Ophiolite Tectonics,Rock Magnetism and Palaeomagnetism,Cyprus |
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Authors: | G J Borradaile F Lagroix T D Hamilton D-A Trebilcock |
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Institution: | 1.Faculty of Science,Lakehead University,Thunder Bay,Canada;2.Equipe de Paléomagnétisme,Institut de Physique du Globe de Paris,Paris Cedex 05,France |
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Abstract: | Magnetic properties of minerals may be sensitive indicators of provenance. Remanence-bearing minerals (RBM) such as iron–titanium
oxides, and matrix-forming minerals such as paramagnetic phyllosilicate or diamagnetic calcite yield different clues to provenance,
strain history and tectonics, and are essential supplements for the full interpretation of palaeomagnetic data. Moreover,
mineral magnetic properties provide magnetic-petrofabric indicators of tectonic strain, determine the suitability of sites
for palaeomagnetism, and permit the restoration of palaeomagnetic vectors in some strained rocks. In the Cretaceous Troodos
ophiolite (~88 Ma) magnetic properties are dictated by the relative importance of mafic silicates and largely primary, ophiolite-derived
RBM. In its cover of deformed pelagic sedimentary rock, magnetic properties are dictated by the balance of clastic RBM versus
matrix calcite and in some cases clay. The two larger Cretaceous ophiolite outcrops (Troodos & Akamas) share a common orientation
of their plutonic flow fabrics, determined by magnetic methods. The dike complex shows fabrics indicating plume-like feeders
spaced along and perpendicular to the spreading axis, with longevities >0.5 Ma. South of the ophiolite, its Cretaceous-Miocene
limestone cover possesses ubiquitous tectonic petrofabrics inferred from anisotropy of magnetic susceptibility (AMS) and anisotropy
of anhysteretic remanent susceptibility (AARM). Its foliation and maximum extension dip and plunge gently northward, sub-parallel
to a common but previously unreported North-dipping stylolitic cleavage. In well-known localized areas, there are S-vergent
thrusts and overturned folds. The S-vergent deformation fabrics are due to Late Miocene (pre-Messinian ~8 Ma) deformation.
The structures are geometrically consistent with overthrusting of the Cretaceous Troodos-Akamas ophiolite, and its sedimentary
cover, onto the underlying Triassic Mamonia terrane. The northern limit of pre-Messinian tectonic fabrics, the Troodos-Mamonia
terrane boundary and the Arakapas-Transform fault form an approximate E–W composite boundary that we term the Troodos Tectonic
Front. Miocene deformation remagnetized the ophiolite and its sedimentary cover in many places and also affects the Mamonia
terrane to the SW, with which the Troodos terrane docked in the late Cretaceous. Magnetic mineralogy, particularly of the
RBM traces the progressive un-roofing of the ophiolite during the deposition of its sedimentary cover. During the submarine
exposure and erosion of the ophiolite, the contribution of RBM clasts to the overlying sedimentary cover changed qualitatively
and quantitatively. Thus, magnetic mineralogy of the sedimentary rock cover records the progressive denudation of the ophiolite
from lavas, down through dikes, to gabbros and deeper mantle rocks. Palaeomagnetic studies previously revealed the anticlockwise
rotation of the Troodos terrane and its northwards migration. Characteristic remanent magnetism (ChRM) is most reliable for
lavas and dikes although it is usually carried by recrystallized RBM. These correspond to the age of greenschist facies ocean-floor
metamorphism, perhaps 7–15 Ma after igneous crystallization with an extent and depth dependent on depth and degree of hydrothermal
circulation. The gabbros and mantle rocks commonly bear young (<12 Ma) remanences probably acquired (or re-acquired) during
uplift of the Troodos terrane. In the cover of pre-Messinian deformed limestone (>8 Ma), the remagnetizing effects of penetrative
strain have been under-estimated. Where strain has occurred, un-tilting procedures produce erroneous restorations for the
remanence vectors, and thus for the associated paleopoles. We find that de-straining of limestone sites most appropriately
restores ChRM vectors to their original orientation. The best-determined and restored ChRMs define an apparent polar wander
path (APWP). Since the APWP terminates at the present N-pole, we inverted it to determine the true plate-motion of the Troodos-terrane. Thus, in present-day coordinates, Troodos rocks moved ~1,000 km South; then ~4,500 km East and finally
~900 km North at an approximate rate of 75 km/Ma 1 km/Ma = 1 mm/a]. This true motion path commenced ~88 Ma ago and rates
of motion since 65 Ma may be too high due to the limited precision of strain-corrections of the ChRM orientations in limestone.
This true motion path is compatible with the eastward and then northward rotation of Africa relative to Europe although other workers show relative motion paths. |
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