Structural and metamorphic relationships between the Mount Ida and Monashee Groups at Mara Lake, British Columbia

Abstract

Mara Lake, British Columbia straddles the boundary between the Monashee Group on the east and the Mount Ida Group on the west. Correlation of units across the southern end of Mara Lake indicates lithologic continuity between parts of the groups. Both groups have experienced four phases of deformation. Phases one and two are tight and recumbent, trending to the north and to the west, respectively. Phases three and four are open to closed and upright, trending northwest and northeast, respectively. Second-phase deformation includes large-scale tectonic slides that separate areas of consistent vergence. Slide surfaces are folded by thud-and fourth-phase structures and outline domal outcrop patterns. Metamorphic grade increases from north to south along the west side of Mara Lake. Calc-silicate reactions involving the formation of diopside are characteristic. From west to east increasing grade is evident in the reaction of muscovite + quartz producing sillimanite + K-feldspar + water. These prograde reactions are related to relative position in the second-phase structure. The highest grade is located near the lowest slide surface. Greenschist conditions accompanied phase-three deformation. Fourth phase is characterized by hydrothermal alteration, brittle fracturing, and local faulting. First-phase deformation appears to be pre-Late Triassic whereas second and third phases are post-Late Triassic and pre-Cretaceous. The fourth phase is part of a regional Tertiary event. The thud folding event is correlated with the development of the Chase antiform and the second-phase folding is related to the pervasive east-west fabric of the Shuswap Complex. The timing of these events indicates that the metamorphic core zone of the eastern Cordillera was relatively rigid during the late Mesozoic foreland thrust development. Ductile deformation significantly preceded thrusting and developed a fabric almost at right angles to the trend of the thrust belt. Le lac Mara en Colombie Britannique recouvre la limite entre le groupe Monashee ti l'est et le groupe Mont Ida a l'ouest. La comklation des unitks lithologiques au travers l'extrkmitk sud du lac Mara indique une continuitk lithologique entre les segments des deux groupes. Les deux groupes ont subi quatre phases de dkformation. La premikre et la deuxikme phases foment des plis s e d s et couchks, respectivement en direction nord et ouest. Les troisikme et quatritme phases foment des plis ouverts B fermks et droits, respectivement en direction nord-ouest et nord-est. La deuxikme phase de dkformation inclut des glissements tectoniques de grandes dimensions qui dparent des rkgions h dkversement constant. Les surfaces de glissement sont plisskes par des structures causkes par les troisikme et quatrikme phases de dkformation et dklimitent les rkpartitions d'affleurements en forme de dame. Le degt-6 d'intensitk du mktamorphisme s'accroit du nord vers le sud le long de la rive ouest du lac Mara. Desr6actions calco-silicatkes menant h la formation de diopside sont caractkristiques. La rkactio...

Structure and tectonic setting of the Okanagan Valley fault system in the Shuswap Lake area, southern British Columbia.

“…Metamorphism of the Silver Creek assemblage in the Larch Hills is upper greenschist facies (garnet zone). Nielsen (1978Nielsen ( , 1982) noted a southward increase in grade within the Silver Creek, which he attributed to juxtaposition of different structural levels across a low-angle fault that here is interpreted as a splay of the Okanagan -Eagle River fault (see lithological descriptions earlier in this chapter).…”

“…The study area encompasses portions of areas mapped at one inch to 4 miles by Jones (1959), Campbell (1963), and Wheeler (1965), and at 1:250 000 by Okulitch (1979). Details of the structure in the Shuswap Lake and Mara Lake areas have been investigated by Fyson (1970) and by Nielsen (1978Nielsen ( , 1982. The northwest corner of the accompanying map (Plate 1) adjoins the 1:100 000 scale map by Schiarizza and Preto (1987).…”

“…The Silver Creek assemblage is the structurally lowest of three units in the respectively. Nielsen (1978Nielsen ( , 1982 interpreted these units as being correlative with 71 other Silver Creek rocks to the north, but displaced relative to them along a low-angle fault. He also proposed that they were correlative with calcareous units and amphibolites in the Hunters Range, across Mara Lake to the east.…”

“…The Eagle Bay and other hangingwall assemblages have been deformed by three phases of penetrative fabric-forming structures. A fourth phase characterized by north-trending upright crenulations and brittle fractures (Fyson 1970, Nielsen 1982) is relatively minor.…”

“…Several lines of evidence indicate Early to Middle Eocene movement on these faults, and associated rapid uplift and exhumation. Among these are truncation and disruption of Eocene volcanic and sedimentary strata (Bardoux 1985(Bardoux , 1993Parkinson 1985;Parrish et al 1985;Tempelman-Kluit and Parkinson 1986), U-Pb ages of deformed and nondeformed intrusions (Parkinson 1985;Carr 1986Carr , 1991Carr et al 1987;), K-Ar dates from the Shuswap complex (Ewing 1980, Mathews 1981, Armstrong 1982 and references therein), and the presence of thick Eocene sedimentary basins (Ewing 1981 (Jones 1959;Nielsen 1978Nielsen , 1982Okulitch 1979). This left open the possibility that the abrupt contrast in metamorphic grade across the boundary of the complex was an artifact of a localized steep paleogeotherm, a view held by earlier workers Mountjoy 1970, Fyson 1970).…”