Comment on “When and where did India and Asia collide?” by Jonathan C. Aitchison, Jason R. Ali, and Aileen M. Davis

1 The principal merit of the article by Aitchison et al. 2007, published recently in the Journal of Geophysical Research, is to remind us how little we know about the early development of the Himalayas, even after several decades of modern geological research Gansser, 1964; Le Fort, 1989; Hodges, 2000. Ironically enough, one of the few facts which appears to be most firmly and consistently constrained by several independent and multidisciplinary lines of direct and indirect evidence is precisely the age of collision between India and Asia, what Aitchison et al. 2007 call “the 55 Ma dogma.” 2 I am personally pleased to read that 25 years after my Ph.D. field days in Zanskar, this date of the India-Asia collision has become so firmly established as to be called a “dogma.” However, I was quite unaware that a dogma was entering the scene when, during joint fieldwork by researchers from the Universities of Grenoble, Lausanne, and Milan, we managed to provide the first direct age of suturing between the Tethys Himalaya (India) and Transhimalaya (Asia) domains. 3 The first onlap of Transhimalayan-derived, volcaniclastic and ophioliticlastic fluviodeltaic sediments onto the Indian passive margin, corresponding to the end of marine sedimentation, was paleontologically dated at the P8 foraminiferal biozone (55 Ma according to the then recent timescale of Berggren et al. 1985). The article, after rejection by the Geological Society of America Bulletin under the consideration that it had “only local implications,” was eventually published Garzanti et al., 1987. Yet after another decade of active geological research along the suture zone, the dogma was still not there. Rowley 1996, p. 1, in his review of stratigraphic data on the age of the India-Asia collision, stated that “The age of initiation of this collision remains poorly constrained. The literature is replete with estimates that range from Late Cretaceous (>65 Ma) to latest Eocene (<40 Ma) with little consensus in between.” He concluded, “A review of the stratigraphic data bearing on the age of initiation of collision between India and Asia shows that it is only well constrained in the Zanskar-Hazara region” Rowley, 1996, p. 11; Rowley, 1998, p. 229. 4 A major breakthrough came in subsequent years, when a joint French-Swiss team managed to date the Tso Morari eclogites at 55 ± 6 Ma de Sigoyer et al., 2000. This age was later refined at 53.3 ± 0.7 Ma by Leech et al. 2005, who inferred first arrival of Indian continental crust at the Transhimalayan trench at 57 ± 1 Ma Leech et al., 2005, p. 83. Stratigraphic evidence from Zanskar (geohistory analysis by Garzanti et al. 1987, Figure 6) had indicated that Indian continental crust reached the Transhimalayan trench between foraminiferal biozones P5 and P6 (i.e., Paleocene-Eocene boundary, 58 Ma according to the Berggren et al. 1985 timescale and 55.8 ± 0.2 Ma according to the more recent Gradstein et al. 2004 timescale). A narrow sea arm where Nummulites thrived separated India and the Transhimalayan arc during foraminiferal biozones P6 to P7, and continental red beds spread across the newly formed suture zone during foraminiferal biozone P8 (∼51 Ma according to the Gradstein et al. 2004 timescale) Garzanti and Van Haver, 1988; Garzanti et al. 1996. 5 This virtually perfect coincidence between metamorphic evidence and geochronological ages at lithospheric depths and sedimentary facies and biostratigraphic ages at the surface Guillot et al., 2003 makes the India-Asia collision one of the best constrained orogenic events in Phanerozoic history. The dogma was established. 6 Aitchison et al. 2007 do not dispute geological evidence for the ∼55 Ma arrival of Indian continental crust at the Transhimalayan trench. On the contrary, they state that underthrusting of the Indian continental edge at that time is documented by geological evidence in South Tibet as well Aitchison et al., 2007, p. 15. Evidence for quasi-synchronous arrival of India at the Transhimalayan trench has accumulated in recent years DeCelles et al. 2004; Najman et al., 2005; Ding et al., 2005; Zhu et al., 2005, although the possibility of diachronous collision is not ruled out and may well explain why calc-alkaline magmatism continued to 37 Ma in South Tibet Aitchison et al., 2007, p.11. Nevertheless, Aitchison et al. 2007 claim that this does not represent the “true” India-Asia collision but simply represents a “minor” collision with an intraoceanic island arc. 7 The idea of “colliding continents” is a useful one in popular geology to attract the imagination of a wider public. In fact, continental collision translates into arrival of buoyant continental crust at an oceanic trench, subsequent choking of the subduction zone, and evolution of an arc-trench system into a high-relief and double-vergent orogen Garzanti et al., 2007. In this sense, the India-Asia collision undoubtedly coincides with attempted subduction of the distal Indian continental margin beneath the Transhimalayan subduction zone. This by no means minor compressional event fully affected the Asian mainland as well e.g., Horton et al., 2002. 8 What distinguishes a minor collision from a major collision, then? And, more interestingly, what exactly do Aitchison et al. 2007 envisage as the “Eurasian margin” if they believe that the Transhimalayan arc (extending from Pakistan to Myanmar Gansser, 1980) does not represent the true margin of Eurasia but simply “an intraoceanic arc?” Several distinct oceanic sutures do exist north of the Transhimalaya, but they have all long been established to be significantly older (and by no means younger) than the Indus-Tsangpo suture e.g., Allègre et al., 1984; Chang et al., 1986; Pudsey, 1986; Rolland et al., 2000. Where did this “true” younger collision between India and Asia take place, then? And what are the continental margins involved? A quite unusual geometry of colliding margins is proposed by Aitchison et al. 2007, Figure 5, who do not take into any consideration radically different and yet well-documented kinematic reconstructions based on detailed structural and tomographic data e.g., Replumaz and Tapponnier, 2003; Replumaz et al., 2004. While waiting for factual geological evidence in support of the new configuration envisaged by Aitchison et al. 2007, I remain more than willing to leave the door open to alternative provocative scenarios, being well aware that countless aspects of early Himalayan history still await understanding. 9 One of these scenarios is indeed the “ophiolite” issue. Formerly interpreted as Neotethyan remnants marking the Indus-Tsangpo suture Gansser, 1980, several ophiolitic allochtons exposed between the Tethys Himalaya and Transhimalaya zones have been subsequently found to be geochemically similar to suprasubduction lithospheric sections Mahéo et al., 2004; Dubois-Cote et al., 2005. Other ophiolites, however, retain geochemical characters suggesting formation at either oceanic-island or mid-ocean ridge settings and only subsequent accretion at a convergent plate boundary Sinha and Mishra, 1994; Ahmad et al., 1996; Pedersen et al., 2001; Miller et al., 2003. The age of ophiolite emplacement, not necessarily synchronous from Tibet to Pakistan and to as far as Oman Gnos et al., 1997, has been repeatedly and vividly debated in the last decades, with estimates ranging from Cretaceous to Eocene Searle, 1986; Kelemen et al., 1988; Aitchison et al., 2000; Corfield et al., 2005; Ding et al., 2005; Garzanti et al., 2005. 10 Aitchison et al. 2007 raise one serious, and yet long-debated, question about the delayed response in foreland basin subsidence and sedimentation. This problem is by no means eliminated by postponing the “true” India-Asia collision to the late Paleogene Aitchison et al., 2007. The 55 Ma event was undoubtedly a major one, comparable to Plio-Quaternary convergence and collision in Taiwan, yet the response in terms of foreland basin subsidence and sedimentation was limited to the Chulung La–Patala–Subathu–Bhainskati clastic wedges Garzanti et al., 1996; Najman and Garzanti, 2000; DeCelles et al., 2004; Najman et al., 2005, which seemingly contain limited volumes of orogenic detritus. The problem of delayed response from sedimentary systems demanded attention since the thick syncollisional succession formerly reported and described from Hazara Bossart and Ottiger, 1989; Critelli and Garzanti, 1994 was seriously questioned by Najman et al. 2001, who found detrital micas of Oligocene 40Ar/39Ar age in what were claimed to be Eocene deposits. 11 It is true that a “Taiwan stage” of clastic sedimentation Dorsey, 1988, dominated by low-rank metamorphiclastic detritus, was reached only at Balakot-Dagshai-Dumri times Najman and Garzanti, 2000; DeCelles et al., 2004. These pre-Siwaliks clastic wedges could only be indirectly dated so far as not older than the youngest 40Ar/39Ar or unreset fission-track ages of detrital minerals (37, 31, and 20 Ma, respectively Najman, 2006; DeCelles et al., 2001). The precise extent of the time lag involved in the underlying unconformity is consequently still undetermined and is the subject of lively debate Bhatia and Bhargava, 2006; Najman, 2007. DeCelles et al. 1998 identified a strongly weathered Oxisol in Nepal and interpreted the unconformity as developing during uplift associated with migration of the flexural bulge. This elegant and viable explanation is, however, dismissed by Aitchison et al. 2007 under the consideration that “the significance of sedimentary rocks below the unconformity may have been misinterpreted” Aitchison et al., 2007, p. 10. 12 But are we sure that morphogenic response to collision should necessarily be “immediate?” Is this a dogma? In fact, stratigraphic evidence from foreland basins associated with well-studied collision orogens unambiguously indicate that arrival of buoyant continental crust at the trench, eventually choking the subduction zone and promoting upward return of subducted material, may precede by a very significant period of time the morphogenic stage, when huge volumes of detritus are eventually produced Gansser, 1982; Schlunegger, 1999; Garzanti and Malusà, 2008. Should we then start from factual observations and build our conceptual models upon them, or should we use our preconceived ideas as a lens through which to evaluate other researchers' results and to decide which suits us best and which does not and is consequently liable to represent a misinterpretation? 13 In conclusion, it appears that, of the numerous unsolved chronical problems that afflict Himalayan geology, the date of arrival of the Indian continental margin at the Transhimalayan trench at ∼55 Ma (i.e., the collision between the Indian passive margin with the active margin of Eurasia Powell and Conaghan, 1973; Patriat and Achache, 1984; Garzanti et al., 1987, 1996; Klootwijk et al., 1992; de Sigoyer et al., 2000; Guillot et al., 2003; Leech et al., 2005) stands as one of the few major events that are robustly constrained by multidisciplinary geological evidence, including radiometric ages from deep lithospheric levels and biostratigraphic ages from sedimentary sequences accumulating at the Earth's surface. 14 Careful revision by Peter DeCelles, Stéphane Guillot, Bruno Lombardo, and Talat Ahmad is gratefully acknowledged.