EMPLACEMENT MECHANISMS OF THE BJORKEDALEN PERIDOTITE, WESTERN NORWAY: IMPLICATIONS FOR MANTLE-CRUST INTERACTION

Mass exchange between the mantle and crust is an important, but poorly understood, aspect of Earth’s geological processes. The crust and mantle can be juxtaposed at convergent boundaries, when a tectonic plate is subducted beneath another resulting in the emplacement of mantle into crust. Once convergent forces cease, these mantle bodies may be exhumed and exposed for study. Knowledge of these interactions can be gained through examining these peridotite bodies and their method of emplacement. The Bjorkedalen Peridotite in western Norway is well exposed, easily accessible and the tectonic framework is well known. The peridotite lens occupies a synform in continental gneiss, and the contact between the peridotite and country rock is sharp and highly deformed; this high strain zone diminishes away from the peridotite. Structural study of the Bjorkedalen Peridotite will be used to distinguish between possible emplacement mechanisms. Three emplacement mechanisms have been proposed: these include i) an upward intruding diapir; ii) brittle sinking intrusion; and iii) ductile sinking intrusion. The diapir model involves an upwelling mantle plume intruding the crust from below. This is unlikely due to the improbability of more dense mantle rising into less dense crust. By contrast, ductile sinking inserts hot, dense mantle into crust subducted underneath, in a reverse diapir manner; this would form high strain zones in the crust. A brittle method involves mantle being thrusted laterally into the crust along shear zones. This constriction process would likely form structural features around the peridotite including folds and boundary shear zones. Though the brittle intrusion model is possible, interpretation of the ductile features observed implies emplacement at high temperature, and suggests ductile sinking best represents the emplacement method of the Bjorkedalen Peridotite.