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

Abstract

S U M M A R YWe 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 g cm −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.

Physical properties and seismic structure of Izu‐Bonin‐Mariana fore‐arc crust: Results from IODP Expedition 352 and comparison with oceanic crust

“…Although Mackenzie et al . [] prefer an interpretation of ∼5 km thick crust underlain by serpentinized mantle, we note that 11 km thick crust with basal velocities of ∼7 km/s compares well with the IBM fore‐arc KT06 profile with 12.5 km thick crust and similar basal velocities.…”

“…Mackenzie et al . [] present two hypotheses for the 6 km thick ∼7 km/s layer: partially serpentinized peridotite or ultramafic cumulates. If the basal layer is serpentinized mantle, then the Troodos crust is ∼5 km thick, whereas if it is composed of ultramafic cumulates, then the crust is ∼11 km thick.…”

“…The IBM fore-arc seismic refraction profile modeled velocities that are slightly lower, and layer thicknesses that are greater, than typical oceanic crust (Figures 9 and 10). The IANGASS seismic refraction profile across the Troodos ophiolite modeled a 1.5 km thick upper layer with velocities 2.5-5.0 km/s, underlain by a 4-4.5 km thick layer with velocities 6.0-6.7 km/s, which is underlain by a 6 km thick layer with velocities of 6.9-7.1 km/s [Mackenzie et al, 2006]. Mackenzie et al [2006] present two hypotheses for the 6 km thick 7 km/s layer: partially serpentinized peridotite or ultramafic cumulates.…”

Geodynamic evolution of Upper Cretaceous Zagros ophiolites: formation of oceanic lithosphere above a nascent subduction zone

“…The Troodos ophiolite crustal sequence contains gabbros (3-6 km thick, Mackenzie et al 2006), a sheeted dyke complex (800 m thick) and overlying lavas and sediments (750 m-1.5 km thick). Lower pillow lavas are IAT while upper lavas are boninitic (Thy & Xenophontos, 1991;Dilek & Thy, 2009).…”

The Hecataeus Rise, easternmost Mediterranean: A structural record of Miocene-Quaternary convergence and incipient continent-continent-collision at the African-Anatolian plate boundary

“…Recently published refraction seismic lines provide insight into the crustal composition of this structure (Welford et al, 2015). Velocity models along two profiles which cross the Hecataeus Rise in NEeSW direction did not reveal crustal velocities as high as measured for the onshore Cyprus Troodos ophiolite (Mackenzie et al, 2006;Welford et al, 2015). In contrast, velocities below 5 km/s are locally detected at depths down to 15 km, suggesting the Hecataeus Rise to be composed of a thick sedimentary cover on top of a thin continental crust (Welford et al, 2015).…”