Palaeomagnetic dating of the earliest continental Himalayan foredeep sediments: Implications for Himalayan evolution

EPSL ELSEVIER

Earth and Planetary Science Letters 128 (1994) 713-718

Express Letter

Palaeomagnetic dating of the earliest continental Himalayan foredeep sediments: Implications for Himalayan evolution Yanina M.R. Najman a, Randolph J. Enkin b Michael R.W. Johnson Alastair H.F. Robertson a, Judith Baker c

a,

a Department of Geology and Geophysics, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK b Geological Survey of Canada, P.O. Box 6000, Sidney, B.C., V8L 4B2, Canada c 1110 Maple Road, Sidney, B.C., V8L 1R8, Canada

Received 6 September 1994;accepted after revision 13 October 1994

Abstract

Between the time of the India-Eurasia collision (50-45 Ma) [1] and the climax of crustal shortening and thrust stacking in the Himalaya when the Main Central Thrust (MCT) was active (21 Ma) [1,2] there is a ca. 30 My gap about which little is known. This paper aims to shed light on this period by dating the initiation of major erosion from the rising Himalaya and the probable start of uplift, a significant event in the orogen's history. This was achieved by accurately dating, for the first time, the Dagshai Formation sediments of northern India, which are interpreted as early Himalayan foreland basin deposits that record initial large-scale erosion of the orogen [3]. Oriented hand samples were collected from six sites and analysed, using palaeomagnetic techniques. Both polarities are represented and the remanence passes a fold test. Fitting the measured palaeolatitude to that expected for the Indian plate dates the Dagshai Formation at 35.5 Ma + 6.7 Ma, and this is taken as the time when the embryonic Himalaya began to be strongly eroded and regionally uplifted.

The Subathu, Dagshai and Kasauli Formations [4] comprise the early Himalayan foreland basin sediments in India. They provide evidence of collisional processes between India and Eurasia [3]. The oldest formation, the Subathu Formation (Palaeocene-mid-Eocene) [5] consists of fossiliferous marine limestones and shales; it bears minimal evidence of terrigenous clastic influence and thus pre-dates Himalayan uplift. The sequence then passes conformably into the clastic red beds of the Dagshai Formation, which consist of sand-

stones, siltstones, mudstones and caliche that are interpreted as continental alluvial deposits [3]. The sandstones of the Dagshai Formation contain significant proportions of clastic terrigenous material (e.g., lithic fragments and mica), which are thought to have been derived from the newly uplifted landmass to the north. A northerly provenance is confirmed by southerly trending palaeocurrent measurements. The sandstone:shale ratio in the Dagshai Formation increases upsection, marking the progressively

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YM.R. Najman et al. /Earth and Planetary Science Letters 128 (1994) 713-718

greater influence of the uplifting landmass; by Kasauli Formation times the lithology was dominantly sandstone [3]. The marine, nonclastic Subathu Formation is thought to have been deposited on the open shelf during the early stages of collision between India and Eurasia. The subsequent sedimentary transition to clastic sedimentation in a continental environment (the Dagshai Formation) is taken to indicate the beginning of erosion, exhumation and uplift, with deposition of the detritus in the basin to the south. Although other variables (e.g., local facies variation, climate change, sea-level fall a n d / o r steady-state exhumation) could possibly also be responsible for a marine to continental transition and a clastic influx, we do not envisage that any of these factors will provide adequate explanations, for the following reasons: The sedimentary transition is not a local but a basin-wide event. Similar facies changes are seen in contemporaneous sediments in Pakistan and N e p a l - - i n Pakistan at the base of the red Lower Murree Formation [6-8] and in Nepal at the base of the red Dumre Formation [9]. This is, therefore, the result of a mountain-belt-wide event, the results of which stretch at least 1500 km laterally along the length of the orogen. Climate change, which is often the cause of increased erosion and deposition is, in this case, intrinsically linked with uplift. It is thought that the uplift of Tibet-Himalaya triggered the intensification of the monsoon in Asia; this major climatic change has been dated as Late Miocene [10], much younger than the Dagshai Formation. The start of this climatic change can indeed be observed in the foredeep sedimentary record, above the Dagshai Formation, at the transition to the overlying Kasauli Formation, where indicators of a semiarid environment (e.g., caliche) cease and evidence for a moist humid environment (e.g., abundant wood and plant material) begin

[3]. A eustatic sea-level fall is indicated for the Oligocene period [11], which does correspond to the time period being studied. Such a sea-level drop could have caused the switch from a shallow marine to an alluvial environment at the base of

the Dagshai Formation. However, this change also correlates with the first major influx of terrigenous sediment (i.e., lithic fragments, mica, etc.) to the basin, implying strong erosion of a source area situated ca. 1000 km to the north [12]. Sea-level change is therefore unlikely to be the sole cause of the sedimentary transition. Steady-state erosion, without regional uplift and increased elevation, could lead to erosion, exhumation and exposure of deep crustal rocks. Steady-state exhumation could be achieved in an orogenic environment by erosion of thickening crust. Indicators of Himalayan crustal thickening at this time include evidence of Barrovian metamorphism at 45-15 Ma [13], anatexis related to crustal thickening beginning at 35 Ma [14], and early thrust stacking which occurred subsequent to the mid-Eocene collision and suturing [1]. However, it is implausible that large volumes of clastic fluvial sediment would be transported ca. 1000 km southwards, in a relatively uniform direction, without significant hinterland relief. Therefore, although the possibility cannot be ruled out that exhumation rather than uplift was the mechanism by which the Dagshai Formation sediments originated, the Dagshai Formation deposits are nevertheless the first preserved sediments deeply eroded from the Himalaya by uplift a n d / o r exhumation, and must record a major event in the region. Dating the unfossiliferous Dagshai Formation is, therefore, of prime importance in elucidating Himalayan evolution. Previous dating attempts involved lithological comparison with the two probable correlatives mentioned above: (1) the Lower Murree Formation of Pakistan, dated palaeontologically as Early O l i g o c e n e - m i d Miocene [6-8], and (2) the Dumre Formation of Nepal, preliminarily dated palaeomagnetically at ca. 40 Ma and palynologically as possibly Palaeogene [9]. Since the breakup of Gondwanaland in the Jurassic, India has been drifting rapidly northwards [15]. From the Indian apparent polar wander path (APWP) [16] it is possible to calculate the expected palaeolatitude for an Indian locality for any given age. The time-averaged inclination of the geomagnetic field is a simple function of

Y.M.R. Najman et al. / Earth and Planetary Science Letters 128 (1994) 713-718

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Fig. 1. Site locations (with inset showing the relative location of India).

Table 1 Site m e a n s from the Dagshai Formation Site

n/N

NRM

%Rev ( m A / m )

Dg

D1 D2 D3 D4 D5 D6

12/13 0/12 0/12 10/10 11/12 11/11

1.04 0.63 1.07 1.67 0.99 1.02

100

134.0 -36.0 Incoherent Incoherent 45.5 54.0 104.2 -61.9 174.7 -73.2

10 82 100

Ig

D~

I~

k

a95

I

180.4

-37.1

15.1

12.1

9.6

25.0 187.4 218.4

22.6 -38.9 -39.6

24.6 59.4 96.1

10.1 6.3 4.7

8.0 5.0 3.7

n / N = n u m b e r of specimens used in m e a n / n u m b e r measured; N R M = geometric mean of natural remanent magnetisations; % R e v = percent of specimens with reversed polarity; D = declination; I = inclination; subscript ' g ' = geographic coordinates; subscript ' s ' = stratigraphic coordinates; k = Fisher precision parameter; a9s = circular 95% confidence i n t e r v a l ; / = marginal 95% confidence interval.

716

Y.M.R. Najman et al. / Earth and Planetary Science Letters 128 (1994) 713-718

geographic latitude; thus in India the palaeomagnetic inclination of a site is a measure of its age, after correcting for post-depositional transport. Oriented hand samples were taken from six Dagshai Formation sites in the sub-Himalayan tectonic belt of Himachal Pradesh province, about 300 km north of Delhi (Fig. 1). All the sites have clear bedding and way-up markers. Standard 2.5 cm cores were measured either on a robotized Schonstedt SSM-1 fluxgate magnetometer or on a Geofyzica JR5-A spinner magnetometer. Demagnetisation was done using a Schonstedt TSD-1 furnace with at least six steps to at least 600°C. In four of the six sites, well-defined high-temperature components of both polarities decaying linearly to the origin or sector-constrained great circles could be isolated. The other two sites had noisy demagnetisation curves and no coherence between specimens. A F demagnetisation was ineffective and mean destructive temperatures ranged from 600 to 650°C, indicating haematite as the main magnetic carrier. Table 1 and Fig. 2a and b give the site means [17]. In orogenic belts, simple untilting around horizontal strike lines will not necessarily restore beds to their original relative o r i e n t a t i o n s - - o n e also expects vertical axis rotations. Therefore, only inclination and not declination is the useful parameter. We studied the inclinations alone by approximating the marginal likelihood distributions of site-mean inclinations with Gaussian distributions [18] (Fig. 2c). Our preferred method of combining the site inclinations is a weighted mean of all four usable sites. The result is not significantly different if the outlying site D4 is omitted a n d / o r if the mean is unweighted. A positive fold test is indicated when minimum dispersion of site means occurs around 100% untilting [19]. The curve of the dispersion (as measured by the X 2) shows a minimum at 120% untilting, and is negligibly larger at 100% untilting. The X 2 value for structurally uncorrected beds is 4.2 times larger, indicating great dispersion. Thus we conclude that the remanence is pre-folding. Along with the reversals, the data support our interpretation that the remanence is primary. Tectonic shortening estimates of the sub-

N

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Y

N

60

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(c) 50

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D6 40

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30

weighted

D1

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mean 20

20

10

10

0 -+--0,0

i

i

0.1

0.2

0

0.3

Likelihood

Fig. 2. Site means of the four usable Dagshai Formation sites. a = upper hemisphere directions; [] = antipodes of lower hemisphere directions. (a) Before and (b) after tectonic correction. (c) Likelihood functions of inclination of the four usable Dagshai Formation sites, after tectonic correction.

Y.M.R. Najman et al. /Earth and Planetary Science Letters 128 (1994) 713-718

documented. Known pre-MCT 'Eo-Himalayan' [23] events i n c l u d e p e a k m e t a m o r p h i s m occurring at pre 30.7 Ma _+ 2.0 M a [24], a n d anatexis related to crustal t h i c k e n i n g b e g i n n i n g at ca. 35 Ma [14]. W e now add a third event, the initiation of uplift at 35.5 M a _+ 6.7 Ma, implying that it took ca. 15 Ma for crustal t h i c k e n i n g to advance sufficiently to allow uplift and s h e d d i n g of clastic s e d i m e n t into the f o r e l a n d basin located less t h a n ca. 1000 km (the length of the restored section) [12] to the south of the suture.

India (31.4N, 77.2E) 40

30 20

10 ~

717

o

Q.

-10 Acknowledgements -20 -30

70

60

50

40

30

20

10

0

Age (Ma) Fig. 3. The observed palaeolatitude of the sampling locality and the expected palaeolatitude as a function of age according to the Indian APWP [16]. The intersection gives the age of the Dagshai Formation.

H i m a l a y a n a l l o c h t h o n o u s units range from 18 to 30 km [12]. U s i n g the value calculated that is closest to this study area (30 km), the c e n t r e of the s a m p l i n g sites adjusted for 30 km southwestward p o s t - d e p o s i t i o n a l t h r u s t i n g is 31.4°N, 77.2°E. (Note that using the 18 km estimate only alters the result by 0.1 My). Fig. 3 shows o u r m e a s u r e d p a l a e o l a t i t u d e a n d the expected p a l a e o l a t i t u d e of the site as a f u n c t i o n of age according to the I n d i a n A P W P . T h e i n t e r s e c t i o n indicates the age of the f o r m a t i o n (35.5 Ma _+ 6.7 Ma, 95% confid e n c e interval). O t h e r c o m b i n a t i o n s of site inclin a t i o n s give ages of 3 3 - 3 9 Ma, showing our result to be robust. This result allows us to date a significant event in the early evolution of the I n d i a n Himalaya. T h e o n s e t of f o r e l a n d basin s e d i m e n t a t i o n was at 35 M a (the E o c e n e - O l i g o c e n e b o u n d a r y ) , which refutes the widely held belief that clastic sedim e n t a t i o n c o m m e n c e d in the M i o c e n e , with all the O l i g o c e n e missing [6,7,20-22]. T h e early history, b e t w e e n collision at 5 0 - 4 5 M a [1] a n d M C T thrust m o v e m e n t active at 21 M a [2], is poorly

Y.N. wishes to t h a n k D. N a j m a n , A. Skelton a n d C. Singh for assistance in the field. This work was f u n d e d by a Shell s t u d e n t s h i p from Shell International Petroleum Company Limited a w a r d e d to the first author. [CL]

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