Gravity interpretation of the jotun nappe of the norwegian caledonides

Abstract

The Jotun nappe in the Norwegian Caledonides consists of a body of dense, mafic granulite-facies Precambrian rocks overlying Cambro-Silurian rocks. Hypotheses to explain the emplacement of the nappe include: 1. (1) an intrusion; (2) low-angle thrust; (3) décollement nappe; (4) fold nappe with Precambrian core; (5) upthrust from below. 2. Hypotheses 2–4 imply a distant root zone, hypothesis 5 implies a local root zone, and hypothesis 1 is generally discredited. Density determinations show that rocks of the nappe have a positive density contrast of approximately 0.12 g/cm 3 with the surrounding rocks; therefore, gravity interpretation should provide useful information concerning the geometry of the nappe. 3. Gravity stations were largely put in by helicopter in order to locate stations as favorably as possible with respect to topography. Bouguer gravity anomalies show a maximum residual anomaly of +58 mGal situated over the Jotun complex. This large anomaly results in a gravity model for the nappe with maximum thickness of 16 km that decreases to the southeast, and this model can only be made to fit the residual anomaly by increasing the density below the surface. Even anomalies over the two northeastern lobes of the nappe can only be simulated by a gravity model 10–15 km thick. The possibility of a dense mass underneath the nappe contributing to the anomaly is considered but ruled out, because of the coincidence of the gravity high with the nappe outcrop. Other possible interpretations indicate that the thickness of the nappe must be at least 8 km. Mineralogical data suggest that initial crystallization of the Jotun complex took place at depths around 25 km followed by a sharp reduction in pressure. Most geologists favor the distant-nappe hypothesis but the shape of the nappe is more consistent with a local root zone. The relation of the nappe to a plate-tectonic framework depends on the position of a Caledonide subduction zone and the position of the root zone. The nappe could represent a slice of the lower crust upthrust from below, similar to the dense rocks found along the Ivrea zone of the Alps.