Tatla Lake Metamorphic Complex: An Eocene Metamorphic Core Complex on the southwestern edge of the Intermontane Belt of British Columbia

Abstract

The Tatla Lake Metamorphic Complex (TLMC), which lies on the southwest side of the Intermontane Belt (IMB) in British Columbia, has characteristics typical of a metamorphic core complex: anticlinorial amphibolite‐grade gneissic and migmatitic core underlying a 1− to 2.5+‐km‐thick zone of mylonite and ductilely sheared metamorphic rocks which is in fault contact beneath an upper plate of low‐metamorphic‐grade cover rocks of the 1MB. Ductile shearing in the TLMC involved tonalitic to granodioritic orthogneiss and structurally overlying amphibolite‐grade metasedimentary rocks and greenschist‐grade chlorite‐actinolite‐albite schist. Structures observed throughout the ductilely sheared rocks include a gently dipping mylonitic foliation (Ss), containing a mineral lineation (Ls) which trends toward 280° (100°) ± 20°. Minor folds of variable trend (Fs), almost exclusively confined to metasedimentary rocks, are interpreted as synductile shear. Vergence of these folds defines a movement sense and direction of top toward 290° ± 20°. Kinematic indicators from rocks not deformed by synductile shear folds indicate a tops‐to‐the‐west sense of shear, while those within metasedimentary rocks (deformed by Fs folds) yield conflicting results, with a tops‐to‐the‐west sense predominating. Calculated a directions from Fs folds which deform Ls lineations indicate nearly horizontal Ds movement, subparallel to 290°–110°. The entire metamorphic core of the TLMC has been deformed by upright, west to west‐northwest trending, shallowly plunging map‐scale folds (F3). The steeply dipping, northwest trending Yalakom fault cuts all units and forms the southwestern margin of the TLMC. U‐Pb zircon geochronology has documented the existence of Cretaceous (107–79 Ma, in the core) and Eocene 55–47 Ma, (in the mylonitic zone) deformation and metamorphism in the TLMC. K‐Ar dates for biotite and hornblende of 53.4–45.6 Ma record the uplift and cooling of the TLMC. During early and middle Eocene time (55–47 Ma) metamorphic rocks of the TLMC were carried to higher crustal levels along the footwall of the TLMC normal ductile shear zone. Final uplift and development of F3 folds (post‐47 Ma) are possibly related to dextral motion along the Yalakom fault The TLMC has structural style and timing of deformation similar to other metamorphic core complexes in southeastern British Columbia. Local and regional evidence is consistent with the formation of the TLMC in a regional extensional setting within a vigorous magmatic arc. Similar interlayered gneisses with near‐horizontal layering and foliation may underlie the entire southern IMB.