Internal deformation and compaction of the Makran accretionarywedge

J. Fruehn, R. S. White and T. A. MinshullDepartment of Earth Sciences, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK

Introduction

The Makran accretionary wedge hasone of the thickest incoming sedimentpiles and forms together with theMak-ran Ranges one of the most extensiveaccretionary complexes in the world.As such, it provides a useful end-mem-ber example of processes such as sedi-ment dewatering, active thrust faultingand sediment underplating/subduction.Seismic investigations imageup to7km

thick sediments in theGulf of Oman andalso resolve the coarse deformation stylewithin the wedge: a series of regularlyspaced thrust sheets is inferred fromvarious, mainly single channel, reflec-tion seismic profiles across the wedge(White and Klitgord, 1976; White andLouden, 1982; Minshull et al., 1992).By reprocessing and prestack depth

migration of a multichannel seismicprofile across the offshore Makran ac-cretionary wedge (CAM30 Fig. 1;Min-shull et al., 1992), we show new detailsof the deformation style in the frontalpart of the wedge and we discuss varia-tions in the physical properties of thesediment as it undergoes compaction,deformation and thrusting.

Tectonic setting and evolution

The Makran accretionary complex hasdeveloped at the convergent margin

between the Arabian and the EurasianPlates throughout the Cenozoic, mainlyby recycling of sediment which waseroded from the India±Eurasia colli-sion belt and from the uplifted olderparts of the Makran accretionary com-plex (Harms et al., 1984).A first evolutionary phase lasting

from late Oligocene to middle Miocenetimes was characterized by turbiditicdeposition of quartzolithic sands andmuds. It is believed that these sedimentsaccumulated in an enormous submar-ine fan on top of igneous oceanic crust(Proto Indus Ð Harms et al., 1984).Analysis of detrital modes of Tertiarysandstones and modern sands acrossthe Himalayan belt suggests that theturbidites travelled south-west alongthe Himalayan Main Central Faultand were deposited in the present-dayGulf of Oman (Garzanti et al., 1996).Between the late Miocene and the

middle Pleistocene these sedimentswere entrained by frontal accretionand underplating (Platt et al., 1985),and theywere covered by huge volumesof shelf-and-slope sands. The accre-tionary wedge not only grew seawardby accretion of trench fill sediments,but also by slope, shelf and coastalplain progradation. It thus forms alargely self-contained system, whereaccretion causes uplift above sea level,and subsequent erosion and sedimenta-tion provides new material for accre-tion (Harms et al., 1984).Since the middle Pleistocene the

coastal Makran has experienced uplift

and normal faulting, while in the sea-ward part accretion has continued. Atpresent, the accretionary complex isabout 1000 km wide and is separatedfrom regions of active continent±con-tinent collision (the Zagros and Hima-laya) by two fault systemsÐ theMinabFault system in the west, and the trans-pressional strike-slip system of the Or-nach-Nal and Chaman Faults in theeast (Fig. 1). The Makran extends400±600 km northward from the defor-mation front to the Baluchistan volca-nic arc. This exceptionally long trench-arc distance is due to a shallow-dippingBenioff zone and is explained by theabundant supply of sediments to thetrench and the accretionary wedge (Ja-cob and Quittmeyer, 1979). The plateconvergence rate is& 40 km Myr71 ina roughlyN±S direction (DeMets et al.,1990), and the wedge is growing sea-ward at an estimated rate of 10 kmMyr71 (White, 1982; Platt et al., 1985).

Reprocessing and prestack depthmigration of CAM30

The two major seismic processing pro-blems were the removal of ringing pro-duced by the collapse of the sourcebubble, and suppression of the multi-ples. A mixed statistical/deterministicapproach proved optimal for signalcompression and filtering of the ring-ing. A wave-equation multiple reduc-tion method was used to suppress thestrong seabed multiple. The remainderwas subsequently filtered in the radon

#1997 Blackwell Science Ltd 101

ABSTRACTThe Makran accretionary wedge is one of the largest on Earth. A7-km-thick column of sands and quartzolithic turbidites areincorporated into this wedge in a series of deformed thrustsheets. We present the results of prestack depth migration andfocusing-error analysis (migration velocity analysis) performedon a profile across the Makran wedge. The depth section showsthe deformation style of the accreted sediments, and themigration velocities allow us to estimate porosity variations inthe sediments. The thrust sheets show evidence of fault-propagation folding, with a long wavelength of deformation(& 12 km) and secondary thrusting in the kink bands of the folds,such that the central part of each thrust sheet is elevated to forman additional ridge. This deformation style and the 158 steep

surface slope of the first ridge suggest a high degree ofconsolidation. Porosities were calculated from the seismicmigration velocities and the ratio of fluid pressure to lithostaticpressure lwas estimated for 5 locations along the profile. Ratherthan being undercompacted and overpressured as in mostaccretionary wedges, the sedimentary input is normallycompacted (exponential porosity decay) throughout almost thewhole wedge. However, a slight increase in porosity and l atdepth, with respect to the normal compaction curve indicates,that the turbiditic sequence might be overpressured landward ofthe deformation front.

Terra Nova, 9, 101±104, 1997

AhedBhedChedDhedRef marker

Fig markerTable markerRef endRef start

Correspondence: J. Fruehn, Fax:+44/ (0)1223

360779; E-mail: fruehn@esc.cam.ac.uk

Paper 119 Disc

domain and an inner trace mute wasapplied locally. Prestack depth migra-tion was performed with a Kirchhoffmultiarrival ray tracing algorithm. Themigration velocities were calculated bydepth focusing-error analysis. The ac-curacy of velocity determination variesbetween 2 and 10% with depth anddegree of deformation. Post-migrationprocessing included fk-filtering of mi-gration smiles, bandpass filtering andautomatic gain control (agc).

Results and interpretation

Abyssal plain segment

The abyssal plain segment exhibitsthree distinct seismic facies (Fig. 2a,c).The upper sequence is characterized bycontinuous subhorizontal reflections,with alternating high and low ampli-tudes, whereas the middle sequence ex-hibits a landward-dipping pattern ofparallel, densely spaced high-amplitudereflections. The lower sequence showscontorted to chaotic reflections.Reflec-tions of the upper sequence onlap thetop of themiddle sequence, indicating amajor change in sedimentation.We interpret the upper sequence to

represent theMakransands (Harmsetal.,1984; Garzanti et al., 1996). Themiddlesequence represents the turbidites ofHimalayan origin and the lower sequenceis attributed to the igneous oceanic crust.Between CDP 500±1100 (Fig. 2a,c) theoceanic crust exhibits a structural highprobably belonging to the buried LittleMurray Ridge (White, 1983).TheMakran sands form a section up

to 3 km thick, which is covered by a thinlayer of aeolian sediments (Stoffers andRoss, 1979). At CDP 2000 the sedi-ments show gentle folding and faultingbelow 2 km depth. The turbidites havean average thickness of about 4 km.

They are slightly faulted above theLittle Murray Ridge. Pairs of shearplanes are observed between CDP1800±2000, probably due to failure un-der increased horizontal compressionalstress (Fig. 2b,d).The seismic velocities shown in Fig.

3(a) (CDP 540 and 1060) were con-verted to porosities using a standardvelocity±density relationship (Hamil-ton, 1978). A pore water density of1050kgm73andanaveragegraindensityfor sandstones of 2690 kg m73 (Carmi-chael, 1984) were assumed. Figure 3(b)shows an exponential porosity decay atCDP 540, which is consistent withAthy's law for normally compactedsediments: n= n0exp(±z/c), where n isthe porosity, n0=43.3% is the porosityat the surface, z is the depth below sea-floor, and c=2.42 km is the compactionlength. Note that the porosities for theMakran sands (open symbols) and theHimalayan turbidites (closed symbols)lie on the same compaction curve. Theporosities for CDP 1060 are generallylower, suggesting increased compaction.

Deformation front and accretionarywedge segment

At CDP 2400 a first major thrust faultrises from a horizon within the turbi-dites and propagates towards the sea-floor (Fig. 2b,d). In the lower half of thesediment pile it displaces the layeringby about 200 m, but in the upper halfonly minor deformation along a proto-thrust is observed. We interpret thisfault as the deformation front and theturbiditic horizon as the basal de colle-ment,which roughly parallels the topofthe oceanic basement landward of theLittle Murray Ridge. The de collementis generally continuous and exhibitsmoderate to bright reflection ampli-tudes. It has also been imaged else-where along the deformation front(White, 1977).Interpretation of the accreted thrusts

was possible by correlation of the dis-tinct seismic pattern of the Makransands. Figure 2(b,d) show the deforma-tional style of the most seaward, bestimaged, thrust sheet. The shape of thereflections indicate a fault-propagationfold. The location of the thrust is in-ferred from reflection terminations andfrom the identification of typical struc-tural elements. Secondary faulting isobserved at CDP 2800, which elevatesthe layers by several hundred metres

and thus produces an additional ridgewithin the thrust sheet. The initial faultspacing of about 12 km, given by thedistance between the deformation frontand the first thrust, is thus reduced toabout half. In the shallow part of thefirst ridge, the sediments have experi-enced substantial deformation by short-ening, shearing and tilting, such that theseismic pattern is no longer identifiable.The1.2kmthrowat this faultproducesanexceptionally steep surface slope of 158.The porosities calculated at CDP

1860, 2260 and 2740 are shown in Fig.3(b). The Makran sands (open sym-bols) again show normal compactionwith amuch smaller compaction lengthof c=1.54 km, which indicates fasterporosity decay than in the abyssalplain. It is remarkable that all threelocations lie on the same compactioncurve, suggesting that the sedimentshave reached their maximum compac-tion seaward of the deformation front.The porosities of the Himalayan turbi-dites (closed symbols) are generallyhigher than is predicted by the compac-tion curve of the Makran sands (thinline in Fig. 3b).

Discussion

The results presented here show (i) thatthe deformation style of the Makranwedge is characterized by secondarythrusting in the kink bands of the fault-propagation fold and (ii) that theMak-ran sands are normally consolidated,whereas (iii) the Himalayan turbiditesdeviate from the normal compactioncurve towards higher porosities.These results indicate that, unlike

other accretionary wedges (e.g. Barba-dos, Cascadia), where expelled fluidleads to fluid overpressure and under-compaction, the sedimentary section ofthe Makran must have experienced ahigh degree of consolidation prior toaccretion (also suggested by Fowler etal., 1985), and that the only sign ofoverpressuring is found in the turbiditicsequence. The main reason for thisunusual compaction history in theMakranwedge is probably thematerialitself: sand is highly permeable, and islikely to lose most of its fluid contentduring preaccretionary normal com-paction. From Fig. 3(b) it is clear thatthe porosities at depth are very low, sothat the deviation from normal com-paction (thin line) could be explainedby a limiting minimum porosity as well

102 #1997 Blackwell Science Ltd

Deformation of the Makran accretionary wedge . J. Fruehn et al. Terra Nova, Vol 9, No. 3, 101±104. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 1 Location map of the seismic profileCAM30 and plate tectonic setting. MF,Minab Fault; MR, Murray Ridge; O±NF,Ornach±Nal Fault. Arrow shows inferredmotion of the Arabian plate with respectto Eurasia (from DeMets et al., 1990).

Paper 119 Disc

as by slight overpressuring. We can,however, estimate the ratio of fluidpressure to lithostatic pressure l fromthe compaction curves presented above,to quantify the pressure conditions inthe abyssal plain and in the accretedsediments.For hydrostatic pressure conditions

Athy's law can be written in terms ofeffective stresss* (vertical compressivestress ± fluid pressure) as n= n0exp{s*/[(r-rw)gc]}, where r is the density of thesediments, rw is the water density and gis the acceleration due to gravity (Allenand Allen, 1990). l is calculated froms* using s*= (1±l) rgz (Davis et al.,1983). In the abyssal plain (Fig. 3bCDP540, numbers at right of symbols),where hydrostatic pressure is indicatedby a good fit of Athy's law, l decreaseswith depth from0.51 at 400mbelow thesea-floor to 0.45 at 3.7 km below thesea-floor.At the deformation front and within

the wedge (Fig. 3b CDP 2260/2740,numbers at left of symbols) l is hydro-

#1997 Blackwell Science Ltd 103

Terra Nova, Vol 9, No. 3, 101±104 Deformation of the Makran accretionary wedge . J. Fruehn et al.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 2 Prestack depth migration and interpretation of an abyssal plain segment (a, c), and an accretionary wedge segment (b, d). Thereflections of theMakran sands onlap the top of theHimalayan turbidites. The deformation front is currently propagating through thesedimentary cover. Secondary faulting is imaged (or inferred) in the kinks of the fold. This deformation style and the 158 steep surfaceslope of the first ridge suggest a high degree of consolidation.Abottom-simulating reflection (BSR) is imaged at about 700mbelow thesea-floor.

Fig. 3 Seismic interval velocities from focusing-error analysis (a) and inferred porosities(b). In the abyssal plain theMakran sands (open symbols) and theHimalayan turbidites(closed symbols) shownormal compaction (thick line). The accretedMakran sands havea smaller compaction length c=1.54 km (thin line). The porosities of the accretedturbidites deviate from normal compaction, which may indicate overpressuring.

Paper 119 Disc

static for the Makran sands. The re-duced compaction length (1.54 kmcompared to 2.42 km in the abyssalplain) indicates increased tectonic stressrelated to accretion. Assuming thatoverpressuring in the turbiditic sequenceis responsible for the deviation of theaccreted sediment porosities from nor-mal compaction, and that the porositywithin the wedge is an unique functionofs*, we estimateds* for the observedporosities as above, using the normalcompaction curve (Fig. 3b). Our ap-proach follows that of Westbrook,1991), except that we use the upper partof the accreted section, rather than theabyssal plain section, for our normalcompaction curve. We infer reducedvalues for s* with respect to normalcompaction and hence l-values in-creasing with depth from 0.49 at 4 kmdepth to 0.60 at 6 km depth. The mudlayers within the turbidites might beresponsible for this slight overpressureby decreasing the permeability. Thede collement horizon, where locally thepore pressure must be close to litho-static (Davis et al., 1983), is also likelyto be controlled by the low basal fric-tion due to fluids trapped in the mudlayers.

Conclusions

Reprocessing and prestack depth mi-gration provide detailed images of theinternal deformation of the Makranaccretionary wedge, and allow us toestimate dewatering parameters suchas the porosity and the ratio of fluidpressure to lithostatic pressure. Fromthese results andanew interpretationofthe nature of the sediment input (mainlysand) we conclude that the Makranaccretionary wedge is at hydrostaticpressure almost throughout. Only thelower part of the Himalayan turbiditesshow slight overpressuring, which islikely tobe causedbydistinctmud layers.

Acknowledgements

This work was funded by the EC HCM-programme and by the NERC. We thankErnst Flueh and Dirk Klaeschen fromGeomar, Kiel for access to computing facilitiesand for the processing support they pro-vided, and Graham Westbrook for a thor-ough review. T.A. Minshull was supportedby a Royal Society University ResearchFellowship. Department of Earth Sciences,Cambridge contribution number 4978.

References

Allen, P.A. and Allen, J.R., 1990. BasinAnalysis: Principles and Applications.Blackwell Scientific Publications, Oxford.

Carmichael, R.S. (ed.), 1984.Handbook ofPhysical Properties of Rocks. CRC PressInc., Boca Raton, FL.

Davis,D., Suppe, J. andDahlen, F.A., 1983.Mechanics of fold-and-thrust belts andaccretionary wedges. J. geophys. Res. 88,1153±1172.

DeMets,C.,Gordon,R.G.,Argus,D.F. andStein, S., 1990. Current plate motions.Geophys. J. Int., 101, 425±478.

Fowler, S.R., White, R.S. and Louden,K.E., 1985. Sediment dewatering in theMakran accretionary prism.EarthPlanet.Sci. Lett., 75, 427±438.

Garzanti, E., Critelli, S. and Ingersoll, R.V.,1996. Paleogeographic and paleotectonicevolution of the Himalayan Range asreflected by detrital modes of Tertiarysandstones and modern sands (Industransect, India and Pakistan). Bull. geol.Soc. Am., 108, 631±642.

Hamilton, E.L., 1978. Sound velocity-density in sea-floor sediments and rocks.J. Acoust. Soc. Am., 63, 366±377.

Harms, J.C., Cappel, H.N. and Francis,D.C., 1984. The Makran coast ofPakistan: Its stratigraphy andhydrocarbon potential. In:MarineGeology andOceanography ofArabianSeaand Coastal Pakistan (B.U. Haq and J.D.Milliman, eds), pp. 3±27. Van NostrandReinhold, New York.

Jacob, K.H. and Quittmeyer, R.L., 1979.The Makran region of Pakistan and Iran:Trench-arc system with active platesubduction. In: Geodynamics of

Pakistan (A. Farah and K.A. de Jong,eds), pp. 305±317. Geological Survey ofPakistan, Quetta.

Minshull, T.A., White, R.S., Barton, P.J.and Collier, J.S., 1992. Deformation atplate boundaries around the Gulf ofOman.Mar. Geol., 104, 265±277.

Platt, J.P., Leggett, J.K., Young, J., Raza,H. and Alam, S., 1985. Large-scalesediment underplating in the Makranaccretionary prism, southwest Pakistan.Geology, 13, 507±511.

Stoffers, P. and Ross, D.A., 1979. LatePleistocene and Holocene sedimentationin the Persian Gulf-Gulf of Oman.Sediment. Geol., 23, 181±208.

Westbrook, G.K., 1991. Geophysicalevidence for the role of fluids inaccretionary wedge tectonics. Phil. Trans.R. Soc. A. 335, 227±242.

White, R.S., 1977. Recent fold developmentin the Gulf of Oman. Earth Planet. Sci.Lett., 36, 85±91.

White, R.S., 1982. Deformation of theMakran accretionary sediment prism inthe Gulf of Oman (north-west IndianOcean). In:Trench and Fore-Arc Geology:Sedimentation and Tectonics on Modernand Ancient Active Plate Margins (J.K.Leggett, ed.), pp. 357±372. BlackwellScientific Publications, Oxford.

White, R.S., 1983. The LittleMurrayRidge.In:Seismic Expression of Structural Styles(A. Bally, ed.).AAPGStud. Geol., 15, 1. 3.19±1. 3. 23.

White, R.S. and Klitgord, K.D., 1976.Sediment deformation and plate tectonicsin the Gulf of Oman. Earth Planet. Sci.Lett., 32, 199±209.

White, R.S. and Louden, K.E., 1982. TheMakran continental margin: structure ofa thickly sedimented convergent plateboundary. In: Studies in ContinentalMargin Geology (J.S. Watkins and C.L.Drake, eds).Mem. Am. Ass. Petrol. Geol.,34, 499±518.

Received 4 March 1997; revised versionaccepted 31 July 1997.

104 #1997 Blackwell Science Ltd

Deformation of the Makran accretionary wedge . J. Fruehn et al. Terra Nova, Vol 9, No. 3, 101±104. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Paper 119 Disc