Pre‐Mid‐Mesozoic tectonic evolution of the Yukon‐Tanana Terrane, Yukon and Alaska

Abstract

Yukon‐Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid‐Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian‐Early Mississippian, mid‐Permian, and Late Triassic‐Early Jurassic time. From Late Devonian to mid‐Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid‐Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid‐Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent‐continent collision. Although the Triassic‐Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other “pericratonic” terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed by transcurrent faulting.

The odyssey of the Cache Creek terrane, Canadian Cordillera: Implications for accretionary orogens, tectonic setting of Panthalassa, the Pacific superwell, and break-up of Pangea

“…Locally referred to as the Nisling, Yukon-Tanana and Kootenay terranes, these assemblages are characterized by quartzose rocks and are assumed to have originated as a continental margin. Voluminous Late Devonian, Mississippian, and Permian volcanic and plutonic suites imply subduction beneath the margin and the development of an Andeantype magmatic arc in the middle to upper Paleozoic [38]. Residence adjacent to a subduction zone is further indicated by the presence of blueschists and eclogites yielding cooling ages ranging from the Carboniferous to the Triassic [39].…”

The Tatshenshini shear zone (new) in southwestern Yukon, Canada: Comparison with the Coast shear zone in British Columbia and southeastern Alaska and implications regarding the Shakwak suture

“…The Nisling terrane is now considered equivalent to the Yukon-Tanana terrane [Mortensen, 1992]. Hence it represents the westernmost margin of the Intermontane superterrane.…”

Geologic Setting of Eclogite-facies Assemblages in the St. Cyr Klippe, Yukon–Tanana Terrane, Yukon, Canada

“…1;Mortensen 1992;Nelson 1993;Nelson et al 2006;Colpron et al 2006a;Piercey et al, 2006Piercey et al, , 2012. This marginal basin was originally termed the Anvil Ocean by Tempelman-Kluit (1979).…”

“…The assemblage also includes ultramafic rocks, carbonaceous pelite, quartzite, volcaniclastic rocks and minor marble (Mortensen 1992;Colpron et al 2006a;Murphy et al 2006). The Finlayson assemblage is unconformably overlain by the Mississippian to Early Permian Klinkit assemblage, consisting of variably metamorphosed mafic to intermediate calcalkaline volcanic and volcaniclastic rocks with minor alkali basalt, limestone/marble and conglomerate (Simard et al 2003;Nelson and Friedman 2004;Murphy et al 2006;.…”

“…Three Late Devonian to Late Permian, unconformity-bounded, geochronologically and geochemically distinct volcanic and volcaniclastic assemblages are built upon pre-Early Devonian metasedimentary basement known as the Snowcap assemblage (Fig. 1;Mortensen 1992;Piercey et al 2002Piercey et al , 2006Piercey et al , 2012Colpron et al 2006a;Piercey and Colpron 2009). The Snowcap assemblage is a heterogeneous mix of psammitic, pelitic and calc-silicate schist, quartzite, marble and amphibolite (Colpron et al 2006a(Colpron et al , 2006bPiercey and Colpron 2009).…”