TECTONIC SIGNIFICANCE OF STRATIGRAPHIC AND STRU ~ CT URAL CONTRASTS BETWEEN THE PURCELL ANTICLINORIUM ANC THE KOOTENAY ARC , EAST OF DUNCAN LAKE
The main goal of this study is to elucidate the wture and tectonic significance of the profound stratigraphic and struttural changes that occur between the crest of the Purcell anticlinorium and the Kootenay Arc. Reconnaissance (12.50 000) mapping by Reesor (1973) outlined conspicuous contrasts between the thick hasal Paleozoic (Hamill-Badshot) succession that overlies the Windermere Supergroup in the western Purcell Range and the thin, condensed early Paleozoic succession with overlapping Upper Devonian strata that occurs in the eastern Purcell Range. Reesor also described the abrupt contrast between the tight upright fold structures in this area and the refolded, west-verging, recumbent isoclinal folds that occur immediately to the west in the Kootcney Arc. He also showed that several small granitic plutons in the area probably were emplaced while the folding was still underway. The rocks exposed in this area (Figure 1-l-l ) record both the Late Proterozoic early Paleozoic birth and development of the Cordilleran miogeoclinal passive margin of North America (Bond and Kominz, 1984; Bond of a/., 1983, and the late Mesozoic -early Cenozoic deformation, regional metamorphism and granitic plutonism resulting from “collisions” between North America and a series of allochthonous terranes that have been accreted to it (Monger et al., 1982). Detailed geological mapping (I:50 000 and greater) was completed during July and August of 199 I and 1992 within an area of about 900 square kilometres in the western Purcell Mountains, between Duncan Lake and the headwaters of Tohy and Jumbo creeks (Figure I-I-2). This work will link the detailed mapping along the Kootcnay Arc by Fyles (I 964) to the detailed mapping by Root (I 987) and Pope (1990) in the central and eastern Purcell anticlinorium. The main objectives of this study are: (I) to establish the nature and tectonic significance of the stratigraphic rclationships within the thick sequence of Windermere, Hamill and Badshot strata in the study area; (2) to investigate the stratigraphic and tectonic relationships between these strata and the condensed onlapping early Paleozoic succession that occurs on “the Windermere high” to the east. below the Mount Forster thrust fault in the central and eastern Purcell Mountains (Root, 19X5: Reesor, 1973); (3) to establish the nature, evolution and regional tectonic significance of the change in structural style between the study area and the adjacent areas in the Kootenay Arc and in the central Purcell Mountains; and (4) to invest&ate the relation ship hetaeeo the initial structural configuration of the rifts d margin and the major structures that develc#ped during Me iozoic tenam: accretion and deformation of the margin.
Structure and Metamorphism
The Brioverian formations of the Armorican Massif have been involved in orogenic events generated by the plate tectonic regime during latest Proterozoic-Early Cambrian times; these gave rise to the Cadomian fold-belt which is contemporaneous with the Pan-African (sensu lato) and Avalonian progenies.
Structural evolution of the Hosmer thrust sheet, southeastern British Columbia; discussion and reply
ABSTRACT The Hosmer thrust sheet is the structurally highest and westernmost component of the Cordilleran foreland thrust zone in the Fernie area. The thrust sheet comprises a sequence of middle Proterozoic to early Mesozoic unmetamorphosed sedimentary rocks up to 18 km thick. It was segmented into five major blocks by differential movements across prominent transverse and longitudinal faults as it was displaced first northeastward (8 km) and then southeastward (12 km) on the Hosmer thrust. The Hosmer nappe forms the leading edge of the sheet, and is a faulted overturned anticline separated by two right-hand tear faults into three segments that strike northwest, northeast and north. The entire thrust sheet was offset by later displacement (8 km) on the southwest-dipping Rocky Mountain Trench normal fault. Individual northeast-plunging structures in the Purcell Mountains west of the normal fault can be matched with their counterparts in the footwall of the normal fault in the Rocky Mountains where they are exposed at a lower structural level. The Hosmer thrust formed across a northwest-dipping (20°) monocline. About 6 km of Cambrian to Middle Devonian strata that accumulated on the north side of the monocline as it formed are overlapped by Upper Devonian strata along an unconformity that bevels the Proterozoic Purcell Supergroup on the crest of the monocline. The sinuous trace of the Hosmer thrust makes it possible to construct geological maps of the hanging wall and footwall. Southeast of the monocline the thrust follows a bedding-glide zone at the base of the Purcell Supergroup that is linked by a single "step" (or ramp) to a bedding-glide zone in Jurassic shales; however, north of the monocline the same bedding-glide zone in the Purcell Supergroup is linked to the glide zone in the Jurassic shales by three successive "steps" separating these bedding-glide zones from others in Cambrian shales and Devonian gypsum (that occur only north of the monocline). This produced a northeastward right-hand deflection of the "step" through the upper Paleozoic rocks in and beneath the Hosmer nappe, and it is responsible for the structural re-entrant that occurs between the northern and southern parts of the nappe. During northeastward displacement of the thrust sheet, the northeast-trending Moyie-Dibble fault formed along the flank of the monocline as the locus of vertical adjustment between the thicker and thinner parts of the sheet. It propagated eastward through the Hosmer nappe as a right-hand reverse fault when the direction of displacement of the northern part of the thrust sheet changed from northeast to southeast, and the part of the Hosmer thrust fault south of the monocline became inactive.