Dawn-Ann Trebilcock MSc Thesis Abstract

Rock magnetic properties of 50 specimens from a continuous section through the lower layers of the Troodos ophiolite in the CY-4 drill hole, reveal an alteration and palaeomagnetic history consistent with ocean spreading. Anisotropy of magnetic susceptibility (AMS), low temperature demagnetization (LTD), high temperature demagnetization (TD), distinct Curie and N©el temperatures and hysteresis were the main rock magnetic techniques and were supplemented by thin section microscopy.

The steeply dipping CY-4 drill hole has a total depth of 2263 m from the surface through the sheeted dike complex, upper and lower gabbro rocks and ultramafic rocks. CY-4 was part of the Cyprus Crustal Study Project in the 1980s as a Cyprus Geological Survey of Canada-NATO collaboration. 

 

The AMS results showed primarily oblate magnetic fabrics reflecting plutonic ductile flow and strain dominated by chlorite, magnetite, biotite and amphiboles. The bulk susceptibility (ĸ) discriminates the four lithological zones very well, with the highest ĸ in the diabase dikes. The AMS tensor means showed the overall magnetic fabric to be a strong L>S fabric with L ~ vertical and the diabase dikes being the main contributor.  The magnetic fabric became less pronounced with depth through the gabbros down to the mantle rocks, indicating weaker strain or flow influences on the fabric in the upper mantle. 

Palaeomagnetism was studied by thermal demagnetization, low temperature demagnetization and detailed measurements of natural remanent magnetism (NRM) on 45 specimens. Characteristic and secondary magnetizations were isolated and palaeolatitudes were compared on the apparent polar wander path (APWP) of the Troodos microplate. Two main vector components were identified from NRM results, a stable and primary component A, with an unblocking temperature (T_ub) >550°C and a secondary viscous remanent magnetism (VRM) component D, with a T_ub <140°C. NRM palaeopoles for the diabase dikes were anomalous and did not contribute to the conclusions.  Palaeopoles for the upper gabbros plot on the APWP as the oldest rocks suggesting that they cooled first, followed by the lower gabbros at about 75 Ma and the ultramafic rocks acquired their characteristic remanence at 55 Ma.  Hydrothermal alteration on the NRM of Troodos varied with time and spreading distance from the ridge. The upper gabbros cooled and acquired their magnetizations before the anticlockwise rotation of the Troodos microplate.  However, the lower gabbros were affected during the first rotation of  about 60° whereas the ultramafic rocks were magnetized in the final approximately 30° rotation.

      
Curie and N©el temperatures identified specific iron and iron-titanium oxide magnetic minerals. These temperatures also indicate the degree of hydrothermal seafloor alteration vertically down the CY-4 drill hole.  Magnetic hysteresis determined the grain size of magnetic minerals and their capacity for palaeomagnetization. Curie Balance results showed magnetite was most common followed by hematite, throughout CY-4. Ti-rich magnetite (TM₆₀) was present as unoxidized and oxidized grains within the diabase dikes and upper gabbro rocks to a depth of 1060 m. Hysteresis results show an obvious trend of decreasing grain size with depth. However, the majority of samples fell within the psuedosingle domain (PSD) range. The ferromagnetic minerals at the bottom of the drill hole had strong, stable remanence value whereas the top of the hole had minerals with weaker memories (H_cr/H_c). These results help to explain the large influence of the diabase dikes on the AMS fabric. Strong magnetization in the diabase dikes is due to TM₆₀, which is identified to the base of the diabase dikes and into the top of the upper gabbroic rocks.  The Curie temperatures also identified the hydrothermal alteration boundary at about 1060 meters; this alteration boundary is found 3500 meters below the sea floor in modern ocean-floor stratigraphy.    

 

A hydrothermal circulation cell model was used to explain the variations in magnetic properties of the four lithological groups found in CY-4.  As the crust spreads further away from the spreading ridge the convection cell is repeatedly established in the same relative proximity to the spreading axis. Therefore, variations in remanent magnetization are largely due to the alteration over time and range in temperatures with depth and distance from the ridge, as the ocean crust moves through the ridge-parallel zone of appropriate temperature and fracture permeability.