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That part of the Kootenay arc, a major structural element in southeastern British Columbia and northeastern Washington, mapped here (lats. 49°30′–49°51′ N., longs, 116°44′–117°00′ W.) spans central Kootenay Lake and includes a metasedimentary and metavolcanic section thicker than 20,000 feet and ranging in age from Proterozoic to Triassic.

The lower Paleozoic sequence, about two-thirds of the total thickness, is unfossiliferous, and basal units, dominantly clastic, are conformable with the Precambrian section. These beds can be correlated with Lower Cambrian formations elsewhere along the Kootenay arc with some confidence on a lithologic basis. The overlying Lardeau Group is somewhat more heterogeneous, and regional correlation less certain as the result of: (1) structural complications unresolved by present incomplete regional mapping, (2) possible sedimentary facies changes across or along the Kootenay arc, and (3) the high contrast in metamorphic grade between the central Kootenay arc (a major part of which lies in the sillimanite zone) and the rest of the Kootenay arc most of which falls below the garnet zone. Separated from the Lardeau Group by a regionally important angular unconformity is the Carboniferous and Triassic Milford Group of great lithologic diversity. Still higher are Triassic metavolcanic (Kaslo Group) and mixed metasedimentary and metavolcanic rocks (Slocan Group) with one or more possible disconformities. The Slocan and Milford Groups can be traced northward into fossiliferous beds and the Slocan Group in the central Kootenay arc is itself sparingly fossiliferous. Equivalence of the Milford, Kaslo, and Slocan Groups with all or part of the Ymir Group just to the south is, however, more problematical. There is no evidence in the central Kootenay arc for unmetamorphosed Carboniferous and younger beds over regionally metamorphosed basement rocks as has been described from the Shuswap terrane and elsewhere; parts of the Slocan Group are in the staurolite zone.

As is typical of other mesozonal plutons, the Nelson and Bayonne batholiths show partially concordant, partially discordant intrusive relationships within the central Kootenay arc. The Nelson batholith locally has also a strongly cataclastic or gneissic border facies leading to some ambiguity concerning the manner and chronology of its emplacement. There are in addition numerous smaller quartz monzonite and granodiorite plutons, many peraluminous, some of which may be syntectonic or even pretectonic on the basis of their fabric and relationships with the enclosing rocks. They are most abundant in rocks of the highest metamorphic grade and some may be a product of partial anatexis.

Macro-, meso-, and microscopic structure and fabric analysis as well as considerations of mineralogical equilibrium suggest that regional metamorphism preceded contact metamorphism associated with some of the major plutons and this primary metamorphic recrystallization was synchronous with the early stages of a strong penetrative deformation divided into three probably overlapping phases each distinguished by a characteristic fold geometry linked to the elastic or rheologic properties of an anisotropic layered sequence. The earliest (Phase I) folds are isoclinal, highly attenuated, similar folds parallel to the local trend of the Kootenay arc and largely control lithologic distribution. Their axial planes commonly dip at low to moderate angles to the west and the axial plunge is low to the north or south. Subparallel to Phase I folds are more open folds (Phase II) of opposite shear sense which have rotated fabrics associated with primary folding, most notably bedding and axial plane schistosity, through varying angles and locally affect the map pattern. Crossfolds or relatively open flexures (Phase III) are believed to have formed pari passu with the first two sets and subperpendicular to them. Strike faults developed locally during Phase I deformation and were apparently utilized for later movements as well. Other faulting and jointing formed still later and is of minor importance. In the vicinity of the Nelson batholith there has been some distortion of pre-existing structures.

The interdependence of rock deformation, metamorphic recrystallization, and igneous intrusion in the context of recent absolute age determinations made on some of the major plutons as well as metamorphic minerals in the Shuswap terrane is of crucial significance in planning future field, petrologic, and isotopic programs. Results to date suggest a far more complex chronology than once postulated on geologic evidence alone. It appears likely that multiple deformation, igneous intrusion, and prograde metamorphism extended over 100 million years or more and was followed by a protracted period of mineralogical and structural readjustments in response to falling temperatures and release of tectonic as well as lithostatic pressures, the latter during rapid erosion or unloading of superjacent rocks. The Shuswap-type rocks of southeastern British Columbia to which the Kootenay Lake metamorphic belt has been traditionally considered genetically related and which flank the Nelson batholith on the north, east, and west, may therefore represent deep-seated penetrative deformation and attendant metamorphism, at conditions approaching partial fusion, of an heterogeneous section of no particular age. Shuswap-type rocks would, on this basis, be unrelated, contrary to an earlier concept, to the emplacement and associated thermal metamorphism of the outer and youngest zones of the Nelson batholith or similar complex multiple intrusions.

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