Paleocene–Eocene high-grade metamorphism, anatexis, and deformation in the Thor–Odin dome, Monashee complex, southeastern British Columbia

The Thor–Odin dome of the Monashee complex, in the southeastern Canadian Cordillera, comprises Paleoproterozoic basement gneiss with infolds of unconformably overlying rocks of a supracrustal cover sequence and is the deepest exposed structural level in the Omineca belt. Orthogneiss and paragneiss of the basement are migmatitic and contain ubiquitous stromatic leucosome and discrete phenocrystic and pegmatitic vein-type leucosome, which are all interpreted to have formed as a result of in situ melting. The stromatic leucosome is infolded with the country rock (F2), contains a weakly developed foliation, and has a biotite-rich melanosome. The phenocrystic and pegmatitic vein-type leucosome crosscut the stromatic leucosome and the transposition foliation (S2). Evidence to support an igneous and anatectic source for the leucosome includes (i) petrography, (ii) major and trace element chemistry, (iii) zircon morphology, and (iv) peak pressure–temperature (P–T) conditions. Sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U zircon dates range from ca. 56 to 54 Ma and are interpreted to represent the age of leucosome crystallization. Zircon commonly contains discrete ca. 2.6-1.8 Ga cores that are interpreted as detrital grains inherited from the host paragneiss. Anatexis was ongoing by ca. 56 Ma, as a result of regional prograde metamorphism, and was coincident, at least in part, with the formation of the penetrative S2 transposition foliation and large recumbent F2 tight to isoclinal folds. Anatexis continued during F3 and F4 folding. Melting may have continued until ca. 51 Ma, driven by decompression reactions, and was concomitant with the D5 extensional deformation.

Brittle faulting in the Thor–Odin culmination, Monashee complex, southern Canadian Cordillera: constraints on geometry and kinematics

Regionally recognized dextral strike-slip faulting is present in the Monashee complex of the southern Canadian Cordillera but is overprinted and partially obscured by subsequent extension. Eocene brittle faults and fractures within the Thor–Odin culmination of the Monashee complex are divisible into three distinct sets. Initial 340°–010° trending strike-slip faults (set 1) were locally overprinted and reactivated by normal faults with a 325°–020° trend (set 2). A third set of 255°–275° trending fractures (set 3) are interpreted as conjugates to set 1, reactivated as transfer faults to the set 2 normal faults. Large regional faults weather recessively, forming topographic lineaments that transect the Monashee complex. The Victor Creek Fault defines one such lineament. Detailed mapping within the northern Thor–Odin culmination reveals piercement points (fold hinges) on the east side of the fault that are not readily matched on the west side. The minimum displacement required on the Victor Creek Fault to down-drop the fold hinge below the level of exposure on the west side is 1370 m, assuming normal down-to-the-west displacement. The geometry of the fault is consistent with a set 1 dextral strike-slip fault, however. Matching the piercement points in the study area with possible equivalents to the north indicates 55–60 km of dextral strike-slip displacement.