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
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)
Climate models and the astronomical theory of the ice ages
Icarus
(1982)- et al.
The Indus clastics: forearc basin sedimentation in the Ladakh Himalaya (India)
Sediment. Geol.
(1988) - et al.
U–(Th)–Pb systematics and ages of Himalayan leucogranites, South Tibet
Earth Planet. Sci. Lett.
(1986) - et al.
Restoration and evolution of the intermontane Indus molasse basin, Ladakh Himalaya, India
Tectonophysics
(1990) - et al.
Sedimentology, petrography and tectonic significance of the shelf, flysch and molasse clastic deposits across the Indus suture zone, Ladakh, NW India
Sediment. Geol.
(1984) - et al.
Sm–Nd studies of Archean metasediments and metavolcanics from west Greenland and their implication for the earth’s early history
Earth Planet. Sci. Letts.
(1983) - et al.
The late Oligocene–early Miocene Himalayan belt; constraints deduced from isotopic compositions of early Miocene turbidites in the Bengal Fan
Tectonophysics
(1996) - et al.
Neogene Himalayan weathering history and river 87Sr/86Sr; impact on the marine Sr record
Earth Planet. Sci. Lett.
(1996) - et al.
A Sm–Nd isotopic study of atmospheric dusts and particulates from major river systems
Earth Planet. Sci. Lett.
(1984) - et al.
The Os isotopic composition of Himalayan river bedloads and bedrocks; importance of black shales
Earth Planet. Sci. Lett.
(2000)