PRESERVATION OF THE LEKA OPHIOLITE COMPLEX IN A PULL-APART SETTING, NORWAY

The island of Leka is located just east of the Trøndelag Platform at ~65° N along the western coast of Norway. This island is composed of the Leka Ophiolite Complex (LOC), while the adjacent mainland and most of the surrounding islands are basement gneisses and metasedimentary rocks unrelated to the ophiolite complex.

The orientation and movement directions of 670 mesoscopic faults were measured at 25 locations within the mafic and ultramafic sections on Leka and the immediately adjacent island of Madsøya. Paleostress inversion of the data revealed two main types of stress tensors, interpreted as small strains: (1) Horizontal extension, where the extension direction was generally E-W, but varied from NE-SW to WNW-ESE; and (2) Horizontal extension and horizontal contraction. In the latter case, extension varied from E-W to NE-SW. Some localities exhibited both types of tensors, reflecting at least two different deformations, although overprinting relations were not observed.

A gravity survey from Norges geologiske undersøkelse indicates that Leka has a strong positive anomaly (25 mGal) due to the high density of the ophiolite complex relative to the surrounding crustal rocks. We utilized this data and completed a three-dimensional gravity inversion. The LOC is confined largely to the surface exposures in Leka, has steep-sided walls, and exhibits a relatively flat bottom located at 6-7 km depth. The gravity data indicate that the LOC has a rhombochasm geometry.

We propose that the LOC represents a pull-apart structure in a sinistral fault system, with the steep-sided walls corresponding to sub-vertical faults. These bounding faults are part of a regional, en echelon, NE-SW trending fault array. Both regional geological constraints and our paleostress inversion support sinistral movement along the bounding faults. This pull-apart geometry also explains the preservation of the LOC, which is part of structurally higher thrust sheets, in comparison to the basement gneisses that currently surround it. The ultramafic rocks within the pull-apart are excellent recorders of fault motion, by preserving fault striae, and their density allows the three-dimensional geometry of the rhombochasm to be determined using gravity inversion techniques.