Technical Comments

Response to Comment on “Tectonic control of Yarlung Tsangpo Gorge revealed by a buried canyon in Southern Tibet”

See allHide authors and affiliations

Science  21 Aug 2015:
Vol. 349, Issue 6250, pp. 799
DOI: 10.1126/science.aaa9636

Abstract

In their Comment, Zeitler et al. do not challenge our results or interpretation. Our study does not disprove coupling between tectonic uplift and erosion but suggests that this coupling cannot be the sole explanation of rapid uplift in the Himalayan syntaxes.

In their Comment on our study (1), Zeitler et al. (2) imply that we interpreted our findings to question the possible existence of feedbacks between surface and tectonic processes, which is not the case. We therefore appreciate the opportunity to clarify potential misunderstandings.

Zeitler et al. (2) suggest that the oldest sediments at the base of the valley fill could be 3 to 4 million years ago (Ma) in age. We have no objection to this assessment. As noted previously (1), our burial age stems from a drill hole located ~150 km upstream of the gorge and therefore represents a minimum age for the onset of deposition. We note, however, that discussing the exact timing of the onset of sediment accumulation was not the main focus of our study.

Zeitler et al. furthermore argue, “Wang et al. assume that a singular impoundment occurred behind a sharply defined structure that became abruptly active at 2 to 2.5 Ma.” This sentence is not correct. First, as mentioned above, we inferred that uplift commenced earlier than the burial agethat we obtained from the base of drill core 3, and therefore we did not assume that a certain structure became “abruptly active at 2 to 2.5 Ma.” Consequently, we have no objections regarding the following paragraph, in which Zeitler et al. cite published evidence for regional exhumation before 2.5 Ma, many of which we have cited in our study (1) already. Second, we did not make any inferences about which structures are responsible for the uplift. From the pattern of bedrock cooling ages that we summarized in figure 2C in (1), we would not infer a sharply defined structure, and we did not do so in the original Report. We also did not comment on whether one or more impoundments occurred, but, in the absence of evidence for a more complicated scenario, our preferred model is that uplift of the Namche Barwa–Gyala Peri massif was a one-directional process with time. Nevertheless, it is conceivable that impounding and upstream sediment accumulation did not occur continuously at a steady rate. For example, if climatic fluctuations during the Quaternary resulted in changing river transport capacities and/or occasional glacial damming and catastrophic flooding (3), the net long-term deposition, which is evident in the valley fill, could have been superimposed with short periods of reincision. Although the details of the valley-filling episode could provide interesting insights into theQuaternary landscape evolution of this region, our observations do not bring any constraints, and therefore, we refrained from commenting on these aspects.

It is clear that we do not question the potential existence of feedbacks between erosion and uplift when we wrote, “Even if positive feedbacks between erosion and uplift nowadays help to maintain these gorges in their current location, our results suggest that rapid incision within the Tsangpo Gorge is the result rather than the cause of rock uplift.” The actual question raised by our observations regards the initiation of these feedbacks. Why do we see exceptionally rapidly uplifting, young metamorphic massifs in the syntaxes regions, where the Yarlung Tsangpo and Indus Rivers traverse the Himalaya, but nowhere else in the Himalaya? Is it because of large-magnitude river incision, as was suggested by Zeitler and colleagues earlier (4), or is it because of the unique tectonic setting in these corner regions of the Himalaya? This, in our view, important question remains unanswered and avoided by Zeitler et al.

In fact, we find comments by Zeitler et al. contradictory when they write, “As regional rock uplift focused into the massif, uplift and erosion rates became locally sufficient to permit feedbacks to develop between coupled tectonic and surface processes,” which suggests to us that focusing of rock uplift into the massif occurred before initiation of feedbacks, seemingly irrespective of surface erosion. Later, Zeitler et al. write, “The tectonic aneurysm develops in response to exhumation-driven thermal thinning of the upper, high-strength portion of the crust in actively deforming regions that have the potential for rapid erosional flushing.” In this case,the “potential for rapid erosional flushing” appears to be a necessary condition. Perhaps we misunderstood Zeitler et al.’s (4) previous use of the word “triggering,” but we still think that it is important to understand under which conditions coupled rapid uplift and exhumation with positive feedbacks between surface processes and tectonics can develop, and if it can initiate solely from below (i.e., by focused uplift) or from above (i.e., by surface erosion). The observation that the Tsangpo-Brahmaputra River was able to carve a deep canyon into southern Tibet and develop a graded profile with no knickpoint before rapid uplift of the Namche Barwa and Gyala Peri massifs suggests the former.

To conclude, we agree with Zeitler et al. that feedbacks between tectonic uplift and erosion play an important role in the development of orogens and that they are at play in the evolution of the Himalayan syntaxes. However, our study makes it unlikely that these feedbacks are the primary cause for the exceptionally rapid uplift that is observed in these regions.

References

View Abstract

Navigate This Article