoid confusion, we will favor the more generic term "skarn-related" to qualify these iron-(copper) deposits in the following sections. The skarn-related Fe-(Cu) deposits in the Aqishan-Yamansu arc belt are reported to have been formed during the Carboniferous before Permian dextral shearing [START_REF] Li | Late Neoproterozoic and Paleozoic tectonic framework and evolution of Eastern Xinjiang, NW China[END_REF]. The Yamansu skarn-related magnetite deposit is one of the most studied in the eastern Tianshan province. It is hosted in Carboniferous limestone and a basaltic formations . Recent studies (e.g. [START_REF] Li | Late Neoproterozoic and Paleozoic tectonic framework and evolution of Eastern Xinjiang, NW China[END_REF]Hou et al, 2014a) show that, despite the massive calc-silicate deposition of Ca and Fe-rich garnets and pyroxenes, these deposits are not iron skarns formed by contact metamorphism as defined by [START_REF] Einaudi | Skarn deposits[END_REF] and [START_REF] Meinert | World skarn deposits[END_REF] because they lack cogenetic intrusions. Moreover, [START_REF] Li | Iron Ore Deposits in the Eastern Tianshan Orogenic Belt (China):the Magnetite-Skarn-Magmatism Association[END_REF] and [START_REF] Li | Comment on "Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit, Eastern Tianshan Mountains, NW China: Constraints on the metallogenesis[END_REF] demonstrate different relationships between basaltic injections, skarn and mineralized sequence than those generally observed in intrusion-related iron skarns. Recently, based on detailed works of Hou et al., (2014a) on the Yamansu deposit, Hou et al. (2014b) proposed a new genetic model for the Chinese skarn-related iron deposits. This "Submarine Volcanogenic Iron Oxide (SVIO)"model involve: i) facies variation in submarine volcanic rocks from a large central-vent composite volcano, ii). mainly stratiform and sub-horizontal ore bodies at the deposition time; iii) convecting aqueous chloride solution deriving from evolved-magma-fluids mixed with seawater; iv) and consequently, a sub-contemporaneity between volcanism, skarnization and iron mineralization.

However, at Yamansu the initial dip of the stratoïd ore bodies is still questionable as Permian deformation overprints the initial structures involved in the genesis of the deposit and rotates the primary geometry. This constitutes a major pitfall for the recently proposed SVIO model, and points toward the necessity of an integration and understanding of such ore deposits within the structural framework and evolution of the eastern Tianshan orogenic segment. To constrain the initial position of the Yamansu iron deposits, the basalts and massive garnet skarns associated with ore deposition have been sampled for paleomagnetic and AMS studies. Moreover, as the AMS method is sensitive to the magnetic mineral texture (Borradaile and Tarling, 1981;[START_REF] Hrouda | Magnetic anisotropy of rocks and its application in geology and geophysics[END_REF] magnetic fabrics are used to trace deformation events [START_REF] Cifelli | An AMS, structural and paleomagnetic study of quaternary deformation in eastern Sicily[END_REF] and/or fluid circulation events in the primary structures (Sizaret et al., 2006a, b). All these data are finally integrated in a genetic model, starting by magmatism associate with metasomatism along a major fault during the late Carboniferous, and its tilting by the dextral strike-slip Permian deformation.

The Yamansu Magnetite Deposit (YMD)

Geological setting

The Yamansu magnetite ore field is located in the Aqishan-Yamansu arc belt (Fig. 1) that experienced north-verging thrusting and folding during the Carboniferous. This former deformation event is characterized by ductile deformation in the Yamansu -Huangshan area and corresponds to a syn-arc thin-skinned tectonic event.

(Laurent- [START_REF] Laurent-Charvet | Palaeozoic late collisional strike-slip deformations in Tianshan and Altay, Eastern Xinjiang, NW China[END_REF][START_REF] Charvet | Palaeozoic tectonic evolution of the Tianshan belt, NW China[END_REF]. Overprinting these former structures, the Yamansu area is affected by a wide and distributed vertical E-NE-trending right-lateral shear zone (Fig. 2a). Like many major shear zones in eastern Tianshan, the dextral shearing along the Yamansu shear zone (Fig. 2a) was thought to be Permian in age [START_REF] Shu | A Large-Scale Palaeozoic Dextral Ductile Strike-Slip Zone:the Aqqikkudug-Weiya Zone along the Northern Margin of the Central Tianshan Belt,Xinjiang,NW China[END_REF][START_REF] Branquet | Synkinematic mafic/ultramafic sheeted intrusions. emplacement mechanism and strain restoration of the Permian Huangshan Ni-Cu ore belt (Eastern Tianshan, NW China)[END_REF]. As a result of the deformation event, the present day geometry of the YMD corresponds to a tilted and deformed monocline that extends E-W and dips south at an average angle of about 60° (Figs. 2b and2c).

Close to the deposit, the formation is locally dominated by Early Carboniferous limestone (Baosteel-Company, 1977;[START_REF] Xbgmr | Xinjiang Bureau of Geology and Mineral Resources:Regional Geology of Xinjiang Uygur Autonomous Region[END_REF], tuff, basalt and arkose with minor lava (Fig. 2a). Basalts have been dated at 34416 Ma from a Rb-Sr age isochron with an 87 Sr/ 86 Sr initial value of 0.70550.0001 [START_REF] Lu | Geochemical characteristics and metallogeny of the Yamansu iron deposit, Xinjiang[END_REF]. More recently, Hou et al. (2014 a) dated the basalts at 324.4 ± 0.94 Ma (LAICP-MS U-Pb on zircon). The basalt has a geochemical signature between Mid Ocean Ridge Basalt and Island Arc Basalt, indicating a back-arc tectonic setting. Moreover, clinopyroxene studies suggest a crystallization temperature of about 1150 °C, suggesting the presence of a shallow magma chamber (4-10km; [START_REF] Hou | Study on the mineralogy of volcanics of the Yamansu group in the Jueluotage tectonic belt , East Tianshan[END_REF].

The Yamansu ore field contains at least six tabular ore bodies. Among them, YMD is the largest one in the easternmost part of the ore field (Fig. 2a). The YMD contains a reserve of 32 Mt with an average grade of 51% Fe and 20,000 t of Cu with a mean grade of 0.06% Cu [START_REF] Mao | Late Paleozoic base and precious metal deposits, East Tianshan, Xinjiang, China. Characteristics and geodynamic setting[END_REF]. The ore bodies are spatially associated with a ~50m thick stratoid garnet skarn and marble lenses intercalated within Early Carboniferous limestones and basaltic sills (Fig. 2). Basalt and skarn yield coeval ages as the skarn gives a 323.47  0.95 Ma age (U/Pb on Zircon, Hou et al., 2014a). It is noteworthy that these ages have been debated (see recent comment and reply by [START_REF] Li | Comment on "Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit, Eastern Tianshan Mountains, NW China: Constraints on the metallogenesis[END_REF]Hou et al., 2014c).

In the walls of the galleries, we observed xenoliths of marbles embedded in the basaltic matrix (Fig. 3). This strongly suggests magmatic stoping and probable carbonate assimilation; basalts at Yamansu being injected sills within limestones rather than surficial lava flows.

To the south of the YMD, a fault branch of the Yamansu shear zone bounds a Permian basin (Fig. 2a). Greywacke beds in the basin trend sub-parallel to the shear zone with a regular southerly dip. Within the Permian basin and in the Yamansu pit, meter-thick steeply-dipping dikes crosscut the skarn, Carboniferous limestone and Permian formations. These dikes have been dated as Permian, coeval with the regional strike-slip shearing [START_REF] Li | Permian mafic-ultramafic complexes on the southern margin of the Tu-Ha basin, East Tianshan Mis en forme : Anglais (États Unis) Mountains. Geological records of vertical crustal growth in Central Asia[END_REF].

The occurrence and the common localization of skarn, magnetite ore bodies, injected basaltic sills and Permian basin border along the Yamansu shear zone are potentially indicative of a common heritage in this fault system before being reactivated during Permian strike-slip tectonics. This study aims to constrain this heritage.

Structure and deformation

In the Yamansu open pit, there are two dominant groups of brittle faults. The fault plane of the first group is subparallel to the ore bodies and limestone bedding (165°∠ 49°, in dip direction/dip notation). In the pit, slickensides associated with these fault planes form two distinct sets (Fig. 4b): (1) E-W oriented with a low to moderate westward plunge. The kinematics criteria indicate dextral sense of shear (Fig. 4c); and,

(2) SSE oriented with a southward steep plunge, shear criteria showing thrusting (Fig. 4d). No clear crosscutting relationships are observable between the two kinematics.

The second group of faults is mostly NE-SW to NNE-SSW trending (Fig. 4a).

The most important of these faults are responsible for decametric offset of ore bodies Finally, as already proposed by several authors [START_REF] Shu | A Large-Scale Palaeozoic Dextral Ductile Strike-Slip Zone:the Aqqikkudug-Weiya Zone along the Northern Margin of the Central Tianshan Belt,Xinjiang,NW China[END_REF][START_REF] Pirajno | Intracontinental strike-slip faults, associated magmatism, mineral systems and mantle dynamics. examples from NW China and Altay-Sayan (Siberia)[END_REF], our structural data at the Yamansu pit scale are consistent with the Permian right-lateral strike-slip and we infer that the Yamansu iron deposit is located within a major Permian shear zone at the eastern Tianshan scale.

Skarn Texture and Paragenesis

The prograde skarn-related minerals are dominated by andradite and grossularite with minor pyroxene and they are extensively altered by chlorite and epidote. Pyrite, chalcopyrite and borosilicate (i.e. tourmaline and axinite) are observed crosscutting massive magnetite, indicating their later deposition during the retrograde stage. The paragenetic succession for the YMD is given in Figure 5.

The main texture within the massive garnet skarn is a centimetre-to metre-scale banding of alternating coarse-grained garnet-rich ribbons with thin green epidote and chlorite-rich ribbons (Fig. 6a). Within the garnet skarn, basaltic relics occur that vary in size from a few to about 30cm. They show irregular rounded shapes with gradational wavy boundaries. In some places, far from the massive garnet ribbons and magnetite bodies, the basaltic sill is partially replaced by andradite growing in the groundmass, whereas euhedral/subhedral plagioclase is still present (Fig. 6b).

Massive magnetite bodies are of two types: (i) massive magnetite lobes replacing marble with a reaction front outlined by garnet growth that can be observed under microscope, leading to the integration of garnet within massive magnetite (Fig. 6); and, (ii) massive magnetite lenses in the centre of Yamansu open pit with a banding pattern composed of alternating magnetite and garnets ribbons. Grandite veinlets crosscutting massive magnetite suggest that magnetite is deposited late but during the prograde stage (Figs 6d and e, Fig. 5). At a microscopic scale the garnet displays a remarkably complex zoning with: (i) a brown andradite core; (ii) a rim of calcite and magnetite inclusions; and, (iii) a border with anomalous birefringence of andratite-grossular composition (45% mol grossular) (Figs. 7a,b) that is classical for iron skarn [START_REF] Einaudi | A special issue devoted to skarn deposits, introduction -terminologie, classification, and composition of skarn deposits[END_REF].

According to the salinity of the fluid inclusions in calcite [START_REF] Lu | Geochemical characteristics of the rare earth elements of the Yamansu iron deposit[END_REF] and the calculated stability limits of stilpnomelane [START_REF] Miyano | Phase equilibria in the system K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-CO 2 and the stability limit of stilpnomelane in metamorphosed Precambrian iron-formations[END_REF]) that altered the garnet in the YMD, the formation temperature of garnet in the prograde stage was above 500°C. Fluid inclusions in magnetite and pyrite give homogenization temperatures of 340-330°C and 220-150°C, respectively, with salinities between 12.9 and 2.7 NaCl equiv [START_REF] Liu | Metallogenic series types of deposits in Xinjiang[END_REF].

Paleomagnetic and Magnetic Fabric Study

Sampling and measurement

Using a portable gasoline drill, 275 cores from 24 sites were sampled in the Yamansu open pit (Fig. 2b). Solar azimuth was measured systematically for all samples to avoid declination deviations caused possibly by the magnetite-rich basaltic sill and magnetite ore bodies. Standard specimens of 2.5 cm in diameter and 2.2 cm in height were prepared in the laboratory.

All specimens were measured for the magnetic fabric study. Twenty oriented cores were taken for paleomagnetic study from two of the 24 sites that were space about 900m apart (Sites 06 and 16 in Fig. 2b). The lithology of these cores were classified into six groups i.e, basalt, massive garnet skarn, ore shoot (magnetite), retrograde stage skarn (epidote, chlorite, K-feldspar, tourmaline), marble and Permian dikes. Sites 06 and 16 are composed of "basalt" with disseminated magnetite and massive garnet skarn, respectively.

Magnetic remanence measurements and alternating field (AF) demagnetization were performed with an Agico JR5A spinner magnetometer and an Agico LDA-3 demagnetizer, respectively, in the Institut des Sciences de la Terre d'Orléans, France.

All specimens were progressively demagnetized by about 10 steps from 0 to 50 mT.

Corresponding remanent magnetic directions were calculated by principal component analysis [START_REF] Kirschvink | The least-squares line and plane and the analysis of palaeomagnetic data[END_REF] and the average of these directions was calculated using Fisher statistics [START_REF] Fisher | Dispersion on a Sphere[END_REF].

The bulk magnetic susceptibility and AMS were measured with a KLY-3S Kappabridge apparatus. The mean orientations of the three principal ellipsoidal axes of AMS (K 1 >K 2 >K 3 ) for each site were computed with ANISOFT software using Jelinek's statistics [START_REF] Jelinek | Characterization of the magnetic fabric of rocks[END_REF]. During the laboratory measurements, some samples showed so high a magnetic susceptibility that they saturated the magnetic instruments. In these cases, the cores were re-cut into cubic specimens of 9mm and/or 6mm in length. As the smaller volume will increase the measurement errors, at least two cubes were taken for each core.

Magnetic mineralogy

In order to identify the magnetic remanence and susceptibility carriers, magnetic hysteresis measurements and thermomagnetic curves were performed. Hysteresis data were acquired at the paleomagnetic laboratory of Institut de Physique du Globe de Paris using a translation inductometer within an electromagnet providing a field of up to 1 T. Thermomagnetic measurements were done in the rock magnetism laboratory of the Orleans University using an AGICO KLY3 Kappabridge-CS3 furnace apparatus.

Hysteresis measurements reveal characteristics of ferromagnetic (sensu lato) minerals in all lithologies (i.e. basalt, massive garnet skarn, ore shoot, retrograde stage skarn and Permian dikes; Figs. 8A and8B) except in marble (Figs. 8c). High Hcr/Hc ratio (3.69-12.2) versus low Mrs/Ms ratio (0.0347-0.14) on a Day plot diagram show the presence of pseudosingle domain magnetite (Fig. 9). The thermomagnetic curves decrease in intensity during heating up to about 350°C, suggesting the possible presence of maghemite or pyrrhotite. The susceptibility shows a sharp drop down at 580°C, revealing the Curie temperature of magnetite (Fig. 8d). A Day diagram shows the presence of multidomain magnetites, suggesting that the magnetic fabrics are normal (Fig. 9; [START_REF] Dunlop | Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data[END_REF]. Such results are consistent with our microscopic observations in which magnetite grains are frequently observed with sizes varying from 10 µm for inclusions to 200 µm in massive magnetite bodies.

The hysteresis curve obtained from the marble sample displays diamagnetic characteristics (Fig. 8c). Diamagnetic calcite with low iron content is known to have an inverse fabric i.e. the calcite <c> axes are parallel to the K 3 axis (inverse fabric) as K 1 , K 2 and K 3 are sorted according to their algebraic values [START_REF] Rochette | Inverse magnetic fabric in carbonate-bearing rocks[END_REF][START_REF] Schmidt | Magnetic anisotropy of calcite at room-temperature[END_REF][START_REF] Essalhi | Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS). An example from Aptian carbonates of La Florida, Northern Spain[END_REF]. Goniometric analyses on marble confirm that the lower value K 3 is parallel to the <c> in the YMD (Fig. 10).

Paleomagnetic results

The progressive AF demagnetization of samples from the basaltic sill and massive garnet skarn reveals two distinct groups of magnetic remanence behaviours (Table 1; Fig. 11).

The first group concerns samples taken mostly in the massive garnet skarn and shows only one magnetic component with a normal polarity that disappears after cleaning with a 15 mT alternate field. The average geographic orientation of the magnetic direction is Dg = 9.7°, Ig = 58.9°, kg = 103.2,  95g = 5.1° with n = 9 (D=declination, I=inclination, k=precision parameter of [START_REF] Fisher | Dispersion on a Sphere[END_REF],  95 =radius of cone of 95% confidence for the site, subscript g indicates geographic coordinates). This direction is close to the dipolar Present Earth's Magnetic Field (PEMF: D = 0°, I = 61°, Fig. 12a). Applying bedding tilt corrections to the average direction gives Ds = 131.4°, Is = 66.1°, ks = 103.2 and  95s = 5.1° in stratigraphic coordinates (Table 1 and Fig. 12b). This result is not comparable with any paleomagnetic direction found in the surrounding areas since the late Paleozoic.

The second group remanence behaviours shows two components. The first is a low-coercivity normal polarity direction similar to the PEMF and cleaned (i.e. removed) by a low AF field up to 7 mT. The second component has a higher coercitivity (7 to 50 mT) and an average remanence direction of Dg = 136.8°, Ig = 15.6°, kg = 29.9,  95g =10.3° and n = 8 in geographic coordinates (Fig. 13a). After bedding tilt corrections this component reveals a reverse paleomagnetic direction of Ds = 134.1°, Is = -27.9°, ks = 29.9 and  95s = 10.3° (Table 1 and Fig. 13b). This direction can be related to the late Paleozoic field (see the discussion).

Magnetic fabrics

The results given in Figures 14 and15 and in Table 2 are presented by lithology and sites. Skarn, magnetite bodies, basalt and samples with retrograde-stage epidote are magnetite rich and, obviously, have a high bulk magnetic susceptibility from 3.10x10 -2 to 6.65x10 -3 S.I. Conversely the Permian dikes displayed relatively low bulk susceptibility from 1.83x10 -5 to 1.2x10 -4 S.I. and the marble samples are diamagnetic.

The majority of the magnetic fabrics are oblate (T>0): 17 sites with positive T values up to 0.724 and 7 sites has negative T values not below -0.379 (Fig. 14a andTable2). The the degree of anisotropy (P J ) is relatively low (11 sites below 1.1) to moderate (Pj maximum value is below 1.3) and it increases with susceptibility (Figs 14b andTable 2).

Figure 15 presents for each site the AMS principal axes (K 1 , K 2 and K 3 ) on an equal-area projection. Results of samples taken from basalt, magnetite bodies, skarn and retrograde stages are presented after bedding correction (165°∠49°), whereas the results from the marble and Permian dikes are presented in geographic coordinates.

The characteristics of the magnetic fabrics are described in the following lines:

(1) The three sites of basaltic sill display a relatively horizontal magnetic lineation (K 1 ) and a foliation with a low anisotropy degree (P J values less than 1.2). After (4) The sites 08 and 10 sampled in the retrograde stage present a horizontal foliation after bedding corrections (N57.2°E 68.8° and N161.2°E 80.5°, respectively) and a flat lineation (N200.7°E 17.4°, N293.7°E 6.4°, respectively), whereas the site 09-2 has a rather vertical foliation (N7°E 17.9) and a lineation oriented N263.2°E 36.4°.

(5) The samples drilled in marble show a near vertical N170°E magnetic foliation (The pole of foliation with no bedding corrections in this case are N339.5°E 19° and N350°E 13.3°, sites 9, 6, respectively). The lineation is horizontal with the following orientation: N73.6°E 11.9° and N80.6°E 2.6°, respectively.

(6) The AMS measured in the Permian dikes show a low degree of anisotropy, a clustered vertical lineation, and variable shape parameters (Table 2). As for samples from Site 04-1, its vertical foliation is dominant and parallel to the dike: the pole of foliation with no bedding correction is N312.6°E 9.3° with a vertical lineation (N187.2°E 74.2°), and for Site 14, the foliation with a pole oriented N53.2°E 6.9° differs from the dike orientation as its shape parameter is close to 0, the lineation is vertical oriented N192.7°E 81°.

Most of the magnetic foliations measured in the basalts, magnetite bodies, skarn and retrograde stage facies are horizontal and show no relationship between the degree of anisotropy P J and the direction of the magnetic foliation (Table 2).

Discussion

Ore body initial position

Field observations provide evidence on the tilting of the Carboniferous sedimentary series, however, the initial position of the skarn and magnetite bodies has been poorly constrained so far this work. The high coercitivity remanence components of the measured basalt and skarn samples give well grouped directions.

The paleomagnetic fold test cannot be applied to these directions because of the monoclinal bedding, however the untilted magnetic directions (i.e. after bedding correction) are reversed and are aligned with the Kiaman Reverse Superchron directions dated from 312 to 262 Ma [START_REF] Merrill | The Magnetic Field of the Earth. Paleomagnetism, the Core, and the Deep Mantle[END_REF]. These directions are similar also to those obtained from late Paleozoic rocks at 324 Ma in the Tianshan region (e.g. [START_REF] Li | Paleomagnetic study and tectonic evolution of Xinjiang and its neighboring regions[END_REF]. The magnetic remanence isolated from the higher coercive component may be considered therefore as primary. Consequently, the "basalt" likely intruded horizontally into the limestone as a sill before being tilted during later Permian strike-slip faulting. In the same way, the skarnization would have occurred before tilting. Though, no dating using paleomagnetic data is proposed in this study because of the small number of samples, the possibility of slightly inclined basalt bedding and the influence of secular variation.

Finally the structural data are coherent and compatible with the transpressional right-lateral strike-slip along the Yamansu shear zone: the horizontal NW shortening is accommodated by coeval dextral and reverse fault planes. In this context, NNE/NE-trending faults might be interpreted as domino (i.e. "bookshelf") faults, but as said above, the antithetic kinematics are not proven. It is noteworthy that this deformation postdates the ore deposition as it is crosscut and offset by the faults.

Interpretation of magnetic fabrics: deformation vs. magmatic flow and fluid flow

In the "basalt", skarn, magnetite bodies and retrograde assemblage, the susceptibility carrier is multidomain magnetite and the magnetic fabrics are normal (i.e. there is a correspondence between the shapes of the magnetite grain and the ellipsoid susceptibility: the long axis of the grain has the K1 direction and the magnetite foliation is parallel to the grain flattening). After bedding corrections, the horizontal magnetic fabrics of the "basalt" together with their low degree of anisotropy is interpreted to be magmatic, and suggests a horizontal N-S magma flow (i.e. perpendicular to the Yamansu fault) during the sill intrusion. The skarn formation, magnetite bodies and retrograde assemblage show two principal types of magnetic fabrics: a mainly N-S oriented vertical foliation and horizontal foliations with a frequent NE-SW lineation with a variable degree of anisotropy (P J ) that depends on the bulk susceptibility values (Km) (Fig. 13b). Thus P J is controlled by the magnetite content within the samples rather than by the deformation. This interpretation is supported also by the absence of a correlation between the variations of the anisotropy degree and the dip of the foliation. Therefore the magnetic fabrics are not related to the deformation. Comparing these fabrics with those measured in basalt could lead to the conclusion that fabrics in the skarn are inherited from the "basalt". However, such a conclusion is not tenable because the magnetite does not have a magmatic origin as iron concentration and bulk susceptibility are both higher in the skarn than in the basalt. The magnetic fabrics measured in those rocks are, therefore, related to hydrothermal magnetite formed during a metasomatic event. In this case it is possible to interpret the magnetic fabric in terms of hydrothermal fluid flow direction. The fabrics carried by minerals with a high susceptibility such as magnetite are related to the shape of the minerals (Stacey and Barnerjee 1974 in [START_REF] Dunlop | Rock magnetism. fundamentals and frontiers[END_REF].

It is known that hydrothermal flow influences the shape of the precipitating minerals, i.e. the shape is elongated along the flow direction (Sizaret et al. 2002;Sizaret et al., 2006a[START_REF] Essalhi | Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS). An example from Aptian carbonates of La Florida, Northern Spain[END_REF]. In such conditions, we propose that magnetite in the Yamansu deposit has recorded the metasomatic flow direction. Horizontal fabrics with NE-SW lineation suggest the metasomatic flow was parallel to the boundaries of the sill and oblique to the Yamansu fault. The N-S oriented vertical foliations with vertical lineation (such as Sites 17-1 in the garnet skarn and 12-2 in the magnetite ore body) are interpreted to be in N-S to NE-SW vertical fractures carrying hydrothermal flows. The study of basaltic and metasomatic flow implies both the pre-existence of a fault that channelized magma and hydrothermal fluids before the deformation events.

The susceptibility carrier in marble is calcite. Its fabrics are controlled by the orientation of the calcite <c> axes that are parallel to K 3 axis (inverse fabric; Fig. 10).

The 340 -350° orientation of the <c> axis is interpreted to be the result of dextral shearing. Previous studies on natural samples show that low temperature (<300°C) shearing may produce a rotation of the <c> axis in an opposite sense to the shearing as the twining of the calcite tends to be parallel to the main stress  1 and to the <c> axis (e.g. [START_REF] Turner | Experimental deformation of calcite crystals[END_REF][START_REF] Wenk | Pure shear and simple shear calcite textures. Comparison of experimental, theoretical and natural data[END_REF][START_REF] Nicol | Structural study of the Zn-Pb mineralzations of the paleozoic of the Pierrefitte dome (Hautes Pyrenee)-Texture goniometry applied to transparent and opaque minerals[END_REF].

Samples taken in the dykes show a vertical lineation that is interpreted to record a vertical magma flow pattern. Previous studies have shown that mafic dykes mostly preserve a vertical K 1 orientation, indicating vertical magmatic flow (e.g. [START_REF] Craddock | Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora-Kabetogama composite dykes (Minnesota and Ontario)[END_REF]. Studies reveal that the vertical-K 1 -type dykes were formed in an extension and fast upward flow environment [START_REF] Callot | Rock texture and magnetic lineation in dykes. a simple analytical model[END_REF][START_REF] Cañón-Tapia | Sampling strategies and the anisotropy of magnetic susceptibility of dykes[END_REF], however imbrications have been observed in some cases and could explain that the foliation is not parallel to the dike (site 14) [START_REF] Tarling | The magnetic anisotropy of rocks[END_REF].

Model and conclusions

In their general reviews on world skarn deposits, [START_REF] Einaudi | Skarn deposits[END_REF] and [START_REF] Meinert | World skarn deposits[END_REF] proposed a continuum between metamorphic hornfels and metasomatic skarn. In the Yamansu skarn, even if the fluids produced by the basalt would have contributed to deuteric alteration, the large amount of iron compared to basalt must have involved a huge contribution of metasomatism with a large fluid flow to provide the necessary chemical fluxes. As discussed above, the pre-existing fault probably channelized deep-sourced, rapidly ascending fluids (Fig. 16). This paleomagnetic study shows that the basaltic sill and the skarn deposit were tilted during Permian dextral shear.

These results are in agreement with the SVIO model proposed by Hou et al., (2014b) supporting the hypothesis of stratoid basaltic bodies being metasomatised soon after emplacement from a shallow magma chamber by seawater circulation through pre-existing faults. However, field observations show the presence of magmatic stoping and the absence of pillows to strongly support sill emplacement rather than submarine basaltic flow. The textural observation supports the idea of a progressive replacement of the protolith by a garnet skarn under the relatively low pressure necessary for andradite and grossular nucleation and growth (Hariya et al., 1978). Moreover, our paragenetic study establishes the magnetite stage during the prograde stage with a mineralizing peak at the end, which is uncommon. Such a characteristic could be linked to the shallow position of skarn and could be the one of the characteristics of SVIO model. In summary, the formation of the YMD can be described in four steps as follow:

1. Deformation of the host Paleozoic formations by north-verging thrusting and folding that was accompanied by calc-alkaline magmatism (Fig. 16a) [START_REF] Wang | Understanding and study perspectives on tectonic evolution and crustal structure of the Paleozoic Chinese Tianshan[END_REF][START_REF] Charvet | Palaeozoic tectonic evolution of the Tianshan belt, NW China[END_REF][START_REF] Wu | Geochronology , geochemistry and tectonic significances of the Hongyuntan granitoids in the Qoaag area Eastern Tianshan[END_REF]. Locally at Yamansu in back-arc environment, basaltic sills are injected into flat-lying limestone along a pre-existing vertical fault (Fig. 16b) Metasomatic fluid was channelized along the same fault and flowed parallel to the sill into the limestone in a N-S direction to produce the garnet skarn and the magnetite deposit in a submarine volcanic environnment (Fig. 16b).

2. Early Permian dextral transtensive strike-slip along the Yamansu shear zone responsible for tectonic subsidence in pull apart area were small Permian basin formed (Fig 16c). Note that mafic/ultramafic magmatism occurred at this time along other major Tianshan shear zones (e.g. [START_REF] Essalhi | Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS). An example from Aptian carbonates of La Florida, Northern Spain[END_REF].

3. Finally, according to our structural data, Late Permian dextral transpressive strike-slip tilted the formations into a positive flower structure that was responsible for the present attitude of the basalt, skarn and ore shoots (Fig. 16d).

According to our paleomagnetic study, the emplacement of the basaltic sill probably occurred in Late Carboniferous before Permian deformation. Finally, this is the first study to use magnetic fabrics to identify the primary flow direction during the genesis of a skarn-related deposit. We posit that the AMS is an efficient tool for determining the fluid flow circulation and front migration in metasomatic processes.

forming processes and adakitic rock in Tuwu-Yandong porphyry copper metallogenic belt, eastern Tianshan mountains. Acta Petrologica Sinica 20, 259-268.

Zhang, Z., Gu, L., Wu, C., Li, W., Xi, A., Wang, S., 2005. Zircon SHRIMP Dating for the Weiya Pluton, Eastern Tianshan. Its Geological Implications. Acta Geologica Sinica English edition 79, 481-490. Zijderveld, J.D.A., 1967. The natural remanent magnetization of the Exeter Volcanic Traps (Permian, Europe). Tectonophysics 4, 121-153 Zou, G., Liang, G., Chen, J., Zheng, Y., Gao, J., Xing, D., Li, S., 2006. Late paleozoic tectonic framword and evolution in the Jiabaishan area, Qoltag, eastern Tianshan, Northwest China. Geological Bulletin of China 25, 48-57. [START_REF] Dunlop | Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data[END_REF]. Fig. 15. AMS results presented in equal-area projections. Squares, triangle and circles stand for K 1 (magnetic lineation), K 2 and K 3 (pole of magnetic foliation), respectively.

FIGURE CAPTIONS

Confidence ellipses at 95% level are drawn around site-mean direction. All sites are tilt-corrected coordinates except the sites of dyke and marble. Tilt correction is based on stratification plane marked by the ripple marks in limestone (165°∠49°). P J values are indicated in red and orange lines in stereoplots give the magnetic foliation. The results of AMS measurements from the Yamansu open pit. Litho: Lithology, n: number of specimens, K 1 magnetic lineation, K 3 pole of the magnetic foliation, D, I,  95min ,  95max are declination, inclination, statistic confidence at 95% level. P J : Corrected anisotropy degree, T: Anisotropy shape parameter [START_REF] Jelinek | Characterization of the magnetic fabric of rocks[END_REF], and Km: Mean bulk magnetic susceptibility (SI*g -1 ); mgt: magnetite rich rock; grt: massive garnet skarn; rss: retrograde stage skarn dominated by epidote, chlorite, tourmaline

(

  Fig 2b). Slickensides and shear criteria are very sparse along this fault family and cartographic offsets indicate both dextral and sinistral relative motion (Fig. 2b). North of the Yamansu open pit, en echelon tension gashes within conjugate shear bands and pressure-solution cleavage are associated within the Carboniferous limestones (Figs. 4e). All of these markers are vertical and clearly indicate an NW directed horizontal shortening.

  bedding corrections, the lineation directions are : N26.6°E 23.6°, N179.5°E 16.5°, N249°E 2.6° and the poles of foliation are N150.2°E 51.7°, N314.2°E 67.1°, N343.9°E 62.4°, for sites 07, 09-1 and 16, respectively (fig.15). .(2) The five sites of massive garnet skarn also show a horizontal magnetic lineation and foliation: after bedding corrections the poles of foliation are the following N118.3°E 76.2°, N191.3°E 20.4°, N347.1°E 65.3°, N155.6°E 79.6°, N193.9°E 84.9° (sites 04-2, 05-2, 13, 15-1, 15-2, respectively, fig.15). The average lineation is well defined with a NE-SW trend (the lineation after bedding corrections are : N213°E 1.1°, N357.4°E 69° , N213.7°E 17.7°, N395.5°E 5.2°, N100.2°E 0.4°, respectively). Two sites reveal high P J values (Sites 15-1 and 04-2) and high susceptibility, probably due to their high content of magnetite (details are addressed in Section 5.2). The site 17-1 displays a vertical magnetic foliation after bedding corrections N 288.8° 7° with a relatively redressed lineation N166.0° 60.9°) and the sites 06 and 05-1 seem ambiguous with an intermediate dip for the foliation (N43.3°E 34° and N216°E 34.4°, respectively) and horizontal lineation (N297.1° 22.5° and N118.2° 11.1°).(3) Three of the six sites from the massive magnetite bodies display a horizontal magnetic foliation (the pole of foliation after bedding corrections are N355.6°E 78.6°, N350.9°E 75.9°, N304.8°E 46.9°, for sites 01-1, 01-2, 03, respectively). The lineation is oriented NE-SW the measures after bedding corrections gives : N243°E 3.6°, N252.2°E 2.1°, N211.7°E 2.9°, respectively). The other three sites have N-S to NE-SW vertical foliation (The pole of foliation after bedding corrections are N330.1°E 5.3°, N283.7°E 7°, N287.6°E 30° for the sites 02, 12-2, 17-2) and the lineations have scattered direction: N60.4°E 1.9°, N31.9°E 68.7°, N49.4°E 42.5° (fig.15).

Fig. 1 .

 1 Fig. 1. Simplified location map showing for the Yamansu iron skarn deposit and the

Fig. 3 .

 3 Fig. 3. Marble fragments within the strongly altered basaltic sill, suggesting mafic

Fig. 4 .

 4 Fig. 4. Schmidt equal-area projection plots in the lower hemisphere of (a) poles of

Fig. 5 .

 5 Fig. 5. Paragenetic sequence for the Yamansu iron skarn.

Fig. 6 .

 6 Fig. 6. Texture and mineralogy of the prograde and massive magnetite stages

Fig. 7 .

 7 Fig. 7. (a) Electron microprobe analyses showing the 573 compositional variations of

Fig. 8 .

 8 Fig. 8. Magnetic hysteresis loops for: (a) basalt, (b) massive garnet skarn (Loop

Fig. 9 .

 9 Fig. 9. Day plot diagram as the ratio of saturation remanence to saturation

Fig. 10 .

 10 Fig. 10. Analysis of the marble texture where the stereogram shows the lattice

Fig. 11 .

 11 Fig. 11. Paleomagnetic measurement results from the Yamansu skarn deposit.

Fig. 12 .

 12 Fig. 12. Equal-area stereoplots for the weaker viscous remanence that is close to the

Fig. 13 .

 13 Fig. 13. Equal-area stereoplots for "hard" remanence components. White (black)

Fig. 14 .

 14 Fig. 14. AMS scalar parameters for sites grouped by lithologies. (a): T shape

Fig. 16 .

 16 Fig. 16. Metallogenic model of the Yamansu iron deposit

Fig. 1 .FIG. 2 .

 12 Fig. 1. Simplified location map showing for the Yamansu iron skarn deposit and the

Fig. 3 .

 3 Fig. 3. Marble fragments within the strongly altered basaltic sill, suggesting mafic

Fig. 4 .Fig. 6 .

 46 Fig. 4. Schmidt equal-area projection plots in the lower hemisphere of (a) poles of

Fig. 7 .

 7 Fig. 7. (a) Electron microprobe analyses showing the compositional variations of

Fig. 8 .

 8 Fig. 8. Magnetic hysteresis loops for: (a) basalt, (b) massive garnet skarn (Loop

Fig. 9 .

 9 Fig. 9. Day plot diagram as the ratio of saturation remanence to saturation

Fig. 10 .

 10 Fig. 10. Analysis of the marble texture where the stereogram shows the lattice

Fig. 11 .

 11 Fig. 11. Paleomagnetic measurement results from the Yamansu skarn deposit.

Fig. 12 .

 12 Fig. 12. Equal-area stereoplots for the weaker viscous remanence that is close to the

Fig. 13 .

 13 Fig. 13. Equal-area stereoplots for "hard" remanence components. White (black)

Fig. 14 .

 14 Fig. 14. AMS scalar parameters for sites grouped by lithologies. (a): T shape

TABLE CAPTIONS Table 1

 CAPTIONS1 Paleomagnetic measurement results from Yamansu magnetite deposit.

Table 2

 2 AMS statistic results and associated parametersTABLES

Table 1

 1 Paleomagnetic measurement results from the Yamansu magnetite deposit.

		N	Dg	Ig	Ds	Is	ks	ɑ95
	Lower coercivity	9 (2 basalt +7
	viscous remanence	garnet skarn)	9.7°	58.9°	131.4°	66.1° 103.2	5.1°
	High coercivity	8 (3 basalt +5
	stable remanence	garnet skarn)	136.8° 15.6° 134.1°	-27.9° 29.9	10.3
	Note: the tilt correction is based on the ripple plane attitude (165°∠49°) of the
	limestone. Paleomagnetic samples are from Sites 06 and 16 of basaltic sill and
	massive garnet, respectively. N: sample number, Dg: geographic declination, Ig:
	geographic inclination, Ds: stratigraphic declination, Is: stratigraphic inclination, 95
	statistic confidence at 95% level, ks: magnetic intensity

magnetite. (e) Rhythmic banding of garnet (reddish) and magnetite (black).

Acknowledgements

We specially thank Baosteel Company for providing access to their mine and unpublished documents. The first author benefits from a scholarship grant from the French Ministry of Education and Research. This study was financially supported by the National 973 Project (2007CB411301), Geological Survey Project (1212011140056) and NNSFC (40603008, 40672040) of China. Thanks a lot to Dr D.T.A Symons, Dr J. Žák, and Dr Santosh associate editor who have reviewed and improved the presented manuscript.


Florence Renucci

L'objectif de cette étude est de mettre en évidence les grandes lignes de l'histoire de l'inspection du travail et son rapport au droit français en Afrique noire tout au long du XXe siècle. Il s'agit d'une recherche introductive à un travail plus approfondi. Sans nier les différences qui peuvent exister d'un territoire à un autre de l'Afrique, nous espérons dessiner une vue d'ensemble qui devra être vérifiée au cas par cas. Elle s'appuie à la fois sur des sources juridiques traditionnelles (législation, jurisprudence, doctrine) et sur les archives des ministères et des gouvernements généraux, conservées au Centre des Archives d'Outre-Mer (CAOM) d'Aix-en-Provence.

L'apparition du corps des inspecteurs est beaucoup plus tardive outre-mer qu'en métropole. L'inspection métropolitaine est d'abord liée au contrôle du travail des enfants 1 . La loi du 2 novembre 1892 fait des inspecteurs des fonctionnaires d'Etat 2 . Elle aura de commun avec l'inspection ultramarine de ne devenir un corps autonome que tardivement. La pression internationale ou l'exemple d'autres pays européens sont également déterminants dans les deux cas.

De fait, la création et le développement de l'inspection du travail outre-mer sont étroitement associés à l'évolution du contexte international. Parallèlement, ses particularismes sont davantage à mettre en relation avec l'histoire coloniale. Quant à son héritage, il mêle les deux influences. C'est du moins ce qu'il semble ressortir de l'étude des deux grandes périodes qu'a connu cette administration avant les indépendances. Fortement limitée dans ses prérogatives jusqu'en 1942, elle acquiert par la suite une importance qu'elle ne possède pas en métropole. L'intérêt pratique et actuel de cette recherche réside notamment dans le legs juridique qu'elle a transmis dans certains territoires français ultramarins et en Afrique, ainsi que dans la dimension d'anticipation du droit qu'elle possède.

Les balbutiements de l'inspection du travail en

Auparavant, cette mission était assurée par des « administrateurs ou des fonctionnaires spécialement commissionnés par le chef du territoire en qualité d'inspecteurs du travail »3 . Le lien étroit qui unissait ces derniers à l'administration coloniale n'était pas sans conséquences sur le contenu et la pratique de leur mission.

Le statut particulier de l'inspecteur du travail ultramarin

Un décret du 22 octobre 1925 pour l'Afrique Occidentale Française (AOF) prévoyait que l'administration prenne en charge l'inspection du travail. Les « autorités administratives ordinaires » étaient compétentes sur ce plan, selon les termes de l'arrêté général du 29 mars 1926. Le 22 janvier 1932 était créée officiellement une « inspection du travail et de la maind'oeuvre indigène en AOF », rattachée tout d'abord au gouvernement général, puis, à partir de 1934, à la direction générale des services économiques. L'inspecteur était nommé par le lieutenant gouverneur.

Cette situation engendrait une double dépendance. En premier lieu, il n'existait pas d'autonomie des fonctionnaires en charge de l'inspection qui demeuraient étroitement subordonnés aux autorités locales, contrairement à ce qui se produisait en métropole. En général, ces fonctionnaires n'avaient reçu aucune formation spécifique et ne possédaient a priori aucune expérience spéciale dans ce type de contrôle. En second lieu, la législation du travail dans les colonies était essentiellement composée de textes pris par les autorités locales. Les gouverneurs avaient dans ce domaine un pouvoir réglementaire. Par conséquent, l'administration contrôlait des textes qu'elle avait elle-même produits et qui pouvaient diverger d'un territoire à un autre.

Le rôle de l'inspection du travail dans un contexte de droits limités

Cette absence de séparation entre les organes de production du droit et de contrôle s'inscrivait dans un contexte historique et juridique bien particulier. Dans la pratique, les mesures en faveur de la liberté du travail et de la protection des travailleurs étaient extrêmement réduites 4 . Au début du XXe siècle, le travail forcé était la règle. L'administration utilisait la contrainte pour la mise en valeur de la colonie au nom de l'intérêt général, mais également d'intérêts privés 5 (comme ce fut, par exemple, le cas lors de la construction du Congo-Océan avec une main-d'oeuvre évaluée à 15.000 hommes en 1927) 6 . Progressivement, à partir des années Vingt, les différentes formes de travail forcé allaient se restreindre, en particulier le travail forcé dans l'intérêt des particuliers. L'administration disposait toutefois de moyens détournés pour les appliquer 7 . Les travaux d'intérêt public persistaient. En 1926, par exemple, le gouverneur général Olivier créait le Service de la Main-d'oeuvre pour les Travaux d'Intérêt Général ou SMOTIG. L'AOF était pourvue d'un service similaire (le service de la maind'oeuvre) en 19288 . Le travail forcé n'était pas uniquement justifié par des motifs économiques, mais reposait également sur un soubassement idéologique selon lequel la liberté du travail ne pouvait se concevoir que lorsque l'obligation de travailler était entrée dans les moeurs9 . Derrière cette affirmation, l'idée d'une différence culturelle et l'image de l'indigène-enfant se dessinaient. Le sujet indigène était assimilée à un enfant parce qu'il préférait, selon les colonisateurs, jouir de la vie, ne rien faire, plutôt que de travailler [START_REF]De son côté, l'inspecteur de Guinée écrivait en 1946 que l'autochtone « répugne généralement au travail salarié, pour lequel il n'a aucun goût. Il fuit le travail régulier, avec ses heures fixes[END_REF] . Il fallait donc le forcer à le faire. Cette attitude s'apparentait à de l'irresponsabilité. Il est intéressant de noter que cette approche de l'irresponsabilité n'était pas uniquement morale, elle pouvait également se rapporter à une définition juridique. En droit, l'enfant est irresponsable. Avec la législation d'outre-mer d'après-guerre toutefois, cette vision semble s'être modifiée puisque le travailleur tout comme l'employeur devaient être, pour le législateur, responsabilisés [START_REF] Ainsi | en cas d'accident dû à une faute intentionnelle de la victime, cette dernière n'a pas le droit de bénéficier des prestations des assurances sociales, contrairement à ce qui se produit en métropole. Concernant l'employeur, ce dernier ne peut contracter une assurance contre les risques de la faute inexcusable dans le but de l'obliger à développer la prévention sur le lieu de travail[END_REF] .

Dans ce contexte, où la notion de main-d'oeuvre primait clairement sur celle de travailleur, l'inspection du travail était limitée dans le contrôle du droit du travail. Ce contrôle n'était d'ailleurs pas considéré comme sa véritable mission. En règle générale, elle était au service des employeurs, son rôle étant d'offrir à ces derniers d'accéder à une main-d'oeuvre souvent réticente, notamment parce qu'elle était obligée de travailler loin de ses terres d'origine. Dans certains cas toutefois, les inspecteurs tentaient de limiter les abus les plus criants, par exemple en Côte d'Ivoire pour mauvais traitements ou pour défaut de paiement de salaires [START_REF] Duperay | L'inspection du travail en AOF et les travailleurs de la Haute-Volta (1932-1960)[END_REF] , et de veiller à ce que les travailleurs soient décemment traités 13 . Le contrôle de l'application du droit était également limité du fait que le droit du travail outremer l'était lui-même. Avec l'arrivée au pouvoir du Front populaire, celui-ci s'étoffa. Si les grandes réformes telle que la réduction du temps de travail ne furent pas appliquées aux colonies, un décret du 20 mars 1937 prévoyait l'introduction de conventions collectives dans les professions industrielles et commerciales en AOF, ainsi que l'institution de délégués élus du personnel dans les établissements ayant plus de dix salariés. De surcroît, les dispositions concernant les syndicats dans le Code du travail métropolitain étaient rendues applicables par le décret du 17 novembre 1937. Mais il s'agissait d'un droit encore très embryonnaire et qui ne concernait que quelques territoires.

Il faudra attendre la fin de la Seconde Guerre mondiale pour connaître une réforme du droit du travail outre-mer, mais également de l'inspection du travail.

Une inspection du travail indépendante aux pouvoirs renforcés

Les balbutiements de la réforme de l'inspection du travail date de 1943. Elle portait sur la question de l'autonomie des inspecteurs vis-à-vis de l'administration et sur la réorganisation de leur mission.

Un nouveau statut

Une quadruple mission : contrôler, conseiller, concilier et produire

L'arrêté du 4 avril 1943 précisait également que la principale mission de l'inspecteur était de contrôler l'application de la réglementation du travail. La production de cette réglementation était conservée par la direction générale des affaires politiques. Par conséquent, il n'y avait plus de confusion entre la production et le contrôle du droit du travail. Toutefois, cette situation allait être modifiée par les textes ultérieurs.

Le décret du 17 août 1944 modifiait l'organisation antérieure, tout en étant présenté comme un texte transitoire. La nouvelle codification du travail applicable outre-mer devait avaliser et préciser la réforme. Toutefois, la promulgation de cette codification se fit attendre plusieurs années 21 , si bien que les inspecteurs du travail se retrouvaient en possession d'un corpus qui n'avait pas encore de valeur légale. Annie Duperay a mis en évidence le rôle de garant de la paix sociale de l'inspection à cette époque en insistant sur sa fonction d'intermédiaire entre la main-d'oeuvre et les employeurs, ainsi que sur celles de contrôle et de formation 22 . Elle ne fait pourtant pas allusion au fait que durant cette période, l'inspection a pu parfois remédier au vide législatif qui existait en s'inspirant de l'esprit des nouvelles réformes. En 1952, peu de temps avant la promulgation du Code du Travail Outre-Mer (CTOM), L. Guelfi indiquait les solutions palliatives utilisées par son administration 23 . Même après la promulgation du CTOM, mais en attente des arrêtés nécessaires, l'inspection a usé de pratiques transitoires, notamment dans le domaine des contrats 24 .

Ce rôle original de production du droit -qui côtoyait la mission de contrôle, de conseil et de conciliation -était confirmé dans le texte de 1952 et dans le décret du 29 décembre 1955. Les inspecteurs généraux avaient en charge l'élaboration des règlements de travail, tandis que les inspecteurs du travail participaient « à l'élaboration de la réglementation locale de leur compétence ». Ces mesures étaient véritablement appliquées pour l'outre-mer et ne demeuraient en rien théoriques 25 . Dans la pratique, le pouvoir des inspecteurs était plus étendu car ils étaient sollicités en tant que techniciens dans l'élaboration des textes. La frontière entre le rôle de conseil 26 et de production est ténue dans ce cas 27 . Certains inspecteurs ont d'ailleurs été sollicités lors des choix à opérer sur des grandes questions du droit du travail ou du droit social dans le CTOM. Cette situation explique qu'un auteur ait pu affirmer que « le titulaire du pouvoir réglementaire [n'avait] pratiquement plus qu'un pouvoir formel de signer. La réalité du pouvoir [appartenait] alors à l'inspection » 28 . On revenait donc pour beaucoup, dans les faits, à la réunion des pouvoirs de production et de contrôle du droit, mais elle se faisait à présent au profit de l'inspection du travail. En matière de contrôle, l'une des principales caractéristiques qui différenciait le système ultramarin du système métropolitain était l'ampleur des compétences des inspecteurs. Outre leur compétence générale relative à la condition des travailleurs, à leur emploi et aux rapports professionnels, ils étaient également en charge, depuis le décret du 29 décembre 1955, de la sécurité et de la prévoyance sociales outre-mer. Enfin, il faut constater que leur rôle de conciliation n'avait pas uniquement pour utilité d'éviter le recours aux tribunaux ou que le conflit ne s'envenime au sein de l'entreprise. Il s'agissait aussi de limiter par ce biais l'emprise des syndicats lors des conflits de travail 29 . Paradoxalement, le gouvernement avait tout intérêt à soutenir l'action sociale de l'inspection dans la mesure où elle concurrençait les syndicats qui étaient, a priori, plus difficiles à encadrer.

Les raisons d'un changement

Comment expliquer ces réformes ? Parmi les raisons avancées, on trouve la carence sociale de l'administration, mais également « la nécessité de détacher l'inspection d'une administration qui est le plus gros employeur outre-mer et le fait que l'inspection doit être l'instrument de réalisation de la politique sociale voulue par le parlement et le gouvernement » 30 prolonger leur service après les indépendances le temps d'assurer la relève grâce à « l'africanisation des cadres » 31 .

Rupture et continuité. L'inspection du travail des indépendances à nos jours

Les ressemblances et les dissemblances entre l'organisation coloniale et postcoloniale se limitent le plus souvent au cadre des pays anciennement occupés. Or, celles-ci doivent également être analysées dans le milieu métropolitain.

L'évolution du rôle de l'inspection du travail en Afrique francophone

Après les indépendances, le système de l'inspection du travail tel qu'il existait auparavant fut en grande partie repris, avec parfois quelques variantes dans la formation et l'organisation. C'est le cas notamment des pouvoirs de l'inspection du travail auxquels certains codes confient explicitement l'élaboration des projets de lois et de règlements concernant le travail, la main d'oeuvre et la sécurité sociale (par exemple, le Burundi 32 et le Sénégal [START_REF]1 du Code du travail du Sénégal. 34 Cour de cassation[END_REF] ). La même remarque vaut pour plusieurs questions tranchées par la jurisprudence. On peut notamment citer le maintien de la jurisprudence de la Cour de cassation qui reconnaissait aux inspecteurs le droit de pénétrer la nuit dans les établissements où le travail était réalisé la nuit 34 et dans des établissements où des indices permettaient de conclure à un travail de nuit [START_REF]Cour de cassation (ch. crim.)[END_REF] .

Parmi les différences avec les anciennes règles, il faut noter l'importance encore accrue de la conciliation. Comme l'indique le professeur Gonidec, « en matière de conflits individuels, tous les codes prévoient une procédure de conciliation préalable devant l'inspecteur du travail sur l'initiative de l'une des parties. De même, en matière de conflits collectifs, le législateur va encore plus loin en imposant la conciliation sous peine d'amende » [START_REF]Cours de droit du travail africain…[END_REF] . Cette particularité serait liée, selon lui, aux « traditions ancestrales » de ces pays 37 .

Le CTOM et, par conséquent, les mesures qu'il contenait traitant de l'inspection, a également continué de s'appliquer en France après les indépendances.

L'évolution du rôle de l'inspection du travail en France

En Polynésie, par exemple, l'inspection du travail et des lois sociales devenait logiquement un service autonome en 1953, possédant des prérogatives étendues, notamment en matière sociale. A partir de 1986, avec les réformes engagées sur le monde du travail, les compétences des inspecteurs furent réduites au point de pouvoir être identifiées à celles de leurs homologues métropolitains. Alors que la défense des salariés était prioritairement le terrain de l'inspection, celle-ci a laissé progressivement sa place aux syndicats. Ces changements

  Afrique noire D'une certaine façon, il a existé une inspection du travail avant…l'inspection du travail. Il faudra en effet attendre 1944 pour que soit créée une inspection du travail indépendante. 1 Cf. la loi du 2 mars 1841 « relative au travail des enfants employés dans les manufactures, usines ou ateliers » (DUVERGIER J.-B., Collection complète des lois, décrets et avis du Conseil d'Etat, Paris, 1841, en particulier p. 52 et s.) et celle du 19 mai 1874 sur « le travail des enfants et des filles mineures, employées dans l'industrie » (Ibid., 1874, en particulier p. 156 et s). L'inspection se trouve en réalité « en gestation » depuis 1827 et on lui connaît un précédent : l'inspection des manufactures créées en 1669 par Colbert. Elle disparaîtra en 1791. Cf. VIET V., Les voltigeurs de la République. L'inspection du travail en France jusqu'en 1914, Paris, CNRS, vol. 1, p. 21 et s. 2 « Loi du 2 novembre 1892 sur le travail des enfants, des filles mineures et des femmes dans les établissements industriels » (DUVERGIER J.-B., op. cit., 1892, Paris, Larose éditeur, en particulier p. 372 et s.).

L

  'arrêté du 4 avril 1943 prévoyait la création d'une inspection générale distincte des autres services administratifs en AOF. Toutefois, cette réforme ne concernait qu'une partie des territoires. De plus, la formation et les conditions de recrutement des inspecteurs étaient inchangées. Enfin, l'inspection restait « sous l'autorité immédiate du gouverneur général ». Plus d'un an après cet arrêté, le décret du 17 août 1944 créait un corps d'inspecteurs du travail aux colonies 14 . Il était véritablement autonome puisque ses membres étaient nommés par décret et dépendaient du service central du travail auquel ils rendaient des comptes. Ce service gérait leurs affectations dans tel ou tel territoire, oblitérant le lien qui existait auparavant avec le chef du territoire.Les inspecteurs acquerraient un statut propre à d'autres égards (formation, rémunération, etc.). A l'origine, il était prévu qu'ils soient recrutés par voie de concours, mais finalement le décret du 30-31 octobre 1950 qui réorganisait l'Ecole nationale de la France d'Outre-Mer (ENFOM) indiquait qu'il s'y trouverait une section de l'inspection du travail. Comment expliquer le choix de l'ENFOM ? La formation qui était dispensée dans cet établissement était adaptée à la spécificité des territoires coloniaux. Cette option répondait également à « une volonté d'intégrer les problèmes sociaux dans la formation des administrateurs coloniaux » 15 . Durant la période intermédiaire, des administrateurs civils ou des colonies ainsi que des inspecteurs du travail métropolitain occupaient les postes du nouveau corps16 .Il faut remarquer que ces inspecteurs se différenciaient d'autres fonctionnaires par le mode de prestation du serment. De fait, « contrairement à la règle qui veut que le serment soit oral le serment [était] prêté par écrit. Il [était] prêté devant la cour d'appel ou le tribunal supérieur d'appel de leur ressort ». Comme en métropole, sa violation était punie « conformément à l'article 378 du Code pénal » 17 . Dans la pratique, le serment devait être de nouveau prêté lors d'une mutation, comme le prouve une lettre du procureur général en AEF à l'inspecteur général du travail et des lois sociales en AEF datée de 195518 . La prestation par écrit 19 est sans entretenaient avec les grandes compagnies. Pour se faire une idée de ce lien dans la pratique, même après la Seconde Guerre mondiale, on peut se référer à MAZENOT G., Le dernier commandant. Mémoires d'outre-mer, Paris, L'Harmattan, 1996, p. 64 et s.17 CHAULEUR P. (docteur en droit, administrateur en chef de la France d'Outre-Mer), Le régime du travail dans les territoires d'Outre-Mer, Paris, éditions de l'Union française, 1956, p. 493. Les conséquences de la violation du serment étaient déjà prévues dans l'article 18 de la loi du 2 novembre 1892. Elle existe toujours sauf qu'elle est punie aujourd'hui conformément à l'article 226-13 du Code pénal. 18 « Lettre du procureur général, chef du service judiciaire de l'Afrique Equatoriale Française Giacobbi, à l'inspecteur général du travail et des lois sociales en AEF, Brazzaville, le 7 juillet 1955 », CAOM, G1 AEF, 1H21. 19 « Inspection du travail et des lois sociales de l'AEF. Serment prêté par devant la Cour d'appel de l'AEF par Monsieur Barraux, Inspecteur adjoint à l'Inspection territoriale du Travail de l'Oubangui-Chari (référence de l'art. 151 de la loi du 15 décembre 1952). Je soussigné, Barraux, Roland, Henri, Inspecteur affecté à l'inspection territoriale de l'Oubangui-Chari par décision (dont copie jointe) n°85/DP-2 du 11 janvier 1952 de Monsieur le Haut-Commissaire de la République en AEF, jure de bien et fidèlement remplir ma charge et de ne pas révéler, même après avoir quitté mon service, les secrets de fabrication et, en général, les procédés d'exploitation dont je doute à mettre en relation avec le problème de distances propres à ces territoires. Cette règle a d'ailleurs été reprise dans les législations de certains pays d'Afrique noire, comme le Sénégal ou le Congo Brazzaville 20 . Dans le premier cas, cette pratique est conditionnée au fait que l'intéressé ne réside pas au siège de la cour d'appel.

  pourrais prendre connaissance dans l'exercice de mes fonctions. Fait à Bangui, le quatre juillet 1953, signature » (CAOM, G1 AEF, 1H21).20 Sur le Congo Brazzaville, cf. N'GAKA P., Le droit du travail au Congo Brazzaville, Paris, L'Harmattan, 2006, p. 26. 21 RENUCCI F., « L'élaboration du Code du travail outre-mer et la durée du travail en Afrique occidentale française », CHATRIOT A., JOIN-LAMBERT O. et VIET V. (dir.), Les politiques du travail (1906-2006), Rennes, Presses Universitaires de Rennes, coll. « Pour une histoire du travail », 2006, p. 59-68. 22 DUPERAY A., op. cit., p. 203 et s. On peut également citer dans son sens COOPER F., op. cit., p. 272. 23 « Note pour le ministre, signée Guelfi, sl, le 28 octobre 1952 », CAOM, Fonds Ministériels (FM), Inspection Générale du Travail (IGT) 97, p. 3. 24 Cf. « Lettre de l'inspecteur général du Travail à l'inspecteur général du travail et des lois sociales de la France d'Outre-Mer, Yaoundé, le 18 avril 1953 », CAOM, FM, IGT 70, p. 2. 25 « Lettre de l'inspecteur général du travail à l'inspecteur général du travail et des lois sociales de la France d'outre-mer, Yaoundé, le 17 juillet 1953 », CAOM, FM, IGT 71, p. 3. 26 Son rôle de conseil consiste également, plus traditionnellement, à éclairer les employeurs et les travailleurs sur les règlements des conflits de travail, etc.

.

  Il est également difficile d'expliquer ces réformes sans tenir compte des pressions internationales. Tout au long du XXe siècle, la Société des Nations (SDN) et l'Organisation Internationale du Travail (OIT) avaient oeuvré pour la restriction puis l'abolition du travail forcé (qui sera finalement interdit définitivement en France par la loi du 6 avril 1946), mais également, concernant l'OIT, pour la création d'une inspection qui soit mise sous le contrôle d'une autorité centrale. Elle insistait également sur l'expérience nécessaire des inspecteurs, sur la création d'un statut à part et sur le fait que ce personnel devait être bien payé pour éviter toute dépendance vis-à-vis des entreprises privées (en 1933 et en 1939). La conférence de Brazzaville reprit les principes exposés par l'OIT et cette dernière, à son tour, refaisait des recommandations en ce sens le 12 mai 1944. Le fait que la France soit sortie affaiblie de la Seconde Guerre mondiale ainsi que les pressions américaines et celles d'autres pays européens accéléra sans doute l'adoption de ces réformes.Un nouveau changement eut lieu avec la loi-cadre de 1956 qui réorganisait l'inspection du travail peu de temps avant les indépendances. Certains des fonctionnaires sur place allaient27 Un exemple de son rôle de conseil auprès des sphères décisionnaires dans : « Le ministre de la France d'Outre-Mer (en fait : pour le ministre et par ordre, l'inspecteur général du travail et des lois sociales de la France d'outre-mer, chef de service, Guelfi) au Haut-commissaire de la République française en Afrique Occidentale Française (inspection générale du travail et des lois sociales), Paris, le 3 octobre 1953 », CAOM, FM, IGT 72, p. 3. 28 GONIDEC P.-F., op. cit., p. 305-306.29 « Lettre du secrétaire général R. Couret pour le gouverneur, chef du territoire du Tchad, au gouverneur général de la France d'Outre-mer, haut commissaire de la République en AEF à l'administration générale (affaires politiques), Brazzaville, le 18 août 1956 », CAOM, G1 AEF, 1H30.

GONIDEC P.-F. (professeur à la faculté de droit de Rennes et professeur de droit du travail à l'Ecole Nationale de la France d'Outre-Mer (ENFOM)), en collaboration avec KIRSCH M. (magistrat, ancien président du tribunal du travail à Dakar), Droit du travail des territoires d'Outre-Mer, Paris, LGDJ, 1958, p. 292.

En théorie, le décret du 24 novembre 1922 pour l'Afrique Equatoriale Française (AEF) proclamait le principe de liberté du travail dans le cas de fin d'intérêt privé.

La division des attributions (administration/secteur privé) sur les chantiers engendra des conflits jurisprudentiels très intéressant en matière de responsabilité liée aux accidents du travail. Cf. sur ce point FABRE M., « Le sort du travailleur expatrié aux colonies. Des juges entre intransigeance et mansuétude », DURAND B. et FABRE M. (dir.), Les roches bleues de l'empire colonial, Lille, Centre d'Histoire Judiciaire (CHJ) éditeur, 2004, p. 345-370.

GONIDEC P.-F., Droit du travail des territoires d'Outre-mer, Paris, LGDJ, 1958, p. 27.

Ibid., p. 29.

Ibid., p. 30.

Le gouverneur général Olivier dans Six ans de politique sociale à Madagascar, cité par GONIDEC P.-F., ibid., p.

Journal Officiel (JO) du 22 août 1944, p. 738 modifié par décret du 9 octobre 1945, n°45, p. 2319.

DUPERAY A., op. cit., p. 201-202.

GONIDEC P.-F., op. cit., p. 294. Le maintien des administrateurs risquait de faire perdurer le lien étroit qu'ils


Catalina Gutierrez
email: catalina.gutierrez@ensam.eu

Philippe Mangin
email: bphilippe.mangin@ensam.eu

Laurent Langlois
email: claurent.langlois@ensam.eu

Régis Bigot
email: dregis.bigot@ensam.eu

Development of a tool design method in cross wedge rolling: description and applications

Keywords:

Till now, the definition of cross wedge rolling dies requires know-how and important expertise from the designers. A decision-making methodology is being developed to provide sequential and logical steps to draw easier and faster the tool geometry. This methodology is based on designing rules found in literature that link the geometrical parameters of the desired rolled part and the geometrical parameters of the tool. Nevertheless, in the literature, the rules are not always consistent because the admissible domain for a parameter can differ from one author to another. In order to take into account this variability, a stability index is associated to each rule and to the designed tool. The methodology allows updating of the existing rules and the implementation of new rules. The set of parameters defining the geometry of the tool can be exported in the CAD/CAM software and FEM software. A case-study is presented to illustrate and validate the methodology.

Introduction

The Cross Wedge Rolling process (CWR) is a metal forming process in which a cylindrical billet is plastically deformed into another axisymmetrical shape by the action of wedge segments, moving tangentially relative to each other. The movement of the wedges causes a progressive spread in the axial direction of the billet and a reduction in its cross-sectional area; it can be of simple diameter reduction or of several reductions. Two main configurations are industrially developed as shown in Fig. 1. The benefits of the CWR process, compared to other traditional forming methods, have been listed by several authors. For [START_REF] Li | Cross wedge rolling failure mechanisms and industrial applications[END_REF], CWR is a material and energy -saving process with lower levels of noise and lower environmental impact. For [START_REF] Li | Investigation of the morphology of internal defects in cross wedge rolling[END_REF] some of the advantages are: the improvements in product quality (fine tolerances, fiber structure), a higher productivity (cycle time up to 5 -10 sec) and a high accuracy and maximum proximity to required dimensions of finished products. In comparison with other metal forming processes, CWR is in commercial terms relatively new. The CWR process has been most likely developed in Eastern Europe (e.g. Russia, Belarus, Poland and Czech Republic) Japan and China. The communicating and translating difficulties are some of the reasons of the low and slow entry of this technology in Western Europe [START_REF] Fu | Past developments, current applications and trends in the cross wedge rolling process[END_REF]. At the Yokohama National University, Masujiro Hayama published his work on optimum working conditions in the CWR of stepped shaft [START_REF] Hayama | Optimum Working Conditions In The Cross Rolling Of Stepped Shaft[END_REF]. He proposed the optimum reduction in cross-sectional area of rod and the effective range of basic angles (forming and spreading angles) of the tool wedge. In 1998, [START_REF] Neugebauer | Optimierte verfahrensparameter und antriebskonzeptionen für das querwalzen[END_REF] from the Fraunhofer Institute for Machine Tools and Forming Technology in Chemnitz-Germany conducted the research work on the optimization of main parameters and the CWR machine's design. At the Lodz Technical University, Pater presented his work on the determination of the ranges of basic parameters for tool segments in order to avoid principal defects presented on the process (e.g necking, central cavities) [START_REF] Zb | Study of the process stability of cross wedge rolling[END_REF]. At the University of Pittsburg in United States, Lovell and Li carried out investigations on the morphology of internal defects in cross wedge rolling and the influence of main parameters [START_REF] Li | Investigation of the morphology of internal defects in cross wedge rolling[END_REF].

Cross-wedge rolling process

Due to the amount of parameters involved and its tridimensional nature, CWR is a complex process only stable within some specific bounds [START_REF] Weronski | Selection of geometric parameters of transverse wedge rolling tools[END_REF]. The difficulty relies in the relationship between the parameters that can be specific and different for each case. In a global scale, the parameters of interest of the CWR process can be classified in two main groups: geometrical parameters, such as the die's geometry (parameters listed in Fig. 2); and forming parameters, such as the required load, rolling rate, friction effect and strain, material, initial temperature of the billet, contact pressure, among others [START_REF] Weronski | Selection of geometric parameters of transverse wedge rolling tools[END_REF]. A wedge can be defined by the following parameters (Fig. 2): The billet is put in position at the beginning of the knifing zone. Once the wedges start to move, the tools bite into the surface of the billet and reduce its diameter to the required portion diameter. Movement is transmitted by the adherence of the billet to the wedges, creating a force torque making the billet to rotate around its longitudinal axis. Trybological phenomena are then of high importance, giving the role of surface texture of the wedges in the magnitude of the transmissible torque, contact condition and slip phenomena. The forming zone, the zone which gives shape to the workpiece, is the most important part of the tool. In this zone the workpiece is plastically deformed, being forced to flow to the ends of the tool. During forming, the surface deformation is propagated to the interior. At this point, some parts of the workpiece are under stress conditions: compressive, tensile and shear strain; and some parts are only in movement: rotation and translation. The forming angle α and the spreading angle β (see Fig. 2) determine the size of the contact area between the tools and the workpiece; and the amount of axial deformation experienced by the workpiece. In this contact strong tangential and normal stresses occur.

Cross-wedge rolling failures. At a global scale, the difficulty relies on the amount of parameters involved in the process that must be correctly chosen in order to avoid failures. Slight variations in basic parameters can have a high impact on the part. According to several authors [START_REF] Li | Investigation of the morphology of internal defects in cross wedge rolling[END_REF] [3], defects in CWR products can be classified in three groups (see Fig. 3): improper forming of a section, external defects and internal defects. According to [START_REF] Li | Cross wedge rolling failure mechanisms and industrial applications[END_REF] these failures are repeatable when encountered at a specific set of operating conditions. This subject is fully described and discussed in the research works done by [2] [3]. Cross-Wedge rolling tool design methods CWR application and evolution remains quite undeveloped in Western Europe, France in particular, despite all its known advantages. Several reasons are listed below:

• CWR tool design has been based on the experience and intuition of designers.

• It has a high degree of technological complexity.

• Little work has been done in the automation and in rational and systematic methods of die design. Moreover, there are no decision-making tools publicly available to the date. • Industry has been developing and working with forging rolling process.

• Very few researches and studies for CWR development. Theoretical and experimental research has been conducted in the aim of solving difficulties in implementing the CWR process in industrial conditions and the automation of the wedge designing method. Some of the different design approaches are: [START_REF] Umbach | Entwicklungsfortschritte zum Querwalzen von Stahlwerkstoffen[END_REF] and [START_REF] Abramov | Preparation of input data for the numerical analysis of CWR process on the basis of the program-methodical complex "Rolling", Advanced methods and technologies for materials manufacture and processing[END_REF]. It should be noted that most of the designing methods are developed privately and access to information related to it, is not easy.

COLT method (Conception d'Outillage de Laminage Transversal COLT for its initials in French), is a decision supporting tool for the designing of the die in cross wedge rolling (CWR). This methodology is being developed to provide sequential and logical steps to draw easier and faster the die geometry. COLT is based on the parametric definition of the desired rolled part, in the first place; and on the parametric definition of the tool, in the second place. The two corresponding sets of parameters are then linked through design rules.

Description of the CWR tool design procedure COLT. The aim of the COLT method is to allow a CWR expert or non-expert user to obtain the geometric parameters of the wedge by inputting the geometric characteristics of the desired part. This procedure is based on a sequence of logical steps as well as a series of criteria to be satisfied. COLT aims to integrate the state of art as much as possible. It also seeks to be a flexible tool, by allowing the updating of the already existing rules and the implementation of new design rules. The main steps of the COLT method according to this latest version (V02) are: Each of these stages encompasses another series of steps, at a more detailed level; and for each of these steps there are: an input data, a defined action, an output data and a number of rules to verify. Rules and stability index. COLT method is based on a synthesis of literature rules as well as rules identified during experimental work. Throughout the method, at each step, this design rules have to be validated in order to proceed to the following step.

The designing rules allow choosing the basic parameters of the wedge. Some of these rules are implemented in order to avoid specific CWR defects, while others correspond to geometrical relations between the parameters. Nevertheless, in the literature, the rules are not always consistent because the admissible domain of variation for a parameter can differ from one author to another. In order to take into account this variability, the concept of stability index is introduced. The stability index is associated to each rule, this one being a function of the parameters concerned by the rule. The maximum stability index will be attributed when all the authors/rules agree the value of the parameter is within the correct limits. Example: the acceptable limits of the forming angle α have been established by several authors: (see Table 2 and Fig. 5) In order to evaluate the different sources, the concept of maturity is used. It allows to weight the sources according to the judgment we make about the author.

Referencing to the geometrical parameters of a wedge, as it is shown in Fig. 2, three axial sections can be observed along the rolling length which are called profiles. These profiles are located at the beginning/end of each zone (i.e. knifing, guiding, forming and sizing zones) and each of them can be assumed as the sum of basic elements e.g. height, length, angles and radius edges and remarkable points. The axial profile of the reduced diameter section in the part matches to the final profile of the wedge.

The COLT method will output the coordinates(x,y,z) of the remarkable points as shown in Fig. 6, calculated through geometrical equations. The full amount of points will define the wedge and will integrate all the geometrical parameters previously enlisted.

Figure 6-Example of remarkable points

If the part consists of more than a simple diameter reduction, then each portion corresponds to one wedge and the relative positions of the different portions will determine de relative position of the wedges.

Demonstration example

A simple reduction part is presented in this article in order to explain the COLT method (see Fig. 7). (2) Definition of the CWR tool structure. If the part is longitudinal asymmetric, the COLT method will demand to perform a symmetry throughout its axial axis. Once the part is longitudinal symmetric, the part is classified. Based on the work by [START_REF] Fu | Development of a knowledge-based sequence design system for cross wedge rolling[END_REF], parts can be classified in three main types as well as wedges can be classified in four main types. A relationship between the type of parts and the type of wedges has been made, consequently once a part has been classified, the corresponding wedges are associated. Following this approach, the structure of the CWR die is one main wedge. A12. Characteristics of the machine. The machine available is a flat type with a maximum length of 220mm. The maximum diameter of the billet accepted is 100mm.

A13. Characteristics of the billet. For calculating the billet's dimensions, the volume of the part is needed. The first assumption made is that the maximum diameter of the part is the diameter of the billet. The length of the billet is solved in the equation of a cylinder, using the volume of the part and the diameter of the billet. The thermal expansion is then considered.

Once the dimensions are selected, it is necessary to confirm that the billet will at least perform 3 laps, in order to guarantee that at least at each rolling zone (knifing, forming and sizing zones) the billet will perform one lap, and so each part of the billet will be subject of the deformation. The number of laps is calculated with the maximum length of the machine and the perimeter of the billet. In this case, the billet can perform 3.3 laps.

From the drawing of the part, it is possible to know the final diameter of the only portion: 9.2mm. At this stage the reduction ratios are validated. Authors use different reduction ratios, for example [START_REF] Hayama | Optimum Working Conditions In The Cross Rolling Of Stepped Shaft[END_REF] use the optimum reduction in cross-section area R op , [START_REF] Neugebauer | New warm forming processes to product hollow shafts[END_REF] use the diameter reduction Ed; and relative reduction δ used by [START_REF] Zb | Study of the process stability of cross wedge rolling[END_REF]. In this example, both Ed and R op were studied. δ is studied in the following stages when also verifying other designing criteria. The reduction ratios are Ed=0.56 and R op =0.60, with stability index Ic=0.42 and Ic=1 respectively. A15 and A16. Definition of wedges. When starting the definition of the wedge, the first profile that can be determined is the one at the end of the sizing zone (profile 3). This profile is imposed by the dimensions of the part, that is to say the geometrical dimensions of the profile 3 are given by the geometrical dimensions of the groove (height, angles, length) of the part. During the sizing zone, the profile is constant, which means that the profile 2, at the beginning of the sizing zone it is the same as profile 3 and so the imposed profile (see Fig. 8).

Figure 8 -Definition of wedges

The profile 1 at the end of the cutting zone has to be chosen. In this case, the height of the profile is imposed by the final diameter of the groove. First, the forming angles need to be chosen following the stability index function. Secondly, the spreading angles are chosen again following the stability index function correspondent. The user can return to the previously decision in order to change the forming and/or spreading angles values if the stability indexes are not acceptable. Thirdly, the length of the three zones (cutting, forming and sizing) will be determined by geometric constraints of: the forming angles, spreading angle, initial diameter of the billet and final diameter of the portion. If the rules imposing: one lap minimum by zone and respecting the maximum length available are not satisfied, the user can return to the previous designing stages, until finding a good combination of parameters. Here below (Table 3), three different configurations of wedges are proposed. At the present COLT is not automated, which means, the user in modifying the parameters manually will search for acceptable stability indexes. In the future, the computer tool will output the optimum selection of parameters, having the higher stability indexes as permitted by the machine restrictions and the part's geometry. For each configuration the stability index of the geometrical parameters were evaluated. In Configuration 2 the length of the cutting zone is less than one lap and in configuration 3 the spreading angle of 12° has a stability index below 0.6. The configuration 1 is chosen (see Table 4). Once the user has chosen the geometrical parameters of the wedge, the local coordinates (x, y, z) of the remarkable points are calculated through geometrical equations. The Fig. 9 shows the remarkable points in the knifing profile of a main wedge and in Table 5 the coordinates of these points are indicated. The procedure is continued to calculate the local coordinates of all the remarkable points of the wedge. Remarkable points

X [mm] Y [mm] Z [mm] PR 01 ∝ ⁄ 0 PR 02 0 PR 03 ∝ ⁄ 0
After obtaining all the local coordinates of each wedge of each reduced section of the part, the global coordinates are calculated according to the relative position of the portions. Also, as it was previously explained, because the part is longitudinally symmetric, only by replicating the designed wedges, the complete matrix is obtained.

Conclusions

A decision supporting tool for the designing of the tool in cross wedge rolling is presented in this paper. The methodology is based on a synthesis of literature rules as well as rules identified during experimental work. The designing rules allow choosing the basic parameters of the wedge. The concept of stability index is introduced in order to take into account the different admissible domain for a parameter that can vary from one author to another. A simple reduction part is used as a demonstration example.

One of the first advantages of this methodology is that the user will be able to have a first approach, still during the designing stage, of the stability of the process. The designing decisions of the user will be taken under the guidance of the COLT method relying on scientific and experimental bases. An additional advantage is the fact that COLT maintains the relationship between the wedge's parameters at all time. So the effect of a chosen parameter can be observed in the stability index of another parameter. This permits the user to observe the interaction and effects of its designing decisions.

A third advantage is the flexibility of the methodology. The user can obtain different wedges by simply modifying one parameter. The coordinates of the remarkable points of the wedge are also modified because of the geometrical relationship established between all the parameters.

COLT seeks to be an interactive method. In the future development of the computer tool the bibliographical and experimental sources, as well as its maturity, can be modified, eliminated or even adding new sources. Subsequently, the experimental user can state its own designing rules.

A first computer tool is being developed; it is expected its implementation for an industrial case of a connecting rod preform.

Figure 1 -

 1 Figure 1 -a) flat-type CWR process, b) roller-type CWR process [1]

Figure 2 -

 2 Figure 2 -Geometrical parameters of die configuration

Figure 3 -

 3 Figure 3-Common defects on CWR products [2]

•

  A11. Collect the specifications of the part. Definition of the CWR tool structure • A12. Characteristics of the machine • A13. Characteristics of the billet • A14. Characteristics of the part • A15 and A16. Definition of wedges • A17 and A18. Definition of remarkable point's coordinates

Figure 4 -

 4 Figure 4-Global approach of the COLT method [10]

Figure 5 -

 5 Stability index function of forming angle α

Figure 7 -

 7 Figure 7 -Simple reduction part A11. (1) Collect the specifications of the part.

Figure 9 -

 9 Figure 9 -Remarkable points in the knifing profile of a main wedge.

Table 1 -Parameters of die configuration

 1

	α	[°]	forming angle
	β	[°]	spreading angle
	ϒ	[°]	ramp angle
		[mm] knifing zone
		[mm] guiding zone
		[mm] forming zone
		[mm] sizing zone
		[mm] billet diameter
		[mm] final diameter
		[mm] wedge height
		[mm] spacing of the wedge

Table 2 -Acceptable limits of the forming angle by different references min [°] max [°] reference

 2

	20	45	[11]
	10	55	[4]
	10	40	[6]
	20	60	[12]
	15	40	[3]
	15	60	[7]
	20	87	[13]

Table 3 -Geometrical characteristics of 3 different wedges

 3

				Profile 1				Profile 2
	Configuration α1	α2	β1	Cp	tour ϒ	α1	α2	β3	Ce	tour	Cc	tour	N
		[°]	[°]	[°]	[mm]	[°]	[°]	[°]	[°]	[mm]		[mm]		total
	1	30 30 8 78.87 1.14 4.6 30 30 8 71.15 1.03 69.97 1.01 3.18
	2	30 30 13 48.01 0.69 7.6 30 30 7 81.44 1.18 90.54 1.31 3.18
	3	20 20 12 82.73 1.20 4.6 30 30 8 71.15 1.03 66.12 0.96 3.18

Table 4 -Configuration 1: list of the parameters with the stability index associated

 4

	Parameter	Value	Stability index
	Cp [lap]	1.14	1
	Ce [lap]	1.03	1
	Cc [lap]	1.01	1
	ϒ [°]	4.6	0.96
	α1 and α2 profile 1 [°]	30	1
	β1 profile 1 [°]	8	1
	α3 and α4 profile 2 [°]	30	1

Table 5 -Local coordinates of the remarkable points in Fig. 9.

 5

Acknowledgement

This paper results from research project which is financially supported by CETIM Forge Committee and Région Lorraine. Special regards to Mr Pierre KRUMPIPE (CETIM) for his cooperative work. The authors are thankful to the IWU of Chemnitz for their scientific and technological support.