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Discussion |
1 1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA (e-mail: pclift@whoi.edu)
2 2Institut de Minéralogie et Géochimie, Université de Lausanne, 1015 Lausanne,Switzerland (e-mail: mschlup@img.unil.ch)
3 3London Thermochronological Research Group, School of Earth Sciences, University & Birkbeck College, Gower Street, London WC1E 6BT, UK
4 4Institut de Minéralogie et Géochimie, Université de Lausanne, 1015 Lausanne, Switzerland
Peter Clift writes: In a recent contribution to the Journal of the Geological Society Schlup et al.(2003) provide new thermochronological controls to record exhumation in the Himalayas of eastern Ladakh, India. Because it is located in the Indus Suture Zone, understanding the tectonic development of this region is crucial to debates on the timing of the India–Asia collision and to models of strain accommodation in the aftermath of that event. As part of this new work the authors record a 23 Ma age fission track age for a zircon grain removed from a sandstone within the Cenozoic Indus Molasse (Indus Group), deposited in the suture zone after the India–Asia collision. Schlup et al. (2003) consider this 23 Ma age to be an original age, i.e. remnant from the source terrain from which it was eroded. As a result these authors conclude that the Indus Group was being deposited until at least 23 Ma (Early Miocene), just before the start of rapid exhumation in the High Himalaya, located immediately south of Ladakh at 22–20 Ma (e.g., Searle et al. 1992; Walker et al. 1999).
The authors state that the sediment is ‘unmetamophosed’ in appearance at outcrop, suggesting that the fission track age was not reset after deposition. However, Indus Group sedimentary rocks, also with a relative unmetamophosed outcrop, in the vicinity of the Zanskar Gorge of central Ladakh showed through an illite crystallinity study that they had been heated >200 °C after deposition (Clift et al. 2002). In this Zanskar area apatite fission track ages of 13–14 Ma, which are totally annealed when burial temperatures exceed c. 110 °C, must be linked to burial and exhumation following the inversion of the Indus Group basin, driven by the northward propagation of the Zanskar Thrust, which in turn is likely linked to the Early Miocene unroofing of the High Himalayas (Searle et al. 1990). Because fission tracks are annealed in zircon at 200–250 °C (e.g., Zeitler et al. 1982) it is possible that the zircon age reported by Schlup et al. (2003) in central Ladakh would have also been reset after deposition. In this case the 23 Ma age would reflect basin inversion in Eastern Ladakh, not the original cooling of the Ladakh Batholith, as proposed. Unfortunately no illite crystallinity data were presented by Schlup et al. (2003) to support their case for only moderate post-depositional burial. However, if the 23 Ma zircon age is reset then Indus Group sedimentation would have finished before 23 Ma, in turn implying an earlier inversion event.
This alternative hypothesis is supported by fission-track ages for the exhumation of the Ladakh Batholith itself, which Schlup et al. (2003) consider to be the source of the grain. Apatite fission track cooling ages from the Ladakh Batholith show a range of 28–44 Ma (Clift et al. 2002), with an age of c. 30 Ma being recorded just under the unconformity between the Indus Group and the granite. At this point in the section illite crystallinity indicates that the sediments do not seem to be heated above the apatite fission track annealing temperature. This implies that the youngest units of the Indus Group are younger than 30 Ma, but also suggests that if the surface of the batholith exposed during the sedimentation of the upper Indus Group had fission track ages of c.30 Ma then a 23 Ma age may well reflect resetting during burial, because a detrital grain eroded from this source would also have a fission track age closer to 30 Ma, and not as young as that reported by Schlup et al. (2003). Sedimentation in the Indus Group is then loosely constrained to have finished between 30 Ma and 23 Ma. In any case isotopic provenance controls on the source of the Indus Group indicate that, contrary to the earlier studies cited by the authors (e.g., Garzanti & van Haver 1988; Sinclair & Jaffey 2001) the Ladakh Batholith is only the dominant source to those sedimentary rocks that were deposited as south-flowing alluvial fans immediately adjacent to the southern edge of the Ladakh Batholith. Instead much of the Indus Group appears to have been eroded from the western parts of the Lhasa Block (southern Tibet), exposed further east than Ladakh, closer to the modern origin of the river near Mount Kailash. Although the Indus Group is also comprised of lesser volumes of material eroded from the Zanskar Himalaya and Ladakh Batholith (Clift et al. 2001) the mixed provenance means that the 23 Ma zircon reported by Schlup et al. (2003) may in fact reflect cooling and erosion of western Tibet. As such it cannot be used to constrain the exhumation of the Ladakh Batholith within the study area. Additional thermochronological work is required to find a unique interpretation to this interesting age determination.
10 June 2003
M. Schlup, A. Carter & A. Steck reply: We welcome Peter Clift's comment as it highlights the importance of thermochronological data in understanding regional tectonic development. Clift rightly questions the significance of a 23 Ma zircon fission-track (FT) age from a sandstone belonging to the Indus Molasse. If the zircon FT age records provenance it would define a young age for Indus Group sedimentation. Conversely if the zircon ages were reset by burial and post-depositional heating it would signify Indus Group sedimentation ended before 23 Ma. In the field the sandstone sample in question (B) displays well-preserved sedimentary structures and no evidence of recrystallization textures, although strongly dipping bedding show the unit has been deformed. Unpublished illite crystallinity data for this sample (determined using the analytical procedures of Jaboyedoff & Thélin 1996), record a value of 0.36 
2
° consistent with lower anchizone grade (200–250 °C in Jaboyedoff & Thélin 1996). Temperatures between 200 and 250 °C correspond to the lower part of the zircon fission track annealing zone (200–310 °C, Tagami et al. 1998) and therefore detrital zircon grain ages should not be completely reset. However, in some cases very old zircon grains and/or grains with high amounts of uranium and thorium can accumulate significant amounts of alpha decay radiation damage that lowers zircon fission track annealing zone temperatures to as low as c. 170 °C, and so in the case of sample B it is conceivable that the zircon grains could be completely reset. Such a process would nevertheless require that the rocks from which the zircons were eroded, remained in the top 5 km of the crust for many hundreds of millions of years. Given the active tectonic setting of the study area, it is needless to say that such history is unlikely.
Lack of detailed stratigraphic data in the Chumatang area prevented us assigning a depositional age and formation name to sample B sandstone. Whilst we consider it, likely this sandstone belongs to the Gongmarula–Hemis–Nurla Molasse deposited by continental meandering and braided rivers and prograding fan deltas (Steck 2003), we cannot rule out a provenance from the western parts of the Lhasa Block. If the sedimentation age is young, i.e. 
23 Ma it would imply that rotation of the molasse transgressional surface to its present position occurred during the Miocene. This rotation is the last deformation event recorded in the Indus Molasse from this region (Steck et al. 1993; Steck 2003). If we assume that the sampled sandstone was not affected by other deformation events, it would mean the former NE vergent folding and thrusting structures would be Eocene in age, based on K/Ar micas ages between 40 and 35 Ma (Van Haver et al. 1986).
It should be borne in mind that this discussion is centered on a single sample which is not sufficient evidence alone to adequately place a unique interpretation on either of the important hypotheses highlighted by Clift. The results of our study show that the evolution and exhumation history of the Indus Molasse and eastern Ladakh was different from other parts of the batholith 100 km to the NW (e.g. Sinclair & Jaffey 2001; Clift et al. 2002). A biotite 40Ar/39Ar plateau age of 32.6 ± 0.2 Ma and apatite fission track age of 5 ± 1 Ma (granite sample A in Schlup et al. 2003) record a different denudation history to that seen in central Ladakh where apatite fission track ages are mostly between 20 and 40 Ma. More extensive detailed thermochonometry, stratigraphic and structural mapping is needed across the entire Ladakh region before we are in a position to provide a solid interpretation for this important part of the Himalaya. In this context our results provide important new constraints for future research in this region.
31 October 2003
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