Combined gravity and magnetic interpretation of the Leka Ophiolite Complex

The Leka Ophiolite Complex (LOC) dated at 497±2 Ma is exposed on the island of Leka in central Norway. Due to the outstanding exposure of a complete ophiolite complex, Leka provides an unusual opportunity to map the magnetic properties of lower crustal and upper mantle rocks, which includes outcrops of both the geophysical and petrologic Moho. The LOC is comprised of five rock units: (1) mantle material that is composed of harzburgite and increases in dunitic composition approaching the Moho (2) ultramafic cumulates that is largely dunite, comprising the lowest crust, which is unconformably overlain by (3) metagabbros, which are cut by (4) metamorphosed basaltic dikes, and covered with(5) pillow lavas. The harzburgite and dunite has been serpentinized to various degrees and the entire island has been deformed with tectonic processes, related to the obduction of the ophiolite on to Laurentia at 475 Ma, and later with surrounding rocks of the Upper Allochthon thrust on Baltica at 400 Ma during the Scandian continental collision. During four field seasons a detailed ground-magnetic survey was made across the island along with sampling of the different rock units for rock magnetic measurements including magnetic hysteresis, NRM and susceptibility. From 109 sites, 565 samples were collected; 371 samples have a Koenigsberger ratio greater than 1, suggesting that the positive magnetic anomaly of 2800 nT above background should be modeled using both the remanent and induced components. The ground-magnetic survey was used in conjunction with aeromagnetic and gravity data acquired by the Geological Survey of Norway to develop a geophysical model of the Leka Ophiolite Complex incorporating the NRM intensities and directions from the samples. The model suggests the structure of the LOC is an open syncline, and has a depth of 1 kilometer and the +85mgal freeair gravity anomaly suggests that the LOC is less serpentinized at depth compared to the surface rock samples.