Interaction between accretionary thrust faulting and slope sedimentation at the frontal Makran...

Interaction between accretionary thrust faulting and slopesedimentation at the frontal Makran accretionary prismand its implications for hydrocarbon fluid seepage

Feng Ding,1 Volkhard Spiess,1 Noemi Fekete,1 Bramley Murton,2 Markus Brüning,1

and Gerhard Bohrmann1

Received 11 December 2008; revised 14 December 2009; accepted 24 February 2010; published 25 August 2010.

[1] Using high‐resolution seismic profiles and other geophysical data, collected during R/VMeteor Cruise M74/2, we investigate the distribution patterns of shallow sediments, theirstructure and deformation processes, and their role in the migration, accumulation andseepage of hydrocarbon‐rich fluids. Here, we show that rapid syn‐kinematic sedimentationat the frontal Makran accretionary prism provides a mechanism by which emergingimbricated thrust packets override the footwall at the seafloor without significant mass‐wasting and destruction of fault‐related anticline in the hanging wall. These anticlines mayrise high above the seafloor to form plate‐boundary‐parallel ridges, and distinguish fromsimple thrust blocks seen at convergent margins elsewhere. With the fast burial of manythrust faults by the syn‐kinematic sediments, anticlinal growth structures form in these syn‐kinematic sediments by continuous thrust activity. The anticlinal structures preserved withinthe cores of the ridges or formed from these syn‐kinematic sediments act as structuraltraps for migrating hydrocarbon‐rich fluids, above which fluid escape structures aregenerated leading to seafloor seeps. Most of the discovered hydrocarbon seeps around SixthRidge are sourced from these traps. Despite the compressional environment and the rapidsyn‐kinematic sedimentation rates, shallow subsurface of the frontal Makran is a normallypressured regime, in which the buoyancy of hydrocarbons may account for the fluidmigration. In this important respect, the Makran accretionary prism differs from many otherconvergent margins and accretionary prisms, where fluid flow is largely driven bytectonically induced overpressure.

Citation: Ding, F., V. Spiess, N. Fekete, B. Murton, M. Brüning, and G. Bohrmann (2010), Interaction between accretionarythrust faulting and slope sedimentation at the frontal Makran accretionary prism and its implications for hydrocarbon fluidseepage, J. Geophys. Res., 115, B08106, doi:10.1029/2008JB006246.

1. Introduction

[2] The Makran accretionary prism (in the Northern Ara-bian Sea) stands out as one of thickest tectonically accreted,convergent margin sediment complexes in the world, withrapid deposition of terrigenous sediment supplied from thesubaerial Makran [e.g., Gaedicke et al., 2002; White andKlitgord, 1976]. In the area immediately land‐ward of thedeformation front (hereafter called the frontal Makran), theaccretionary prism is distinguished by a series of elevated,plate‐boundary‐parallel ridges that stand several hundreds ofmeters above the surrounding seafloor. These ridges are thesurface expressions of imbricated and anticlinally folded pre‐and syn‐kinematical sediments, beneath which many thrust

faults are continuously active. They are continuous alongstrike over several tens of kiolometers. The steepness of theirflanks is close to maximum angle of repose [e.g., Fruehnet al., 1997; Kukowski et al., 2001; Schlüter et al., 2002].[3] Because plate convergence and tectonic shortening is

the major driving force for the thrust faulting, most previousstudies of the frontal Makran have focused on the deepdécollement‐level faulting and sediment underplating andassumed or implied that new sediments deposited on theaccretionary wedge merely form a passive infill [e.g., Plattet al. 1985; Kopp et al. 2000; Grando and McClay, 2007].Few investigations have been made on the interplay betweenthrust faulting and syn‐kinematic sedimentation in the frontalMakran and its consequence on the tectonic, morphologicaland fluid‐related processes. However, the volume of syn‐kinematic sediments (hereafter called slope sediments) in thefrontal Makran is significant, and inmany cases exceeds 1 kmin thickness. Rather than being a passive infill, these sedi-ments are often overridden by the hanging walls of thrustfaults and are deformed by syn‐depositional thrust faulting at

1Marum, University of Bremen, Bremen, Germany.2National Oceanography Centre Southampton, Southampton, UK.

Copyright 2010 by the American Geophysical Union.0148‐0227/10/2008JB006246

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B08106, doi:10.1029/2008JB006246, 2010

B08106 1 of 16

http://dx.doi.org/10.1029/2008JB006246

depth. Hence they affect the geometric configuration ofthe thrusts, influencing the structural and morphologicalexpressions of the continued fault activity. As a result,subsurface fluid migration processes and the location ofseepage through the seafloor is also affected by this high‐sedimentation‐rate environment.[4] The Makran is a fluid seepage province. Tectonic

thickening and under‐plating of sediments generates anexcess of fluids, such as expelled pore water, diagenetic waterand hydrocarbon‐rich fluids. In the subaerial and shallow‐water parts of the Makran prism, fluid expulsion is expressedby the pervasive occurrence of mud volcanoes [Schlüter et al.2002]. At the more deeply submerged frontal Makran,hydrocarbon‐rich fluids are indicated at depth by pronouncedbottom simulating reflectors (BSR), high‐reflectance hor-izons (bright spots), and on the seafloor by authigenic car-bonate crusts, the release of gas bubbles (flares) and cold‐seep biological communities [Minshull and White, 1989; vonRad et al., 2000; Bohrmann et al., 2008; Spiess et al., 2008].Traditionally, thrust faults at the frontal Makran are assumedto be the major pathway for fluid seeps, but the newly dis-covered seep sites do not occur along the surface traces ofthese thrusts but are instead located on crests of the anticlinalridges [Bohrmann et al., 2008].[5] In this contribution, we examine the relationship

between thrust faults, sedimentary deposition, deformationand subsurface fluid migration in an attempt to explain theseepage distribution pattern at the frontal Makran. Usinghigh‐resolution seismic and swath bathymetric data (col-lected during Cruise M74/2 to the frontal Makran) this studyaddresses: (1) the interplay between thrust faults and slopesedimentation, (2) shallow subsurface (down to 1.5 km belowseafloor) sediment structure patterns as a result of this inter-action, and (3) the control of such sediment structures on thefluid seepage. To do this, we interpret the evolution of themargin, from the deformation front to the anticlinal ridgeslocated ∼40 km further landward. First, the deformation frontis used as a precursor for the initial geometry of the thrustfaults and sediments, to demonstrate how these thrusts andimbricate slices are generated. Our study then examines howthis initial geometry evolves through continued thrust activityand rapid syn‐kinematic sedimentation in the area ∼40 kmfurther landward. Afterwards, we use seismic features such asBSRs and high amplitude reflectors (bright spots) to identifyindications of fluid activity, and discuss the fluid activitypattern in the shallow subsurface. Finally we show how fluidmigration and seepage processes are controlled by thestructural pattern and the evolution of syn‐kinematic sedi-ments at the frontal Makran, and compare this with otherconvergent margins.

2. Methodology

[6] The 2D high‐resolution seismic data set was acquiredduring R/V Meteor Cruise M74/2. The seismic source was a4.1 L GI gun. The streamer had a total active length of 400 m,with 64 channels. Additional lead‐in and stretch sectionsincreased the maximum gun‐receiver offset to ∼480 m. Partof the seismic data were collected simultaneously with deep‐towed, 30kHz (TOBI [Murton et al., 1992]) side scan sonarimagery. The seismic data set has a dominant frequency range

of 50–200 Hz, allowing for a vertical resolution of 10–2.5 m[Yilmaz, 1991], given a seismic velocity of ∼2000 m/s. Thesedata were processed using the ‘Vista’ seismic processingpackage and included band‐pass filtering, common mid‐point (CMP) binning (with a 5 m bin spacing), normal moveout (NMO) correction, and CMP stacking based on pickedvelocities. The stacked profiles were time‐migrated using afinite difference method.[7] Our bathymetric data were collected at an average

speed of 2kts, using a Simrad EM120 hull‐mounted multi-beam system on R/V Meteor. For data processing, theMBsystem software [Caress and Chayes, 1996] and Caraibes(courtesy of IFREMER) were used. Procedures includedsound‐velocity modeling, trace editing and gridding with aspacing of 50 m. The GMT software [Wessel and Smith,1991] was used for bathymetry gridding and seismic linelocation displays.

3. Geological Setting

[8] The Makran prism locates the site of a convergentaccretionary margin, where oceanic crust in the Gulf of Omanis subducting under the continental margin of the EurasianPlate. The prism is bordered to its west by the Zagros Rangeand to its east by the Himalayas. It trends largely east‐westand is slightly curved at each end (Figure 1a). Currently, theconvergence rate is 30–50 km/Ma [DeMets et al., 1990;Quittmeyer and Kafka, 1984], in a direction that is approxi-mately normal to the strike of the prism. Its deformation frontmigrates seaward at a rate of 11 km/Ma into the Gulf of Oman[Platt et al., 1985]. TheMakran prism is extraordinarily wide,extending ∼150 km offshore and 200–300 km onshore [Whiteand Klitgord, 1976]. The history of accretion dates back tothe Late Cretaceous [Arthurton et al., 1982; Harms et al.,1984], and substantial under‐plating since at least the lateMiocene [Platt et al., 1985] has uplifted a large portion of theprism above sea level.[9] Turbidite sediments in the Gulf of Oman and the

Makranwere first sourced from the Indus River in the east. Bythe Late Tertiary, the predominant supply had shifted to thenorth, from the hinterland of the Makran [Gaedicke et al.2002]. Throughout both depositional episodes, and sincethe Oligocene, the Gulf of Oman has continued to be a regionof rapid sediment accumulation. Presently, the total sedimentthickness directly seaward of the Makran prism is estimatedto be 5–7 km [Fruehn et al., 1997; Schlüter et al., 2002].[10] In the vicinity of our study area (E62°45′ to E63°8′,

Figure 1a), the décollement has been calculated to be locatedat a depth of ∼4 km beneath the current deformation front[Fruehn et al., 1997;Kopp et al., 2000;Kukowski et al., 2001;Grando and McClay, 2007]. Sediments above the décolle-ment are accreted to the overriding plate by thrust faulting[Fruehn et al., 1997;Grando andMcClay, 2007]. The frontalMakran comprises a series of lower slope ridges immediatelyinboard of the deformation front and a mid‐slope terrace(MST [Kukowski et al., 2001]). The lower slope ridges are aseries of near‐parallel, steep‐sided features marking the sur-face expression of imbricate slices and related anticlines. Theamplitude of these ridges varies along‐strike and rise up toseveral hundred meters above the surrounding seafloor. TheMST is a relatively flat area reflecting burial by sediments

DING ET AL.: SHALLOW SEDIMENTS AND COLD SEEPS, MAKRAN B08106B08106

2 of 16

derived from the north [Kukowski et al., 2001], while furtherlandward the frontal Makran is bounded by a steep upper‐continental slope.[11] The Makran prism is arguably dissected by a strike‐

slip fault, the Sonne Fault, which separates the underlyingoceanic crust into to two plates: the oceanic part of the Ara-bian Plate and the Ormara Plate [Kukowski et al., 2000]. Thefault causes a left‐lateral offset to the sediment ridges occu-pying the lower slope. In addition, several canyons traversethe frontal Makran and flow onto the abyssal plain. They bothzigzag and cut through the uplifted ridges or are linear andfollow the surface trace of the Sonne Fault [Kukowski et al.,2001].

4. Study Area and Its Bathymetric Features

[12] The study area here forms a section of the frontalMakran, located offshore fromWestern Pakistan (Figure 1a),and is divided into two separate subareas. One is focused onthe deformation front (DF) and the other is ∼40 km landwardof the deformation front, and centered around the MST. Thelatter is designated as Area MST, to distinguish it from theMST itself (Figure 1b).[13] In the DF, the First Ridge is elevated along most of its

strike and locally rises to more than 1000 m above the abyssalplain. However, its elevation reduces to ∼800 m toward theeast where it is cut by the Sonne Fault and a deep canyon. Thiscanyon (hereafter called Sonne Canyon), which follows thetrace of the Sonne Fault, has incised a deep gulley at itscanyon mouth (Figure 2a). To the southwest of this gulley,some undulations on the abyssal floor indicate sedimentwaves (Arrow 1, Figure 2a) that strike largely parallel to thegulley, decreasing in elevation and wavelength with distanceto the west. The ‘Nascent Ridge’ is the most seaward ridge ofthe entire frontal Makran. It reaches its maximum elevation(∼400 m) in the western part of the DF (Figure 2a), butdecreases to only few tens of meters toward the east.

[14] The AreaMST contains a continuous ridge (Figure 2d,called the ‘Sixth Ridge’ after Kukowski et al. [2000]) andbounded to the north by another ridge (the ‘Seventh Ridge’,Figure 2d) that stands only a few tens of meters above thesurrounding floor. Perched between these ridges, and locateddirectly to the northeast of the Sonne Fault and Canyon, is arelatively flat area (Figure 2d) that forms the main part of theMST. To the east, the MST is intersected by another canyon(Figure 2c, the ‘Save Canyon’ [Kukowski et al., 2001]). Thereis another relatively flat area seaward of the Sixth Ridge,which is defined as southern terrace (Figure 2d). Furtherdownslope, the Save Canyon kinks to the west, cuttingthrough the southern boundary of the southern terrace. TheSixth Ridge maintains a constant elevation, several hundredsof meters higher than any of the other ridges, although at itswestern end its crest is cut by many small scarps (Arrow 2,Figure 2c) including a 1 km wide slump scarp cutting into itscentral section (Arrow 3, Figure 2c). This ridge bounds theMST and the southern terrace and is thus isolated fromthe other ridges in the frontal Makran. Both the MST and thesouthern terrace are slightly tilted toward the west where theygrade into the floor of the Sonne Canyon or one of itstributaries (Figure 2c). In addition, some small seafloorundulations (Arrow 4, Figure 2c), only several meters inamplitude, are visible in the eastern parts of both terraces.

5. Seismic Observations and PrimaryInterpretation

[15] In this section we describe the structure and features ofthe frontal Makran, starting with the most seaward part (theDF) and then move landward to the Area MST.

5.1. The Deformation Front (DF)

[16] At its most elevated position, the Nascent Ridge con-tains a broad and open anticline in its core (Figure 3a) that isconsiderably more open than other anticilines at the frontal

Figure 1. Makran prism and the cruise M74/2 survey area: (a) Location of Makran prism and the studyarea, (b) Bathymetry of the study area and locations of the two subareas: The Deformation Front (DF)and the Area MST. Note: The morphology and bathymetry data of frame Figure 1a is extracted from Gebco1 min global data. The bathymetry of frame Figure 1b is collected from M74/2.


3 of 16

Makran e.g., the anticline landward to the First Ridge(Arrow 4, Figure 3a). In addition, the Nascent Ridge anticlineis highly asymmetrical, with a gently inclined backlimb(landward side) and a steeply dipping forelimb (up to 30°).Indeed, the steepness of the dip of the forelimb is consistentwith an influence by thrust faulting (Arrows 1, Figure 3a), apossibility that is discussed later. Shallow sediments onlapthe backlimb of the Nascent Ridge (Arrow 3, Figure 3a),and is thus characteristic of syn‐kinematic sediments. Theyhave accumulated to a recognizable thickness (up to 200 msTWT, two‐way travel time). For convenience, we name theaccreted sediments that were deposited before the beginningof folding or faulting (and which are namely now folded and/or faulted) as imbricate slices, while the sediments deposited

thereafter are called slope sediments, which are likewise alsodeformed.[17] The First Ridge is much more elevated than the

Nascent Ridge. The forelimb of this ridge has at least twosteep and straight planes that appear as scarps (Arrows 2,Figure 3a). Reflections from within these forelimb sedimentsare very chaotic and structureless, indicating that the forelimbhas been affected by piecewise normal‐faulting or sliding.Further ∼4 km to landward from the First Ridge is anotherridge, comprising a complete asymmetrical anticline withinits core (Figure 3a). This fold is considerably tighter than theone forming the core of the Nascent Ridge. Further east (fromthe location of Figure 3a) the First Ridge decreases in ele-

Figure 2. The study area: (a) Bathymetry of the DF; (b) Interpretation of major features; (c) Bathymetry ofthe Sixth Ridge and the AreaMST; (d) Interpretation of major features. The track lines of all processed seis-mic data are shown on the Figures 2c and 2d. The locations of fluid seep sites at the Area MST, which werediscovered during cruise M74/3, are shown in Figure 2c as stars with the names of the seep sites besidethem. The S in the names “S” is for seeps and the number is for the order of their discovery during the cruiseM74/2. Note that S6 is not on the location of profile shown in Figure 7, and instead it is located a bit further tothe north. In the interpretation frame Figure 2b and Figure 2d, “A” is for the anticlines picked from seismicdata (see text for details). The locations of seismic sections presented in this work are also shown, with “F.”for figure, followed by the figure numbers called out in the text. Also shown: Arrow 1: the sediment waves inthe abyssal basin; Arrow 2: small scale scarps on the Sixth Ridge; Arrow 3: central slump scarp on the SixthRidge; Arrow 4: sediment waves at the Area MST.


4 of 16

Figure 3. Seismic profiles from the deformation front: (a) Profile GeoB07‐364, (b) Profile GeoB07‐329(see Figure 2b for the locations of the two profiles). Arrow 1: apparently faulted Nascent Ridge forelimb;Arrow 2: collapsed forelimb of the First Ridge; Arrow 3: Syn‐kinematic slope sediments; Arrow 4: foldabove a second thrust fault (see section 5.1 for details).


5 of 16

vation and its forelimb appears to be intact and not to havecollapsed or have been faulted (Figure 3b).

5.2. The Sixth Ridge and the Area MST

5.2.1. Structural Variation Along the Strikeof the Sixth Ridge[18] The ∼30 km long section of the Sixth Ridge in

Figure 2c is characterized by a relatively constant elevation,

but our seismic data show that this section displays significantalong‐strike variations in its internal structure, particularly,the burial depth of the imbricate slide, thickness of slopesediments and dip of strata. At the westernmost part(Figures 4a and 5c), and the landward side of the Sixth Ridge,shallow sediment units, defined as Units IV to I in Figure 4and 5, thin toward and often onlap the ridge. Furthermore,the dips of the reflectors decrease up‐section, from Unit IV to

Figure 4. Sixth Ridge seismic profiles: (a‐1) Profile GeoB07‐413, (a‐2) Its structure interpretation.(b‐1) Profile GeoB07‐343, (b‐2) Its structure interpretation. (c‐1) Profile GeoB07‐354, (c‐2) Its structureinterpretation; (see Figure 2d for the location of the profiles). Arrow 1: triple BSRs; Arrow 2: collapsedfrontal slope of the Sixth Ridge; Arrow 3: high amplitude reflections (bright spots) in Unit III; Arrow 4:chimney‐like structures in the subsurface beneath the Sixth Ridge. The position and geometry of Unit V andVI in Figure 4c‐2 is based on interpolation of reflector picking of these two units in the neighboring profiles.


6 of 16

I, and hence show progressive unconformities typical of syn‐kinematic deposition, thus slope sediments by our definition.This indicates that the Sixth Ridge was continuously upliftingduring deposition of the slope sediments. In contrast, theolder sediments forming the core of the ridge (defined as UnitVI and V in Figure 4 and 5) have parallel reflectors of con-stant thickness and without onlaps. Hence they are more likethe sediments defined as imbricate slices, deposited beforethe uplift of the ridge. These two units are exposed at the crestof the ridge and at the frontal (basin‐ward) side of the SixthRidge, they have been displaced downdip by normal‐faultingand sliding (Arrow 2, Figure 4a). About 10 km along the

strike of the Sixth Ridge to the east, the imbricate slice (UnitsVI and V) forms a tight anticline under the crest of the SixthRidge (Figure 4b and 5b) and is buried by four shallowerslope sediment units (Units IV to I). Note the small sedimentundulations in the shallowest part of Unit I (Arrow 1 inFigure 5b, Arrow 4 in Figure 2c for map view location),which are interpreted here as the subsurface continuation ofthe seafloor undulations in the bathymetry (Figure 2c).[19] In its easternmost part (Figure 4c and 5c), the Sixth

Ridge is completely comprised of slope sediments (Units IVto I). Here, the sediment undulations in Unit I (Arrow 1 inFigure 5c), which are barely visible in Figure 5b, become very

Figure 5. Seismic profiles on the mid slope and the southern terrace: (a) Profile GeoB07‐329, (a‐2) Itsstructure interpretation. (b) Profile GeoB07‐361, (b‐2) Its structure interpretation., (c) Profile GeoB07‐364, (c‐2) Its structure interpretation: (see Figure 2d for the locations of the profiles). Arrows 1: sedi-ment waves in Unit I; Arrow 2: high amplitude reflection on the top of Anticline I. The position andgeometry of Unit V and VI in Figure 5c‐2 is based on interpolation of reflector picking of these two units inthe neighboring profiles.


7 of 16

pronounced. These undulations have an elevation of 60 msTWT and a wavelength typically about 2 km. In the locationof Figures 4c and 5c, the imbricate slice (Units VI and V) is sodeeply buried that it is almost out of the penetration limits ofour seismic signal. However, the subsurface depth and geom-etry of these two units is inferred from the same two units,observed in the neighboring profiles where they are not sodeeply buried. The geometry of the two units is outlined byinterpolation of reflector picks in those neighboring profiles(Figures 4c‐2 and 5c‐2), showing that the anticline of UnitsVI and V becomes much wider with gently dipping limbs.[20] In summary, to the west of the study area, the imbricate

slice (Units VI and V) in the Sixth Ridge is highly elevated,steeply dipping, and forms a tight anticline in the core of theridge. The forelimb of the ridge is often normal‐faulted andhas slid downdip to the south. To the east, the imbricate slicedips more gently, forming a wider anticline inside the core ofthe ridge. In addition, the imbricate slice plunges to the eastand becomes deeply buried by the slope sediments (Units IVto I). These slope sediments also dip more gently than theirwestern counterparts, and become gradually thicker from thewest to the east.5.2.2. Anticlines Buried Under the Two Terraces[21] From its bathymetry, the Sixth Ridge seems to be an

isolated topographic high, bound by the relatively flat MSTand southern terraces. However, these two terraces revealfairly complicated subsurface structures: in the eastern part(Figure 5c), a small anticline (Anticline I) located seaward ofthe Sixth Ridge, is buried by some 500 ms TWT of slopesediments. Only 10 km further to the west (Figure 5b) Anti-cline I is much shallower in depth. At the southern limit of thesouthern terrace, a further anticline (Anticline III) is foundand which has chaotic structures in its shallowest parts,although more coherent folds are visible at greater depth.Further to the west (Figure 5a), erosion of the slope sedimentsby the Sonne Canyon exposes both Anticline I and III at thesurface, revealing another anticline (Anticline II) located inbetween these two anticlines. Furthermore, beneath the MST,another anticline (Anticline IV) is found, located ∼5 km northto the Sixth Ridge.[22] In total there are four anticlines in this area: three

beneath the southern terrace and one beneath the MST.Mapping of their trends is shown in Figure 2d and suggeststhat Anticline III could be the continuation of a sedimentridge located further to the east. Except for this anticline, allthe others reduce in elevation and becomemore deeply buriedby slope sediments from west to east, similar to the plunge ofthe imbricate slice (Units VI and V) in the core of the SixthRidge.5.2.3. Seismic Indications of Fluids Beneaththe Area MST5.2.3.1. BSRs and Their Depths[23] A BSR (bottom simulating reflector) occurs exten-

sively beneath the Sixth Ridge, with clear reversed polarity(Figures 4 and 5) and a depth of 500–600ms TWT bsf (belowthe seafloor ‐ Figure 6). Although there are several otherdeeper BSR‐like reflectors (Arrows 1 of Figure 4a), only theshallowest, most widely distributed and highest‐amplitudeBSR is discussed here. Figure 6 shows this BSR’s depthalong five, long profiles in the Area MST (see Figure 2d forthe locations of these profiles). The BSR depth varies little

throughout the study area, located between 500–600ms TWTbelow the seafloor (bsf). Only at three locations does the BSRshoal (by about 100 ms TWT): the western section of Anti-cline IV (Anomaly 1, Figure 6) and Anticline II (Anomaly 6,Figure 6), and the middle section of Anticline I (Anomaly 2,Figure 6). The BSR is absent along the entire frontal(southern) side of the Sixth Ridge (Anomaly 3, Figure 6), andat eastern section of the Sixth Ridge, BSR is also absent atthe back (northern) side of the ridge (Anomaly 4, Figure 6).The BSR is also absent, or substantially shallower, in partsof Anticline III (Anomaly 5, Figure 6).[24] In a morphologically complicated area, it may not be

possible to clearly image a BSR beneath a rugged seafloor, orit may shoal due to morphological affects on the thermal field[Delisle and Berner, 2002; Grevemeyer et al., 2004]. Thus,whether Anomalies 5 and 6 are indicative of fluid activity ornot is ambiguous. Even at BSR shoaling of Anomaly 1 and 6,the shoaling of ∼100 ms indicates only ∼5 mW/m2 of heatflow uplift, based on a similar calculation method to that usedby Minshull and White [1989] and Kaul et al. [2000]. Thusthe BSR anomalies do not seem to indicate significant localuplift of flow rate. However, Anomalies 1 to 4 do coincidewith seismic high amplitude anomalies (i.e., bright spots,Figures 4 and 5), which are presented in the next section.5.2.3.2. High Reflectivity (Bright Spots) Beneaththe Sixth Ridge and the Two Terraces[25] In the core of the Sixth Ridge, Unit VI shows strong

reflectivity (e.g., at Anomaly 3), and is clearly distinguishedfrom the transparent Unit V (Figures 4 and 5). At the west-ernmost part of the study area, such high amplitudes furtherextend into the frontal, collapsed side of the ridge (Arrow 2 inFigure 4a). The polarity of the high reflectivity is ambiguous.Nevertheless, the high amplitude zone (bright spot) doesshow reduced frequency in the anticlinal core of Figure 4b,suggesting the presence of free gas. Moreover, during theM74/2 Cruise, 6 m long gravity cores, recovered from thesame area, indicate a very shallow (∼3 m) methane‐sulphatetransition (SMT) zone, thus suggesting higher methane fluxesabove Unit VI [Spiess et al., 2008]. These observations favorthe interpretation that the high amplitude reflectors inUnit VI,under the summit of the anticline, are at least partially causedby gas accumulation.[26] At Anomaly 4, a section of high amplitude and low

frequency reflectors appear to be strictly confined inside/under a weakly reflective interval of Unit III (Arrow 3 inFigure 4c‐1), which is shallower than the expected BSRdepth. This high amplitude zone extends along Unit III untilunder the crest of the Sixth Ridge, where many small fault‐ orchimney‐like features are present (Arrow 4 in Figure 4c‐1),and where gas seeps (seep site S3 in Figure 2c) were ground‐truthed above these features during CruiseM74/3 [Bohrmannet al., 2008].[27] Where the BSR slightly shoals at Anticline I (Anomaly

2), high amplitude reflections (bright spots) are also observedat the anticline crest (Arrow 2 in Figure 5b). In the immediatevicinity of Figure 5b, a narrow seismic chaotic zone extendsalmost vertically from the top of the anticline to a depressionon the seafloor (Figure 7), resembling a diatreme structureand hence indicating a previous fluid/gas escape event. Assuch, this supports the interpretation that the high amplitudereflectors within Anticline I are the fingerprints of fluid/gas


8 of 16

accumulations. Similar high amplitude reflections (brightspots) also appear in Anticline IV (Figure 5a), where the BSRdepth also shoals slightly (Anomaly, 1, Figure 6). Althoughactive seepages are not observed directly above this anomaly,they were found on the seafloor above the western extensionof Anticline IV (seep site S7 in Figure 2c). The spatial rela-tionship between the anticlines, seismic features, BSRanomalies and seep sites are summarized in Table 1, whichwill be further discussed in section 6.3.2.

6. Discussion

[28] The seismic data presented here illustrate the internalstructure of the Deformation Front (DF) and theAreaMST, as

well as indicating the distribution pattern of BSRs and otherhigh amplitude reflections (bright spots) in the subsurface ofthe MST. In the following discussion, we suggest a structuralmodel for the DF (i.e., at the Nascent and the First Ridge).This model of the Nascent and First Ridges is regarded as ananalog for the initial, paleo‐structure of the Sixth Ridge andthe Area MST. We then discuss how this initial structure ismodified by continuous fault activity and rapid accumulationof slope sediments at the Sixth Ridge and the Area MST(section 6.2). This structural pattern and its evolution modeare then combined with the BSR distribution and highamplitude bright‐spots beneath the Area MST, to discuss the

Figure 6. BSR depth picked in five seismic profiles around Sixth Ridge. The five curves high‐lighted inthe main frame are for the BSR depth in five seismic profiles (Profiles GeoB07‐327, 328, 361–363). Thelocations of these profiles are marked in the small, lower right‐hand frame which is, in turn, simplified fromFigure 2d. Y‐axes in the main frame are for depth in TWT. x axis is the offset from the crest of the SixthRidge, with negative offset to the basin‐ward (southward) direction, and positive offset to the landward(northward) direction. The trends of the four anticlines are marked in the main frame to illustrate their spatialrelationship with the BSR Anomalies (see text).


9 of 16

link between the evolving structure of the frontal Makran andfluid accumulation, migration and seepage (section 6.3).

6.1. Imbricate Folding and Faulting at the DeformationFront

6.1.1. Upward Propagating Thrust Faults and TheirInfluence on the Nascent and First Ridges[29] If the ridges at the frontal Makran follow similar

evolutionary paths, then the Nascent Ridge’s structureevolves from its initial state to one similar to that of the FirstRidge. As deformation proceeds, the Nascent Ridge becomesuplifted further and its open anticline structure tightened.There are two scenarios for such uplift and folding: onepossibility is that the Nascent Ridge could be continuouslyfolded without any thrust faulting. Only when the fold

becomes locked does a thrust fault develop, forming ‘break‐thrusts’ or faulted detachment folding [e.g., Mitra, 2002;Thorbjornsen and Dunne, 1997]. Alternatively a thrust faultmight branch from the décollement, developing a splay faultbelow the Nascent Ridge. Such a splay fault, offsettingdeeper strata, and would gradually propagate into shallowersediments by generating simple folds or tri‐shear folds in theshallower strata above the fault tip [Allmendinger, 1998;Erslev and Mayborn, 1997; Suppe and Medwedeff, 1990].With increasing fault displacement at depth, the folds in theshallower strata become even tighter, while the deeper thrustfaults propagate upwards until break through to the surface.[30] Geometric constraints are still insufficient to deter-

mine the exact fold evolution mode applicable to thedevelopment of the Nascent Ridge, but the second scenario ‐

Table 1. Spatial Relationship Between the Shallow Anticlines and Fluid and Seepage Featuresa

Anticlines Figures, With Seismic Features Seismic Features BSR Anomalies Seep Sites Other Indications

Anticline I Figure 7a Diatreme‐like conduits Anomaly 2Anticline IV Figure 5a‐1 Figure 7b High amplitudes Anomaly 1 S7 (10km westward offset

from the Figure 5a)Anticline in the core

of Sixth Ridge(1) Figure 4b‐1

(2) Figure 4c‐1 (Arrow 4)(1)High amplitudes(2) Gas chimney

(1)Anomaly 3(2) Anomaly 4

(1) N/A (2) S3 (1) Very shallow MST(Methane SulphateTransition) depth

Anticline in the coreof Seventh Ridge

Figure 5b‐1 Figure 7c High amplitudes S11

aThe gray area stands for non‐geophysical features of seep and hydrocarbon activity.

Figure 7. Diatreme structure and some additional gas chimneys. (a) Diatreme above Anticline I, (b) Gaschimney approximately underneath seep site S7, (c) Gas chimney underneath seep site S11. Arrows showthe location of gas chimney like features. See Figures 2c and 2d for the locations.


10 of 16

synchronous development of folding and faulting ‐ is morelikely based on the following considerations: If a fold has tobe sufficiently tightened to allow the development of thrusts,a relatively thick ductile layer overlying the décollement isrequired to infill the void in the core of the tightening folds[e.g., Stewart, 1999]. However, there is little evidence that athick ductile layer (such as over pressurised and fluidisedmud) exists beneath the Nascent Ridge. Although mudvolcanoes occur in the interior Makran, at the DF most ofthe sediment above the décollement is normally compacted[Fowler et al., 1985; Fruehn et al., 1997]. Furthermore,previous seismic surveys very close to the location ofFigure 3a indicate that the deep strata below the forelimb ofthe Nascent Ridge are cut by a large offset fault indicatingbrittle deformation processes [Fruehn et al., 1997; Koppet al., 2000]. These data show that the shallow sedimentsforming the forelimb of the Nascent Ridge are only rotatedand not offset [e.g., Kopp et al., 2000, Figure 8]. In the end,the less elevated sections of the Nascent Ridge in Figure 2lay close to the First Ridge: a phenomenon that is typicalfor fault‐propagating fold (one style of synchronous foldingand faulting). In such a fold, the fold axis always locatesabove the upward tip of a propagating blind thrust at depth.Where the deeper thrust fault has a smaller displacement, italso just emanated from the décollement and still lays closeto the First Ridge. Accordingly, the shallow folds above thefault also have lower amplitude, with its axis closer to theFirst Ridge. The structure model in the Area DF thus isillustrated in Figure 8.[31] If we assume that the First Ridge evolves structurally

from a state represented by the Nascent Ridge, then theslumped sides of the First Ridge (Arrows 2, Figure 3a) canalso be explained by over‐steepening or even overturning ofthe forelimb of the shallow fold, caused by upward propa-gation of the blind thrust. Alternatively, when the thrust faultreaches the seafloor and continues to move, it can extrude thefolded forelimb into an open space, forming a ‘snake‐headed’forelimb, which would fail under gravity (a progress illus-trated in Figure 9). Likewise, where the First Ridge has only agentle and open anticline at its core (Figure 3b), the dis-placement of the thrust fault at the surface will also be smalland the forelimb has not yet become ‘snake‐headed’. In thissituation the forelimb does not fail and the anticlinal fold inthe ridge core is better preserved.

6.1.2. Development of Secondary Thrust Faults[32] The upward propagating thrusts beneath the Nascent

Ridge are regarded as splay faults, emanating from thedécollement [Fruehn et al., 1997], that incorporate abyssalbasin sediments into the accretionary prism. We define suchthrust faults as primary thrust faults, while sediments accretedbetween two primary thrust faults is defined as a horse(Figure 8). Based on both geometric considerations andseismic observations, the horizontal distance between thesurface trace of two primary thrust faults in the frontalMakran is typically about 12 km [Kopp et al., 2000]. Whilethe structural horse forming the Nascent Ridge (Figure 8) isnot internally faulted [Fruehn et al., 1997, Figure 2; Koppet al., 2000, Figure 8], the horse of the First Ridge(Figure 8) apparently does, based on the following. Theanticlinal ridge at Arrow 4 in Figure 3a and the thrust faultunderneath it (Figure 8) are separated by only 4 km of hori-zontal distance from the First Ridge. This fault is “too close”to the First Ridge to be a primary thrust fault, since the nextlandward primary thrust should be ∼12 km distant from FirstRidge. Thus, it is most likely to be an internal fault, dissectingthe horse of the First Ridge. Such thrusts are defined as sec-ondary thrust fault. A similar secondary thrust, albeit withmuch less displacement, is also visible in the seismic profileto the east of the study area: SO122‐04a [e.g., Schlüter et al.,2002, Figure 3]. These secondary thrusts may also initiate atdepth and propagate upward, resulting in hanging wall anti-clines (Arrow 4, Figure 3a), which share similar structures tothose related to the primary thrusts.[33] The basinward vergence of the secondary thrusts in-

dicates that they are not simply back‐thrusts. Although it isunclear where andwhen these thrust initiated, their absence inthe Nascent Ridge indicates that they develop later than theprimary thrusts. We infer that theymay be resulted from rampfaulting, evolving after the primary thrust has incurred asubstantial displacement. In this case, these secondary thrustsmay also initiate inboard of the deformation front; forexample at the Sixth Ridge (discussed in the next section).

6.2. Fate of Primary and Secondary Thrust Faultsin the Area MST

6.2.1. Nature of the Anticlines Beneath the Area MST[34] The Sixth Ridge is located ∼37 km landward from the

First Ridge, and has an estimated age difference of ∼2 Ma

Figure 8. Interpretation and inference of the structure beneath the Nascent and First Ridge, down to thedécollment level. This is based on, and extrapolated from, Figure 3a and previously reported, deeperpenetration seismic reflection data. It has no vertical exaggeration. The primary thrust fault under theNascent and the First Ridge bounds the horse (defined in section 6.1.2) of the Nascent Ridge. This horse isnot internally faulted, while the one containing the First Ridge is cut by a secondary thrust fault.


11 of 16

based on the plate convergence rate ofKukowski et al. [2000],or ∼3.4 Ma based on the prism progradation model of Plattet al. [1985]. With the average sedimentation rate for thefrontal Makran, an estimated ∼1 km of sediments would havebeen deposited in the abyssal plain during this time. Thus, theimbricate slices at the Area MST should be about 1 kmthinner than those at the DF, but may still amount to ∼3 km.Because the level of the décollement is thought to have beenstable during this time [Fruehn et al., 1997; Kukowski et al.,2001], the regularly shaped and consistently spaced bathy-metric ridges at the frontal Makran (Figure 2a) indicate astable plate convergence rate [Platt et al., 1985]. In all, thethick imbricate slice, the same décollement level with that ofthe First Ridge, and a stable convergence rate all suggest thatit is likely the Sixth Ridge had a structure similar to that of thepresent‐day First Ridge when it was first uplifted at its paleo‐deformation front. Moreover, the sediment strata and thrustfaults at the Area MST still have a similar dip angle to that ofthe First Ridge, supporting this notion.[35] Using the First Ridge as an analog, Anticlines I to IV

and the anticline in the core of the Sixth Ridge are most likelyto be thrust‐induced, with a blind thrust fault underlying eachanticline. The horizontal distance between adjacent anticlines(and hence thrust faults) is about 3 to 4 km, i.e., much shorter

than the distance between the primary thrust faults of 12 km.If we regard the thrust faults beneath the Sixth Ridge as aprimary thrust fault, then the thrust faults beneath Anticlines Iand II are probably secondary thrust faults within the horse(i.e., within the accreted sediments laying between two pri-mary thrusts) located basinward of the Sixth Ridge. Similarly,Anticline IV is probably a secondary thrust within the horselocated landward of the Sixth Ridge. Although it is anassumption to regard the thrust fault underneath the SixthRidge as a primary thrust, the pronounced elevation of theSixth Ridge compared with those elsewhere in the Area SMTsupports its validity.6.2.2. Role of Slope Sediments6.2.2.1. Slope Sediments “Supporting” Thrust Fault[36] As seen already at the First Ridge (Figure 3a and

Figure 9a), continuous thrusting can extrude the hanging wallblock over its footwall and the seafloor, causing its frontallimb to collapse (Figure 9a). Thrust faulting has been locatedat the Sixth Ridge for much of its history, causing significantextrusion of its hanging wall (i.e., forming a much moreprominent ‘snake‐head’ compared with the First Ridge).Here, because the actual imbricate slice (i.e., sedimentsdeposited before the onset of thrust activity) is either equal toor deeper than the top of Anticline I, the elevation difference

Figure 9. Illustrative structural development of thrust faulting: (a) without the slope sedimentation,(b) with slope sedimentation. The geometry of the fault and fold is based on the model of Erslev andMayborn, 1997. This is shown only to qualitatively demonstrate the different fate of overriding thrusthanging wall, with the absence or presence of slope sedimentation, and is not meant to imply the exactgeometry or evolution of the thrusts in the study area. Since themodel applies to both primary and secondarythrust faults, only one fault is illustrated in the diagram. For Figure 9b, the sedimentation rate is envisaged tobe exactly in pace with the protrusion of the imbricate wedge resulting from thrust faulting. If the sedi-mentation rate was much higher, it would bury both the thrust fault and its overlying anticline. This case isdiscussed in the text (section 6.2.2.2.).


12 of 16

between the top of Unit V in the Sixth Ridge (i.e., imbricateslice at hanging wall) and that of Anticline I (i.e., shallowestpossible top of imbricate slice at footwall) reflects a minimumvertical displacement of the thrust fault beneath the SixthRidge of ∼800m, Figure 5b. Since the fault dips ∼45° beneaththe Sixth Ridge (Figure 5b), 800 m of vertical displacementcorresponds to ∼800 m of horizontal movement, causing thehanging wall to form a typical ‘snake‐head’ thrust over itsfootwall and the seafloor. Yet, the Sixth Ridge (Figure 5b)still has a complete anticline in its core, and has not collapsedby gravity‐driven mass wasting, because its ‘snake‐head‘must have been bolstered by a thick packet of underlyingslope sediments. These slope sediments reveal a wedge‐shaped geometry indicating that they were deposited whilethe Sixth Ridge was being uplifted and extruded, i.e., they aresyn‐kinematic deposits (Figure 9b).[37] Where such syn‐kinematic sedimentation rates do not

keep apace with the extrusion rate of a ‘snake‐headed’ thrustblock, the accumulated thickness of the slope sediments arenot thick enough to support the forelimb of the sedimentridge, allowing gravity‐driven slumping. This is seen in thewestern section of the study area (Figure 4a and Figure 5a),where a high‐degree of shortening (as indicated by steeperUnit VI to I), and large thrust fault displacements have out‐paced sedimentation rates resulting in the thrust blocks beingunsupported by the underlying slope sediments which hasallowed gravity collapse and sliding of the forelimb of theSixth Ridge. The contrast between the fates of anticlinal coreof Sixth Ridge in its western and central sections highlightsthe importance of slope sediment in preserving anticlinesabove the primary thrusts.[38] In the other extreme, if the uplift rate is out‐paced by

the sedimentation rate, the thrust and ridge would be buried, aprocess for the next section.6.2.2.2. Slope Sediments Above the Thrust Faults[39] In the eastern section of the Sixth Ridge (Figures 4c

and 5a), the gentler dip of Unit VI to I indicates that com-pression and thrust faulting is less active than for the sameridge further west. Here, Units VI and V have a lower ele-vation are covered by the thick slope sediments of Unit IV toI, which are uplifted, forming the folded core of the SixthRidge. In addition, the dips of these slope sediment units

increase up‐section (Figure 5). Together, these featuresindicate that thrust faulting is still active in this less‐pro-nounced, eastern part of the Sixth Ridge. Overall, the core ofthe ridge changes from a thick but less elevated Unit IV to I(slope sediments) in the east, to the highly uplifted Units VI toV (i.e., the imbricate slice) in the west (Figure 5c to 5a), whilethe entire Sixth Ridge maintains a quasi‐constant elevation(Figure 2b). This is an example of slope sediments participatethe ridge formation.[40] Secondary thrusts, developed in the less elevated parts

of the imbricate slices, appear to be rapidly buried by slopesediments, as shown by Anticline I to VI (Figure 5). Theseblind thrusts propagate upwards causing the sediment aboveto fold; the samemechanism that caused the folding at the DF.Anticlines I, II and IV exemplify this mechanism, in whichthe slope sediments are folded rather than passively onlap andbury the anticlinal core of the ridge (Figure 5b and Figure 10).

6.3. Fluid Accumulation and Seepage

6.3.1. Thermal Field and Fluid Expulsion[41] Previous studies of the frontal Makran have indicated

the presence of BSRs, representing gas hydrate and under-lying free gas [e.g., Kaul et al., 2000; Minshull and White,1989; Schlüter et al., 2002; White, 1984]. The depth of theBSR has also been used to infer heat flow at the frontalMakran, where an increase of 20–25% over that predicted bya conductive heat dissipation model has been found [Minshulland White, 1989]. This anomaly has been ascribed to lowrates of advective flow from depth, as a result of tectonicthickening and dewatering of the accreted sediments. Lowadvective fluid flow rates are also indicated by the accumu-lation of a thick free‐gas column below the BSR [Grevemeyeret al., 2000; Sain et al., 2000;Haacke et al., 2007]. Refractionseismic studies generally indicate a normally compactedaccretionary prism [Fowler et al., 1985; Fruehn et al., 1997;Kopp et al., 2000], only with the exception of a deep, lowvelocity zone near the Sixth Ridge that may be related toundercompaction of deeply buried sediments [Kopp et al.,2000]. By summarizing these arguments, we infer that theentire sediment column beneath Area MST is overall nor-mally compacted, although low‐rates of dispersive fluid flowstill pervade the region.

Figure 10. Structure interpretation of Profile GeoB07‐364 around Sixth Ridge: Note this profile, which ispartly presented in Figure 5c, shows features within the full depth penetration of the seismic record andpresented with a lesser vertical exaggeration. It highlights the structural mechanisms that focus migratinghydrocarbons into the shallow anticlines, associated with deeper thrust faults, that act as structural traps.The sediment wedge under the Sixth Ridge bolsters the ridge, preventing its hanging wall and associatedanticlinal imbricate wedge from further collapse.


13 of 16

[42] In the AreaMST, the BSR depth is mostly at a depth of500–600 ms TWT bsf, with little systematic west‐east vari-ation (Figure 6). This in turn implies that the advective flowrate varies little around the Sixth Ridge, despite the signifi-cant west‐east variations in deformation intensity as indicatedby the large difference of sediment thickness, structure anddeposition/erosion patterns from the west to the east. Hence,we infer that the rate of tectonic dewatering in this locationmust be minimal as the fluid flow flux does not appear toincrease where compression is more intensive. Instead, wesuggest the advective flow may be driven by the fast sedi-mentation rate and associated compaction of newly depositedslope sediments. Despite this, shallow subsurface fluid/hydrocarbon accumulations do exist in the Area MST, thedistribution of which appears to be controlled by the avail-ability of shallow fluid traps.6.3.2. Structures Trapping Fluids[43] As in Section 5.2.3.3, BSR anomalies are found at

various locations within the shallow anticlines, above whicheither hydrocarbon seeps are present (e.g., S3 and S7) orsignificant seismic features (e.g., Figure 7) and geochemicaldata (e.g., Shallow MST, Table 1) suggest hydrocarbon‐richfluid activity. Additionally, the seep site S11 is located on aridge further landward than the Sixth Ridge (e.g., 7th Ridge inFigures 2c and 2d and Figure 5). Although this ridge is notdiscussed in this study, it is almost certainly located above theanticline of the next landward primary thrust fault because itlies only ∼12 km from the Sixth Ridge: the typical distancebetween primary thrust faults. The majority of identifiedhydrocarbon seeps are located above various anticlines,which act as structural traps for hydrocarbon accumulations.The only exception is seep site S6, which lies close to thesurface trace of the thrust fault underneath Sixth Ridge.[44] The stratigraphic belongings of the reservoir units in

the anticlinal traps can vary but are commonly shallow.Where the slope sediments are thin, the folded imbricateslice acts as a structural trap and hydrocarbon fluid reservoir(e.g., Anomaly 3, at the core of Sixth Ridge Figure 6 andFigure 4b). Also, where imbricate sediments are more deeplyburied, permeable intervals of slope sediments form the res-ervoir units (e.g., at Anomalies 1, 2 and 4 of Figure 6,corresponding to Anticline IV in Figure 5a, Anticline I inFigure 5b and Sixth Ridge in Figure 4c, respectively). As aresult, the fluid reservoirs below the identified seep sites arecommonly shallow, typically buried by only a few hundredms TWT of slope sediments. At these shallow depths, dif-ferences between hydrostatic and lithostatic pressures areconsidered to be too small to sustain significant overpressure.[45] Because the bulk of the accreted sediments are largely

normally compacted, the resulting pervasive advective flowmust be slow.We therefore consider it unlikely that such flowcan transport substantial volumes of hydrocarbon fluids tosupport persistent seepage. Buoyancy of hydrocarbon maythus play an important role for hydrocarbon migration fromgreater depth. A deep, thermal origin for the seeping hydro-carbon is suggested by the lowC1/C2 ratios [Bohrmann et al.,2008] of sampled methane and from natural oil slicksobserved at the sea surface. (I. MacDonald, personal com-munication, 2008). Similarly, biogenic origins for the hy-drocarbons is considering less likely, given the low totalorganic carbon (TOC) content and low hydrocarbon yieldpotential of the rapidly deposited slope sediments.

[46] To summarize, we proposes a seepage scenario inwhich hydrocarbon fluids originate at depth, migrate buoy-antly through normally compacted sediments and accumulatein shallow structural traps. In addition, several deeper struc-tures appear to focus the flow of hydrocarbon fluids into theshallow reservoirs: the tilted imbricate slices, the bottom ofthe low permeable gas hydrate zone and the thrust faults(Figure 10). Where the shallow traps are sufficiently chargedwith fluids, the overlying strata can be breached (e.g., bydensity differences between the fluid and overlying sedimentcolumn) allowing the fluids to rapidly escape (e.g., formingdiatremes and gas chimneys ‐ Figures 4 and 7).[47] Several additional implications can be drawn from this

study: First, the coincidence between the location of brightspots and fluid seeps with the crests of buried anticlinessuggests that fluid pathways and accumulations are notrelated to of the presence of mud volcanoes. Nor do thrustfaults play a major role for fluid seeps (fluids appear to bedirectly sourced through thrust faults at only one of the manyseep sites ‐ S6, Figure 2). Thus the frontal Makran is quitedifferent from other accretionary prisms such as the Medi-terranean Ridge [Kopf et al., 2001], Gulf of Cadiz [Murtonand Biggs, 2003], Cascadia [Zühlsdorff and Spiess, 2004;Screaton et al., 1995], or even the more landward part ofMakran [Harms et al., 1984; Schlüter et al., 2002, Grandoand McClay, 2007], where intensive, tectonic‐induced,overpressure‐driven migration of fluidised mud from theunderplated décollement plays an important role in mudvolcanism and seepage. In this respect, the frontal Makranacts more like a passive continental margin in which deephydrocarbon‐rich fluids supply shallow structural traps.Finally, these shallow anticlines are both produced and pre-served by the interplay between thrust faulting and slopesedimentation. Without rapidly accumulated, syn‐kinematicslope sediments to burymany of the thrust faults, the resultinggrowth‐anticlinal structures would not form or be preserved.Likewise, without the bolstering support from the slopesediments, many anticlines would form “snake‐headed”protrusions that would become slumped and destroyed. Thuswithout the fast slope sediments, most shallow traps wouldnot be preserved, and the picture of hydrocarbon seepage inthis area would be very different.

7. Conclusion

[48] At the Deformation Front of the frontal Makran,sediments above the décollement are imbricated by upward‐propagating thrust faults. Shallower sediments above blindthrusts are also folded and form anticlinal ridges on the sea-floor. The earliest of these ridges are the Nascent Ridge andthe First Ridge. Where the thrust faults are directly linked tothe décollement zone, as splays, they are defined as primarythrust faults. Secondary thrust faults occur during the rotationof the imbricate slices through which they dissect. Thesesecondary thrust faults also cause anticlinal folding of thesediments above them, with a spacing that is much less thanthat of the primary thrust splays.[49] The folds resulting from the primary and secondary

thrust faults can also be recognized below the Area MST:Anticlines I to IV and the one in the core of the Sixth Ridge.Thrust faulting and compression are still active at the AreaMST, uplifting the Sixth Ridge. Where the accumulation rate


14 of 16

of the slope sediments fails to keep apace with the uplift rateof the anticlinal ridges (e.g., the Sixth Ridge), the protrudinghanging walls form “snake‐headed“ structures that extrudeanticlinal cores of the ridges over the seafloor where they areunsupported and collapses under gravity (e.g., the westernsection of the Sixth Ridge). Where the accumulation rates ofthe slope sediments keeps apace with the uplift, the slopesediments bolster the protruding hanging walls and anticlinalcores of the ridges, preserving the structures within theemerging ridges (e.g., the central section of Sixth Ridge).Where the accumulation rate of the slope sediments is fasterthan the uplift, the syn‐kinematic sediments bury the ridge, itsprotruding hanging wall and its anticline, causing the thrustfault to remain blind. In these situations, the thrusts continueto be active, causing folding of the overlying syn‐kinematicslope sediments (e.g., the eastern section of Sixth Ridge, andAnticlines I, II and IV beneath the AreaMST). In this respect,the slope sediments play an important role in the forma-tion and preservation of the folds in the shallow subsurface.Without the slope sediments, most thrust‐induced anticlinesand ridges would collapse and be destroyed.[50] The shallow anticlines beneath the Area MST form

near‐surface structural traps for hydrocarbon‐rich fluids. Thetilted imbricate slices, BSRs and slope sediments providefurther structures that focus deep‐sourced fluids into thesetraps. When the pressures of the buoyant fluid‐column in theanticlinal traps exceed the yield strength of the overlyingsediments, they induce diatremes and gas chimneys abovethem, allowing fluid seepage at the seafloor. We suggest thisis the major style of fluid seepage in the frontal Makran and isexemplified by the newly discovered seeps and fluid‐flowactivity above many of the anticlinal ridges at the Area MST.In addition, accreted sediments of the frontal Makran arelargely normally compacted, and lack mud volcano‐likefeatures that are indicative of the vigorous ascent of tectoni-cally over‐pressurized fluids.We postulate that, in addition toslow, pervasive, advective flow at the shallow reservoirdepths, hydrocarbon‐rich fluids may be driven from greaterdepth by their own density‐induced buoyancy rather thantectonically driven overpressure.

[51] Acknowledgments. We thank the crew of the R/VMeteor duringthe Cruise M74/2 for their excellent and professional work. We are alsograteful to the Pakistani government for the permission to survey Pakistaninational waters. Special acknowledgments are given to Tim Minshull andTim Le Bas of National Oceanography Centre, Southampton, for their dis-cussions and sharing of data. We sincerely thank Christoph Gaedicke ofBGR, for his sharing of data and consideration during our cruise planning.We appreciate the careful and inspiring reviews by the associate authorand three anonymous reviewers. F. D.’s research visits to NOC weresupported by Bremen International Graduate School for Marine Sciences(GLOMAR). The M74/2 cruise and this study are funded through DFG‐Research/Exzellenz Cluster, “the ocean in the Earth system”.

ReferencesAllmendinger, R. W. (1998), Inverse and forward numerical modelling oftrishear fault‐propagation folds, Tectonics, 17(4), 640–656, doi:10.1029/98TC01907.

Arthurton, R. S., A. Larah, and W. Ahmed (1982), The late Cretaceous‐Cenozoic history of western Baluchistan, Pakistan‐the northern marginof the Makran subduction complex, in Trench and Fore‐Arc Geology:Sedimentation and Tectonics on Modern and Ancient Active Plate Mar-gins, Geological Society of London, Special Publications, No. 10, editedby J. K. Leggett, pp. 373–385.

Bohrmann, G., G. Ohling, and other cruise participants (2008), Report andpreliminary results of R/V Meteor Cruise M74/3, Fujairah ‐ Male, 30October–28 November, 2007. Cold seeps of the Makran subduction zone(continental margin of Pakistan), Berichte, Fachbereich Geowissenschaften,Nr.266, Universität Bremen, Bremen, Germany.

Caress, D. W., and D. N. Chayes (1996), Improved processing of Hydro-sweep DS Multibeam data on the R/V Maurice Ewing, Mar. Geophys.Res., 18, 631–650, doi:10.1007/BF00313878.

Delisle, G., and U. Berner (2002), Gas hydrates acting as cap rock to fluiddischarge in the Makran accretionary prism? in The tectonic and ClimaticEvolution of the Arabian Sea Region, Geological Society London, Spe-cial Publications, No. 195, edited by P. D. Clift, D. Kroon, C. Gaedicke,and J. Graig, pp. 137–146.

DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein (1990), Current platemotions, Geophys. J. Int., 101, 425–478, doi:10.1111/j.1365-246X.1990.tb06579.x.

Erslev, E. A., and K. R. Mayborn (1997), Multiple geometries and modesof fault‐propagation folding in the Canadian thrust belt, J. Struct. Geol.,19(3–4), 321–335, doi:10.1016/S0191-8141(97)83027-1.

Fowler, S. R., R. S. White, and K. E. Louden (1985), Sediment dewateringin the Makran accretionary prism, Earth Planet. Sci. Lett., 75, 427–438,doi:10.1016/0012-821X(85)90186-4.

Fruehn, J., R. S. White, and T. A. Minshull (1997), Internal deformationand compaction of the Makran accretionary wedge, Terra Nova, 9,101–104.

Gaedicke, C., A. Prexl, H. Schlüter, H. Meyer, H. Roeser, and P. Clift(2002), Seismic stratigraphy and correlation of major regional unconfor-mities in the northern Arabian Sea, in The tectonic and Climatic Evolu-tion of the Arabian Sea Region, Geological Society London, SpecialPublications, No. 195, edited by P. D. Clift, D. Kroon, C. Gaedicke,and J. Graig, pp. 25–36.

Grando, G., and K. McClay (2007), Morphotectonics domains and struc-tural styles in the Makran accretionary prism, offshore Iran, Sediment.Geol., 196, 157–179, doi:10.1016/j.sedgeo.2006.05.030.

Grevemeyer, I., A. Rosenberger, and H. Villinger (2000), Natural gashydrates on the continental slope off Pakistan: Constrains from seismictechniques, Geophys. J. Int., 140(2), 295–310, doi:10.1046/j.1365-246x.2000.00009.x.

Grevemeyer, I., et al. (2004), Fluid flow through active mud dome MoundCulebra offshore Nicoya Peninsula, Costa Rica: Evidence from heat flowsurveying, Mar. Geol., 207, 145–157, doi:10.1016/j.margeo.2004.04.002.

Haacke, R. R., G. K. Westbrook, and R. D. Hyndman (2007), Gas hydrate,fluid flow and free gas: Formation of the bottom‐simulating reflector,Earth Planet. Sci. Lett., 261, 407–420, doi:10.1016/j.epsl.2007.07.008.

Harms, J. C., H. N. Cappel, and D. C. Francis (1984), The Makran coast ofPakistan: It’s stratigraphy and hydrocarbon potential, in Marine geologyand oceanography of Arabian Sea and coastal Pakistan, edited by B. U.Haq and J. D. Milliman, pp. 3–26, VNR‐SAE.

Kaul, N., A. Rosenberger, and H. Villinger (2000), Comparison of mea-sured and BSR‐derived heat flow values Makran accretionary prism,Pakistan, Mar. Geol., 164, 37–51, doi:10.1016/S0025-3227(99)00125-5.

Kopp, C., J. Fruehn, E. R. Flueh, C. Reichert, N. Kukowski, J. Bialas, andD. Klaeschen (2000), Structure of the Makran subduction zone fromwide‐angle and reflection seismic data, Tectonophysics, 329, 171–191,doi:10.1016/S0040-1951(00)00195-5.

Kukowski, N., T. Schillhorn, E. R. Flueh, and K. Huhn (2000), Newlyidentified strike‐slip plate boundary in the northeastern Arabian Sea,Geology, 28(4), 355–358, doi:10.1130/0091-7613(2000)28<355:NISPBI>2.0.CO;2.

Kukowski, N., T. Schillhorn, K. Huhn, U. von Rad, S. Husen, and E. R.Flueh (2001), Morphotectonics and mechanics of the central Makranaccretionary wedge off Pakistan, Mar. Geol., 173, 1–19, doi:10.1016/S0025-3227(00)00167-5.

Minshull, T., and R. White (1989), Sediment compaction and fluidmigration in the Makran Accretionary Prism, J. Geophys. Res., 94(B6),7387–7402, doi:10.1029/JB094iB06p07387.

Mitra, S. (2002), Structural models of faulted detachment folds, Am. Assoc.Pet. Geol. Bull., 86(9), 1673–1694.

Murton, B. J., and J. Biggs (2003), Numerical modelling of mud volcanoesand their £ows using constraints from the Gulf of Cadiz, Mar. Geol., 195,223–236, doi:10.1016/S0025-3227(02)00690-4.

Murton, B. J., I. P. Rouse, N. W. Millard, and C. G. Flewellen (1992),Multi‐sensor, deep‐towed instrument explores ocean floor, Eos, 73,225–228.

Platt, J. P., J. K. Leggett, J. Young, H. Raza, and S. Alam (1985), Large‐scale sediment underplating in the Makran accretionary prism, southwestPakistan, Geology, 13, 507–511, doi:10.1130/0091-7613(1985)13<507:LSUITM>2.0.CO;2.


15 of 16

Quittmeyer, R. C., and A. L. Kafka (1984), Constraints on plate motionsin southern Pakistan and the northern Arabian Sea from the focalmechanisms of small earthquakes, J. Geophys. Res., 89(B4), 2444–2458,doi:10.1029/JB089iB04p02444.

Sain, K., T. A. Minshull, S. C. Singh, and R. W. Hobbs (2000), Evidencefor a thick free gas layer beneath the bottom similating feflector in theMakran accretionary prism, Mar. Geol., 164, 3–12, doi:10.1016/S0025-3227(99)00122-X.

Schlüter, H. U., A. Prexl, C. Gaedicke, H. Roeser, C. Reichert, H. Meyer,and C. von Daniels (2002), The Makran accretionary wedge: Sedimentthickness and ages and the origin of mud volcanoes, Mar. Geol., 185,219–232, doi:10.1016/S0025-3227(02)00192-5.

Screaton, E. J., B. Carson, and G. P. Lennon (1995), Hydrogeologic prop-erties of a thrust fault within the Oregon accretionary prism, J. Geophys.Res., 100, 20,025–20,035, doi:10.1029/95JB02152.

Spiess, V., and other cruise participants (2008), Report and preliminaryresults of R/V Meteor Cruise M74/2, Fujairah ‐ Fujairah, 7 October–28 October, 2007. Cold seeps of the Makran subduction zone (continentalmargin of Pakistan), Berichte, Fachbereich Geowissenschaften, Uni-versität Bremen, Bremen, Germany.

Stewart, S. A. (1999), Geometry of thin‐skinned tectonic systems in rela-tion to detachment layer thickness in sedimentary basins, Tectonics,18(4), 719–732, doi:10.1029/1999TC900018.

Suppe, J., and D. Medwedeff (1990), Geometry and kinematics of fault‐propagation folding, Eclogae Geol. Helv., 83(3), 409–454.

Thorbjornsen, K. L., and W. M. Dunne (1997), Origin of a thrust‐relatedfold: Geometric vs kinematic tests, J. Struct. Geol., 19(3–4), 303–319,doi:10.1016/S0191-8141(96)00090-9.

von Rad, U., U. Berner, G. Delisle, H. Doose‐Rolinski, N. Fechner, P. Linke,A. Lückge, H. A. Roeser, R. Schmaljohann, and M. Wiedicke, and theSonne 122/130 Scientific Parties (2000), Gas and fluid venting at theMakran accretionary wedge off Pakistan, Geo‐Mar. Lett., 20, 10–19,doi:10.1007/s003670000033.

Wessel, P., and W. H. F. Smith (1991), Free software helps map and dis-play data, EOS, 72, 445–446.

White, R. S. (1984), Active and passive plate boundaries around the Gulfof Oman, north–west Indian Ocean, Deep‐Sea Res., 31(6–8), 731–745,doi:10.1016/0198-0149(84)90038-4.

White, R. S., and K. Klitgord (1976), Sediment deformation and platetectonics in the Gulf of Oman, Earth Planet. Sci. Lett., 32, 199–209,doi:10.1016/0012-821X(76)90059-5.

Yilmaz, Ö. (1991), Seismic data processing, Society of ExplorationGeophysics, Tulsa.

Zühlsdorff, L., and V. Spiess (2004), Three‐dimensional seismic character-ization of a venting site reveals compelling indications of natural hydraulicfracturing, Geology, 32(2), 101–104, doi:10.1130/G19993.1.

G. Bohrmann, M. Brüning, F. Ding, N. Fekete, and V. Spiess, Marum,University of Bremen, Klagenfurter Str., D‐28359 Bremen, Germany.([email protected]; [email protected]; dingfeng@uni‐bremen.de; nfekete@uni‐bremen.de; vspiess@uni‐bremen.de)B. Murton, National Oceanography Centre Southampton, Empress Dock,

Southampton SO14 3ZH, UK.


16 of 16

Interaction between accretionary thrust faulting and slope sedimentation at the frontal Makran...

Documents

Transcript of Interaction between accretionary thrust faulting and slope sedimentation at the frontal Makran...