Geophysical investigation of the Honningsvåg igneous complex, Scandinavian Caledonides

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Abstract
The Honningsvåg igneous complex within the Mageroy nappe, northern Norway, has marked magnetic and gravimetric signatures. Palaeomagnetic studies reveal a Siluro-Devonian (Scandian) dual-polarity remanence (NE and up—SW and down; mean declination = 045°, inclination = -38° and a 95 = 8.4°). Rock-magnetic and petrological studies indicate a pure magnetite host for the remanent magnetization component. Magnetic-fabric ellipsoids delineate Dl and D2 Scandian structures affecting the area, and demonstrate an internal reclined fold-structure of the Honningsvåg igneous complex. Forward-modelling demonstrates that a short-wavelength, poúitive aeromagnetic anomaly (c. 700 nT) over the northwestern part of the complex is associated with an area of reverse remanent polarity (SW and down). Conversely, areas of normal remanence polarity (NE and up most common) are associated with magnetic ‘lows’ or ‘quiet’ zones. The gravity field in the vicinity of the complex displays a symmetrical positive anomaly of approximately 200 g.u. (wavelength 15 km). This can be attributed to a density contrast of approximately 0.20 Mg/m 3 between the gabbros of the Honningsvåg complex (2.93) and surrounding metasediments (2.73). An optimal geophysical and geological model demonstrates that the complex has been tilted through 90°, and now has the form of a steeply dipping reclined fold-structure extending to a depth of at least 6 km. Palaeomagnetic remanence acquisition post-dates emplacement of the Magerøy nappe (late Dl), and probably originated during post-Dl uplift. Brittle, upper-crust, D2 deformation which continued into Devonian times, folded the Honningsvåg igneous complex and introduced a small but systematic dispersion in the palaeomagnetic remanence directions. The magnetization is, therefore, post-Dl but pre-D2 in age, and can be termed ‘syn-Scandian’. Palaeomagnetic and isotopic evidence suggests a syn-tectonic magnetization age of c. 410 Ma. The palaeomagnetic pole (N7° and E344°) resembles that from the underlying Kalak nappe complex. It is therefore inferred that the Scandian emplacement of the Magerøy nappe was synchronous with a thermal event which affected the older and underlying Finnmarkian-defor-med Kalak nappe complex.