Modes of extension at oceanic spreading centers: evidence from the Solea graben, Troodos ophiolite, Cyprus

Abstract

The Solea graben in the Troodos ophiolite, Cyprus, represents a preserved Cretaceous-age axial valley formed during a brief period of amagmatic seafloor spreading. Structural analysis of all crustal levels of the graben reveals two periods of deformation; an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the Tertiary-Recent. Amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike-plutonic contact and oriented subparallel to ophiolite pseudostratigraphy. Individual fault segments are essentially planar but link to approximate listric geometries locally. Bulk structural rotation of fault blocks bounding the graben occurred about a NNW-trending axis, parallel to the present graben trend. Average dips in the sheeted complex within the central portions of the graben suggest 40–50° of structural rotation, although this is a minimum figure because present dike dips are not a simple function of rotation magnitude. Tensor analysis of fault populations associated with extensional deformation shows that σ 3 was oriented approximately perpendicular to the graben axis. Late, near-axis extensional deformation segmented the detachment fault along NNW-trending normal faults. Obduction-related uplift and doming of the Troodos ophiolite is expressed in the Solea graben by minor thrust, reverse and strike-slip faults principally developed during reactivation of pre-existing low-angle fault zones formed during seafloor deformation. Tensor analysis of this later phase of faulting shows that σ 1 was oriented approximately N—S and that σ 2 and σ 3 alternated position during the deformation. The style of deformation documented in the Solea graben supports recent speculation for Basin and Range-style detachment faulting at slow-spreading ridge crests with low magma budgets, such as parts of the Mid-Atlantic Ridge near large-offset transform faults. Natural cross-sectional exposure of the Solea graben shows how extensional structures might link at depth beneath such slow-spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large-scale (ore-forming) hydrothermal circulation cells.