THE TROODOS (CYPRUS) AND KIZILDAG (S TURKEY) OPHIOLITES AS STRUCTURAL MODELS FOR SLOW-SPREADING RIDGE SEGMENTS

The Troodos (Cyprus) and Kizildag (southern Turkey) ophiolites in the eastern Mediterranean region represent remnants of the Neotethyan ocean, which evolved as a result of seafloor spreading north of Afro-Arabia in Mesozoic time. Both ophiolites display numerous planar to listric normal faults and structural grabens in their sheeted dike complexes suggestive of tectonic extension, and complex intrusive relations in their plutonic units indicate recurring and intermittent magmatic activities. Detachment surfaces within the lower crustal sequence (Troodos) or at the boundary between the crustal and mantle sequences (Kizildag) define a brittle-plastic transition zone along which tectonic thinning and extension within the crustal units above was accommodated by mylonitization and denudation in lower crustal units or mantle rocks below. The variation in depth to these accommodation zones is inferred to be a function of the vertical temperature gradient in the oceanic lithosphere, which may in turn point to the presence (or absence) of an active magma chamber during tectonic extension. An asymmetric tectonic extension model is inferred for development of the detachment surfaces and associated structures in these ophiolites. The structural architecture and the interpreted tectonic evolution of both ophiolites are reminiscent of the slow-spreading ridge segments, where episodic magmatism and amagmatic and asymmetric extension occur. In contrast, the modern oceanic lithosphere generated at fast-spreading ridge segments and its ancient analogues have much thicker crustal units and display no evidence of crustal attenuation, extension, or detachment, suggesting a continuous magma supply and the existence of a perennial magma chamber beneath the spreading center. The Moho in fast-spreading oceanic lithosphere is defined by a several hundred meter thick transition zone of interlayered mafic-ultramafic rocks and thus represents an igneous compositional boundary with lateral mutability in character as well as depth; in slow-spreading oceanic lithosphere, however, the Moho is defined by a gently to moderately dipping detachment surface or a fault, and thus represents a sharp tectonic boundary whose location at depth is determined by the thermal budget and the distribution of isotherms.