Rock Physical Properties

In-situ measurements of magnetic susceptibility and electric resistivity (Fig. 7, for location see Fig. 2) were made on outcrops of gneiss and dolerite. The gneisses have low susceptibility of ca 1.8-10"4 SI and the dolerite has >100 times higher susceptibility, 3-10"2 SI ( Fig. 7a). Rock samples were measured with respect to density, magnetic susceptibility, and remanent magnetization. The gneisses and granites are generally low-magnetic, in contrast to the high-magnetic dolerite, whose magnetic anomalies can be followed under the bay from outside the shore to the north and south. The dolerite has a typical remanent magnetization with declination 40o and inclination ranging from +10° to -10o, acquired during its formation in the Jotnian (Riphean) period (Fig. 8). The ratio of remanent to induced magnetization is 1.35, resulting in a total magnetization with low inclination (ca. +25o) and northeast directed declination (ca. 30o).

Physical Properties Gniss
Fig. 7. Rock physical properties. Numbers indicate type values. a. (top) Magnetic susceptibility of gneiss + granite, and dolerite, respectively, from regions A and B in Fig. 2. b. (bottom) Radial variation of in situ electrical resistivity. For location see Fig. 2.

Fig. 8. Directions of remanent magnetization in dolerite from oriented rock samples in regions A and B. The present geomagnetic field is shown within the small circle. Negative inclination is marked with + and positive with dots, respectively. The total magnetization vector (induced + remanent magnetization) would be located at T in the diagram. Radial variation of electric resistivity collected in regions C and D and at scattered localities shown with + in Fig. 2. On average a reduction with a factor 2.5 is observed towards the center of the structure. The lines represent the uncertainty interval due to unisotropy.

The electric resistivity (Fig. 7b), is generally low for the gneisses with a normal resistivity of 2.5 kQm outside the Avike Bay structure and decreasing towards 1 kQm at the shore of the Bay and on nearby islands. The low resistivity of the gneiss is related to the content of conductive minerals (potentially graphite and pyrrhotite). An additional decrease in resistivity is related to an increased fracturing, similar to that observed in large impact structures (Henkel 1992 and Backstrom, this volume).

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