Tracing patterns of erosion and drainage in the Paleogene Himalaya through ion probe Pb isotope analysis of detrital K-feldspars in the Indus Molasse, India

https://doi.org/10.1016/S0012-821X(01)00346-6Get rights and content

Abstract

The Indus Molasse is a pre- and syn-tectonic sedimentary sequence situated in the Indus Suture Zone of the western Himalaya. Spanning in time the collision of India and Asia, this deposit is well placed to record the evolving uplift and erosion history of the early Himalayan orogen. Nd isotope analyses from clay extracted from shales interbedded within the dominantly alluvial sequence indicate a low negative ϵNd (−1.64 to 0.72), in the basal Paleocene Chogdo Formation, slightly more negative than measured values from the Transhimalaya and Kohistan/Dras Arc. Up-section ϵNd becomes more negative, as low as −10.05, indicating influence of a different, more enriched source. Ion microprobe Pb isotopic analyses of single K-feldspars help constrain this source as being either the Lhasa or Karakoram Blocks, with westward paleo-current flow favoring the former. 207Pb/204Pb ratios are too low to be consistent with known Indian plate sources, a conclusion supported by the lack of muscovite or garnet that would be indicative of a High Himalayan contribution. Given the known age of rapid cooling of the High Himalaya at ∼20 Ma, and the lack of exposure of suitable lithologies prior to that time, an age of sedimentation prior to ∼20 Ma is inferred. The post-collisional change in paleo-flow and provenance is suggested to reflect the initiation of the Indus River during the Early Eocene. This study demonstrates the power of combined bulk sediment and single grain analyses in resolving provenance in tectonically complex settings.

Introduction

The Early Cenozoic collision of India with Eurasia and the consequent uplift of the Himalaya and Tibetan Plateau have created the most dramatic relief on earth. Understanding the growth of this system is not only important to understanding the processes of orogeny, but is also crucial to testing models of climate–tectonic coupling in South Asia. The height and extent of the Tibetan Plateau and High Himalaya disrupts atmospheric circulation on a global scale [1] and hence the Cenozoic growth of the plateau, coupled with chemical weathering in the Himalaya, may ultimately be responsible for the global cooling that led to the Plio–Pleistocene Ice Age [2], [3]. Moreover, the Tibetan Plateau appears to play an important role in driving a strong summer monsoon [4], [5], [6]. Determining the uplift history of the system is crucial to testing such models. Knowledge of the Miocene to Recent uplift and erosion history is now constrained in outline, due to work on sediments from the foreland basins [7], the Bengal Fan [8], [9], and from direct measurements on the crystalline basement of the High Himalaya (e.g. [10]). However, the early development of the system has remained obscure. This is because those rocks exposed at or close to the surface at that time have been eroded away leaving only the relatively insensitive, high temperature paleothermometers to record the cooling history at that time. Peak metamorphism in the Pakistan Himalaya postdates collision by ∼10 Myr [11] so that no record of Himalayan orogenesis preceding ∼45 Ma can be found in this tectonic unit. Further east, in the High Himalaya of Zanskar and Lahaul, rapid cooling dates from 20–25 Ma [12], [13], limiting the record of earlier orogenesis even further. Uplift history is therefore best charted through study of the detrital sedimentary record that spans this period. Unfortunately access to Eocene–Oligocene sediments is limited due to the difficulty in drilling the great thicknesses of the Indus or Bengal Fans, and due to the fragmentary nature of the sedimentary record of this age in the Indian foreland [14], which in any case was located far from the zone of active collision during the Eocene.

In this study we present data on the erosion history of the early Himalaya recorded in the Indus Molasse Basin, located in the Indus Suture Zone in the Indian Himalaya, which help constrain the nature of early Cenozoic erosion and the development of a post-collisional drainage system. To do this we investigate the source of the detrital minerals that comprise this succession using a combination of bulk sediment and single grain isotopic analyses.

Section snippets

Geologic setting

The Indus Molasse is a folded and thrusted sequence of dominantly clastic formations which is observed locally to rest unconformably over the Ladakh Batholith, where this contact is not rethrusted. The Indus Molasse also unconformably overlies Indian passive margin units (Lamayuru Group) south of Upsi [11], ophiolitic mélanges west of Chilling [15], and the Cretaceous forearc of the Kohistan/Dras Arc (Nindam Formation), also west of Chilling [15], [16]. However, the Indus Molasse is observed to

Stratigraphy

We choose to follow the defined stratigraphy of Searle et al. [22], based on the Indus Molasse section exposed in the Zanskar Gorge (Fig. 1, Fig. 2), because this section is the focus of this study. In this scheme the base of the section is marked by a well-cleaved, light, buff-colored shalely carbonate sequence, the Sumda Formation, in practice the upper part of the Jurutze Formation [17], [23]. The Sumda Formation is clearly marine, having a foraminifer fauna that constrains the Maastrichtian

Detrital mineral compositions

The provenance of the Indus Molasse can be constrained in part through its mineralogy. While epidote is common throughout the section, hornblende is only found in significant amounts in the upper parts of the Choksti Formation (Fig. 2). There is also apparent decrease in the dominance of biotite up-section. What is most noticeable in backscattered electron probe images is how sandstone located above the Nummulitic Limestone contains grains of large, single K-feldspar and quartz minerals,

Paleo-current indicators

Paleo-flow directions can be useful in constraining possible sediment sources. Since most of the sediments that comprise the Indus Molasse are braided river facies sandstones [24], this means that cross bedding is the most common form of recognizable paleo-current indicator. Brief turbidite intervals in the Choksti Formation, presumably of lacustrine origin, show scour structures within channel complexes. The anastamosing nature of braided streams means that there is an inherent spread of

Nd isotopes

The source of the Indus Molasse can be further constrained using the Sm–Nd isotopic system. The technique is based on the assumption that the finest fraction of detrital material represents a good average composition of the source area drained. Since weathering and the sediment transport process are not expected to result in isotopic fractionation, the measured isotopic signature of the shale fraction should reflect the bulk composition of the source.

We compare modern Nd isotopic character of

Pb isotopes of detrital feldspars

Although the mineral assemblage observed in the Indus Molasse and the positive ϵNd values argue against the modern High Himalaya as a plausible source, further discrimination is difficult because the mineralogy is insufficient to distinguish between possible Transhimalayan, Lhasa Block and Kohistani sources. It is also possible that the isotopically negative Nd source needed to account for the evolving Nd compositions was in part from the Indian Zanskar Shelf, if not the High Himalaya. We

Discussion

The ion probe isotopic data combined with the geologic constraints and the clay Nd data provide an image of the erosion history of the early Himalaya. It is important to remember that erosion is not the same as tectonic uplift, since this can be triggered by other factors, such as climate change. For the detrital history to be used to track rates of denudation thermochronological work would need to be performed on individual grains. This approach is most effective when depositional age can be

Conclusions

This study demonstrates that despite significant analytical uncertainties the in situ analysis of Pb isotopes within single K-feldspar grains is effective at constraining provenance in tectonically complex areas when used in conjunction with bulk mineral analyses, such as the Nd work on clays presented here. The approach allows end members to mixed sedimentary sequences to be constrained. Whole rock analyses only provide an average measurement of source composition. Without such single grain

Acknowledgements

P.C. thanks JOI/USSAC and WHOI for financial support to perform fieldwork in the Indus Suture and for some analytical support. P.C. is indebted to M.P. Searle for his introduction to the geology of the suture zone and to Fida Hussein Mittoo of Leh and Rockland Tour and Trek for all their help. M.P. Searle and Y.M.R. Najman are thanked for their advice on Himalayan erosion. J.P. Burg, C. France-Lanord and an anonymous reviewer provided helpful reviews that improved the quality of the work. The

References (61)

  • C. Gariepy et al.

    The Pb-isotope geochemistry of granitoids from the Himalaya–Tibet collision zone: implications for crustal evolution

    Earth Planet. Sci. Lett.

    (1985)
  • P. Vidal et al.

    Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya, Nepal)

    Geochim. Cosmochim. Acta

    (1982)
  • D. Ben Othman et al.

    The geochemistry of marine sediments, island arc magma genesis, and crust–mantle recycling

    Earth Planet. Sci. Lett.

    (1989)
  • W.F. Ruddiman, M.E. Raymo, H.H. Lamb, J.T. Andrews, Northern Hemisphere climate regimes during the past 3 Ma; possible...
  • M.E. Raymo et al.

    Tectonic forcing of the late Cenozoic climate

    Nature

    (1992)
  • S. Manabe et al.

    The effects of mountains on the general circulation of the atmosphere as identified by numerical experiments

    J. Atmos. Sci.

    (1974)
  • W.L. Prell et al.

    Sensitivity of the Indian monsoon to forcing parameters and implications for its evolution

    Nature

    (1992)
  • F. Fluteau et al.

    Simulating the evolution of the Asian and African monsoons during the past 30 Myr using an atmospheric general circulation model

    J. Geophys. Res.

    (1999)
  • D.W. Burbank, R.A. Beck, T. Mulder, The Himalayan foreland basin, in: A. Yin, T.M. Harrison (Eds.), The Tectonic...
  • P. Copeland et al.

    40Ar/39Ar single-crystal dating of detrital muscovite and K-feldspar from leg 116, southern Bengal fan: implications for the uplift and erosion of the Himalayas

    Proc. Ocean Drill. Prog. Sci. Res.

    (1990)
  • C. France-Lanord, L. Derry, A. Michard, Evolution of the Himalaya since Miocene time: isotopic and sedimentological...
  • M.P. Searle, Cooling history, erosion, exhumation and kinematics of the Himalaya–Karakorum–Tibet orogenic belt, in: A....
  • S. Tonarini et al.

    Eocene age of eclogite metamorphism in Pakistan Himalaya; implications for India–Eurasia collision

    Terra Nova

    (1993)
  • P.J. Treloar et al.

    K–Ar and Ar–Ar geochronology of the Himalayan collision in NW Pakistan constraints on the timing of suturing, deformation, metamorphism and uplift

    Tectonics

    (1989)
  • J.D. Walker et al.

    Metamorphism, melting, and extension; age constraints from the High Himalayan slab of Southeast Zanskar and Northwest Lahaul

    J. Geol.

    (1999)
  • Y.M.R. Najman, P.D. Clift, M.R.W. Johnson, A.H.F. Robertson, Early stages of foreland basin evolution in the Lesser...
  • A.H.F. Robertson, Formation of mélanges in the Indus Suture Zone, Ladakh Himalaya by successive subduction-related,...
  • P.D. Clift, A. Carter, M. Krol, E. Kirby, Evidence for ophiolite obduction and continental collision within the Indus...
  • R. Weinberg

    The disruption of a diorite magma pool by intruding granite; the Sobu Body, Ladakh Batholith, Indian Himalayas

    J. Geol.

    (1997)
  • A. Steck, L. Spring, J.-C. Vannay, H. Masson, H. Bucher, E. Stutz, R. Marchant, J.-C. Tieche, The tectonic evolution of...
  • Cited by (0)

    View full text