Circum-Troodos limestone succession of south and west Cyprus : a magnetic fabric and magnetic mineralogical study
Hamilton, Thomas Derek
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Calcite petrofabrics align easily with weak strains, possibly being the most sensitive classical petrofabric indicator. Thus, calcareous sediments may reveal stress trajectories in neotectonic environments. Calcite aligns by crystal-plastic deformation and pressure solution to produce corresponding alignments in accessory clay minerals and magnetite (possibly fossil-bacterial). Their alignments are rapidly and precisely detected by anisotropy of low field magnetic susceptibility (AMS). These net magnetic fabrics blend diamagnetic contributions from matrix calcite (diamagnetic bulk susceptibility (see document) Considering their relative abundances and different anisotropies, their orientation distributions of AMS axes are sensibly interpreted as paleostress trajectories in Neogene and Quaternary strata at the regional and sub-area level (each ~400km2 and ~30km2 respectively). The AMS axes may be correlated with the orientation of faults, plate-motion vectors and seismic solutions. A large sample (1090 specimens from 419 sites) was treated by different statistical approaches (“standardization”) to emphasize or suppress the contribution of sub-fabrics with anomalous mean susceptibility. A subsample of 254 specimens from 219 sites, from different sub-areas was also investigated by anisotropy of anhysteretic remanence (AARM), which isolates the orientation distributions of magnetite. AMS and AARM magnetic fabrics are mostly of the L-S kind with the magnetic lineations compatible with gravitational stretching of the sedimentary cover away from the Troodos massif and orthogonal to the strike of principal faults and graben. The L-direction (kMAX) shows a smooth variation in orientation, through the subareas, directed radially from the Troodos massif and the S-components of the magnetic fabrics are inclined gently to the bedding, compatible with vergence toward the Cyprean Arc that lies offshore to the South and South-West of Cyprus. From the original set of 1090 specimens, two smaller sets of samples were further studied using different magnetic techniques to examine differences in magnetic mineralogy and granulometry in different lithologies and through time. The first set of 100 specimens was divided into pelagic and non-pelagic sub-sets and microhysteresis showed that these samples contained magnetite in the appropriate size ranges for simply interpretable AMS fabrics (“normal fabrics”) and also exhibited possible contributions from titanomagnetite (TM60) in the non-pelagic samples. A sub-set of 55 samples in stratigraphic sequence with approximately known age determinations (54-6 Ma) shows systematic variations in bulk susceptibility (k), anhysteretic susceptibility (kARM), saturation isothermal remanence (SIRM), and thermal demagnetization unblocking temperatures (Tub)- Some combinations of these magnetic parameters have demonstrated that TM60 is present in appreciable amounts in the youngest of the Cyprus limestones (due to uplift and early erosion of the Troodos massif), and in some of the oldest rocks (due to distal submarine volcanism). Furthermore, the lack of TM60 in the middle of the sequence and the magnetic granulometry shows that magneto tactic bacteria dominate the chalk units of Cyprus.
- Retrospective theses