Seismic Velocities from the Kohistan Volcanic Arc, Northern Pakistan

Journal of the Geological Society, London, Vol. 146, 1989, pp. 971-979,6 figs, 1 table. Printed in Northern Ireland

Seismic velocities from the Kohistan Volcanic Arc, northern Pakistan

P. N. CHROSTON' & G. SIMMONS2 'School of Environmental Sciences, University of East Anglia, Norwich NR4 7IJ, UK 2Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstrad: The rocksbounded by theMain Mantle Thrust and the Northern Suture in northern Pakistanconstitute an exhumed section through a Cretaceousvolcanic arc. Samples have been collected from all the principal lithological groups of the arc, and P and S-wave seismic velocities have been measured in the laboratory with the prime objective of comparing the velocities with those determined by seismic refraction experiments on modern volcanic arcs. Velocities were measured at up to 0.7 GPa and consideration has been given to the effect of confining pressure, pore pressure and temperature and to the deformation and metamorphism involved in the Himalayan collision. Thereconstructed velocity section through the arc shows a distinct'upper crust' comprising granitic-dioritic intrusions, metasediments and volcanin with a P-wave velocity of 6.2-6.4 km S-' dependmg on the parameters used. Beneath, the 'lower crust'of amphihlites and pyroxene granulite has a velocity mainly about 6.4-6.7 km S-', though the garnet granulites extend to 7.8 km S-'. The more mature arc with a higher proportion of granitic rocks would show a slightly lower upper crustal velocity than the younger arc. Velocity inversions might be expected with a thermal gradient of as little as 10-15 "C km-', depending on the pore pressure. In general, theproposed velocity structure is comparable with that of volcanic arcs and with many other sections of the continental crust.

Seismic refractionstudies using complexexperiments and paper we describe the results of laboratory measurements of refined interpretation techniques in the last two decades compressional andshear wave velocities of samples from have provided a wealth of detailedinformation onthe this suite, which is commonly referred to as the Kohistan velocity structure of many sections of the continental crust. Volcanic Arc, and then compare the velocities of the units Satisfactory geological interpretation of the velocities is in in the suite to the refraction seismic velocities from modem most cases however rather poor or lacking completely. This volcanic arcs. may be due to a number of factors, including the complexity and variability of the structure, thedifficulty or failure of the experimental design to take advantage of geological features The Kohistan Volcanic Arc which would aid the interpretation, and the lack of general The Kohistan sequence lies in northern Pakistan and is information on seismic velocities of many crustal lithologies. effectively sandwiched between theNorthern Suture or For the oceaniccrust, the geological interpretation of Megashear and the Main Mantle Thrust (MMT) or Southern seismic refraction data has been greatly aided by a data set Suture. The two sutures represent the division westwards of based on velocities of samples from ophiolitesuites the Indus Suture which separatesthe Indianfrom the measured in the laboratory (e.g. Christensen 1978). Eurasian plates (Gansser 1980). Earthquake epicentre and Comparablestudies forthe continentalcrust include, for focal mechanism studies(Seeber & Armbuster 1979) example, the reconstruction of the velocity structure indicate that the area is underplated by the Indian plate and through theIvrea Zone (Fountain 1976) and through this is also supported to some extent by the results of the Calabria (Kern & Schenk 1988), but because of the varying deep seismic sounding line northwards from Nanga Parbat evolutionary histories of different areas of the crust such (Finetti et al. 1979) which suggests a low velocity zone with data cannot generally be used tointerpret all velocity upper crustal velocities beneath the Kohistan sequence. The sections. However an example of a crustal section where this sequence is remarkably well exposed along the Karakorum kind of exercise may be valid is a volcanic arc. The role of Highway, which follows the Indus river and then traverses arcs in the formation of continental crust is now generally northwards to Gilgit andthe Karakorum Range.The accepted (Windley 1977) and in addition, the structure of general features of the geology of the arc can be found in many volcanic arcshas been determined by seismic Tahirkeli (1979, 1982), Tahirkeli & Jan (1979), Coward et refraction studies. al. (1982, 1986, 1987) andare briefly summarized below. In the Karakorum range of the Western Himalaya in N From south to north the Kohistan sequence is made up of Pakistan, a suite of rocks bounded by the Main Mantle the following rock groups (see Fig. 1). Thrust (MMT) andthe Northern Suture (NS) hasbeen Jijal complex proposedas anexhumed completesection through a volcanic arc (Tahirkeli 1979; Bard et al. 1980), with Close to the MMT is an ultramafic complex which consists lithologies ranging from ultramafic rocks and granulites to dominantly of clinopyroxenites with some dunitesand sedimentary, volcanic and intrusive rocks. It hasprovided peridotites and thinchromite bands (Jan & Howie 1981). an excellent opportunityto examine the lithology and Most of the ultramafics are serpentinized to some extent and structure of an arc, and also its physical properties. In this there are extensive mylonites due to the close proximity of 971

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\- L

KEY

Kohistan batholith and undifferentiated sediments Yasin group

Indian Plate Amphibolites . )Manthra Chilas complex Garnet granulites )Jiial complex Peshawar \*6.r" Abbotabad Ultramafics

Fig. 1. Simplified geological map of the Kohistan Arc (mainly after Coward et al. 1982) and sample localities.

the MMT. The northern part of the complex is occupied by Chalt volcanic rocks and sediments a distinctive belt of garnet granulite. Some of the rocks are To the north of the Chilas Complex, more amphibolites are strongly bandedand some massive layers of noriteare present. The metamorphicgrade decreases towards the found. Hornblendite occurs throughoutthe complexas Northern Suture, so that the northern part of the sequence lenses, boudins and dykes. consists of volcanic and sedimentary rocks which are usually deformed but some retain primary textures and structures. Kamila amphibolites The volcanic rocks include extensive tuffs, amygdaloidal basalts (often with pillow structure), fragmental basic and This broad belt some30 km wide has a faulted contact to the some rhyolitic and andesitic rocks. In some localities they Jijalcomplex. The rocks are amphibole-rich butthe are intruded by thick gabbro layers. The metasedimentary proportion of true amphibolites may be quite low. They sequencesin thearea are sometimesmappable units but havebeen divided intothree kinds; massive and often occur in small isolated pockets between the igneous homogeneous, banded and sheared, and a bedded variety rocks. The units mapped show variable lithologies including (Tahirkheli 1979), but a broad variety of rocks is slates, phyllites, schists, quartziticsandstones, crystalline represented including amphibolite with or without garnet, limestones and conglomerate and also show variable hornblendites, hornblende schists, garnetgabbros, diorite, metamorphism according to their proximity to the Northern and also some small amounts of tonalites and granites Suture or to the Nanga Parbat Syntaxis. (Coward et al. 1982). Tonalites and diorites The Chilas Complex These are commonly referred to as the Kohistan batholith This is an extensive stratiform cumulate body over 300 km (Coward et al. 1986) and they occupy an extensive area in long and greater than 8 km thick. It is dominantly norite but the northernpart of thearc intruding intothe also includes, particularly at the lower levels, hypersthene rnetasediments and volcanics. They vary from mafic to felsic gabbro,gabbro, chromite layersin dunite,dunite and andfrom early, foliatedand gneissic types, tolater hartzburgite. Rhythmically alternating cumulate layers up to discordant bodies. lOcm thick are a feature near Chilas, and graded units are Radiometric dates from the batholithrange from 40 to found elsewhere. 100 Ma (Petterson & Windley 1985). The uplift and present

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N S Sampling Samples were collected from all of the rock groups described above from the localities shown in Fig. 1. Most were collected at or close to the Karakorum Highway. On the roadcuts, areas of obvious blast effects wereavoided, but it is possible that the velocities at low pressure for some samples may be affected by microcracks introducedas a result of the road engineering. Ideally a large number of 2. Structure of the arc according to Coward et al. (1982). G/D, Fig. samples is required to provide thetrue average velocity granodiorites and diorite intrusions; V/S volcanic and metasedi- througheach suite, but the practicalities of the time mentary rocks; CC, Chilas Complex; KA,Kamila amphibolites; JC, Jijal Complex; NS, Northern Suture; "T, Main Mantle Thrust. consuming laboratorymeasurements do notpermit this. The schematic sections showing the development of the arc can be Field choice of samples was based on advice from previous found in Coward et al. (1987), fig. 4. workers and information from the literature, but principally from personalselection of the best likely representative structure of the arc resulted from its collision firstly with the samples. Eurasian continent and then with India (Coward et al. 1987) although earlier work suggest the collision with India to be Velocity measurements first (Tahirkeli et al. 1979). The proposed subduction zone dipped to the north (Coward et al. 1986). For the majority of the samples velocity measurements were Originally, Bard et al. (1980) interpreted the northward made on three orthogonal cores of 2.54cm diameter. If a sequence from the MMT as representing a complete section foliation was present these directions were perpendicular to through a volcanic arc, with the JijalComplex ultramafic the foliation (Z), parallel tothe foliation and to any rocks representing upper mantle, the Kamila amphibolites lineation if present (X),and parallel to the foliation and representing the metamorphosedoceanic crust, the Chilas perpendicular to any lineation (Y). Compressional and igneous complex a magma chamber which fed the volcanic shear velocities were measured using a pulse transmission rocks andthe felsic plutons which intrudedinto the technique with a mercury delay line (similar to Birch 1960) sedimentary and volcanic pile. Moredetailed structural andLead Zirconium Titanate tranducersmounted against studies by Coward et al. (1982, 1986, 1987) have indicated the end of the sample. The samples were jacketed in copper that thearc is much more complex (Fig. 2). The Chilas and most were saturated in water, though it is unlikely that Complex probably represent the deepest partof the arc, and there would be any significant difference in velocity between the Kamila amphibolites represent,at least in part,the dry and wet samples above about 0.1 GPa (Nur& Simmons lateralequivalent of theChalt volcanic rocks. TheChalt 1969). For these wet samples a fine mesh screen was volcanic rocks and volcaniclastic sediments intruded by the inserted between the jacket and the sample to allow pore Chilas complex form the earliestphase of the arc's fluid to escape and thus reduce its effects. All of the samples development. Thelate phaseincludes the Kohistan had extremely low measured porosity. Estimated total error batholith. Part of the latter intrusive rocks are foliated but is fl% forthe P wave, and f3% forthe shear wave most are undeformed andrepresent intrusions afterthe velocity. All of the velocity measurementswere made at collision with the Asiancontinent. Thegarnet granulites Massachusetts Institute of Technology and, for most samples, were made as a function confining pressure up may be equivalent tothe Chilas complex(Coward et al. of 1982) butalternatively they may representanother arc to 0.7 GPa. which hasbeen overthrust by the mainKohistan arc (Coward et al. 1987). Results A summary of the results is shown in Table 1, with Laboratory measurement of seismic velocities from lithology, mean density and mean velocities of the samples the arc measured at the highest confining pressure. Full details of all the velocity measurements at variouspressures for each core, deduced elastic properties and thin section analysis for Rationale each sample have been deposited with the Society Library Estimates of the velocities for the lithologies from the arc and British Library at Boston Spa, W. Yorkshire, U.K., as could beobtained in different ways, including velocities Supplementary Publication NO. SUP 18058. from comparable lithologies in the literature, from densities, Table 1 also shows the calculated Poisson's ratio foreach from aggregate (crack free) mineral velocities applied to sample based onthe mean P-wave velocity and mean pointcounts of thin sections, and also fromlaboratory S-wave velocity from the cores. Although the combined P measurements on samples from the suite. The decision to and S wave velocities (or Poisson's ratio and P-wave use the latter was based on the difficulty of finding in the velocity) from seismic investigations should potentially literature sufficient and comparable lithologies (particularly provide more information on rock composition than P-wave for the highest and intermediate grade metamorphic rocks) velocity alone, the Poisson's ratios here should be viewed and the need to improve the data set on seismic velocities. with cautionas only one determination of the S-wave In addition, dataon seismic anisotropy andon crack velocity was made for each core. In addition porosity, crack structure was required for other studies and this information structure and pore pressure may also modify Poisson's ratio maywell beunique to thissuite and could notbe (see e.g. O'Connell & Budiansky 1974; Toksoz et d. 1976). ascertained from the velocities based on thin section The error in Poisson's ratio is probably about f0.01. For analysis. the granitic-dioritic rocks andthe metasediments the

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Table 1. Summary of lithologies and velocities measured at maximum pressure

Sample No.Sample of Pressure Density anisotropyVpPoisson’s Vp Vs no. cores no. Lithology (GPa) Mgkm m-3 s-l ratio (a) %

JIJAL COMPLEX: ULTRAMAFIC ROCKS Pool 1 Pyroxenite 0.7 3.12 7.14 4.03 0.266 - PO05 3 0.7Pyroxenite 3.33 8.12 4.63 0.259 2.6 P126 3 Peridotite 0.5 3.29 8.16 4.64 0.260 4.0 3 Pyroxenite 0.6 PyroxeniteP127 3 3.16 7.75 4.41 0.260 2.3 P128 3 Dunite 0.7 3.29 7.96 4.45 0.273 6.7 P129 3 0.7 Peridotite 3.18 7.51 4.22 0.270 3.1 P130 3 Pyroxenite 0.5 3.08 7.54 4.24 0.269 3.4 P133 3 P133 0.7Pyroxenite 3.24 8.04 4.57 0.262 4.1

JIJAL COMPLEX: GARNET GRANULITES PO08 3 granuliteGt. 0.7 3.29 7.67 4.20 0.286 2.3 Poll 3 granuliteGt. 0.7 3.76 8.60 4.84 0.269 1.2 PO12 3 Gt. granulite 0.7 3.42 7.95 4.52 0.261 2.8 P013 3 granuliteGt. 0.7 3.37 7.35 4.12 0.270 5.7 PO14 3 Gt. granulite 0.7 3.59 8.24 4.59 0.275 2.0 PO16 3 Gt. granulite 0.7 3.18 7.74 4.14 0.299 5.6 PO10 2 Pyroxenite 0.6 3.14 7.62 4.10 0.297 7.2

KAMILA AMPHIBOLITES PO19 3 Amphibolite 0.7 2.95 7.06 3.91 0.279 1.2 p023 2 Amphibolite 0.7 3.16 7.13 3.93 0.285 17.2 PO24 3 Amphibolite 0.6 3.10 7.45 4.23 0.262 4.4 PO27Cataclastic 2 amphibolite 0.6 2.95 6.56 - - 10.9 PO29 3 Amphibolite 0.6 3.01 7.12 4.01 0.269 12.9 P224 3 Diorite 0.7 2.82 6.50 3.52 0.292 3.6 PO28 3 amph.Qtz. schist 0.7 2.85 6.59 3.83 0.245 4.5

CHILAS COMPLEX PO30Pyroxene 3 granulite 0.7 2.98 6.85 3.83 0.272 1.5 PO32 1 Px. granulite 0.7 3.03 6.77 3.73 0.282 - PO33 Px. 2 granulite 0.7 3.01 7.14 3.91 0.286 1.5 PO36 Px. 2 granulite 0.7 2.96 6.88 3.76 0.287 0.6 P108 Px. 2 granulite 0.7 2.94 7.14 3.89 0.289 3.7 P114 Px. 3 granulite 0.7 2.81 6.86 3.78 0.282 1.9 P115 Px. 3 granulite 0.7 2.94 6.86 3.79 0.280 3.0 P111 2 Dunite 0.6 3.31 7.74 4.31 0.276 0.0

VOLCANIC ROCKS PO46 2 Metabasalt 0.7 2.83 6.66 3.80 0.260 7.3 P047 3 Metabasalt 0.7 2.95 7.08 4.20 0.230 10.5 PO49 3 Metabasalt 0.7 2.93 6.87 3.92 0.258 3.3 PO61 3 Metabasalt 0.7 2.85 6.70 3.82 0.259 4.7 PO78 3 Metabasalt 0.7 3.03 7.02 3.92 0.274 9.0 p095 3 Metabasalt 0.7 2.81 6.54 3.66 0.272 1.4 P100 3 Metabasalt 0.7 3.21 7.81 4.51 0.250 9.1 PO79 2 Tuff 0.7 2.94 6.58 3.47 0.307 11.7

GRANITES/DIORITES PO44 3 Granite 0.7 2.67 6.36 3.69 0.246 0.3 PO45 3 Granite 0.7 2.69 6.08 3.61 0.227 7.1 PO69 3 Gran. pegmatite 0.7 2.60 6.45 3.53 0.287 1.9 p041 3 Granodiorite 0.7 2.74 6.28 3.46 0.282 4.4 PO62 3 Granodiorite 0.7 2.77 6.58 3.56 0.293 2.6 PO64 3 Granodiorite 0.7 2.76 6.40 3.52 0.284 4.5 P107 3 Granodiorite 0.7 2.70 6.32 3.60 0.259 4.0 P073 2 Granodiorite 0.7 2.65 6.33 3.56 0.268 1.1 P102Monzodiorite 3 0.7 2.66 6.51 3.70 0.262 2.0 PO66 2 Diorite 0.7 2.98 7.04 3.88 0.282 5.6 P104 2 Diorite 0.7 2.91 6.76 3.69 0.288 0.0

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Table 1. (continued)

~ ~~~ Sample No. of Pressure Density vp vs Poisson's Vp anisotropy no.Lithology cores (GP4 Mg m-3 km S-' ratio (a) % -- METASEDIMENTS PO55 3 Schist 0.7 2.78 6.465.53.65 0.266 PO56 3 Calcareous siltstone 0.7 2.91 6.80 3.78 0.276 5.2 PO60 3 Psammite 0.7 2.70 6.238.5 3.76 0.234 PO63 3 Psammite 0.4 2.73 6.119.0 3.38 0.279 PO81 3 Psammite 0.7 2.74 6.19 3.67 0.229 6.4 PO84 3 Psammite 0.5 2.90 6.19 3.67 0.229 7.1 PO91 3 Pelite 0.7 2.76 6.54 3.68 0.268 8.2

importance of quartz content is shownin the trend from uppercrustal velocity andstructure no doubt reflects the medium velocity, high Poisson's ratio of the diorites to low varying proportion and types of volcanic, sedimentary and velocity, low Poisson's ratio of the granites. The scatter in intrusive rocks. The boundary between the upper and lower the volcanic rocks and Kamila amphibolites is attributed to crustal layers is likely to be complex; for the lesser Antilles the range of lithology and metamorphic grade. The Chilas its depth varies by over 15 km (Boynton etal. 1979) and Complex samples give very similar results to each other. suggests that it is not simply the boundary between the arc The ranges of velocities in both the garnet granulite and material and the older oceanic crust. It may be the top of ultramafic rocks are largely due to varying garnet content mafic intrusions that occurred during the arc's development. and alteration respectively. The estimates of P-wave anisotropyshown for each sample are liable toerror because the maximum and Reconstructing the velocity section through the arc minimum velocities may not liein themeasurement directions, and becauseheterogeneity in the rock sample can result in different velocities between cores. The latter Structural considerations effect no doubt contributes to the high apparent anisotropy In order to reconstruct the velocity structure of the Kohistan of 17% in sample number PO23. However the well foliated arc in its original pre-collision form, the present structural samples from the Kamila Amphibolites and Chalt Volcanic complexities have to be considered and unravelled in order rocks are distinctive in showing significantly higher to estimate the thicknesses of the various units, and these anisotropies than the other suites. will depend on the model adopted. The overall thickness of the arc is difficult to calculate, but following a comparison with modem volcanic arcs (Fig. 4), a value of 30 km is Seismic structure of modem volcanic arcs adopted herefor the model of Bard et al. (1980) and Data are available on the seismic velocity structure for most estimates of the various units have been made to fit, based present active volcanic arcs including the LesserAntilles on outcrop dimensions and regional dip. It is recognized, (Boynton et al. 1979), Greater Antilles (Officer et al. 1959), however, that it is an oversimplification. The model of Aleutians(Shor 1964), Japan (Yoshii 1979), New Ireland Coward et al. (1982) probably represents the upper 20 km or and New Britain (Finlayson etal. 1972), Bali andSunda so of the arc. Figure 4 shows the simplified model of the arc (Curray et al. 1977), Kermadec & Kamchatka (Anasov et al. used for calculating the velocity structure. Although the 1980), Izu-Mariana (Marauch & Yasui 1968; Sugimura & error for each unit may be several kilometres it is not of Uyeda 1973), Sumatra (Kiechhefer et al. 1980), and South greatconsequence to the overall adopted velocities. In Sandwich arcs(Ewing etal. 1971). The quality of the estimating the overall average velocity for various depths in refraction experiments vary somewhat, with only the more the crust the relative proportion of the various lithologies recent ones having used ray tracing and synthetic must be estimated. For the interlayered metasediments and seismograms as interpretational aids. Figure 3 shows a summary of the simplified seismic structure of these arcs, LA GA J NI NB B A K IM S adapted from that produced by Boynton et al. (1979). All of themature arcs are 25-30 km thick, and although the velocity structure typically shows two principal velocity layers, the structure is quite variable, at least superficially. The arcs have lower-crust velocities between6.6 and 7.1 km S-' (locally 7.5 kms-'for the South Sandwich Islands), and a mean velocity of 6.9 km S-'. This lower layer varies considerably in thickness between arcs but is typically over 10 km thick and can be up to 20 km (Greater Antilles, Fig. 3. Summary of velocitystructure based on seismic refraction Officer et al. 1959). The lower crustal velocity in some areas experiments for volcanic arcs, after Boyntonet al. (1979). LA, is high in comparison to other, more mature,sections of the Lesser Antilles; GA, Greater Antilles; J, Japan; NI, New Ireland; continental crust, particularly in view of the high thermal NB, New Britain; B, Bali, Sunda arc; A, Aleutians; K, Kermadec; gradient and thus reduced velocity in arcs. The variation in IM Izu-Marian; S, South Sandwich.

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that there are only a few samples and the range of velocities is considerable (Table 1).

Physical parameters For the pressure coefficient (dV/dP),, we have adopted the mean value obtained from all samples forthe higher pressures (0.2-0.7 GPa).The gradientsrange from 0.2 toabout 0.9 km S-' GPa-' with an overallmean of S-' 61 : 0.4 km GPa-'. The means of the different groups are close to this value, only the Kamila amphibolites being noticeably higher,though with the wide rangeand small sample size the difference is not statistically significant. l - 20 There is more difficulty in estimating a satisfactory value for the low pressures at shallow depths. Here, notonly will microcracks in the rocks be open but also macrocracks in the form of joints and fractures, and the latter cannot be modelled in the laboratory.This is a vexed problem in geophysics and is not easily resolved. The predicted low velocity near the surface has been observed on very detailed - 30 seismic experiments (e.g. Hall 1978), butalternatively, km Smithson & Shive (1975) have shown in another case that Fig. 4. Simplified geological models of the arc used as a basis for seismic lines of only a few kilometres length can produce calculation of velocity structure. almost the 'crack-free' laboratory velocities. To predict the velocities at relatively low pressures we have concentrated on simply extending the gradient found at higher pressures to low pressures but have notattempted to estimate the volcanics the proportion observed in the well-mapped area velocity for depths less than about 3 km. around Gilgit has beenused (see maps inCoward et al. 1982), but there is a greater difficulty when considering the Relevant to the pressure coefficient is the way in which granitic and dioritic intrusions in the upper part of the arc. possible pore pressure is treated. The effective pressure (Pe) The earlier intrusions are more mafic and later ones more is usually defined as felsic, and inaddition most of the granitic/granodioritic P, = P, - n P, intrusions have been emplaced after the collision with the Eurasian continental Mass. The pre-collision intrusions are where P, is the confining pressure, P, is the pore pressure, distinguished because they contain some foliation, and the and n is a constant. Theoretical studies have concluded that proportion of these in thecountry rock (excluding the n is slightly less than unity (Brandt 1955). Experimental non-foliated intrusions) canbe used as an initial simple determinations for crystalline rocks are few with Christensen measure for the 'pre-collision' arc. (1984), for example, finding a range of 0.64 to 0.92 depending on the pore pressure, differential pressure and lithology, though most values areabout 0.9. Inthe Metamorphic effects laboratory we have attemptedto keep a negligible pore For thetop part of thearc there is nodoubt that the pressure and therefore the effective pressure is approxim- measured velocities for the sedimentary and volcanic rocks ately the confining pressure of the oil medium. How it is will now be too high for comparison with the equivalent equatedto depth depends on the assumption used in position in a modem volcanic arc. This is partlybecause determining P,, n and P,. We assume that Pc, is simply the they have become deformedand compacted, with a lithostatic head. The effect of pore pressure is to reduce resultant loss of porosity and therefore increase in velocity. seismic velocities by opening microcracks and in this paper Partly it is also becausealthough much of the sediment we have assumed two alternatives, either it is equal to the retains primary features, elsewhere the metamorphic grade hydrostatic head, or it is zero. reaches greenschist facies. Velocity measurementsindicate Laboratorymeasurements of the thermal coefficient thatthe velocity is only slightly higherfor the psammitic (dV/dT), vary considerably in theliterature, and it is rocks than the likely originalconstituent minerals, and possible thatthe nature of the laboratoryprocedure thereforethe reductionin porosity is the greatest effect. accounts for at least part of the variation. We have adopted Similarly, many of the volcanic rocks retain original textures the value of 0.6 X 10-3 km S-' OC-' used by Hall & Simmons and mineralogy but vesicles are filled and primary voids (1979) which is similar to the average value found by between pillows are lost, hence increasing velocity. Christensen (1979) and by Chroston & Evans (1983). The granulite facies metamorphism of the Chilas Thermal gradients in active volcanic arcs can be high, with a complex and the amphibolite facies metamorphism of the gradient of above 25 "C km-' required to produce the high Kamila amphibolites predate the collision of the arc with T-low P metamorphism (Miyashiro 1972). Inan inactive the Asiancontinent. In anycase there is little difference arc it may be much lower, with perhapsthe 'ancient between the velocities of a norite and a pyroxene granulite continental geotherm' of Sclater et al. (1980) being taken as as the mineralogies are similar. Further, the results show no the lowest possible value. Velocity profiles are shown here significant difference between the mean velocity of the Chalt with three gradients, 25°Ckm-', 10°C km-', andno volcanic rocks and the Kamila amphibolites. Note however temperature effect.

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Seismic anisotropy Vp 1 km S-! 6.0 6.5 7.0 7.5 8.0 A strong foliation is developed in many of the rock samples and most samplesshowed significant seismic anisotropy (Table 1). There is nodoubt that the Himalayan deformation was a major factorin development of the foliation, and samples of the Chalt Volcanic rocks and the metasediments takenfrom close tothe Northern Suture show significant anisotropy with the maximum velocity paralleling the plane of thesuture. However a 'primary' fabric (from the original development of the arc) may well 10 be preservedin some areas. Because of the difficulty in determining whetherthe fabric is 'primary' in origin, the velocities here are based on mean velocities for each sample (i.e. assuming isotropy). In practice this might mean that if the foliation in the original arc was dominantly horizontal, 15 then the velocities shown hereare underestimatedwhen ? compared with refraction velocities from seismic refraction lines. For example the Kamila amphibolites could show a I 1 velocity about 0.3 km S-' higher than the mean velocity used 20- here.

Velocity models 25- Figure 5 shows how velocities will vary with depth for each unit, based on the mean velocity for the volume of each lithology present and with differentassumptions of pore water pressure and temperature. We have assumed that the presentproportions forthe upper part of thearc are: 30- granites and granodiorite 30%, diorite and dioritic gneiss 20%, volcanic rocks 30%, sediments 20%, although accurate estimates are extremely difficult to make. Only the Fig. 6. Composite velocity model of the arc based on data from Fig.

"P *m .-1 5. Curve A represents a model assuminga dry rock (zero pore pressure) and a thermal gradient of 10 "C km-', and Curve B 'lJpi)*r Crust' represents a model with hydrostatic pore pressure and thermal gradient of 25 "C km-'. Key for dots at about 14 km depth is the same as for Fig. 4. The model represents a complete crustal section as proposed by Bard et al. (1980) but onlythe top 20 km of the model equate to the interpretation of Coward et al. (1982). The error bars for each suite are one standard error of the mean.

velocity functions for this 'mature' (post collision) arc are shown. The velocities for the pre-collision arc will be about 0.2 km S-' more than those shown. Forthe Kamila Amphibolites and Chilas Group, there is no post collision model. From these velocity functions it is now possible to construct a velocity modelthrough thearc which incorporates the various units intheir estimated pre-collision positions. Two composite models are shown in Fig. 6, with A being the model based on a zero pore pressure and a thermal gradient of 10 "C km-', and B having a hydrostatic pore pressure andthermal gradient of 25 "C km-'. As expected a clear low velocity 'upper crust' and a higher Fig. 5. Plots of velocity against depth for: (a) lithologies at the top velocity 'lower crust' aredemonstrated, but the possible of the arc ('upper crust'); (b) the Kamila Amphibolites; and (c) the range of velocity, particularly in the lower crust, is Chilas Complex. Solid lines assumeno pore pressure, and dashed considerable. assume hydrostatic pore pressure. The number at the bottom of the lines indicate the thermal gradients assumed in "C km-'. For (a) an Discussion and conclusions average velocity of the lithologies present (intrusive, metasedimentary and volcanicrocks) has been assumed but G and V represent Although the Himalayan collision provides the tectonic the likely extreme values of the mean velocitiesof the setting to allow an investigation of thedeep roots of a granites/granodiorites and volcanic rocks withthermal gradient of volcanic arc, the deformation and metamorphism involved 25 "C km-' and hydrostatic pore pressure (0)and gradient of in that collision mean thatthe Kohistan arc doesnot 10 "Ckm-' and no pore pressure (0). represent the perfect solution for studying physical

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properties. As a result, despite our careful choice of samples Comparison withthe velocity structure of island arcs and our attempts to unravel the structure of the arc, there will be some error in the ultimate velocities obtained, from Our velocity model shows an ‘uppercrust’ with P-wave sampling error, from oversimplifying the structure, and from velocities about 6.2 to 6.3 km S-’, and a deeper ‘lower crust’ reconstructing the original depth. Nevertheless the exercise with velocities about 6.4 to 6.8 km S-‘, depending on the allows us tomake severalpoints relevant otthe velocity thermalgradient adopted, and assuming the model of structure of arcs and other sections of continental crust. Coward et al. (1982). The velocity structure is comparable to that found for the continental crust in general and there is fair comparison to the upper and lower crustal velocities of The relationship between pressure and thermal arcs (see Fig. 4). A very detailed comparison with the gradient profiles through individual arcs is unjustified because of Laboratory measurements by Christensen (1979) demonstr- genuine local differences in structure and also because of the ated the range of values of the parameters (dV/dP),and varied experimental andinterpretational techniquesfrom (dV/dT), for a variety of lithologies and he was able to which the velocity profiles were derived. calculate individual critical thermal gradients above which The comparison with the arcs indicates that the upper there would be a velocity inversion.This is also crustal constituents of the Kohistan arc can explain the demonstrated in our model using averagevalues forthe upper crustal P-wave velocities of other volcanic arcs. There pressure and temperature effects. The assumptionrelating is very little data on shear wave velocities for arcs but the to pore pressure is also important, in that it controls the way Poisson’s ratio of 0.267 f 0.007 determined for the upper in which the pressure gradient is related to depth. For our crust of the LesserAntilles (Boynton et al. 1979) is ‘upper crustal’model, we would expect toget a velocity comparable to that for the Kohistan Arc. The ‘lower crust’ inversion above a gradient of about 15 “C km-’ if no pore in the Kohistan arc comprises the amphibolites plus the pressure is assumed, andat above 10 “C km-’ with a Chilas igneous complex with the latter (in the Coward et al. hydrostatic pore pressure. Our model however assumes a 1982 model) representing the deepest part seen of the arc. uniform lithology within each of the units. In practice the The precise nature of the transition here is not absolutely proportion of more mafic rocks (andtherefore higher clear, because the ‘lower crust’ is possibly notin a true P-wave velocity) may increasedownwards thus cancelling vertical section below the exposed‘upper crustal’ rocks. out any thermallyinduced velocity inversion. In addition, However it appears to be ata much deeper level, and in the compaction, lithification and metamorphism will lead to a Kohistan arc the change from lower velocity upper crust to greater value of (dV/dP), than measured on samples in the higher velocity ‘lower crust’ is marked essentially by the loss laboratory. of felsic igneous rocks and metasediment,as the velocity Nevertheless, the higher thermal gradients likely to be of the volcanic rocks is similar tothe velocity of the found in arcs will have a significant effect on the velocity, amphibolites and gabbros/pyroxene granulites. This mecha- perhaps as much as -0.3 kms-’ in the lower crust and this nism is rather different from that proposed to explain the effect is important when comparing refraction velocities with velocity change for some other parts of the continental non-temperaturecorrected laboratory velocities fordeter- crust, where it has been argued that the increase in velocity mining crustal composition. can be due to an increased metamorphic grade (Ringwood & Green 1966) and this is supported by some field and Velocity-age relationship seismic velocity evidence(Fountain 1976; Fountain & Salisbury 1981). Other exhumedsections of continental The geological history of the arc demonstrates theincreasing crust, however, do contain a significant amount of amount of felsic intrusives with development of the arc. This amphibolite orother mafic igneous rocks (Fountain & is more obvious when comparing thearc which was Salisbury 1981; Chroston & Evans 1983). developed before its collision with the Asian continent to its The seismic velocities of the lower crust in volcanic arcs present form, with the vast Kohistan batholith. Because of are however higher in some cases than the Kohistan velocity the generally lower P-wave velocity of the granitic/ and without any reduction due to the higher temperatures granodioriticrocks ascompared tothe earlier mafic would approach values of about 7.4km S-’. This implies intrusives andthe volcanic countryrocks, this leads to a large amounts of ultramafic material emplaced in the lower progressively loweraverage P-wave crustal velocity. A crust and/or a much higher garnet content than found in the similar effect has also been noted by Smithson et al. (1981) amphibolites and pyroxenegranulites of the Kohistan. when comparingaverage refraction crustal velocities with Seismic anisotropy would also be a contributingfactor in the crustal age, although in their case it illustrated reduction accounting for the difference in velocities. of velocity associated with continental development over a The role of the garnet granulite and the ultramafic rocks much longer time scale. of the Jijal complex is controversial. Coward et al. (1982) Table 1 demonstrates that the diorites and volcanic rocks regard the granulites as representingpart of the Chilas have Poisson’s ratios over a wide range but aremainly about complex whichwas subducted to a greater depth in the 0.26-0.29. The granodioriteshave a similar valuebut the arc-India collision thus resulting in the higher grade of granitesrange below 0.25. The overall proportion of the metamorphism. The ultramafics do not consist simply of latter in the area studied is relatively small and would not peridotites and serpentinite,but also include patches of greatly affect the average Poisson’s ratio in the upper crust, pyroxene granulite and amphibolite, and are dominated by but with the further long term development of the arc the diopsidite (Jan & Howie 1979). They probably originated in reductionin P-wave velocity would beaccompanied by a the upper mantle(Jan 1979) but most are heavily altered reduction in Poisson’s ratio as theproportion of granites and mylonitized. Whether the garnetgranulite/ultramafic increased. boundaryrepresents a palaeo-Moho is uncertain, butthe

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Received 13 January 1988; revised typescript accepted 17 January 1989.

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