Aitchison, JC

Ali, JR

Davis, AM

Journal of Geophysical Research Atmospheres

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Title Reply to comment by Eduardo Garzanti on "When and where didIndia and Asia collide"Author(s) Aitchison, JC; Ali, JR; Davis, AMCitation Journal Of Geophysical Research B: Solid Earth, 2008, v. 113 n.4Issued Date 2008URL http://hdl.handle.net/10722/73056Rights Creative Commons: Attribution 3.0 Hong Kong LicenseReply to comment by Eduardo Garzanti on ‘‘When and where didIndia and Asia collide?’’Jonathan C. Aitchison,1 Jason R. Ali,1 and Aileen M. Davis1Received 10 October 2007; revised 1 February 2008; accepted 5 February 2008; published 18 April 2008.Citation: Aitchison, J. C., J. R. Ali, and A. M. Davis (2008), Reply to comment by Eduardo Garzanti on ‘‘When and where did Indiaand Asia collide?,’’ J. Geophys. Res., 113, B04412, doi:10.1029/2007JB005431.[1] We thank Eduardo Garzanti for his comment[Garzanti, 2008] on our recent paper [Aitchison et al.,2007]. A traditional approach is to apply Occam’s razorand view our planet’s most significant orogenic zone as aresponse to closure of a one-ocean, two-continent conver-gence system. However, a vast body of new data has beenobtained in the decades following widespread acceptance ofthis conservative model by the scientific community. Wecontend that significant details are lost with this conven-tional view. Critically, it is now clear that the northernmargin of India experienced not one but two quite discretecollisions during the Paleogene.[2] The importance of definitions notwithstanding,semantic discussions regarding the definition of continent-continent collision and what is meant by the India-Asiaevent obfuscate the important issue of how this massiveorogen really developed. In particular, the possibility of‘‘preconditioning’’ the northern margin of India prior to itsimpact with Asia is easily overlooked. If, as we suggest,India experienced other collisions prior to that with Asia,then this would have affected the nature of its leading edgeprior to the terminal episode. We certainly do not dispute theexistence of geological phenomena that record a 55 Maevent in the Himalaya-Tibet orogen and elsewhere region-ally; we dispute only their interpretation. All aspects of the55 Ma data set are consistent with a collision between anintraoceanic island arc and the northern edge of GreaterIndia.[3] We contend that any Himalaya-Tibet orogen evolu-tion model has to have an India-Asia contact (t0) that iscompatible with what is known from both a plate tectonicperspective and the geology of the suture zone. There is noa priori reason why the 55 Ma event was a collision betweenIndia and Asia. Indeed, a critical test of the existing 55 Mahypothesis is whether or not the relative positions of Indiaand Asia at that time could have allowed collision betweenthese continents. Plate tectonic studies can be used (1) toconstrain India’s motion history, (2) to constrain India’sprecollision size, (3) to constrain the position of ‘‘stable’’Asia, and (4) to constrain the position of Asia’s southernedge; geological investigations from the Indus-YarlungTsangpo suture zone allow us (5) to examine the elementsinvolved in the convergence system and (6) to obtain directinformation concerning the timing of collision.[4] In summary, the first point has been tightly fixed forat least a decade [e.g., Acton, 1999; Besse and Courtillot,2002]; the second point was recently evaluated by Ali andAitchison [2005]; the third point has been addressed by Aliand Aitchison [2004, 2006]; our knowledge of the fourthpoint is limited, but with the third point now much betterknown we can be confident of where the edge of southernAsia likely lay during the early Cenozoic and mid-Cenozoic(and perhaps more importantly where it definitely was notlocated). Both Kohistan-Ladakh and Lhasa terranes amal-gamated with Eurasia prior to the early Cenozoic along theShyok (northern) and Bangong-Nujiang sutures, respectively.The Late Cretaceous–Eocene Transhimalayan/Gangdesebatholith developed above the continental convergent mar-gin, which marked the southern edge of Eurasia and isbounded by the Indus–Yarlung Tsangpo suture along itssouthern margin [Ahmad et al., 2005]. The fifth point isnow much better understood following almost 3 decades ofintensive research along the India-Asia suture zone insouthern Tibet [Aitchison et al., 2000, 2002; Aitchisonand Davis, 2004; Davis et al., 2002]; the sixth point canbe deduced principally from the age dating of calc-alkalinevolcanic rocks erupted onto the southern Lhasa block,indicating a continuation of subduction-related activity wellinto the late Eocene [Chung et al., 2005; Miller et al.,2000], and the oft-cited classic constraint [e.g., Searle et al.,1987], ‘‘the youngest marine sediments,’’ identified from anumber of studies as being of late Priabonian (35 Ma) age[Aitchison et al., 2007; Li et al., 2002; Li and Wan, 2003; Liet al., 2000; Wang et al., 2002; Xu, 2000; H. Willems,personal communication, 2007]. In their recent Tectonicscommentary Searle et al. [2007, p. 1] reiterated this keyissue for determining the timing of continent-continentcollision by stating: ‘‘We believe that the strongest evidencefor timing of the start of India-Asia collision remains theending of the final marine sedimentation in the suture zoneand along the north Indian passive continental margin[Searle et al., 1988, 1990, 1997].’’ While the youngestsediments may not have been documented from localities inNW India, marine sediments as young as Priabonian areindeed present at localities we have visited in the center ofthe orogenic system in Tibet [see Aitchison et al., 2007, andreferences therein].JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B04412, doi:10.1029/2007JB005431, 20081Tibet Research Group, Department of Earth Sciences, University ofHong Kong, Hong Kong, China.Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JB005431$09.00B04412 1 of 3[5] From a plate-modeling perspective (i.e., drawingupon the first through fourth points), collision of India withAsia at 55 Ma is impossible, but ~35 Ma is perfectlyfeasible [see Aitchison et al., 2007, Figure 5], which isconsistent with the geological record in the Indus-Yarlungsuture zone. The fifth and sixth points (principally geologicalstudies of the suture zone rocks with additional constraintsprovided by seismic tomography [Van der Voo et al., 1999])tell us that the 55 Ma event was a Taiwan-style arc-continent collision, with India playing a role analogous tothat of the present SE margin of China, which in essence iscolliding with and being overridden by, albeit in a geologi-cally temporary configuration, the Luzon arc.[6] To emphasize the robustness of our model, we drawattention to the fact that for India to have collided withTibet, part of Asia at 55 Ma would require a majormodification to one or more of the key plate-modelingand suture zone parameters (see above). Perhaps the mostflexibility we can offer adherents of the 55 Ma collisionmodel is to suggest the development of a model with amassively distended Greater Asia (second point). However,following collision, there would have to have been anenormous amount of sideways extrusion and/or shorteningacross Tibet (principally the Lhasa terrane) to ‘‘remove’’ acrustal block or blocks very roughly 2.5  106 km2 in area,for which no evidence exists. Incidentally, prior to thecollision the protuberance would need to have had effec-tively the same E-W width as northern Greater India, again,a quite unlikely scenario requiring special pleading.[7] Evidence of a 55 Ma collision between an intra-oceanic island arc and India is supported by the geologicalrecord. Paleontological evidence from Tibet clearly indi-cates that marine conditions persisted between India andAsia until almost the end of the late Eocene. Furthergeological evidence, in particular the sedimentary record,points to a second, more significant collision event and theelimination of oceanic space between India and Asia at thattime. Thus, applying Searle et al.’s [2007] primary criterion,continent-continent collision must postdate these sediments.[8] We do not dispute E. Garzanti’s earlier (1980s) data,instead preferring to build on his work. The past quartercentury has seen many new studies, particularly in areas ofsouthern Tibet where access was previously more difficult.Indiscriminant grouping of geological information poten-tially attributable to more than one collision event does notnecessarily provide the best answer. New data and enhancedstratigraphic resolution permit recognition of two majorevents separated by 20 Ma, the latter of which occurredaround the Eocene-Oligocene boundary when tectonicmodeling predicts India and Asia had converged. Wehypothesize that this event is the India-Asia collision in astrict sense. Our new hypothesis is consistent with presentlyavailable data and is entirely testable.[9] Clearly, continent-continent (India-Asia) collision isprecluded at 55 Ma, and available constraints serve tofalsify the existing hypothesis. We contend that recognitionof late continental collision is a major advance in under-standing of the India-Asia collision system. Regardless oftests of the 35 Ma interpretation, as the 55 Ma continent-continent collision hypothesis has already been falsified,this can no longer serve as a ‘‘fallback’’ position. We are notaware of any reason to suspect that Tethys differed markedlyfrom modern ocean basins and to suggest that it likely alsocontained numerous positive bathymetric features such asintraoceanic island arcs, plateaux, etc. Given the nature oflarge ocean basins it would come as little surprise if furtherstudy of the Himalayan-Tibet orogenic system were toreveal other collision events affecting either the southernmargin of Asia or the northern margin of India.[10] Acknowledgments. We thank our collaborators in the TibetanGeological Survey whose help made this research possible. This ongoingwork is financially supported by the Research Grants Council of the HongKong Special Administrative Region, China (projects HKU 7001/03P7001/04P and HKU 7001/07P).ReferencesActon, G. D. (1999), Apparent polar wander of India since the Cretaceouswith implications for regional tectonics and true polar wander, in TheIndian Subcontinent and Gondwana: A Palaeomagnetic and Rock Mag-netic Perspective, edited by T. Radhakrishna et al., Mem. Geol. Soc.India, 44, 129–175.Ahmad, T., N. B. W. Harris, R. Islam, P. P. Khanna, H. K. Sachan, and B. K.Mukherji (2005), Contrasting mafic magmatism in the Shyok and Indussuture zones: Geochemical constraints, Himalayan Geol., 26, 33–40.Aitchison, J. C., and A. M. Davis (2004), Evidence for the multiphasenature of the India-Asia collision from the Yarlung Tsangpo suture zone,Tibet, in Aspects of the Tectonic Evolution of China, edited by J. G.Malpas et al., Geol. Soc. Spec. Publ. 226, 217–233.Aitchison, J. C., Badengzhu, A. M. Davis, J. Liu, H. Luo, J. G. Malpas, I.R. C. McDermid, H. Wu, S. V. Ziabrev, and M. Zhou (2000), Remnantsof a Cretaceous intra-oceanic subduction system within the Yarlung-Zangbo suture (southern Tibet), Earth Planet. Sci. Lett., 183, 231–244,doi:10.1016/S0012-821X(00)00287-9.Aitchison, J. C., A. M. Davis, Badengzhu, and H. Luo (2002), New con-straints on the India-Asia collision: The lower Miocene Gangrinbocheconglomerates, Yarlung Tsangpo suture zone, SE Tibet, J. Asian EarthSci., 21, 251–265, doi:10.1016/S1367-9120(02)00037-8.Aitchison, J. C., J. R. Ali, and A. M. Davis (2007), When and where didIndia and Asia collide?, J. Geophys. Res., 112, B05423, doi:10.1029/2006JB004706.Ali, J. R., and J. C. Aitchison (2004), Problem of positioning PaleogeneEurasia: A review; efforts to resolve the issue; implications for the India-Asia collision, in Continent-Ocean Interactions Within East Asian Mar-ginal Seas, Geophys. Monogr. Ser., vol. 149, edited by P. D. Clift et al.,pp. 23–35, AGU, Washington, D. C.Ali, J. R., and J. C. Aitchison (2005), Greater India, Earth Sci. Rev., 72,169–188, doi:10.1016/j.earscirev.2005.07.005.Ali, J. R., and J. C. Aitchison (2006), Positioning Paleogene Eurasia pro-blem: Solution for 60–50 Ma and broader tectonic implications, EarthPlanet. Sci. Lett., 251, 148–155, doi:10.1016/j.epsl.2006.09.003.Besse, J., and V. Courtillot (2002), Apparent and true polar wander and thegeometry of the geomagnetic field over the last 200 Myr, J. Geophys.Res., 107(B11), 2300, doi:10.1029/2000JB000050. (Correction, J. Geo-phys. Res., 108 (B10), doi:10.1029/2003JB002684, 2003).Chung, S.-L., M.-F. Chu, Y. Zhang, Y. Xie, C.-H. Lo, T.-Y. Lee, C.-Y. Lan,X. Li, Q. Zhang, and Y. Wang (2005), Tibetan tectonic evolution inferredfrom spatial and temporal variations in post-collisional magmatism, EarthSci. Rev., 68, 173–196, doi:10.1016/j.earscirev.2004.05.001.Davis, A. M., J. C. Aitchison, H. Luo, and S. Zyabrev (2002), Paleogeneisland arc collision-related conglomerates, Yarlung-Tsangpo suturezone, Tibet, Sediment, Geol., 150, 247 – 273, doi:10.1016/S0037-0738(01)00199-3.Garzanti, E. (2008), Comment on ‘‘When and where did India and Asiacollide?’’ by Jonathan C. Aitchison, Jason R. Ali, and Aileen M. Davis,J. Geophys. Res., doi:10.1029/2007JB005276, in press.Li, G. B., and X. Q. Wan (2003), Eocene microfossils in southern Tibet andthe final closing of the Tibet-Tethys, J. Stratigr., 27, 99–108.Li, G. B., X. Q. Wan, Qiherige, D. Y. Liang, and W. C. Liu (2002), Eocenefossil carbonate microfacies and sedimentary environment in Gamba-Tingri, southern Tibet, Geol. China, 29, 401–406.Li, X. H., C. S. Wang, X. M. Hu, X. Q. Wan, Y. L. Xu, and W. J. Zhao(2000), The Pengqu Formation: A new Eocene stratigraphical unit inTingri area, Tibet, J. Stratigr., 24, 244–248.Miller, C., R. Schuster, U. Klo¨tzli, W. Frank, and B. Grasemann (2000),Late Cretaceous-Tertiary magmatic and tectonic events in the Transhima-laya batholith (Kailas area, SW Tibet), Schweiz. Mineral. Petrogr. Mitt.,80, 1–20.B04412 AITCHISON ET AL.: COMMENTARY2 of 3B04412Searle, M. P., et al. (1987), The closing of Tethys and the tectonics of theHimalaya, Geol. Soc. Am. Bull., 98, 678 – 701, doi:10.1130/0016-7606(1987)98<678:TCOTAT>2.0.CO;2.Searle, M. P., D. W. J. Cooper, and A. J. Rex (1988), Collision tectonics ofthe Ladakh-Zanskar Himalaya, Philos. Trans. R. Soc. London, Ser. A,326, 117–150, doi:10.1098/rsta.1988.0082.Searle, M. P., K. T. Pickering, and D. J. W. Cooper (1990), Restoration andevolution of the intermontane Indus molasse basin, Ladakh Himalaya, India,Tectonophysics, 174, 301–314, doi:10.1016/0040-1951(90)90327-5.Searle, M. P., R. I. Corfield, B. Stephenson, and J. McCarron (1997),Structure of the North Indian continental margin in the Ladakh-ZanskarHimalayas: Implications for the timing of obduction of the Spontangophiolite, India-Asia collision and deformation events in the Himalaya,Geol. Mag., 134, 297–316, doi:10.1017/S0016756897006857.Searle, M. P., M. R. St-Onge, and N. Wodicka (2007), Reply to com-ment by Jason Ali and Jonathan C. Aitchison on ‘‘Trans-HudsonOrogen of North America and Himalaya-Karakoram-Tibet Orogen ofAsia: Structural and thermal characteristics of lower and upper plates’’by M. R. St-Onge et al., Tectonics, 26, TC3019, doi:10.1029/2007TC002101.Van der Voo, R., W. Spakman, and H. Bijwaard (1999), Tethyan subductedslabs under India, Earth Planet, Sci. Lett., 171, 7–20, doi:10.1016/S0012-821X(99)00131-4.Wang, C. S., X. H. Li, X. M. Hu, and L. F. Jansa (2002), Latest marinehorizon north of Qomolangma (Mt Everest): Implications for closure ofTethys seaway and collision tectonics, Terra Nova, 14, 114 –120,doi:10.1046/j.1365-3121.2002.00399.x.Xu, Y. L. (2000), Early Tertiary calcareous nannofossils from southernTibet and the closing time of east Tethys in Tibet, Geosci. J. Grad.Sch. China Univ. Geosci., 14, 255–264.J. C. Aitchison, J. R. Ali, and A. M. Davis, Tibet Research Group,Department of Earth Sciences, University of Hong Kong, Pokfulam Road,Hong Kong, China. (jona@hku.hk)B04412 AITCHISON ET AL.: COMMENTARY3 of 3B04412

Reply to comment by Eduardo Garzanti on "When and where did India and Asia collide"

Aitchison, Jonathan C.

Ali, Jason R.

Davis, Aileen M.

University of Queensland eSpace

Reply to comment by Eduardo Garzanti on “When and where did India and Asia collide?”

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