Comment on “Trans‐Hudson Orogen of North America and Himalaya‐Karakoram‐Tibetan Orogen of Asia: Structural and thermal characteristics of the lower and upper plates” by M. R. St‐Onge et al.

[1] The recent paper by St-Onge et al. [2006, paragraph 32] cites a 2005 work by ourselves [Ali and Aitchison, 2005] as evidence that the initial contact of India with Asia (southern Tibet) took place at ‘‘magnetic Anomaly 22 (circa 53 Ma) times. ’’ It is a nontrivial point for St-Onge et al. to define the T0 condition since it underpins the overarching continent-continent collision model presented later in their manuscript. We are, however, unsure how such a statement on the collision timing can be drawn from our publication; the Asian continent was not shown in any of our reconstructions in that paper, nor did we mention or even assert that the event happened then. Indeed, in earlier works [e.g., Aitchison et al., 2002; Davis et al., 2002; Aitchison and Davis, 2004; Ali and Aitchison, 2004] we have stated specifically, via text and figures, that initial contact of India with Asia (in Tibet) took place in the Oligocene (that is 33.9 Ma). If St-Onge and colleagues are keen to draw upon literature supporting a 53 Ma collision, there are other more appropriate references to cite [e.g., Rowley, 1996; Hodges, 2000; Zhu et al., 2005]. It is also noted that the most widely used Late Cretaceous–Cenozoic geomagnetic polarity timescales date Anomaly 22 as either 49.714– 49.037 Ma [Cande and Kent, 1995] or 49.427–48.599 Ma [Gradstein et al., 2005], both of which are somewhat younger than the 53 Ma age St-Onge et al. [2006] assign to the anomaly. [2] Regarding our view on the timing of India-Asia collision, Figure 1 shows our reconstructions of the two continents in the latest Cretaceous and early Cenozoic based on the work by Ali and Aitchison [2004, 2005, 2006]. Critically, our modeling shows that the margins of the two crustal blocks which were to collide in what is now southern Tibet were nowhere near one another circa 53 Ma. Even if it is assumed that several hundred kilometers of shortening Figure 1. Model of the India-Asia collision system in latest Cretaceous and early Cenozoic times. The position of Eurasia at 55 Ma is based on the work by Ali and Aitchison [2004, 2006]. The site of India at 70 and 55 Ma is derived from Acton [1999], which we note is in close agreement with other recent works [Besse and Courtillot, 2003; Schettino and Scotese, 2005]. The size and shape of Greater India (craton in yellow; craton’s precollision northern extension in gray) is based on the work by Ali and Aitchison [2005]. The intraoceanic island arc immediately to the north of 55 Ma Greater India is based on the work by Davis et al. [2002], Aitchison and Davis [2004], and Abrajevitch et al. [2005]. The images were made using the GMAP software package [Torsvik and Smethurst, 1999]. Note that the stencil to draw Eurasia (‘‘Eurasiayoung.c97’’) has been modified slightly by extending Tibet south to include all of the ground north of the Yalung Tsangpo suture. The positions of Indochina and western Sundaland relative to Eurasia are based on the work by Hall [2002]. Eastern Sundaland (Borneo, western Sulawesi, etc.) is not shown. TECTONICS, VOL. 26, TC3018, doi:10.1029/2006TC002043, 2007