Geophysical constraints on the crustal architecture of the Troodos ophiolite: results from the IANGASS project

Abstract

SUMMARY We present results from modelling a combined 160 km seismic wide-angle reflection/refraction and gravity profile sited across the sheeted dykes, lavas and sediments of the northern part of the Troodos ophiolite, Cyprus. The P-wave seismic velocity and density model, including five separate layers, is defined to a depth of ∼10 km. This shows the ophiolite sequence dipping to the east beneath the central, well resolved, 85 km section of the line between Kambos in the west and Kelia in the east. The upper layer (average velocity 2.83 km s −1 ; density 2.21 g cm −3 ) is interpreted as consisting of sediments and the upper extrusive group. Beneath this, accepting that layer boundaries may be gradational beyond the experiment resolution, the layers are identified as the lower extrusive and basal groups (3.57 km s −1 ; 2.31 gc m −3 ), the sheeted dyke complex (SDC) (4.57 km s −1 ; 2.72 g cm −3 ), a gabbroic layer (6.31 km s −1 ; 2.92 g cm −3 ), and an ultramafic cumulate layer and/or serpentinized peridotite (7.00 km s −1 ; 3.00 g cm −3 ). While the thickness (∼6 km) of this layer beneath the proposed gabbroic layer is defined, positive identification of its composition is not possible, owing to the lack of discriminatory S-wave velocity information. The underlying material provides an unreversed velocity of 8.0 km s −1 beneath the western end of the profile. These modelled velocities and densities are compared with seismic models of oceanic crust and their implications for models of the tectonic evolution of the Troodos ophiolite are discussed. Several major faults are interpreted from the models, with throws of up to ∼500 m offsetting both the extrusive-SDC and the SDC-gabbroic layer boundaries. These are all downthrown to the west, there being no evidence from the data to suggest any major faults downthrown to the east. Our results therefore do not support existing tectonic models for a multiple graben structure within the ophiolite recording either the progress of an eastward stepping spreading centre or off-axis graben formation. Instead, assuming that the faults are downthrown towards the ridge axis, the data are consistent with a single spreading centre located to the west of the recording profile.