Geologie und Bau des Sithonia-Ophioliths (Chalkidiki,NE-Griechenland): Anmerkungen zur Bildung ozeanischer Krusten

The Upper Jurassic Sithonia ophiolite of Chalkidiki (NE Greece) provides the opportunity to study the processes of the formation of fossil oceanic crusts in detail. This ophiolite consists from top to base of:

1. an over 900 m thick succession of shallow-water sediments frequently interlayered with basic volcanics;
2. an about 700 m thick basaltic layer which predominantly consists of hyaloclastics;
3. an excellently developed sheeted dyke complex which is at least 1.2 km thick. It is the main scope of this study.

The sheeted dyke complex consists entirely of dykes. They have intruded each other preferably along their margins. Two groups of dykes may be distinguished. The first group comprises those dykes which were obviously formed within the active spreading zone. These dykes make up the structure of the sheeted complex. They are in average 3.9 m thick and do not display a clear polarity of their chilled margins. The dykes of the second group were formed later, i.e. off-ridge. They are up to 0.8 m thick and almost always symmetrically chilled. The general strike of the dykes is N 54° E. This direction is almost perpendicular to the major tectonic units of Chalkidiki. The geometrical relations between the dykes can be best explained by formation at an oscillatory spreading axis. Available geological and geochemical data suggest a low to very low spreading rate. A periodic process of dyke injection is recognized. The dyke injection periods are of variable duration. This together with the restricted and variable magmatic differentiation within the different periods points towards a nonsteady state magma chamber beneath the spreading axis. The off-ridge dykes impart valuable information as regards the spreading direction and the relative position of the paleoridge. Some ideas about the spreading rate may also be obtained from these dykes. The shallow-water conditions during the sedimentation may be explained as due to the tectonic uplift of the Upper Jurassic oceanic crust possibly along a transcurrent fault. This interpretation is supported by other geological observations.