In our 2019 paper on the Salt Range of Pakistan (Hughes et al., 2019, p. 1108), we wrote, “Recognizing significant geological differences between neighboring rocks separated by a major high-strain zone is, according to Martin (2017, p. 62), fundamental to terrane recognition.” Martin’s (2017) contention had been that the Salt Range thrust provides such a case, but his response to our paper reports no data suggesting significant geological differences across this structure. To do so would be challenging because, as we reported in our paper (Hughes et al., 2019), two boreholes drilled on either side of the thrust, located less than 25 km apart, reveal the same stratigraphy in terms of the rock succession, unit thickness, detrital zircon spectra, and depositional age of both hanging-wall and footwall rocks (Siddiqui, 2012, fig. 5; Hughes et al., 2019, figs. 3, 4, 11, and 12). The suite of rocks shared on both sides of the fault includes a distinctive 1000-m-thick evaporite succession, an overlying 200-m-thick sandstone, and then a transition to carbonate. Accordingly, by Martin’s own definition of a terrane boundary, the Salt Range thrust is excluded. In his response to our paper, and in order to preserve the terrane model, Martin relaxes the requirement for significant geological differences between adjacent terranes and instead claims that lateral displacement of several thousand kilometers along a transform fault resulted in two successions being juxtaposed—two successions that are so similar that they cannot be distinguished. A parsimonious alternative explanation, that there is no terrane boundary along the Salt Range thrust, is the explanation that we offered in our paper. We stand by that interpretation for the reasons presented in our paper and expanded upon below. We also correct a typographic error in the conclusion to our paper, noting that our Salt Range detrital zircon spectra contain abundant zircons with 1000–550 Ma ages, not 100–550 Ma ages, as printed (Hughes et al., 2019, p. 1110).
Martin’s response affords an opportunity to examine the implications of his ideas in more detail, and we thank him for this. Our work has long concentrated on the Late Neoproterozoic and Cambrian, because of all chronostratigraphic intervals, these are the best represented along and across all sectors (i.e., all lithotectonic units) of the Himalayan margin and on the Indian craton itself. Given the degree of facies variation across the Himalayan margin demonstrated in our paper (for example, the 1000-m-thick Neoproterozoic evaporite succession occurs only on the craton and in the Salt Range), it would be remarkable indeed for movement along Martin’s putative transform fault, now represented by the Salt Range thrust, to have juxtaposed two sets of rocks so similar as to provide no independent evidence of significant lateral displacement between them.
Given the above, the only other support for a Salt Range thrust terrane model would come from along the strike of the equatorial Gondwana margin. Specifically, Martin’s model predicts lateral displacement of his putative Himalayan terrane several thousand kilometers from an original Neoproterozoic and Cambrian position along the northern Australian margin. It suggests that the geology of the putative Himalayan terrane will have parallels with northern Australian geology that are as close as those with cratonic India, or more so. However, comparison of relevant north and northwestern Australian stratigraphy, biota, and detrital zircons with those of the Himalayan margin shows notable differences. For example, in our paper, we discussed the similarities among the Neoproterozoic Salt Range Formation, the Hanseran Evaporite on the Indian craton, immediately to the south across the Salt Range thrust, and the Ara Group of Oman. This is because these are the only areas along the equatorial Gondwanan margin in which extensive evaporites of this age are known. Such rocks are not known in northern Australia. Even more tellingly, northern Australian lower and middle Cambrian stratigraphic successions, biota, and detrital zircon signatures are markedly distinct from those immediately north of the Salt Range thrust, and indeed from any part of the Himalaya. Much of the lower Cambrian section of the northern Australian basins is made up of thick basaltic successions of the Antrim Plateau volcanics (Shergold et al., 1985); such rocks are absent from the Salt Range Cambrian section and, aside from reports of sparse Himalayan Cambrian volcanic rocks (Bassi and Chopra, 1984; Tangri and Pande, 1994), do not form a significant part of the Cambrian succession of any part of the Himalaya. Australian detrital zircon populations are also not closely comparable to those of the Salt Range or any part of the Himalayan Cambrian section. Australian profiles from Cambrian sandstone units (McKenzie et al., 2014, their fig. 2) lack the dominant peak of grains in the age interval 2.0–1.5 Ga that is typical of all Cambrian samples from the Indian subcontinent, whether cratonic or Himalayan (Hughes et al., 2019, their fig. 11). A study of detrital zircons and trilobite faunas from middle Cambrian rocks in Henan Island was able to locate the boundary between original Australian crust and that of the South China block (the region with the strongest stratigraphic and biotic affinity with the Himalayan margin; Hughes, 2016) within the island using the criteria that Martin (2017) previously considered essential for terrane recognition, i.e., significant geological differences across the fault. In this case, biostratigraphically dated middle Cambrian rocks on either side of the shear zone show strikingly different detrital zircon populations and fauna. In particular, the early middle Cambrian genus Xystridura was present in southeastern Henan Island, a form that is prevalent in Australia and Antarctica, but one that is not known in South China or in the Himalaya (Xu et al., 2014). This fossil occurrence matched detrital zircon data showing a grain age population characteristic of a western Australian source and a markedly different spectrum from that of western Henan Island. Hence, it is evident that there were significant regional variations along the equatorial Gondwana margin during the Cambrian, and that there is no evidence of a stronger link between Martin’s putative Himalayan terrane and the northern Australian margin than between the putative terrane and the Indian craton itself. In fact, compelling evidence directly contradicts that association. These data thus refute Martin’s (2017) prediction of geological continuity between northern Australia and the Himalayan margin at this time. Accordingly, northern Indian Neoproterozoic and Cambrian geology provides no evidence for the terrane model, and the evidence from northern Australia speaks directly against it.
We agree with Martin that there are significant differences between his model and that of DeCelles et al. (2000). DeCelles et al. (2000) stated that metasedimentary rocks of the Greater Himalaya were accreted onto the northern margin of India during the Cambrian-Ordovician boundary interval, and thus that the Greater Himalaya would not have been part of cratonic India prior to this event. The DeCelles et al. (2000) model differs from that of Martin (2017) in that the protolith of the putative Greater Himalayan terrane accumulated close to the Indian craton in the former model. In our view, both the Martin (2017) and DeCelles et al. (2000) models are incorrect, for reasons stated in our 2019 paper, as both studies overlooked Neoproterozoic and Cambrian rocks deposited directly on the craton itself. We agree with Martin that there are several difficulties with the DeCelles et al. (2000) model, including that deposition of the Tethyan strata was portrayed as having resulted from collision during the Cambrian-Ordovician boundary interval, yet the Tethyan strata extend further back into the late Proterozoic. More generally, we have demonstrated that detrital zircon spectra are consistent from the craton to the Tethyan Himalaya, and that facies are consistent with a continuous, northern-deepening margin (i.e., a margin without terrane boundaries; see McKenzie et al., 2013; Myrow et al., 2016, 2019; Hughes et al., 2019; and references therein).
With regard to the stratigraphy of the northwestern Himalaya, as noted by Martin, the Mandhali Formation occurs both in the hanging wall of the Tons thrust (i.e., the “outer” Lesser Himalaya) and in the footwall (i.e., the “inner” Lesser Himalaya). In the Tons Valley, the Mandhali Formation is stratigraphically the highest unit in the inner Lesser Himalaya, sitting depositionally above the Deoban Formation. It is also the lowermost stratigraphic unit of the outer Lesser Himalaya, and it is exposed in the hanging wall of the Main Boundary thrust. The depositional contact between the Mandhali and Deoban formations has been described in numerous publications (e.g., Valdiya, 1980; McKenzie et al., 2011, and references therein; see Fig. 1 herein). Furthermore, we do not treat the Paleoproterozoic–lowermost Mesoproterozoic as “part of one depositional succession” with the Neoproterozoic–Cambrian strata, but rather as two distinct successions within the inner Lesser Himalaya that are separated by a prominent unconformity, but not by a high-strain zone (see Fig. 1).
The issue of how much Cambrian and Neoproterozoic rock has been eroded in the Himalaya, and when, is important for several reasons. The Cambrian was a time of global sea-level highstand, and so it is no surprise that Cambrian rocks are known to extend far onto the craton: This is typical of other cratons (Peters and Gaines, 2012). We have argued that erosion of the Cambrian Tal Formation, and its lateral equivalents such as the Pakistani Tarnawai Formation, provided a significant source of radiogenic Os introduced into the world’s oceans beginning at 16 Ma, and that this was responsible for a major global change in ocean chemistry at that time (Myrow et al., 2015). Recent thermochronometric studies strongly support this idea (Colleps et al., 2018). Thus, in some parts of the orogen at least, substantial erosion of Cambrian rocks has occurred, and relatively recently. Our paper showed that thousands of meters of Neoproterozoic and Cambrian rock on the Indian craton are in depositional contact with underlying Mesoproterozoic rocks, correlative with similarly aged rock in the Himalaya, and formed as part of a continuous margin. Such rocks are exposed in the Marwar region and are present at depth in the Panjab-Ganganagar–Nagaur basin, and also in the Indo-Gangetic basin (Xiao et al., 2016; Tang et al., 2017). Accordingly, Neoproterozoic and Cambrian deposits originally extended far onto the craton itself, and we have presented strong evidence for their relatively recent erosion within the Lesser Himalaya.
In summary, due to their widespread distribution in the northern Indian subcontinent, Neoproterozoic and Cambrian rocks have special significance for evaluating tectonic models of the history of the northern Indian subcontinent prior to the onset of the Himalayan orogeny. In defending his terrane model, Martin (2019) violates his own predictions about the terrane’s affinities with northern Australia. Rather, available evidence indicates continuity between the northern Indian craton and all parts of the Himalaya at that time.