Evidence for a new regional NW–SE fault and crustal structure ......Geophysical Prospecting, 2015...

Geophysical Prospecting, 2015 doi: 10.1111/1365-2478.12248

Evidence for a new regional NW–SE fault and crustal structurein Tunisia derived from gravity data

Mohamed Arfaoui1,3∗, Alan Reid2,4 and Mohamed Hedi Inoubli31Office National des Mines, BP no. 215, 1080 Tunis, Tunisia, 2Reid Geophysics Ltd, 7 Keymer House, Michel Grove, Eastbourne, BN211JZ, U.K., 3Unité de Recherche de Géophysique Appliquée aux Matériaux et aux Minerais, Université Tunis-El Manar, Faculté des Sciences deTunis, 2092 El Mana, Tunis, Tunisia, and 4School of Earth & Environment, University of Leeds, Leeds, LS2 9JT, U.K.

Received July 2013, revision accepted January 2015

ABSTRACTA new Tunisian gravity map interpretation based on the Gaussian filtered residualanomaly, total horizontal gradient, and Moho discontinuity morphology establishedfrom gravity data exhibit a new regional northwest–southeast fault extending fromEastern Kairouan to Ghardimaou (Algeria–Tunisia Boundary). It presents a horizon-tal gradient maximum lineament that terminates the north–south Jurassic structures inthe Kairouan plain. Further, this interpretation reveals other known fault systems andcrustal structures in Tunisia. The new regional northwest–southeast fault constituteswith the north–south axis and Gafsa–Jefara faults the deepest faults coinciding withthe Moho flexures, which had an important role in their initiation. They constitutethe border intra-continental crust faults of the Mesozoic rift. The newly recognizeddeep fault has critical implications for mineral and petroleum perspectives.

INTRODUCTION

Crustal strike-slip faults are of considerable structural im-portance and have an indirect economic implication becausethey are of significance for recognition of sedimentary struc-ture evolution. Crustal faults play a significant role in theformation of:

- folds and faults, which commonly constitute structural trapsto accumulate hydrocarbons;

- economic mineral deposits by controlling the migration ofhydrothermal fluids.

The delineation of economic reserves of valuable mineral de-posits and hydrocarbon accumulations include generally thestudy of the relation between basement morphology and sed-imentary basins and the relation between shallow and deepfaults.

Examination of Tunisian deep structures and their evo-lution has required a combination of geological studies andlarge-scale regional geophysical surveys. Thus, Tunisia has

∗E-mail: [email protected]

been the focus of several regional geophysics investigationssince 1940. The first gravimetric campaigns were completedin 1947, and a regional aero-magnetic survey covered all ofTunisia in 1964. The requirement for a gravity map coveringthe entire country was fulfilled by compiling data from vari-ous gravimetric campaigns carried out by oil companies, theresults of this work mainly discussed the structure of the litho-sphere (Jallouli and Mickus 2000; Mickus and Jallouli 1999).Knowledge of deep structures in Tunisia was enormously en-riched by the results of a seismic survey carried out by GeoTraverse in 1985 (Buness et al. 1992). Thus, regional geophys-ical surveys have contributed greatly to the characterization ofdeep structures in Tunisia. However, some interpretations areaffected by poor quality data, and others are still hypotheticalor have been based on a single profile model; therefore, theycannot be extended to the entire Tunisian surface.

In spite of the poor data quality in certain profiles andthe low sampling density, the airborne magnetic survey resultsreveal the shape and depth of a magnetic basement situated ata depth of 2 km in the extreme North of Tunisia, which may bein concordance with Archaean granitic basement outcrops inEastern Algeria. In the Atlas, the magnetic basement appears

1C⃝ 2015 European Association of Geoscientists & Engineers

2 M. Arfaoui, A. Reid and M. H. Inoubli

with a convexity at a depth of 4 km, which separates twodifferent tectonic zones in the South. The magnetic basementincreases to a depth of 4 km in the Eastern Mednine regionand south of Chotts, where its structure is interpreted as ananticline with NW–SE axis.

Several authors have discussed the results of the seismicreflection/refraction profile observed by the European Geotra-verse Project for lithosphere recognition. Morelli and Nicol-ich (1990) presented a cross section of the lithosphere alongthe European Geotraverse southern segment from the Alps toTunisia, which showed a Moho that deepens from a depthof 20 km on the north coast to 35 km in the extreme northand 40 km in the Atlas. Buness et al. (1992) proved in theirseismic model of an N–S profile segment of Tunisia that thicksediment reached 14 km in the Tunisian furrow and 12 kmin the Gafsa furrow. The authors presented a tentative Mohocontour map of the surveyed area of Tunisia, in which thecrust thickness was approximately 22 km in the north and 37km in the Atlas.

Mickus and Jallouli (1999) and Jallouli and Mickus(2000) remodelled the N–S profile based on Tunisian gravi-metric data and provided new information about the deepstructure that remained associated with the profile, which can-not be generalized elsewhere. In those studies, it was revealedthat:

(i) a thickened crust in the South of the Saharan flexure (32–38 km thick) coupled with a reduction of the Paleozoic andMesozoic sediment thicknesses (2–3 km) and a crust thick-ening in the Atlas of 35 km that decreases towards the north(26 km) contributing to thick Palaeozoic, Mesozoic, andCenozoic sediments (12 km);

(ii) a thick Cenozoic filling of the basins in the Tunisian furrow(1–2 km);

(iii) the Triassic is significantly thicker in the north than thesouth and begins to increase in the north.

A residual anomaly model presented by Jallouli and Mickus(2000) shows sediments and upper crust thickness variationsfrom the north to the south of Tunisia without any indicationof the gravity effect from major faults. None of the Tunisiagravity studies discuss the large Bouguer anomaly gradientlineaments.

None of the geophysical studies used for the recogni-tion of Tunisian deep structure indicate the presence of deepfaults, although the major fault systems that affect the crustsuch as the Jeffara–Gafsa, Teboursouk, and N–S faults havebeen the subject of many geological studies since the workof Glangeaud (1951), Dubourdieu (1956), Castany (1951),

Jauzein (1967), Zargouni (1985), Boukadi (1996), Martinezand Truillet (1987) and Bouaziz et al. (2002).

In this paper, we use Woollard’s (1959) empirical relationbetween crust thickness and Bouguer anomaly to determinethe depth, shape, and flexure of Moho discontinuity over allof Tunisia. Then, horizontal gradient grid peaks of Bougueranomaly and optimal upward continuation have been per-formed to determine, respectively, superficial and deep faultsystems.

TUNIS IAN GEOLOGY SETTING

Sited in North Africa, Tunisia includes geologic outcrops thatextend in age from the first era to the recent quaternary. Out-crop diversity results from the juxtaposition of two differentAfrican domains sharply separated by the Jefara–Gafsa faultsystem: the Alpine domain, including the Tell and Atlas zonesstructured essentially at the Mesozoic and Cenozoic, and thestable Saharan platform that belongs to the old African base-ment. The nearest Archaean granitic basement outcrop ap-pears in Eastern Algeria, which may be in concordance withthe basement of Northern Tunisia.

The tectonic setting of Tunisia is an integral part of NorthAfrican tectonics recognized mainly by:

(i) The Tethyan extensional event corresponding to EarlyMesozoic rifting of the North Africa margin (Snoke,Schamel, and Karasek 1988) and massive subsidence dur-ing the Jurassic, Triassic and early Cretaceous. This exten-sion has largely controlled sedimentation by the activation ofNE–SW, NW–SE, and N–S deep faults; limited extensionalevents were recognized equally at Late Cretaceous and Ter-tiary times (Martinez and Truillet 1987).

(ii) The compressional tectonic event associated with theAlpine orogeny remained active from the late Cretaceousto recent time. It caused the inversion of normal faults, reac-tivation of uplifts, and intrusion of Triassic evaporate diapirs(Ben Ferjani, Burrolet, and Mejri 1990; Bouaziz et al. 2002).

Tectonic movements have controlled sedimentation bythe activation of NE–SW, NW–SE, and N–S deep paleo-faultsand fashioned structures by erecting anticlines, synclines, andcollapsed troughs. Thus, five major structural zones can bedistinguished in Tunisia by their historical geology and con-stituents, known from north to south as: the nappe zone,diapiric zone, Atlas zone, the N–S Axis, eastern platform, andSaharan platform (Fig. 1a). The boundaries of these zones aregenerally marked by faults.

C⃝ 2015 European Association of Geoscientists & Engineers, Geophysical Prospecting, 1–12

New regional fault and crustal structure 3

Figure 1 (a) Geological map of Tunisia. (b) Major structural provinces.

In the extreme north, the nappe zone proposed by Jauzein(1967) and Rouvier (1977) is characterized by allochthonousunits formed by Numidian clays and sandstones. Local out-crops of acid volcanic rocks and basalts occur in this zone(Rouvier 1977; Ouazaa 2000).

A diapiric zone formed in the Tunisian furrow dur-ing the Mesozoic (Bolze, Burollet, and Castany 1952) andlargely deformed during the compressional tectonic eventis distinguished by Triassic evaporite outcrops that occupyanticline cores and follow major faults (Perthuisot 1978,1981).

The Tunisian Atlas comprises thin platform sediments(Burollet 1956), which have been folded into large NE–SW an-ticlines and synclines. Thereafter, they have been cut by NW–SE border faults of Mio-Plio-Quaternary troughs (Jauzein1967; Turki 1985).

The N–S axis composed of N–S structures correspondsto a sedimentary limit between two domains, which have dif-ferent paleogeographic settings, i.e., the Atlas zone and theEastern platform. This alignment formed a paleogeographicfeature since the Jurassic as the reductions or condensations ofthe sedimentary sequences often show gaps and discordances.

The Eastern platform, extending east of the N–S axis,is characterized by a slow subsidence during the Mesozoicand more active subsidence during the Cenozoic (Burolletand Byramjee 1974). At surface, the eastern platform presentsfolding tectonics marked by large Mio-Plio-Quaternary foldsand appears unaffected by faulting but, in depth, shows astructure of horsts and grabens, which are the result of NE–SW, NW–SE, and N–S fault activities (Fig. 1b) (Haller 1983).

The Saharan platform zone, fringed to the north by theGafsa fault and the huge endorheic salted basins of El Jerid and



El Fejej Chotts, remained stable during the whole Mesozoicand Cenozoic, and was affected only by epeirogenic move-ments (Bishop 1975).

A review of the vicinity zones of Tunisian major faultsystems reveals important contrasts and gaps in sedimentaryseries, strike-slip structure, overlapping, flexures, unconfor-mities, and facies changes. In some cases these faults haveconstituted an important paleogeographic limit. Other faultsextend by Triassic salt structures (such as the Teboursoukfault) or associate with volcanism and hydrothermalism. Fol-lowing their directions, these major faults can be grouped asfollows.

NE–SW major faults essentially dominate the Alpine do-main. The Zaghouane fault shows an overlapping to thesoutheast. This fault was primarily active in the Jurrassic.The Tunis–Elles fault continued to Bakaria in Algeria, witha length of 250 km (Jauzein 1967). This fault presents gaps,thinning, and local overlapping caused by reverse faults. TheTeboursouk fault extends by the Triassic salt structures atLansarine, Fej Lahdhoum, Kebouche, and is characterizedby the overlapping of the Tebousouk. The Guardimaou–CapSerat fault separates the Algerian and Tunisian blocs and ismarked by volcanic rock manifestations and diapirism.

NW–SE major faults appear in the southern part ofTunisia. They include the Jefara–Gafsa fault system that sep-arates the Atlas zone from the Saharan platform. This sys-tem contains the Gafsa and Mednine faults. The Mio-Pliocenebasin border faults also follow the NW–SE direction (Fig. 2).

T U N I S I A N G R A V I T Y M A P

Between 1948 and 1967, oil companies conducted many sur-veys over their permits, e.g., SEREPT, i.e., Sahel (1948), CapBon (1950) and Kairouane (1967) permits; AGIP, i.e., Sudand Bir Aouine permits (1951); and Mobil, i.e., Gafsa andGabès permits (1951). There are many more. To establish theregional Tunisian gravity map from these data, Petroleum Ac-tivity Tunisia Enterprise (ETAP, 1982), completed a gravitysurvey in the extreme north and compiled all the data to es-tablish the gravity map with approximately one station per25 km2.

The compilation operation was used to homogenize allthe data. Field gravity data were corrected for Earth tides andinstrumental drift and converted to absolute gravity values.The international gravity Formula of 1967 is used to reducethe field data. Then, the usual free air, Bouguer, and ter-rain corrections were applied; the reduction density used was2.67 g/cm3.

Figure 2 Tectonic Tunisia map adapted from Bouaziz et al. (2002)and Tunisia geological map.

The Tunisian gravity map used in this paper was estab-lished using a grid generated from the digitized points of theBouguer anomaly map presented by ETAP (1982). This gridis accepted after a cross-validation of data based on a sta-tistical analysis of the differences between digitized Bougueranomaly and calculated grid values, which gives an averageof the differences of 0.01 mGal and a standard deviation of1.06 mGal. The values of the Bouguer anomaly vary between−85 mGal in the western part of central Tunisia and 40 mGalin the northern part of Tunisia. They are generally compa-rable with those of the map established in 1982 (−86 mGal



Figure 3 Bouguer anomaly map of Tunisia.

and 47 mGal). The Bouguer anomaly map adopted from thisstudy is dominated by:

- long-wavelength negative anomalies in the Tunisian Atlas,the Chotts Zone, and the Oriental Erg Basin;

- long-wavelength positive anomalies associated with the Tell,the Sahel, and the southeastern coastal zones. Other positiveanomalies of maximum amplitude of -10 mGal occupy theAlgerian anticlinorium (Fig. 3).

High gradients of the Bouguer anomaly are associated withsignificant structural alignment. It is clearly observed in thelevel of the N–S axis, the South of the Chott El Jerid, and be-tween the Algerian anticlinorium and the Oriental Erg Basin.The most outstanding high-gradient zone is the Kairouane–Ghardimaou NW–SE alignment, which appears affected inthe north by other alignments. This high gradient constitutesthe gravity effect of a major fault that can be expected to af-fect the upper and lower crusts. We discuss this alignment inmore details later.

The regional aspect of the Tunisian Bouguer anomalymap is characterized by long wavelengths that reflect only thegravity effects of the thick sedimentary units, combined withmajor faults and density contrast effects inside the lithosphere.Thus, the Bouguer anomaly map is largely correlated with themajor structural province map of Tunisia (Jallouli and Mickus2000). The gravity anomalies of the Tell and the TunisianAtlas are mainly due to variations in the thickness and densityof the sedimentary sequences.

The positive anomaly associated with the Sahel is limitedto the West by the high gradient defining the North–Southaxis. This axis is observed throughout the Sahel. It is not sim-ply a response to a change in density of Mio-Plio-Quaternaryoutcrops dominating this region. It is due probably to the ris-ing of the basement towards the east and to a thinning of thecrust.

The Oriental Erg Basin presents an anomaly of −60 mGalcrossing the image of the Tunisian part of the Illizi Basin,which extends from the Southeast of Algeria in the Westthrough to Libya in the east (Jallouli and Mickus 2000). TheAlgerian Anticlinorium is represented by a relatively positiveanomaly of −10 mGal.

According to the above discussion, we identify fiveanomalous zones on the Bouguer anomaly map where bound-aries are indicated by at least one high gradient zone. Thesezones are: the Tell, the Tunisian Atlas, the Sahel, the AlgerianAnticlinorium, and the Oriental Erg Basin.

To better identify the major outcrop structures, a Gaus-sian regional/residual filter with a standard deviation of0.007 km−1, is applied in the wavenumber domain to theBouguer anomaly to determine the residual anomaly associ-ated with relatively shallow sources.

The Gaussian regional/residual filter is a low-pass fil-ter and a high-pass filter characterized by a smoother cutoffprocess. Residual and regional results from Gaussian filter-ing are similar to high-pass and low-pass filters, respectively,in that they attenuate low- and high-wavenumber compo-nents. Mathematically, a Gaussian regional filter anomaly isobtained by convolution of the Bouguer anomaly with a Gaus-sian function in the space domain. The regional and residualGaussian filters are implemented in the wavenumber domainand are given, respectively, in the following:

Regional filter : L(k) = e

(k2

2k20

)

, (1)

Residual filter : L(k) = 1 − e−

(k2

2k20

)

(2)



Figure 4 Residual anomaly map after Gaussian filtering. A white lineindicates the Triassic outcrops boundary.

where L(k) is the Gaussian filter operator in spectral domain,k is the wavenumber in cycles km−1, and k0 is the standarddeviation of the Gaussian function in km−1.

The Gaussian residual anomaly appears to be the mostuseful to explain Tunisian geological outcrops. The residualand geological maps present significant similarities.

The residual anomaly shows short-wavelength anomaliesthat show values ranging from −15 mGal to 9 mGal, with thepresence of strong gradients (Fig. 4). This map reveals certaindominant anomaly directions associated with the principalstructural zones: the NE–SW direction characterizes northernTunisia and the Sahel; the NW–SE tendency dominates thenorth part of the Atlas and the Saharan flexure; the E–Wdirection characterizes the Oriental Erg and the Chotts zone.

Thus, the structural zones of Tunisia are clearly iden-tifiable on the residual anomaly map (Fig. 4). The extreme

northern zone is occupied by NE–SW anomalies having gen-erally positive amplitudes. They are limited towards the southby an E–W band of negative anomalies, which also consti-tute the northern limit of a succession of negative and positiveNE–SW anomalies, translating the effects of the structuresof the Tunisian furrow. The spatial shift of these anomaliesfollowing the NE–SW direction is probably due to densitydiscontinuities, which particularly affect the NW–SE and N–Sdirections. The majority of the Triassic outcrops of NorthernTunisia are in relation to the anomaly boundaries occupiedby NE–SW discontinuities (Fig. 4).

The NW–SE negative anomalies of the northern partof the Atlas coincide with Mio-Plio-Quaternary basin fill-ings; they are separated by excess mass anomalies associatedwith uplift structures. The Sahel, dominated by Mio-Plioceneand Quaternary outcrops (Fig. 1b), is represented by deficitsand excesses in mass anomalies that are oriented NE–SW and,respectively, associated with collapse and uplift structures af-fected by faults. The Sfax area is occupied by a relativelybroad anomaly of 1 mGal. In the west, an N–S anomaly witha maximum amplitude of 9 mGal is associated with the N–Saxis structures. This feature disappears towards the north inthe level Kairouan area; it appears to be stopped by a NW–SEnegative anomaly in relation to the zone of a high Bougueranomaly gradient described previously (Figs 3 and 4).

The Chotts zone is represented by E–W anomalies, gen-erally uncorrelated with the salt nature of the outcrops. Thus,except for the Chott El Gharsa, which is superimposable withan E–W negative anomaly of minimum of −6 mGal, Chott ElJerid and Chott El Fejej are related partially or completely topositive anomalies.

The northern part of the Chott El Jerid is superimpos-able on three negative anomalies organized on a flattened arcshape, which is continuous with the southern part of ChottEl Fejej. These anomalies could be explained as basins sep-arated by faults, which is an interpretation that is in agree-ment with the asymmetrical syncline under the Chott (WesternPetroleum Corporation 1967; Guederi 1980). The southernpart of Chott El Jerid is superimposed on a positive anomalyindicating a depth structure signal different to that observedin the northern part. Chott El Fejej is associated with a posi-tive anomaly reaching a maximum of 5 mGal according to aneroded anticline substratum of the Chott (Western PetroleumCorporation 1967; Guederi 1980).

Starting from Gabès, the coast of southern Tunisia showsa broad positive residual anomaly uncorrelated with outcrop-ping Quaternary deposits. It confirms the presence of the densestructures associated with the Medenine fault. The anomalous



arc of a deficit of mass on the level of the northern part of theChott El Jerid, as previously discussed, continues towards theSE simultaneously with this anomaly. The Triassic outcropin this zone is superimposed on a negative anomaly, and itappears to be continuous in depth under the Sebkha of OumEl Khielat.

The Oriental Erg is presented by two negative residualanomalies: an E–W anomaly in the North and an oval formanomaly in the South; they are the gravity response to twosedimentary basins separated by an uplift zone.

T U N I S I A N C R U S T A L S T R U C T U R E

Isostatic anomalies

The comparison between Tunisia Bouguer anomaly map andthe topographic one shows that long-wavelength Bougueranomalies correlate inversely with long-wavelength topogra-phy: A significant negative anomaly covers the Tunisian Atlasmountains and a positive anomalous band along the coastalzones of subdued topography. This supports the notion ofisostatic compensation. The gravity effect of the relief is com-pensated at depth by a deficit of mass reflecting a thick crustwith a density that is relatively low compared with mantledensity. The effects of the mass deficits at the base of the crustgenerally mask the gravity signal associated with shallowercrustal sources.

The regional isostatic anomaly map of Tunisia is estab-lished using the local compensation Airy model based on to-pography, with a density of the land of 2.67 g/cm3, a contrastof density through the Moho of 0.35 g/cm3, and a thicknessof the normal crust of 30 km. This value is averaged fromprevious crust studies and the Moho map below determinedfrom gravity data (Fig. 6). The regional isostatic map generallypresents long-wavelength anomalies associated with densitycontrasts between the mantle and the lower crust (Fig. 5a). Itshows a broad anomaly of mass deficit on the Tunisian Atlasreaching −74 mGal, closed in the east at an alignment coin-ciding with the N–S axis and continuous in the West (SaharianAtlas in Algeria). This isostatic anomaly is the response to thesignificant change of crustal thickness in this zone. AnotherN–S negative anomaly is observed in the south with relativelylow amplitude (−40 mGal). The remainder of the country iscovered by a positive regional isostatic anomaly.

The residual isostatic anomaly was calculated by elimi-nating the gravity effects of large wavelength caused by theisostatic variations associated generally with the lower crustsources.

Table 1 Difference range between Moho depths determined from thethree formulas.

Formulas Difference range

Woollard 1959 – Demenitskaya 1958 0.84 – 7.33Woollard and Strange 1962 – Woollard 1959 0 –3.35Woollard and Strange 1962 – Demenitskaya 1958 2.19–3.98

The long wavelength of some isostatic residual anomaliesexplains the gravity effects of the upper crust (Fig. 5b). Thus,the Atlas trend (NE–SW) appears in the north zone. The Sahel,north zone, and the Algerian Anticlinorium are occupied bypositive anomalies, whereas the Chotts Zone and the OrientalErg are associated with negative anomalies, which reveal thelow density nature of the sedimentary sequences (Fig. 5b).

Moho morphology

The Bouguer anomaly usually contains the effect of all litho-spheric density variations. It is therefore normal to seek theeffects of crustal structure and major deep faults in the grav-ity data. In this paper, crustal structure relating essentially tothe depth and morphology of the Moho discontinuity is ap-proached by the empirical relationship, which is deduced bycomparing the gravity anomaly and seismically determinedcrustal thickness proposed by Woollard (1959) and used byLiu and Yen (1975), Ram Babu (1997), Rivero, Pinto, andCasas (2002), Demenitskaya (1967), Arslan, Akýn, and Alaca(2010), and many others.

The formula proposed by Woollard (1959) is one amongvarious empirical formulas that are generated from the corre-lation between gravity and Moho depth deduced from seismicobservations in distinct regions.

We determine the Moho depth using two formulas pro-posed by Demenitskaya (1958) and Woollard and Strange(1962). We compare these results with those obtained usingthe proposed formula below (Woollard 1959) (Table 1).

The Moho depth calculated by the Woollard formulais the closest to the Moho seismic depth following the N–Srefraction/reflection profile, from the Geotraverse Europeanproject (Buness et al. 1992), and we have therefore chosen touse this model for further work.

The Moho discontinuity depth is estimated by thefollowing empirical Woollard (1959) formula (3) deducedexperimentally from the linear relation between seismic



Figure 5 (a) Regional isostatic anomaly map, (b) Residual isostatic anomaly map.

measurement depth to the Moho discontinuity and Bougueranomaly proved in various parts of the world.

Hm = 32 − 0.08g, (3)

where Hm is the Moho depth in km, and g is the regionalgravity anomaly in mGal.

The depth of Moho varies between 29 km and 39 km. Thecrust has a maximum thickness in the Tunisian Atlas region,where the Moho discontinuity deepens gradually towards theWest to reach 39 km. This thickening of the crust continues inAlgeria (Saharian Atlas). The Moho depth map also shows arelative thinning of the crust in the North of Tunisia (29 km)and in the Sahel (31 km) (Fig. 6).

The comparison of the Moho depth estimated by thepresent study with the Moho depth estimated by Buness et al.(1992) and Mickus and Jallouli (1999), according to an N–S

profile, shows a similarity between values with a differencesof 2 km in the South and the Atlas and approximately 5 kmin the North. On the other hand, the values of the Mohodepth presented are roughly comparable with those of theMoho depth map of Europe based on seismic measurements(Molinari and Morelli 2011), which show a variation of Mohodepth between 30 km and 40 km beneath Tunisia.

The Moho map shows a typical morphology character-ized by a depression in the Tunisian Atlas and in the south.Elsewhere, the surfaces present an uplift zone (Fig. 6). Thepassage between the depression and uplift zones is markedby a flexure of the Moho surface that coincides in the eastand the south with known faults, namely the N–S axis faultand the Gafsa fault. The north flexure must mark an un-known fault, which we name the Kairouan–Ghardimaou fault(Fig. 6).



Figure 6 Depth of the Moho discontinuity. NDF: new deep fault,NSAF: North South axis fault, GJF: Gafsa Jeffara fault. The Gafsaand Mednine faults constitute the Gafsa -Jefara fault.

SUPERFICIAL A N D DE EP DEN SI T YDISCONTINUI T I ES

The horizontal gradient map computed from the Gaussianresidual grid shows high gradient features, which constitutezones of density discontinuity (Fig. 7). The Blakely methoduses the horizontal gradient peaks to delineate abrupt lateraldensity changes. They are generally interpreted as faults orcontacts (Blakely and Simpson 1986). Taking into accountthe inhomogeneity of the measurement distribution and thegravity map scale, horizontal gradient peaks revealed by theBlakely method are likely to be responses to major deep faultsassociated with a significant density contrast.

Thus, the major known normal fault systems of Tunisiashould be and are detectable on the horizontal gradient peakmap: the Mednine fault with an NW–SE direction appears par-allel to the southern coast and continues beyond the Tunisian–Libyan border. The absence of the alignment correspondingto the Gafsa fault is due to the poor gravity data cover of thiszone. The Chotts zone is characterized by E–W, NW–SE, andN–S alignments related the faults affecting geologic structuresat depth (Fig. 7).

Figure 7 Horizontal gradient map. The black lineaments correspondto faults and discontinuities of density. NDF: new deep fault, NSAF:North-South axis fault, GF: Gafsa fault, MF: Mednine fault. TheGafsa and Mednine faults constitute the Gafsa Jefara fault.

The southern part of the Tunisian Atlas is dominated byE–W, N–S, and NE–SW faults, whereas the northern part ofthis area is marked by NW–SE discontinuities coinciding withbasin border faults. These NW–SE faults, cut by the NE–SWfaults in the NW of the Meknassi area, appear in relationto the discontinuities of the same direction and alignment inthe Sfax–Mahres area. To the north of the latter area, NE–SWand NW–SE discontinuities affect the non-outcropping Creta-ceous. An N–S discontinuity extends in the West of the Sahel;it corresponds to the N–S axis and constitutes a structurallimit at which the NE–SW and NW–SE faults of the Atlas andthe Sahel terminate (Fig. 7).

The N–S discontinuity disappears in the Kairouan plainarea, and the previously described NW–SE discontinuity fromKairouan to Ghardimaou takes its place. It represents a



Figure 8 (a) Cross-correlations between continuations to two successive heights versus the height, (b) Deflection (c) versus the upward contin-uation height.

regional deep fault. This lineament coincides with the high-gravity-gradient zone in the north of El Kef, which has con-stituted a paleogeographic boundary during Barremian time(Chikhaoui 2002; Arfaoui et al. 2011). This non-outcroppingfault is not described by the geologists; it affects deep struc-tures and crust (Fig. 7).

Tunisia presents a tectonic regime typically characterizedby NW–SE and NE–SW faults. The majority of the Triassicoutcrops are associated with NE–SW discontinuities (Triassicalignments of Dabadib, Tibar, and Lansarin, among others).

To determine deep faults or crustal faults, we determinethe horizontal gradient peaks of the upward continuation ofthe Bouguer anomaly at an optimum height estimate by themethod of Zeng, Xu, and Tan (2007). This method is basedon the cross-correlation factor r between Bouguer anomalyupward continuations to two successive heights. The cross-correlation factor between two Bouguer anomaly continu-ations g1 and g2 is evaluated by the formula (4) given byAbdelrahman et al. (1989):

r (g1, g2) =∑M

i

∑Nj g1(xi , yj )g2(xi , yj )√∑M

i

∑Nj g

21(xi , yj )

∑Mi

∑Nj g

22(xi , yj )

, (4)

where M and N are the number of columns and rows ofBouguer anomaly grids, respectively.

The cross-correlation is plotted as a function of the up-ward continuation height (Fig. 8a). The deflection definedby the difference between the cross-correlation curve and thechord joining its two end points is plotted equally as func-tion of the height. Optimal upward continuation height cor-responds to maximum deflection is deduced directly from thedeflection curve (Fig. 8b).

In our case, the upward continuation is calculated forheights varying from 5 km to 95 km with a step of 5 km.The optimum upward continuation height estimated by themethod of Zeng et al. (2007) was 30 km (Fig. 8b).

The different regional anomalies inferred from lesser andgreater heights, compared with the optimal one, are domi-nated by the gravity effects of near-surface sources and deep-seated geological bodies, respectively (Zeng et al. 2007).

The NW–SE Kairouane–Ghardimaou, N–S axis, and theMednine faults persist and constitute the deep structural pat-tern of Tunisia. The Kairouane–Ghardimaou fault, revealedin this study, continues into Algeria to the west and into theMediterranean Sea to the east. The three faults constitute asignificant limit in the Moho depth map that separates a thincrust zone in the east and north from a thick crust zone inthe west, from which it can be deduced that this limit consti-tutes a Moho flexure origin of these major faults (Fig. 9). TheMednine fault and associated E–W faults affect the Palaeozoicbasement (Ben Ayed 1986). The two deepest NW–SE faultsare equally associated with the Sirte Basin opening.

In order to determine the crustal deformation caused bythe current collision in the Alpine–Himalaya orogeny, Sok-outis et al. (2000) built an experimental rheological modelcomposed of three layers: upper crust with a change in thethickness, lower crust, and asthenosphere mantle. Then, theysubmitted the model to a shortening force. Thus, four ma-jor belt systems appeared at different stages of the shorteningexperiment. The new deep fault (Kairouane–Gardimaou) cor-responds to the strike-slip fault affecting the N–S axis in thetectonic Tunisia map presented by Sokoutis et al. (2000) thatcaused the deviation of the N–S axis direction to the east ofthe part sited north of Kairouan.



Figure 9 Horizontal gradient after optimal upward continuation ofthe Bouguer anomaly (30 km). The black dashed lines indicate deepfaults.

The superficial effects of the NW–SE deep fault aremarked by the E–W collapsed structures of the Gaafour–ElAroussa graben and the Mejarda basin bordered by normalfaults and marked at their southern borders by structures withno detected anticlinal ending (Figs. 1a and 2).

The tectonic sketch of Tunisia and the Pelagian Sea pre-sented by Burollet (1991) shows an offshore of Eastern Tunisiadominated by NW–SE strike-slip faults and grabens orientedin the same direction as the new deep fault. Burollet (1991) re-vealed that the NW–SE strike-slip faults and grabens resultedfrom basement extension associated with shear faults. Thissuggests and confirms the presence of the new deep fault.

The new NW–SE fault with the N–S axis and the Med-nine faults constitute an old conspicuous crustal limit of theNorthern and Eastern Tunisia margins, which resulted from

early Mesozoic extensional events. They represent equally thewest border faults or the intra-continental crust faults of theMesozoic rift whose centre is apparently in an offshore thinnercrust zone (Molinari and Morelli 2011).

Thus, Mesozoic rifting in the Sahel platform and in thenorth is confirmed by volcanic rocks found in the Trias-sic, Jurassic, and Cretaceous series in oil wells and outcrops(Ouazaa 2000). In these active crust-thinning stages, N–S andNW–SE rifting control faulting occurs, respectively, in theSahel platform and in Northern Tunisia. An active post riftsubsidence is recognized during the Mesozoic north of thenew Kairouane–Ghardimaou deep fault in the Tunisian Fur-row where more then 5000 m of Cretaceous series are ac-cumulated (north of El Kef) (Burollet 1956; Jauzein 1967;Chikhaoui 2002). Post-rift subsidence is recognized mainlyduring Cenozoic time in the Eastern platform (Burollet andByramjee 1974).

CONCLUSIONS

The major deep faults known in Tunisia as the Mednine andGafsa faults are recognized by the horizontal gradient peaks,with a new deep fault oriented NW–SE, which runs betweenKairouan and Ghardimaou. Crustal structure extended tothe entire Tunisian territory based on the morphology of theMoho discontinuity shows conspicuous flexure zones in thesurfaces of the latter, which coincide with the deepest faults:NW–SE Kairouan–Ghardimaou, N–S axis, and the Medninefaults. Thus, the new NW–SE Kairouan–Ghardimaou fault re-vealed by this study is a deep crust fault; it is an important ge-ological significance with critical mineral and oil explorationimplications.

ACKNOWLEDGEMENTS

We would like to thank the Editor Prof. Maurizio Fedi, and theAssociate Editor and anonymous reviewers for their numerousconstrictive suggestions that improved the final version of thismanuscript substantially.

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Evidence for a new regional NW–SE fault and crustal structure ......Geophysical Prospecting, 2015...

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