Paleomagnetic constraints on the tectonic history of the Foreland Belt, southern Canadian Cordillera: preliminary results

In the southern Canadian Cordillera, the paleomagnetic memory of Paleozoic carbonate strata in the Front Ranges and Inner Foothills of the Foreland thrust and fold belt retains no record of their known deposition at low latitudes. Instead, each folded structure exhibits a similar, but asynchronous, sequence of events including an eastwardly progressing, predeformational chemical remagnetization during the Cordilleran orogeny. The remagnetization of a "western Front Ranges" structure occurs during a period of normal polarity before 130 Ma. The paleomagnetic pole requires that the subsequent deformation of the western Front Ranges is Jurassic or younger. The remagnetization of a Front Ranges structure in the Lewis thrust sheet occurs during a period of normal polarity after 130 Ma but before deformation which, from other evidence, occurred around 75 Ma. The predeformational remagnetization of an "Inner Foothills" structure occurs during a reversed magnetic period that we interpret to be after 75 Ma. An Early Cretaceous sill in the Lewis thrust sheet was remagnetized during a reverse-polarity chron prior to the end of Lewis thrust deformation, when about 70% of the present dip of the sill was acquired. Remagnetization consistently predates deformation, whereas it occurs later at more easterly localities. There are also similarities in character and style of the remagnetizations among localities. When coupled with the eastward progression of the deformation, our observations suggest that an important and pervasive, but hitherto unrecognized and unappreciated, orogenic chemical process affected Paleozoic carbonate strata in the van of the deforming Cordilleran tectonic wedge.