BIRTH OF THE NORTHERN CORDILLERAN OROGEN, AS RECORDED BY DETRITAL ZIRCONS IN JURASSIC SYNOROGENIC STRATA AND REGIONAL EXHUMATION IN YUKON

Whitehorse trough (Laberge Group) is an Early to Middle Jurassic marine sedimentary basin that overlaps the Intermontane terranes in the northern Cordillera. Detrital zircon age dates from eight samples of Laberge Group sandstones are similar, with a major peak at 220-180 Ma and a minor peak in the mid-Paleozoic (340-330 Ma), and correspond exactly with known igneous ages from areas surrounding the trough. Source terrains generally have Early Jurassic (ca. 200-180 Ma) mica cooling ages and the petrology of metamorphic rocks and Early Jurassic granitoid plutons flanking the trough indicates rapid exhumation during emplacement. These data suggest that foundering of the trough and coarse clastic sedimentation occurred concurrently with rapid exhumation of the basin’s shoulders. Isolated occurrences of similar sandstone and conglomerate units with identical detrital zircon signatures occur west and east of the trough, as well as overlapping the Cache Creek terrane, indicating that either the trough was more extensive or isolated smaller basins tapped similar sources. Development of these sedimentary basins and accompanying rapid exhumation in the northern Cordillera are coeval with documented onset of orogenic activity in the hinterland of the southern Canadian Cordillera and onset of subsidence in the western Canada sedimentary basin. The Intermontane basins are interpreted as piggyback and foreland basins developed atop the nascent orogen and their sediment record witnessed the birth of the Cordilleran orogen. Late Jurassic fluvial deposits overlapping the Whitehorse trough have detrital zircons that were mainly derived from recycling of the Laberge Group, but also contain exotic zircons interpreted to reflect wind-blown detritus from Late Jurassic-Early Cretaceous arc developed atop the approaching Insular terranes to the west.