Diachronous deformation and a strain gradient beneath the Selkirk allochthon, northern Monashee complex, southeastern Canadian Cordillera

Structural relationships of granitoid rocks dated by the U-Pb method indica te that deformation was diachronous and a strain gradient exists in a 6-km- thick section beneath the Selkirk allochthon, in the northern Monashee comp lex, one of the deepest structural exposures in the southern Canadian Cordi llera. At high structural levels, immediately beneath a crustal-scale thrus t zone that transported the allochthon eastward, a metasedimentary-dominate d cover sequence was strongly affected by kilometre-scale east-verging isoc linal folds (F-1) and outcrop-scale folds (F-2) that are associated with th e dominant foliation and lineation. The F-2 folding occurred, at least in p art, after 58 Ma and ceased by 55 Ma. In deeper levels of the cover sequenc e and the underlying orthogneiss-dominated basement, F-2 folding occurred, at least in part, after 52 Ma and ceased by 49 Ma. Proterozoic dykes in the basement were locally weakly affected by D-2. These new findings require t hat: (i) D-2 compression youngs structurally downward, synchronous with the thermal peak of metamorphism; (ii) D-2 in deeper levels is synchronous wit h extension above the complex that was partly responsible for its exhumatio n; and (iii) a D-2 strain gradient lies between strongly deformed cover roc ks and weakly D-2-deformed basement rocks. We propose a model in which rock s that were tectonised at different places and times within the orogen were juxtaposed, likely during east-verging kilometre-scale F-1 folding and she aring along the isocline limbs (a similar model was previously proposed to explain a pattern of downward younging thermal peak ages and an inverted me tamorphic sequence in higher rocks). The rapid downward decrease in deforma tion intensity suggests that the lower limit of significant Cordilleran str ain lies in the exposed basement. Cessation of deformation at this revel is attributed to the fact that the basement attained elevated temperatures an d began straining when the Cordilleran tectonic regime changed from compres sional to extensional. (C) 2001 Elsevier Science Ltd. All rights reserved.