Construction of a sheeted dyke complex: evidence from the northern margin of the troodos ophiolite and its southern margin adjacent to the arakapas fault zone

Abstract

The structure of a sheeted dyke complex in relation to intrusion is important for an understanding of sea-floor spreading. Here, we provide data for a previously undescribed ~ E-W cross-strike section along the northern margin of the Troodos massif. For comparison, we also provide new information for a ~NS section along the southern margin of the Troodos massif, adjacent to the Arakapas Fault Zone, an inferred oceanic transform fault. The N-Troodos margin section is divided into two main segments with opposing dip directions, corresponding to the Solea graben in the east and the Stravros graben in the west. High-temperature, hydrothermally altered dykes occur relatively early in a relative intrusive chronology of both grabens, followed by gabbroic bodies and ubiquitous grey sheeted dykes; plagiogranite dykes intruded at relatively later stages. Chemical analysis reveals a trend towards a boninite-type composition, both in the sheeted complex as a whole and in several specific sections for which crosscutting relations were documented in detail. The relatively steeply dipping, late-stage, boninitic-type dykes are interpreted as feeders to the Upper Pillow Lavas. To the N of the Arakapas Fault Zone, large plagiogranite bodies intruded relatively early and were in turn transected by grey dykes, small gabbroic intrusions and late-stage isolated brown dykes. Chemical analyses show that both early-stage grey dykes and the later-stage brown dykes were fed by chemically similar magma. Highly depleted boninite-type dykes occur close to the Arakapas Fault Zone. The relatively late-stage brown dykes show a swing in orientation towards ~ E-W near the transform zone, reflecting an inferred up to ~ 90° relatively clockwise, vertical-axis rotation. The earlier-stage grey dykes are indicative of lesser amounts of vertical-axis rotation but show complex rotations about low-angle axes. Small-scale faults are mainly of normal and left-lateral strike-slip type. Taking alternatives into account, we infer that the faulting in the dyke section took place near a left-offset spreading axis within an overall right-slipping, transtensional transform fault zone. In addition, depleted mantle harzburgites were exposed and eroded on the seafloor within an inferred southward extension of the transform zone (now within the Late Cretaceous Moni Melange), proving additional evidence of a transtensional setting near or within the Arakapas transform. The combined evidence suggests that the Troodos sheeted dyke complex as a whole developed in response to unstable, asymmetrical spreading above a subduction zone in which relatively late-stage amagmatic extension is likely to have played an important role. The spreading crust was bounded to the south (in present coordinates) by a relatively broad transtensional transform fault zone, along which mantle rocks were locally exposed and eroded on the seafloor.