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Home , Geology of Nepal, Himalayan foreland basin, Kali Gandaki Gorge

Earth and Planetary Science Letters 183 (2000) 215^229 www.elsevier.com/locate/epsl

Predicting paleoelevation of and the Himalaya from N18O vs. altitude gradients in meteoric water across the Himalaya

Carmala N. Garzione a;*, Jay Quade b, Peter G. DeCelles b, Nathan B. English b

a Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA b Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA Received 26 May 2000; received in revised form 3 August 2000; accepted 8 September 2000

Abstract

The N18O value of meteoric water varies with elevation, providing a means to reconstruct paleoelevation if the N18O 18 18 values of paleowater are known. In this study, we determined the N O values of water (N Omw) from small tributaries 18 along the Seti River and Kali Gandaki in the Nepal Himalaya. We found that N Omw values decrease with increasing 18 altitude for both transects. N Omw vs. altitude along the Kali Gandaki in west-central Nepal fit a second order polynomial curve, consistent with increasing depletion of 18O with increasing elevation, as predicted by a Rayleigh-type 18 fractionation process. This modern N Omw vs. altitude relationship can be used to constrain paleoelevation from the 18 18 N O values of ^ carbonate (N Oc) deposited in the Thakkhola graben in the southern . Paleoelevations of 3800 þ 480 m to 5900 þ 350 are predicted for the older Tetang Formation and 4500 þ 430 m to 6300 þ 330 m for the younger Thakkhola Formation. These paleoelevation estimates suggest that by V11 Ma the southern Tibetan Plateau was at a similar elevation to modern. ß 2000 Elsevier Science B.V. All rights reserved.

Keywords: O-18/O-16; altitude; streams; surface water; Nepal;

1. Introduction s 5000 m [1]. Understanding when and how the Himalaya and Tibet attained their current eleva- Altitude across the topographic front of the tions is important for evaluating the e¡ects of Himalaya increases from approximately 200 m high topography on south Asian ecology, regional in the Gangetic to 8000+ m at and global climate, and ocean chemistry [2^8]. the tallest peaks. North of the high Himalaya, Previous work has demonstrated that oxygen the Tibetan Plateau has an average elevation isotopes of lacustrine and carbonate can be used as a gauge of paleoelevation (e.g., [9^11]) because they are partially determined by 18 18 the N O value of meteoric water (N Omw), which * Corresponding author. Tel.: +1-716-273-4572; is largely dependent on elevation. As a vapor Fax: +1-716-244-5689; E-mail: [email protected] mass rises in elevation, it expands and cools, caus-

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S0012-821X(00)00252-1

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18 ing rainout, and producing lower N Omw values maintaining rain collection stations across the Hi- at progressively higher elevation. Modern world- malayan -thrust belt would provide better es- 18 wide gradients of N Omw (Vienna Standard Mean timates of average meteoric water composition, Ocean Water (VSMOW)) vs. altitude fall in a sampling stream water is much more feasible. range from 30.1x to 31.1x/100 m, with a global mean gradient of 30.26x/100 m [12]. Oxygen isotopic partitioning trends generally ¢t 2. Geology Rayleigh-type fractionation models [13,14], com- plicated by processes such as evapotranspiration 2.1. Himalayan fold^thrust belt and southern and vertical mixing in a cloud system (e.g., Tibetan Plateau [15,16]). 18 18 The N O values of carbonate (N Oc) from Ti- The Himalayan fold^thrust belt in Nepal con- bet and the Himalaya are potentially useful for sists of four regional lithotectonic packages sepa- understanding the development of high elevation rated by major zones. The northern bound- on the plateau. Carbonate from the Thakkhola ary of the fold^thrust belt is the Indus zone graben on the southern Tibetan Plateau yield (Fig. 1A), which sutures the Eurasian and Indian 18 N Oc (Vienna Peedee belemnite (VPDB)) values plates [22,23]. South of the suture zone lies the between 316x and 323x, consistent with for- Tibetan Himalaya, which consists of ^ mation at very high elevation [11]. However, pa- sedimentary rocks of the Tethyan Series leoelevation reconstructions rely on knowledge of that are incorporated into numerous south-verg- 18 the N Omw vs. altitude relationship, which is ing thrust sheets [23^25]. The southern boundary sparsely documented in the Himalaya. Two stud- of the Tibetan Himalaya is the South Tibetan ies of river waters in report linear gradients detachment system, a system of north-dipping, 18 of N Omw vs. altitude of 30.19x/100 for the normal-sense detachment faults that juxtapose headwaters of the between 300 and the Tethyan Series against Greater Himalayan 3000 m [17] and 30.14x/100 m for surface rocks [24,26]. Greater Himalayan rocks consist waters in the Gaula river catchment in Kumaun, of late to early Paleozoic [27,28] para- India between 915 and 2150 m [18]. Another , orthogneisses, and that were study of thermal waters determined a range of metamorphosed to amphibolite grade during the 0.2 to 0.3x/100 [19]. In this study, we measured orogenic event. These rocks have been N18O values of small streams along two transects thrust southward along the Main Central thrust across the Himalayan orogen in Nepal, from the system upon lower to upper greenschist grade foothills to the Tibetan Plateau (Fig. 1). Our ob- metasedimentary rocks of the Lesser Himalayan jective is to determine an empirical relationship zone [29^31]. Lesser Himalayan rocks, which con- 18 for the modern N Omw vs. altitude gradient and sist of early to middle Proterozoic passive margin to compare it to the paleocarbonate record from sequences have been thrust southward along the the Tibetan Plateau to understand the elevation Main Boundary thrust system. The frontal, Sub- history of the plateau. We chose these transects himalayan zone of the fold^thrust belt comprises because they provide a N^S pro¢le across the Hi- an imbricate thrust system in foreland malaya and southern Tibet, allowing sampling basin deposits of the Siwalik Group. along elevation gradients across the approximate The kinematic history of the Himalayan fold^ trajectory of rainfall as it moves over the Hima- thrust belt followed a generally southward pro- layan range. Streams more closely represent mete- gression of emplacement of the major thrust sys- oric water compositions of average annual rainfall tems [31^33]. Tibetan thrusts were active mainly if sampled during the dry season when ground- during Eocene and time [23,25], and water dominates the stream water budget, because produced regional Barrovian of groundwater contains a mixture of rainfall underlying Greater Himalayan rocks [34]. Ther- throughout the annual cycle [20,21]. Although mochronologic data from Greater Himalayan

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Fig. 1 (continued).

rocks suggest rapid cooling during late Oligocene^ early Miocene time, presumably during emplace- ment of the Main Central thrust system (e.g., [35,36]). At about the same time, the South Tibe- Fig. 1. (A) General tectonic map of the southern Tibetan tan detachment system became active (e.g., Plateau and Nepal Himalaya. Hachures are north-south [36,37]). Since time the deforma- striking normal faults. Lines with ball and stick ornaments tion front has migrated southward into the Lesser are South Tibetan Detachment system (STDS). Barbed lines Himalayan and Subhimalayan zones [38,39]. Esti- are Main Central thrust (MCT) and Main Boundary thrust (MBT). Thakkhola graben is shown. Boxes show locations mates of regional shortening rates in the Hima- of study areas detailed in 1B and 1C. (B) Map of the Seti layan fold^thrust belt in Nepal during the Neo- River showing tributary sample localities. (C) Map of the gene [39^41] and the [33] are similar to Kali Gandaki showing tributary sample localities. Small modern rates of shortening determined by space sampled tributaries are shown as medium gray and large geodetic measurements [42]. Observations that (1) tributaries sampled in the Thakkhola graben are shown as light gray. crustal thickening su¤cient to drive Barrovian metamorphism in Greater Himalayan rocks was underway during Eocene and Oligocene time and (2) the has been ¢lled above sea level since at least late Oligocene time

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Table 1 Oxygen isotopic data and elevation information for sampled tributaries along the Seti River and Kali Gandaki Sample name Tributary/river Sampling elevation Tributary elevation range N18O(VSMOW) ND(VSMOW) (m) (m) (x)(x) Seti River 42 Gadpai Gad 2350 2740 310.4 310.5 33 Ramus Khola 2040 3110 310.5 30 Panai Gad 1770 2200 310.2 27 Lisni Khola 1710 1400 39.7 26 Unnamed 1590 900 39.1 24 Talkoti Gad 1460 1500 38.4 14 Dungri Khola 1310 900 39.7 12 Dil Gad 1220 1300 39.5 39.5 9 Jidear Gad 1130 2050 39.3 6 Juili Gad 910 900 38.8 49 Nayagari Khola 820 1450 37.7 54 Dhung Gad 720 1550 37.8 58 Rori Gad 610 900 37.4 59 Kali Gad 570 1000 37.7 61 Unai Gad 530 1000 37.7 63 Gandi Gad 490 1050 37.7 66 Bheri Khola 430 1050 37.2 69 Chairo Khola 370 1450 36.8 Kali Gandaki 99kg9 Unnamed 3900 1850 317.5 3128 317.4 99kg8 Unnamed 3800 1450 318.3 99kg10 Giling Khola 3750 250 317.4 99kg11 Qumona Khola 3700 2200 320.5 99kg12 Unnamed l3700 1550 319.3 3143 99kg6 Samar Khola 3670 1800 317.6 3128 99kg16 Ghechang Khola 3550 2450 319.8 99kg15 Puyon Khola 3500 2000 318.5 99kg1 Ghidiya Khola 3450 1800 317.7 3113 99kg18 Yak Khola 3410 2350 318.8 3139 99kg13 Kali Gandaki 3300 * 319.5 99kg14 Tsarang Khola 3300 2500 321.2 99kg17 Tange Khola 3280 2450 319.1 99kg5 Unnamed 3250 2000 317.5 97kg1 Kali Gandaki 3000 * 318.2 99kg3 Kali Gandaki 3000 * 319.5 99kg2 Narsing Khola 2960 2750 318.8 99kg4 Chele Khola 2960 2750 317.5 99kg20 Panda Khola 2800 2500 316.5 99kg22 Unnamed 2750 1700 315.7 3115 99kg21 Unnamed 2700 600 313.7 99kg23 Unnamed 2600 1000 311.2 376 99kg24 Yamkin Khola 2500 1400 313.4 392 99kg25 Unnamed 2500 1400 312.8 99kg26 Largung Khola 2440 1800 312.2 384 99kg27 Unnamed 2300 1800 313.0 99kg28 Kaiku Khola 2000 2300 311.9 382 99kg29 Ghasa Khola 1850 1400 311.2 99kg30 Unnamed 1800 1600 312.0 384

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Table 1 (continued) Sample name Tributary/river Sampling elevation Tributary elevation range N18O(VSMOW) ND(VSMOW) (m) (m) (x)(x) 99kg31 Rupse Khola 1700 2300 311.4 99kg32 Dana Khola 1400 1550 310.0 368 99kg33 Bhalu Khola 1350 2250 310.6 310.7 99kg34 Unnamed 1240 1060 39.6 367 99kg35 Thado Khola 1140 1540 310.7 99kg36 Beg Khola 1040 1840 39.8 368 99kg37 Unnamed 870 210 39.5 99kg48 Hulandi Khola 855 500 39.0 99kg49 Unnamed 820 330 39.2 367 99kg40 Dundure Khola 800 320 39.6 368 99kg42 Buchchha Khola 790 1210 39.4 367 99kg38 Damuwa Khola 760 1140 39.6 370 99kg41 Luha Khola 720 780 39.8 99kg43 Phoksin Khola 720 1280 39.7 99kg45 Unnamed 720 180 38.7 99kg39 Malyandi Khola 680 920 39.3 99kg50 Unnamed 660 160 38.7 99kg44 Unnamed 620 160 38.2 99kg47 Unnamed 620 660 38.8 362 99kg51 Unnamed 480 220 38.1 360 Data for each location appear in order from highest to lowest sampling elevation. Tributary elevation range (m) is the number of meters of elevation above the sampling elevation over which the tributary £ows. Seti River tributary data also appear in [52].

[43,33], suggest that the Himalayan fold^thrust tion of the Tetang Formation [11]. The overlying belt attained high elevations by Oligocene time. Thakkhola Formation (up to 1000 m thick) is exposed throughout the basin and consists of al- 2.2. Thakkhola graben luvial fan deposits along the western basin-bound- ing fault, and £uvial and lacustrine deposits in the Thakkhola graben is located in north-central central and eastern parts of the basin. The pres- Nepal, on the southern edge of the Tibetan Pla- ence of C4 vegetation reconstructed from the soil teau, between the South Tibetan Detachment sys- carbonate record of the Thakkhola Formation tem and the Indus Suture zone (Fig. 1A). The constrains its age to 6 7 Ma [11]. Paleocurrent Kali Gandaki drains the southern Tibetan Plateau indicators suggest southward axial drainage since in this region, £owing southward along the axis of the beginning of Thakkhola Formation deposi- the graben and through the fold^thrust belt to the tion [46]. More recently, the Kali Gandaki has Himalayan foreland basin. The Thakkhola graben incised through at least 1000 m of basin ¢ll, into is bound by faults to the east and west, with Tethyan Series rocks. Several cycles of Tethyan Series rocks in its footwalls [44]. Basin- Pleistocene deposition within the incised valley ¢ll deposits are exposed between 3000 and 4200 m indicate that the basin has experienced multiple in the basin. Two depositional sequences, sepa- phases of Pleistocene damming and incision rated by an angular unconformity, make up the [45,47,48] basin ¢ll [45,46]. The older Tetang Formation (up to 230 m thick) crops out in the southeastern part of the basin and consists of £uvial deposits that 3. Hydrology and meteorology of the Himalaya grade into lacustrine deposits toward the top of and southern Tibet the section. Magnetostratigraphic data suggest an age of V11 to 10.5 Ma for the onset of deposi- Most rainfall in the Himalaya and southern

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tan Plateau, north of the South Tibetan Detach- ment system and south of the Indus Suture zone. A drainage pro¢le along the Kali Gandaki [51] has a large nick point associated with the Main Central thrust and high Himalaya (Fig. 2A). To a large extent, this steep stream gradient is a result of the more resistant rock types in the Greater Himalaya.

4. Sampling and analytical methods

Eighteen tributaries to the Seti River (Fig. 1B) and 46 tributaries to the Kali Gandaki (Fig. 1C) were sampled during foot traverses in March^ Fig. 2. Elevation pro¢les, tributary N18O values, and rainfall April 1998 and late September^October 1999, re- pro¢le across the Kali Gandaki drainage plotted against dis- spectively. We collected during the dry season in tance from source of the Kali Gandaki. (A) Elevation pro¢le order to sample base-level £ows, which give a of the Kali Gandaki is shown as shaded region and average closer approximation of mean annual preci- crest elevation is shown as black line (from [51]. Tributary N18O values are plotted at location of sampling along the pitation [20,21]. Generally, we sampled tribut- Kali Gandaki. Principle bedrock provinces: Tethyan Series aries that £ow perennially over an elevation range (TS), Greater Himalaya (GH), Lesser Himalaya (LH), and of 6 2000 m (henceforth termed `small tributa- fault systems: South Tibetan Detachment System (STDS), 18 ries') for the N Omw vs. altitude transect. Tribu- Main Central Thrust (MCT). (B) Average annual precipita- taries with perennial £ow over an elevation range tion across the Kali Gandaki drainage pro¢le (from [51]). s 2000 m we term `large tributaries.' We sampled both large and small tributaries north of along the Kali Gandaki traverse, in order to char- 18 acterize the complete range of modern N Omw values in the Thakkhola graben. Tibet occurs during the summer monsoon. Rain In the ¢eld, water samples were sealed in 15 ml stations at New Delhi, Kathmandu, and Lhasa polyethylene bottles. Samples were transported record more than 85% of annual precipitation and stored in darkness and refrigerated during from May to September, during the summer mon- storage prior to analysis. Elevations were deter- soon [49]. The monthly average discharge of ma- mined with a Garmin Model 45 GPS unit and jor rivers that £ow southward out of the fold^ cross-checked with 1:25,000 to 1:125,000 topo- thrust belt increases by an order of magnitude graphic maps for accuracy along the Kali Ganda- during the summer monsoon [50]. Precipitation ki. Elevations were determined from 1:63 360 varies greatly between the Himalaya and Tibet scale maps along the Seti River. as a result of the high Himalayan rainshadow For oxygen isotope analysis, 5 ml of water was (Fig. 2B). South of the high Himalaya, across equilibrated with CO2 at 16³C for 8 h. Gasses the central Nepal fold^thrust belt, the mean an- were analyzed on a Finnegan MAT Delta gas-ra- nual rainfall is between V100 and 500 cm/yr, tio mass spectrometer. Precision was þ 0.07x whereas north of the high Himalaya, in the Thak- for N18O of ¢ve replicate pairs of water. Twenty- khola graben, rainfall is 6 40 cm/yr [50]. seven internal standards run during the time of The Kali Gandaki and Seti River £ow south- analysis yielded standard deviations þ 0.05. On ward through the Himalayan fold^thrust belt, the basis of these data, the overall external preci- joining the Ganges River system in the foreland. sion was 6 þ 0.1x (1c). All water results are These rivers drain the southern edge of the Tibe- presented in VSMOW.

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5. Results caused an increase in tributary N18O values (Fig. 3). During collection of the ¢nal set of samples 5.1. Seti transect below Jiuli Gad (49^69), no rainfall events oc- curred. The addition of rainfall with high N18O Tributaries to the Seti River were sampled be- values to upstream tributaries (24^42) decreases tween 365 and 2350 m (Table 1 and Fig. 3). the gradient of N18O vs. altitude estimated for 18 N Omw plotted against altitude yields a best-¢t far-western Nepal. Excluding these data, a linear line with a slope of 30.18x/100 m, with a re- regression through tributaries from 365 to 1310 m gression coe¤cient (R2) of 0.85 (n = 18). We yields a best-¢t line with a slope of 30.29x/100 sampled from Jiuli Gad (6) north to Gadpai m and R2 = 0.92 (n = 12) (Fig. 3). At the 95% con- Gad (42), and then from Nayagari Khola (49) ¢dence limit, the slope varies between 30.23x/ south to Chairo Khola (69 ^ just o¡ of map) 100 m and 30.35x/100 m and the y-intercept (Fig. 1B). The ¢rst samples (6, 9, 12, and 14) between 35.4x and 36.4x. were collected prior to several large rainfall events. The ¢rst rainfall event lasted V12 h and 5.2. Kali Gandaki transect raised the water level in the Seti River substan- tially. After rainfall 1, the ¢rst tributary sampled Tributaries to the Kali Gandaki and Tinau upstream (24) had a N18O value 1.3x higher than Khola (just south of the eastward bend in the the last downstream tributary (14), sampled just Kali Gandaki) were sampled between elevations before the storm (Fig. 3). This suggests that rain- of 480 and 3900 m. (Fig. 1C). We sampled both fall run-o¡ had higher N18O values than base-level large and small tributaries north of Kagbeni, in £ow, and is consistent with measurements of high- the Thakkhola graben, to characterize the full er N18O values for winter/spring rainfall compared range of N18O values of river water. Tributaries to summer monsoon rainfall in the Himalaya [53]. of the Thakkhola graben, sampled between 2960 Over several days of water sampling subsequent and 3900 m, yield N18O values ranging from 18 to rainfall 1, N Omw values declined as run-o¡ 315.7x to 321.2x (Table 1 and Fig. 1C). decreased. Rainfall 2, which lasted V8 h, also Small tributaries have higher N18O values (315.7x to 319.3x) than large tributaries sampled at the same elevations (316.5x and 321.2x), because the larger tributaries have catchment areas that reach higher elevations and therefore carry a greater proportion of high ele- vation precipitation (Fig. 4). Large tributaries in- crease the scatter in the data, producing a spuri- 18 ous N Omw vs. altitude gradient and are therefore 18 excluded from the N Omw vs. altitude regression. We determined ND values from 20 water sam- ples, representing the full range of elevations, so that we could evaluate the possibility of kinetic evaporation e¡ects as moisture moves northward beyond the high Himalaya onto the Tibetan Pla- teau. A linear regression through the data produ- Fig. 3. N18O vs. altitude for tributaries sampled along the ces a local meteoric water line of: Seti River in far-western Nepal during March/April 1998. Rainfalls 1 and 2 show the ¢rst samples collected after rain- ND ˆ 7:59 Æ 0:14†N18O ‡ 5:67 Æ 1:9† 1† fall events. The positive shift in tributary N18O values subse- quent to rainfall events indicates that spring rainfall is rela- 2 tively more positive than base-level £ow. See text for yielding an R = 0.99. The excellent line-¢t to a discussion of regression lines. slope of V8 indicates that there are no signi¢cant

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tion or the stream carries a greater proportion of winter rainfall than other sampled tributaries. A second order polynomial provides a better ¢t to the data (R2 = 0.96) (Fig. 5), yielding the following relationship:

N 18O ˆ 34:02U1037 Æ 1:26U1037†h2‡

3 0:0012 Æ 0:0005†h3 8:02 Æ 0:46† 2†

Sample 99kg23 has been omitted from the regres- sion because it is clearly anomalous, and it signi¢- Fig. 4. N18O vs. altitude for all tributaries sampled along the cantly modi¢es the best-¢t curve. We choose a Kali Gandaki and Tinau Khola in west-central Nepal during second order polynomial ¢t because it allows for September/October 1999. Large tributaries ( s 2000 m eleva- tion range) north of Jomsom are shown as open squares and variation from pure Rayleigh fractionation, all other tributaries are shown as ¢lled diamonds. Labels for present in the data, that can result from processes tributaries north of Jomsom indicate the tributary elevation such as evapotranspiration and mixing of vapor range. More negative N18O values of large tributaries indicate masses. they carry a greater proportion of high-elevation precipita- Two minor rainfall events occurred in the late tion. afternoon between Larjung and Beni (Fig. 5). On both afternoons, drizzle fell over a 2^3 h period kinetic evaporation e¡ects during rainfall in the and stream discharge increased. Little scatter in arid region north of the high Himalaya. the data between Larjung and Beni indicates Tributary N18O values plotted next to the Kali that these events did not signi¢cantly modify trib- Gandaki stream pro¢le indicate there is a strong utary N18O values. This suggests that tributary correlation between oxygen isotopic fractionation discharge during the months after the monsoon and local sampling elevation (Fig. 2A). Where the season is dominated by groundwater discharge 18 Kali Gandaki stream gradient is steeper, N Omw of monsoon rainfall. values decrease more rapidly. The correlation be- tween the Kali Gandaki gradient and tributary N18O values suggests that elevation changes are the dominant cause of rainout and that the N18O value of small tributaries represents rainfall at lo- cal elevations, rather than at high elevations on the surrounding peaks (black line, Fig. 2A). Even in the arid region north of the high peaks, the decrease in tributary N18O values re£ects the Kali Gandaki stream gradient, which suggests that the e¡ects of evaporation are negligible. A plot of N18O values of tributaries vs. altitude yields a best-¢t line with a slope of 30.29x/100 m and R2 = 0.93 (n = 38) (Fig. 5). One data point (99kg23) falls signi¢cantly above the line. This data point ¢ts the local meteoric water line, which Fig. 5. N18O vs. altitude for tributaries sampled along the indicates that disequilibrium evaporation process- Kali Gandaki and Tinau Khola in west-central Nepal during es are not responsible for the high value. Possible September/October 1999. Only those tributaries with perenni- al £ow over an elevation range of 6 2000 m are plotted in 18 causes of the higher N O value are that the the arid region north of the high Himalaya. Line ¢t is shown stream water has undergone equilibrium evapora- in gray, and second order polynomial ¢t in black.

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6. Discussion y-intercept of 36.0x, approaching the value for New Delhi rainfall. Above 1000 m there is no 6.1. Mean rainfall elevation in tributary drainage signi¢cant change in the y-intercept of the best- basins ¢t second order polynomial. An elevation di¡er- ence of 1000 m between sampling elevation and Tributaries carry meteoric water that fell over a average elevation of the drainage basin seems rea- range of elevations in the drainage basin. The sonable given that the streams we sampled source elevation of sampling is lower than the average water over a range of elevations from 160 to 2300 elevation of the drainage basin and N18O values m above the sampling elevation (Table 1). The therefore re£ect higher elevation rainfall than the second order polynomial that ¢ts New Delhi pre- rainfall at the elevation of sampling. For this rea- cipitation and de¢nes a 1000 m elevation di¡er- son, the y-intercept of the Kali Gandaki regres- ence above the sampling elevations is de¢ned by: sion (38x) is more negative than the N18O value of mean annual precipitation in the modern Hi- N 18O ˆ 32:61U1037 Æ 0:84U1037†h2‡ malayan foreland (Fig. 5). We assume that the N18O value at New Delhi (elevation = 212 m, 30:0013 Æ 0:0005†h3 6:00 Æ 0:69† 3† N18O=35.81, weighted mean [53]) is a represen- tative value of precipitation in the foreland source (termed `Kali Gandaki relationship'). Tributaries region. We can estimate and correct for the di¡er- of the Seti River fall within the range of scatter of ence between sampling elevation and the average the Kali Gandaki data, assuming an average ele- elevation of the drainage basin by determining the vation of rainfall 600 m above the sampling ele- di¡erence in elevation that yields a curve that ¢ts vation (Fig. 7). The di¡erence in the apparent both New Delhi rainfall and N18O values of Kali average elevation of rainfall probably resulted Gandaki tributaries (Fig. 6). At an elevation dif- from a greater proportion of more positive win- ference of 1000 m, the best-¢t polynomial has a ter/spring rainfall in Seti River tributaries because there were anomalously large rainfall events dur- ing our sampling. However this is di¤cult to re- solve because the sample set is small and repre- sents a narrow range of elevations.

6.2. N18O vs. altitude relationship

The Kali Gandaki transect provides the best 18 constraints on the regional N Omw vs. altitude relationship because it covers the largest range of elevations, and tributary N18O values did not experience perturbations from rainfall events. Rainout by a Rayleigh fractionation process pre- 18 dicts that N Omw will become increasingly more Fig. 6. N18O vs. altitude for Kali Gandaki tributaries cor- negative with each rainout event. The second or- rected for the di¡erence between sampling elevation and der polynomial ¢t to the N18O vs. altitude rela- average rainfall elevation. Diamonds show N18O values of mw tributaries at the sampling elevation. Note the y-value at 200 tionship for Kali Gandaki tributaries displays this m (approximate elevation of the Gangetic foreland) does not depletion and is consistent with a Rayleigh-type equal the average annual rainfall at New Delhi (N18O= fractionation process. Rowley and Pierrehumbert 35.81x, weighted mean [53]) in the foreland. Squares show used atmospheric thermodynamic relations, based 18 N O values of tributaries at 1000 m higher than the sam- on relative humidity and temperature, in a Ray- pling elevation. The best-¢t second order polynomial to both the New Delhi value and elevation-corrected data is shown. leigh-type fractionation model to predict the oxy- See text for discussion. gen isotopic composition of rainfall with increas-

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to quantify. In lacustrine carbonate deposits rela- tive changes in evaporation can be assessed using changes in Mg/Ca ratios and by comparing per- cent aragonite to percent calcite. Comparing dif- ferent types of carbonate can help in determining whether one type has been enriched in 18Oby evaporation or other processes. Oxygen isotopic fractionation between calcite and water (Kcw) depends on the temperature of calcite precipitation:

6 32 1000lnK cw ˆ 2:78 10 T †32:89 4† Fig. 7. The Kali Gandaki relationship (black line) compared to the N18O/altitude model of Rowley and Pierrehumbert [54] 18 16 where the fractionation factor Kcw =( O/ O)c/ (crosses) and elevation-corrected data from the Kali Gandaki 18 16 and Seti River transects. See text for discussion. ( O/ O)w and T is absolute temperature [55]. Assumptions must be made about average annual temperature and the season of carbonate precip- ing elevation in the tropics [54]. Data presented itation in order to use this equation to calculate 18 here provide an empirical relationship that can be N Omw. An uncertainty of þ 10³C only introdu- 18 1 compared with the model of Rowley and Pierre- ces an error of þ 2.3x to the N Omw estimate , humbert. By setting the y-intercept to 0x, the which is minor compared to the degree of frac- Kali Gandaki relationship can be compared to tionation associated with changes in elevation. their model or any region with di¡erent source Garzione et al. [11] inferred that the Thakkhola moisture values (Fig. 7). The agreement between graben in the southern Tibetan Plateau has been our data and the model of Rowley and Pierre- at an elevation similar to modern since the onset humbert is striking. of deposition in the graben, based on very low 18 All of the small tributaries we sampled along N Oc values from lacustrine and paleosol carbo- the Kali Gandaki (except 99kg23) fall within nate in the ^Pliocene basin ¢ll (data þ 1.5x of the Kali Gandaki relationship (Fig. available in EPSL Online Background Data- 7). We therefore take þ 1.5x as the range of set2)[11]. A range of paleoelevations can be esti- error in the y-axis of the relationship. This yields mated from these carbonates using the Kali Gan- errors in elevation estimates of less than þ 560 m daki relationship. Today the average annual for regions over 3000 m. temperature of Lo Manthang, in northern Thak- khola graben is 6³C3 (3750 m above sea level) 6.3. Modern meteoric water vs. the carbonate [57]. In lacustrine systems, the average annual record

18 The modern N Omw vs. altitude relationship 1 We use the equation of Friedman and O'Neil [55] instead of can be used to reconstruct the paleoelevation dur- the more recently published equation by Kim and O'Neil [56] ing carbonate formation. A variety of factors in- to better compare calculated meteoric water values to past £uence the N18O values of carbonate, such as the studies that use the Friedman and O'Neil equation. Using the Kim and O'Neil equation would produce calculated mete- 18 N Omw, evaporation, diagenetic e¡ects, and tem- oric water values up to 0.4x lower, which would increase perature of calcite/aragonite precipitation. Diage- paleoelevation estimates by up to V100 m. netic processes can be gauged independently through petrographic observations and by analyz- 2 http://www.elsevier.nl/locate/epsl, mirror site: http://www.el- ing diagenetic carbonate to determine the isotopic sevier.com/locate/epsl composition of diagenetic £uids. The degree of 3 Mean of average monthly maximum and minimum temper- fractionation as a result of evaporation is di¤cult atures from data available between 1974 and 1994.

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tures of calcite precipitation in the Thakkhola 18 graben (Fig. 8). The N Oc values from the Thak- khola Formation (data available in EPSL Online Background Dataset2) are superimposed on this (vertical line pattern). Elevations of 4500 þ 430 m to 6300 þ 330 m are predicted by the Kali Gan- daki relationship for calcite that precipitated at temperatures between 6 and 14³C. These eleva- tions represent the source region of meteoric water in the basin and are in agreement with the elevations of the peaks surrounding the basin to- day (up to 6700 m). 18 Modern N Omw from the Thakkhola graben 18 can be compared with predicted N Omw values from Thakkhola graben carbonate. It is likely that monsoon conditions have prevailed in south- ern Asia since at least 8 Ma during deposition of 18 Fig. 8. N Oc values calculated from the Kali Gandaki rela- the Thakkhola Formation ( 6 7 Ma) [59,60]. In tionship for calcite that precipitated from water at tempera- 18 tures between 2 and 18³C plotted against altitude. Solid lines addition, the N Oc values of paleosol carbonate show 6^14³C, which is the most reasonable range of temper- from Siwalik foreland basin deposits do not atures for the precipitation of Thakkhola graben carbonates. change signi¢cantly over the last 7 Ma [61] and Boxes show the range of elevations predicted from the range compare well with source moisture from the mod- 18 of N Oc values of Thakkhola Formation carbonates (vertical ern Himalayan foreland, indicating that the N18O lines) and Tetang Formation carbonates (horizontal lines). 18 value of source moisture has remained fairly con- Tetang Formation N Oc values have been shifted by 3x to re£ect the di¡erence in source moisture values prior to 7 Ma. stant. We compare both large and small tributa- ries in order to gauge the full range of water de- rived from basin £oor precipitation to snow melt temperature is a reasonable minimum tempera- run-o¡ from the peaks that £ank the basin. The ture for calcite precipitation because most lacus- range of tributary N18O values is 315.7x to trine carbonate precipitates during the warm sea- 321.2x, from 2960 to 3900 m (Table 1). Kali son when lakes are most productive. The average Gandaki water sampled in September 1997 and temperature for July (warmest month) in Lo 1999 (97kg1, 99kg3, and 99kg13) from two loca- Manthang is 14³C [57], which represents a reason- tions in the graben ranges from 318.2 to 319.5, able maximum temperature for lake water and which re£ects the average of tributary N18O val- 18 carbonate precipitation. Paleosol carbonate prob- ues. However, predicted N Omw values from car- ably precipitated over a similar 6^14³C tempera- bonate of the Thakkhola Formation range from ture range because soil carbonate is thought to 317.8x to 323.2x (calculated at 12³C), which 18 form during the growing season as soil is dewa- is V2x more negative than modern N Omw val- tered by evapotranspiration [58]. Average ues. This di¡erence can be explained by changes monthly temperatures in Lo Manthang are be- in local elevation. We sampled tributary water tween 6 and 14³C from April to October [57], within the (average sampling and the assumption is that most lacustrine and elevation = 3480 m), which has incised the Thak- paleosol carbonate precipitated during these khola graben ¢ll by V1000 m. Assuming the ba- months. sin £oor elevation was approximately 4000 m dur- To incorporate the error associated with the ing deposition of the Thakkhola Formation, the temperature of calcite precipitation, we deter- elevation di¡erence accounts for V1.7x of the 18 mined the N Oc values from the Kali Gandaki 2x increase in modern meteoric water. There- 18 relationship over the inferred range of tempera- fore modern N Omw values agree very well with

EPSL 5606 3-11-00 226 C.N. Garzione et al. / Earth and Planetary Science Letters 183 (2000) 215^229 meteoric water values predicted from Thakkhola water and the paleotemperature of calcite precip- graben carbonate. itation. These two variables o¡set each other. As- Additional climatic variables must be taken suming some degree of evaporative enrichment into consideration in determining paleoelevation would increase the paleoelevation estimate, from Tetang Formation carbonate (data available whereas a higher temperature of calcite precipita- in EPSL Online Background Dataset2). Several tion as a result of warmer temperatures in the lines of evidence suggest that the regional weather Miocene would decrease the estimate. Oxygen iso- patterns may have been di¡erent [2,59^61] prior topic data of planktonic foraminifera from the to 8 Ma. In addition, the N18O values of soil car- late Miocene suggest that equatorial sea surface bonate from the Siwalik Group display an V3x temperatures were similar to Holocene sea surface increase at V7 Ma, suggesting that source mois- temperatures [63], which may indicate that equa- ture traveling northward from the foreland basin torial continental temperatures were similar to to- could have been as much as 3x more negative day. Even if calcite precipitation occurred at tem- prior to 7 Ma [61]. Dettman et al. documented peratures of 10³C warmer, the paleoelevation strong seasonal variations in oxygen isotopes of estimates would only be reduced by V500^600 fossil mollusks from the Siwalik Group (back to m. The role of evaporation cannot be discounted V10.7 Ma), similar to variations observed under and may cause an underestimation of paleoeleva- modern monsoon conditions [62]. They inferred tion. that the monsoon existed prior to 7.5 Ma, but that monsoon rainout was more intense and the dry season was less dry. Under these conditions, 7. Conclusions the V3x increase in the soil carbonate record at 18 18 V7 Ma would have resulted from higher N O Values of N Oc of paleocarbonate can provide values of monsoon rainfall (from less intense rain- useful insights into paleoelevation, where sup- out) and more evaporation from soils during the ported by an understanding of the modern varia- 18 more arid dry season. In the simplest case, we tion in N Omw with altitude. This study deter- 18 assume that the N Omw vs. altitude gradient was mines an empirical relationship for the modern 18 the same as the modern gradient and that source N Omw vs. altitude gradient across the Himalaya moisture (at V200 m elevation) was 3x lower by analyzing water from small tributaries across 18 during Tetang deposition. In Fig. 8 the N Oc val- two thrust belt transects. A second order polyno- ues of Tetang Formation carbonate (317.2x to mial (Eq. 2) provides the best ¢t to N18O values of 323.4x) are shifted by 3x to account for dif- tributary water sampled over a 3200 m range of ferent source moisture values prior to 7 Ma. Ele- elevations along the Kali Gandaki in west-central vations of the source regions of meteoric water Nepal. An elevation correction that ¢ts both aver- are estimated to be 3800 þ 480 m to 5900 þ 350 age annual meteoric water at New Delhi and the m for calcite that precipitated at temperatures be- N18O values of Kali Gandaki tributaries suggests tween 6 and 14³C. Absolute paleoelevation esti- that rainfall in the tributary basins occurs on mates for the Tetang Formation are 400 m to 700 average 1000 m higher than the sampling eleva- m lower than those predicted for the Thakkhola tion. This elevation-corrected equation is termed Formation, but agree within error. High elevation the `Kali Gandaki relationship' (Eq. 3) and is during the late Miocene is consistent with obser- used to estimate the paleoelevation of the Thak- vations from the Himalayan fold^thrust belt, khola graben from carbonate deposited in the which indicate that the region south of the Indus graben. 18 Suture Zone had experienced major crustal thick- Very negative N Omw values inferred from car- ening by Oligocene time. bonate of the Thakkhola graben on the southern Two signi¢cant unknowns that introduce error edge of the Tibetan Plateau suggest that this re- into the paleoelevation estimates are the degree of gion has been at a high elevation since the onset evaporative enrichment of 18O in paleometeoric of deposition in the basin between 10.5 and 11

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Ma. Elevations of 4500 þ 430 m to 6300 þ 330 m [7] P. Molnar, P. England, J. Martinod, Mantle dynamics, are predicted by the Kali Gandaki relationship for uplift of the Tibetan Plateau, and the Indian monsoon, Rev. Geophys. 31 (1993) 357^396. Thakkhola Formation carbonate. Carbonate of [8] L.A. Derry, C. France-Lanord, Neogene Himilayan the Tetang Formation produces paleoelevation es- weathering history and river 87Sr/86Sr: Impact on the ma- timates between 3800 þ 480 m to 5900 þ 350 m. rine Sr record, Earth Planet. Sci. Lett. 142 (1996) 59^74. Tetang and Thakkhola formation paleoelevation [9] C.N. Drummond, B.H. Wilkinson, K.C. Lohmann, G.R. estimates are within error of each other and are Smith, E¡ect of regional topography and hydrology on the lacustrine isotopic record of Miocene paleoclimate in similar to modern elevations on the Tibetan Pla- the Rocky Mountains, Palaeogeogr. Palaeoclimatol. Pa- teau. High elevation since the onset of deposition laeoecol. 101 (1993) 67^79. in the Thakkhola graben is consistent with data [10] D.L. Dettman, K.C. Lohmann, Oxygen isotope evidence on the kinematic and metamorphic history of the for high-altitude snow in the Laramide Rocky Mountains Himalayan fold^thrust belt, which suggest signi¢- of north America during the Late Cretaceous and Paleo- gene, Geology 28 (2000) 243^246. cant elevations were attained by Oligocene time. [11] C.N. Garzione, D.L. Dettman, J. Quade, P.G. DeCelles, R.F. Butler, High times on the Tibetan Plateau: paleo- elevation of the Thakkhola graben, Nepal, Geology 28 Acknowledgements (2000) 339^342. [12] M.A. Poage, C.P. Chamberlain, Empirial relationship be- tween elevation and the stable isotope composition of We thank D. Hodkinson, D. Robinson, B. Up- precipitation: considerations for studies of paleoelevation reti, B. LaReau, T. Ojha, and P. Kaminski for change, Am. J. Sci., in review. their enthusiastic assistance in the ¢eld. Thanks [13] U. Dansgaard, Stable isotopes in precipitation, Tellus 16 also go to D. Dettman for valuable discussions (1964) 436^468. and comments on an early version of the manu- [14] U. Siegenthaler, H. Oeschger, Correlation of 18O in pre- cipitation with temperature and altitude, Nature 285 script and for assistance in the lab. The manu- (1980) 314^317. script was improved by reviews from C. Drum- [15] K. Rozanski, C. Sonntag, Vertical distribution of deute- mond and B. Wilkinson. This project was rium in atmospheric water vapour, Tellus 34 (1982) 135^ supported by donors to the Geostructure Indus- 141. trial Partnership at the University of Arizona, [16] N.L. Ingraham, B.E. Taylor, Hydrogen isotope study of large-scale meteoric water transport in northern Califor- NSF Grant EAR-9725607 (to DeCelles and nia and Nevada, J. Hydrol. 85 (1986) 183^197. Quade), and the Institute for the Study of Planet [17] R. Ramesh, M.M. Sarin, Stable istope study of the Ganga Earth at the University of Arizona.[RV] (Ganges) river system, J. Hydrol. 139 (1992) 49^62. [18] S.K. Bartarya, S.K. Bhattacharya, R. Ramesh, B.L.K. Somayajulu, N18O and ND systematics in the sur¢cial waters of the Gaula river catchment area, Kumaun Hi- References malaya, India, J. Hydrol. 167 (1995) 369^379. [19] J. Grabczak, M. Kotarba, Isotopic composition of the [1] E.J. Fielding, B.L. Isacks, M. Barazangi, C. Duncan, thermal waters in the central part of the Nepal Hima- How £at is Tibet?, Geology 22 (1994) 163^167. layas, Geothermics 14 (1985) 567^575. [2] J. Quade, T.E. Cerling, J.R. Bowman, Development of [20] P. Fritz, River waters, in: J.R. Gat, R. Gon¢atini (Eds.), Asian monsoon revealed by marked ecological shift dur- Stable Isotope Hydrology: D and 18O in the Water Cycle, ing the latest Miocene in northern , Nature 342 IAEA, Vienna, Techn. Rep. Ser., no. 210 (1981) 177^199. (1989) 163^166. [21] C.J. Yonge, L. Goldenberg, H.R. Krouse, An isotopic [3] M.E. Raymo, W.F. Ruddiman, Tectonic forcing of late study of water bodies along a traverse of southwestern Cenozoic climate, Nature 359 (1992) 117^122. Canada, J. Hydrol. 106 (1989) 245^255. [4] J.M. Edmond, Himalayan , weathering processes, [22] A. Gansser, Geology of the Himalayas, Interscience, Lon- and the strontium isotopic record in marine , don, 1964, 289 pp. Science 258 (1992) 1594^1597. [23] M.P. Searle, Structural evolution and sequence of thrust- [5] W.L. Prell, J.E. Kutzbach, Sensitivity of the Indian mon- ing in the high Himalaya, Tibetan Tethys and Indus su- soon to forcing parameters and implications for its evo- ture zones of Zanskar and Ladakh, western Himalaya, lution, Nature 360 (1992) 647^652. J. Struct. Geol. 8 (1986) 923^936. [6] T.M. Harrison, P. Copeland, W.F.S. Kidd, A. Yin, Rais- [24] J.P. Burg, G.M. Chen, Tectonics and structural zonation ing Tibet, Science 255 (1992) 1663^1670. of southern Tibet, , Nature 311 (1984) 219^223.

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