Extrusion of high-pressure Cache Creek rocks into the Triassic Stikinia–Quesnellia arc of the Canadian Cordillera: implications for terrane analysis of ancient orogens and palaeogeography
Jaroslav Dostal, J. Duncan Keppie, Filippo Ferri, 2009. "Extrusion of high-pressure Cache Creek rocks into the Triassic Stikinia–Quesnellia arc of the Canadian Cordillera: implications for terrane analysis of ancient orogens and palaeogeography", Ancient Orogens and Modern Analogues, J. B. Murphy, J. D. Keppie, A. J. Hynes
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The volcanic Triassic Takla Group constitutes a significant part of Stikinia and Quesnellia, two major terranes of the Canadian Cordillera that are separated by high-pressure rocks of the Cache Creek terrane containing Asian fauna. The geochemical and isotopic characteristics of the Takla Group in Quesnellia and Stikinia are similar, that is, tholeiitic basalts characterized by low abundances of strongly incompatible trace elements, negative Nb anomalies, +6 to +8 ɛNd values, the low initial Sr isotopic ratios, and relatively horizontal chondrite-normalized heavy REE patterns, all features typical of relatively primitive arcs with little or no continental crust involvement. These similarities have led to several geometric models: post-Middle Jurassic duplication by strike-slip faulting, and oroclinal or synformal folding. However, they are all inconsistent with either palaeomagnetic or faunal data, and the presence of a Triassic overstep sequence, which indicates amalgamation c. 50 ma before emplacement of the youngest oceanic rocks of the Cache Creek terrane. An alternative model is proposed: oblique eastward subduction of the Cache Creek accretionary prism and fore-arc producing high-pressure metamorphism, followed by extrusion into the arc and exhumation by the Middle Jurassic. This model implies that these high-pressure rocks, rather than marking an oceanic suture between disparate arc terranes, support a para-autochthonous origin.
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Plate tectonics provide a unifying conceptual framework for the understanding of Phanerozoic orogens. More controversially, recent syntheses apply these principles as far back as the Early Archaean. Many ancient orogens are, however, poorly preserved and the processes responsible for them are not well understood. The effects of processes such as delamination, subduction of oceanic and aseismic ridges, overriding of plumes and subduction erosion are rarely identified in ancient orogens, although they have a profound effect on Cenozoic orogens. However, deeply eroded ancient orogens provide insights into the hidden roots of modern orogens. Recent advances in analytical techniques, as well as in fields such as geodynamics, have provided fresh insights into ancient orogenic belts, so that realistic modern analogies can now be applied. This Special Publication offers up-to-date reviews and models for some of the most important orogenic belts developed over the past 2.5 billion years of Earth history.