structural evolution of the northwestern zagros, kurdistan region

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Structural evolution of the northwestern Zagros, Kurdistan Region,Iraq: Implications on oil migration

Lszl Csontos, goston Sasvri, Tams Pocsai,Lszl Ksa, Azad T. Salae and Athar Ali

ABSTRACT

The studied area in Kurdistan Region o Iraq lies across an important topographic/structural boundary between the southern lowlands and the northern, oldedand imbricated Zagros Mountains. It also encompasses a prominent change instructural orientation o the northern Zagros, rom a general NW-SE Zagros toan E-W Taurus trend. Geological mapping and structural observations, bothin the mountains (MesozoicPalaeogene) and in the lowlands (Neogene), led tothe ollowing conclusions. (1) The oldest recorded deormation is a layer-parallelshortening, coupled with southwest-vergent shear that was ollowed by majorolding o ca. 10 km wavelength and ca. 1,000 m amplitude. Even the Upper

MiocenePliocene Bakhtiari Formation has steep to overturned beds in some parts,and synclines preserve syn-tectonic strata o NeogenePliocene age. Box olding isassociated with crestal collapse, internal thrusting in the core and with ormationo systematic joint sets. (2) On the southern limb o the major olds, thrusting ovariable oset can be observed. The thrusts on the southern and northern limbsare considered responsible or the major uplit during main olding. (3) En-chelonold-relay patterns suggest let-lateral shear along the EW-oriented segment andright-lateral shear along the NW-oriented segment. (4) A quick-look qualitativeanalysis o striated ault planes suggests a variable shortening trend rom NE-SW to N-S, and some rare NW-SE shortening all associated with thrust aults.(5) The general structural setting o the area is linked to the north-eastwards tonorthwards propagation o the Arabian Margin beneath Eurasia. The ca. 30 bend

in the mountain chain may be explained by the original shape o the ArabianMargin, or by pre-existing tectonic zones o E-W orientation in the northernpart. Several observations suggest that there was no oroclinal bending (i.e. majorrotation) o dierent parts o the chain, but the structures simply molded on theirlocal buttress (almost) according to present orientations. However, a limitedamount o rigid-body rotation in the dierent segments cannot be ruled out. Thechanging shortening directions generated several structural combinations on boththe NW-SE Zagros and the E-W Taurus segments o the arc, many o which are stillpreserved. (6) Spectacular bitumen seepage in Upper Cretaceous and Palaeocenelimestone originates rom ractures or geodes o these ormations. Many o thesebitumen-lled voids are linked to the above-described Late NeogeneRecentshortening-olding process; thereore hydrocarbon migration into these voids is

interpreted to be very young. This contradicts earlier ideas about massive LateCretaceous breaching and bleeding o o hydrocarbons in this region.

INTRODUCTION

The studied area is located around the city o Aqra about 100 km north o Erbil, the capital o KurdistanRegion o Iraq (Figure 1). The area was the subject o a structural analysis and mapping work includingthe analysis o satellite photos and visits to selected outcrops. The study area is important or tworeasons:

1. It contains a sharp topographic boundary between the southern lowlands and abruptly risingnorthern mountains, possibly corresponding to the Mesopotamian Foredeep and the SimplyFolded zones o the Zagros Mountains (e.g. Bretis et al., 2011; Chalabi et al., 2010);

2. It is located where the strike o the chain changes rom NW-SE Zagros Trend to E-W TaurusTrend (Figure 1).

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GeoArabia, 2012, v. 17, no. 2, p. 81-116Gul PetroLink, Bahrain


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See facing page for continuation.

40 504535

40 5550

40N

35 35

30

a

ThrustNormal

TranstensionalTranspressional

Strike-Slip

HighZagrosThrust

Figure 1: (a) Location of the study area projected on regional Digital Terrane Map with recentstress trends and stress environments (from Heidbach et al. (2008) available on-line atwww.world-stress-map.org). Main geological elements aer Kaymakci et al. (2010), Sissakian(1997) and Spaargaren (1987). NAF and SAF = North and South Anatolian Fault; DSF = Dead Sea

Fault.

SAF

Zagros

Lurestan

Fars

Dezhful

MesopotamianForedeep

SimplyFoldedZone

MainRecent

ThrustMain

Frontal

Flexure

DSF

NAF

Taurus

IRAN

Caspian Sea

IRAQ

TURKEY

SAUDI ARABIA

N5000

kmEGYPT

SYRIA

Sinjar

Bitlissuture

TURKMENISTAN

JORDAN

LEVANT

KUWAIT

Cyprus

UAEQATAR

BAHRAIN

LEBANON

Yksekova B.

Fig 1c

Fig 1b

Mardin High

KurdistanRegion

The study area also comprises the Bekhme (Peris) Gorge and Zonta Gorge (Figure 1), which containkey exposures in Iraq as well as bitumen shows in Cretaceous and Palaeocene carbonates. The mainaims o the paper are to: (1) give a more detailed description o the observed structural orms, (2)describe a possible structural evolution o the regions, and (3) link these aspects with observations onhydrocarbon migration.

GENERAL GEOLOGY

Main Structural Trends

The study area comprises the rst mountain range between the Mesopotamian Foredeep and theZagros Mountains, in which a marked change in structural style occurs rom north to south (Figure1a, b). A prominent eature o the area is a bend o ca. 30 in the structural trend between the northweststrike o the Zagros Mountains to the south and an E-W strike o the Taurus Mountains towards theBitlis Suture o southern Turkey and the Sinjar Mountains o Iraq and northern Syria. This changein trend occurs in the heart o the study area, practically within the Bekhme Anticline. The broaderneighbourhood o the study area can be divided into three belts rom north to south.

The rst belt lies to the north and is characterised by the highest mountains (Figure 1a). Near theTurkish-Iranian border the main structural unit is the ophiolite nappe and related deep-sea sedimentsemplaced along the High Zagros Thrust (Figure 1a). Thrust emplacement was initiated in the Turonianand ended in the Late Neogene, since the ront o the nappe lies upon Neogene sediments (Spaargaren,1987; Sissakian, 1997). The other main structural lineament, the right-lateral Main Recent Thrust (e.g.Blanc et al., 2003; Navabpour et al., 2007), does not appear to continue in Iraqi Kurdistan; or at least itssharp topographic expression in Iran ades away. This zone seems to splay into smaller aults towardsTurkey, to be terminated in the Yksekova Basin (Figure 1a).


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Northwestern Zagros, Kurdistan Region, Iraq

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IRAQ Erbil

IRAN

TURKEY

37N

3650

4443E 4350

N600

km

Fig 22a, b

Fig17

Fig 22 c,d

Figure 9, Maulian Figure 18a, Naokelekan

Figure 18a, Barzani

Figure 1c

Shaikan Aqra

SwaraTikka

Khoskha

AinSifni Rovi

Bardarash

b

Permam-Sarta

Dinarta

Bekhme

Bijell

Harir

ShahRook

Safeen

Mirawa

BinaBawi

ChiaiBrandoz

ZagrosTrend

TaurusTrend

Figure 1: (c) Geological map of the area mapped in detail. Thetwo major anticlines displaying Mesozoic formations in the

north correspond to the Aqra (western) and the Bekhme (eastern)anticlines.

Figure 1: (b) Location map of Kurdistan Region of Iraq, based on 90 m digital terrane model(DTM) downloaded from NOAA. Study areas are marked by rectangles.

Bekhme Gorge

Zonta Gorge

Gunduk Gorge

BekhmeAnticline

BijellAnticline

Aqra

Dinarta

Bijeel

Gunduk

Bakrman

4420'

4420'

4410'

4410'

440'

440'

4350'

4350'E

3650'

3640'

c

AqraAnticline

N100

km

Bakhtiari

UpperFars

Lower Fars

Pila Spi

Gercus

Khurmala

Quaternary

SarmordBekhme

Shiranish

Tanjero

Qamchuka

Bakhtiari

Upper Fars

Lower Fars

Shiranish

Aqra

Bekhme

Qamchuka

Quaternary

Sarmord

Chia GaraPila Spi

Khurmala

Gercus

Tanjero

Anticline

Syncline

RoadTown /build-up area

Bekhme

The second belt is named in Iran the Simply Folded Zone (e.g. Blanc et al., 2003; Homke et al.,2004). It is characterised by large regions o exposed Mesozoic rocks, deeply incised down to Triassiclevels (Spaargaren, 1987; Sissakian, 1997). Most o the olds o this belt are thrust upon their southernneighbour. The Chia-i-Brandoz Anticline (Figure 1b) is thrust upon the Neogene Fars Formation, andthereore the age o nal thrusting is Neogene. The southernmost elements o this second belt arethe Bekhme and Aqra anticlines (Figures 1b and 1c) with a locally emergent thrust at their southern


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termination. The emergent anticlines near Bina Bawi (Figure 1b) also belong to the second belt. Themain boundary corresponds to the topographic step between the plains and the mountains andis possibly a SW-vergent, deep and blind thrust. This hypothetic thrust ault is termed the MainFrontal Fault or Flexure (Figure 1a) in Iran (e.g. Blanc et al., 2003; Lacombe et al., 2006).

The third belt is located south o the Simply Folded Zone, and is characterised by relatively fat

topography with smaller anticlines. This belt, named the Mesopotamian Foredeep, has Eocene agerocks as the oldest exposed ormations (Figure 1a, b, Figure 2). Based mainly on proprietary seismicexamples, it seems that this zone is ormed by more-or-less concentric olds, which are linked toinverted high-angle aults. Possibly there is a deep detachment, which may be located along evaporitestrata o Early Jurassic, Late Triassic or Late Proterozoic age. The sharp topographic contrast betweenthe Mesopotamian Foredeep and Simply Folded Zone does not seem to be refected in the near-surace geology (see below).

General Stratigraphy of Kurdistan Region of Iraq

The stratigraphy o the area (see e.g. van Bellen et al., 1959/2005; Jassim and Go, 2006; Figure 3) ischaracteristic or the whole o Iraqs Simply Folded Zagros. The sedimentary succession is possibly

more than 10 km thick and quite probably begins with a ductile Upper Precambrian series. Thisis topped by a several thousand metre thick PalaeozoicLower Mesozoic succession, o which theshallow-water carbonates o the Permian Chia Zairi and Triassic Kurrachine ormations orm thicker,more rigid units. These are intercalated with thinner evaporitic and shaly beds that may be potentialdcollement horizons, near their base and top.

The Jurassic succession begins with a several hundred metres thick neritic carbonate, also generallyrigid. In the Middle Jurassic this dolomitic platorm passes laterally to evaporites (Alan and Adaiyahormations). In the upper Middle Jurassic and Upper Jurassic there is a widespread, yet thin basinal

IRAQ

IRAN

TURKEY

Chia Gara, SargeluSekhaniyan, Sarki

Bakhtiari

Pila Spi, Gercus, Khurmala

Upper and Lower Fars

Tanjero, Shiranish,Aqra, Bekhme, Qamchuka

Baluti, Kurrachine

Ophiolites

Rocks associatedwith Ophiolites

Anticline axis

Thrust fault

N400

km

320000 380000360000 420000400000 460000440000 500000

4140000

Khoskha

SwaraTikka

RoviAinSifni

Bardarash

Permam-Sarta

Bijell

Dinarta

Bekhme

Harir

Mirawa

Safin

ShahR

ook

4080000

4060000

4040000

4020000

4000000

3980000

480000

4140000

4080000

4060000

4040000

4020000

4000000

3980000

320000 380000340000 360000 420000400000 460000440000 500000480000

DokhanLake

Figure 2: Schematic geologic map aer Sissakian (1997), reinterpreted and changed based on ourobservations. The frame is identical to Figure 1b. Anticline axes and names marked on map.

Aqra


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acies, divided into the Sargelu and Naokelekan ormations, which is believed to be one o thesource rocks o the area. In regions exposed to compression these two ormations might be eectivedcollements. In the Upper Jurassic the Sargelu and Naokelekan ormations are overlain either byneritic dolomites (Barsarin Formation) or evaporites (Gotnia Formation) in this region.

In the Early Cretaceous the basinal sedimentary Chia Gara shale and marl was deposited. Locallythis unit can also be a dcollement. It passes upwards into the Sarmord/Balambo marl and thenthe Qamchuka neritic carbonate. Above a minor unconormity in the middle Cretaceous, an Upper

210

134

67

54

36

25

5

Ma

Formations

exposedtothe

north-northeast

ofthemappedarea

F

ormationsexposedonthemappedarea

Bakhtiari

Upper Fars/Agha Jari

Quaternary gravels

Lower Fars/GachsaranEuphrates, Jeribe/Asmari

Kirkuk

Pila Spi/JahrumPabdeh

Gercus/Sachun

KhurmalaSin

Tanjero/Gurpi

Kholosh, Aaliji/Gurpi

Aqra/Tarbour

Bekhme/Ilam

Qamchuqa/Sarwak

Sarmord, Balambo/Khazdoumi

Fahliyan Chia Gara/Gadwan

Najmah/Surmeh

BarsarinGotnia/Gotnia

Naokelekan

Sargelu/Sargelu

Alan/AlanMus/Mus

Adaiyah/AdaiyahSekhaniyan

ButmahSarki

Neyriz Baluti

Kurrachine/Dashtak

Main Zagros thrusting

Folding

Folding

Obduction, Forebulge formation

Cap rock Reservoir rock Source rock

Inner Platform Outer Platform

Shiranish

Upper

Upper

Upper

L

L

L

M

M

U

U

U

Low

er

Lower

Middle

Triassic

Jurassic

Cretaceous

Eocene

Miocene

Pliocene

Palaeo-cene

Quaternary

Olig

o-

cen

e

0

250

Figure 3: Stratigraphy of the study area. Note that the column is proportional to time, not tothickness. Formation names aer van Bellen et al. (19592005). Sin = Sinjar. North Iraqi namesmarked by regular characters, their Iranian equivalents are marked by italics. The lowest exposedformation in the study area sensu-stricto is the Chia Gara Formation, but older formations are alsoexposed to the north. Tectonic events marked by black arrows. Source rocks, reservoirs, sealsmarked by diamonds, circles and box shape, respectively.


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Cretaceous platorm carbonate represented by the Bekhme and Aqra carbonate was deposited. Thisplatorm passes laterally and upwards into basinal sediments (Shiranish and Tanjero marl). The upperpart o the deep-marine marl may also be Palaeocene in age (Kholosh Formation; Sarbazheri et al.,2009). The Cretaceous neritic carbonates (Qamchuka, Bekhme and Aqra ormations) orm a ca. 600m thick rigid and weathering resistant structural level that can be used to determine the wavelengtho olds. The Qamchuka and Bekhme ormations orm the core o most olds in the Kurdistan region

(Figure 2).

A more rigid carbonate unit (Palaeogene Khurmala-Sinjar Formation) is overlain by very characteristic,brick-red Eocene clays orming a detachment horizon (Gercus Formation) and by a thin and chalky-dolomitic Eocene carbonate (Pila Spi Formation). This rigid unit orms very characteristic outcropexposure patterns and is easily recognised even on satellite photos.

The Neogene is represented by (1) the sometimes evaporitic, variegated Middle Miocene Lower FarsFormation; (2) the mostly sandy, fuvial MiddleUpper Miocene Upper Fars Formation; and (3) theconglomeratic Upper MiocenePliocene Bakhtiari Formation (8 Ma and younger). All these ormationsare rarely and poorly dated (Jassim and Go, 2006). Modern magneto-stratigraphic-isotope data (likeHomke et al., 2004) are lacking. These Neogene ormations have a cumulative thickness o more than

1,500 m.

OBSERVATIONS

Layer-parallel Shortening

Thrust FaultsSeveral sites indicate a layer-parallel shortening episode. The best examples can be seen in the PilaSpi Limestone, because multi-coloured clays orm interlayers that separate the mechanically morebrittle limestone beds. In the southern oothills o the Bekhme Gorge, and along the Bijeel-Dinartaroad (Figure 1c), the Pila Spi Formation is olded with steep to subvertical layers (Figures 4a and 4b).The sti limestone layers carry smaller internal thrusts, which detach in adjacent clay layers. Whenbedding was re-tilted to horizontal, thrusts ormed a conjugate set and dened a SW-directed, layer-parallel shortening (Figures 4c and 4d).

CleavageIn some cases, marly layers show traces o incipient cleavage, which is irregular and dense (Figure5). The cleavage planes are parallel to each other and oblique to layering. Penetration depends on thecompetence o the aected material. Layering may be tilted, olded, but the characteristic angularrelationship is preserved (Figures 5a and 5b). Geometrical relationships suggest that when beddingwas re-tilted to horizontal a NE-SW layer-parallel shortening with a SW-oriented shear componentoccurred along the layers (red traces on the stereograms o Figure 5d and 5h). The suggested sequenceo events is explained on Figure 5j. Layer-parallel shear may have also been generated by fexuralolding, but observed angular relationships are not conormable with this interpretation (Figure 5i).

Comparing the cleavage in the Bekhme and Aqra anticlines (Figures 5c and 5d versus 5g and 5h)indicates a slight rotation o ca. 1520 between the two regions and shear directions. The sheardirections inerred rom cleavage correspond to a direction perpendicular to the local old axis (seebelow). A dense racture network / racture cleavage is perpendicular to bedding and is interpretedas a systematic joint set (see below).

Layer-parallel CavitiesThe Bekhme Formation and at least three massive horizons within the Qamchuqa Formation arepervasively dolomitised and ractured. The ractures are oriented in well-organised networks, eitherperpendicular, or parallel to layering. The ractures are apparently conducting fuids, since they hostlighter colour overgrown crystals in cavities. Almost as a rule, these cavities are nely coated with


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a b

c dNN

Figure 4: Pila Spi Limestone at the southern mouth ofBekhme Gorge, with subvertical dip. Small insert map withstar shows the location of the exposure. (a) Uninterpretedfield photo, and (b) with interpretation. Note the smallthrusts (red) repeating individual layers and detaching inshaly interlayers. (c) Stereoplot (equal area projection; lowerhemisphere) marks original measurements of bedding andshear surfaces. Dot = pole to bedding; Traces = measuredthrust faults. Small arrows = slickenslide lineations with relative movement. Outward: normal;inward: thrust. (d) Stereoplot marks measurements of shear surfaces tilted back to bring beddingto horizontal. Red traces = back-tilted fault traces. Small arrows = same legend, as for (c).

4420'4410'440'4350'E

4420'4410'440'4350'E

3650'

N 100

km

Bekhme Gorge

Aqra

Index Map (Fig 1c)

North South North South


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Cleavage

La

yering


ba

dc N N

See facing page for continuation.

4420'4410'

4420'4410'440'4350'E

3650'

N 100

km

Aqra

Index Map (Fig 1c)

Bekhme Gorge

Figure 5: (a) Marlier facies of the Aqra Formation near thesouthern mouth of Zonta Gorge, uninterpreted. Small insertmap with star shows the location of the exposure. (b) Sameinterpreted. Yellow marks layering, red marks incipientcleavage. Note the oblique paern of cleavage with respect tobedding. Top-down shear direction along layers is deduced.(c) Stereoplots mark original measurements of bedding andcleavage. Same legend as for Figure 4. Crosses = pole tocleavage. (d) Cleavage dip back-rotated until bedding becomes horizontal.

Zonta Gorge

euhedral crystals o calcite or dolomite. This means that crystals were precipitated rom uids aterthe racturing process, in an open space, and were ree to grow. The innermost part o these cavitiesis flled by bitumen (Figure 6).

While layer-perpendicular racture sets are easily explained by joint ormation induced by sedimentary/tectonic load or incipient olding, layer-parallel ractures need a more detailed explanation: it is veryhard to open up ractures against normal stress induced by sedimentary load. The only plausible wayto open such ractures is through layer-parallel shear. Indications or such shears were ound in somewell-developed, layer-parallel racture zones (Figure 6). Most rhomb-shaped cavities opened up bylayer-parallel shear were compatible with top-SW movement (Figures 6c and 6d, see explanationon Figure 6e), but some were compatible with top-NE shear (Figures 6a and 6b). The ormer ft intoa general pre-olding layer-parallel shear event, while the latter can be explained by layer-parallel


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Cleavage

Layering


e

a

fe

hg

i j

N

e

a

N

Figure 5: (continued). (e) Sarmord marl in the heart of Bekhme Gorge, uninterpreted. (f) Same,interpreted. Marlier beds carry penetrative incipient cleavage. Same colour code as in Figure (b).(g) Stereoplot marks original measurements of bedding and cleavage. (h) Same withreconstructed cleavage dip with bedding rotated to horizontal. Note top-southwest shear

direction in both cases. Note also small divergence in orientation, that may correspond todifferent fold orientation. (i) Flexural fold model. Layering marked by thick, cleavage marked bythin lines. Cleavage is roughly parallel to axial surface. There is a shear (red arrows) on the limbsof the fold. (j) Model in which cleavage precedes folding. Same signs as for i. Le figure inoriginal position, right figure in folded position. Small quadrangles mark the possible locationsof photos of Figure 5.

slip due to exural olding (see explanation Figure 5j). Although high uid pressures may havecontributed to racturing, our uid inclusion studies did not record any abnormal overpressure, butindicated deep burial o all Mesozoic ormations; ca. 3,600 m or the Cretaceous ormations.

Summary Layer-parallel Shear

When all above mentioned eatures are assessed, deormation can be interpreted as roughlyhomogenous i the layers are retro-olded until horizontal. In other words this shortening, coupledwith mostly SW-oriented shear occurred in a pre-tilted and pre-olded state. Deormation should


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be younger than Eocene (youngest observed eatures in Eocene) and hydrocarbon migration shouldhave occurred synchronously or should have happened ater the layer-parallel shear (voids andstructures invaded by bitumen).

b

d

a b

c

e

d

Northeast Southwest Northeast Southwest


Figure 6: Layer-parallel cavities in Qamchuqa dolomite on the southern limb of the Bekhmeanticline, in road cut. Note that all filled cavities make rhomb-shaped open gashes due tolayer-parallel slip along non-separated parts of the fractures. (a) Overview photo of top-NE shearassociation. Yellow line marks layering. Insert box and (b), shows detail of such rhomb-shapedgashes. Red shear arrow indicates deduced shear direction. Yellow outlines mark the shape ofrhomb-shaped cavity walls, that moved away due to shear. (c) Neighbouring exposure ofQamchuka dolomite on same southern limb, same location. Yellow lines mark layering. Insert boxand (d), shows detail of rhomb-shaped gashes giving top-SW shear. Red shear arrow indicatesdeduced shear direction. Yellow outlines mark the shape of rhomb-shaped cavity walls. Thecavities are coated with calcite-dolomite minerals and inner parts are filled by bitumen. (e) Modelfor formation of rhomb-shaped cavities. Original situation in le hand side, final situation in righthand side picture. Upper layer is pushed to the right, and small irregularity in slip surface results

in a rhomb-shaped opening.

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Bekhme Gorge

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Index Map (Fig 1c)


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Major Folds

Folding o layers into major, roughly E-W- to NW-oriented olds is the most prominent deormation inthe studied area. The topography closely resembles the olding in the study area (insert on Figure 1b).Only minimal erosion has occurred and all slopes are practically parallel to layering. The crestal areasare unaltered. A couple o gorges, up to 1,000 m deep, are incised into the antiorms perpendicularto their strike, near their periclinal terminations (Figures 1b and 2). Synclines are represented byregional valleys with young clay-rich sediments. The anticlines to the north o the study area are moredeeply eroded and their cores better exposed.

Geomorphologic Character of FoldsThe olds in our study area have lengths o 2532 km and widths o 45.5 km, corresponding to alength-to-width aspect ratio o 4.57.5. These ratios plot in the transitional zone between detachmentolds and ramp-related olds in the geomorphologic analysis o olds in Iran (Burberry et al., 2010,their fgure 9). The symmetry o the olds is complex; their axial trace is curvilinear, and the oldschange acing at dierent segments o the old. In the ollowing discussion it will be shown that the

olds are linked with thrusts, requently on both limbs.

Box FoldsMost o the olds are box-olds, with steep, relatively planar limbs that dip at 4070 (Figures 7a and

7b). The limbs pass into sharp, poorly rounded hinges. The anticlinal shape in the Mesozoic successionis airly simple. However, anticlines ormed by the Khurmala or the Pila Spi Limestone are somewhatdiverging rom that simple shape (Figures 7c and 7d). The reason is that sometimes the Tanjeromarl and more requently the Gercus clay behave in a ductile manner, thus eventually causing thecomplete detachment o the old geometries below and above. From Bijeel towards Aqra and Bakrman(Figure 1c) it is quite common to fnd steep southerly dips in the Mesozoic section on the southernlimb o the anticline, while extra olds or steeply overturned (northerly) dips, are ound in the Pila Spi,Fars, or even Bakhtiari ormations (Figures 7c and 7d). Accordingly, the internal deormation o theGercus clay can be very chaotic, and can range rom tight to open olding, internal thrusting, to ductile

thickness changes. Theoretically the divergence o geometric orms in the Palaeogene strata romthe sti Mesozoic core can be the result o gravity collapse o sediments gliding down on a ductile(Gercus) detachment. However, overturned beds are possibly not the result o such a gravity-drivenprocess, because in some examples (e.g. southern mouth o Zonta Gorge) they are associated withthrust aults (see below). These divergent geometric orms are rather the expressions o thrust shear.

A box-old shape creates space problems in both external and internal parts o the hinge zone. Theaxis-perpendicular, layer-parallel elongation in the external, and the similarly directed shorteningin the internal part, should be accommodated by some structures. Extension in the outer arc(Figure 8i) was accommodated by large normal aults, creating crestal collapse (Figures 8a and 8b). Theinternal part o the olds is characterised by gentle disharmonic olding and a multitude o internalthrust suraces (Figures 8c to 8). These thrusts are repeating layers, or orming duplex structures.As a consequence, higher layers may have higher curvatures and steeper dips, whereas lower layersmay have more gentle dips. The internal thrusts generally detach on more ductile layers within thesame succession. The Sarmord and Balambo ormations or Chia Gara shale preerentially orm thesedetachment suraces.

Dip MeasurementsDip measurements o bedding on stereoplots suggest that the olds are curved rom a NW trend in theeastern part, towards an E-W trend in the western part o the area (Figure 9). When the measured orconstructed old axes are taken into account, there is a strong correlation between the local directiono the curvilinear axial trace and the measured local axes. However, even in the case o simpletransects (like in the case o Gunduk Gorge or Bekhme Gorge), the measured bedding poles do not ftperectly on a single girdle, i.e. the olds are not ully cylindrical. They rather show a distinct spread.Although the dierence between these old orientations is not large, it is still notable, and mightsuggest superposition o two dierent old orientations.


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Pila

Sp

i

PilaSpi

Gercus Khurmala

NorthSouth

NorthSouth

b

a

d

cNorthSouth

NorthSouth

Figure 7: (c and d) Aqra Anticline near Bakrman Gorge, with and without interpretation. Locationmarked by index map. Note that layering in Gercus-Pila Spi (yellow full line) is not conformable tolayering in Khurmala (yellow stippled). There are extra folds in the Pila Spi, enabled by the ductilebehaviour of the Gercus shale.

Figure 7: (a and b) Aqra Anticlinenear Gunduk Gorge, with andwithout interpretation. Location

marked on index map. Layeringis marked by a yellow line. Notesteep southern limb and smallsharp hinge.

Gunduk Gorge4420'4410'440'4350'E

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3650'

N 100

km

Bekhme Gorge

Aqra

Index Map (Fig 1c)

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Aqra

Index Map (Fig 1c)

Bakrman Gorge


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BekhmeTanjero

marl

Shiranish

marl

dolomite

Shiranishmarl

Normal fault

Thrust fault

Southwest Northeast Southwest Northeast


South North South North

Southwest Northeast Southwest Northeast

iFigure 8: Structural features associated to major folding.(a) Crestal collapse in the core of Bekhme Anticline,uninterpreted. (b) Same, interpreted. Note small offset

normal faults (red lines) in well-layered Qamchu qaForma tion (Bekhme Gorge, NW face) . (c) Smallerinter nal thrusts in the core of Bekhme Anticlin e,Sarmord Fm, uninterpreted. Same location as above.(d) Interpreted. Thrusts marked by red. (e) Duplexingrelated to major folding in Zonta Gorge, uninterpreted.(f) Same, interpreted. Internal thrusts repeatingindividua l layers in Qamch uqa Formation. Internalthrusts: red; layering: yellow. (g) Thrust affecting thenorthern limb of Bekhme anticline, uninterpreted. (h) Same, interpreted. Yellow markslayering, red marks faults. Note south dipping thrust fault juxtaposing flat layers of BekhmeDolomit e above steeply dipping Tanjer o and Shiran ish Marls. Note also normal fault

dissecting the axis of anticline. (i) Model for orthogonal flexure. Le hand side figure showsextens ion in outer arc, shorten ing in inner arc (arrows); right hand side figure showscorresponding structural forms as crestal collapse in outer arc, disharmonic folding and smallerthrusts in inner arc.

4420'4410'440'4350'E

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3650'

N 100

km

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Index Map (Fig 1c)

Zonta Gorge

Bekhme Gorge

8ad

8e/f

8g/h

g h

e f

dc

ba


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Bijell Anticline

Aqra

Dinarta

Bijeel

Gunduk

4420'

4420'

4410'

4410'

440'

440'4350'

3650'

3640'

Aqra Anticline

N

100

km

N

Zonta Gorge

N N

Aqra East

N

Gunduk Gorge Maulian Gorge

N

Bakrman

N

Bekhme West Bekhme Gorge

N

N

Bijell Fold

N

Zonta West

Figure 9: Stereoplots of the measured dips in Aqra and Bekhme anticlines, all equal area, lowerhemisphere projections. Measurements marked by blue asterisks on map. Ellipses mark the

outcrops used for plot construction. Poles to bedding are marked by black dots. Measured axesmarked by black barbed dots. Great girdles marked by red traces. Constructed axes marked byred barbed dots.

On some o the clearly E-W-oriented olds, like the Shaikan Anticline (Figure 1b), there is a set osmaller amplitude superposed olds, which have axial trends oblique to the main axis (Figure 10).These smaller olds orm let-lateral en-chelon sets arranged along the main E-W trend (idea o MikeHawkins, ex GKPI). The smaller superposed olds have outcrop expressions. Stereoplots o beddingpoles rom Central segment in the Shaikan Anticline also suggests a combination o E-W and WNW-ESE axis olds (Figure 10).

Relay of AnticlinesOne o the characteristics o major olds in the study area is that they display let-lateral relays onthe E-W-oriented segment, right-lateral relays on the NW-oriented segment o the mountain chain.The Bekhme and Aqra anticlines have a clear let-lateral en-chelon separation at the Khurmala anddeeper levels (Figures 11a to 11d). The map relationship o the Bekhme and Harir anticlines (Figure1b) is not so straightorward, since at the eastern end the Bekhme Anticline apparently has twopericlinal terminations (Figure 1c). However, in the eld the northeastern termination o Bekhme andthe southwestern termination o Harir are more pronounced and thereore the separation is right-lateral (Figures 2, 11e and 11). In the wider surroundings o the study area the observed tendency isreinorced by more en-chelon congurations. The Harir Anticline, in turn, passes with right-lateralen-chelon relay to the Shah Rook Anticline (Figure 1b), which is itsel relayed by another set o right-lateral anticlines (Figures 2, and 12). On the E-W trend, the Swara Tikka, the Khoskha and the Dinartaanticlines (Figures 1b and 2) orm a set o let-lateral en-chelon olds. It is also noted that the let-lateral relays are more apparent, than the right-lateral relays.


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an)anticline

NLalish Gorge

NCentral Segment

NEastern Segment

Figure 10: Stereoplots of the measured dips in Shaikan anticline (for locatioin see Figure 1b), allequal area, lower hemisphere projections. Base map is a Google Earth satellite photo.Measurements marked by red asterisks on map. Ellipses mark the outcrops used for plotconstruction. Poles to bedding are marked by black dots. Great girdles marked by red stippledtraces. Constructed axes marked by red barbed dots.

Mapping and dip measurements o Tertiary ormations in the southern lowlands reveal olds southo the exposed olds discussed above (Figure 2). Several such olds are indicated by periclinal Pila Spicarbonate ridges (like the Mirawa Anticline) or more resistant outcrops o Lower Fars beds (like theBardaras and Permam (Sarta) anticlines (Figures 1b and 2). However, divergent dips even in Bakhtiaribeds can indicate olds (like the Bijell Anticline, just south o and overthrust by the exposed AqraAnticline). Most o these olds in the lowlands have a WNW-ESE axial orientation and a let-lateralen-chelon arrangement along E-W zones south o the topographic boundary (Figure 12). At least twosuch zones can be recognised: the Ain Sini-Rovi-Bijell Zone and the Bardaras-Permam (Sarta)-Sanand Mirawa zones. In this latter zone all the Tertiary anticlines are oset in a let-lateral sense romtheir respective Mesozoic exposed cores.

JointsSystematic joints are generally associated with the main olds. These joint sets are parallel,perpendicular to the old axis and symmetrically oblique to it (Figure 13). Systematic joints arethought to be generated in the incipient phases o olding, during the rst layer-parallel compressionepisodes (e.g. Twiss and Moores, 1992). The parallel/perpendicular elements are tied to extensionalcracks, while the oblique couple is imagined as a pair o shear-induced Mohr sets or conjugates. Sinceincipient compression is generally perpendicular to the uture old axis, all the measurements wereback-tilted to horizontal bedding. In this case the acute angle bisector o the conjugate set or Mohrcouple can be used to dene the incipient shortening directions o the uture old (Figure 14).

When plotted with the old axes, the back-tilted systematic joint sets show a remarkable rotation(larger composite stereoplots with olded bedding poles and measured axes on Figure 14). In theeastern Bekhme Anticline, the racture set is symmetrical to a SE-trending old axial plane. Here the

axis-parallel joints can be so dense, that they orm layer-orthogonal racture cleavage. These maysomewhat diverge rom the mapped axial directions, but the angular dierence is minimal. In the


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Figure 11: En-chelon arrangement of Bekhme, Aqra andHarir anticlines. For names see Figure 1. (a) Satellite view(Google Earth) towards the west of the en-chelon paernand fold closures of Bekhme (lower part of picture) and Aqra(higher part) anticlines. (b) Interpreted version. Yellowmarks bedding; red stippled arrows mark plunging axes. (c)Field photo from Bekhme Anticline (foreground) towardspericlinal closure of Aqra Anticline (background). Locationmarked by red asterisk on insert, photo looking west. (d)Interpreted. Yellow marks bedding; red stippled arrowsmark plunging axes. (e) Field photo from Bekhme Anticline(foreground) towards periclinal closure of Harir Anticline

(background). Location marked by yellow asterisk on insert,photo looking east. (f) Interpreted. Red stippled arrowsmark plunging axes.

4420'4410'440'4350'E

4420'4410'440'4350'

3650'

N 100

km

Aqra

Index Map (Fig 1c)

Bakrman Gorge

a

c

b

e

d f

Aqra Anticline

Bekhme Anticline

Bijeel

North

NorthSouthNorthSouth

South

Aqra Anticline

Bekhme Anticline

Bekhmeanticline

Hareeranticline

SouthwestNortheast

NorthSouth SouthwestNortheast


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9797

Figure 12: Main tectonic elements of the study area. Tectonic interpretation on NOAA DTMimage. The main anticline axes are marked by blue. The interpreted le-lateral shear zones aremarked in orange, and right-lateral shear zones in green. Stippled deep purple line marksimportant blind thrusts zone that explains the topographic break. Note the major change in strike

of the structures.

340000320000 380000360000 420000400000 460000440000 500000

4140000

4080000

4060000

4040000

4020000

4000000

3980000

4140000

4080000

4060000

4040000

4020000

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340000320000 380000360000 420000400000 460000440000 500000480000

IRAQ

Al Mawsil

Dahuk

Abril

IRAN

TURKEY

N400

km

4000000

western Aqra Anticline (more precisely the transitional zone rom the Bekhme to the Aqra anticlines,including the Zonta Gorge) the interpretation o the racture set is not so straightorward. We canobserve a set symmetrical to a WNW-ESE axis and another one, symmetrical to an E-W axial plane(let larger stereoplot in Figure 14). In the more E-W domain o the western Aqra Anticline (GundukGorge and Bakrman) the plot shows that also here both symmetries can be ound. These observationsmight also support intererence o two dierent shortening episodes, related to olding.

Bitumen ShowsHydrocarbons oten invade the systematic joint sets described above. Bituminous suraces can evenbe ound in the more ductile marls o the Shiranish Formation (Figures 13a and 13b). It is remarkablethat in the Bekhme Gorge the Tanjero Formation above the Shiranish marl does not contain shows,and the Shiranish shows are solely in joints. This act suggests that the Tanjero Formation and/orthe Shiranish Formation was an eective seal, and hydrocarbons were pressure-injected into theotherwise closed racture system, just at the top o Cretaceous dolomite reservoir section. Anotherconclusion is that hydrocarbon migration should have occurred ater (incipient) old ormation.

Age of FoldingThe youngest ormation aected by olding is the Upper Miocene and Pliocene Bakhtiari Formation(Figure 2). These young conglomerates ll all the major synorms (Figure 15a). Layering in the Bakhtiariis fatter than in the adjacent Upper Fars: a progressive unconormity (onlap o the uppermost UpperFars and Bakhtiari during old growth) is observed (Figure 15b). The onlap associated with oldgrowth may be heterochronous, which is not unique in the Zagros (see Homke et al., 2004).


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a b

dc

Northwest Southeast Northwest Southeast

East West East West

Figure 13: Systematic joints associated to folding. (a) Road cutat southern slope of Bekhme anticline, uninterpreted. Locationmarked by red asterisk in insert map. (b) Same, interpreted.Note axis- parallel/perpendicular (yellow) and oblique (violet)sets in Shiranish Marl. Fold axis is marked by orange stippledline. The acute angle bisector is perpendicular to the fold axis.Note that bitumen (oil) migrated along these surfaces. (c)Fractured Pila Spi limestone layer along the Bell-Dinartaroad. Location marked by green asterisk in insert map. (d)Same, interpreted. Acute bisectrix is perpendicular to fold axis.

4420'4410'440'4350'E

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Aqra

Index Map (Fig 1c)

Bakrman Gorge


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9999

Be

khmeGorge

Bekh

me

Anti

clin

e

BijellA

nticline

Aqra

Bijeel

Gunduk

4420'

4420'

4410'

4410'

440'

440'

4350'

3650'

3640'

3640'

AqraAnticline

N

Shiranish

N

LowerF

ars

N

LowerFars

N

N

N

Khurmala

Khurmala

Khurmala

Khurmala

N

N

N

Qamchuka

N

B

ekhme

N OT

N

N

Bekhme

N

N

N

N

N

N

N

N

N

N

Khurmala

N

N

Gercus-PilaSpi

Sarmor

d

Shiranish

Aqra

N

N

Gund

ukGorge

Zo

ntaGorge

BekhmeWest

BekhmeGorge

N

Dinar

ta

N N

Figure14:Pairsofstereoplotsofthesy

stematicjoints

inthes

tudyarea,allequalarea,lowerhemisphere

projections.Measurementlocationsm

arkedbyblue

asterisk

sonmap.Originalmeasureme

ntsmarkedby

blacktraces,polestobeddingarema

rkedbyblack

dotson

leftstereograms.Back-tilted

joints(poleof

beddingtocentre)aremarkedbyredtracesonrightstereograms.Bi

ggerstereoplotsintegratere-tiltedjointsoflargerareas.Polestobeddingand

measuredaxesfromsameareasarecopiedonsameplotsasblackdotsandblackbarbeddots.Interpretedbisectricesaremarkedasblackarrowsonouter

periphe

ryofplots.

N

10

0

km


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4420'4410'

4420'4410'440'4350'E

3650'

N 100

km

Aqra

Index Map (Fig 1c)

Bekhme Gorge

Zonta Gorge

d

South North

Bakhtiari

Bakhtiari

Upp

erF

ars

Upper Fars

Upper Fars

UpperFars

a

b

c

Figure 15: Gradual onlaps in Upper Fars-Bakhtiari syncline. (a)Syncline within Upper Fars and Bakhtiari formations. Picturetaken to the west of Dinarta. Location marked on insert map.(b) Same, interpreted. Yellow marks layering. Note dipdifference between older and younger formations. Red cagemarks location of C, D. (c) Blow up of northern limb. (d) Same,interpreted. Continuous dip change within Upper Fars and

Bakhtiari formations. Younger layers of Bakhtiari onlap thefolded/tilted older ones.


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Map-scale Trusts

Observed ThrustsSeveral thrust exposures were ound at the southern limbs, and one at the northern limb, whichsuggest that thrusts underlie the limbs o major olds. Here two exposures are shown to illustrate

the nature and appearance o these thrusts. The best mappable thrust is located at the southern exito the Zonta Gorge (Figure 1c), where steep, subvertical Mesozoic ormations are thrust on top ooverturned (down to 40) Upper Fars and Bakhtiari ormations (Figures 16a and 16b). The thrust isalso evidenced by a gradual cut-o o younger ormations against the Bakhtiari towards the north(Figure 16c).

The northern limb o the Bekhme Anticline hosts a thrust ault cutting through Mesozoic ormations(Figures 1c, 8g and 8h). It superposes a normal, at-lying succession o Bekhme-Shiranish-Tanjerobeds on top o steeply north-dipping Shiranish and Tanjero marls o the northern limb. Thrusts onboth anks o anticlines are common. Analogue experiments suggest that they are more a rule thanan exception (e.g. McClay and Whitehouse, 2004, their fgure 3a).

Thrust aults generally ramp into ductile clays intercalated in the Tanjero, Gercus, and Lower Farsormations. These orm detachment levels within the stratigraphic sequence. Ductile ow o materialwithin a detachment zone might also be responsible or the dierent dips o strata on either side othe detachment.

Geoseismic SectionA geoseismic section was constructed based on proprietary seismic data and on surace dipmeasurements (Figure 17). This section shows that olds in the less deormed lowland originate romthe inversion o earlier normal aults that defne ault-bound wedges. These wedges are then pushedup against the aults to orm the box olds observed at surace. As a rule, one o the confning aultshas a greater oset, but this is not necessarily the southern one. In the Bekhme and Aqra anticlines,the top-to-the-north thrust seems to be generally more important. This ault detaches below the

Triassic Kurrachine dolomite (base o violet layer on Figure 17), most probably in an evaporite. Alsoas a rule, the same horizon is the upper detachment o a duplex o Palaeozoic rigid units, which haveyet a deeper lower detachment (Hormuz Salt?). This duplex is generally south-vergent, and may bethe equivalent o the Main Frontal Fault. The top-to-the-north surace thrusts act as a hanging wallduplex above the leading edge o the Palaeozoic duplex.

Fault Measurements

General MethodsWherever possible, ault attitude and ault-slip measurements were perormed. Data were plotted oreach outcrop, in present attitudes and with bedding attitudes re-tilted to horizontal (Figure 18). Specialcare was taken when overturned beds were encountered. We separated aults ound in the Neogene

Lower and Upper Fars, and Bakhtiari ormations rom aults ound in Palaeogene and Cretaceousrocks (Figures 18a and 18b). No major dierence is seen in the ault pattern. The more numerousobservations in older rocks are related to the better preservation o aults in brittle carbonates, than insoter siliciclastics and shales.

Individual outcrops do not yield sufcient data to perorm a meaningul numerical tensor analysisor each outcrop, like e.g. Navabpour et al. (2008). Another difculty is that the time o ormationo individual aults or striations could not be fxed. In some outcrops, it seemed that ault timingin a re-tilted position makes more sense; in other outcrops it seemed more logical that aults wereormed in the present (oten steeply dipping) bedding position. This is a problem, since even theyoungest beds may be overturned, and all evidence suggests that aults were created during theolding phase(s). Since no direct proo or ormation time o aults could be given, we decided not

to perorm calculations on amalgamated sets, or to deal with average stress tensor directions, but topresent just the data and to draw frst-hand conclusions.


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Bakhtiari

75

80

89

UpperFars

KhurmalaGe

Aqra

Pila Spi

LowerFars

Bekhme

900

800

700

600

600

500

500

a

b

c

North South

North South

Bekhm

e

Shira

nish

Uppe

rAqra

andK

hurm

ala

Gercu

s

Bakh

tiari

Uppe

rFars

4065500

4066000

4065500

4066000

408500408000

408500408000

N0.20

km

Figure 16: Thrust at the southern mouth of Zonta Gorge. (a) Uninterpreted photo, marked byblack line on C. Location marked by red box on insert map. (b) Same, interpreted. Khurmala and

Gercus formations are thrust upon Upper Fars and Bakhtiari. All beds are overturned. Overthrustis younger than Bakhtiari. (c) Geological map from the southern mouth of Zonta Gorge. Contourinterval = 100 m. Note the cut-off of Pila Spi Limestone by the thrust surface (red). This surface isslightly offset by later strike-slip faults.

4420'4410'440'4350'E

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Aqra

Index Map (Fig 1c)


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Tanjero Formation

Bekhme, Qamchuka,Sarmord formations

Bakhtiari

Upper Fars

Lower Fars

Thrust faults

Jurassic

Triassic

Palaeozoic

Paleogene

1,000

Depth(m)

0

-1,000

-4,000

-5,000

-6,000

-7,000

-8,000

-3,000

SouthwestNortheast

Bekhme anticline Safeen anticline

Figure 17: Geoseismic section across Bekhme anticline, based on surface dip measurements andproprietary seismic section transformed to depth. Location on Figure 1.

Separation and Analysis of Measured FaultsAter plotting individual ault data, we frst separated sites that were least tilted. We considered(especially in case o Neogene) that these aults were only slightly rotated ater being active. Amanual separation into potential synchronously active ault amilies was perormed on this data(Figure 19). Subsequently, we analysed the ault sets with large bedding dips and analysed i intheir present position the aults ftted any o the ault amilies defned in the less tilted strata. Finallyeducated guesses about the potential shortening directions were made. Unortunately, no superposed

striations were ound, and thereore no clear chronological relationship between the identifed aultpatterns could be established.

We tried to group the data around two major regions: the surroundings o Bekhme Gorge and thesurroundings o Zonta Gorge. These two areas also correspond to the somewhat dierently orientedold segments (Figure 9). Both in the younger and older rocks we can observe thrusts, normal aultsand strike-slip aults o similar attitudes and osets.

Three sets o thrusts might be observed (Figure 19): ones possibly generated by NE-SW compression;others possibly generated by N-S compression. Since the striations do not orm distinct clusters, itseems that there is a gradual transition between these shortening directions, and that this deormationaected the whole study area in a homogenous way. The third set o thrusts was possibly generated by

a NW-SE compression that apparently did not produce map-scale structures. Even more interesting,both western and eastern sectors show hints o NW-SE shortening, also in a quite homogenous way.

Two strike-slip ault sets might be dierentiated: one with N-S right-lateral and ENE-WSW let-lateralaults, possibly created by NE-SW compression and NW-SE extension, and another one with NE-SWlet-lateral and NW-SE right-lateral aults, possibly created by N-S compression and E-W extension.E-W let-lateral aults might belong to the frst stress feld. Some E-W right-lateral aults might belongto NW-SE compressional feld.

Normal aults were also observed and measured. There is one set o crestal collapse normal aults(NE-SW extension) in the core o the Bekhme Anticline. There are two other sets o normal aults:ones generated by E-W extension and others generated by NNW-SSE extension. This latter set might

have been active in the pre-olding phase.


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Figure 19: Separation of the fault data into sub-families. Method described in the text. The faultsare separated into two regions (i.e. two columns): Zagros trend (surroundings of Bekhme Gorge)and Taurus trend (surroundings of Zonta gorge). Three phases may be separated: a NE-SW; aN-S; and a NW-SE compression dominated one. Data recorded on Neogene and younger (le

stereoplot) and on Palaeogene and older (right stereoplot) are also differentiated. Black and greensigns indicate data in measured (present) position; red signs indicate re-tilted data (bedding tohorizontal) For all symbol explanations see Figure 18.

SarmordShiranishTanjeroPila Spi

KhurmalaShiranishSarmord

Lower Fars

Lower Fars Tanjero

Upper Fars

GercusKhurmala

Bakhtiari

BekhmeAqra

KhurmalaPila Spi

AqraKhurmala

Taurus trend

Aqra, W Bekhme

Zagros trend

E Bekhme

N N N N

N

N

N N N

NNNN

Bakthtiari

Bakthtiari

BakthtiariLower Fars

There seems to be no substantial rotation between the aults and related potential stress felds othe eastern (Zagros trend) and western (Taurus trend) regions, although some minor angulardierence between the two regions is detected.

Bitumen and FaultsBitumen staining is very oten ound along thrust aults in dierent Mesozoic ormations. Faultsuraces with ault gauge breccia, or non-flled, displaced thrust ault cavities (dilatation jogs) werealso flled by bitumen (Figures 20a and 20b). This suggests that thrusting was synchronous to, orollowed by hydrocarbon migration.


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CONCLUSION ON STRUCTURAL OBSERVATIONS

Structural Pattern in Kurdistan Region of Nortwest Iraq

In our study area there are several main characteristics or the deormation pattern. The most obviousis the change o trend o the major olds rom the NE-SW Zagros trend to the E-W Taurus trend(Figure 1a, b). These segments are also dierent, because there are systematic right-lateral en-chelonrelays along the Zagros trend, and let-lateral en-chelon relays or intererence olds along the Taurustrend. Several sets o let-lateral en-chelon olds are ound in the southern lowlands, along E-W-oriented zones.

Like other authors, we also propose that the marked topographic rise corresponds to a top-to-the-south-southwest blind thrust, that carries the whole Mesozoic section (Figure 12, 17). The smalleremergent thrusts are on one hand too steep, and on the other, they have variable throw, and thereorecannot be the basic reason or the topographic dierence (Figure 17). The olds are accompanied byconjugate thrusts that may create larger osets and even overturned layers in Tertiary sediments.

A

Northwest Southeast

SouthNorth

a

b

Figure 20: Bitumen in faults.(a) Big fissure leakingoil/bitumen at the southernentry of Bekhme Gorge, alongroad, opposite bridge. Locationgiven by red asterisk in insertmap. Note that fissure

obliquely transects layering(yellow). (b) Young thrust faultfilled in the separated cavitiesby bitumen in Zonta Gorge.Location given by blue asteriskin insert map.

4420'4410'440'4350'E

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Index Map (Fig 1c)

*


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However, these are possibly inverted earlier normal aults. Such structures are also present beneaththe lowland (Figures 12 and 17). The change in shape o the topographic rise may reect a change inthe position o this blind thrust (Figure 12). It is suggested that this is an inherited ault, linked to theoriginal shape (or earlier structures) o the Arabian Margin.

Based on our section (Figure 17) the blind thrust may create duplications within the Palaeozoic sectionand may have its upper detachment at a Permian or Triassic evaporitic horizon. The conjugate thrustsentwining the major anticlines may rise rom that lower detachment and run into the PalaeogeneGercus or Neogene Lower Fars shales. The top-to-the-north thrusts within the Mesozoic section maybe interpreted as hanging-wall backthrusts to the lower top-to-the-south blind thrust.

Sequence of Deformation Events

A pre-olding NE-oriented shortening, coupled mostly with top-to-the-southwest shear, can bedocumented in both the Zagros and Taurus trends. This early deormation is however, gently counter-clockwise rotated in regions o the Taurus trend.

Major olds are present in both segments, with their corresponding systematic joint sets, orientedaccording to the old axis (Figure 14). These major olds do not necessarily orm at the same time.While there seems to be a unique phase o joints in the Zagros trend olds, there also seems to be asuperposition o a gently counter-clockwise-rotated NE-SW symmetrical and a N-S symmetrical jointset along the Taurus trend.

Because the northern structures are deeper incised and expose deeper stratigraphic levels than thesouthern structures, it seems they were subject to longer erosion, and thereore it is probable that theolds and thrusts propagated towards the southwest. Stratigraphic sequences trapped in synclinesbetween the dierent anticlines allow age determinations in the Iranian Zagros (Hessami et al.,2001; Homke et al., 2004), which also indicate a southeast thrust propagation. Hessami et al. (2001)suggested that convergent deormation may have started in Eocene, but certainly rom Early Mioceneand propagated SW in the Pliocene. Our study area alls approximately in their Middle Miocene

Pliocene zone o convergence, which is also supported by our observations (Figure 15).

Fault-related Deformations

Faults can be grouped in deormation events created by three main compression directions. Eventhe youngest Bakhtiari Formation records many o these aults, hence deormation is younger thanMiddle-Late Miocene. The relation o aults and olds is controversial: some ault patterns appear aspre-olding, others appear as post-olding patterns. There might even be aults that were created inan interim stage o olding.

NE-SW compression can create conjugate NW-striking thrust aults, shear zones, as well as N-S-oriented right-lateral and ENE- to E-W-oriented let-lateral aults.

N-S compression can create conjugate E-W-striking thrusts, shear zones, as well as NW-SE right-lateral, NE-SW let-lateral strike-slip aults and N-S-oriented normal aults. The latter may gently cutup the anticlines in a transverse direction.

In some recorded data, a gradual transition between NE-SW compressive and N-S compressivedirections is evident.

An enigmatic NW-SE compression is also present, apparently not creating map-scale structures. NE-striking conjugate thrust aults, together with N-S-striking, let-lateral and E-W-striking, right-lateralaults and eventually NW-oriented normal aults may be created by this compressional stress feld.

Although the ew collected ault data do not allow a relative timing o events, other evidence pointsto a temporal change rom NE-SW compression to N-S compression. Shortening preceding oldingappears NE-oriented; thrusting during or ater olding appears oriented rom NE-SW to N-S.


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The observed sequence o deormation is very similar to the sequence o events in the Iranian part oZagros (Lacombe et al., 2006; Navabpour et al., 2007, 2008). Lacombe et al. (2006) set up dierent models,with and without strain partitioning, to explain the observed deormation pattern and sequence.Navabpour et al. (2007) recorded most o their data within the High Zagros Belt, around the MainRecent Thrust (which is a principal right-lateral displacement zone providing strain partitioning), yetthey ound the same events with similar stress directions. Navabpour et al. (2008) suggested that in

Lurestan (Figure 1a) the NE-SW compression beore olding was acting in Early Miocene, the mainolding occurred in Middle-Late Miocene. Some NNE-SSW compression ater olding happened inLate MiocenePliocene and the change o stress feld to N-S compression occurred in Pliocene. Theyalso evidenced a NW-SE compression ater main olding.

Kaymakci et al. (2010) collected and analysed ault slip data on both the Arabian Platorm and on theallochthons o the Tauride segment. In the Turkish part o the Arabian Platorm they interpreted theirdata as having a (1) PalaeoceneEocene NE-SW extension; 2) EoceneEarly Oligocene ENE-WSWcompression; (3) latest OligoceneMiddle Miocene NW-SE extension; (4) Middle MiocenePlioceneNNW-SSE compression; and (5) a Middle Pliocene NNE-SSW to NE-SW compression. Evidently, ourdata set is not as refned as theirs, and absolute timing o the structural events in our study region isnot yet possible. However, in their youngest data set they have the same thrust directions as observed

by us, smeared between NE-SW and N-S. Thereore we propose that both areas, the Zagros andTaurus trends, underwent the same deormation, that may be interpreted in dierent ways, but thathas very similar signatures.

DISCUSSION ON OROCLINAL BENDINGVERSUS PRE-FORMED BUTTRESS

Fold orientations, together with their linked racture sets, diverge in the Zagros and Taurus segmentso our study area. There are basically two explanations or this change in structural orientation: (1) arotation o the western or eastern segments (oroclinal bending); or (2) moulding on the buttress o theArabian Platorm edge.

Oroclinal bending o the Taurus trend is not probable, because neither westward increase in thrusting,nor increased topographic uplit relative to eastern areas is seen. Preliminary palaeomagnetic resultsseem to contradict any large rotation in the Mardin High (Figure 1a) region, i.e. the Taurus trendpart o the mountain chain (Peynircioglu, 2010). Sparse ault data also suggest that the two dierentsegments did not suer major relative rotation (see e.g. the NW-SE shortening and the minimalangular dierence in these deormation patterns between the two sectors).

Moulding suggests that structures are oriented by the pre-existing buttresses. That system does notcreate space problems and implies that local orientation closely ollows the shape o the ArabianPlatorm (Figure 12).

DISCUSSION ON STRAIN PARTITIONING

VERSUS SUCCESSION OF DEFORMATION PhASES

Based on GPS and earthquake ocal mechanism data, an overall Recent N-S convergence o theArabian Plate towards Eurasia is evident (Blanc et al., 2003; Vernant et al., 2004; Lacombe et al., 2006).In northern Iraq the direction o convergence is NNW-SSE, rather than true N-S. This Eurasian-Arabian plate convergence vector is thought to be the reason or all observed structures (e.g. Allenet al., 2004). Plate tectonic reconstruction (Cochran, 1981; McQuarrie et al., 2003) suggests that therotation o Arabia versus Eurasia gradually changed in time. This resulted in a ca. NE-SW trendingshortening during earlier phases o convergence (Oligocene to Early Miocene) gradually shiting toN-S convergence in Late MiocenePresent.


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Several authors have suggested strain partitioning to explain the dierent shortening directionsobserved mainly in Iran (e.g. Talebian and Jackson, 2002; Blanc et al., 2003; Allen et al., 2004; Lacombeet al., 2006; Navabpour et al., 2008). In that model the southern oreland should accommodate onlyNE-SW shortening, while the Main Recent Thrust should accommodate an important right-lateralshear (Figure 1a). In Lurestan and Fars this concept works: with rare exceptions the present-daydeormation patterns o the two dierent parts (SW o Main Recent Thrust and NE o it) obey this

rule (e.g. Navabpour et al., 2008, their Figure 4). However, adaptation o this concept to the KurdistanRegion o Iraq raises some problems.

1) Te exact extent and potential nortward termination of te Main Recent TrustThis ault seems to splay o and diminish in oset towards the Kurdistan Region o Iraq (Figure1a). It may extend until the Yksekova Basin o southeast Turkey (stippled portions o the MainRecent Thrust on Figure 1a), but its surace expression is diminished. Along this hypotheticportion the earthquakes (big enough to yield a ocal mechanism) become quite rare (e.g. Heidbachet al., 2008; Talebian and Jackson, 2004, Figure 1a). Only one earthquake with ocal mechanism isound east o our study region. I the Main Recent Thrust ends north o the Kurdistan Region oIraq (dense line on Figure 1a) this might explain the presence o mostly NE-SW convergence tothe south o the Main Recent Thrust (Zagros trend) versus the mainly N-S convergence in the

region where its eects as right-lateral ault are diminished (Taurus trend).

2) Superposed structuresI strain partitioning works in our region, then one should expect only NE-SW shortening in theblock SW o the Main Recent Thrust and only N-S shortening in the Taurus trend, where theMain Recent Thrust does not exist. However, our data suggest that in both areas both NE-SWand N-S convergent structures are present. The existence o such structures in the Taurus trendseems to be the more problematic, since there is no obvious reason to explain structures related toNE-SW shortening. The present-day and Neogene plate convergence was perpendicular to thistrend; the present stresses are also perpendicular to the trend.

3) Different tectonic pases

This is in act the same problem as in point 2. In dierent parts o Zagros (Fars: Lacombe et al.,2006; High Zagros: Navabpour et al., 2007; Lurestan: e.g. Navabpour et al., 2008; and our studyarea) the same brittle stress patterns were ound with very similar timing. Both NE-SW and N-Scompression were recorded and these were clearly linked to dierent episodes (Lacombe et al.,2006; Navabpour et al., 2008). Moreover, these dierent stress episodes were ound both in theZagros trend and in the Taurus trend. Rigid body rotation might be a solution to this problem(i.e. the block recording deormation rotates under a stable external stress feld), but Navabpouret al. (2008) and also our study did not reveal major block rotation.

Navabpour et al. (2008) suggested that the zone o deormation partition (i.e. the active majorright-lateral shear zone) shited south with time. In early phases, when the partitioning boundaryis ar towards the north, the whole southern region is subject to NE-SW compression. When the

partitioning boundary moves to the south, its closer vicinity experiences N-S shortening, while theregion to the south o this new boundary still experiences NE-SW shortening.

Translating that concept to our study area, in early phases the precursor main right-lateral shearzone was much more to the north, in Iranian territory. In this episode, the whole Kurdistan Region oIraq, including the Taurus trend, experienced NE-SW shortening. When the right-lateral shear zoneshited to its present, partly hypothetical, location along the Iranian-Iraqi borderland, the Zagrostrend experienced N-S compression with right-lateral shear. Since the Main Recent Thrust endsbeore reaching the Taurus trend, this area also experienced N-S compression.

An alternative to the above solution is that the tectonic activity along the Main Recent Thrust wasnot continuous in time, neither was its lateral extent constant. This would mean an alternation othe two compressional phases. In periods when the Main Recent Thrust was inactive, we expectN-S compression in the whole area. In periods when the Main Recent Thrust was active, the region


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south o it experienced NE-SW shortening, while there was right-lateral shear along the Main RecentThrust. This concept is not probable, because once a well-established ault system becomes active itwill create a weakness zone that is unlikely to be let inert in a subsequent deormation episode.

Another alternative is that the dierent recorded tectonic phases do represent dierent convergenceperiods o Arabia against Eurasia. Early (to Middle) Miocene NE-trending underthrusting o the

Arabian Platorm would create ortho-structures parallel to the NW-SE margin, while the samedeormation would induce shortening coupled with let-lateral shear and en-chelon structures on theEW-oriented Taurus trend margin and in the southern lowland (Figure 21a). N-S shortening wouldbe related to latest MiocenePliocene structures. This would create ortho-structures in the Taurustrend, while the same phase would generate right-lateral shear and superposed olding along theZagros trend (Figure 21b). Although that concept does not exclude strain partitioning (especially inLate MiocenePliocene), it is not a necessity either. This concept could explain the observed eatures,although it is likely, that some sort o strain partitioning existed in the Zagros rom early deormationphases onwards (Navabpour et al., 2008).

Foreland Foreland

a b

Figure 21: Models for the structural development of Kurdistan Region of Iraq. Green stripemarks the thrusted-folded part of Zagros. Black arrows indicate main directions ofEurasia-Arabia convergence. (a) Earlier NE-SW convergence direction generateshead-on structures in NW-oriented segments and E-W oriented en-chelon structuresand le-lateral shear on E-W oriented segments. (b) Recent N-S convergence directiongenerates head-on structures along E-W segments, while it generates en-chelon foldsand right-lateral shear zones along NW-oriented segments. Interference structures are notshown; just the freshly formed structures are schematically indicated.

NN

CONCLUSION AND DISCUSSION ON hYDROCARBON MIGRATION

Summary of Observations on Bitumen Seeps

Several geode- or cavity-types were identifed in the feld (Figure 22). Some o these were non-conclusive in determining the timing o petroleum migration; others gave useul constraints on timing.Most o the cavities had an identical mineral coating o dierent generations o calcite, dolomite, andmore rarely quartz, precipitated rom hydrothermal uids o dierent temperature and salinity. Thismineral coating was observed irrespective o the age o the host rock.

In deep gorges across CretaceousPalaeocene reservoirs some o these bitumen-flled cavities wereobserved in macroossils. In most cases, moldic porosity was reduced by mineral coating. These

cavities do not provide any age constraint.


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Some exposures in Palaeogene marl and dolomitic marl (especially on the Bijell-Dinarta road)revealed cavities flled by angular splinters o wall-rock ragments, that might be assembled to orma more integral original rock volume (Figures 22a and 22b). The cavities were ormed by desiccation,dissolution or mineral transormation resulting in volume loss. Later, bitumen flled these cavities.At a quick glance, this type o cavity might be mistaken or a snowball pebble o bitumen, collectingdierent rock debris as it is rolled by waves across the seashore. This cavity-type and its infll does not

give a strong age constraint either, except that this is a diagenetic eature linked to uid movement,that occurred most probably ater deposition.

Some cavities are linked with tectonic suraces, such as joints (Figure 13), ractures, ault breccias(Figure 20), and shear-related cavities (Figure 6). These cavities have a mineral coating identicalto tectonic suraces without bitumen. Some joints just have a bitumen flm, and no coating. Thesetectonic eatures, as earlier shown, are linked to several phases o the main NE-SW to N-S shorteningo the area, which is dated by the syn-tectonic structures in the Upper Fars and Bakhtiari ormations.So these cavity suraces do provide a timing constraint or migration, i.e. they occurred during orater the Middle MiocenePliocene main shortening episode.

a c

d e

b

Figure 22: Apparent bitumen clasts in Palaeocene rocks. (a) Black, rounded bitumen in whitishKhurmala marl layers; northern limb of Aqra Anticline near Dinarta. (b) Close-up of one of thebitumen balls indicating pieces of rock-wall encased in bitumen. These pieces can be assembledwith some space deficit, due to some volume-loss process. It is inferred that dessication cavitieswere formed first, later infilled by bitumen. (c) Apparent bitumen pebbles (arrows) in a coarsepolymict conglomerate of Kholosh Formation (Palaeocene) near MKerr village, close to Hareer(location in Figure 1b). (d) One of these bitumen pebbles indicating a remaining rock crust andthe inner space occupied by bitumen. (e) Cut surface of the same pebble indicating thatdissolution cracks were formed inside the carbonate pebble, later filled by bitumen.


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Even apparently young, non-cemented and brecciated racture zones are flled with bitumen (Figure20b). Several large, massive bitumen seeps, and accounts o hillsides burning or several years, suggestthat racturing and hydrocarbon migration to the surace is still ongoing.

The general absence o bitumen in ormations younger than the Eocene Gercus Formation speaks ora very eective Gercus seal, and not or migration time preceding Gercus shale.

Cretaceous versus Miocene Migration

Earlier interpretations by Dunnington (1958) suggested a Late CretaceousPalaeocene migrationand massive surace seeps. This interpretation might have been based on the presence o roundedbitumen objects, interpreted as bitumen pebbles, in the Palaeocene Khurmala marl-dolomite and inthe Palaeocene Kholosh ormations (Figures 22a and 22c).

Near MKerr village, in the Mirawa Anticline (Figure 1b), polymict Palaeocene conglomerate (KholoshFormation) apparently contains bitumen pebbles (Figure 22c). A closer look reveals however, that allthese bitumen pebbles have a whitish calcite coating on the external surace o the pebble, growingtowards the inner, dissolved part. Bitumen flled the inner space defned by this mineral coating/

rock wall (Figure 22d). In other words, the bitumen pebbles are in act cavities, which ormed aterdissolution o pebbles, and were later flled by overgrown calcite minerals and fnally bitumen.

An additional actor suggesting young maturity and migration is ormation thickness. There is awidespread agreement amongst earlier and recent petroleum geologists that the main source rock othis region is the Middle Jurassic Sargelu-Naokelekan Formation (e.g. Dunnington, 1958; Pitman etal., 2004). The ca. 1,000 m o overburden provided by just the Cretaceous ormations in the study areais insufcient to bring that source rock to maturity with recent heat-ow rate (Ameen, 1992; Pitmanet al., 2004; in-house modelling). The uppermost Cretaceous, Palaeocene and Eocene succession canadd another 8001,000 m, but still this thickness is insufcient to initiate maturity due to the lowheat ow (Pitman et al., 2004). The thickest sediments were deposited in Late Miocene, Pliocene andQuaternary (Fars and Bakhtiari ormations). This late overburden may have been sufcient to initiate

hydrocarbon generation rom the Sargelu-Naokelekan source rocks (below ca. 3,500m, e.g. Pitman etal., 2004, in-house modeling).

ACKNOWLEDGEMENTS

MOL Plc, Kalegran Ltd and Gul Keystone Petroleum International are thanked or permission topublish the paper. Gyrgy Kovcs, Kalegrans and Jzse Grill MOLs country manager are thankedor continuous support and encouragement and helpul contributions to feldwork. The Ministry oMineral Resources o Kurdistan Regional Government o Iraq and his Eminence Dr Ashti Havrami arethanked or giving us the permission to present the data. The GeoArabia team is acknowledged ortechnical eorts to take care o the paper. Moujahed Al Husseini, Joerg Mattner and Kathy Breiningare especially thanked or prooreading the manuscript. Nino Buhay is greatly acknowledged or the

high-quality graphics. Great thanks is due to anonymous Reviewer A, who pushed us to present datain a more precise and better understandable way.

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ABOUT ThE AUThORS

Lszl Csontos obtained an MSc in Geology in 1983 at the EtvsUniversity o Sciences, Budapest, Hungary, and deended his PhD inJune 1988 at the University o Lille I, France. He spent three months inBritish Columbia with a mapping team o the Geological Survey o Canada.

Ater his degree he was employed by the Etvs University, Departmento Geology, where he taught structural geology, geological mapping, andgeneral geology. From the early 1990s he was involved in numerous studiesor petroleum exploration companies. These studies mostly reported onseveral aspects o Carpathian geology, but he also participated in overseasworks o MOL PLC, Hungary. His expertise covers most o Middle Eastand Far East, including the Arabian Peninsula, India, Pakistan, Indonesia, and Malaysia. In2006 he let teaching or a ull-time exploration job at MOL. He was charged with feld work inthe Oman Mountains and in Kurdistan Region o Iraq. He is particularly interested in structuralgeology, tectonics, and their application in Petroleum Industry.

[email protected]

goston Sasvri holds an MSc in structural geology rom Etvs LorndUniversity o Sciences, Budapest, Hungary, in 2003. Ater his PhD studiesin the same university, he worked as Exploration Geologist at MOLHungarian Oil and Gas Company ocusing on Middle East region. Sasvriis interested in the structural and petroleum geology based on feld work,geological mapping and structural analysis in Hawasina Window (Oman),Margala Hills (Pakistan) and Kurdistan Region (Iraq). Additional interestsare in structural inversion methods and models. Recently he changed job toFugro-Robertson

[email protected]

Tams Pocsai obtained an MSc in Geology in 2004 at the EtvsUniversity o Sciences in Budapest, Hungary. Ater he graduated hewas employed by the Geological Institute o Hungary. In 2006 he let theInstitute and joined the MOL Middle East, Arica & Caspian ExplorationDepartment. He was immediately involved to the feld work in the OmanMountains and later to Pakistan and Kurdistan Region o Iraq. His mainfelds o interest are structural geology and sedimentology.

[email protected]


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Lszl Ksa obtained MSc in Geology in 1998 at the Etvs LorndUniversity o Sciences Budapest, Hungary. He immediately joined MOLExploration & Production Division, Domestic Exploration Department,which he changed or the Middle East Arica & Caspian Exp

structural evolution of the northwestern zagros, kurdistan region

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