Amagmatic Extension and Tectonic Denudation in the Kizildağ Ophiolite, Southern Turkey: Implications for the Evolution of Neotethyan Oceanic Crust

Abstract

The Cretaceous Kizildag ophiolite in southern Turkey represents a remnant of oceanic lithosphere formed in a southern strand of the Mesozoic Neotethys. The northeast-southwest trending ophiolite complex consists, from bottom to top, of serpentinized peridotites, cumulate and isotropic gabbros, sheeted dikes, and massive and pillow lava flows. The well-preserved sheeted dike complex and underlying plutonic rocks are best exposed in a topographically and structurally defined narrow graben on the southeastern flank of a laterally extensive antiform the core of which is an axial high of serpentinized peridotites. High-angle normal faults associated with local horst-graben structures and low-angle normal faults associated with rotated blocks are common within the sheeted dike complex and are reminiscent of spreading related structures at slow-spreading ridge segments. The fault contact between sheared gabbros and serpentinized peridotites dips away from peridotites and beneath gabbros in a fashion similar to detachment surfaces documented in several other ophiolite complexes and in modern oceanic crust. The extrusive sequence of the ophiolite is commonly underlain by serpentinite and gabbro and includes highly tilted massive and pillow lavas locally intruded by sheeted and isolated dikes. These structures and contact relationships in the Kizildag ophiolite are atypical of a characteristic ophiolite template and are interpreted to have resulted from a period of tectonic extension and denudation following magmatic construction of ophiolitic crust. A low-angle fault dipping southeast (in present coordinates) toward the axial graben (inferred spreading axis) is interpreted to have accommodated tectonic extension at crustal levels and uplift and exposure of upper mantle rocks. This postulated master normal fault is locally preserved on the northwest and southeast flanks of the antiform, which probably developed during isostatic rebound of upper mantle rocks as a result of excessive serpentinization and diapiric activity during and after displacement of oceanic crust from its original spreading environment. We envision a model of asymmetric exten-sion, analogous to that suggested for magma starved segments of the Mid-Atlantic Ridge around 23°N latitude. The geometry of the inferred asymmetric extension was parallel to an extensional shear zone that may have cut through the entire lithosphere of the northern edge of Arabia resulting in continental break-up in Late Triassic time. The Kizildag ophiolite represents a fragment of a Neotethyan oceanic spreading center that was subsequently emplaced onto the passive margin of Arabia possibly by conversion of a ridge-parallel normal fault to a thrust fault (incipient subduction zone?) as a result of a change in relative plate motion in late Cretaceous time.