Reply to comment by Jason Ali and Jonathan C. Aitchison on “Trans‐Hudson Orogen of North America and Himalaya‐Karakoram‐Tibet Orogen of Asia: Structural and thermal characteristics of lower and upper plates” by M. R. St‐Onge et al.

Correlation of Archaean and Palaeoproterozoic units between northeastern Canada and western Greenland: constraining the pre-collisional upper plate accretionary history of the Trans-Hudson orogen

“…Recently, St-Onge et al (2006c) have argued that the tectonic record of collision and indentation of the Superior craton into the upper plate collage of cratons and terranes (Churchill domain) that formed northeastern Canada in the late Palaeoproterozoic Period (c. 1830-1785 was similar to the record of collision and indentation of India into the upper plate collage of cratons and terranes that made up central Asia in the early Eocene, beginning at c. 50.6 Ma (Rowley et al 2004;Searle et al 2007). Using the Asian template, the Archaean and Palaeoproterozoic geological record of western Greenland and northeastern Canada can thus be compared and sequenced (ordered) from north to south in order to document: (1) the amalgamation and growth of a northern composite upper plate (Churchill) domain mostly through terrane accretion; (2) the collision of the lower plate Superior craton with the upper plate Churchill domain; and (3) the pre-to syncollisional accretion of juvenile magmatic arcs along the southern margin of the growing Laurentian craton.…”

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

“…The fifth point is now much better understood following almost 3 decades of intensive research along the India‐Asia suture zone in southern Tibet [ Aitchison et al , 2000, 2002; Aitchison and Davis , 2004; Davis et al , 2002]; the sixth point can be deduced principally from the age dating of calc‐alkaline volcanic rocks erupted onto the southern Lhasa block, indicating a continuation of subduction‐related activity well into 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 a number of studies as being of late Priabonian (∼35 Ma) age [ Aitchison et al , 2007; Li et al , 2002; Li and Wan , 2003; Li et al , 2000; Wang et al , 2002; Xu , 2000; H. Willems, personal communication, 2007]. In their recent Tectonics commentary Searle et al [2007, p. 1] reiterated this key issue for determining the timing of continent‐continent collision by stating: “We believe that the strongest evidence for timing of the start of India‐Asia collision remains the ending of the final marine sedimentation in the suture zone and along the north Indian passive continental margin [ Searle et al , 1988, 1990, 1997]. ” While the youngest sediments may not have been documented from localities in NW India, marine sediments as young as Priabonian are indeed present at localities we have visited in the center of the orogenic system in Tibet [see Aitchison et al , 2007, and references therein].…”

“…Further geological evidence, in particular the sedimentary record, points to a second, more significant collision event and the elimination of oceanic space between India and Asia at that time. Thus, applying Searle et al 's [2007] primary criterion, continent‐continent collision must postdate these sediments.…”