Formation of the Shuswap metamorphic core complex during late-orogenic collapse of the Canadian Cordillera: Role of ductile thinning and partial melting of the mid-to lower crust

Abstract

The early Tertiary evolution of the Shuswap metamorphic core complex is characterised by low-angle crustal detachments and nearly isothermal decompression followed by rapid cooling of rocks in the footwall of the detachments. Previous work as well as our own observations suggest that Paleogene late-orogenic extension produced the main tectonic features of the region. Furthermore, structural analysis of the migmatites and published geochronological data indicate that partial melting of the mid-to lower crust was coeval with extension in the upper crustal levels, suggesting that these two processes are linked genetically. Consequently, we propose that the formation of the Shuswap metamorphic core complex corresponds to late-orogenic gravitational collapse of the Canadian Cordillera accommodated by normal faulting of the brittle upper crust and by ductile thinning of the mid-to lower crust. The initiation and amplification of extension during the Paleocene in the Shuswap metamorphic core complex are tentatively related to partial melting of the thickened crust which caused drastic mechanical weakening of the crust. RESUME. -L'evolution durant Ie Tertiaire du complexe metal1'lorphique du Shuswap est marquee par la formation de detachements crustaux subhorizontaux au-dessous dcsquels les roches ont subi une decompression quasi-isothennique, suivie d'un refroidissement tres rapide. L'etude des travaux publies sur Ie complexe metamorphique du Shuswap, ainsi que nos .observations de terrain, montrent que I'extension Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 USA tardi-orogenique paleogene est 11 l'origine des structures majeures de cette region. L'analyse structurale des migmatites et les donnees geochronologiques indiquent que la fusion crustale fut synchrone de l'extension de la croute superieure, ce qui suggere que ces deux processus sont lies genetiquement. En consequence, nous proposons que la formation du complexe metamorphique du Shuswap corresponde 11 I'effondrement gravitaire de la Cordillere canadienne accommode par extension fragile de la croute superieure et par I'amincissement ductile de la croute mediane 11 inferieure. Nous proposons egalement que l'initiation et l'amplification de I'extension orogenique durant Ie Paleocene dans Ie complexe du Shuswap sont liees 11 la fusion de cette croute epaissie qui entralne un amollissement catastrophique de la croute.Mots des: Effondrement tardi-orogenique, metamorphic core complex, Shuswap, fusion partielie, migmatites, Cordillere canadienne, Colombie Britannique.

Exhumation of orogenic crust: Diapiric ascent versus low-angle normal faulting

“…A thermal boundary is placed at 15 km depth, the base of the rigid upper crust. This depth corresponds to fi eld observations in the Shuswap core complex (Vanderhaeghe and Teyssier, 1997).…”

Deciphering orogenic evolution

“…The impact of the thermal evolution of the crust and lithospheric mantle on the lithosphere's rheology and mechanical behavior is a major issue that is rarely taken into account. This point has been stressed by several authors ( [Vanderhaeghe and Teyssier, 1997] , [Vanderhaeghe et al, 2003] , [0275] and [Schulmann et al, 2008] ). This makes a big difference in rheological and deformation regimes governing "thermally mature" orogens such as the Himalayan and Variscan ones with respect to "thermally immature" ones (e.g., Western Alps).…”

The thermal–mechanical evolution of crustal orogenic belts at convergent plate boundaries: A reappraisal of the orogenic cycle

“…Others are characterized by shallow-dipping foliations indicative of gravitational collapse as it was proposed for the Variscan belt of western Europe (Rey et al, 1991(Rey et al, -1992Vissers, 1992;Burg and Vanderhaeghe, 1993;Doblas, 1994;Gardien et al, 1997), the Mesozoic North American Cordillera (Foster and Fanning, 1997;Vanderhaeghe and Teyssier, 1997;Kruckenberg et al, 2008), and part of the Cenozoic Alpine belt (Dinter and Royden, 1993;Sokoutis et al, 1993;Gautier and Brun, 1994;Vanderhaeghe, 2004;Duchêne et al, 2006). In these examples, migmatites and granites are juxtaposed to upper crustal units along low-angle detachment zones associated with granitic laccoliths (Lister and Baldwin, 1993;Foster and Fanning, 1997;Vanderhaeghe et al, 1999a;Teyssier et al, 2005).…”

“…6). Examples of such a superposition of tectonic-metamorphic conditions encompass the Caledonides exposed in Norway (Andersen, 1998;Labrousse et al, 2002), the Hercynian belt of western Europe (Vanderhaeghe et al, 1999a;Lardeaux et al, 2001;Ledru et al, 2001;Solgadi et al, 2007;Gardien et al, 2011), the CircumPacific Canadian Cordillera (Brown et al, 1986;Vanderhaeghe and Teyssier, 1997;Vanderhaeghe et al, 1999b;Norlander et al, 2002;Mihalynuk et al, 2004), the Aegean domain in Greece (Lister et al, 1984;Buick and Holland, 1989;Buick, 1991;Gautier and Brun, 1994;Avigad, 1998;Duchêne et al, 2006) and the higherHimalayan Crystallines . The duration of the HT conditions and partial melting of the orogenic crust is recorded by the age spread of granites issued from the mid to lower crust and emplaced at higher structural levels.…”