Pull-apart formation and strike-slip partitioning in an obliquely divergent setting, Leka Ophiolite, Norway

摘要

The Leka Ophiolite Complex (LOC) is located on the island of Leka, Norway, and belongs to the Uppermost Allochthon of the Scandinavian Caledonides. The rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex. Paleostress analysis, gravity inversion, and regional geology support a fault-bounded rhombochasm geometry for the LOC. The paleostress inversions revealed two types of tensors, interpreted as small strains: (1) horizontal extension, generally E-W to NE-SW, and (2) horizontal extension in the same direction with an added component of perpendicular horizontal contraction. A strong positive gravity anomaly (25 mGal) is centered on Leka, and gravity inversion indicates that the LOC lies directly below its surface exposures with steep-sided walls and a flat bottom located at similar to 7 km depth. The faults bounding the LOC probably initiated during postorogenic extension in the Scandinavian Caledonides. The faults are regional in scale and are parallel to other NE-SW trending en echelon faults along the Norwegian coastline and on the adjacent mainland. A pull-apart structure explains the down-dropping and subsequent preservation of the LOC, as it is surrounded by rocks from lower structural positions within the nappe stack. The paleostress directions from Leka support a sinistral component of shear along these faults. The gravity inversion is consistent with a fault-bounded geometry. This pull-apart structure, as uniquely recorded by the dense ophiolitic rocks, suggests that strike-slip partitioning was active in an obliquely divergent setting. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 64]