| dc.description.abstract | 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. | |
