DETAILS OF DAY 4

 The aim of the day is to study the dynamics of spreading ridges, as afforded by sections in the Troodos ophiolite. Transform faults are a major global feature of spreading ridges, and in the Limassol Forest area we have the unique opportunity to explore a fossil transform fault. The Arakapas Fault Belt defines the main area of a transform fault active during the spreading history of the ophiolite. We will examine volcanic, plutonic, and minor intrusive igneous rocks, tectonic effects, and volcaniclastic sediments and assess the evidence in support of the transform fault hypothesis.

 The day will be spent at the eastern end of the Troodos Massif, along the Pitsylian ridges and Machairas Forest area. Because the geology of this part of the ophiolite in the AFB and to the south is quite different in detail to the main, High Troodos part, this area is denoted as the Limassol Forest Complex (LFC). The igneous succession here is designated as the transform sequence (TS) to distinguish it from the axis sequence, generated along the ridge crest, one of which is centred on Mt. Olympus.

 The transform sequence displays the same igneous units as the axis sequence. Geochemistry, tectonics, and emplacement characteristics differ significantly however. The orientation of the SDC is the most immediately noticeable difference. Dyke strike in the axis sequence is N-S, but along the AFB dykes are rotated till aligned E-W when seen close to the transform (figure 5). This is interpreted as the effect of block rotation due to transform movement during the emplacement of the dykes (Murton, 1986; Morris et al., 1998). As block rotation decreases away from the transform, so the synmagmatic rotation of originally N-S striking dykes lessens. Multiple phases of magmatism are also identified in the LFC. Original axis sequence mantle tectonites are seen intruded by high level gabbros, crustal ultramafic cumulates are seen invaded by cross-cutting later ultramafic cumulates, and axis sequence dykes are intruded by later dykes and high level gabbros. In places, gabbro bodies can be related to later magma chambers, since some are seen to feed cross-cutting dykes, while some of the later gabbro bodies cut earlier ones. Overlapping, multiphase cross-cutting magma chambers are thought to be common under slow to medium-spreading ridges.

 The TS magmas are somewhat unique in their composition. In comparison with axis sequence magmas the TS suite are characterised by a greater proportion of very depleted and primitive geochemical compositions. All Troodos magmas are LILE enriched, high magnesian, high silica, hydrous melts. The TS melts show a greater proportion of magmas with very low Ti, Zr, Y, and Nd, and unique REE profiles. They show considerable similarity with boninites from Cape Vogel, Papua New Guinea, and particularly with respect to their REE profiles and high calcium content, to boninites from the Izu-Bonin-Mariana fore-arc system ( Rogers et al., 1989). For the TS magmas such depleted melts are thought to have been derived from a portion of mantle that had previously been involved in magma generation on two earlier occasions before the TS magmas were extracted. All the TS igneous suite can be related to the same primitive overall magma source, although the apparent abundance of depleted boninitic magmas relative to the axis sequence is more likely to be the result of variations in magma plumbing which allowed frequent eruptions of fresh magmas with very little opportunity for ponding prior to release (Coogan et al., 2003).

 Tectonic effects are also very noticeable in the LFC. Dykes are fractured, and pillow lavas extensively brecciated. In many parts of the extrusive sequence volcaniclastic sediment acts as infill between pillows. Multiple E-W shear planes are developed, and particularly where tectonised mantle harzburgites are involved, the shear planes are extensively serpentinised. Serpentinites mobilised by transtensional tectonics are seen to flow and invade along the shear zones. Cataclastites are also developed in places within the shear zones, with mylonitisation and mineralization noticeable.

 The volcaniclastic sedimentary infill is the result of brecciation of extrusive volcanic products during fault movement. Movement along the transform fault also produces other sedimentary features. Submarine fault scarps will be developed along the transform, and debris shed from submarine highs will be deposited at lower levels on the ocean floor, over the volcanic crust. Submarine gravity slides can be triggered seismically, and turbidity flows deposited downslope. At a larger scale parts of the volcano-sedimentary succession on the submarine highs can become dislodged and redeposited as debris flows and fan deposits at lower levels. Some debrites form extensive deposits, and can comprise rafts of sediment many metres thick, deposited within finer debris and volcanics.