Renaud Jaunatre
email: renaud.jaunatre@irstea.fr

Noellie Fonvieille

Thomas Spiegelberger

Elise Buisson

Thierry Dutoit

Recovery of arbuscular mycorrhizal fungi root colonization after severe anthropogenic disturbance: four species assessed in old-growth Mediterranean grassland

Keywords: Cultivation, Diversity, Plant Community, Steppe, Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi (AMF) interact continuously with vegetation and soil and thus shape the dynamics of plant communities. Yet the recovery of AMF after severe anthropogenic disturbance such as cultivation has rarely been assessed. Here, to determine whether AMF root colonization recovers after such disturbance, we compared AMF root colonization in abandoned fields last cultivated 2, 35, and 150 years ago in the La Crau area (south-eastern France) with that of a grassland several thousands of years old (considered as the reference ecosystem). We measured AMF root colonization of four species (Carthamus lanatus L, Carduus pycnocephalus L, Brachypodium distachyon (L.) P. Beauv, and Bromus madritensis), and performed surveys of plant communities and soil chemical properties. AMF root colonization was still significantly lower 35 years after disturbance for one species (B. distachyon) and 2 years after disturbance for two species (B. distachyon and B. madritensis). The main soil chemical properties (soil pH, phosphorus and potassium content) were similar to the reference ecosystem 35 years after disturbance. Average vegetation height and cover recovered after 35 years, whereas species richness recovered only on the field abandoned for 150 years. Vegetation composition and structure did not recover in any of the abandoned fields. Our results suggest that recovery of AMF root colonization is very low after a severe anthropogenic disturbance, despite the recovery of soil chemical properties.

Introduction

The importance of taking into account arbuscular mycorrhizal fungus (AMF) interactions when exploring plant community dynamics is increasingly acknowledged [START_REF] Grime | Floristic diversity in a model system using experimental microcosms[END_REF]O'Connor et al. 2002;[START_REF] Van Der Heijden | Socialism in soil? The importance of mycorrhizal fungal networks for facilitation in natural ecosystems[END_REF][START_REF] Bever | Rooting theories of plant community ecology in microbial interactions[END_REF][START_REF] Koziol | Mycorrhizal Response Trades off with Plant Growth Rate and Increases with Plant Successional Status[END_REF]. However, there are far fewer studies of AMF than of plants because access to AMF data is more difficult [START_REF] Bever | Arbuscular mycorrhizal fungi: more diverse than meets the eye, and the ecological tale of why[END_REF]. Mycorrhizas can be found in most plant communities [START_REF] Van Der Heijden | Toward and old-growth concept for grasslands, savannas, and woodlands[END_REF], principally arbuscular mycorrhizal fungi (AMF), and interact with 80% of plant species [START_REF] Wang | Phylogenetic distribution and evolution of mycorrhizas in land plants[END_REF]. AMF increase plant water and phosphorus uptake mainly through the increased soil volume available for foraging [START_REF] Allen | Influence of Vesicular-Arbuscular Mycorrhizae on Water Movement Through Bouteloua gracilis (HBK) Lag Ex Steud[END_REF][START_REF] Bolan | A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants[END_REF][START_REF] Koide | Nutrient supply, nutrient demand and plant response to mycorrhizal infection[END_REF][START_REF] Augé | Water relations, drought and vesicular-arbuscular mycorrhizal symbiosis[END_REF]) and may thus impact growth of plant individuals and of their descendants [START_REF] Koide | Mycorrhizal infection of wild oats: maternal effects on offspring growth and reproduction[END_REF][START_REF] Heppell | The effect of mycorrhizal infection of Abutilon theophrasti on competitiveness of offspring[END_REF].

Consequently, AMF modify competition between plants, thereby affecting their coexistence [START_REF] Grime | Floristic diversity in a model system using experimental microcosms[END_REF][START_REF] Smith | nteracting influence of mycorrhizal symbiosis and competition on plant diversity in tallgrass prairie[END_REF][START_REF] Mariotte | Arbuscular mycorrhizal fungi reduce the differences in competitiveness between dominant and subordinate plant species[END_REF] as well as the structure [START_REF] Wilson | Effects of mycorrhizae on plant growth and dynamics in experimental tall grass prairie microcosms[END_REF]O'Connor et al. 2002), species richness and composition of plant communities [START_REF] Gange | Vesicular-Arbuscular Mycorrhizal Fungi: A Determinant of Plant Community Structure in Early Succession[END_REF][START_REF] Francis | The contributions of mycorrhizal fungi to the determination of plant community structure[END_REF][START_REF] Zobel | Interspecific competition and arbuscular mycorrhiza: importance for the coexistence of two calcareous grassland species[END_REF]. This makes AMF dynamics a key to improving our comprehension of ecosystem functioning, and a topic that clearly warrants further investigation. Ecosystems in stressful environments are known to favor biotic interactions such as facilitation within plant species [START_REF] Callaway | Positive interactions among alpine plants increase with stress[END_REF]He et al. 2013) or with AMF (van der [START_REF] Van Der Heijden | Different arbuscular mycorrhizal fungi alter coexistence and resource distribution between co-occurring plant[END_REF], especially old-growth grasslands. These ancient grassland ecosystems characterized by high species richness, high endemism, and unique species composition [START_REF] Van Der Heijden | Toward and old-growth concept for grasslands, savannas, and woodlands[END_REF] have had time to develop species-specific relationships with soil organisms [START_REF] Tscherko | Plant succession and rhizosphere microbial communities in a recently deglaciated alpine terrain[END_REF][START_REF] Bauer | Plant-soil feedbacks as drivers of succession: evidence from remnant and restored tallgrass prairies[END_REF].

Such old-growth grasslands (e.g. dehesa in Spain or continental steppes in Siberia) are particularly threatened.

Technical progress in agronomy in the last century allowed agricultural intensification in low-productivity ecosystems [START_REF] Huston | The three phases of land-use change: implications for biodiversity[END_REF]. Understanding how such changes in land-use can alter these ecosystems is important, especially when environmental authorities aim to stop biodiversity loss and target 15% ecosystem restoration (Millennium Ecosystem Assessment 2005;Convention on Biological Diversity 2011). In this context, abandonment of intensive land-use can be viewed as an opportunity both to restore former ecosystems and to study ecosystem natural recovery [START_REF] Prach | Four opportunities for studies of ecological succession[END_REF]. While old-growth grasslands usually show poor recovery after severe anthropogenic disturbance [START_REF] Forey | Vegetation, soils and seed banks of limestone grasslands are still impacted by former cultivation one century after abandonment[END_REF][START_REF] Van Der Heijden | Toward and old-growth concept for grasslands, savannas, and woodlands[END_REF], little seems to be known about the recovery of other ecosystem components, such as soil or AMF.

Cultivation can be seen as a severe anthropogenic disturbance for natural or semi-natural plant communities: during cultivation, disturbance occurs over a large scale exceeding plant community processes [START_REF] Peterson | Ecological resilience, biodiversity, and scale[END_REF][START_REF] Huston | Local Processes and Regional Patterns: Appropriate Scales for Understanding Variation in the Diversity of Plants and Animals[END_REF] and its strength (i.e. the force of the disturbance; Sousa 1984) is high. The severity of disturbance (i.e. the damage caused by the disturbance; Sousa 1984) is also high, as the majority of mature plants initially present are killed prior to and during a cultivation event. As a consequence, anthropogenic disturbance such as cultivation is followed by low recovery in many ecosystems, such as grasslands, heathlands or forests [START_REF] Bellemare | Legacies of the agricultural past in the forested present: an assessment of historical land-use effects on rich mesic forests[END_REF][START_REF] Dupouey | Irreversible Impact of past Land Use on Forest Soils and Biodiversity[END_REF][START_REF] Römermann | Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management[END_REF][START_REF] Elmore | Agricultural Legacies in the Great Basin Alter Vegetation Cover, Composition, and Response to Precipitation[END_REF][START_REF] Gustavsson | Land use more than 200 years ago explains current grassland plant diversity in a Swedish agricultural landscape[END_REF]).

In the medium term (i.e. decades), vegetation recovery has been proven to be very poor [START_REF] Römermann | Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management[END_REF]Buisson et al. 2006). Cultivation disturbance generally leads to leaching of amended nutrients and therefore a progressive recovery of soil chemical properties, except for phosphorus, which is known to remain in soil [START_REF] Smits | Longterm aftereffects of fertilisation on the restoration of calcareous grasslands[END_REF][START_REF] Henkin | Long-term productivity of Mediterranean herbaceous vegetation after a single phosphorus application[END_REF]. Due to their low dispersal and competitive abilities, most steppe plant species can recolonize only a few meters from remnant steppe patches (Buisson et al. 2006). This means recovery of vegetation composition is usually poor even when abandoned fields are surrounded by remnant steppe patches.

To address AMF recovery dynamics, we chose the La Crau area of south-eastern France, an old-growth Mediterranean dry grassland or steppe plant community impacted by thousands of years of interaction between [START_REF] Bellemare | Legacies of the agricultural past in the forested present: an assessment of historical land-use effects on rich mesic forests[END_REF] Mediterranean climate, (2) a particularly well-draining soil and (3) extensive sheep grazing [START_REF] Devaux | Notice de la carte phyto-écologique de la Crau (Bouches du Rhône)[END_REF]Buisson and Dutoit, 2006). Almost 20% of this species-rich plant community was once cultivated and then abandoned. As AMF root colonization depends not only on AMF dispersal abilities but also on host abundance and soil conditions, our hypothesis was that the more recent and intense the disturbance is, the less AMF root colonization recovers.

We assessed AMF root colonization on four plant species from this Mediterranean steppe after three disturbance events (a vineyard abandoned for 150 years, a melon field abandoned for 35 years and an intensive orchard abandoned for 2 years), compared to an undisturbed ecosystem, to determine whether AMF root colonization recovers after cultivation disturbance.

Materials and methods

Study area

The La Crau area is the last xeric steppe of south-eastern France (ca. 10,000ha). It is shaped by i) a dry Mediterranean climate, ii) a 40cm-deep soil with about 50% of siliceous stones overlying an almost impermeable conglomerate layer, making the alluvial water table unavailable to plant roots and iii) itinerant sheep grazing over a period of several thousand years [START_REF] Devaux | Notice de la carte phyto-écologique de la Crau (Bouches du Rhône)[END_REF]Buisson and Dutoit 2006). This has led to a unique and species-rich vegetation association composed mainly of annuals and dominated by Brachypodium retusum Pers. and Thymus vulgaris L.. The steppe, which experienced recurring anthropogenic disturbance until 400 BP from sheep grazing and pastoral fires, has lost more than 80% of its original 45,000 ha area due to other anthropogenic disturbances such as intensive cultivation (Buisson and Dutoit 2006). Our study focuses on three types of cultivation, all initiated on the original steppe, applied for several years and today abandoned: (1) a vineyard abandoned approximately 150 years ago (AF-150) as revealed from consultation of old cadastral maps and land registers [START_REF] Dutoit | Rémanence des utilisations anciennes et gestion conservatoire des pelouses calcicoles de France[END_REF], (2) two melon fields, cultivated for one year only before being abandoned 35 years ago (AF-35) [START_REF] Römermann | Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management[END_REF]) and an orchard cultivated for 17 years, abandoned in 2006 and which underwent its last disturbance comparable to ploughing in 2009, two years before sampling [START_REF] Jaunatre | Can ecological engineering restore Mediterranean rangeland after intensive cultivation? A large-scale experiment in southern France[END_REF] (AF-2) (Fig. 1). The control was the surrounding steppe, whose available history [START_REF] Cassini | Early post-fire regeneration dynamics of Brachypodium retum Pers. (Beauv.) in old fields of the Valencia region (eastern Spain)[END_REF] showed no severe anthropogenic disturbance. The three types of cultivation represent a gradient both of time since abandonment and of disturbance intensity: the vineyard probably underwent only one ploughing and no herbicide applications [START_REF] Dutoit | Rémanence des utilisations anciennes et gestion conservatoire des pelouses calcicoles de France[END_REF], the melon field underwent one ploughing and one fertilizer and herbicide application [START_REF] Römermann | Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management[END_REF], while the orchard underwent repeated ploughing, fertilizer and herbicide applications [START_REF] Jaunatre | Can ecological engineering restore Mediterranean rangeland after intensive cultivation? A large-scale experiment in southern France[END_REF].

Sampling

We concentrated our sampling areas around the former vineyard (AF-150, n=3) and the former orchard (AF-2, n=3) as they are unique in the La Crau area (Fig. 1). Two steppe sites (ST-6000, n=6) and two sites in fields abandoned for 35 years (AF-35, n=6) were selected, one of each in proximity to either AF-150 or AF-2. The imbalance in design is due to the fact that there was only one site abandoned for 150 years before sampling and only one site abandoned for 2 years before sampling in the entire La Crau area (10 000 ha). We considered the distance between sampling areas (>70m; Fig 1c) sufficient because of the very weak dispersal ability of La Crau plant species (i.e. after 19 years of abandonment, half the species had colonized less than a few meters, [START_REF] Buisson | Colonisation by native species of abandoned farmland adjacent to remnant patch of Mediterranean steppe[END_REF]Buisson et al., 2006). In each of the six sites selected, three sampling areas were set up to sample AMF root colonization, vegetation and soil (Fig. 1d).

AMF root colonization

Colonization by AMF was assessed from roots of four species occurring over almost the entire gradient described above: i) Carthamus lanatus L, an Asteraceae more abundant in the steppe, ii) Carduus pycnocephalus L, an Asteraceae more abundant in the abandoned fields, iii) Brachypodium distachyon (L.) P. Beauv, a Poaceae more abundant in the steppe and iv) Bromus madritensis L, a Poaceae more abundant in the abandoned fields. When possible, in each sampling area three individuals of each species were collected; however, some species were not present in some sampling areas. Air-dried roots were colored with black Schaeffer ink using the vinegar coloration method [START_REF] Vierheilig | Ink and Vinegar, a Simple Staining Technique for Arbuscular-Mycorrhizal Fungi[END_REF]. For total percentage AMF root colonization, internal hyphae, vesicles or arbuscules were counted with the magnified intersections method using 100 intersections [START_REF] Mcgonigle | A New Method which Gives an Objective Measure of Colonization of Roots by Vesicular-Arbuscular Mycorrhizal Fungi[END_REF]).

Soil analyses

In each sampling area, three 70g sub-samples of soil were taken from depths of 1-10cm (Fig. 1d) and pooled to constitute one soil sample. Soil was sieved (2mm mesh) for analyses carried out by INRA (Institut National de la Recherche Agronomique, Arras, France). Granulometry without decarbonation (percentage content of clay (<0.002 mm), fine silt (0.002-0.02 mm), coarse silt (0.02-0.05 mm), fine sand (0.05-0.2 mm) and coarse sand (0.2-2 mm)), nutrients (organic C, total N, P2O5 [START_REF] Olsen | Estimation of available phosphorus in soils by extraction with sodium bicarbonate[END_REF], CaO, and K2O) and pH were measured according to the methods described in [START_REF] Baize | Guide des analyses en pédologie[END_REF].

Vegetation survey

Vegetation relevés were performed on three 2x2m quadrats for each sampling area using Braun-Blanquet coefficients [START_REF] Braun-Blanquet | Les groupements végétaux de la France Méditerranéenne[END_REF]). In addition, average vegetation height (i.e. height of the dominant stratum) and cover (in percentage) were estimated in each quadrat.

Data analysis

Generalized Linear Mixed Models (GLMM) were used to determine whether the cultivation disturbance continued to affect AMF root colonization, soil chemical variables and plant community characteristics. We compared two models: in the first, age was implemented as a categorical fixed factor and blocks (i.e. around the AF-2 or around AF-150; Fig. 1) as a categorical random factor; the second contained neither a fixed factor nor blocks as a categorical random factor. If the first model containing age had a lower Aïkake Information Criterion (AIC) with a difference greater than 2, age effect was declared significant [START_REF] Burnham | Multimodel inference Understanding AIC and BIC in Model Selection[END_REF] and difference between ages was assessed by a multiple comparison test using Tukey's method. To obtain a global overview of soil parameters and of plant communities, we ran a Principal Component Analysis (PCA) for soil parameters and a Correspondence Analysis (CA) for plant community [START_REF] Borcard | Numerical Ecology with R[END_REF]. To measure similarity of plant communities to those of the undisturbed control, we used the normalized Community Structure Integrity Index (CSIInorm) and the Higher Abundance Index (HAI) to distinguish the percentage of recovery of reference species abundances (CSIInorm) from new abundances not occurring in this reference (HAI) [START_REF] Jaunatre | New synthetic indicators to assess community resilience and restoration success[END_REF].

All analyses were performed with R 2.13.0 (R Development Core Team 2011), univariate analyses with its package "lme4" and "multcomp" [START_REF] Hothorn | Simultaneous Inference in General Parametric Models[END_REF]Bates et al. 2012) and multivariate analyses with its packages "ade4" [START_REF] Chessel | The ade4 package-I-One-table methods[END_REF]Dray and Dufour, 2007;Dray et al. 2007) and "vegan" [START_REF] Oksanen | vegan: Community Ecology Package. R package version 1[END_REF].

Results

AMF root colonization

AMF root colonization recovered to ST-6000 values (approximately 90%) in all three abandoned fields for forbs (Carthamus lanatus and C. pycnocephalus ; Fig. 2, Table 1). However, both grasses did not recover in all the locations: Bromus madritensis shows lower AMF root colonization in AF-2 (40.7±3.8) than in ST-6000 (63.9±4.2) (Fig. 6, Table 1) and B. distachyon shows a significant difference in AMF root colonization between AF-35 (51.4±5.9) and ST-6000 (64.2±2.1) (no B. distachyon was found in AF-2).

Soil analyses

The PCA of soil properties (Fig. 3) clearly discriminates among the four locations. The first axis (35.2%) shows a clear distinction in age between the field abandoned 2 years previously (AF-2) and the three other locations, with higher concentrations of P2O5, pH and K2O (Fig. 4), although only pH is significantly higher (Table 1). The second axis (24.5%) discriminates the steppe (ST-6000) from the longest-abandoned field (AF-150), with higher carbon and nitrogen content (Fig. 4), although only carbon differences are significant (Table 1).

Vegetation survey

Compared to the reference, all three abandoned fields have significantly different values for composition and structure (Table 1, Fig. 5). Their CSIInorm is lower: AF-150 and AF-35 have recovered less than 60% of the vegetation structure, while AF-2 has recovered less than 20%. Their HAI is higher: in AF-150 and AF-35, 50% of the structure is composed of species whose abundances are higher than in ST-6000 (both new species and the same species); while in AF-2, more than 70% of species have abundances higher than in ST-6000. These differences in plant community variables are also discernible on the correspondence analysis (Fig. 6): AF-2 is dominated by Bromus madritensis L, Bromus lanceolatus Roth and Carduus pycnocephalus L.. AF-35 is also characterized by Poaceae and Asteraceae (Bromus hordeaceus L, Bromus rubens L, Carthamus lanatus L. etc.). AF-150 and ST-6000 share many species (e.g. Aegilops ovata L, Brachypodium distachyon (L.) P. Beauv, Carlina corymbosa L, Erodium cicutarium (L.) L'Hérit. or Plantago bellardii All.), although i) some dominant ST-6000 species are absent from AF-150 (e.g. Brachypodium retusum (Pers.) P. Beauv.) and ii) some AF-150 species are absent from ST-6000 (e.g. Bothriochloa ischaemum (L.) Keng, Crucianella angustifolia L.). The species richness of AF-150 is not significantly different from that of ST-6000; however, it is significantly lower for AF-35 and AF-2. All the abandoned field locations except AF-2 have recovered ST-6000 average height and vegetation cover, while AF-2 shows significantly higher values (Table 1, Fig. 5).

Discussion

It takes a very long time to recover ecosystem characteristics. This has been proved for many different old-growth grassland types, such as Midwest prairie, Scandinavian semi-natural grasslands, Mediterranean steppe or limestone grassland [START_REF] Tomanek | Natural Revegetation on a Field Abandoned for Thirty-Three Years in Central Kansas[END_REF][START_REF] Bonet | Species richness and cover along a 60-year chronosequence in old-fields of southeastern Spain[END_REF][START_REF] Öster | Plant colonization of ex-arable fields from adjacent species-rich grasslands: The importance of dispersal vs. recruitment ability[END_REF][START_REF] Forey | Vegetation, soils and seed banks of limestone grasslands are still impacted by former cultivation one century after abandonment[END_REF], and our study confirms it. Most of these low-resilient ecosystems have similar characteristics: harsh environmental conditions, which are attenuated by the disturbance event (i.e. nutrient or water availability is increased). In such a context, some plant species adapted to higher nutrient content can colonize and generate relatively stable mutualistic associations with soil organisms, such as AMF or microbial communities, thus slowing down the potential recovery of the original state [START_REF] Spiegelberger | Long-term effects of short-term perturbation in a subalpine grassland[END_REF][START_REF] Wardle | Long term resilience of above-and below-ground ecosystem components among contrasting ecosystems[END_REF]). Our study shows that it is not only vegetation and soil components that are durably affected by a severe anthropogenic disturbance such as cultivation:

there is an impact on AMF root colonization too. The results also confirm our hypothesis that the more recent and intense the disturbance is, the less AMF root colonization recovers. Our findings are in line with those from studies on different components under harsh environmental conditions, which reacted in a similar way [START_REF] Hejcman | The Rengen Grassland Experiment: Plant species composition after 64 years of fertilizer application[END_REF][START_REF] Spiegelberger | Resilience of acid subalpine grassland to short-term liming and fertilisation[END_REF][START_REF] Spiegelberger | Long-term effects of short-term perturbation in a subalpine grassland[END_REF]. However, new evidence is provided here on ecosystem recovery via the recovery of AMF root colonization, rarely studied in parallel with soil and plant communities.

AMF root colonization

AMF root colonization can decrease due to any soil disturbance [START_REF] Jasper | Soil disturbance reduces the infectivity of external hyphae of vesiculararbuscular mycorrhizal fungi[END_REF]). Agricultural practices in particular are known to impact AMF communities, modifying their abundance, their diversity and their composition [START_REF] Douds | Effect of tillage and farming system upon populations and distribution of vesicular-arbuscular mycorrhizal fungi[END_REF][START_REF] Jansa | Soil tillage affects the community structure of mycorrhizal fungi in maize roots[END_REF], and their reproduction dynamics [START_REF] Oehl | Distinct sporulation dynamics of arbuscular mycorrhizal fungal communities from different agroecosystems in long-term microcosms[END_REF]). Areas cultivated with more fertilizer show less AMF root colonization [START_REF] Douds | Effect of tillage and farming system upon populations and distribution of vesicular-arbuscular mycorrhizal fungi[END_REF][START_REF] Mäder | Arbuscular mycorrhizae in a long-term field trial comparing low-input (organic, biological) and high-input (conventional) farming systems in a crop rotation[END_REF]. In our study, the AMF root colonization rate increases with time since cultivation abandonment for two plant species out of four. It has previously been shown that AMF root colonization can recover with time [START_REF] Eriksson | Arbuscular mycorrhiza in relation to management history, soil nutrients and plant species diversity[END_REF][START_REF] Gibson-Roy | Do arbuscular mycorrhizal fungi recolonize revegetated grasslands?[END_REF] but that this depends on plant community composition [START_REF] Fitzsimons | Scale-dependent niche axes of arbuscular mycorrhizal fungi[END_REF]. This is supported by our results: AMF root colonization recovered and was no longer significantly different from the steppe after 35 years, except for B. distachyon, and the field abandoned for 35 years showed over 50% recovery of plant community structure (Figure 5). It should be noted that the major finding here is the difference between AMF root colonization in abandoned fields and in the reference. The similar values found here for AMF root colonization may in fact mask an absence of recovery of certain AMF species, since it is likely that different AMF species are involved. This hypothesis is supported by the fact that AMF community composition can be altered by a change in plant community composition [START_REF] Klamer | Fungal diversity in set-aide agricultural soil investigated using terminal-restriction fragment length polymorphism[END_REF], and such a change occurred in our abandoned fields.

We did not find any difference in root colonization recovery between species more abundant in the steppe (C. lanatus and B. distachyon) and species more abundant in the abandoned fields (C. pycnocephalus and B. madritensis). We found, however, that grasses (B. distachyon and B. madritensis) have higher average mycorrhizal root colonization and higher recovery of mycorrhizal root colonization than forbs (C. lanatus and C. pycnocephalus), which confirms that plant functional type is the most important driver of plant response to mycorrhizal fungi [START_REF] Hoeksema | A meta-analysis of context-dependency in plant response to inoculation with mycorrhizal fungi[END_REF].

Soil parameters

The different abandoned fields are found to have contrasting soil properties. The former vineyard, which is the longest abandoned field, has the lowest pH and potassium and phosphorus content. However, the difference in fertilization practices can also introduce biases; fertilizer use was more sparing and more organic-based 150 years ago than 3 years ago [START_REF] Dutoit | Rémanence des utilisations anciennes et gestion conservatoire des pelouses calcicoles de France[END_REF]. Annual, and therefore cumulative, fertilizer quantities are difficult to estimate. Despite these potential biases, our results are consistent with findings from other studies: as in [START_REF] Wong | A conceptual model of plant community changes following cessation of cultivation in semi-arid grassland[END_REF], potassium and phosphorus content tend to decrease with time since last cultivation, and as in studies on Mediterranean or calcareous grasslands [START_REF] Smits | Longterm aftereffects of fertilisation on the restoration of calcareous grasslands[END_REF][START_REF] Henkin | Long-term productivity of Mediterranean herbaceous vegetation after a single phosphorus application[END_REF], high soil phosphorus content is maintained even 35 years after cessation of fertilization. Soil properties are known to be very important in determining plant community diversity, structure and composition. For instance, [START_REF] Kulmatiski | Soil history as a primary control on plant invasion in abandoned agricultural fields[END_REF] showed how soil legacies impact invasive plant species distributions and [START_REF] Janssens | Relationship between soil chemical factors and grassland diversity[END_REF] suggested that 5 mg of phosphorus per 100 g of dry soil is a threshold value limiting establishment of species-rich plant communities.

Despite the fact that in our abandoned fields current soil phosphorus content remains close to (AF-2) or below this level (AF-35 and AF-150), it is possible that a previous higher phosphorus content had a long-term effect which can still be observed, even after the recovery of lower values [START_REF] Semelová | The Grass Garden in the Giant Mts. (Czech Republic): Residual effect of long-term fertilization after 62 years[END_REF].

Plant communities

As with soil, the longer the time since abandonment, the more plant community characteristics in abandoned fields differ from the reference. More recently abandoned fields have lower species richness or CSII, while they have higher HAI, average vegetation height and cover. Moreover, vegetation composition, which is mainly dominated by grasses such as Bromus spp. in the most recently abandoned fields, gains in species richness and in the number of forbs with time since abandonment. Similar slow recolonization was observed in other ecosystems after severe disturbances [START_REF] Tomanek | Natural Revegetation on a Field Abandoned for Thirty-Three Years in Central Kansas[END_REF][START_REF] Coffin | Recovery of Vegetation in a Semiarid Grassland 53 Years after Disturbance[END_REF][START_REF] Meiners | Exotic plant invasions over 40 years of old field successions: community patterns and associations[END_REF][START_REF] Bonet | Species richness and cover along a 60-year chronosequence in old-fields of southeastern Spain[END_REF]. However after 150 years, while the vegetation composition of the abandoned field is close to that of the steppe it is still slightly different, notably due to the absence of B. retusum, the dominant steppe species. Interestingly, the species with the lowest AMF root colonization recovery rate (B. distachyon) is phylogenetically closest to B. retusum (The Angiosperm Phylogeny Group 2009). B. retusum is not present in any part of this long-abandoned field, although it is the dominant steppe species and occurs at the abandoned field boundaries, and despite the fact that all measured environmental characteristics are not significantly different. Low seed production and fertility, and hence poor dispersal abilities, have already been hypothesized (Buisson et al. 2006;[START_REF] Coiffait-Gombault | Are old Mediterranean grasslands resilient to human disturbances?[END_REF]. The slow recovery of AMF root colonization rates in a phylogenetically close species would suggest that B. retusum not only has low seed production (Caturla et al. 2000) but also suffers from a lack of AMF interaction. This interaction would likely enhance its growth by higher phosphorus uptake [START_REF] López-Sánchez | Influence of vesicular-arbuscular mycorrhizal infection and P addition on growth and P nutrition of Anthyllis cytisoides L. and Brachypodium retusum (Pers.) Beauv[END_REF], enabling it to establish in abandoned fields.

Ecosystem recovery

Our results on vegetation are consistent with previous studies, where: vegetation was still affected 70 years after a fertilization event, even though the soil almost recovered its properties [START_REF] Spiegelberger | Long-term effects of short-term perturbation in a subalpine grassland[END_REF]; only half the species were able to colonize abandoned fields 50 years later (Öster et al. 2009); or differences in plant communities were still significant more than 2000 years after a cultivation event in a forest community [START_REF] Dupouey | Irreversible Impact of past Land Use on Forest Soils and Biodiversity[END_REF]. The filter model is often used to describe vegetation dynamics [START_REF] Keddy | Assembly and Response Rules: Two Goals for Predictive Community Ecology[END_REF][START_REF] Lortie | Rethinking plant community theory[END_REF]): i) plant species have to be able to disperse, which depends on species dispersal abilities and proximity of source site [START_REF] Gibson | The nature and rate of development of calcareous grassland in Southern Britain[END_REF][START_REF] Pärtel | Small-scale plant species richness in calcareous grasslands determined by the species pool, community age and shoot density[END_REF][START_REF] Lindborg | Historical Landscape Connectivity Affects Present Plant Species Diversity[END_REF][START_REF] Herault | How habitat area, local and regional factors shape plant assemblages in isolated closed depressions[END_REF]; ii) plant species have to be able to withstand environmental constraints, which depends both on historical environmental conditions and on disturbance legacies [START_REF] Foster | The Importance of Land-Use Legacies to Ecology and Conservation[END_REF]) and iii) the first two filters will be modified by biotic interactions and will depend on the presence of facilitators or competitors in the community [START_REF] Bruno | Inclusion of facilitation into ecological theory[END_REF].

This model explains the low resilience of plants after cultivation in the La Crau area: species have low dispersal abilities and no permanent seed bank [START_REF] Graham | A Field Investigation of Germination from the Seed Bank of a Chalk Grassland Ley on Former Arable Land[END_REF][START_REF] Römermann | Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management[END_REF], soil nutrient content is still different in recently abandoned fields, and finally, some species are better competitors under higher nutrient conditions [START_REF] Baeten | Low recruitment across life stages partly accounts for the slow colonization of forest herbs[END_REF][START_REF] Öster | Plant colonization of ex-arable fields from adjacent species-rich grasslands: The importance of dispersal vs. recruitment ability[END_REF], especially if they arrived first due to chance or better dispersal abilities [START_REF] Fukami | Species divergence and trait convergence in experimental plant community assembly[END_REF].

It should be feasible to apply a similar model to AMF, to explain their recovery after a severe anthropogenic disturbance, as suggested by [START_REF] Lekberg | Role of niche restrictions and dispersal in the composition of arbuscular mycorrhizal fungal communities[END_REF]. If AMF have been eliminated during the cultivation event [START_REF] Douds | Effect of tillage and farming system upon populations and distribution of vesicular-arbuscular mycorrhizal fungi[END_REF][START_REF] Jansa | Soil tillage affects the community structure of mycorrhizal fungi in maize roots[END_REF], they have to disperse through the disturbed area. Wind seems to be a poor long-distance disperser [START_REF] Egan | Detection of arbuscular mycorrhizal fungal spores in the air across different biomes and ecoregions[END_REF], but faunal agents may also provide some local-scale dispersal [START_REF] Warner | Dispersal agents of vesicular-arbuscular mycorrhizal fungi in a disturbed arid ecosystem[END_REF][START_REF] Allen | The restoration of disturbed arid landscapes with special reference to mycorrhizal fungi[END_REF][START_REF] Harinikumar | Potential of earthworms, ants, millipeds, and termites for dissemination of vesicular-arbuscular mycorrhizal fungi in soil[END_REF]. Hyphae not killed by cultivation can also be a rich source of AMF; however, they are highly affected by soil disturbance [START_REF] Jasper | Soil disturbance reduces the infectivity of external hyphae of vesiculararbuscular mycorrhizal fungi[END_REF][START_REF] Brundrett | Mycorrhizal fungus propagules in the jarrah forest[END_REF]. AMF root colonization thus depends on environmental conditions: the more available nutrients are, the less roots are colonized by AMF [START_REF] Koide | Nutrient supply, nutrient demand and plant response to mycorrhizal infection[END_REF]. Our findings support this: AMF root colonization was higher in the abandoned fields with lower soil nutrient content. Finally, AMF root colonization depends on biotic interactions.

In our study, the abandoned field with the highest vegetation average height and the highest vegetation cover (AF-2) was found to have the lowest mycorrhizal root colonization. These results are in accordance with the negative relationship between vegetation biomass and AMF biomass already found in abandoned fields [START_REF] Hedlund | Plant Species Diversity, Plant Biomass and Responses of the Soil Community on Abandoned Land across Europe: Idiosyncracy or Above-Belowground Time Lags[END_REF]. Despite the fact that mechanical soil disturbance plays a greater role than plant communities in shaping AMF communities [START_REF] Schnoor | Mechanical soil disturbance as a determinant of arbuscular mycorrhizal fungal communities in semi-natural grassland[END_REF], plant species richness has been shown to increase the diversity and fitness of AMF [START_REF] Burrows | Arbuscular mycorrhizal fungi respond to increasing plant diversity[END_REF]. Moreover, the diversity of AMF infecting an individual plant depends on the diversity of the whole plant community (van de [START_REF] Van De Voorde | Comparing arbuscular mycorrhizal communities of individual plants in a grassland biodiversity experiment[END_REF], and the composition of the plant community has a significant effect on the composition of the AMF community [START_REF] Johnson | Can Fertilization of Soil Select Less Mutualistic Mycorrhizae?[END_REF][START_REF] Eom | Host plant species effects on arbuscular mycorrhizal fungal communities in tallgrass prairie[END_REF]Hiiesalu et al., 2014).

Our study is a first step towards exploring the relationship between a proxy of AMF (AMF root colonization), soil conditions and plant communities. In accordance with our hypotheses, recovery of these three ecosystem components differs, increasing from vegetation (lowest recovery) to AMF root colonization (intermediate recovery), to soil conditions (highest recovery). All three components assessed are moving towards the undisturbed state, but full, unassisted recovery is highly unlikely within a human lifetime.

Feedbacks occurring between different ecosystem components, as in a panarchy model, need to be taken into account to understand the overall recovery process. One component starts to recover while being influenced by another component; based on these dynamics, the recovery of the first component is affected, and in turn the recovery of other components is further influenced [START_REF] Allen | Panarchy: Theory and Application[END_REF]). To our knowledge, although few studies have measured the recovery of different components of an ecosystem after disturbance, most of them found, like us, staggered responses for each component [START_REF] García-Tejero | Differential responses of ecosystem components to a low-intensity fire in a Mediterranean forest: a three-year case study[END_REF]Wardle and Jonsson, 2013). Determining the limiting components able to affect, slow down or stop the recovery dynamics of the whole ecosystem would be of particular interest in a restoration context. AMF communities affect plant communities in complex ways: like plants, not all AMF species play the same role in ecosystems [START_REF] Hart | Plant coexistence mediated by arbuscular mycorrhizal fungi[END_REF]. The effects of AMF on plants are species-specific [START_REF] Hoeksema | A meta-analysis of context-dependency in plant response to inoculation with mycorrhizal fungi[END_REF][START_REF] Endresz | Arbuscular mycorrhizal colonisation of roots of grass species differing in invasiveness[END_REF]), but also depend on environmental conditions [START_REF] Grime | Floristic diversity in a model system using experimental microcosms[END_REF][START_REF] Hartnett | Mycorrhizae influence plant community structure and diversity in tallgrass prairie[END_REF][START_REF] Kytöviita | A test of mutual aid in common mycorrhizal networks: established vegetation negates benefit in seedlings[END_REF], and higher AMF root colonization is not always linked with better vegetation recovery [START_REF] Richter | Assessment of arbuscular mycorrhizal fungal propagules and colonization from abandoned agricultural fields and semi-arid grasslands in riparian floodplains[END_REF]. Further research is required to explore how different ecosystem components can affect overall ecosystem recovery, and how they could be used to facilitate or to accelerate this recovery in a restoration context [START_REF] Allen | The restoration of disturbed arid landscapes with special reference to mycorrhizal fungi[END_REF][START_REF] Herrera | Inoculation of woody legumes with selected arbuscular mycorrhizal fungi and rhizobia to recover desertified Mediterranean ecosystems[END_REF][START_REF] Callaway | Compensatory growth and competitive ability of an invasive weed are enhanced by soil fungi and native neighbours[END_REF][START_REF] Kardol | Soil Organism and Plant Introductions in Restoration of Species-Rich Grassland Communities[END_REF]. Table 1: Aikaike Information Criterion of Generalized Linear Mixed Models, one including Age as a fixed effect and Block as a random effect, the other including only Block as a random effect. Stars preceding variable names indicate that the model including age is better (i.e. ΔAIC > 2 ; [START_REF] Burnham | Multimodel inference Understanding AIC and BIC in Model Selection[END_REF]

Tables

Figure Captions Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6

 Captions123456 Figure Captions

  and age can therefore be considered as affecting the variable.

		Variable	AIC Age+Block	AIC Block
		*Brachypodium distachyon	276.1	289.8
	Mycorrhizal root	*Bromus madritensis	344.2	368.5
	colonization	*Carthamus lanatus	274.7	286.0
		*Carduus pycnocephalus	187.0	198.1
		*Species-richness	364.7	472.6
		*CSII	-50.5	40.8
	Vegetation	*HAI	-110.5	-14.9
		*Average height	351.5	410.0
		*Vegetation cover	430.7	474.5
		*Carbon	69.2	76.8
		*pH	9.5	19.5
		C:N	23.2	25.0
	Soil	Nitrogen	3.1	-6.9
		Phosphorus	-54.8	-69.8
		Potassium	-30.8	-40.3

Acknowledgement

This study was supported by CDC Biodiversité, CEN PACA, the Réserve Naturelle des Coussouls de Crau, the Conseil Régional de Provence Alpes Côtes d'Azur and CNRS RTP Ingecotech funding. The author would like to thank Daniel Pavon (IMBE) for help with fieldwork and Marjorie Sweetko for English language editing.


Dominique Abtout

B Gibert

B Henry

M E M Bouyahiaoui

Derder

H Boukerbout
email: s.boukerbout@craag.dz

A Abtout

D Gibert

B Bouyahiaoui

M E M Derder

Identification of deep magnetized structures in the tectonically active Chlef area (Algeria) from aeromagnetic data analyzed with 2-D and 3-D imaging derived from the wavelet and ridgelet transforms

Keywords: Aeromagnetic data, Chlef, wavelet transform, ridgelet transform, 2D -3D imaging, deep magnetized structures

    Identification of deep magnetized structures in the tectonically active Chlef area (Algeria) from aeromagnetic data analyzed with 2-D and 3-D imaging derived from the wavelet and ridgelet transforms H. Boukerbout, A.

Introduction

The Chlef region (Figure 1) is located in North of Algeria around coordinates: 00°33'-02°00'E & 35°48'-36°56'N. It is one of the most seismically active zones of the western Mediterranean Sea [START_REF] Meghraoui | Tectonique plioquaternaire de la chaîne tello-rifaine et de la mer d'Alboran. Une zone complexe de convergence continent-continent[END_REF][START_REF] Bezzeghoud | Faulting mechanism of the El Asnam (Algeria) 1954 and 1980 earthquakes from modelling of vertical movements[END_REF][START_REF] Yelles | La tectonique active de la région nord-algérienne[END_REF] related to the collision between African and European plates since the Upper Cretaceous [START_REF] Dewey | Kinematics of the western Mediterranean[END_REF][START_REF] Le Pichon | Plate kinematics and tectonic leading to Alpine belt formation: a new analysis, processes in continental lithospheric deformation[END_REF][START_REF] Ricou | Tethys reconstructed: plates continental fragments and their boundaries since 260 Ma from Central America to South-eastern Asia[END_REF]. Kinematic models based on the study of the magnetic anomalies in the Atlantic Ocean; showed that this convergence is related to a counter clockwise rotation of Africa with respect to Eurasia [START_REF] Dewey | Kinematics of the western Mediterranean[END_REF]Mazzoli and Hetman, 1994;[START_REF] Rosenbaum | Relative motions of Africa, Iberia and Europe during Alpine orogeny[END_REF]. The seismic activity in this area is directly associated with the plate boundary between Europe and Africa. This region was affected by two very destructive seismic events: the 1954 9 th September Orleansville earthquake with a 6.5 magnitude and the El Asnam one of 1980 10 th October (7.3 magnitude) (Karnik, 1969;[START_REF] Dewey | The 1954 and the 1980 Algerian earthquakes :implications for the characteristic-displacement model of fault behavior[END_REF]. Studies of structural geology, seismicity and seismotectonics were therefore strongly developed in this region to better understand the presently active deformation mechanisms (e.g. [START_REF] Rothé | Les séismes de Kherrata et la séismicité en Algérie[END_REF][START_REF] Perrodon | Etude géologique des basins néogènes sub-littoraux de l'Algérie nord occidentale[END_REF]Tapponier, 1977;[START_REF] Girardin | La séismicité du Nord de l'Algérie[END_REF][START_REF] Philip | Structural analysis and interpretation of the surface deformations of the El Asnam earthquake of October 10[END_REF][START_REF] Thomas | Géodynamique d'un bassin intra montagneux : le bassin du Bas Chlef occidental (Algérie) durant le Moi-Plio-Quaternaire[END_REF][START_REF] Ouyed | Le tremblement de terre d'El Asnam du 10 octobre 1980. Etude des répliques[END_REF][START_REF] Meghraoui | Géologie des zones sismiques du nord de l'Algérie (paléosismicité, tectonique active et synthèse sismotectonique[END_REF]Yielding et al., 1989; A C C E P T E D M A N U S C R I P T [START_REF] Boukerbout | Identification of Western Algeria faults using aeromagnetic data[END_REF][START_REF] Domzig | Searching for the Africa-Eurasia Miocene boundary offshore western Algeria (MARADJA'03 cruise)[END_REF]Aifa and Zaagane, 2014;[START_REF] Abtout | Gravimetric evidences of active faults and underground structure of the Chlef seismogenic basin (Algeria)[END_REF].

With the same purpose, the aeromagnetic data supply further information on the causative geological structures of the magnetic anomalies. The use of methods developed recently such as the wavelet and the ridgelet transforms, applied to process the magnetic field data, allows establishing 2-D and 3-D images of the geological bodies both at surface and at depth as well inshore as offshore.

The present study is a contribution in a large project including geophysical and geological surveys in the Chlef region (CMEP Project 08MDU752). The main objectives of this study are to identify the geometry of the major tectonic contacts bounding the Chlef basin, to evaluate the depth of the different identified structures and to have a better knowledge of the basin basement. Geophysical methods and particularly aeromagnetic approach are adapted for obtaining a realistic model of the deep tectonic structures. This work leads to 2D and 3D representation from a set of sections across the geological units and magnetic field anomalies, to the geometric relationships existing between the different geological features. It also provides additional information enabling for example geologists to refine the structural interpretation of the volcanic domains and then improving the knowledge of the studied region.

Figure 1.

A C C E P T E D M A N U S C R I P T

4

Geological setting

The studied region (Figure 1) includes a large area from Mediterranean Sea to the North to the Ouarsenis Mountains to the South. Its central part is an intra mountainous depression of the Tellian chain: the post-nappes Chlef basin. This basin is filled by Mio-Plio-Quaternary rocks, lying on ante-Neogene basement. The latter outcrops as flyshoid and epi-metamorphic massifs, like Temoulga and Doui. These massifs are mainly composed of Siluro-Devonian schistosed limestones with a Jurassic dolomitic limestones cover [START_REF] Kireche | Etude géologique structurale des massifs de la plaine du Chélif (Doui, Rouina, Temoulga)[END_REF]. The basin has an elongated shape, suggesting compressional movements [START_REF] Meghraoui | Seismotectonics of the lower Chelif basin: structural background of the El Asnam (Algeria) earthquake[END_REF] related to the Alpine orogeny [START_REF] Perrodon | Etude géologique des basins néogènes sub-littoraux de l'Algérie nord occidentale[END_REF]. The neo-tectonics studies show that the main deformation is a NNW-SSE compression giving overthrusting reverse and strike-slips faults (Groupe de Recherche néotectonique de l'arc de Gibraltar, 1977;[START_REF] Philip | Détermination de la direction de raccourcissement de la phase de compression quaternaire en Oranie (Algérie)[END_REF] related to African and European plate movements [START_REF] Philip | Détermination de la direction de raccourcissement de la phase de compression quaternaire en Oranie (Algérie)[END_REF][START_REF] Minster | Present-day plate motions[END_REF][START_REF] Anderson | Active tectonics of the Adriatic region[END_REF].

Volcanic activity in the basin is evidenced by stratified cineretic deposits of upper Miocene age in the SW of the studied area [START_REF] Perrodon | Etude géologique des basins néogènes sub-littoraux de l'Algérie nord occidentale[END_REF], a small rhyolitic outcrop (0°35'E, 36°05'N), microgranite dykes cutting the upper Miocene deposits in the Dahra [START_REF] Anderson | Geology in the coastal atlas of western Algeria[END_REF] and biotitic andesites in the Beni-Chougrane.

Along the coast, east of Tenes, rhyolites and andesitic dacites [START_REF] Hernandez | Pétrographie des laves liées aux bassins de l'Ouest algérois. Comparaison avec le volcanisme du Rif et de l'Oranais[END_REF] have ages varying between 15 and 9 Ma. The calc-alkaline volcanism is known all over the coastal border of the northern Tell in a band parallel to the coast line. It might evidence an ancient subduction zone [START_REF] Girod | Contribution de la pétrologie à la connaissance de l'évolution de la Méditerranée occidentale depuis l'Oligocène[END_REF]Girod, 1977, Maury et al., 2000). These volcanic episodes would occur between lower Miocene compressional phases related to the formation of the nappes [START_REF] Mattauer | Etude géologique de l'Ouarsenis oriental[END_REF][START_REF] Delteil | Tectonique de la chaîne alpine en Algérie d'après l'étude du Tell oranais oriental : Monts de la Mina[END_REF] and the Plio-Quaternary compression. This situation is probably due to a period where local relaxation of compressional stresses allows volcanic

A C C E P T E D M A N U S C R I P T 5 eruptions.
More to the West, the volcanic activity was continuous during the Plio-Quaternary in contrast to its absence in the Chlef basin [START_REF] Bellon | Le volcanisme alcalin plio-quaternaire d'Algérie occidentale. Etude radiométrique et paléomagnétique[END_REF]. Nevertheless E-W extensional movements may be simultaneous to the N-S compression [START_REF] Bousquet | Les caractéristiques de la Néotectonique en Méditerranée occidentale[END_REF]) that is the main deformation mechanism during the Plio-Quaternary period in the western Tellian Atlas [START_REF] Thomas | La phase de compression Pléistocène en Algérie nord occidentale : âge, premiers éléments cinématiques, relation avec les mouvements en distension[END_REF][START_REF] Philip | Détermination de la direction de raccourcissement de la phase de compression quaternaire en Oranie (Algérie)[END_REF][START_REF] Meghraoui | Etude néotectonique de la région Nord-Ouest d'El Asnam : Relation avec le séisme du 10 octobre[END_REF].

Therefore, the Plio-Quaternary alkaline volcanic manifestation west of the Chlef basin, likely does not characterize an independent extensional episode.

At a local scale, remarkable tectonic structures can be observed across the basin.

-The Sara El Marouf broken fold correspond to the El Asnam fault, reactivated during the October 10, 1980, El Asnam earthquake [START_REF] Ouyed | Seismotectonics of the El Asnam earthquake[END_REF][START_REF] King | Active folding in the Algerian earthquake of 10 October 1980[END_REF]. It has a NE-SW direction and it is limited to the SE by a reverse active fault since the early Quaternary [START_REF] Meghraoui | Etude néotectonique de la région Nord-Ouest d'El Asnam : Relation avec le séisme du 10 octobre[END_REF][START_REF] Meghraoui | Seismotectonics of the lower Chelif basin: structural background of the El Asnam (Algeria) earthquake[END_REF]. It crosses the eastern part of the basin separating the Oued Fodda valley from El Asnam valley.

-The Boukadir broken fold is located about 40 km in the WSW of El Asnam, separating the El Asnam valley from Relizane valley. The Boukadir fault shows recent activity affecting Quaternary deposits. It corresponds to a thrust striking N065 with a NW.

-A third structure crossing the basin is the Dahra fault, which separates the Relizane valley from the Mostaganem valley. It affects Quaternary deposits that are strongly deformed, clearly evidencing an active flexure.

-The southern limit of the basin which is the southern limit of the Quaternary sediments is defined by a tectonic linear contact between recent Quaternary silts and upper Miocene limestones. Its strike is N070 and it runs for some 150km from El Asnam to Relizane [START_REF] Meghraoui | Seismotectonics of the lower Chelif basin: structural background of the El Asnam (Algeria) earthquake[END_REF].

A C C E P T E D M A N U S C R I P T 6

Aeromagnetic data

The aeromagnetic map used in this study outcome from the airborne survey which cover Algeria during the period from 1970-1974 by Aero Service Corporation, at a constant flight altitude of 150m along N20W lines 2km apart with perpendicular tie-lines at 5 km intervals;

sampling distance along the profiles was about 46m. The survey data were originally recorded in analogue form [START_REF] Paterson | Réinterprétation des levés aéromagnétospectrométriques de l'Algérie. 3 volumes[END_REF]. The digital dataset used for this study was obtained by digitizing the paper map each 2nT contour line at a scale of 1/200,000. The digitizing method is based on recognition of shape. There is no spatial error introduced in the automated vectorization process. The vector data is faithfully reproduced in vector form. 1871 points were obtained. The survey was then processed to compute magnetic anomaly map through the subtraction of a normal field, hereby established for the period of the airborne survey [START_REF] Boukerbout | Le champ aéromagnétique algérien: champ magnétique total ou champ d'anomalie ?[END_REF], by the use of Algerian repetitive network data. The values cover the area at coordinates: "00°33'-02°00'E" & "35°48'-36°56'N" and are interpolated to a spacing regular grid of 325m x 325m. Each grid node holds a calculated value of the field.

The scattered magnetic data were interpolated onto grid using the nearest neighbour interpolation technique. This method seeks to generate the smoothest surface which closely reproduces the observed data.

Processing and interpretation of aeromagnetic field maps

Because of the inclination of about 54° of the magnetic field at latitude of survey for the period of 1974, there is a slight discrepancy between the magnetic anomalies and the corresponding causative magnetized bodies. In order to replace the anomalies above their geological sources, the magnetic anomaly map was reduced to the pole. For this purpose, we

A C C E P T E D M A N U S C R I P T 7 need to know, the values of inclination and declination of normal earth's field (In, Dn) and those of the magnetization (Im, Dm). The difficulty is to determine Im and Dm. From measurements made on 73 samples from the andesites in the studied area, [START_REF] Idres | Modélisations gravimétriques et magnétiques des structures profondes du bassin du Chélif (Algérie)[END_REF] point out that remnant magnetization is negligible [START_REF] Derder | Transpressive tectonics along a major E-W crustal structure on the Algerian continental margin: Blocks rotations revealed by a paleomagnetic analysis[END_REF], so only the induced magnetization is taken account, therefore, the used parameters of the magnetization were the same that the ones of normal field (54° for the inclination and -4° for the declination). The aeromagnetic map reduced to the pole (Figure 2) is not very different in shape and amplitude from the original map. The reduced map shows a slight northward offset of the magnetic anomalies, an increase in the amplitude of the positive magnetic anomalies and an attenuation of the negative ones.

The long wavelength magnetic anomalies can be separated from the short wavelength ones by calculating an upward continuation at different altitudes, for instance, we choose to show the upward continued by 5 km (Figure 3).

To highlight the short wavelength anomalies that reflect the signature of superficial or sub outcropping geological structures, we use the" technique of shading relief" (Mc Donald et al., 1992). This technique consists in representing the magnitude of the field as shades of gridded data. Produced images are obtained by treating the image as a topographic surface and simulating illumination either by using a simple mathematical model or by filtering the image to create a shaded relief image. Shaded relief image convey changes in values and do not contain any information on the magnitude of the field. This is particularly useful for enhancing subtle linear features which maybe related to geological structures (Mc Donald et al., 1992). The shading relief technique is applied to the reduced to pole anomaly map illuminated from the northeast, to show up the short wavelength of the magnetic anomalies and enhance features with a northwesterly trend (Figure 4). The advantage of this method: it

A C C E P T E D M A N U S C R I P T
8 doesn't modify the strength of the high frequency signature, but as the illumination is directional, some anomalies may be unfavourably illuminated and thus not show up.

We also use the technique of calculating gradient of magnetic anomalies: it has no directional bias but by amplifying the short wavelengths and attenuating the long wavelengths, it can produce some oscillations which can have effect of distorting the legibility and may be lead to erroneous interpretations. The vertical gradient map is shown in Figure 5.

Figures 2, 3, 4 and 5.

Description of the aeromagnetic field maps

The examination of different magnetic maps shows that the strongest anomalies are those located in the North: all along the coast deriving from volcanic formations and in the Mediterranean Sea but, also in the Chlef basin and in the South part of the studied area. A set of N80E positive magnetic anomalies occurs between Gouraya and Tenes and follows the coast. This set is made up of a series of short wavelength anomalies (Figures 4 and5) that arise over a group of calc-alkaline volcano formations but coincide with no precise geological boundary. In addition, these small anomalies contain a number of small offsets that reflect strike-slip-faults [START_REF] Schettino | Plate kinematics of the Western Mediterranean region during the Oligocene and Early Miocene[END_REF][START_REF] Domzig | Searching for the Africa-Eurasia Miocene boundary offshore western Algeria (MARADJA'03 cruise)[END_REF][START_REF] Yelles | Plio-quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank[END_REF][START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF]. In Northward of this set, a large and intense magnetic anomaly, trending E-W, occurs offshore. The feature giving rise to this anomaly may be buried very deep. It can be shown on upward extensions maps at different altitudes (Figure 3), whereupon this magnetic anomaly remains very strong indicating a relatively deep rooting, whereas those derived from the signature of the basin disappear between altitudes of 4 and 5 km, indicating that the basin is not flat with a mean thickness of 5 km. However the overview

A C C E P T E D M A N U S C R I P T

9 of all processed maps show that the magnetized structure is not simple and will reflect the geological complexity of the studied area.

The anomalies on the magnetic map (Figure 2) are correlated to geological data (Figure 1); they correspond, in the West part of the studied area, to bodies within the Chlef basin. The Boukadir positive magnetic anomaly is made up of two lobes disrupted by a discontinuity in the NW-SE direction. This anomaly may correspond to a succession of basaltic sills embedded between the calcareous formations (Aït [START_REF] Hamou | Etude pétrologique et géochimique du volcanisme d'âge Miocène de la région de Hadjout (Ouest algérois)[END_REF][START_REF] Meghraoui | Géologie des zones sismiques du nord de l'Algérie (paléosismicité, tectonique active et synthèse sismotectonique[END_REF][START_REF] Idres | Modélisations gravimétriques et magnétiques des structures profondes du bassin du Chélif (Algérie)[END_REF]. In a northward, there is a negative magnetic anomaly corresponding to the Ouled Fares anticline. This anomaly could be produced by a magnetized source located at the boundary substratum-basin or by a succession of basic sills deeply rooted [START_REF] Idres | Modélisations gravimétriques et magnétiques des structures profondes du bassin du Chélif (Algérie)[END_REF].

In the East of the region, there are two positive magnetic anomalies and one negative in their North appears. This magnetized substratum limits the Chlef basin in the South. The positive anomaly in the East is identified on the Doui massif, which is better defined.

The Chlef basin is composed of mio-Pliocene and quaternary formations lying on ante-Neogene basement. According to the compressional phase from Pliocene to Quaternary [START_REF] Meghraoui | Géologie des zones sismiques du nord de l'Algérie (paléosismicité, tectonique active et synthèse sismotectonique[END_REF], this substratum is outcropping in some places such as Temoulga, Rouina and Doui, in the form of epi-metamorphic and Jurassic massifs of different sizes. This outcropping basement is made up of Mesozoic and Palaeozoic formations. The Palaeozoic formations are composed of a metamorphic substratum over which lies a volcanic complex composed from lavas and tuffs. The Mesozoic formations are composed of Triassic and Cretaceous structures (Kirèche, 1977).

The long wavelength positive anomalies were probably caused by structures located in the recognized outcropping Palaeozoic substratum which limit the basin in South, since they are not superimposed on known outcropping Jurassic massifs. On the other hand, the long

A C C E P T E D M A N U S C R I P T
10 wavelength negative anomaly is located over the outcropping Jurassic massifs of Temoulga and Rouina. As this anomaly remains negative, its origin is in the Palaeozoic substratum which is more magnetized and deepest than the Cretaceous one. This is also confirmed by the identification of the sources of the magnetic anomalies in 2-D along the profile and in 3-D (see section 6, below) and by the gravity study of the same area [START_REF] Idres | Modélisations gravimétriques et magnétiques des structures profondes du bassin du Chélif (Algérie)[END_REF][START_REF] Abtout | Gravimetric evidences of active faults and underground structure of the Chlef seismogenic basin (Algeria)[END_REF] Some contacts are identified on the gradient map (Figure 5). The NW-SE one is thrusting in the sinistral way the Boukadir anomaly. The second one is perpendicular to the first and is identified in the South of the Boukadir anomaly. It shows the SW limit of the Chlef basin and the Palaeozoic substratum located in the South. It coincides with a part of the Relizane Fault.

More in the North of this latter, two contacts appear and may be correspond to some deep aftershocks of El Asnam and Relizane Faults as they are parallel. In the East, between the negative magnetic anomaly and the positive one, appears a contact which may be correspond to the Palaeozoic substratum known in the South and which becomes more deepest in the North.

Identification of magnetic anomalies causative bodies with the continuous wavelet transform

The main application in geophysical potential fields studies is to identify the geological structures responsible of the anomalies by quantifying their depth, inclination, dip, geometry, and so on. These characterization and localization of the sources of geophysical potential fields anomalies lead to development of refined inversion and analysis techniques. Among these developed methods, the continuous wavelet transform [START_REF] Grossmann | Decomposition of Hardy functions into square integrable wavelets of constant shape[END_REF] which assists automatic interpretation of large amount of potential fields data, through many

A C C E P T E D M A N U S C R I P T
11 applications such as in filtering or inversion [START_REF] Fedi | Wavelet analysis for the regional-residual and local separation of potential field anomalies[END_REF][START_REF] Ridsdill-Smith | The wavelet transform in aeromagnetic processing[END_REF][START_REF] Trad | Wavelet filtering of magnetotelluric data[END_REF][START_REF] Boschetti | Wavelet based inversion of gravity data[END_REF][START_REF] Leblanc | Denoising of aeromagnetic data via the wavelet transform[END_REF][START_REF] Li | Fast inversion of large-scale magnetic data using wavelet transforms and a logarithmic barrier method[END_REF].

In this study, we use methods based on the continuous wavelet transform to identify and localize the causative bodies of magnetic anomalies. The wavelet transform uses the property of the homogeneity degree of the analyzed function which allows detecting and characterizing homogeneous singularities in signals [START_REF] Grossmann | Detection of abrupt changes in sound signals with the help of wavelet transforms[END_REF][START_REF] Holschneider | On the wavelet transformation of fractal objects[END_REF][START_REF] Mallat | Singularity detection and processing with wavelets[END_REF]). This technique is described and detailed by [START_REF] Alexandrescu | Detection of geomagnetic jerks using wavelet analysis[END_REF][START_REF] Alexandrescu | Worldwide wavelet analysis of geomagnetic jerks[END_REF] with an application to geomagnetic time series. The wavelet theory can be found in the book of Holshneider (1995) and the theory of its application to potential fields data can be found in [START_REF] Moreau | Wavelet analysis of potential fields[END_REF][START_REF] Holschneider | Analysis of potential field data in the wavelet domain[END_REF]), Hornby et al., (1999), [START_REF] Sailhac | Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana[END_REF], [START_REF] Sailhac | Identification of sources of potential fields with the continuous wavelet transform: Two-Dimensional wavelets and multipolar approximation[END_REF], [START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Two-dimensional ridgelet analysis[END_REF], [START_REF] Sailhac | The theory of the continuous wavelet transform in the interpretation of potential fields: a review[END_REF], [START_REF] Fedi | 2D Continuous wavelet transform of potential fields due to extended source distributions[END_REF], Fedi and Cascone ( 2011) and so on. Many studies illustrate application of this method, for instance we can refer to applications to aeromagnetic and magnetic data [START_REF] Sailhac | Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana[END_REF][START_REF] Pouliquen | Wavelet analysis of deep-two magnetic profiles: Modeling the magnetic layer thickness over oceanic ridges[END_REF][START_REF] Boschetti | Feature removal and isolation in potential field data[END_REF][START_REF] Vallée | Estimating depth and model type using the continuous wavelet transform of magnetic data[END_REF][START_REF] Yang | Continuous wavelet transform, theoretical aspects and application to aeromagnetic data at the Huanghua Depression, Dagang Oilfield, China[END_REF], electromagnetic data [START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Application to VLF data[END_REF], spontaneous electrical potential [START_REF] Gibert | Identification of sources of potential fields with the continuous wavelet transform: Application to self-potential profiles[END_REF][START_REF] Sailhac | Analytic potentials for the forward and inverse modelling of SP anomalies caused by subsurface fluid flow[END_REF][START_REF] Saracco | Localization of self-potential sources in volcano-electric effect with complex wavelet transform and electrical tomography methods for an active volcano[END_REF][START_REF] Gibert | Comment on ''Self-potential signals associated with preferential groundwater flow pathways in sinkholes[END_REF][START_REF] Mauri | Depth determinations of shallow hydrothermal systems by self-potential and multi-scale wavelet tomography[END_REF] and gravity data (Martelet et al., 2001;Fedi et al., 2004;[START_REF] Cooper | Interpreting potential field data using continuous wavelet transforms of their horizontal derivatives[END_REF][START_REF] Chamoli | Crustal configuration of the Northwest Himalaya based on modeling of gravity data[END_REF][START_REF] Abtout | Gravimetric evidences of active faults and underground structure of the Chlef seismogenic basin (Algeria)[END_REF]. [START_REF] Grossmann | Decomposition of Hardy functions into square integrable wavelets of constant shape[END_REF]Grossmann et al., 1985) 

The 1-D continuous wavelet transform,    a b g w , , 0  of a function     x 0  is defined as convolution product
       b g D dx x a x b g a a b g w a 0 0 0 1 , ,                (1) A C C E P T E D M A N U S C R I P T 12
Where the function  

x g is called the analyzing wavelet,    a is the dilation parameter, b is the translation parameter, and the dilation operator a D is defined by the following action [START_REF] Goupillaud | Cycle-octave and related transforms in seismic signal analysis[END_REF],

         a x g a x g D a 1 (2)
In the case of 2-D potential fields anomaly data measured in the horizontal plane, the 1-D wavelet transform given by equation ( 1) may be generalized to give ridgelet transform [START_REF] Candès | Ridgelets: Theory and Applications[END_REF][START_REF] Candès | Ridgelets : A key to higher-dimensional intermittency ?[END_REF],

                               dxdy y x a x b g a dx dxdy y x s y a x b r a s a b r R , , , , 1 , , , 0 0 0 2         a b s RT g W , , , // 0   (3)
where  s is a unit vector perpendicular to the anomaly strike, // s is a unit vector in the direction of the elongated anomaly, the analyzing ridgelet is obtained by steering a 1-D Poisson wavelet   x g in the perpendicular direction y , RT is the Radon transform of the potential field anomaly. Equation (3) shows that the ridgelet transform of 2-D anomalies is

given by the 1-D wavelet transform applied in the Radon domain. In practice, this is done by computing a 1-D wavelet transform for each direction in the Radon domain.

Explicitly the extension of the method in 3-D is to add an angular parameter in the wavelet domain by using the ridgelet transform which consists in a Radon transform (in 2-D)

prior to a 1-D wavelet transform (Boukerbout, 2004;[START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Two-dimensional ridgelet analysis[END_REF][START_REF] Sailhac | The theory of the continuous wavelet transform in the interpretation of potential fields: a review[END_REF]. More than localization, this method leads to an image of the anomalies causative structures both in the 2-D and 3-D cases.

In the present study, we use the complex ridgelet transform since we use complex analyzing wavelets [START_REF] Sailhac | Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Application to VLF data[END_REF]. The imaginary part of the latter is the Hilbert transform of their real part, thus the wavelets are analytic signals whose

A C C E P T E D M A N U S C R I P T

13 envelop (or modulus) and phase can be computed. The inclination of identified sources is obtained from the phase of the complex continuous transform [START_REF] Sailhac | Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana[END_REF]. Both the modulus and the phase of the ridgelet transform display conspicuous cone-like patterns associated with each analyzed anomaly [START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Two-dimensional ridgelet analysis[END_REF]. The cone-like appearance apex is pointing onto the source depth and its likelihood is practically evaluated with entropy criteria [START_REF] Tass | Detection of n:m phase locking from noisy data: application to magneto encephalography[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Application to VLF data[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Two-dimensional ridgelet analysis[END_REF].

The results obtained from the aeromagnetic data analysis in the case 3-D, are shown in figure 6. The structures responsible of magnetic anomalies are shown in the top of figure while their location in the crust is shown in bottom. The 3-D image of the Chlef region is complex as well as its geological context and seismotectonic sketch. [START_REF] Bezzeghoud | Faulting mechanism of the El Asnam (Algeria) 1954 and 1980 earthquakes from modelling of vertical movements[END_REF][START_REF] Ouyed | Seismotectonics of the El Asnam earthquake[END_REF]. These reverse faults and related folding are disposed on right lateral echelon and should be coupled with NW-SE to E-W trending strikeslip deep active faults [START_REF] Meghraoui | Etude néotectonique de la région Nord-Ouest d'El Asnam : Relation avec le séisme du 10 octobre[END_REF][START_REF] Meghraoui | Seismotectonics of the lower Chelif basin: structural background of the El Asnam (Algeria) earthquake[END_REF][START_REF] Meghraoui | Géologie des zones sismiques du nord de l'Algérie (paléosismicité, tectonique active et synthèse sismotectonique[END_REF][START_REF] Thomas | Géodynamique d'un bassin intra montagneux : le bassin du Bas Chlef occidental (Algérie) durant le Moi-Plio-Quaternaire[END_REF][START_REF] Chiarabba | Tomographic images of the El Asnam fault zone and the evolution of a seismogenic thrust-related fold[END_REF].

The NE-SW faults are associated with asymmetric folds and the different tectonic structures define some NE-SW blocks [START_REF] Thomas | Géodynamique d'un bassin intra montagneux : le bassin du Bas Chlef occidental (Algérie) durant le Moi-Plio-Quaternaire[END_REF]Morel & Meghraoui, 1996). A kinematics model of block rotation related to a transpression with NNW-SSE direction of plates convergence, is proposed in the Chlef basin [START_REF] Meghraoui | Tectonique plioquaternaire de la chaîne tello-rifaine et de la mer d'Alboran. Une zone complexe de convergence continent-continent[END_REF] where the blocks rotation was previously studied with paleomagnetic investigations by Aïfa et al., 1992 and recently by [START_REF] Derder | Tectonic evolution of the active 'Chelif' Basin (Northern Algeria) from paleomagnetic and magnetic fabric investigations[END_REF][START_REF] Derder | Transpressive tectonics along a major E-W crustal structure on the Algerian continental margin: Blocks rotations revealed by a paleomagnetic analysis[END_REF]Aifa, 2014).

Figure 7.

The compilation of all these identified magnetized features from different analyzing methods is reported on the shaded map and correlated with the seismicity of the area (Figure 7). Many of identified structures are located on active zones. The recognized lineaments show polygonal shapes at different scales and corroborate the theory of structure in blocks of the Chlef basin as defined by many authors.

We would like to sketch out the shape of the topography of the magnetized substratum and how are connected the identified structures in depth. So we choose to process an N-S profile running across the magnetic anomalies from the Mediterranean Sea until Ouarsenis mounts, throughout the sedimentary basin in the Oued Fodda area. This profile extracted from the magnetic anomaly map, is located at longitude of 1°40'E. We use the complex one dimensional wavelet transform [START_REF] Sailhac | Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Application to VLF data[END_REF] to analyze it.

The modulus and the phase values of the complex wavelet coefficients allow us to identify both the depth and the dip of the structures.

A C C E P T E D M A N U S C R I P T

Figures 8a, 8b, 8c, 8d, 8e.

The figure "8 a" represents the corresponding topographic and bathymetric profile.

The altitudes in this area vary from 500 to 1500m onshore and reach -2700m offshore. This suggests that the area is very steep.

The N-S structural cross-section (figure 8 with an inclination of 90°. The third fault identified in this area is located at latitude of 4002 km at depth of 10km with an inclination of 50°. In the Oued Fodda-El Abadia region, the first fault is identified at latitude of 4010km (O. Fodda) at depth of 6km with an inclination of 30°, this is confirmed by the results of dislocation model of vertical movements for this area [START_REF] Bezzeghoud | Faulting mechanism of the El Asnam (Algeria) 1954 and 1980 earthquakes from modelling of vertical movements[END_REF]. The second fault is located at latitude of 4020km at depth of 14km with an inclination of 54°. In the Bou Maâd-Damous area, the identified magnetized

A C C E P T E D M A N U S C R I P T 16
substratum is located at latitudes between 4030km (Bou Maâd) and 4047 km (Damous) and depth ranging from 20km to 29km (Volcanism of Damous) with an inclination varying from 90° to 160°. The last part of this profile is over the Mediterranean Sea. Here we depict three contacts or faults located at latitudes of 4050, 4056 and 4066km and at depth of 16, 18 and 24km and inclination of 180°, 170° and 100° respectively. The first idea without any assumption we have, in modeling this part of the profile, is that, the deep structure offshore is a basaltic structure where, some mantle material more susceptible than basalt, intrudes through the contacts and faults. The second interpretation for this offshore part of profile according to the identified structures and different studies in the Mediterranean Sea [START_REF] Galdeano | Assemblage à altitude constant de cartes d'anomalies magnétiques couvrant l'ensemble du bassin occidental de la Méditerranée[END_REF][START_REF] Mauffret | Indice de la compression récente en mer Méditerranée : un bassin losangique sur la marge nord-algérienne[END_REF][START_REF] Gelabert | A new proposal for the Late Cenozoic geodynamic evolution of the western Mediterranean[END_REF][START_REF] Mauffret | E-W opening of the Algerian Basin (Western Mediterranean)[END_REF][START_REF] Cavazza | The Transmed Atlas -The Mediterranean Region from Crust to Mantle[END_REF][START_REF] Domzig | Searching for the Africa-Eurasia Miocene boundary offshore western Algeria (MARADJA'03 cruise)[END_REF][START_REF] Leprêtre | Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and reflection seismic data off Tipaza, Algeria[END_REF][START_REF] Medaouri | The transition from Alboran to Algerian basins (Western Mediterranean Sea): chronostratigraphy, deep crustal structure and tectonic evolution at the rear of a narrow slab rollback system[END_REF][START_REF] Bouyahiaoui | Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean[END_REF], that the identified structures may correspond to the boundary between two blocks with different susceptibilities, and could be the limit of African and Eurasian plates in the Miocene, when the opening of the Mediterranean Sea happened and the AlKaPeCa blocks were drifted and collided with the African plate.

Along this profile, some deep identified structures are well correlated with the seismicity along the profile and the observed structures on the structural cross-section (Figure 8 e), either by merging, extending those in the same direction or shifting those in the perpendicular one. Whereas many others complete the structural information. There is also a good correlation with other geophysical and geological studies [START_REF] Ouyed | An aftershock study of the El Asnam (Algeria) earthquake[END_REF][START_REF] Meghraoui | Géologie des zones sismiques du nord de l'Algérie (paléosismicité, tectonique active et synthèse sismotectonique[END_REF][START_REF] Avouac | On the growth of normal faults and the existence of flats and ramps along the El Asnam active fold and thrust system[END_REF][START_REF] Bezzeghoud | Faulting mechanism of the El Asnam (Algeria) 1954 and 1980 earthquakes from modelling of vertical movements[END_REF]Aifa et al., 2003;Yelles et al., 2006;[START_REF] Afalfiz | Mise en évidence d'une minéralisation à Ba ± (Cu, Fe, Ni)[END_REF][START_REF] Beldjoudi | The Tadjena Earthquake (Mw=5.0) of December 16, 2006 in the Chlef Region (Northern Algeria): Waveform Modeling, Regional Stress, and Relation with the Boukadir Fault[END_REF][START_REF] Abtout | Gravimetric evidences of active faults and underground structure of the Chlef seismogenic basin (Algeria)[END_REF].

A C C E P T E D M A N U S C R I P T 17

Correlation between magnetic and gravity results

We compare the results obtained from magnetic data analysis with those obtained with the gravity ones [START_REF] Abtout | Gravimetric evidences of active faults and underground structure of the Chlef seismogenic basin (Algeria)[END_REF], especially in the Chlef basin, where there is a good gravity data curvature.

The common anomalies observed both on the gravimetric and magnetic maps correlated to geological data, correspond, in the West part of the studied area, to bodies within the Chlef basin. The Boukadir positive magnetic anomaly, which is negative in gravity, is made up of two lobes disrupted by a discontinuity in the NW-SE direction. In a northward, there is a negative magnetic anomaly corresponding to the Ouled Fares anticline and appears also negative on the gravity map. The positive anomaly in the East is identified on the Doui massif, which is better defined, but appears very weak on the gravity map. The negative one coincides with the positive gravimetric anomaly and is located upon the thrusting Jurassic massifs of Temoulga and Rouina.

Many discontinuities are identified (Figure 7). by the latitude axis. The colour scale corresponds to the maximum entropy criteria [START_REF] Tass | Detection of n:m phase locking from noisy data: application to magneto encephalography[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Application to VLF data[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Two-dimensional ridgelet analysis[END_REF][START_REF] Sailhac | The theory of the continuous wavelet transform in the interpretation of potential fields: a review[END_REF] for selecting sources location.

ACCEPTED MANUSCRIPT

A The colour scale corresponds to the maximum entropy criteria [START_REF] Tass | Detection of n:m phase locking from noisy data: application to magneto encephalography[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Application to VLF data[END_REF][START_REF] Boukerbout | Identification of sources of potential fields with the continuous wavelet transform: Two-dimensional ridgelet analysis[END_REF][START_REF] Sailhac | The theory of the continuous wavelet transform in the interpretation of potential fields: a review[END_REF] Figure 8 e. Correlation between identified structures with the continuous wavelet transform, the seismicity along the profile and the structural cross-section.

Figure 6 .

 6 Figure 6.

  b) along the profile shows that the area displays in surface folds and thrusting accidents, trending from NE-SW to ENE-WSW directions. The folds result from transpressive phases related to thrusting accidents. In subsurface the basin is affected by sub-vertical accidents. The substratum (African and Maghrebides) outcrop in the North and is rising in the basin. The intensity of the magnetic anomaly (Figure 8 c Top) varies from -40 to +40nT. The modulus of the continuous wavelet transform (Figure 8 c Middle) shows the signature of many contacts or faults. The depth of the identified magnetized structures along this profile, using the maximum entropy criteria (Figure 8 c Bottom), is ranging between 6 and 30km. The very deep magnetized structures are located mainly in the Ouarsenis Mounts area and in the Mediterranean Sea, while the less deep ones are located in the sedimentary basin and give an estimation of its thickness. From South to North along the profile, in the Ouarsenis Mounts area, the first identified fault is located at the latitude of 3975km and at depth of 8 km. The phase of the wavelet transform (Figure 8 d) identifies this fault with an inclination of 90°. The second fault is located at latitude of 3996km and at depth of 14km

  Figure1. Geological map of the Chlef region (Modified from[START_REF] Meghraoui | Seismotectonics of the lower Chelif basin: structural background of the El Asnam (Algeria) earthquake[END_REF].

Figure 2 .

 2 Figure 2. Aeromagnetic map of Chlef region: total field anomaly reduced to the pole.

Figure 3 .

 3 Figure 3. Aeromagnetic map of Chlef region: total field anomaly 5 km upward

Figure 4 .

 4 Figure 4. Aeromagnetic map of Chlef region: shaded map of total field anomaly

Figure 5 .

 5 Figure 5. Aeromagnetic map of Chlef region: vertical gradient of the total field

Figure 6

 6 Figure 6. 3-D imaging of magnetized structures identified from the wavelet and

Figure 7 .

 7 Figure 7. The compilation of identified magnetized features (black lines) from

Figure 8 a

 8 Figure 8 a. The topographic and bathymetric profile along the N-S aeromagnetic

Figure 8 b

 8 Figure 8 b. Structural cross-section along the N-S profile (Dahra-Bou Maad, Chlef

Figure 8 c

 8 Figure 8 c. Identification of the magnetized substratum along the N-S profile using the

Figure 8 d

 8 Figure 8 d. The phase of the wavelet transform gives the inclination of the identified

Figure 8 eFigure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .

 812345 Figure 8 e. Correlation between identified structures with the continuous wavelet

Figure 7 .

 7 Figure 7. The compilation of identified magnetized features (black lines) from

  Figure 8 a. The topographic and bathymetric profile along the N-S aeromagnetic

Figure 8

 8 Figure 8 b. Structural cross-section along the N-S profile (Dahra-Bou Maad, Chlef

Acknowledgements

This work was supported by CMEP Project 08MDU752. We are grateful to A. Yelles, H. Djellit, A. Boudiaf, F. Aït Hamou, D. Belhaï, A. Afalfiz and S. Maouche for different discussions. We especially thank Pr. Li Zhen Cheng for his useful suggestions. We also thank the editor and the reviewers for their constructive remarks and comments.

Different processing methods are used to highlight short and long wavelengths.

The deep E-W structures limiting the area are identified.

2-D and 3-D images of emerging and deep structures both onshore and offshore are done.

A correlation between geology, seismicity and identified structures is done.