THE ROLE OF TRANSVERSE BASEMENT STRUCTURES AND OBLIQUE TERRANE BOUNDARIES ON THE ARCHITECTURE OF LARGE-HOT LONG OROGENS - EXAMPLES FROM THE HIMALAYA-TIBET AND GRENVILLE OROGENS (Invited Presentation)

The Mesoproterozoic Grenville and Cenozoic Himalaya-Tibet orogens are two archetypal examples of large-hot long orogens. While the Grenville Orogen provides a complete picture of continental collision from initiation to decay, the Himalaya-Tibet system informs us on processes that occur during the early to peak collisional stages. In the Himalaya, cross-strike structures in the Indian basement influence the crustal architecture on both sides of the India-Asia suture zone, from the foreland fold-thrust belt of southern Nepal to presently-active extensional graben in Tibet. The cryptic effects of cross-strike structures on the mid-crust have been determined through detailed petrochronology and phase equilibria modelling. Despite the apparent lateral continuity of the exposed Himalayan metamorphic core, its diachronous burial and exhumation on each side of a lateral ramp in the Himalayan basal detachment is likely linked to a cross-strike Indian basement structure interpreted from satellite gravity data. The influence of cross-strike structures on orogenic collapse, however, cannot be studied in the ongoing Himalaya-Tibet orogen. In the Grenville orogen, several structures accommodating late-Ottawan extensional collapse strike at a high angle to the Grenville Front and Allochthon Boundary thrust. In the Mauricie area, the eastern Taureau and Tawachiche shear zones exhumed mid-crustal units of the Laurentian crust against a previously accreted terrane. They coincide with a terrane boundary imaged at depth with aeromagnetic data that extend >200 km northwards. In the Lac Saint-Jean area to the NNE, the Saint-François-de-Sales shear zone displays similar structural characteristics, suggesting that it is one of several parallel late oblique extensional structures. Characterizing the control of cross-strike inherited basement structures and oblique deep crustal terrane boundaries from early mountain building to late extensional tectonics, through combined field, petrochronology, and geophysical studies, provides important information to help understand the internal geometry of large-hot long orogens.