Himalayan Elevation-Somnolence to Hyperactivity

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Himalayan Elevation

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Somnolence to Hyperactivity

V. RAIVERMAN

708 Majtri Apartment, 255 NSC Bose Road, Kolkata- 700 047 Email: [email protected]

Abstract: This short note attempts to illustrate the efficacy of multiple-probe investigations in solving compiex geoIogicaI problems. The Cenozoic is the era of Alpine-Qimalayan orogeny and the uplift pattern is one of long somnolence followed by brief hyperactivity. Nowhere else has this pattern been studied in greater detail than perhaps in the Himalaya. C - L a d why,. ffisd - ~ ~ r d s nf -&fa ar>d fauna, 2nd s&jmtnto~ugjjc, pa~tocIimatjc and geomorpho~ogic data

offer ample evidence in this direction. Modern techniques of radiometric geochronology of the Himalayan crystallines also support this contention.

Keywords: Himalayan uplift, Biostratigra~hy, $edirnentology, Geochronology, Pattern recognition.

Biostratigraphic and Sedimentologic Indicators of Himalayan Elevation

The closure of the Tethys sea at about SO m.y. based on the age of youngest marine fossils in the hdus-Tsangpo Suture Zone (van Haver in Searle, 1988; Mathur and Juyal, 1996), and likewise at 40 m.y. in the Himalayan Foreland (Mathur, 1978) indicates that the uplift process moved from north to south (see Fig. 1 for tectonic elements of \IVestern Himalayan region). TerrestriaI life forms on either side of the ITS2 maintained family affinities for a lobg time following. A large variety of land vertebrates comprising bison, deer, giraffe, hippopotamus, rhinoceros, horse, elephant, a host of carnivores, and their evolution have been well documented in the Siwalik Basin by pilgrim (1910, 19 13), Colbert (1935) and other paleontoIogists. Observations since the early nineteenth (circa 1839) century (H. Falconer, in Lydekker, 188 1) till the twentieth (Pilgrim, 1939; Li et al. 1981; Savage and Russel, 1983; Shackelton and Chang Chengfa, 1988) reveal the similarity of vertebrates in the Siwalik and the Tibetan basins and lead to the inevitable conclusion that the Central Himalayan axis had a low elevation which permitted free migration paths to the animals, a*d circul~&o;oi, of moist air into Tibet from the southern seas during the Oligocene through to at least Early Pliocene times. Late Pleistocene strata in eastern Tibet have yielded fossil Bibos - an animal found in low-altitude tropics today (Wan-Po and Hong-Xiang, 1981). Tibetan climate had remained hospitable enough for the s t ~ ~ e - a g e man whose archeological' remains are t"ond there (Wan-po and Hong-Xiang, 198 1; Valdiya, 1993). Gansser (1964) '

believes that the early man witnessed main elevation of the Himalaya.

Takkhola-Mustang half-graben on the northern slopes of the Central Himalaya in Western Nepal, a little north of the Annapurna Detachment (equivalent to the Trans- Himadri Fault, or South Tibet Detachment), provides sedimentological evidence on the elevation of the Central Himalayan axis during the earlier part of the Pliocene. The graben contains -3000 m thick Pliocene-Pleistocene sediments resting over a floor of Mesozojc successidn of the Tibetan Series (Brown and Nazarchuk, 1993). Presence of red paleosol in the basal part and the occurrence of warm- loving Pliocene ostracod Ilyocypris in the sediments suggest that the floor elevation was not in excess of 1500 m in an extant warm, humid climate (Fort et al. 198 1 a,b).

Xu Ren (1981) provides the best estimate of elevation gain as well as the rate of elevation from a study of fossil plants occurring on the northern slopes of the Himalaya and on the Tibetan Plateau. He deduced the habitat of fossil plants by comparing them with their modem relatives. The Tibetan Plateau seems to have risen very gently for the first 50 million years of the Cenozoic, then it started picking up speed from the Late Miocene(-10 m.y.) and moved up at spectacularly rapid rate during the last 3 million years (Xu Ren, 1981). The elevation history of the Tibetan Plateau is as follows (Fig. 2a):

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Trans-Himalaya

Sub-Himalayan Parautochtho~~

Sub-H ilnalayan

Authochthon

Indo-Gangetie Foredeep Basement

of Indian Shield

0

-

100 200k111

Fig.1. Tectonic subdivisions of Western Himalaya and adjacent regions (adapted from Raiverman, 2002). SSZ - Shyok Suture Zone; MKT

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Main Karakorvm Thrust; NPHM - Nanga Parbat-Haramosh Massif; ITS2 - Indus-Tsangpo Suture Zone; MMT - Main Mantle Thrust; TSZ - Tethyan Shear ZonetZSZ - Zanskar Shear Zone (

1

Trans-Himadri Fault / South Tibet Detachment ); MCT I1

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Main Central Thrust I1

(1

VT - Vaikrita Thrust); MBT - Main Boundary Thrust; MBF - Main Boundary Fault; HFB - Himalayan Frontal Boundary. Areas of radiometric geochronological studies stippled. see FEg.2e for age frequency.

A genetic support for the exponential rate of Himalayan elevation comes from the process-response model popularised by Krumbein for stratigraphic studies(see Krumbein and Graybill, 1969). The deposition of elastic sediments in the foreland basin resulted as a direct response to the rising mountain chain, so the rate of sedimentation as well as the coarsening upward of grain size would reflect the status of uplift (Raiverman, 2002). Figure 2b is a plot of time against thickness of enseqs -the stratigraphic units of the foreland basin chronologically redefined to correspond with tectonic pulsations as reflected in grain size cycles (Raiverman, 2 0 2 ; Raiverman et al. 1983). The resultant curve, proportional to the rate of sedimentation, rises exponentially in keeping with the uplift rate of the Tibetan Plateau and the Central Himalaya, Mktivier et al. (1999) who integrated the rate of sedimentation for the Cenozoic

basins of India present also a very similar picture, reproduced with thick broken line in F i g . 2 ~ for comparison. The lithology of the foreland sediments expressed as percentage of three main components - shale, sandstone and conglomerate - plotted parallel to time-axis for each enseq in Fig.2d graphically represents how grain size increased with time, keeping pace with Himalayan elevation. The youngest enseq (Sarda / Indo-Gangetic Alluvium) has been omitted in this diagram (Fig. 2d), while the preceding three (Sadhot, Batwan, Wah Devi) have been grouped together for convenience of plotting. Brookefield (1989) observes in a sediment budget study that sediment volume delivered to the Indian ocean increased progressively in post- Oligocene times by erosion of the rapidly rising Himalaya, and the exponential rate of elevation remarkably corresponds with radiometric geochronology and paleobiologic evidence.

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Present

Late Pleistocene

I'

,

, l : r E n s e q

1 .O Jawalarmukhi Es

60 50 40 30 30 t r o

0.81 /~akreri Enseq

:

1

2

vYhattl

yseq

-;/

-

"E Oharmpur Enseq J

2'0.4

w 60 50 40 30 , r * * * 10

Sdt-

JmiKld

ef-

100 Dharm~ur Enseq _r Kin Es I Mak Es I Es r Es r WD rl

Rg.2. Paleobotanical, sedirnentological and geochronoIogical evidence from the Tibet-Himalaya region on the pattern of Himalayan

elevation, (a) from altitude changes in Tibetan Plateau based on fossil flora (after Xu Ren, 1981), (b) from time-thickness plot of enseqs in Western Himalayan foreland during the Cenozoic (after Raiverman, 2002, fig. 6.13), (c) from rate of sedimentation in the Cenozoic basins of India (afcer Mktivier et al. i999), (d) from gross lithological changes through time (afler Raiverrnan,

2002), (e) from frequency plot of radiometric ages in rn.y,, area shown in Fig.1, data from north of MMT-ITSZ excluded for frequency computation. Horizontal scale in Figs. 2(a) to (e) same for easy comparison.

Radiometric Geochronology on Himalayan Elevation Some,authors assign the main Himalayan elevation to an age around 20 m.y. based on radiometric dating of and geochemical observations on the Central Crystalline Complex and its clastic products redistributed on land and in ocean basins (Cerveny et al. 1988; France-Lanord et al. 1993; Valdiya, 2005, and other authors reviewed therein). Such ideas not only bypass the classical disciplines of stratigraphy and paleobiology, as described above, but also

miss the full information potential of radiometric geochronology, having utilised only a narrow window thereof. Igneous intrusion, high grade metamorphism and their exhumation have been a continuous process in the Himalayan terrain both before and after 20 m.y., and the later events appear to be even more potent. A frequency plot of cooling ages of minerds in the western Lesser, Higher and Tethyan Himalaya (Fig. 2e) provides a regional picture of the uplift waves. The frequency curve represents whole-

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rock and mineral ages based on Rb-Sr, K-Ar and Ar-Ar

systematics and fission-track annealing temperatures, taken out of a report compiled by the geochronology laboratory of the Geological Survey of India (Bhanumati and Roy, 2000). These data were generated for the Western Himalayan region during the second half of twentieth century by a number of authors like A. Desio, Nand Lal, K.K. Nagpaul, V.B. Bhanot, K.K. Sharma, G.D. Ashgirei, A.K. Sinha, P. K. Mehta, M.P. Coward, P.J. Treloar, M.P. Searle, Ashok Kumar, R.B. Sorkhabi,

P.K.

Zietler and many others $or details see Bhanumati and Roy, 2000). The cut-off date selected at 90m.y. in the frequency diagram (Fig.2e), about 40m.y. preceding the orogeny, offers precise identification of the change forced on the frequency pattern with the initiation of orogenic movement. The cooling age frequency curve starts rising since 50m.y. (closing of Indus Suture), followed by several smaller peaks till the sharp rise at 20m.y. This date, corresponding to HOM (Himalayan Orogenic Movement) - 2 (Shanker et al. 1989), in spite of its strong signal, marks only a modest Himalayan elevation (1.4 km, Fig. 2a) but its significance lies elsewhere, in the articulation of foreland tectonics and stratigraphy. An analysis of the f~reland~architecture and its evolution reveals that transverse basement ridges which partitioned the foreland basin so far into a number of disconnected or partially connected depressions, got completely submerged at

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18m.y, and, for the first time during the Cenozoic, the Himalayan strike was fully imposed over the foreland depocentre (Raiverman, 2002, fig.7.15, p.132; Raiverman, 2003; Narula et al. 1989). Sedimentation in the frontal basin achieved lateral continuity, end to end from Potwar in the west to Ganga and perhaps Brahmaputra Basin in the east, ushering in the deposition of the SiwaIik Group. Secondly, even as a modestly elevated geomorphic feature the Central Himalayan Crystallines had been making their contribution felt geochemically as far away as1 the Bengal Fan sediments (France-Lanord et al.

1993).

The frequency peak of cooling ages between 6 and 4 million years is higher still than at 20 m.y. (Fig.2e). It Iies close to the sharp inflection points in the rising curves of Himalayan elevation (Fig.2a) and foreland sedimentation rates (Fig. 2b) and coincides with the appearance of conglomeratic strata in the basin (Fig.2d). Its termination marks HOM-3 (3.5 .c l m.y.) of Ravi Shanker et al. ( 1989). Accelerated elevation continued in the Himalaya even after 4 m.y. Considering the average elevation of Tibetan Plateau at 5 km today and 2.6 km at 4 may. (Fig.2a), the upward movement since 4 m.y. should have been a minimum of 2.4 km, possibly more, if we take into account the denudation factor. This phase of rapid uplift that disrupted

the faunal affinity and climatic parity between Tibetan and Siwalik basins is confirmed by similar indications from mineral cooling ages, noted specifically by some authors like Zeitler (1985) in the Nanga Parbat-Haramosh Massif and Ashok Kumar et al. (1995) in the Kishtwar Dome.

Why is there a sharp drop in the cooling age frequency after 4 m.y. (Fig.2e)? It certainly could not mean cessation of uplift and exhumation with Quaternary neotectonic activities abounding (Narula et al. 1989; Valdiya, 2005). In general, uplift has far outweighed denudation, the reason for amazing elevation of the mountain system (Zeitler, 1985). Only the elevated mountain levels appear to be the current repository of older dates, in this case 4m.y. and older. Younger dates are visible only in some river gorges that have incised deeply into the Central Crystallines, like the Indus gorge north of Nanga Parbat, average elevation -1200 m above MSL, which has yielded F-T ages as low as 0.5 - 0.4 m.y. (Zeitler, 1985; Coward et at. 1986). The tectonics-supported uplift is now getting further boosted by isostatic uplift as a result of accelerated mass removal from the elevated mountain block (Ravi Shanker et al. 1989). The high rate of erosion along steep mountain slopes contrasts sharply from the slow mass wastage over the plateau surface, purportedly causing differential movement between the two, and the Higher Himalayan Crystallines slip upward past the Tibetan Series of sediments along the Trans-Himadri Fault/South Tibet Detachmen t.

Conclusions

This short communication on the Himalayan elevation pattern attempts to illustrate the efficacy of multiple-probe investigations in solving complex geological problems, avoiding the pitfalls inherent in restricted option on search tools. The principle of synergy has particularly become a catchword in one exploration geology sector, e.g. oi 1, though by no means exclusive to it, as the "anticlinal theory" was superceded by integrated basin analysis to meet the rapidly growing global demand for oil after the second World War. Synergistic vision is, however, often missing in structural geological works. A very gentle uplift rate of the Central Himalayan Range for the first 50 million years of the Cenozoic, followed by a moderately rapid speed till about

3 million years and then a severely accelerated uplift in the final stage is the pattern of Himalayan elevation, which is compatible with all available observations in stratigraphy, paleontology, paleobotany, paleoclimatology, structural geology and radiometric geochronology.

Acknowledgements: Inspiration came from friends and erstwhile colleagues - Arnitava Mukherjea of Oil and

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Natural Gas Commission, and Ravi Shanker who held office geochronological data readily available and to Bhanumati first in ONGC and then in the Geological Survey of India. Radhakrishnan of the same organisation for discussions. Special thanks are due to Tarun Raybarmon of the Author alone is responsible for the opinions expressed in Geochronology Laboratory of G.S.I. for making the the paper.

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(Received: I I July 2005; Revised f o m accepted: 9 Seprernber 2005)