Geophysical Investigations of the Troodos Ophiolite, Cyprus

The IANGASS 95 geophysical survey involved the acquisition of a wide-angle reflection/refraction seismic and gravity profile across the Troodos Ophiolite, extending from the centre of the ophiolite complex eastwards into the circum-Troodos sedimentary succession. The seismic data has recently been modelled using 2-D tomographic and ray tracing methods followed by modelling of the Bouguer gravity anomaly based upon the seismic model through simple velocity-density conversion. Both velocity and density models indicate a 5 layered structure. A thin, intermittent, low density layer with velocity 2.9km/s is interpreted as a combination of sediments and the upper pillow lava sequence. This is underlain by a 0.5km thick layer with average velocity of 3.56km/s and density 2.3g/cm³ that is consistent with the lower pillow lava sequence consisting of pillows and massive flows. A 1 km thick layer of velocity 4.6-5.0km/s outcropping at the western end of the profile and c. 2km depth in the east corresponds with the sheeted dyke complex and has a modelled density of 2.7g/cm³. The boundary between the lavas and sheeted dykes is defined primarily by seismic diving waves and does not preclude the presence of a gradational `basal layer'. The layer beneath the sheeted dykes is c. 3-6km thick with velocity 6 - 6.7km/s and density 2.9g/cm³ and is interpreted as comprising gabbroic rocks, possibly including ultramafic plutonics. Several large (up to ~500m) offsets modelled in these upper layers, some extending into the gabbroic layer, are interpreted as faults, interestingly apparently downthrown to the west, and presumably associated with spreading related extension in the ophiolite. The base of the gabbroic rocks at ~4-8km depth from west to east is likely to be a relic petrological Moho and is underlain by a very thick (c. 5km) layer of > 7km/s. We interpret this to be serpentinized harzburgite having a density of 3.0g/cm³. This is thought to be underlain by normal mantle although velocity control at this depth is limited. A deep reflector is imaged at c. 60km depth but its cause is unknown. One possibility is that it may result from the present-day downgoing slab in the subduction zone to the south of Cyprus.