Journal,fS,'ucmral Geology. Vol ~. Nos, 3/4. pp. 255 to 268. 1986 0191-8141/86 $()3.(~)+ 0.(KI Printed in (irea! Britain © 1986 Pergamon Press Ltd Obduction, backfolding and piggyback thrusting in the metamorphic hinterland of the southeastern Canadian Cordillera RICHARD L. BROWN, J. MURRAY JOURNEAY, LARRY S. LANE, DONALD C. MURPHY and CHats J. REES Ottawa-Carleton Centre for Geoscience Studies, Carleton University. Ottawa, Canada Abstract--The (paleo-)continental margin of the southeastern Canadian Cordillera was deformed and metamorphosed mainly in the Jurassic and Cretaceous, beginning with the obduction of a composite oceanic terrane, eastwards at least 60 kin, onto the western part of the continental margin in the late Early Jurassic. In the Middle Jurassic. W-verging backthrusts and backfolds developed in this region, the folds becoming increasingly overturned and attenuated at deep structural levels. This deformation of the hinterland produced major crustal thickening and regional metamorphism, which peaked in the late Middle Jurassic. The thickening mass of the hinterland curtailed westward vergence. The focus of crustal shortening stepped to the east in the form of E-verging piggyback thrusting, which carried the deformed hinterland eastwards. Displacements propagated from the hinterland into the foreland on major detachments and thrusts. The initial shear zone, the Monashee drcollement, decoupled the cover from its basement and accounted for at least 80 km of shortening by the middle Late Jurassic. Continued thrusting on lower shear zones that rooted at the base of the crust, led to the development of a basement duplex and major uplift of the hinterland. We present two balanced cross-sections; each based on available structural, petrologic, geophysical and geochronologic data. Both sections are internally consistent, and demonstrate that development of a basement duplex beneath the hinterland could have accommodated contemporaneous thin-skinned shortening of the Rocky Mountain Foreland. INTRODUCTION OBDUCTION A BALANCED cross-section and palinspastic reconstruc- tion of the North American Rocky Mountain Thrust and Fold Belt was first produced by Bully, Gordy and Stewart in 1966. Similar sections have since been drawn across the Canadian segments of the foreland region (Price & Mountjoy 1970, Dahlstrom 1970, Thompson 1979, Price 1981, Price & Fermor 1983). Reconstruction westward from the foreland into the hinterland (Omineca Belt) has been hindered by poor stratigraphic control (caused by complex deformation) and by uncertainties concern- ing the depth and nature of basement involvement. However, in recent years our understanding of the hinterland in the vicinity of the Shuswap Metamorphic Complex (Fig. 1) has approached the point where rigor- ous structural sections and at least partial palinspastic reconstructions may be drawn. Most recently a section of the southern Canadian Cordillera has been compiled for Transect B2 of the Decade of North American Ge- ology Program (Monger et al., in press). This section crosses the hinterland between latitudes 51°N and 52°N and extends eastward to the front of the Rocky Mountain Foreland. The purpose of this paper is to document the Mesozoic and early Cenozoic structural history of the southern Omineca Belt, and present two composite cross-sections between latitudes 50°N and 53°N (Figs. 1 and 2). Geo- logical constraints from surface data are combined with the principles of balancing cross-sections (Dahlstrom 1969) to produce as realistic a subsurface geometry as possible. In the late Early to Middle Jurassic, a composite oceanic terrane, Terrane I of Monger et al. (1982), accreted to the North American (paleo-)continental margin. Terrane I consists of four smaller terranes, which were assembled by the end of the Triassic (Figs. 3 and 4). Stikinia, a dominantly volcanic terrane, was emplaced against Quesnellia, an upper Paleozoic to Lower Jurassic island arc terrane, in the latest Triassic (Fig. 4; Monger & Price !979). The suture between them is marked by a Mississippian to Triassic subduction mrlange, the Cache Creek terrane (Monger et al. 1982). From the Mississippian to the Early Jurassic, the Eastern terrane, probably a small oceanic basin, separated Ques- nellia from the continental margin (see Monger 1977). Easterly directed thrusting began within this basin in the Early Jurassic (or earlier, see Klepacki & Wheeler 1985), leading to the obduction of the Eastern terrane and part of Quesnellia onto the continental margin, which is now represented by the Omineca Belt (Monger 1977; Fig. 3). The timing of the obduction is bracketed between the late Early Jurassic when the youngest marine sediments in Terrane I were deposited (see Monger 1984), and the Middle Jurassic when both Terrane I and the Omineca Belt suffered regional deformation, metamorphism and plutonism (Read & Wheeler 1976, Archibald et al. 1983). The obducted Eastern terrane is generally composed of ophiolitic rocks throughout its length in southern British Columbia (Monger et al. 1972, Monger 1977). In one locality, serpentinized ultramafic cumulates, am- 255