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Allochthonous Mesozoic and/or upper Paleozoic siliceous mylonite. sheared ophiolite and cataclastic granitic rocks are preserved in klippen above autochthonous, structurally imbricated Paleozoic and lower Mesozoic North American shelfstrata in central Yukon. The siliceous mylonite may be a trench melange the ophiolite oceanic forarc floor and the granitic rocks the roots of a northeast facing Early Mesozoic arc later accreted to North America. If so they imply that the arc system was partly thrust over the early Mesozoic North American margin. Northeast polarity of the arc suggests that it formed above southwestward subducting oceanic lithosphere and that the arc - continent suture marks a major closed ocean. This ocean may have spread in the Late Paleozoic and closed in the mid-Jurassic. Northeast abduction provides one continuous model for accreting the Early Mesozoic arc to North America for forming the [ore land [old and thrust belt, and for forming late-tectonic quartz monzonite plutons. It relates variations in the width o[ the [old belt to the degree of arc abduction. The duration and rates of the events are comparable to those in other mountain belts and subdue lion complexes

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The physiography of southern Yukon is dominated by upland plateaux and plateau remnants that probably evolved in the Tertiary, culminating in a mature erosion surface about Miocene time. Variations in the elevation of this surface are thought to result from uneven uplift and faulting in the Late Miocene or Pliocene. The Tintina and Shakwak Trenches are young grabens superposed on the upland and the mountain ranges are youthful dissections of raised parts of the plateau.In the Miocene, water from central Yukon probably drained to the coast across the region occupied by the St. Elias and Coast Mountains. About the late Miocene, general uplift occurred and the St. Elias and Coast Mountains rose unevenly above interior parts of the Yukon. As a consequence, the drainage became entrenched. During deglaciation, ice in the St. Elias outlasted that in the interior and this forced derangements of the entrenched stream system. It was abandoned in favour of the less direct northwest drainage of today. The northwest drainage persists, but is unstable and in danger of capture by more vigorous streams that flow directly to the coast.La physiographic du sud du Yukon est dominee par les hauts plateaux et des vestiges de plateaux qui se sont probablement developpes au Tertiaire pour arriver a une surface d'erosion de maturite au Miocene. Les variations dans I'elevation de cette surface sont probablement le resultat du soulevement inegal et du faillage a la fin du Miocene ou au Pliocene. Les fosses de Tintina et de Shakwak sont de jeunes grabens superposes aux plateaux et les chaines de montagnes sont des dissections plus jeunes dans les parties soulevees du plateau.Au Miocene, I'eau provenant du centre du Yukon se drainait probablement vers la c6te a travers la region occupee par les monts Cbtiers et St-Elie. Vers la findu Miocene, un soulevement general s'est produit et les monts St-Elie et Cbtiers se sont eleves inegalement au-dessus des parties interieures du Yukon. Comme consequence, le drainage s'est retranche. Durant la deglaciation, la glace de St-Elie a persiste apres la fonte de celle de I'interieur et celaa provoque des derangements dans le systeme de cours d'eau retranches. Ce systeme a ete abandonne en faveur du drainage moins direct vers le nord-ouest qu'on observe aujourd'hui. Le drainage au nord-ouest persiste mais il est instable et en danger de capture par des cours d'eau plus vigoureux qui coulent directement vers la cBte.[Traduit par le journal]Can.