Deformational history of an outlier of metasedimentary rocks, Coast Plutonic Complex, British Columbia, Canada

Abstract

The Khutzeymateen assemblage records a portion of the polyphase deformation experienced by rocks within the core of the Coast Plutonic Complex. This series of deformational events probably took place during Late Cretaceous to Early Eocene regional orogenic activity. The Khutzeymateen assemblage is dominated by metamorphosed graywackes and volcaniclastic material. The earliest recognizable deformation involves thrust faulting that juxtaposed rocks of the Khutzeymateen assemblage and Central Gneiss Complex. The next deformational event produced isoclinal folds (F1), a penetrative foliation (S1), and a strong mineral lineation (L1). Both F1 and L1 have a 340°, 15 °orientation. Peak metamorphism (P = 450 ± 50 MPa, T = 650° ± 50 °C) was synchronous with this isoclinal folding event. F1 folding was followed by a brittle chevron folding event (F2) with a 335°, 20° orientation. There is a strong lithologic control on the development of F2 minor folds, which are developed predominantly within regularly layered quartzo-feldspathic lithologies. Open F3 folds (065°, 35°) may have developed by buckling related to differential uplift on the Larch Creek Fault. Post-F3 faults and minor shear zones are developed mostly in the eastern half of the area. The different deformational styles associated with the different deformational events probably reflect variations in the position of this group of rocks with respect to the surface during a single orogenic episode.