Birth of the northern Cordilleran orogen, as recorded by detrital zircons in Jurassic synorogenic strata and regional exhumation in Yukon

Abstract

The Whitehorse trough is an Early to Middle Jurassic marine sedimentary basin that overlaps the Intermontane terranes in the northern Cordillera. Detrital zircon dates from eight Laberge Group sandstones from various parts of the trough all display a major Late Triassic-Early Jurassic peak (220-180 Ma) and a minor peak in the mid-Paleozoic (340-330 Ma), corresponding exactly with known igneous ages from areas surrounding the trough. Source regions generally have Early Jurassic (ca. 200-180 Ma) mica cooling dates, and the petrology of metamorphic rocks and Early Jurassic granitoid plutons flanking the trough suggests rapid exhumation during emplacement. These data suggest that subsidence and coarse clastic sedimentation in the trough occurred concurrently with rapid exhumation of the shoulders. Isolated occurrences of sandstone and conglomerate units with similar detrital zircon signatures occur west and east of the trough, as well as overlapping the Cache Creek terrane, indicating that either the trough was once more extensive, or isolated basins tapped similar sources. Development of these sedimentary basins and accompanying rapid exhumation in the northern Cordillera were coeval with the onset of orogenic activity in the hinterland of the southern Canadian Cordillera, and subsidence in the western Canada foreland sedimentary basin. The Whitehorse trough is interpreted as a forearc basin that progressively evolved into a collisional, synorogenic piggyback basin developed atop the nascent Cordilleran orogen. Upper Jurassic-Lower Cretaceous fluvial deposits overlapping the Whitehorse trough have detrital zircons that were mainly derived from recycling of the Laberge Group, but they also contain zircons exotic to the northern Intermontane terranes that are interpreted to reflect windblown detritus from the Late Jurassic-Early Cretaceous magmatic arc that developed either atop the approaching Insular terranes to the west or southern Stikinia.

Seismic and gravity constraints on the crustal architecture of the Intermontane terranes, central Yukon

“…More recently, however, with the identification of a regional unconformity between these two units (Lowey 2008), White et al (2012) and Colpron et al (2015) associated the Whitehorse trough only with the Laberge Group (Fig. 3), which is an overlap assemblage deposited on both Stikinia and Quesnellia, as well as the Cache Creek terrane further south, and we have followed this usage.…”

“…2). These were exhumed during development of the trough, and constitute the main source of sediments for Laberge strata (Colpron et al 2015). The…”

“…Jurassic sedimentary strata that were deposited in a forearc setting, and subsequently shortened as the basin evolved into a synorogenic piggyback basin at the start of the Cordilleran orogeny (White et al 2012;Colpron et al 2015, and references therein).…”

“…In the northern Whitehorse trough, the Laberge Group comprises the Tanglefoot formation, which contains shallow marine to fluvial sandstone and conglomerate, together with a small amount of shale containing coal, and the Nordenskiold facies of volcaniclastic rocks. The Middle Jurassic to Lower Cretaceous Tantalus Formation containing fluvial conglomerate, sandstone and coal disconformably overlies the Laberge Group, and delimits the final stage of deposition in the Whitehorse trough (Long 2005;Colpron et al 2015).…”

“…3), is an overlap assemblage that extends across the inferred contacts between the Stikinia, Quesnellia, and Cache Creek terranes (Colpron et al 2015). The Laberge Group unconformably overlies the Triassic Lewes River Group, and block faulting and erosion of Stikinia prior to deposition of the Laberge Group has been inferred from its contact with different parts of the Lewes River Group (Colpron et al 2007); the Laberge Group is usually in contact with the Aksala formation, but southeast of Carmacks the Laberge Group immediately overlies the Povoas formation.…”