L Driehaus
email: lena.driehaus@univ-rennes1.fr

T Nalpas

P R Cobbold

B Gelabert

F Sàbat

Effects of margin-parallel shortening and density contrasts on back-arc extension during subduction: Experimental insights and possible application to Anatolia

Keywords: Analogue Modelling, Subduction, Back-Arc Extension, Anatolia

So as to investigate the parameters influencing subduction and back-arc extension, we have done three series of laboratory experiments (32 in all) on physical models.

Introduction

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In other models (Mantovani, 1997;[START_REF] Mantovani | Back arc extension: Which driving mechanism[END_REF][START_REF] Gelabert | About the origin of the northern hemisphere Pàcific arcs[END_REF][START_REF] Gelabert | A new proposal for the late Cenozoic geoynamic evolution of the western Mediterranean[END_REF][START_REF] Ferrer | On the origin of the North Pacific arcs[END_REF], back-arc extension helps to drive subduction. One of the possible reasons for such back-arc extension is compression parallel (or slightly oblique) to a continental margin. This might explain why some orogenic arcs start as straight features in plan view and then acquire curvature as their back-arc basins develop. Although a few physical experiments have tested this model (Faccenna el al., 1996), we know of none that have tested the relationship between back-arc extension and the density ratio between oceanic and continental plates. Therefore in this paper, we describe three series of experiments (32 in all), in which we tested the effects of (1) the density ratio between oceanic and continental plates and (2) shortening parallel to a continental margin.

Our experimental design follows that of [START_REF] Faccenna | The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea[END_REF], who modelled the Tyrrhenian-Apennine system in a framework of collision between the African and Eurasian plates. [START_REF] Faccenna | The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea[END_REF] in their experiments tested the influence of margin-parallel compression, but only within a continent. In contrast, we tested the effects of compression within oceanic and continental plates simultaneously, as well as the effects of a wider range of density ratios and velocities of plate convergence.

Finally, we have compared our experimental results with the history of back-arc extension in Anatolia.

Experimental Procedure

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We did the experiments in the modelling laboratory at Géosciences Rennes (University of Rennes 1), using the experimental procedure of Davy andCobbold (1988, 1991), which is based on simplified strength profiles of the lithosphere. For boundary conditions, we followed partially [START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF].

The models simulated the formation of subduction zones and deformation at lithospheric scale. Each model consisted of one continental plate (CP) and one oceanic plate (OP).

Between them was the continent-ocean boundary (COB).

Strength profiles

To calculate the vertical strength profile of the continental or oceanic lithosphere, we assume that the upper crust is brittle and that the lower crust is ductile. To model the mantle lithosphere, the number of layers should depend on the temperature: two-layer lithosphere for high temperatures; three-layer lithosphere for intermediate temperatures;

four-layer lithosphere for low temperatures [START_REF] Davy | Experiments on shortening of a 4-layer model of the continental lithosphere[END_REF]. For our purposes we assumed high temperature and therefore a two-layer lithosphere for Aegean system (see [START_REF] Gautier | Timing, kinematics and cause of Aegean extension: a scenario based on a comparison with simple analogue experiments[END_REF][START_REF] Tesauro | A new thermal and rheological model of the European lithosphere[END_REF].

We consider that the oceanic lithosphere is a thermal boundary layer and its base is an isotherm. To calculate shapes of isotherms, we follow Turcotte and Schubert (1982). If the lithosphere cools in the same way as a semi-infinite half-space, a lithospheric segment that forms between 10 Ma and 35 Ma is up to 100 Km thick, moves at 3 cm/yr and, if the base of the lithosphere is at 1400⁰ K, has an average slope of about 3⁰. In our experiments, the base of the oceanic plate had a slope of 4⁰.

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Materials

Following previous authors [START_REF] Faccenna | The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea[END_REF][START_REF] Faccenna | Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments[END_REF][START_REF] Faccenna | Lateral slab deformation and the origin of the western Mediterranean arcs[END_REF][START_REF] Faccenna | Slab detachment beneath eastern Anatolia: A possible cause for the formation of the North Anatolian fault[END_REF][START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF][START_REF] Regard | From subduction to collision: Control of deep processes on the evolution of convergent plate boundary[END_REF][START_REF] Regard | Slab pull and indentation tectonics: insights from 3D laboratory experiments[END_REF][START_REF] Regard | Laboratory experiments of slab break-off and slab dip reversal: insight into the alpine oligocene reorganization[END_REF] we used sand to model the brittle upper crust, silicone putty for the ductile lower crust (Tables 1 and2) and honey for the asthenosphere.

The sand was a pure dry variety from Fontainebleau, France. This consists of more than 95% quartz. Grains are well-rounded and their size is between 2 x 10 -4 and 4 x 10 -4 m.

The material yields according to a Mohr-Coulomb envelope, with negligible cohesion and an angle of internal friction between 30° and 40⁰ (Krantz, 1991). The pure sand has a density of 1.56 g/cm³ (± 0.6 %) and this is what we used for the oceanic plate. In contrast, for the continental plate the density was 1.36 g/cm³ (± 1.5 %), because we mixed the sand with ethyl-cellulose powder. This did not change the mechanical properties of the sand. We measured the density of the sand by first sieving it into a dish of known volume and then recording its weight. This we did 3 times, so as to obtain an average value.

The pink silicone putty (Silbione Gomme 70009, Rhône Poulenc, France) is a Newtonian fluid, in other words, it has a linear dependence of strain rate upon stress. At room temperature (20 ± 2⁰C), the density of the pure silicone is about 1.2 g/cm³ and the viscosity is about 10 4 Pa s. The viscosity varies with temperature (Weijermars, 1986). To investigate the effects of various densities in the models, we mixed the silicone with heavy mineral powders, so that the density of the mixture for the continental plate was between 1.175 and 1.346 g/cm³ and the viscosity was between 31700 and 56800 Pa s

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For the oceanic plate, the density of the silicone was between 1.33 and 1.67 g/cm³ (± 1 %), and the viscosity was between 26800 and 153000 Pa s. To measure the density, we first weighed a sample of silicone and then dropped it into water, to record the increase in volume. This we did 3 times, so obtaining an average value. To measure the viscosity and check the flow law, we used a thick-ring rotary viscosimeter, of the kind that was in use at Uppsala University in 1981 (see Weijermars, 1986;[START_REF] Cobbold | Gum rosin (colophony): A suitable material for thermomechanical modelling of the lithosphere[END_REF]. In such a viscometer, both the stress and the flow rate vary radially, allowing the operator to obtain a flow law, by observing the change in shape of a radial line on the free surface. The precision of this method is ± 5% and we repeated all measurements 3 times to decrease errors.

The density of the natural honey that represented the asthenosphere was 1.43 g/cm³ and the viscosity was 10² Pa s. We measured and carefully controlled these values. We obtained the viscosity and the flow law from a thick-ring rotary viscometer, as well as by dropping spherical objects into a column of honey and using Stokes' Law [START_REF] Lamb | Hydrodynamics[END_REF].

Scaling

We adopted the experimental procedure of Davy andCobbold (1988, 1991) for lithospheric modelling (see Table 2 for the parameters and model ratios). The length ratio was 3 ×10 6 (1 cm representing 30 km in nature). The density ratio was between 2 and 2.5, therefore, according to [START_REF] Davy | Experiments on shortening of a 4-layer model of the continental lithosphere[END_REF], the stress ratio was about 8 × 10 6 , and the time ratio was 6 × 10 9 .

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8

In nature, the density of oceanic lithosphere varies, but in our models we did not take this into account. Instead, we used a silicone layer of uniform density, but variable thickness, having an average basal slope of 4°.

Apparatus and procedure

Each model consisted of two plates, an oceanic plate (in blue, Fig. 1) and a continental plate (in dark grey).

We built and deformed all models inside a rectangular tank (40 × 40 cm, corresponding to 1200 × 1200 km in Nature; Table 1). The base and walls of the tank were of rigid transparent material. The models were built up layer by layer.

To deform the models, we used two kinds of pistons (Fig. 1). In experiments of Series A, the piston was as wide as the model (40 cm), its leading edge was in contact with the oceanic plate and it advanced in a direction perpendicular to the COB. In experiments of Series B, the piston was also wide (40 cm), but its leading edge was perpendicular to the COB and it advanced in a direction parallel to the COB, compressing both the oceanic and continental plates. Finally, in experiments of Series C, the piston was narrower (20 cm), so that its leading edge spanned the continental pate. As in Series B, the piston was perpendicular to the COB and it advanced in a direction parallel to the COB. However, it compressed the continental plate only.

To record progressive deformation at the surface of the model, we laid down a grid of thin white lines of sand at a spacing of 5 cm and photographed it every 15 minutes. We

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9 also took photographs of the lateral boundaries of the model, visible through the transparent sidewalls.

To measure the amount of extension, we calculated local changes in surface area of the continental and oceanic plate, by counting pixels automatically on the photographs. This procedure yields the percentage areas of piston, continental plate and oceanic plate.

Each of these values changed with time, the area of the piston varying at the same rate in all the experiments having the same piston size (see Fig. 6a, b, c, for Series A).

In our experiments, we did not investigate the possible effects of other parameters, such as (1) the viscosity and flexural resistance of the oceanic plate [START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF][START_REF] Regard | From subduction to collision: Control of deep processes on the evolution of convergent plate boundary[END_REF][START_REF] Funiciello | Trench migration, net rotation and slab-mantle coupling[END_REF], (2) conditions at the base of a subducting slab [START_REF] Regard | From subduction to collision: Control of deep processes on the evolution of convergent plate boundary[END_REF][START_REF] Guillaume | Variations of slab dip and overriding plate tectonics during subduction: Insights from analogue modelling[END_REF], (3) slab break-off [START_REF] Regard | Laboratory experiments of slab break-off and slab dip reversal: insight into the alpine oligocene reorganization[END_REF][START_REF] Regard | From subduction to collision: Control of deep processes on the evolution of convergent plate boundary[END_REF], (4) relative thicknesses of oceanic and continental plates [START_REF] Faccenna | Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments[END_REF]Nikolaeva et al., 2010), or (5) free lateral boundaries of subducting plates [START_REF] Funiciello | Role of lateral mantle flow in the evolution of subduction systems: insights from laboratory experiments[END_REF].

Experimental results

Of the three series of experiments, those of Series A were to determine which was the most appropriate piston velocity, whereas those of Series B and C were to investigate the influence of the density contrast, for two different widths of piston.

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To simplify the description (Figs. 2 to 5), we will use geographical coordinates, such that North is upwards and the piston is in the West. This means that the displacement of the piston is always eastwards.

Following previous workers (e.g. [START_REF] Faccenna | The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea[END_REF][START_REF] Faccenna | Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments[END_REF][START_REF] Regard | From subduction to collision: Control of deep processes on the evolution of convergent plate boundary[END_REF][START_REF] Faccenna | Slab detachment beneath eastern Anatolia: A possible cause for the formation of the North Anatolian fault[END_REF][START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF][START_REF] Regard | Slab pull and indentation tectonics: insights from 3D laboratory experiments[END_REF][START_REF] Regard | Laboratory experiments of slab break-off and slab dip reversal: insight into the alpine oligocene reorganization[END_REF], we used a rectangular box, which had rigid lateral boundaries. Inevitably, these led to some boundary effects, resulting from rigidity and from sidewall friction. In our experiments, the effects of friction were visible over a distance of 2 to 3 cm from the sidewalls. However, we realize that their mechanical effects may have extended further across the model, and they might have contributed to increase normal faulting near the north-eastern corner.

When continental material was spreading over the oceanic plate, the deformation affected the entire continental plate (sand and silicone).

We stopped the piston after 12 hours in almost all the experiments. For a piston moving at 0.5 cm/h, 12 hours corresponded to 6 cm of shortening, in other words, 120 km in nature, which is a reasonable value.

Series A

In experiments of Series A, the piston was as wide as the model (40 cm), its leading edge was in contact with the oceanic plate and it advanced in a direction perpendicular to the COB. The piston velocities were 0.1, 0.5 or 2.5 cm/h. Surface views before deformation show an initially linear COB (Fig. 2, Series A). The total displacement was 2.4 cm for model A1 and 6 cm for models A2 and A3.

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In Model A1 the velocity was 0.1 cm/h. Extension initiated in the continental plate, next to the COB, and propagated westward toward the piston. After 20 hours (2 cm of piston displacement), the oceanic plate had subducted almost completely, while the continental plate had extended from 20 cm to about 35 cm. After 24 hours (2.4 cm of piston displacement), there was no further change and we stopped the experiment.

In Model A2 the velocity was 0.5 cm/h. Folding of the oceanic plate started after 2 hours

(1 cm of piston displacement); subduction of the oceanic plate, after 4 hours (2 cm of piston displacement); and extension of the continental plate, after 7 hours (3.5 cm of piston displacement). At the end of the experiment, after 12 hours (6 cm of displacement), a zone in the continental plate near the COB was still undergoing extension, while the oceanic plate was shortening and subducting.

In Model A3 the velocity was 2.5 cm/h. Folding of the oceanic plate started after 0.5 hours (1.25 cm of piston displacement); subduction of the oceanic plate, after 1 hour (2.5 cm of piston displacement); but no extension was visible in the continental plate.

When the piston stopped, after 2.4 hours (6 cm of piston displacement), the oceanic plate was still shortening and subducting.

In both models A1 and A2, extension of the continental plate and shortening of the oceanic plate occurred synchronously. However in model A2 displacement of the piston was balanced by shortening and subduction of the oceanic plate with minor deformation of the continental plate. Thus, after this first series of experiments, using different velocities, we decided to adopt a piston velocity of 0.5 cm/h (Fig. 2), corresponding to

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1.5 cm/yr in nature (Table 2). In the future it might be interesting to test other velocities and to compare the results.

Series B

For this series, we again used a wide piston (40 cm), which moved eastwards, perpendicularly to the COB (Figs. 2 and3). In the early stages (0 to 12 hours) the piston moved at a steady velocity of 0.5 cm/h, whereas in later stages (12 to 18 hours) it was stationary. Before deformation, the COB was straight (Fig. 2, Series B).

We will describe six stages in the development of Model 7 (after 3, 6, 9, 12, 15 and 18 hours, Fig. 3).

Stage 1 (3 hours)

. By this stage, the COB, especially its western part, had moved southwards. Slight bending, next to the piston and opposite sidewall, was a boundary effect, due to friction, and was also visible at later stages. In the south-western part of the continental plate, where the displacement of the COB was larger, elements of the reference grid had become somewhat rectangular, so that the total area of the continental plate had increased by about 2.7%. In the north-western part of the oceanic plate, near the COB, E-W-trending folds (fo1) and reverse faults had appeared.

Elsewhere in the oceanic plate, the square grid had not deformed. At this stage, the total area of the continental plate had increased by about 26.3% (8.4%

since Stage 3). In the oceanic plate a new East-trending fold (fo4) had formed to the SE of the COB. Apart from the sharp fold (dashed line) within the oceanic plate, the square grid showed little or no deformation.

Series C

For this series, we used a narrow piston (20 cm), which moved eastwards, perpendicularly to the COB (Figs. 2 and4). In the early stages (0 to 12 hours) the piston moved at a steady velocity of 0.5 cm/h, whereas in later stages (12 to 18 hours) it was stationary. Before deformation, the COB was straight (Fig. 2, Series C), after shortening the asymmetric curvature of the COB may be due to the size of the piston.

We will describe 6 stages in the development of Model 8 (after 3, 6, 9, 12, 15 and 18 hours, Fig. 4).

Stage 1 (3 hours)

. By this stage, the COB, especially its western part, had moved

Experiments of Series B and Series C for various density ratios

We will now describe experiments of Series B and C (small or large piston) after 12 hours (6 cm of displacement), when the piston had stopped moving, and for various values of the density ratio (R =  o / c ), where  o is the density of the oceanic plate and  c is the density of the continental plate (Fig. 5).

Density ratio: 1.13 -1.16

Model 5, Series B. For this density ratio (1.13 -1.16), the pattern of grid lines at the COB In the oceanic plate, where the displacement of the COB was larger, a sharp fold (dashed line) appeared within the oceanic plate, parallel to the COB.

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Effect of density contrast

In a first group of experiments (Model 5 of Series B and Model 4 of Series C), where the density ratio (between oceanic and continental plate) was 1.1, the percentage area of continental plate changed very little, while the piston was in motion. Until the end of each experiment (percentage area of 2.2 % for a wide piston and -2.2% for a narrow piston, after 18 hours) there was no extension and no oceanic subduction (Fig. 6a).

In a second group of experiments (Model 7 of Series B and Model 6 of Series C), where the density ratio was 1.3, the percentage area of continental plate changed little (8.7%

for a wide piston and 20.8% for a narrow piston), while the piston was moving, but changed much more (17.4% for a wide piston and 32.3% for a narrow piston) after the piston had stopped (after 12 hours) and until the end of observations (18 hours for series B and 15 hours for series C). After 6h of compression, the oceanic plate started to subduct; the slab sank slowly, while the piston was in motion, but much more quickly, after the piston had stopped (Fig. 6b).

In a third group of experiments (Model 9 of Series B and Model 8 of Series C), where the density ratio was 1.4, the percentage area of continental plate increased steadily from the beginning to the end of the experiment (by 42.8% for a wide piston and by 67.8% for a narrow piston, after 18 hours). After 3h of compression, the oceanic plate started to subduct; the slab then sank continuously and fast (Fig. 6c). However, the surface area

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24 increased to higher values for the narrow piston than it did for the wide piston. A narrow piston provided more space for a continental plate to spread over the adjacent oceanic plate.

For all these experimental results (Series B and C), we infer that the density ratio (between oceanic and continental plates) was a very important factor in facilitating extension in the continental plate during subduction (Fig. 7). When the density ratio was lower than 1.2-1.25, no extension occurred in the continental plate. When the density ratio was between 1.25-1.35, extension occurred in the continental plate, although mainly after compression stopped. Finally, when the density ratio was higher than 1.35, extension occurred in the continental plate from the very beginning of the experiment.

Most authors have assumed that an oceanic plate is denser than the underlying asthenosphere and some have investigated the effect of systematically varying the density ratio between the two, either in physical models [START_REF] Faccenna | Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments[END_REF][START_REF] Shemenda | Physical modeling of slow seafloor spreading[END_REF] or in numerical models [START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF]Nikolaeva et al., 2010).

When other parameters remained constant, the rate of subduction was higher for larger density ratios and we agree with that conclusion.

The dimensionless buoyancy number, F [START_REF] Houseman | Deformation of subducted oceanic lithosphere[END_REF]Faccenna et al., 1999) is a measure of the density contrast between oceanic lithosphere and asthenosphere, as well as the thickness and ductile resistance of the oceanic lithosphere:

(1)

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Here L is the thickness of the oceanic plate (h sand + h silicone), ɳ is the viscosity (of what) (which we have denoted as µ), is the velocity of convergence of the piston, g is the gravitational acceleration, and is the density contrast between the oceanic lithosphere and the asthenosphere.

We have calculated the buoyancy number for all of our experiments (Table 1). For the second and third group of experiments, F>1 and we observed subduction, whereas for model 5 of the first group of experiments, F<1 and there was no subduction. These results are in agreement with the observations of [START_REF] Faccenna | Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments[END_REF]. In contrast, for Model 4, F>1, yet there was no subduction. We infer that, although the buoyancy number is important in determining if subduction will occur, it is not the only factor. We suggest that the density ratio between oceanic and continental plates is also important.

In our experiments, subduction did not occur, even for F>1, if the density ratio was not high enough (Table 1 and Fig. 7).

Piston velocity

In previous experimental studies, as in our own, it is clear that piston velocity was one of the most important parameters controlling subduction and continental extension [START_REF] Faccenna | The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea[END_REF][START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF]. When the piston velocity was fast, no extension was visible in the continental plate. In contrast, when the velocity was slow, extension of the continental plate and subduction of the oceanic plate occurred synchronously. However, these two processes did not always balance, so that in some experiments there was much extension, for almost no piston displacement (Fig. 2).

development of viscous instabilities [START_REF] Regard | From subduction to collision: Control of deep processes on the evolution of convergent plate boundary[END_REF]. A very thick layer favours the formation of instabilities, but we did not study this parameter. In all our models the initial thickness of honey was the same.

Comparing models and nature

The Aegean-Anatolian orocline and back-arc system have developed in a context of convergence of the African plate with Eurasia since the Upper Jurassic [START_REF] Sengör | Tethyan evolution of Turkey: A plate tectonic approach[END_REF][START_REF] Robertson | Introduction: aspects of the geological evolution of the Eastern Mediterranean[END_REF]. The present-day Aegean-Anatolian system is bounded to the north by the right-lateral North Anatolian Fault (NAF), to the east by the left-lateral East Anatolian Fault (EAF) and to the south by the Hellenic and Cyprus Subduction Zones (Fig. 8).

Although both the Hellenic Subduction Zone and the Cyprus Subduction Zone have accommodated large amounts of plate convergence between Africa and Eurasia, the former has a back-arc area where extension is large -the Aegean Sea -whereas the latter has a back-arc area -Central Anatolia -where extension is small.

The back-arc area of the Hellenic Subduction Zone, North of Crete, includes the Aegean Sea and surrounding areas. Late Cenozoic extension has been responsible for exhumation of several metamorphic complexes (Cyclades, Menderez and at least some part of Rhodope) [START_REF] Lister | Metamorphic core complexes of Cordilleran type in the Cyclades, Aegean Sea, Greece[END_REF]Gautier at el., 1999 andJolivet et al., 2012).

Extension in a roughly NNE-SSW direction occurred by southward retreat of the Hellenic et al., 1989;[START_REF] Le Pichon | The Aegean Sea[END_REF][START_REF] Gautier | Timing, kinematics and cause of Aegean extension: a scenario based on a comparison with simple analogue experiments[END_REF]Van Hinsberger andSchmid, 2012). This was coeval with southward migration of magmatism, from the Rhodope Massif to the Cyclades (since about 35 Ma according to [START_REF] Jolivet | Aegean tectonics: Strain localisation, slab tearing and trench retreat[END_REF].

The back-arc area of the Subduction Zone, north of Cyprus, includes the region of Central and East Anatolia, a Mesozoic to Tertiary collision zone. This region consists of three main terrains with ophiolitic sutures. These terrains are, from north to south, the Pontides, Central Anatolian Crystalline Complex and Taurides (Moix et al., 2008). The Central Anatolian Crystalline Complex consists of high-grade metamorphic rocks intruded by late granitoids [START_REF] Floyd | Geochemical character and tectonic environment of Neotethyan ophiolitic fragments and metabasites A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 34 in the Central Anatolian Crystalline Complex, Turkey[END_REF]. This complex was formerly covered by ophiolitic nappes, which are now widespread in the Anatolian terrains (Moix et al., 2008).

For this region, South of the North Anatolian Fault, the tectonic setting during the Cenozoic is debatable. It could be a strike-slip system (Koçyigit and Beyham 1998) a compressional system (Gülyüz et al., in press) or an extensional system [START_REF] Genç | Coeval extension and compression in Late Mesozoic-Recent thin-skinned extensional tectonics in central Anatolia, Turkey[END_REF]. In any case, unlike in the Aegean area, there has not been so much extension as to produce a metamorphic core complex (see discussion in [START_REF] Gautier | Dating the exhumation of a metamorphic dome: geological evidence for pre-Eocene unroofing of the Niğde Massif (Central Anatolia, Turkey)[END_REF].

The question is: why, in one geodynamic setting, are the tectonic responses so different? According to [START_REF] Jolivet | Aegean tectonics: Strain localisation, slab tearing and trench retreat[END_REF], extension in the Aegean began at 35 Ma or before. In contrast, westward extrusion of Anatolia may not have started before 10-13

Ma (Sengör et al., 1985). Therefore lateral shortening may have helped to control subduction and continental extension only during the later stages.

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29

A map of Bouguer gravity anomalies for Turkey shows a residual positive anomaly in Central Anatolia [START_REF] Ates | New gravity and magnetic anomaly maps of Turkey[END_REF], which could be due to the high density of deep crustal rocks. In contrast, the rocks forming the Aegean Sea are mainly sedimentary or metasedimentary, so of low density. Exceptions are metamorphic rocks in the Cyclades, Menderez and the Rhodope Metamorphic Complex, of which the exhumation partly resulted from Aegean extension. Thus, prior to the extension, the density of the continental lithosphere could have been higher, north of the Cyprus Subduction Zone, than it was north of the Hellenic Subduction Zone. If so, and if the oceanic lithosphere had the same density south of both subduction zones, the density ratio between the oceanic and continental lithospheres was higher, across the Hellenic subduction zone, than it was across the Cyprus subduction zone. We suggest that a high density ratio has been an important driving force for Aegean extension, while in Central Anatolia a low density ratio may have prevented extension. Likewise in our models, density ratio has been a key factor.

In general, three other factors may influence the lithospheric density variations between subducting lithosphere and continental lithosphere in order to favour back-arc extension.

1) Initial properties of the oceanic plate. Variations in the properties of the subducting lithosphere (more oceanic versus less oceanic) may explain the difference in resulting back-arc extension. A more oceanic composition means that the density is higher, so that the density ratio between subducting lithosphere and continental lithosphere is higher, so favouring back-arc extension.

A C C E P T E D

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2) Hydration of the lithospheric upper plate. During subduction processes, dehydration of the downgoing plate [START_REF] Peacock | Large-scale hydration of the lithosphere above subducting slabs[END_REF][START_REF] Ringwood | The petrological evolution of island arc systems[END_REF]) causes hydration of the upper mantle wedge [START_REF] Hyndman | Serpentinization of the forearc mantle[END_REF][START_REF] Arcay | Numerical simulations of subduction zones: Effect of slab dehydration on the mantle wedge dynamics[END_REF]. This will change its physical properties, including density and seismic velocity [START_REF] Bostock | An inverted continental Moho and the serpentinization of the forearc mantle[END_REF][START_REF] Hacker | Subduction factory, 2, Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?[END_REF]. A decrease in density of the upper plate, due to hydration of the mantle wedge, may contribute to back-arc extension [START_REF] Arcay | Numerical simulations of subduction zones: Effect of slab dehydration on the mantle wedge dynamics[END_REF]Levander et al., 2011) 3) Initial continental thickness. The initial crustal thickness could have been large

enough to favour gravity spreading.

Conclusions

For the experiments, we draw the following conclusions.

-In every experiment, where there was extension in the continental plate, there was also subduction of the oceanic plate.

-However, in some experiments, there was subduction of the oceanic plate, but no extension in the continental plate. Therefore subduction was not enough to explain continental extension.

-Compression parallel to the continent-ocean boundary did not favour continental extension.

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-In contrast, the density ratio between the oceanic plate and continental plate was a key factor: the higher the density ratio, the greater the amount of extension in the continental plate during subduction.

For the application to Anatolia, we draw the following conclusions.

-Despite similar geological settings, the areas north of the Hellenic and Cyprus subduction zones differ, in that extension is larger in the former and much smaller in the latter.

-In the light of our experiments, an important control on extension and retreat of the trench should be the density ratio between oceanic and continental plates. Accordingly, we suggest that Aegean extension was due to a high density ratio between subducting oceanic lithosphere and a Hellenic-Balkanic upper plate of relatively low density, whereas in the Anatolian upper plate (of relatively high density) the density contrast was lower and there was less extension.

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Piston Velocity

cm/h 0,1 0,5 2,5 0,5 0,5 0,5 0,5 0,5 0,5

Buoyancy number, F  We tested deformation velocity and density ratio (oceanic vs continental plate).

 We examine the factors generating extension during subduction.

 The density ratio is a key factor to facilitate extension during subduction.

 We propose an application to the Anatolian system.

  By this stage, the COB had rotated counter-clockwise and its western part had moved further southward. At 21.5 cm from the piston, a slight change in strike defined an inflexion point (Inf). More generally, offsets of the grid lines demonstrated a small amount of right-lateral shear along the COB. In the continental -trending folds (fc1) appeared near the piston in the north-western corner of the model. Also, several major right-lateral faults (d) and minor conjugate left-lateral faults appeared in the south-western and central area of the continental plate. At the end of one right-lateral fault, a zone of extension (e1) developed. In western areas (at 5 to 15 cm from the piston), where the displacement of the COB was larger, elements of the reference grid had become more rectangular and, next to the COB, the sand layer had thinned and faulting was diffuse. At this stage, the total area of the continental plate had increased by about 2.4% since the start of the experiment, but had decreased by 0.2% since Stage 1. In the oceanic plate, new North-trending folds (fo2) had appeared near the piston in the western area. Elsewhere in the oceanic plate, the square grid showed little or no deformation.Stage 3 (9 hours). By this stage, southward displacement of the COB had increased.The inflexion point (Inf) had migrated eastward through the material, remaining at 21.5 cm from the piston. Right-lateral slip along the COB had increased. In the continental plate, new North-trending folds formed in the north-western corner of the model (fc1) and more centrally (fc2). Strike-slip faults formed a triangular pattern, with a new major left-lateral fault (s) in the North and right-lateral fault (d) in the South. At the eastern ends of these faults, in the centre of the continental plate, the original zone of extension (e1) had become more visible, while a new one (e2) had developed further north. Where the displacement of the COB was larger, near the piston, the zone of diffuse faulting had become wider. The greatest distortion of the surface grid was in an area about 5 to 15 cm in front of the piston. To a first approximation, elements of the grid had rotated about A C C E P T E D M A N U S C R I P T since the start of the experiment, and by 3.4% since Stage 2. In the oceanic plate, the North-trending folds (fo2) had propagated to the South, while the COB had overridden them in the North. Where the displacement of the COB was larger, a sharp fold (dashed line) appeared within the oceanic plate, parallel to the COB. Elsewhere in the oceanic plate, the square grid showed little or no deformation. Stage 4 (12 hours). At this stage we stopped the advance of the piston. Southward displacement of the COB had increased even more. The inflexion point was at 23 cm beyond the piston. Right-lateral slip along the COB reached a maximum at a point between 12 and 20 cm in front of the piston. In the continental plate, some of the folds, which had formed at Stage 3, were still active. The left-lateral fault (s) had lengthened and the right-lateral fault (d) had shortened. The previous zones of extension (e1 and e2) had become even more visible, while another one (e3) had appeared further South. Where the displacement of the COB was greater, in the western area, the zone of diffuse faulting had enlarged. The greatest distortion of the surface grid was in an area about 5 to 17 cm in front of the piston. To a first approximation, elements of the grid had rotated about 14⁰ clockwise in the south-western area (8⁰ since Stage 3), and 22⁰ counter-clockwise in the south-central area (11⁰ since Stage 3). At this stage, the total area of the continental plate had increased by about 8.8% (2.9% since Stage 3). In the oceanic plate a new North-trending fold (fo3) had formed in front of the southern area of maximum curvature of the COB, the previous fold (fo2) had developed further, while the A C C E P T E D M A N U S C R I P T Stage 5 (15 hours). By this stage the piston was stationary. Southward displacement of the COB had increased even more. The inflexion point was still at 23 cm beyond the piston. Left-lateral slip had occurred along the first 4 cm of the COB in front of the piston and right-lateral slip was present further away. In the continental plate the left-lateral fault (s) had lengthened and almost traversed the model, from the piston to the opposite wall. The previous zones of extension (e1, e2 and e3) had increased, while another small one had appeared (next to e2). Where the displacement of the COB was greater, in the western area, the zone of diffuse faulting had enlarged. The greatest distortion of the surface grid was within 23 cm of the piston. Elements of the grid had rotated, by about 25⁰ counter-clockwise in the south-central area (3⁰ since Stage 4). At this stage, the total area of the continental plate had increased by about 17.9% (9.1% since Stage 4). Apart from the sharp fold (dashed line) within the oceanic plate, the square grid showed little or no deformation. Stage 6 (18 hours). By this stage the piston was stationary. Southward displacement of the COB had increased even more. The inflexion point was still at 23 cm beyond the piston. Left-lateral slip had occurred along the first 4 cm of the COB in front of the piston and right-lateral slip was present further away. In the continental plate, the left-lateral fault zone (s) had widened and shortened (the fault had an extensional component). Right-lateral faults (d in the previous stage) had vanished. The previous zones of extension (e1, e2 and e3) had increased in area. Where the displacement of the COB A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 16 was greater, in the west-central area, the zone of diffuse faulting had enlarged. The greatest distortion of the surface grid was within 26 cm of the piston. Elements of the grid had rotated about 29⁰ counter-clockwise in the south-central area (4⁰ since Stage 5).

  southwards. Slight bending, next to the piston and to the opposite sidewall, was probably due to friction. These boundary effects were visible also at later stages. At 23 cm from the piston, a slight change in strike defined an inflexion point (Inf). Left-lateral slip had occurred along the first 12 cm of the COB in front of the piston and right-lateral slip was present further away. In the continental plate, two elongate extension zones (e 1 and e2) developed parallel to the COB, in northern and central areas. At the eastern end of the extension zones, we observed left-lateral faults (s) striking NW-SE. In western areas, where the displacement of the COB was larger, elements of the reference grid next to the COB had become somewhat rectangular, the sand layer had thinned and faulting was diffuse. In the eastern area, where the displacement of the COB was smaller, a group of small normal faults had appeared, striking NW-SE. At this stage, the total area of the continental plate had increased by about 13.3%. Elsewhere in the oceanic plate, the square grid had not deformed.Stage 2 (6 hours). By this stage, southward displacement of the COB had increased and had become symmetric. Left-lateral slip had increased along the first 12 cm of the COB in front of the piston and right-lateral slip was present further away. In the continental plate, the extension zones (e1 and e2) had propagated and branched out sideways. At the eastern end of these extension zones, left-lateral faults were still active.In front of the piston, strike-slip faults formed a triangular pattern, with a new left-lateral fault (s) in the North and a right-lateral fault (d) in the South. Normal faults appeared, next to the area of diffuse faulting, and propagated in a direction almost perpendicular to the COB. The zone of diffuse faulting had become wider, forming a belt across the continental plate, north of the COB. The greatest distortion of the surface grid was in an area about 10 to 20 cm in front of the piston. To a first approximation, elements of the grid had rotated about 19⁰ clockwise in the south-western area and 17⁰ counterclockwise in the south-central area. At this stage, the total area of the continental plate had increased by about 30.6% since the start of the experiment (17.3% since Stage 1). plate, where the displacement of the COB was larger, a sharp fold (dashed line) had appeared within the oceanic plate, parallel to the COB. Elsewhere in the oceanic plate, the square grid showed little or no deformation.Stage 3 (9 hours). By this stage, southward displacement of the COB had increased.Left-lateral slip had increased along the first 15 cm of the COB in front of the piston and right-lateral slip was present further away. In the continental plate, the extension zones (e1 and e2) had become larger. In front of the piston, strike-slip faults still formed a triangular pattern, with a left-lateral fault (s) in the North and a right-lateral fault (d) in the South. New normal faults appeared, next to the area of diffuse faulting, and propagated in a direction almost perpendicular to the COB. The zone of diffuse faulting, north of the COB, had become narrower. To a first approximation, elements of the grid had rotated about 30⁰ clockwise in the south-western area and 25⁰ counter-clockwise rotation in the south-central area. At this stage, the total area of the continental plate had increased by about 39.8% since the start of the experiment (9.2% since Stage 2). Apart from the sharp fold (dashed line) next to the COB, the square grid showed little or no deformation in the oceanic plate. Stage 4 (12 hours). At this stage we stopped the advance of the piston. Southward displacement of the COB had increased. Left-lateral slip had increased along the first 15 cm of the COB, in front of the piston, and right-lateral slip was present further away. In the continental plate, the extension zones (e1 and e2) had propagated further, surrounding rigid blocks, which had moved towards the oceanic plate. In front of the piston, left-lateral faults (s) were still active. The normal faults, which had formed roughly A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 19 perpendicular to the COB, had become wider and somewhat longer. The zone of diffuse faulting had become narrower. To a first approximation, elements of the grid had rotated about 39⁰ clockwise in the south-western area and 38⁰ counter-clockwise in the southcentral area. At this stage, the total area of the continental plate had increased by about 49% since the start of the experiment (9.2% since Stage 3). Apart from the sharp fold (dashed line) next to the COB, the square grid showed little or no deformation in the oceanic plate. Stage 5 (15 hours). By this stage the piston was stationary. Southward displacement of the COB had increased even more. Left-lateral slip had increased along the first 16 cm of the COB in front of the piston and right-lateral slip was present further away. In the continental plate, the extension zones had become larger, isolating more blocks. Leftlateral faults (s) were still active. The normal faults, which had formed roughly perpendicular to the COB, had become wider and longer, isolating a few blocks. The zone of diffuse faulting had become narrower. To a first approximation, elements of the grid had rotated about 48⁰ clockwise in the south-western area and 49⁰ counterclockwise in the south-central area. At this stage, the total area of the continental plate had increased by about 52.9% since the start of the experiment (3.9% since Stage 4).In the oceanic plate, where the displacement of the COB was larger, a sharp fold (dashed line) appeared within the oceanic plate, parallel to the COB. Elsewhere, the square grid showed little or no deformation in the oceanic plate.Stage 6 (18 hours). By this stage the piston was stationary. Southward displacement of the COB had increased even more. Left-lateral slip had increased along the first 17.5 cm , in front of the piston, and right-lateral slip was present further away. In the continental plate, the extension zones had become larger, isolating more blocks. The zone of diffuse faulting in central areas had become so narrow, that continental blocks had reached the COB. To a first approximation, elements of the grid had rotated about 50⁰ clockwise in the south-western area and 52⁰ counter-clockwise rotation in the southcentral area. At this stage, the total area of the continental plate had increased by about 67.8% since the start of the experiment (14.9% since Stage 5). Apart from the sharp fold (dashed line) next to the COB, the square grid showed little or no deformation in the oceanic plate.

  showed bilateral symmetry, but little deformation. Offsets of the grid lines demonstrated a small amount of right-lateral shear along the eastern part of the COB. In the continental plate, North-and Northwest-trending folds developed on the western side of the model, near the piston. Strike-slip faults formed a triangular pattern, with a major leftlateral fault in the North and a right-lateral fault in the South. Another left-lateral slip fault northern boundary of the model. In the north-eastern part of the model, an extension zone developed. No zone of diffuse faulting was visible. In the oceanic plate, North-trending folds formed in three places: (1) in the western area, next to the piston, (2) in the centre, where the curvature of the COB was a maximum, and (3) in the eastern area, next to the sidewall. Furthermore, East-trending folds appeared in the southern area.Model 4, Series C. For this density ratio (1.13 -1.16), the pattern of grid lines at the COB showed very little deformation. Offsets of the grid lines demonstrated a small amount of right-lateral shear along the COB. In the continental plate, Northwest-trending folds formed in the northern area of the model and North-trending folds in the south-eastern area. A major Northwest-trending left-lateral fault formed between these two fold families. No zone of diffuse faulting was visible. At the northern edge of the oceanic plate, near the COB, E-W-trending folds appeared. Series B. For this density ratio (1.33 -1.34), the COB bulged southwards as far as the inflexion point. Right-lateral slip occurred along the COB. In the continental plate, North-trending folds appeared in the north-western area and major strike-slip faults crosscut the plate, forming a zone of extension where they intersected. A zone of diffuse faulting appeared, where the displacement of the COB was greater. In the oceanic plate, a North-trending fold formed next to the COB and a sharp fold (dashed line) developed parallel to the COB, where its displacement was larger. Series C. For this density ratio (1.33 -1.34), the COB bulged symmetrically southward. Slip was left-lateral along the western part of the COB and right-lateral along its eastern part. In the continental plate, there was a large area of diffuse faulting, where a series of minor left-lateral faults were visible. Where the displacement of the COB was larger, a sharp fold (dashed line) appeared within the oceanic plate, parallel to the COB. Series B. For this density ratio (1.41 -1.42), the COB underwent a large southward displacement. Right-lateral slip occurred along the COB. In the continental plate, extension zones linked, isolating rigid blocks, which also moved southwards, rotating clockwise in the south-western area and counter-clockwise in the south-central area. In the North, left-lateral faults were active. The zone of diffuse faulting formed a long narrow belt next to the COB. In the oceanic plate, where the displacement of the COB was larger, a sharp fold (dashed line) appeared within the oceanic plate, parallel to the COB. Model 8, Series C. For this density ratio (1.41 -1.42), the COB underwent a large southward displacement. Left-lateral slip occurred along the western part of the COB and right-lateral slip along its eastern part. In the continental plate, extension zones linked, isolating rigid blocks, which moved southwards, rotating clockwise in the southwestern area and counter-clockwise in the south-central area. In the North, left-lateral faults were active. The zone of diffuse faulting formed a long narrow belt next to the COB.

Fig. 1 .

 1 Figure captions

Fig. 2 .

 2 Fig. 2. Surface views of Series A, B and C before piston displacement (upper part of

Fig. 3 .

 3 Fig. 3. Surface views (photographs and their interpretations) of Model 7 (Series B) after

Fig. 4 .

 4 Fig. 4. Surface views (photographs and their interpretations) of Model 8 (Series C) after

Fig. 5 .Fig. 6 .

 56 Fig. 5. Surface views (photographs and their interpretations) of Models 5, 7 and 9

Fig. 7 .

 7 Fig. 7. Change in surface area (%) with time, for continental plates of 6 models, versus

Fig. 8 .

 8 Fig. 8. Map of Aegean Sea, Anatolia and surrounding areas, showing main structural

Figure 1 A

 1 Figure 1

Table 1 .

 1 ). Inset (top left) shows location with respect to Europe and Africa and the main tectonic elements of Turkey. Abbreviations in figure are KFZ, Kžrklareli Fault Zone; NAFZ, North Anatolian Fault Zone; WAGS, West Anatolian graben System; EFZ, Eskis¸ehir Fault Zone; FBFZ, Fethiye Burdur Fault Zone; IASZ, I™zmir Ankara Suture Zone; EAFZ, East Anatolian Fault Zone; DSFZ, Dead Sea Fault Zone; KTJ, Karlžova Triple Junction. (modified from[START_REF] Gürer | Relation between electrical resistivity and earthquake generation in the crust of West Anatolia, Turkey[END_REF]. Values of experimental parameters for each of models 1 to 12 (h for layer thickness,  for density and  for viscosity).

Table 2 .

 2 Characteristic values of parameters in nature and models and corresponding model ratios.

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6⁰ clockwise in the south-western area and 11⁰ counter-clockwise in the south-central area. At this stage, the total area of the continental plate had increased by about 5.9%

Acknowledgements:

The authors would like to thank J.-J. Kermarrec for his valuable technical assistance. This work was part of the Spanish Research Project, "El significado del Promontorio Balear" (CGL2008-05724BTE), which provided funding. Drs. Pierre Gautier and Romain Bousquet provided valuable comments on the tectonic context of Anatolia. We thank Reviewers 1 and 3 for their very detailed reviews and constructive suggestions that helped us to improve the manuscript.

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Having said this, we would like to make it clear that, in our experiments, the following parameters were equally important, if not more so, in controlling the amount of continental extension.

Convergence history

Although some authors have described what happened when a converging piston stopped moving [START_REF] Faccenna | The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea[END_REF][START_REF] Faccenna | Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments[END_REF][START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF], we have investigated this more fully. For various density ratios between continental and oceanic plates, we found that extension continued to accumulate, even after the piston stopped (Fig. 6).

Moreover we found that the density ratio between continental and oceanic plates had important effects on the styles of subduction and continental extension, for all directions of piston convergence.

Viscosity and flexural resistance of the oceanic plate, slab break-off

Previous authors [START_REF] Becker | The development of slabs in the upper mantle: Insights from numerical and laboratory experiments[END_REF][START_REF] Regard | From subduction to collision: Control of deep processes on the evolution of convergent plate boundary[END_REF][START_REF] Regard | Slab pull and indentation tectonics: insights from 3D laboratory experiments[END_REF][START_REF] Regard | Laboratory experiments of slab break-off and slab dip reversal: insight into the alpine oligocene reorganization[END_REF][START_REF] Funiciello | Trench migration, net rotation and slab-mantle coupling[END_REF] have concluded that the rheological properties of the slab are important, in that they control the development of the subduction zone. According to [START_REF] Regard | Laboratory experiments of slab break-off and slab dip reversal: insight into the alpine oligocene reorganization[END_REF], viscous removal of the denser parts of the slab decreases the slab-pull force. However, if the slab is strong, it does not deform and the slab-pull force remains active. The other way for a slab to deform is by breaking off entirely. Pyskywec et al. (2000) have modelled this process numerically.

Conditions at the base of a subducting slab


Keywords: Flame spread, Slope, Heat release rate, radiant and convective fractions

Abstract:

The study of the heat transfers and the energy released during the fire spread through beds of pine needles (Pinus pinaster) was conducted under slope and no slope conditions by using a Large Scale Heat Release (LSHR) calorimeter. The results revealed two regimes of propagation. For experiments along a flat surface, the heat release rate (HRR) reached a quasi-steady state whereas for experiments taking place under slope conditions, the HRR increased during the fire spread. This difference was due to a distortion of the fire front. Under no slope conditions, the fire front was indeed linear and propagated with a nearly constant rate of spread. On the contrary, under 20° slope conditions and with a fuel bed of 2 m long and 1 m wide, the fire front had a V-shape leading to an increase of the burnt surface per unit of time and therefore to a rise of the heat release rate during the fire spread. The study was then devoted to the characterization of the heat transfers. Firstly, the convective fraction was determined by using the temperature recorded in the smoke exhaust duct of the LSHR. Then, the radiant fractions of the flame and the embers were calculated from the measurements of radiant heat fluxes, the HRR and the geometrical properties of the fire front. The contribution of each mechanism of heat transfer to the fire spread was therefore quantified. The results showed an increase of the radiation heat transfer, when the fire spread under slope condition, which was mainly due to an increase of the radiation emitted by the flames. All these data give global information on the fire spread and thus they are very useful to improve and validate physical models of wildland fires.

Introduction

Determining the parameters that control the spread of forest fires is a major objective for the management and the prevention of forest fires. This knowledge is not only crucial for understanding the phenomena but 2 also for developing and improving models of forest fires. Among all the parameters involved in the fire spread, the role of heat transfer modes still arises many uncertainties. Several studies have reported measurements of radiant or total heat transfers focusing on the heat flux having an impact on the fuel ahead of a fire front [START_REF] Silvani | Fire spread experiments in the field: Temperature and heat fluxes measurements[END_REF][START_REF] Morandini | Experimental investigation of the physical mechanisms governing the spread of wildfires[END_REF][START_REF] Dupuy | Slope effect on laboratory fire spread: contribution of radiation and convection to fuel bed preheating[END_REF]. Morandini and Silvani [START_REF] Morandini | Experimental investigation of the physical mechanisms governing the spread of wildfires[END_REF] showed the existence of two different regimes of fire spread (plume-dominated and wind-driven) that were either dominated by radiation or governed by mixed radiantconvective heat transfer; but they underlined the difficulties for quantifying their proportion. Other studies [START_REF] Knight | Radiation above bushfires: Report to State Electricity Commission of Victoria and Electricity Trust of South Australia[END_REF][START_REF] Freeborn | Relationships between energy release, fuel mass loss and trace gas and aerosol emissions during laboratory biomass fires[END_REF][START_REF] Kremens | Radiant flux density, energy density and fuel consumption in mixed-oak forest surface fires[END_REF] have focused on the percentage of total energy released as radiation and convection. The measurements obtained by Knight and Dando [START_REF] Knight | Radiation above bushfires: Report to State Electricity Commission of Victoria and Electricity Trust of South Australia[END_REF] on a bushfire exhibit radiant fraction close to 20 %. Freeborn [START_REF] Freeborn | Relationships between energy release, fuel mass loss and trace gas and aerosol emissions during laboratory biomass fires[END_REF] middle infrared measurements. He obtained a radiant fraction of 12.4 % and suggested a convective fraction of about 52%. Kremens [START_REF] Kremens | Radiant flux density, energy density and fuel consumption in mixed-oak forest surface fires[END_REF] used a two-band infrared radiometer to determine the fire radiative energy and found a radiant fraction of 17 %. Tewarson [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF] determined the convective fraction by using the Gas Temperature Rise (GTR) calorimetry. For pine wood, a convective fraction of 70.2 % was found, which leads to a radiant fraction equal to 29.8%. In these studies except that of Tewarson [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF], the authors assumed a complete combustion and used the heat of combustion instead of the net heat of combustion leading to uncertainties in the estimation of the total heat released. The aim of our study is to understand how the energy is released during fire spread and to quantify the radiant and convective fractions for different conditions (with or without slope). For this, we propose to investigate the effect of the slope on the propagation of a fire across a bed of Pinus pinaster needles by using a Large Scale Heat Release Rate Calorimeter (LSHR). We focused mainly on the geometry of the fire front, the heat release rate and the heat transfers.

Materials and methods

Experimental device

Experiments were performed on a 2 m long and 2 m wide inclinable table placed under a 1 MW Large Scale Heat Release (LSHR) calorimeter (Fig. 1). Table inclination was set either to 0° or 20°. At least four experimental replications for each condition of the table were carried out. Fuel beds were made up of pine needles of Pinus pinaster covering a surface of 2 m long and 1 m wide, with a loading of 0.9 kg/m². The mean moisture content of pine needles was equal to 7.4 %. The ignition was performed with 4 ml of alcohol over the entire width of the fuel bed edge. The fire experiments were recorded by using two digital cameras: one was located perpendicularly to the fire spread direction (Camera 1) while the other was placed behind the fire front in the direction of propagation (Camera 2). Two radiant heat flux transducers (MEDTHERM 64P-02-24T) were also used to measure heat fluxes during the fire spread (Fig. 1). They were equipped with a Sapphire window (view angle of 150°) and were cooled with circulating water. The first radiant heat flux gauge (RG1) was oriented towards the flame and was placed at the end of the experimental bench, 20 cm above the bed and in the middle of the fire front. The second radiant heat flux gauge (RG2) was located at 1 cm from the end of the experimental bench and halfway up the fuel bed. Finally, a thermocouple (TK) was used to measure the temperature of the combustion gases in the exhaust smoke duct.

FIG. 1 -Experimental device for the fire spread study 3

Methods

The energy released by the combustion of the pine needles was measured by using the LSHR based on the principle of oxygen consumption calorimetry. The exhaust flow velocity and gas volume fractions were measured during the experiments. Then, the heat release rate (HRR) was calculated from these data, according to the formulation derived by Parker [START_REF] Parker | Calculations of the heat release rate by oxygen consumption for various applications[END_REF]. The calculation procedure was detailed in [START_REF] Santoni | Determination of fireline intensity by oxygen consumption calorimetry[END_REF]. The characterization of the fire front was carried out from its geometry and position. The time needed to the head of the fire front to cover intervals of 0.25 m along the bench was recorded during the experiments. Flame length (L f ) and the angle between the flame and the vertical axis (β) were determined from photographs taken by camera 1 (Fig. 1). The length of the fire front (L) was obtained from photographs recorded by camera 2 (Fig. 1). When the front was curved (that is to say, for the experiments with a slope), other data were added to complement the information on the fire front. The rear position of the flanks was determined over time by using the pictures taken by the cameras. The angle between the flank and the propagation axis (δ) were obtained from photographs taken by camera 1 (Fig. 1). As the flames on the flanks of the fire front were smaller than in the head and were inclined inwardly of the fire front, we also introduced the length L h which corresponds to the length of the front, where the flames were the highest. The measurement L h was performed with camera 2. For each experience, mean values of L, L h , L f , β and δ were computed from 30 to 50 images. Thus, the flame length increased with increasing slope. However, the flame length exhibit variations along the fire perimeter. Thus, the region of the fire front where the flames was the highest (L h ) corresponds only to a length equal to 27.8 % (s.d. 8.7) of the total length of the fire front (Fig. 2c). On the remaining part of the fire front, the flames were smaller and inclined inwardly of the fire front.

Heat release rate

For experiments across a flat surface, after 15 s corresponding to ignition phase, the heat release rate became almost constant (about 87 kW (s.d. 21.5)) during about 350 s (Fig. 3a). For our experiments with a 4 slope of 20°, a quasi-steady state was not observed. The heat release rate continued indeed to increase 120 s until the arrival of the fire front to the end of the table (Fig. 3b). The mean maximum value recorded for the different test fires was 245 kW (s.d. 36.5). The burning time was thus lower for experiments with a slope, because the fire front spread more rapidly (Fig. 3b). Despite these differences, the total heat released did not vary with the slope. The same average value (32.6 MJ (s.d. 1.5)) was indeed obtained regardless propagation conditions. As for the two configurations, the fuel consumption efficiency was equal to 0.97, we obtained the same combustion efficiency. However, for experiments without slope, the heat was released weakly but during a longer time. For our experiments under 20° slope conditions, the heat release rate was much more significant. This difference was due to the fire front shape. Under no slope conditions, the fire front propagated linearly leading to a constant fuel consumption over time and thus to a constant HRR. For our upslope experiments, because of the V-shape of the fire front, the length and depth of the flame front increased during the fire spread. This induced an increase of the surface burnt by the fire per unit of time which in turn produced an increase of the heat release rate.

Heat transfers

The proportion of convection and radiation in the heat transfers was quantified by using the convective and the radiant fractions. The convective fraction corresponds to the ratio between the convective heat release rate [START_REF] Dahlberg | Error analysis for heat release rate measurement with the SP Industry calorimeter[END_REF] and the heat release rate:

HRR dT T c M HRR HRR g a T T mix p mix conv conv ∫ = = ). ( . , & χ (1)
Where T a is the ambient temperature, T g is the gas temperature at duct entry, mix p c , is the specific heat of the mixture at temperature T and mix M & is the mass flow rate of the chemical compounds-air mixture, determined from the volumetric flow rate of the mixture through the duct. The density and the specific heat of the mixture were taken equal to those of dry air. The gas temperature T g was calculated from the measurements recorded by the thermocouple TK placed in the gas measuring section (Fig. 1) by considering the heat losses in the duct before the measurement section. According to the propagation conditions, the convective fractions varied significantly. For experiments conducted without slope, the mean convective fraction was equal to 80.2 % (s.d. 4.6), whereas for experiments with a slope, the mean value corresponded to 61.1% (s.d. 2.4) of the heat transfers. The convective fraction was therefore 1.3 times higher for experiments performed along a flat surface than along a slope of 20°.

To determine the fraction of radiation in the heat transfers, the contribution of the flames was distinguished from that of the embers. For the flame, the radiant power was calculated from the heat flux density measurements ( 1 RG q ) performed with the radiant heat flux gauge RG1 (Fig. 1) by considering the view factor (F) and the emission surface of the fire front (S fl ):

5 F HRR S q fl RG fl rad . . 1 , = χ (2)
The determination of the view factor F was based on the Contour Double Integral Formula (CDIF) method, which calculates the view factor between planar polygons. Figure 4 shows a sketch of the configuration adopted for these calculations. For experiments under slope conditions, as the flanks of the fire front consisted of small discontinuous flames, inclined inward the fire front, only the front part where the flames were higher was considered for the calculation of the radiant fraction.

For the embers, the radiant power was calculated from the heat flux density ( 2 RG q ) measured with the radiant heat flux gauge RG2. The view factor was assumed to be equal to 1, because the radiant heat flux gauge and the embers were very close (distance equal to 1cm). Embers were modeled by a rectangular parallelepiped having a width of 1 m for experiment without slope. In the case of slope experiments, they were modeled by a trapezoidal prism, with the large base equal to 1 m and the small base of 0.8 m (Fig. 4). Thus:

HRR S q em RG em rad . 2 , = χ (3)
where HRR is the heat release rate measured when the head of fire front was at the end of the table and S em the embers surface.

FIG. 4 -Diagrams describing the modeling of the fire front for the calculation of the view factors

For experiments conducted without slope, the mean radiant fractions for the flame and the embers were both equal to 9.9 % (s.d. 0.9 for the flame and 2.0 for the embers). The total radiant fraction corresponds therefore to 19.8%. These results are consistent with literature [START_REF] Knight | Radiation above bushfires: Report to State Electricity Commission of Victoria and Electricity Trust of South Australia[END_REF][START_REF] Freeborn | Relationships between energy release, fuel mass loss and trace gas and aerosol emissions during laboratory biomass fires[END_REF][START_REF] Kremens | Radiant flux density, energy density and fuel consumption in mixed-oak forest surface fires[END_REF]. For experiments with a slope of 20°, the radiant for the flames and the embers were equal to 24.1 % (s.d. 2.6) and 11.8 % (s.d. 2.6), respectively. The overall radiant fraction for upslope fires corresponds to 35.9%. The contribution of the radiation is therefore more significant, when the fire spread occurs under slope conditions. This result is consistent with the decrease observed for the convective fraction. The increase of the radiant fraction of the embers is due to an increase of the volume of embers. For the flames, the radiant fraction was higher because of an increase of the thickness of the flame, which leads to a raise of the emissivity [START_REF] Boulet | Experimental Investigation of Radiation Emitted by Optically Thin to Optically Thick Wildland Flames[END_REF]. The sum of the radiant and convective fractions was equal to 100% without slope and to 95.4% with a slope of 20°. We did not obtain 100% with a slope because of an underestimation of the convective fraction. During 6 the propagation, the suction flow of the hood was not sufficient to evacuate immediately the gases released by the fire and a layer of smoke appeared inside the hood that increased heat losses in the device. This part was not taken into account for the calculation of T g . For a fire spreading along a flat surface, the ratio of convection to radiation was 4:1 whereas for experiments with a slope of 20°, the of the radiation increases leading to a ratio of 1.8:1.

Conclusion

In this study, oxygen consumption calorimetry and heat transfer measurements were used to investigate the slope effect on the energy released in the combustion of a bed of pine needles and on the contribution of radiation and convection in fire spread. The main results can be summarized as follows. The total heat released does not vary with the slope. However, this energy is not released in the same way. For experiments without slope, the energy is steadily released during a longer time but reaches a smaller value. For our experiments under 20° slope conditions, the heat release rate increases during the fire spread. These variations are due to the fire front shape. Under no slope conditions, the fire front propagates linearly leading to constant fuel consumption over time. For our experiments under slope conditions, because of the V-shape of the fire front, the length and depth of the flame front increases during the fire spread leading to a raise of the fuel consumption over time. The contribution of convection and radiation to the fire spread varies following the experimental configuration. For experiments across a flat surface, the convective fraction represents about 80% of the heat released by the fire. The remaining part corresponds to the radiation of the flames and of the embers, which participated in an identical manner of the radiation (about 10%). For the experiments with a slope of 20°, the convection represents less than 65% of the heat transfer, released by the fire. The part of the radiation is indeed more significant. The radiation of the flames constitutes about 24%, whereas that of the embers represents about 11% of the heat transfers. Therefore, the common assumption of 30% of radiation must be taken with caution for vegetative fuels. All these results can be helpful for the improvement and validation of physical models of wildland fires.

FIG. 2 -3 Results and discussion 3 . 1

 231 FIG. 2 -Geometric descriptors of the fire front a) no-slope lateral view b) slope lateral view c) slope rear view

FIG. 3 -

 3 FIG.3 -a) Heat release rate b) Mean position of the head of the fire front

1 Heat transfers and energy released in the combustion of fine vegetation fuel beds

  Les transferts thermiques et l'énergie libérée lors de la propagation d'un feu à travers un lit d'aiguilles de pin (Pinus pinaster) ont été étudiés avec et sans pente à l'aide d'un calorimètre à consommation d'oxygène à grande échelle (LSHR). Les résultats ont révélé deux régimes de propagation. Pour les expériencesconduites sur une surface plane, la puissance dégagée (HRR) atteint un état quasi-stationnaire tandis que pour les expériences qui se déroulent en pente, la puissance du feu augmente au cours de la propagation.

	SPE-UMR 6134 CNRS, Université de Corse, Campus Grimaldi, BP 52, 20250 Corte, France
	Résumé :

V. TIHAY, Y. PEREZ-RAMIREZ, F. MORANDINI, P.A. SANTONI, T. BARBONI Cette différence est due à une déformation du front de flamme. A plat, le front de feu est en effet linéaire et se propage avec une vitesse presque constante. Au contraire, pour les propagations avec une pente de 20° et un lit de 2m x 1m, le front de feu prend la forme d'un V ce qui entraîne une augmentation de la surface brûlée par unité

de

temps et donc une augmentation de la puissance du feu lors de la propagation. L'étude a ensuite été consacrée à la caractérisation des transferts thermiques. Dans un premier temps, la fraction convective a été déterminée en utilisant la température enregistrée dans le conduit d'extraction des fumées. Ensuite, les fractions radiatives de la flamme et des braises ont été calculées à partir des mesures de densités de flux radiatifs, de la puissance dégagée et des propriétés géométriques du front de feu. La contribution de chaque type de transfert thermique a ainsi été quantifiée. Les résultats ont montré une augmentation des transferts thermiques par rayonnement lorsque le feu se propage en une pente. Ceci est principalement dû à une augmentation du rayonnement issu des flammes. Toutes ces données donnent des informations globales sur la propagation d'un feu et sont donc très utiles pour améliorer et valider les modèles physiques de propagation de feux de forêt.