The magnetic fabric of metasediments in a detachment shear zone: the example of Tinos Island (Greece)

We present a magnetic fabric study (12 sites) in strongly deformed metasedi ments (micaschists and calcschists) from a well-exposed detachment shear zo ne at Tinos Island (Cyclades, Greece). A gradient of deformation is observe d from the southwest side to the northeast side of Tinos. The detachment sh ear zone is contemporaneous with a greenschist retrogression episode (ca. 2 5-20 Ma). High-pressure (HP) (45 Ma) metasediments are preserved in the sou thwest side of Tinos. We compared the anisotropy of magnetic susceptibility (AMS, 158 samples) to the anisotropy of isothermal remanent magnetization (AIRM, 40 samples). AMS is primarily dominated by phyllosilicates whilst AI RM reflects the contribution of mixture of ferromagnetic grains (magnetite, hematite, goethite and pyrrhotite). Interestingly, we observe the appearan ce of goethite (alpha FeO . OH) is observed when approaching the detachment area. The orientation of the AMS and AIRM axes shows a general consistency . On the other hand, AIRM is more prolate and anisotropic than AMS. We docu ment a close agreement between the AMS magnetic foliation and the structura l cleavage that has been measured from the core sample. The magnetic lineat ion is generally parallel to the stretching lineation observed on the field . However, composite magnetic lineations suggest well-preserved petrofabric s contemporaneous to the 45 Ma HP episode. An increase in the AMS foliation anisotropy parameter is observed in micaschists when deformation increases . Unfortunately, no quantitative estimate of finite deformation is possible using AMS due to the dominant role of phyllosilicates and the complex ferr omagnetic mineralogy. (C) 2000 Elsevier Science B.V. All rights reserved.