Pre- to post-Cordilleran transposition history of Joss Mountain: Insights into the exhumation of the Shuswap complex, southeastern Canadian Cordillera

Abstract

We present new multiscale structural, mineral chemical, and U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data in order to unravel part of the tectono-metamorphic evolution of the Shuswap complex in the southern Canadian Cordillera. We reconstructed the pressure-temperature-deformation-time ( P - T - d - t ) history of the Joss Mountain domain within the Shuswap complex. The west-dipping Greenbush shear zone separates the Joss Mountain domain from the structurally lower Thor-Odin culmination to the east, the southern culmination of the Monashee complex, and one of the structurally deepest parts of the Shuswap complex. At Joss Mountain, the protolith of an orthogneiss crystallized at ca. 360 Ma which is consistent with Late Devonian arc magmatism along the western paleomargin of North America. Joss Mountain metasedimentary rocks and orthogneiss were transposed at ∼21–29 km depth over a period of at least 20 m.y., and possibly more than 38 m.y., during Late Cretaceous to Paleocene mature stages of Cordilleran continental collision. This mature collision took place while slow detachment of the subducted oceanic lithosphere occurred and thermal conditions were approaching those of a crust undergoing postorogenic thermal relaxation. Transposition at Joss Mountain ended earlier and exhumation started earlier than in the Monashee complex. Exhumation occurred under conditions of near-isothermal decompression and geothermal gradients consistent with lithospheric thinning. Earlier and slower exhumation of the Joss Mountain domain than of the adjacent northwestern Thor-Odin culmination may have resulted from normal movement along the Greenbush shear zone contributing to the exhumation of the Shuswap complex.