Fault Block Rotations in Ophiolites: Results of Palaeomagnetic Studies in the Troodos Complex, Cyprus

Abstract

Results of palaeomagnetic surveys in the Troodos ophiolite indicate the existence of major fault block rotations, about both sub-horizontal and steeply inclined net tectonic rotation axes, apparently related to deformation at a ridge and at a transform fault.

The palaeomagnetic results from dykes in the sheeted complex and the extrusives are analysed by a technique that restores the dykes to vertical, and the measured stable magnetisation to a previously deduced mean Troodos magnetisation directon (dec.=276°; inc.=32°). The method produces two solutions for the initial dyke strike that are symmetric about the mean Troodos magnetisation direction, and two corresponding solutions for the net tectonic rotation that effected the site. The appropriate solution has to be selected from external criteria. It is assumed that dykes from a given area will have the same initial strike. The Solea graben is a possible fossil axial valley that lies directly to the north of the Mount Olympos plutonic centre. The results of a palaeomagnetic survey across the structure give a best solution giving dykes a north-westerly initial strike and net tectonic rotation axes parallel to the initial dyke strike, and sub- horizontal. They are also parallel to the dominant fault trend in the area. These results are consistent with rotational normal faulting.

Palaeomagnetic and structural studies on the western flank of the Larnaca graben, at the eastern edge of the ophiolite, give similar results to those obtained in the Solea graben. The north-westerly initial dyke solution,with sub-horizontal rotation axes is preferred. The maximum recorded rotation is 115°, from flat- lying dykes. The sense of rotation is variable; both clockwise and anticlockwise about north-westely directed axes. Four east-west elongate domains with similar senses and degrees of dip can be identified, each bounded to the north and south by transfer faults. The main normal faults in this area trend north-west, parallel to the initial dyke strike, suggesting that the block rotations occurred by movement on a set of ridge-parallel normal faults.

The Lefkara area, in the eastern part of the ophiolite, immediatly to the north of the east-west trending Arakapas fossil transform fault, has dykes in both the sheeted complex, and the extrusives, with a northeasterly present-day strike. Palaeomagnetic evidence suggests that these had an initial north-westerly strike, and were rotated some 110° clockwise about steeply inclined axes. Cross-cutting dyke relationships suggest that dykes were intruded during this rotation. At least two generations of dominantly strike-slip faults can be identified. The rotations were apparently accommodated by movement on both macrofaults (spacing 1–2km) and mesofaults (spacing l–10m), associated with dextral slip on the Arakapas transform fault.