Gravity and tectonic model across the Sulaiman fold belt and the Chaman fault zone in western Pakistan and eastern Afghanistan

ELSEVIER

Tectonophysics 254 (1996) 89-109

Gravity and tectonic model across the Sulaiman fold belt and the Chaman fault zone in western Pakistan and eastern Afghanistan Ishtiaq A.K. Jadoon a.,, Akbar Khurshid b.1 a Department of Earth Sciences, Quaid-i-Azam University, Islamabad, Pakistan b Geological Suroey of Pakistan, Quetta, Pakistan Received 16 September 1994; accepted 3 July 1995

Abstract

Gravity data from the western boundary of the Indian subcontinent have been analysed to infer the gross crustal structure across the Indian/Afghan collision zone. Seismic reflection profiles reveal the gross structural (duplex) geometry of the Sulaiman fold belt. These data show that the wedge, which is 10 km thick at the deformation front, thickens northwestward to attain a tectonic thickness (thickness due to deformation) of about 20 km in the hinterland. Gravity modeling depicts the depth of the Moho between 33 and 35 km at the deformation front of the Sulaiman fold belt. The Moho depth shows an upward convexity along an E-W profile. It decreases northward with a gentle gradient of 1.1° (20 m/km) below the Sulaiman fold belt, and then deepens abruptly with a gradient of about 7.8° (136 m/km) across the Chaman fault zone, attaining a depth of about 57 km in eastern Afghanistan. The model suggests that the Sulaiman fold belt is underlain by transitional crust (15-27 km thick), in contrast to the continental crust (38 km thick) underneath the fold belt of the Himalayan collision zone in northern Pakistan. The about 57-km-thick crystalline crust in eastern Afghanistan may be due to: (1~ underplating by crust of the Indian subcontinent; and (2) structural thickening within the Afghan block.

I. Introduction Collision of continental crust begins with the impingement of a passive margin on an active one. The orogeny may stop at a stage where rift-related features and a thinner crust could still be intact underneath the mountain belts, as in the Paleozoic

* Corresponding author. Present address: Institute of Geology, University of T~bingen, c / o Prof. Dr. W. Frisch, Sigwartstr. 10, 72076 Tiibingen, Germany. Phone: (49) 7071-295240/14; Fax: (49) 7071-5059. J Present address: Directorate General of Petroleum Concessions, Islamabad, Pakistan.

Ouachitas of Arkansas and Oklahoma of the southcentral United States (Lillie, 1985, 1991; Keller et al., 1989). In other cases, ongoing collision may have progressed to an advanced stage such that all the oceanic crust and features related to a thinner passive margin are highly deformed, as in the main Himalayas (Searle, 1986; Duroy et al., 1989; Malinconico, 1989). Like the other fold-and-thrust belts of Pakistan (Salt Range/Potwar Plateau, Kirthar Range), the Sulaiman fold belt is a consequence of the convergence of the Indian subcontinent and Eurasia that resulted in the ongoing Himalayan collision (Powell, 1979). This system is conspicuous because of its

0040-1951/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0040-195 1(95)00078-X

90

LA.K. Jadoon, A. Khurshid / Tectonophysics 254 (1996) 89-109

700 TIBET

Fig. 1. Simplified tectonic map of Indian/Eurasian collision zone, along the western transprcssional boundary of the Indian subcontinent. Arrow indicates relative drift of the Indian plate with respect to the Afghan block (Minster et al., 1974; Minster and Jordan, 1978). Box shows the location of Fig. 4. A-A' and B-B' locate the Bouguer gravity profiles modeled in Figs. 6 and 7, respectively. C-C' and D - D ' locate the Bougucr gravity profiles shown in Fig. 3. J = Jandran; KB = Kabul Block; M S 0 = Muslimbagh Ophiolitcs; MST= Main Boundary Thrust; MKT = Main Karakoram Thrust; MMT = Main Mantle Thrust; Q = Quetta; SMB = Sibi Molasse Basin; SRPP = Salt Range//Potwar Plateau.

LA.K. Jadoon, A. Khurshid/ Tectonophysics 254 (1996) 89-109

widespread extent over the foreland (Fig. 1), primarily due to effective decoupling of the sedimentary sequence from the underlying basement (Sarwar and De Jong, 1979; Humayon et al., 1991; Jadoon et al., 1992; Davis and Lillie, 1994). Several studies have been carried but to understand the deep structures of the Himalayas. In particular, gravity interpretations by Marussi (1976), Karner and Watts (1983), Verma and Subrahmanyam (1984), Lyon-Caen and Molnar (1983, 1985), Molnar (1984), Malinconico (1986, 1989) and Duroy et al. (1989) provide ideas about the nature and structure of the crust underneath the high Himalayas. In northern Pakistan, the crust is nearly twice the normal continental thickness (about 70 km). To the south, in the Salt Range/Potwar Plateau (Fig. 1), about 3-8 km of sediments are thrust over continental crust (Duroy et al., 1989). However, in the Sulaiman region little is known about the underlying crust. In this paper, an E - W and a N-S gravity profile across the Sulaiman fold belt were evaluated to study the configuration of the Moho. These profiles are constrained by structural sections based on seismic reflection interpretations from the Sulaiman foreland (Fig. 2). The configuration of the Moho was useful in interpreting the overall nature and structure of the crust and to propose a kinematic model of crustal evolution at the western margin of the Indian subcontinent. In the first part of the paper free-air, Bouguer and Airy isostatic anomalies are presented, and some important considerations regarding the Moho configuration and isostatic state of the region are discussed. These results, along with assumptions developed about the shallow crust, are then used as constraints for the construction of 2-D gravity models. For this purpose, forward gravity modeling was done using the GM-SYS computer modeling program. The preferred models are interpreted to show a transitional crust (about 15-27 km thick) related to Mesozoic passive continental margin of the Indian subcontinent, involved in early stages of active collision. Finally, possible physical mechanisms responsible for deformation in the region are discussed and a tectonic/kinematic model, based on the preferred 2-D gravity models, is proposed.

91

2. Tectonic setting The Himalayas and the related structures of Southwest Asia are a result of accretion of island arcs and fragments of Gondwana to the Eurasian landmass, and later closing of the Tethys Ocean prior to collision between the Indian subcontinent and Eurasia at about 55 Ma (Powell, 1979). Paleomagnetic data show that since collision, the Indian subcontinent has moved about 2000 km northwards into Asia (Klootwijk et al., 1985). This ongoing collision has produced the active Himalayan mountain system which extends in a NW-SE direction for about 2500 km through Burma, Nepal, India and Pakistan. The continent-continent collision is interpreted by gravity modeling across the Himalaya in northern Pakistan (Lillie et al., 1987; Duroy et al., 1989) which shows a crust of 38 km (continental thickness) extending from the Jhelum plain beneath the Salt Range/Potwar Plateau and Peshawar Basin to the Main Mantle Thrust (MMT), and a crust of about 65 km below the Kohistan Arc (C-C' in Figs. 1 and 3). Direct collision in northern Pakistan gives way to transpression along the Chaman fault (Lawrence et al., 1981a; Farah et al., 1984). This oblique convergence (Fig. 1) produced the lobate Sulaiman fold belt southeast of the Chaman fault (Sarwar and De Jong, 1979; Klootwijk et al., 1981). The Afghan block, northwest of the Chaman fault, is considered to consist of microfragments, which were detached and drifted from Gondwana, and accreted to the Eurasian landmass from late Paleozoic through Cenozoic times (Bordet, 1978; Boulin, 1981; Tapponnier et al., 1981). The left-lateral strike-slip Chaman fault marks the western boundary of the Indian subcontinent. However, its nature as a crustal shear or a feature restricted to the upper crust is not clear. The broad Sulaiman fold belt southeast of the Chaman fault manifests prograde deformation toward the foreland. It involves Neogene continental molasse strata in the foreland part and a Permian to Paleogene platform sequence in the hinterland (Kazmi and Rana, 1982). On the north and west side of the Sulaiman fold belt Khojak Flysch (dominantly deep-water turbidites) and ophiolites are present (Lawrence et al., 1981 a). Facies changes in the Mesozoic to Paleogene

LA.K. Jadoon, A. Khurslu'd / Tectonophysics 254 (1996) 89-109

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I.A.K. Jadoon, A, ghurshid / Tectonophysics 254 (1996) 89-109

sequence, from west to east and north to south indicate deep-marine to shallow-water shelf environments, reflecting evolution of a passive continental margin during Mesozoic times. Collision processes from Paleocene to Recent times were accompanied by emplacement of the ophiolites along the western margin of the Indian subcontinent (Abbas and Ahmad, 1979). These ophiolites encased in flysch may represent pieces of oceanic lithosphere thrust over the platform sequence during Paleocene to Early Eocene times (Allemann, 1979; Otsuki et al., 1989).

3. Previous work Despite its intriguing geometry and tectonic importance in the framework of collision structures along the Himalayan convergence zone, little geological/geophysical information is available on the Sulaiman structure and evolution of the Sulaiman fold belt. Hunting Survey Corporation mapped the area on a scale of 1:253,000 (Jones, 1961). Structural studies, predominantly based on LANDSAT images, seismicity, magnetostratigraphy and field studies, were done by Abdel-Gawad (1971), Hemphill and Kidwai (1973), Menke and Jacob (1976), Rowlands (1978), Quittmeyer et al. (1979, 1984), Verma et al. (1980), Klootwijk et al. (1981, 1985), Waheed and Wells (1990) and Bannert et al. (1992). These studies interpret the active structure of the area as dominated by N-S-trending, left-lateral strike-slip faults and compressional arcuate folds and faults. Recently, the structure of the Sulaiman fold belt has been more thoroughly investigated, interpreting a duplex geometry, based on the study of seismic reflection lines from the Sulaiman foreland (Banks and Warburton, 1986; Humayon et al., 1991; Jadoon et al., 1992, 1994a).

93

Previously, Rahman (1969) presented a very simplified Bouguer gravity model along a NW-SEtrending profile, along the western margin of the Indian plate (D-D' in Figs. 1 and 3). His model depicts a crustal thickening from about 30 km in the western Sulaiman foredeep to about 55 km (> continental crust of 40 km) along the Chaman fault. Rahman's model could not account for the enormous pile of sedimentary strata (15-20 km thick) from the Sibi molasse basin and the Kirthar fold belt (Banks and Warburton, 1986). Thus, his interpretation may simply be incorrect (Fig. 3). In contrast to the interpretation of Rahman (1969), S-wave studies of earthquake data by Chun (1986) suggest that the crust beneath the Sulaiman fold belt has an oceanic affinity. In this paper gravity data along an E - W and a N-S gravity profile across the Sulaiman fold belt are used to infer the crustal structure at the western margin of the Indian subcontinent (Fig. 2).

4. Gravity data In order to interpret the gross density structure of the Sulaiman fold belt, a Bouguer gravity anomaly map (Fig. 4) has been prepared based mainly on maps of Marussi (1976) and Oil and Gas Development Corporation (OGDC) (unpublished) at contour intervals of 2 mGal. Observed Bouguer gravity values obtained from Marussi (1976) agree well with values given by McGinnis (1971) in Afghanistan and with recently collected data along profile A-,~ in Fig. 2 (Khurshid, 1991). The general trend of the Bouguer gravity anomalies is northeast-southwest along the Indian/Afghan collision zone. Near zero and negative Bouguer gravity values in the Sulaiman foredeep become more negative toward the edge of the Indian subcontinent. The linear gravity anomalies in Fig. 4 agree well with the tectonic features, like

Fig. 2. Bouguer gravity anomaly profiles and 2-D geological sketch along lines A-A' and B - B ' modeled in Figs. 6 and 7. respectively. The values between km 0 and about 150 along the E - W profile are incorporated from AMOCO Pakistan Exploration Company map (unpublished) and new data obtained from km 150 m 470 near the Chaman fault (Khurshid, 1991). The values between km 0 and 250 along the N-S profile are from the Oil and Gas Development Corporation (OGDC) map (published in part for the southern Indus basin by Quadri and Shuaib (1986) and between km 250 and 800 from Mamssi (1976). Depth to the basement and structural interpretation in the foreland of the eastern and southern Sulaiman fold belt is from Humayon et al. (1991) and Jadoon et al. 0992, 1994a,b). CF = Chaman Fault; KFL ~ Katawaz Flysch Basin (Neogenc); MBO = Muslimbagh Ophiolite; UD = undifferentiated.

94

LA.K. Jadoon, A. K h u r s h i d / Tectonophysics 254 (1996) 89-109

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LA.K. Jadoon, A. Khurshid / Tectonophysics 254 (1996) 89-109

95

Fig. 4. Bouguer gravity anomaly map of Indian/Afghan collision zone. The map is compiled mainly from Mamssi (1976) and Oil and Gas Development Cm'lg~tion of Pakistan data, partially published by Quadri and Shuaib (1986). Contour interval is 25 mGal. A-A' and B-B' locate the Bouguer gravity profiles modelled in Fig. 6 and Fig. 7, respectively. See Fig. 3 for D-D'.

96

I.A.K. Jadoon, A. Khurshid / Tectonophysics 254 (1996) 89-109

the Chaman and Herat faults, but the gravity anomalies at contour interval of 25 mGal are too coarse to reveal any anomalies related to shallow structures. For more detailed modeling (to incorporate shallow structures), an E - W gravity traverse was run along the road from D.G. Khan to Chaman in order to interpret the gross density structure of the Sulaiman fold belt (Fig. 5). Along a new profile ( A - g in Fig. 5), gravity data were observed at 2-2.5 km intervals (Khurshid, 1991). Both Marussi (1976) and Khurshid (1991) used a reduction density of 2.67 g / c m 3 for Bouguer corrections. To the east, between D.G. Khan and Marot, the gravity values were incorporated from the Bouguer gravity map of AMOCO Pakistan Exploration Company (unpublished). The observed Bouguer gravity values are projected on profile A-,~ in Fig. 5 running almost parallel to the road at 5 km intervals. Short-wavelength anomalies along the profile are useful to constrain the basement structures. The elevation of each gravity station along profile A-,~ was measured by an altimeter and was considered correct within 5 m, as these points were tied to local triangulation points established by the Survey of Pakistan. Average elevation in the fold belt is about 1400 m and in the foredeep it is just over 100 m. Terrain corrections were applied; however, they only range from 0 to 5 mGal. The Bouguer gravity anomalies in Fig. 4 agree

well with profile A-,~ in Fig. 5. However, discrepancies occur with Bouguer gravity values of Rahman (1969) (D-D' in Figs. 3 and 4). Rahman did not mention what density he used for Bouguer reduction. However, the low Moho contrast (0.15 g / c m 3) used in his model to calculate crustal thickness suggests that a reduction density higher than the standard 2.67 g / c m 3 was used, leading to relatively low Bouguer gravity values. In this paper, the Bouguer gravity modeling along profiles A - g and B-B' (Figs. 1 and 2) is presented to enable discussion of the crustal structure along the western margin of the Indian subcontinent.

5. Constraints and assumptions A unique interpretation based on gravity observations is almost impossible because of the superimposed effects of many mass distributions; the free-air and Bouguer gravity fields represent the combined effects of shallow and deep sources. Since the primary objective of this paper is to interpret the gross crustal structure and configuration of the Moho, constraints and assumptions on the shallow crust facilitate gravity modeling for the crustal variation along the western boundary of the Indian subcontinent. As most of the major structures in the Sulaiman

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LA.K. Jadoon, A. Khurshid/ Tectonophysics 254 (1996) 89-109

fold belt are linear, 2-D modeling is a reasonable approach to interpret the Bouguer gravity anomalies observed in the area. The location of the profiles was chosen on the basis of seismic reflection interpretations and balanced sections from the southern (Jadoon et al., 1992, 1994a) and eastern (Humayon et al., 1991) Sulaiman foreland (Fig. 2). These data provide constraints for sediment thickness and the top of the crystalline basement in the Sulaiman foreland. However, deep inside the fold belt sediment thickness and top of the crystalline basement can only be estimated from surface geology and by extrapolating information from the seismic data in the foreland (Jadoon et al., 1994b). Thin sediments and shallow depth of the basement surface in the Afghan block northwest of the Chaman fault along profile B-B' (Fig. 2) are based on surface geology and magnetic modeling (Andfitzky et al., 1971; Wittekindt and Weippert, 1973; Bordet, 1978; Boulin, 1981; Tapponnier et al., 1981). Profile A - , g is generally oblique to the structures, while profile B - B ' is across strike and almost parallel to the plate convergence vector (Fig. 1; Minster et al., 1974; Minster and Jordan, 1978). At the Marot well (Fig. 1) the basement is encountered at about 2 km depth and dips at about 2.5 ° towards the west (Humayon et al., 1991). Near the deformation front in the eastern Sulaiman foreland it is about 8 km deep and extends off the bottom of seismic lines at 5 s two-way travel time data. Similarly, basement is about 10 km deep near the deformation front in the southern Sulaiman foreland (Jadoon et al., 1992) and its depth increases to about 15 km near Jan&an along profile B-B' in Fig. 1 (Jadoon et al., 1994a). If one extrapolates the basement surface inside the fold belt, assuming the same gentle dip along both the eastern and southern Sulaiman profiles, it deepens to over 20 km in the Sulaiman hinterland. This is consistent with observations from the western Sulaiman fold belt (Banks and Warburton, 1986). The Mesozoic-Cenozoic sediments are primarily composed of limestone, dolomite, sandstone and shale, the densities of which vary between 2.17 and 2.77 g / c m 3 (Dobrin and Savit, 1988). Interval velocities from seismic profiles, lithological descriptions and density logs from the Marot well (Humayon et al., 1991; Jadoon et al., 1992) provide estimates for

97

Table 1 Estimated averagedensitiesof geologicand structuralunits Geologicand Approximate Approximate structural units velocity density (kin/s) (g/cm 3) Molasse (Tertiary) Flysch(Tertiary) RoofSequence (Eocene-Cretaceous) Duplex Sequence (Jurassic and older) Crust

2.5-3.0 no seismic available 2.8-4.5

2.3 2.55-2.65

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Note: The P-wavevelocitiesare from seismicreflectioninterpretations fromthe southern(Jadoonet al., 1992)and eastern(Humayon et al., 1991) Sulaiman fold belt. The approximatedensities are estimated from the Nafe and Drake (1963) curve. Crustal density is based on earthquake seismic studies (Menke and Jacob, 1976; Seeber and Armbruster, 1979; Kaila, 1981).

subsurface densities used in the gravity models. In addition Nafe and Drake (1963) curves were utilized in estimating densities. Table 1 shows estimates of densities for different rock units encountered along the gravity profile. The densities assumed for the crust is 2.8 g / c m 3. This Value is below the density range determined by Chun (1986) for the western part of the Indian subcontinent crust. He determined shear wave velocities to be between 3.1 and 3.8 k m / s for the crust, which he associated with densities between 3.0 and 3.08 g / e r a 3, leading him to suggest that the crust beneath the Sulaiman fold belt has an oceanic affinity. On the same basis, densities (Churl, 1986) determined for the upper 10 km of sediments are between 2.66 and 2.72 g / c m 3. These densities appear to be systematically high. Menke and Jacob (1976), Seeber and Armbruster (1979) and Kaila (1981), on the basis of earthquake seismic studies, interpreted Pwave velocities of 5.8 to 7 k m / s for the crust. Based on these values, the Nafe and Drake (1963) curves suggest densities from 2.65 to 3.00 g / c m 3 for the crust. We have used an average density of 2.8 g / c m 3 for the crust, a standard density contrast of +0.4 g / c m 3 between the crust and upper mantle, and - 0 . 1 5 to - 0 . 5 g / c m 3 for sediments.

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LA.K. Jadoon,A. Khurshid/ Tectonophysics254 (1996)89-109 6. G r a v i t y a n o m a l i e s

6.1. East-west profile A-A' 6.1.1. Free-air gravity anomalies Gravity anomalies relative to each other and to topography along profile A-/~ are displayed in Fig. 5. Free-air anomalies generally mimic the topography of an area. The highest free-air gravity values ( + 100 mGal) are observed in areas of high elevation in the fold belt, with two prominent highs (about 135 reGal) over the Fort Munro anticline and Ziarat, while the lowest values (about - 100 mGal) are just east of the deformation front(near D.G. Khan) in the foredeep. Comparison of highest anomalies to topography (1.6 and 2.6 km, respectively) suggests some contribution from high-density material at depth in the Fort Munro-Loralai area. On the western end of the profile, free-air gravity values are near zero, suggesting that roots corresponding to topography produce isostatic compensation in this region. 6.1.2. Bouguer gravity anomalies Bouguer gravity anomalies along profile A-,~ generally decrease toward the west, with a low over the foredeep and a high over the eastern Sulaiman foreland, separated by a gradient of 1.6 mGal/km (Figs. 2 and 5). In the foredeep region, short-wavelength variations are primarily due to shallow-depth structures (including sedimentary), which are apparent on seismic reflection profiles (Humayon et al., 1991). Values decrease to - 1 1 6 mGal near D.G. Khan and abruptly increase to - 3 7 mGal at Fort Munro. They remain relatively high for some distance to the west, decreasing slightly at a rate of about - 0 . 3 mGal/km. Still further west, between Loralai and Ziarat, Bouguer anomalies decrease rapidly from - 7 5 to - 150 reGal at a rate of about

99

- 1.0 mGal/km. West of Ziarat the Bouguer gravity field is relatively flat, with three short-wavelength apparent anomalies.

6.1.3. Airy-isostatic anomalies To compute Airy isostatic anomalies, Bouguer anomalies were calculated that would result if the topography along the profile was compensated only by a crustal root (Fig. 6A). These computed anomalies were then subtracted from the observed Bouguer profile to yield the Airy Isostatic anomalies in Fig. 5. Isostatic gravity anomaly values in the area range from - 7 2 to + 56 mGal, and the average value is + 3 mGal, suggesting very general compensation of the area; however, there are zones that systematically deviate from the equilibrium (Fig. 5). Major patterns are a negative anomaly in the foredeep region, the steep gradient between D.G. Khan and Fort Munro, a long-wavelength positive anomaly west of Fort Munro and various short-wavelength anomalies. The steep gradient between D.G. Khan and Fort Munro separates low values over the foredeep from higher values over the fold belt. The Airy isostatic anomaly values increase by 118 mGal over this zone. Similar isostatic gravity anomalies are noticed at many oceanic-continental boundaries (Rabinowitz and Labrecque, 1977). In the foredeep region, close similarity of shortwavelength anomalies with free-air and Bouguer gravity anomalies appears to be due to dominance of structures at shallow depths. However, the isostatic anomaly is calculated by assuming the density contrast between the crust and mantle, and subtracting the root anomaly from the Bouguer gravity anomaly (Fig. 6A); deviations of the anomaly from zero may represent variations in Moho depth. In the Fort Munro-Loralai region the broad-wavelength positive isostatic anomaly may suggest undercompensation

Fig. 6. 2-D densityand tectonicmodels for observedBouguergravity anomalyalong the E-W profile across the Sulaiman fold belt. (A) Roots calculated corresponding to surface topography, for normal crust of 35 km thickness at sea level, using the Airy Isostatic compensationmodel.Comparisonof calculated root anomalywith the observedBouguerprofilesuggests that rootsexplain only parts of the observed profile. Subtractionof the calculatedroot anomalyfrom the observedBouguerprofilegives the airy isostatic anomalyshown in Fig. 5. (13)Comparisonof the observedBousneranomalyand the calculatedanomalyof the 2-D densitymodel drawn, using the constraints and assumptions discussed in the text and the Moho depth obtained in (A). (C) Preferred 2-D density model showing the presence of transitional crust below the thick sedimentsover the basement. Densitycontrasts are in g/cm 3 relativeto the crystallinecrust. Densityfor the crystallinecrust, includingophiolites (black), was assumed to be 2.8 g/em 3 based on P-wavevelocitiesfrom earthquakes(Menke and Jacob, 1976; Kaila, 1981)(text for discussion).

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due to shallower than expected mantle material. It may be a southward extension of the shallow mantle identified in the Muslimbagh region (Fig. 1; Farah and Zaigham, 1979), possibly inherited from Mesozoic rifting. In the western extremity, beyond Ziarat, the isostatic anomalies are near zero, suggesting local isostatic compensation of the region. Superimposed, three short-wavelength anomalies may represent a shallow, high-density mass in the Ziarat region and isolated ophiolite bodies in the subsurface (Farah and Zaigham, 1979; Gansser, 1981). The free-air anomalies also approach zero west of Ziarat. The relatively low free-air values for such high topography at Ziarat may suggest mass deficiency in the deep subsurface. On the Bouguer anomaly, a steep westward gradient, west of Loralai, suggests deepening of the Moho and subsequent thickening of the crust to the west. The short-wavelength Bougner anomaly (positive isostatic anomaly as well) present immediately at the lower end of the westward gradient, at Ziarat, is likely to be from a shallow source. Jurassic to Cretaceous strata are exposed in the Ziarat region and topography is high. It is possible that this shallow-depth, high-density source beneath Ziarat may be a basement high, while the remaining two short-wavelength anomalies are due to subsurface ophiolite bodies (Fig. 6). 6.2. North-south profile B-B' The Bouguer gravity anomalies along profile B-B' (Fig. 2) consistently decrease northwestward from near zero regal in the Sulalman foredeep, to about - 1 9 0 mGal along the Chaman fault and - 2 6 5 regal in central Afghanistan (Fig. 2). The Bouguer gravity profile has a general gradient of -0.35 mGal/km towards the north. This can be interpreted as a combined result of the sediment thickness and the Moho depth variation, suggesting that the gravity effect of low-density sediments is compensated by high-density mantle material southeast of the Chaman fault. In the foreland region south of the Sui, a low-amplitude and long-wavelength anomaly may be due to a Mesozoic rift-related structural high (Raza et al., 1989; Kemal et al., 1991). The gravity effect of a similar high is indicated by close contours of 0 to

101

+ 25 mGal in the foredeep region of the Sulaiman fold belt (Fig. 4). North of the Sui, another low-amplitude and long-wavelength anomaly may be due to structural uplift of relatively high density duplex related sediments (Figs. 2 and 7). Further to the north, the Bouguer gravity effect of the exposed Muslimbagh and other hidden ophiolites is apparent from two relatively short-wavelength anomalies. It is striking that, despite over 20 km of sediments to the south and exposed crystalline crust to the north of the Chaman fault, the Bougner gravity profile generally remains smooth. This is attributed to the Moho configuration above and discussed later in this paper. However, juxtaposition of high-density crystalline crust of the Afghan block against the low-density sediments is reflected by a Bouguer gravity low and high across the Chaman fault (Figs. 2, 4 and 7).

7. Construction and discussion of models

7.1. East-west profile A-A' The result of gravity modeling along E-W profile A-/¢ is revealed in Fig. 6. The roots necessary for the region to attain Airy isostatic equilibrium according to the assumptions made are shown in Fig. 6A. The comparison of the observed Bouguer anomaly with the calculated root anomaly suggests that portions of the region are compensated in a manner that differs significantly from simple Airy isostatic equilibrium. Subtraction of the calculated root anomaly from the observed Bouguer anomaly in Fig. 6A thus yields the Airy isostatic anomaly shown in Fig. 5. A 2-D density model constructed using the above-mentioned constraints, assumptions and the shape of the Moho from the crustal root calculated in Fig. 6A, is depicted in Fig. 6B. The calculated anomaly from the model shows a low toward the foreland and high over the hinterland. This anomaly is in agreement with the observed Bouguer anomaly in the eastern and western extremity of the gravity profile, but the long-wavelength discrepancy between the foreland and hinterland might suggest shallowing of the Moho to match the relatively high Bouguer anomaly. The same 2-D density model as in Fig. 6B is depicted in Fig. 6C, but the Moho adjusted so that

102

LA.K. Jadoon, A. Khurshid / Tectonophysics 254 (1996) 89-109

the observed and calculated Bouguer anomalies are in close agreement. A large discrepancy in the observed and calculated Bouguer anomalies between Fort Munro and Loralai is in the region where gravity values could not be observed in the field due to tribal problems. Hence, the values in this region were approximated between the values observed along the roads and the nearby gravity contours. The Moho configuration in the preferred model (Fig. 6C) is dominated by a long-wavelength upward convexity of the crust below the eastern Sulaiman foreland and associated foredeep. In the foredeep region gravity anomalies are of low amplitude and there are no major differences between the free-air, Bouguer and isostatic anomalies (Fig. 5). Shortwavelength anomalies are caused by local structures identified on seismic profiles in the region (Humayon et al., 1991). West of the deformation front, positive free-air and isostatic anomalies suggest undercompensation, perhaps by mantle material that is shallower than expected in the region west of D.G Khan (Fig. 6C). In the central part of the model, the Moho descends steeply between Loralai and Ziarat and then flattens out west of Ziarat near the Chaman fault. Short-wavelength Bouguer anomalies in this region are interpreted as the result of a basement high below Ziarat and isolated bodies of ophiolites near the Chaman fault region. The ophiolites have been mapped in the field in that region (Abbas and Ahmad, 1979). Generally flat configuration of Moho west of Ziarat suggests local isostatic equilibrium in the region, which is evident from near zero free-air and Airy isostatic gravity values (Fig. 5). Although no evidence is available to substantiate the existence of the Ziarat basement high, it is plausible considering the nature of the crust in the region. Outer margin basement highs have developed on many rifted continental margins, such as the Gulf of Mexico (Hall et al., 1983), along the southwest African continental margin (Gerrard and Smith, 1983), and in the Carolina Trough region (Hutchinson et al., 1983). Such highs are relatively narrow features (10-50 km) and may extend roughly parallel to the margin for hundreds of kilometers (Hall et al., 1983). The Ziarat basement high may be a similar feature, thus forming a ramp for the emplacement of ophiolites and thrust sheets during compres-

sional orogeny (R.D. Lawrence, pers. commun., 1990). Alternately, the arching of the basement occurred during later compressional movement, in a manner analogous to the Benton uplift in the Ouachita mountains (Lillie et al., 1983). Reverse faulting of the crystalline basement might have resulted in vertical movement of a portion of the continental margin as the Ziarat basement high. This reverse movement could have been accomplished by the reactivation of old normal faults developed during Mesozoic tiffing, as in the Zagros region (Jackson, 1980). The interpretation shows Moho at a depth of about 36 km below the Marot where basement surface is encountered at about 2 km depth. The Moho is modeled at a depth of about 33 and 42 km near the deformation front and the Chaman fault where the top of the basement is assumed at a depth of about 8 and 20 km, respectively. Thus, about 34-km-thick crust below the Marot in the eastern Sulaiman foredeep thins to 25 and 22 km near the deformation front and the Chaman fault, respectively. Below Loralai the crust is about 15 km thick. The roots necessary for the region to attain simple Airy isostatic equilibrium according to the assumptions made, are shown in Fig. 6A. In contrast, the preferred model in Fig. 6C depicts transitional crust below the Sulaiman fold belt. The transitional crust region is wide and varies from about 15 to 27 km in thickness, as along many passive margins. For example rift-stage crust with thickness ranging from 7 to 28 km has been observed along the Atlantic margin of North America (Grow and Sheridan, 1981; Hutchinson et al., 1983). The transitional crust (1527 kin) below the wide Sulaiman subbasins, filled with marine to shallow-marine sediments (Raza et al., 1989), may suggest continuation of a similar rift-stage crust adjacent to the western margin of the Indian subcontinent. Two levels of deformation can be associated with the model in Fig. 6C. East of Ziarat deformation appears to be thin skinned, consistent with seismic reflection interpretations from the Sulaiman foreland (Banks and Warburton, 1986; Humayon et al., 1991; Jadoon et al., 1992, 1994a). This is supported by the existence of a wide zone of foreland thrusting with gentle topography over a weak decollement (Davis and Lillie, 1994), shallow seismicity (Quittmeyer et

I.A.K. Jadoon, A. Khurshid/ Tectonophysics 254 (1996) 89-109

al., 1979, 1984), paleomagnetic studies from the internal (Klootwijk et al., 1981, 1985) and external (Ahmad and Khan, 1990) part of the fold belt, and by paleocurrent directions (Waheed and Wells, 1990). Thick-skinned deformation may occur within the interior of the fold belt, forming the Ziarat basement high and other structures further west. However, present gravity modeling is not sufficient to substantiate this hypothesis.

7.2. North-south profile B-B' The result of Bouguer gravity modeling along N-S profile B-B' is revealed in Fig. 7. The Moho configuration is dominated by upward convexity in the foreland, a general gradient of -0.35 mGal/km towards the north and a steep gradient below the Chaman fault. This can in general be interpreted as a combined result of the sediment thickness and Moho depth variations (Figs. 2 and 7). The effect of these contributions on the observed Bouguer gravity profile is shown in Fig. 7A, B; Fig. 7A (sediment contribution) shows the superimposed gravity lows and highs due to low-density molasse sediments in the Sulaiman foredeep, high-density Muslimbagh ophiolites, and the .crystalline crust of the Afghan block against the Khojak flysch north of the Chaman fault; Fig. 7B (mantle contribution) is obtained by subtracting gravity effect of sediments in Fig. 7A from Fig. 7C and shows the negative northward gradient due to the northward-dipping Moho with a gentle inclination of about 1°. This overall gradient is modified by a slight upward convexity of the Moho in the Sulaiman foredeep region and a steeper Moho gradient at the margin of the Afghan block. These effects are interpreted as a result of flexural bending in the Sulaiman foredeep and thickening of the crust of the Afghan block. The best match between observed and calculated anomalies is shown in Fig. 7C. This is obtained by combining the effect due to sediment and mantle contributions (Fig. 7A, B). The gravity model depicts the depth to the Moho as about 35 km at the deformation front of the Sulaiman fold belt. The Moho is flexed upward in the foredeep. North of the foredeep it has a gentle northward dip of about 1.1° (20 m/kin) until it approaches the Chaman fault zone. The depth to the Moho below the Chaman

103

fault zone is about 42 km along profile B-B'. It deepens abruptly across the Chaman fault zone from about 42 km south of the Chaman fault to about 57 km north of the fault. A steep northward dip of 7.8° (136 m/km) of the Moho below the Chaman fault zone results from this model. The Moho regains its gentler northward dip north of the Chaman fault system. A 57 km Moho depth in eastern Afghanistan is similar to previously interpreted crustal thicknesses of 53 km in central Afghanistan (McGinnis, 1971). Twenty-seven-km-thick crystalline crust in the southern Sulaiman foredeep, similar to the eastern Sulaiman foredeep is shown in Fig. 7C. The crust thins northward and is modeled to show a thickness of about 20 km east of the Chaman fault and about 57 km below the Afghan block. A preferred tectonic interpretation of the density model in Fig. 7C is represented by Fig. 7D. The crustal model proposed in Figs. 6C and 7D from the Sulaiman foredeep across eastern Afghanistan (B-B' in Fig. 1) has several important tectonic implications listed below: (1) The two profiles (A-,g and B-B' in Figs. 6(2 and 7D, respectively) suggest that the Sulaiman fold belt overlies a broad ( > 300 km wide) transitional crust related to the western, pre-collisional, passive margin of the Indian subcontinent. Crust closer to oceanic thickness is apparently underthrusting the Afghan block beneath the Chaman fault zone (Fig. 7D) as the distal end of the Indian plate crust, similar to as inferred by Izatt (1990). (2) Across the Chaman fault, crystalline crust thickens dramatically to about 57 km in eastern Afghanistan due to (a) structural thickening within the Afghan block and/or (b) underplating by oceanic crust of the Indian subcontinent. (3) Bouguer gravity modeling along A-,~ (Fig. 6) and B-B' (Fig. 7) constrains the attitude of the Moho below the Sulaiman fold belt. Crustal variation along these profile suggests a general dip of 1°, toward N57°W, for the Moho below the Sulaiman fold belt. Seismic and well data show nearly the same dip direction for the top of the basement, but a larger dip of about 3° (Jadoon et al., 1992). Thus, the strikes (N33°E) of the top and of the base of the northwestward thinning crust are about the same (Fig. 8).

104

l A X . Jadoon, A. Khurshid / Tectonophysics 254 (1996) 89-109

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(4) The precollisional (Cretaceous) western passive margin of the Indian subcontinent could have resembled the Blake Plateau Basin of the US Atlantic continental margin (Grow and Sheridan, 1981; Klitgord et al., 1988) in that both margins have a post-rift platform sequence more than 7 km thick, and a broad (about 300 km wide) transitional crust with an average thickness of about 20 km. (5) The intact transitional crust (15-27 km) under the Sulaiman fold belt contrasts with the full thickness of crystalline crust (about 38 km) underneath the Salt Range/Potwar Plateau in northern Pakistan (Duroy et al., 1989). This suggests a smaller amount of convergence along the western margin of the Indian subcontinent. (6) A simplified 3-D model (Fig. 8) based on observations from gravity modeling illustrates the thickness of the crust and geometry of the surface and deep structures along the western margin of the Indian subcontinent. It shows underplating of the Indian crust below the Afghan block and transpression-related structures over a deep d~collement in the Sulaiman fold belt. 8. Kinematic model of crustal development The transitional crust below the Sulaiman fold belt would have been part of Gondwana prior to its

breakup, during late Paleozoic to Early Miocene times. The breaking away of the Indo-Pakistani block from the Gondwana Supercontinent is documented by an Early Permian unconformity (Raza et al., 1989; Humayon et al., 1991). This unconformity probably marks a period of erosion associated with thermal and tectonic uplift during the rifting stage of the craton (e.g., Falvey, 1974). Subsequent tectonic processes, such as subsidence caused by thermal cooling of the lithosphere and sediment loading of thin extended basement, led to the development of the thick wedge of Mesozoic sediments along the western margin of the Indian subcontinent. An evolutionary diagram of tectonic development along the western margin of the Indian subcontinent is presented in Fig. 9. It shows an Atlantic type passive continental margin in the Jurassic (Fig. 9A). Deformation of the margin started along a northward subducting slab of Tethys oceanic lithosphere along the southern margin of paleo-Asia (Fig. 9B). This produced the mid-Cretaceous Kandahar andesitic arc (Lawrence et al., 1981b; Farah et al., 1984; Debon et al., 1986). Oblique collision of the northwestern margin of the Indian subcontinent, the future Sulaiman area, started by the Paleocene to Early Eocene emplacement of the Muslimbagh ophiolites (A1lemann, 1979; Otsuki et al., 1989). This event is constrained by the emplacement of ophiolites over

I.A.K. Jadoon, A. Khurshid/ Tectonophysics 254 (1996) 89-109

Maastrichtian sheff sediments and onlap of Eocene platform strata (Fig. 9(2; Allemann, 1979; Otsuki et al., 1989). During the emplacement of the ophiolites, distal, deep-marine facies of Triassic rocks were scraped from the downgoing plate and were transported southeastwards beneath the translating oceanic lithosphere (Otsuld et al., 1989). These authors suggest that these exotic facies, which include some basalt flows, were deposited near a mid-ocean ridge in

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105

relatively shallow water. They may have travelled 200-300 km during emplacement of the Muslimbagh ophiolite. Deposition of the Khojak flysch occurred on the remaining oceanic lithosphere between the Eocene and the Late Oligocene, with the early Himalayan uplift as the most likely sediment source (R.D. Lawrence, pers. commun., 1991). Continued oblique convergence in the Late Oligocene to Early Miocene (25 ± 5 Ma?) resulted in the final closure of the

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106

I.A.K. J adoon, A. Khurshid / Tectonophysic s 254 (1996) 89-109

ocean, the initiation of the left-lateral strike-slip Chaman fault system, and deformation of the Khojak flysch (Fig. 9D). Convergence of the Indian subcontinent along its western margin is responsible for the development of the Sulaiman fold belt and the enormous thickness of the molasse sediments (Banks and Warburton, 1986; Waheed and Wells, 1990). Since the Miocene 353 :t: 25 km of shortening in a NNWSSE direction has been observed in the cover sediments of the Indian subcontinent (Jadoon et al., 1994b). The present model (Fig. 9E), based on density modeling (Fig. 7), shows that the structures at the surface (including the sinistral Chaman fault and the Sulaiman fold bel0 may be restricted to a brittle flake above a d~collement zone. Below the d~collement oceanic/transitional crust related to the former Mesozoic passive margin of the Indian plate is preserved and is underthrnsting the Afghan block. This suggests deformation partitioning with transpression in the thin-skinned brittle wedge above and pure translation of the distal end of the rigid Indian plate below the d~collement. The above model supports the previous studies suggesting deformation partitioning between upper and lower crust (Nakata et al., 1990) and continental escape tectonics (Treloar and Coward, 1991) at the western margin of the Indian subcontinent. It is similar to the flake tectonic hypothesis (Oxburgh, 1972) proposed for the transverse ranges (Yeats, 1981), central California margin (Crouch et al., 1984) and northern Pakistan (Seeber, 1983). Crouch et al. (1984) have proposed that the shortening along the strike-slip and thrust faults in the upper crust is separated from a lower, possibly oceanic crustal layer, along an aseismic zone of dtcollement. The Pacific/North American plate boundary is located to the east of the San Andreas fault in their model, similar to the presence of rigid Indian plate to the west of the Chaman fault in the Sulaiman model (Figs. 8 and 9E).

9. Conclusions Gravity modeling from the Sulaiman foredeep in Pakistan across the Chaman fault zone in eastern Afghanistan suggests that the Sulaiman fold belt is a northward thickening wedge of sediments thrust over

transitional crust of the western passive margin of the Indian subcontinent. Comparison of gravity anomalies (free-air, Bouguer, isostatic) suggests that the Sulaiman fold belt deviates significantly from ideal Airy isostatic equilibrium. Its foreland (Fort Munro-Loralai topography) lacks crustal roots due to shallowing of the Moho, suggesting undercompensation (mass excess), while the hinterland (Ziarat and northwestern topography) does have roots and is generally in a state of local isostatic equilibrium. Gravity modeling suggests that a crystalline crust of about 34 km below Marot in the eastern Sulaiman foredeep thins westwards to become about 25 and 22 km thick below the deformation front and the Chaman fault, respectively. Crystalline crust of about 27 km thickness in the southern Sulaiman foredeep thins northward to become 20 km thick below the hinterland of the Sulaiman fold belt. The Moho that generally dips gently to the northwest, steepens abruptly along the Chaman fault system. The resulting geometry is crystalline crust thickening to about 57 km in eastern Afghanistan. This thick crystalline crust may be due to: (1) structural thickening within the Afghan block; and (2) underplating by crust of the Indian subcontinent. A geological model suggests that the western Mesozoic passive NNE-trending and NW-dipping margin of the Indian subcontinent is underthrusting the Afghan block along a d6collement at about 15 km depth in the hinterland of the Sulaiman fold belt. Deformation partitioning is occurring along the Indian/Afghan collision zone by pure translation of the rigid Indian plate below the d~collement, transpression with internal deformation (including the sinistral Chaman fault and the Sulaiman thrust system) in the thin-skinned brittle wedge above, and buttressing by the relatively rigid Afghan block.

Acknowledgements We thank R.J. Lillie and R.D. Lawrence for being excellent supervisors at the Oregon State University where we were funded by United States Agency for International Development (USAID) for a Ph.D and a M.Sc. dissertation. This work was compiled for publications at the University of Ttibingen during a post doctoral research fellowship (1994-1995)

I.A.K. Jadoon, A. Khurshid / Tectonophysics 254 (1996) 89-109

awarded to I. Jadoon by Alexander von Humboldt Stiftung (AvH), Germany. Russell Nazuruilah of the Geological Survey of Pakistan is thanked for the use of gravity data he recorded across the Sulaiman fold belt. Northwest Geophysical Associates of Corvallis donated the GM-SYS program to Oregon State University for gravity modeling. The manuscript benefited from pre-submission reviews, comments and discussions with R.J. Lillie, R.D. Lawrence, L. Seeber, K. De Jong, Bob Yeats, Alan Niem, Jon Kimerling, V. Langer, M. Ali, R. Ahmed, Jochen B., Mirza S. Baig, Azim Ali Khawaja and Gary Huftile. C.McA. Powell, E. Banda and an anonymous reviewer are acknowledged for constructive criticism for publication in Tectonophysics. I. Jadoon thanks W. Frisch (T'Sbingen) for hospitality and laboratory facilities, and P. O'Shea for final upgrading of the English.

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