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. …