Gravity Field of the Buried Shield in the Punjab Plain, Pakistan

Gravity field of the buried shield in the Punjab Plain, Pakistan

ABUL FARAH MOHAMMAD A. MIRZA MOHAMMAD A. AHMAD Geological Survey of Pakistan, Quetta, Pakistan MOHAMMAD H. BUTT

ABSTRACT out by the Geological Survey of Pakistan, the Oil and Gas Devel- opment Corporation of Pakistan, and American Oil Company, The regional Bouguer anomaly data of the Punjab Plain, Salt Pakistan Exploration. The coverage provides one gravity observa- Range, and Potwar Plateau, covering an area of about 135,000 km2 tion for every 5 km2 along profiles separated by an average of 3 to 4 (lat 29°00' to 33°30'N, long 71°00' to 74°30'E), have been deter- km. mined. Inferences have been drawn on the assumption that the The Punjab Plain is bordered by the Salt Range in the north, gravity anomalies reflect the buried features of the Precambrian which separates the plain from the Potwar Plateau. In the north- basement, with density of 2.7 g/cm3 as compared to an average of east, the plateau joins the northwest-striking range belonging to the 1.9 g/cm3 for the sedimentary cover. The Sargodha-Shah Kot ridge Sub-Himalayas (Fig. 2). In the west the plain is bordered by the of the Punjab Plain is inferred to be a horstlike block of raised con- Trans- Indus-Marwat Ranges. The average elevation of the Punjab tinental crust extending northwestward with gentle dips. Our in- Plain is 200 m above sea level, of the Salt Range 1,000 to 1,200 m, terpretation of the gravity data indicates that the shield elements and of the Potwar Plateau 400 to 500 m. with increasing sedimentary cover can be followed northward be- The entire region of the Punjab Plain is covered by alluvial depos- neath the southward thrust of the Himalayas, as suggested by its. Inliers of upper Siwalic sedimentary rocks (Kharian Inliers) Gansser. occur immediately south of the Jhelum River. Scattered outcrops of Precambrian crystalline basement are found near Sargodha, INTRODUCTION Chiniot, Sangla, and Shahkot. The hills near Sargodha and Chiniot strike northwestward and those near Sangla and Shahkot extend Geological and geophysical studies in Pakistan have been largely northeastward. The rocks consist of fine- and medium-grained devoted to exploration for oil, mineral, and water resources. Ac- massive quartzite and thinly bedded shale and commonly have cumulation of regional geological and geophysical (gravity and igneous intrusions. Sedimentary rocks occupy the Salt Range and magnetic) data make it possible to apply these data to conceptual the Potwar Plateau. geodynamic problems. Two such problems stand out in Pakistan geology: (1) Himalayan geology in the context of the relation be- PROCEDURE OF REDUCTION tween the Peninsular shield and the southern margin of Asia — AND INTERPRETATION whether there was a link between the two old platforms or whether the junction arose along a geosuture as a result of plate tectonics; The gravity anomalies have been reduced in absolute terms by (2) the meaning of the radically contrasting tectonics of the Kirthar determining the relation of each observed station to the value of the and Makran ranges. Karachi gravity base (978.9630 cm/s2), which in turn is linked to It is essential to have a panoramic picture of the distribution of the Teddington and Washington pendulum stations (Woollard, gravity and magnetic fields and seismic velocities in the northern 1950). A density value of 1.8 g/cm3 has been used for Bouguer cor- Punjab Plain, through the Salt Range and Potwar Plateau, and a rection. It may be somewhat higher than the actual value, especially little beyond the Karakorum-Kashmir Himalayan front to deter- in the areas of the plain. However, as this value was used by vari- mine the geologic relationships and to analyze the orogenic wave- ous oil companies and the Oil and Gas Development Corporation, front of the Himalayan geosynclinal belt and the northern pro- the same value has been used by the Geological Survey of Pakistan. montory of the Gondwana foreland. Gansser (1964, p. 20) noted The accuracy of the density value for Bouguer correction is as- that "the direct relations of the shield elements with the Himalayas sumed to be ±0.2. The elevations in the plain are considered to be are masked today by the extensive Quaternary foreland cover; in correct to within 1.5 m and those in the Salt Range and Potwar this the most important geological facts to be ascertained are the Plateau to within 3 m. The locations of stations are considered to depth and the configuration of the basement, as well as the amount be accurate to within 5 to 10 s. The maximum error for most of the of younger sediments." Toward this end, study of regional gravity Bouguer anomalies is estimated to be less than 3 mgal, and the con- data of the Punjab Plain, Salt Range, and Potwar Plateau has been tour interval of 5 mgal on the Bouguer anomaly map is considered undertaken. In this report, the gravity data (Bouguer anomaly) of reliable. the region between Rawalpindi in the north and Bahawalpur in the The Bouguer anomaly map (Fig. 1) and the profiles (Figs. 3, 4, 5) south are presented, and inferences relating to the subsurface have been interpreted with the assumption that they reflect the configuration of the Precambrian basement are discussed. buried features of the Precambrian basement; density of the Pre- cambrian basement rocks (quartzite and granite gneiss) is consid- AREA OF BOUGUER ANOMALY MAP ered to be about 2.7 g/cm3, and that of the sedimentary cover 1.9 g/cm3. The assumed density of the Precambrian rocks is similar to The area of the Bouguer anomaly map (Fig. 1) is approximately the average density values for the metamorphic focks of most of the 135,000 km2, extending from Rawalpindi in the north to Bahawal- continental crust (Smithson, 1971). Further, the value of 2.7 g/cm3 pur in the south. The gravimetric coverage of the area was carried is supported by Woollard's suggestion (1969) that 2.74 g/cm3 is a

Geological Society of America Bulletin, v. 88, p. 1147-1155, 9 figs., August 1977, c. no. 70811.

1147

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1147/3433893/i0016-7606-88-8-1147.pdf by guest on 30 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1147/3433893/i0016-7606-88-8-1147.pdf by guest on 30 September 2021 10 0 10 20 30 40 50 60 MILES I 1—1 I I I I 1 1

Figure 1. Bouguer gravity anomalies in Punjab Plain, Salt Range, and Potwar Plateau, Pakistan. Diagonal-ruled part of index map shows area of survey. Gravity values tied to Karachi; airport value 978.630 gal. Contour interval 5 mgal. Elevation datum is mean sea level; elevation correction factor 0.071 mgalift. Profiles A-A', B-B', and C-C' shown in Figures 3, 4, and 5, respectively. Diagonal-ruled area near Sargodha is Precambrian basement outcrop. Broken line = concealed ridge. Solid dots = earthquake epicenters.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1147/3433893/i0016-7606-88-8-1147.pdf by guest on 30 September 2021 Figure 2. Main structural features of Himalayas (Gansser, 1964).

+ 50-

* X Observed

0- A Calculated v

= -50 • S /

• - -X. -150-

30Km South of Mjanwali Rawalpindi | -m Forelond jSalt Ronge Fore deep Sea level sw t A' NE"

Basement (Precambrian) Igneous -25Km— Intrusion I I 11 I n I Depth points from Seismic 8 Aeromagnetic Data

Figure 3. Bouguer gravity anomaly curve and interpreted geological structure along profile A-A'.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1147/3433893/i0016-7606-88-8-1147.pdf by guest on 30 September 2021 GRAVITY FIELD OF THE BURIED SHIELD, PUNJAB PLAIN, PAKISTAN 1151

reasonable density for the upper crust. Prominent positive closures We have attempted a correlation of the observed Bouguer have been attributed to density contrast in the basement arising anomalies with the depth of the Precambrian basement as deter- from igneous intrusions (dolerite, diabase). A contrast of +0.3 has mined from drill holes and seismic and aeromagnetic surveys in the been taken for calculating the gravitational effect of intrusive Jhelum-Multan region (Fig. 6). bodies. The formulas developed by Heiland (1946) have been used Crustal thicknesses at Rawalpindi, Jhelum, near Mianwali, for computing the theoretical gravity curves generated by geologi- Lahore, and Multan have been calculated using the formula of cal models corresponding to a vertical cylinder and to a two- Demenitskaya (in Rahman, 1969). The calculated thicknesses have dimensional semi-infinite horizontal slab with a sloping edge. Our been compared (Fig. 7) with the curve showing the variation of computations would be improved if computer facilities were avail- crustal thickness relative to the Bouguer anomaly (Case and Mac- able. donald, 1973).

+ 50 —

0 -

S -100-

-15 0 -

50Km North East of Multan t B Jhelum Sea level 180 « B S w _1 r - . 1- > NE

Basement I Igneous (Precambrian) intrusion -2 5 Km—

Depth Points from Seismic 8 Aeromagnetic Data

Figure 4. Bouguer gravity anomaly curve and interpreted geological structure along profile B-B'.

+ 50- X X Observed

Calculated 0 -

-50- \ V < -100-

-150- X.

Near Shahkot Sea level sw NE' Basement ( Precambrian) -25 Km-

Depth points from Seismic 8 Aeromagnetic Data

' Depth points from Drilling

Figure 5. Bouguer gravity anomaly curve and interpreted geological structure along profile C-C'.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1147/3433893/i0016-7606-88-8-1147.pdf by guest on 30 September 2021 1152 FARAH AND OTHERS

GEOLOGICAL INTERPRETATION northeastward-sloping gravity gradient over the vast alluvial stretch between Jhelum and Lahore is about 1.1 mgal/km, which is The Bouguer anomalies of the Rawalpindi-Bahalwapur region comparable to that observed in the northern reaches of the Gan- are linear and identifiable in three distinct strips. The central strip, getic plain in India (Qureshy and others, 1974). The central ano- with a number of positive gravity closu res, has a trend from Mian- maly strip, with a number of positive gravity closures, represents wali in the northwest to Patokki in the southeast. This central strip the Sargodha-Shahkot arch, comprising scattered inliers of rocks of is flanked by a nearly east-west strip of decreasing gravity in the the Purana Group along a distance of about 100 km. The expo- north and by a nearly north-south strip« of decreasing gravity in the sures of these rocks near Sargodha, Chiniot, Sangla, and Shahkot, south and southwest. The gravity gradient on either side of the cen- as monadnocks in the Punjab Plain, are characterized by well- tral strip is moderate, 1.4 mgal/km to 1.6 mgal/km. In places a defined gravity closures. Closures of about +10 mgal have been in- comparatively high gradient of 3 mgal/km has been noted. The terpreted as igneous intrusions in the quartzite and slate. Such clo-

- 6.5

•7.0

Figure 6. Approximate correlation of Bouguer gravity anomaly with basement depth, based on seismic aeromagnetic and drilling results in Punjab Plain (from Jhelum in north to Multan in south).

Figure 7. Crustal thickness versus Bouguer anomalies for areas largely in isostatic equilib- rium, modified by Case and Macdonald (1973, p. 2913) from Demenitskaya and Belyaevsky (1969) and Woollard and Strange (1962). Solid circles indicate calculated crustal thickness at dif- ferent places in Punjab Province, Pakistan.

6 200 0 - 200

BOUGUER ANOMALY (mgal)

sures coincide with magnetic closures of +500-y to 600y (total the large thickness of salt deposits in the Salt Range, which com- field), which may be caused by mafic and ultramafic bodies. pensates for the excess mass of sedimentary rock in the uplift. The The rocks constituting the Sargodha-Shahkot arch, although less seismic reflection surveys near Chakwal in the southern Potwar than 80 km south of the Salt Range, are similar to those of the Plateau indicate that the salt is about 1.4 km thick and the base- Aravalli Hills in India, which are more than 400 km to the south- ment depth is nearly 6 km. By interpolating this basement depth to east. The geologic features of the central gravity anomaly strip the Sargodha outcrop, a slope of 3° on the basment surface is ob- strongly suggest that the Sargodha-Shahkot ridge is a continuation tained; this supports the model adopted from the gravity profiles of the deeply denuded Aravalli Hills. Such a possibility was first in- (Figs. 3, 4, 5). The inferred crustal model of the Sargodha-Shahkot ferred by Wilsdon and Bose (1934), on the basis of gravity mea- ridge of the Punjab Plain, based on geophysical data, conforms to surements. They postulated a direct continuation of the buried the inference drawn by Gansser (1964, p. 10) that shield elements chain of the Aravalli Hills northward to the Salt Range and more can be followed northward under the southward thrust of the than 1.2 km of sedimentary rock on the buried ridge between Himalayas. Shahpur and the Salt Range in the north. These postulations have An interesting problem in the subsurface geology of the foreland been confirmed by the results of detailed geophysical investiga- region extending from Sargodha to the northern margin of the Salt tions. In the crustal model (Figs. 3, 4, 5), based on the interpreta- Range is whether the Attock Slate (Precambrian-Paleozoic?) of the tion of gravity data supported by seismic and magnetic results, a folded belt is represented in the foreland area. The geological evi- horstlike block of raised continental crust has been depicted ex- dence indicates that a basement of folded slate extends southward tending northwestward and gently sloping (<5°) on the sides. In beneath the Marghala Hills of Rawalpindi. The location and na- places, the block has been traversed by dikelike bodies of mafic ture of the basement toward the southern margin of the foredeep is rocks and by normal faults. This interpretation is supported by a matter of speculation. There is a change in the gravity gradient comparison of the observed horizontal gravity gradient with the north of the Salt Range (lat 33°N). This may mark the margin of calculated gravity gradients corresponding to three specific the foredeep but probably has no relation to the distribution of the geologic features (Table 1). slate zone. The possibility that a thin slate zone (equivalent to the Whereas a particular gravity gradient generated by a faulted Attock Slate geosynclinal facies) overlies the Precambrian basement block is useful in evaluating relative vertical displacement, it yields rocks near Sargodha cannot be excluded. However, the description no definitive information on large horizontal displacements that of the basement rocks in the drill logs for holes in the vicinity of the may be associated with some of the "megashears" in the Purana Sargodha-Shahkot ridge (Fig. 8; Kidwai, 1962; Kazmi, 1964) do massif. The gravity gradient southwest of the central strip is steeper not support this possibility. than in the northeast. A refinement of the geological implications of Crustal thicknesses at Lahore and Mirpur, 32 km north of the gravity gradient is not really justified in view of the ambiguity Jhelum, are closely comparable, on the basis of Bouguer gravity of the gravity data. It is possible, however, that the flanks of the anomalies and the seismic data obtained from explosions. The buried Precambrian Sargodha-Shahkot ridge slope about 10° Bouguer anomaly at Mirpur is —150 mgal and that at Lahore is southwest and 2° to 5° northeast. The steep dip of the beds noted in —15 mgal. The crustal thicknesses, estimated from the gravity some of the exposures of the Precambrian rocks is not reflected in anomalies using the formula of Demenitskaya (in Rahman, 1969), the gravity curves, implying that the basement surface is regular are 52 km at Mirpur and 38 km at Lahore and are comformable and gentle. The emplacement of the northwest-trending ridge, with with the relationship of crustal thickness and Bouguer anomaly de- gentle slope, into the foreland realm may be explained by the con- rived by Demenitskaya and Woollard and modified by Case and cept of L. L. Fermor (in Wadia, 1931) that block faulting in pre- Macdonald (1973, Fig. 7). The depth of the Mohorovicic discon- Talchir or early Paleozoic time produced horst mountains in the tinuity calculated from the records of seismic shocks generated by northwestern part of the Peninsula, reflected in the central strip of explosions at Mirpur in 1969 and at Lahore in 1970 are 48 and 38 positive gravity closures. km, respectively (A. Ahmad, 1969, unpub. data; K. U. Siddiqui From Sargodha northward across the lower Jhelum Valley, a and M. Idris, 1970, unpub. data). The crustal thicknesses calcu- negative anomaly gradient is persistent, indicating plunge of the lated from the gravity anomaly are identical to those calculated basement. The Salt Range uplift does not significantly interfere from the seismic records. The additional 10 km of crust at Mirpur with the northerly gravity gradient. However, a weak residual high may be explained as excess sedimentary and granitic layers, each is suggested over the Salt Range (Fig. 3). This may be explained by about 5 km thick. There may be an excess of light sedimentary rocks 3 to 4 km thick at Mirpur, compared to Lahore, that may account for most of the difference in gravity anomalies ( — 135 TABLE 1. GEOLOGIC STRUCTURES mgal) at the two places. The evidence is not conclusive because the AND GRAVITY GRADIENTS seismic data with which comparison of crustal thicknesses has been made is not of high enough, quality. However, a uniform density Structure Gravity gradient contrast between crust and upper mantle materials (cr = 0.3) and Observed Calculated variation in crustal thickness appear more plausible, because they (mgal/km) (mgal/km) indicate uniform and regular thickening of the crust toward the 1. Two-dimensional semi-infinite 0.7 to 1.2 0.6 to 1 orogenic sedimentary rocks and ophiolites (Fig. 2). horizontal slab, with edge An important unresolved issue is the relation of the Peninsular sloping at 2° to 5° to 5-km Shield to the Himalayan orogeny. There are two contrasting con- depth; density contrast 0.8 cepts on this subject. In one concept, the proximity of the Peninsu- 2. Vertical fault in basement 2.5 1.8 to 2.5 lar Shield to the present site of the Himalaya Mountains during at 2-km depth, truncated Paleozoic and Mesozoic time is advocated, implying no consider- against sedimentary rock able horizontal movement of the Peninsular Shield (Saxena, 1971; with 2-km throw; density Kats and Sokolov, 1971; Khain and others, 1975). This concept in- contrast 0.8 volves the "classic" or "orthodox" contact relations between the 3. Dike, like vertical cylinder; 4.0 4.0 northern periphery of the Peninsular platform and the folded struc- igneous intrusion in basement; ture of the Tethys, as schematically depicted in Figure 9. The prin- 5 km wide; 10-km depth; density cipal requirement of such a contact relation is "a transversely contrast 0.3 asymmetrical structure with a narrow folded edge and a wide,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1147/3433893/i0016-7606-88-8-1147.pdf by guest on 30 September 2021 1154 FARAH AND OTHERS

72° 73° ï£L / E2CV60 2V,5 2 VAL QADIRABAD*B-¿^Jj^T ^J°R ° *BHAI # ¿[17/602 , fVANIKE J EI6/500°v. CKOT MOMIN J^f aGUJRANWALA Jj E 14/550*1 (/ „ oElíl/1092 // ? ? s / SARGODHA VCH RANJABJr^ »jalalPUR nau ¿HAFIZABAD JJ ILIBWALA// E-KAMOKE T I / ' 1ÍL* ffpiNDI BHATTIAN Wjt b—J PUR >)CHUND» // \ LYALL * \2/°72lVZ^0^ E6/6ÌKq,4/680

il (< ^BR7/98Ò )) // OJHANG \ /V770*6/882,000--"'' \ «24/970 ri* \ 27°903 «GOJRA \ X^-^SYEDWALA . .X ÇP^1 VV£FÍ TANDUANWALA A)fi A v /(1 03I/88I O29/965 y -£-1200 /P=J

8 64 20 4 8 12 16 MILES I I I I

EXPLANATION

Figure 8. Locations of drill holes, with depth to bed rock, in Punjab Plain •2/721 (from Kidwai, 1962). TEST HOLES DRILLED TO BEDROCK (CRYSTALLINE BASEMENT) WELL NUMBER/DEPTH TO BEDROCK(CRYIATALLINE BASEMENT) IN FEET BELOW LAND SURFACE O30/982 TEST HOLES IN WHICH BEDROCK (CRYSTALLINE BASEMENT) NOT ENCOUNTERED! WELL NUMBER/DEPTH gently sloping platform edge" (Kats and Sokolov, 1971, p. 880). DRILLED, IN FEET BELOW LAND SURFACE The gravity field of the Punjab Plain depicts a wide, gently sloping 400 platform. However, the implication of this evidence of a wide, i"' gently sloping basement into the foreland region should be evalu- APPROXIMATE CONTOURS ON BEDROCK (CRYSTALLINE BASEMENT) ated in the light of a general observation that "observable foreland SURFACE. IN FEET BELOW LAND SURFACE CONTOUR INTERVAL 200 FEET basement deformation is rare along most of the world's orogenic belts" (Lowell, 1974). In the modern and more elegant concept, the Himalayan orogeny BEDROCK (CRYSTALLINE BASEMENT) OUT CROPS and the current tectonic activity of the region are explained within the framework of continental collision, satisfying the constraints of plate tectonics (Dietz and Holden, 1970; Powell and Conaghan, 1973; Molnar and Tapponnier 1975). Gansser (1975, written Peninsular Himalayan folded range Shield commun.) stressed that "neither Kats and Sokolov nor Powell and Conaghan seem to be accepting the fact that the northern Indian Plate boundary is the Indus suture zone with its ophiolite belt and not the south border of the Himalayan orogen. The greater part of the Himalayas seems to consist of northern Indian Shield material with its northwards increasing sedimentary cover. All these units were subsequently thrust to the south during the collision of the In- Figure 9. Classic contact relation between northern edge of Peninsular dian and Asian plates." The gravity field of the Punjab Plain is platform and folded Tethys. broadly in agreement with this general hypothesis.

CONCLUSIONS in Rechna Doab, West Pakistan: Geol. Survey Pakistan Recs., v. 10, pt. 3, p. 9-10. Analysis of the Bouguer gravity anomaly data of the Punjab Khain, V. Ye., Sokolov, B. A., and Kats, Ya. G., 1975, Nature of the con- Plain, including the Salt Range and Potwar Plateau, indicates that nection of continental and oceanic platforms and the concept of "plate tectonics": Internat. Geology Rev., v. 17, p. 528. the scattered outcrops in the Sargodha-Chiniot-Sangla-Shahkot Kidwai, Zamiruddin, 1962, Geology of Rechna and Chaj Doabs, West area constitute the northwestern continuation of the Aravalli Pakistan: West Pakistan Water &c Power Development Authority block. The regular and gentle slope of the Archean basement sur- Water & Soils Inv. Div. Bull. 5, p. 12-16. face, from the foreland region northward to the foredeep region Lowell, J. D., 1974, Plate tectonics and foreland basement deformation: and beyond, is evident from the gravity data. The evidence dis- Geology, v. 2, p. 275-278. cussed in this paper broadly supports Gansser's hypothesis about Molnar P., andTapponnier, P., 1975, Cenozoic tectonics of Asia: Effects of the relation of the Indian Peninsular Shield and the Himalayan a continental collision: Science, v. 189, p. 419-426. orogen. Powell, C. M., and Conaghan, P. J., 1973, Plate tectonics and the Himalayas: Earth and Planetary Sci. Letters, v. 20, p. 1-12. Qureshy, M. N., Venkatachalam, S., and Subrahmanyam, C., 1974, Verti- REFERENCES CITED cal tectonics in the middle Himalayas: An appraisal from recent gravity data: Geol. Soc. America Bull. v. 85, p. 921-926. Case, James E., and Macdonald, W. D., 1973, Regional gravity anomalies Rahman, Aziz ur, 1969, Crustal section across the Sibi-syntaxial bend, and crustal structure in northern Colombia: Geol. Soc. America Bull., West Pakistan, based on gravity measurements; Jour. Geophys. Re- v. 84, p. 2905-2916. search, v. 74, p. 4367-4370. Demenitskaya, R. M., and Belyaevsky, N. A., 1969, The relation between Saxena, M. N., 1971, The crystalline axis of the Himalaya; the Indian the Earth's crust, surface relief, and gravity field in the USSR, in Hart, Shield and continental drift: Tectonophysics, v. 12, p. 433—447. P. J., ed., The Earth's crust and upper mantle: Am. Geophys. Union Smithson, Scott B., 1971, Densities of metamorphic rocks: Geophysics, Geophys. Mon. 13, p. 312-319. v. 36, p. 691-693. Dietz, R. S., and Holden, J. C., 1970, Reconstruction of Pangea; break-up Wadia, D. N., 1931, The syntaxis of the northwest Himalaya: Its rocks, and dispersion of continents, Permian to present: Jour. Geophys. Re- tectonics and orogeny: Geol. Survey India Recs., v. 65, pt. 2, search, v. 75, no. 26. p. 190-195. Farah, A., 1972, Preliminary statement on initiation of geodynamics project Wilsdon, B. H., and Bose, N. K., 1934, A gravity survey of the sub-alluvium in Pakistan; recommendations and proposals: Paris, Secretariat of the of the Jhelum-Chenab-Ravi Doabs and its application to problems of Inter-Union Commission on Geodynamics, p. 237-240. waterlogging: Punjab Irrigation Research Inst. v. 6, no. 1, p. 6. Gansser, A., 1964, Geology of the Himalayas: London, Interscience Pub., Woollard, G. P., 1950, The gravity meter as geodetic instrument: Geophys, p. 8-20, 235-256. v. 15, p. 1-29. Heiland, C. A., 1946, Geophysical exploration: New York, Prentice-Hall, 1969, Regional variations in gravity, in Hart, P. J., ed., The Earth's p. 150-153. crust and upper mantle: Am. Geophys. Union Geophys. Mon. 13, Kats, Ya. G., and Sokolov, B. A., 1971, Articulations of Alpine- p. 320-341. Mediterranean folded belt with Gondwana platforms: Tectonic dis- tinctions of the zone: Internat. Geology Rev., v. 13, no. 6, p. 878- MANUSCRIPT RECEIVED BY THE SOCIETY MAY 29, 1975 883. REVISED MANUSCRIPT RECEIVED JUNE 1, 1976 Kazmi, A. H., 1964, Report on the geology and ground water investigations MANUSCRIPT ACCEPTED JULY 1, 1976

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1147/3433893/i0016-7606-88-8-1147.pdf by guest on 30 September 2021