Structural Evolution of the Hosmer Thrust Sheet, Southeastern British Columbia

Abstract

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.