C. T. Aubourg

Magnetic fabric and petrographic investigation of hematite-bearing sandstones within ramp-related folds: examples from the South Atlas Front (Morocco)

An investigation of the anisotropy of magnetic susceptibility (AMS) of hematite-bearing sandstones has been carried out along two fault-propagation folds located at the front of the High Atlas Belt (Goulmima area, Morocco). It reveals two types of unusual magnetic fabrics. The first one is always located in the forelimb of the anticlines and is characterised by an oblique magnetic foliation with respect to bedding. The relationship between bedding and magnetic foliation indicates a sense of shear, which is consistent with the one expected from kinematic modelling. It is proposed that such an oblique magnetic foliation is the result of a weak strain that develops during folding. The other striking feature of the AMS results is that 40% of the magnetic lineations are parallel to the regional shortening direction. From structural and petrographical data, it is suggested that this magnetic lineation corresponds to the intersection lineation between two planar elements that are, respectively, the bedding and iron oxides mineralised veins. The latter result from iron-rich fluid circulations through a fracturing network.

Magnetic fabrics and oblique ramp-related folding: A case study from the western Taurus (Turkey)

Abstract An analysis of magnetic fabric has been performed in weakly deformed Paleocene-Eocene limestone adjacent to two subparallel ramp-related folds (Akseki and Ormana folds) from the western Taurus (Turkey). The magnetic fabric of tectonic origin records two trends of well-defined magnetic lineation: N160 ° ± 7 ° and N130 ° ± 14 ° in front of the Akseki and Ormana folds, respectively. The trend of the magnetic lineation is oblique to the Akseki fold, whereas in the Ormana fold, magnetic lineation and fold axis are subparallel. The observed change in the trend of magnetic lineation is probably linked to a change of the shortening direction that occurred during the development of the thrust system. This shows that the two subparallel folds result from two different translation vectors.

Magnetic monitoring of hydrothermal magnetite nucleation-and-growth: Record of magnetic reversals

Abstract Hematite reduction in the presence of pyrite and CaO led to the formation of anhydrite and magnetite at 30 MPa and 723 K under hydrothermal conditions in a sealed gold container. For the first time, hydrothermal magnetite production was monitored magnetically at the run conditions. Sample magnetic susceptibility was found to follow an Avrami kinetic law with k = 1.5 10-5/s and n = 2.07. Application of up to two external field reversals in the course of the magnetite production reaction shows that partial Chemical Remanent Magnetizations (CRM) are recorded by the sample. The intensity of each partial CRM acquired within a given time interval (under a given field intensity) is proportional to the amount of magnetite produced during that time interval. Alternating magnetic field demagnetization failed at separating these partial CRMs, which are believed to superpose in the (V, Hc) diagram of Néel. Additional TEM, SEM, and magnetic characterizations suggest that continuous nucleation and fast magnetite growth (to a maximum size of ca. 10 mm), compared to the field polarity duration, can account for the evolution of the sample CRM with time. This study demonstrates that one should be cautious when using natural CRM apparent polarity to infer timing of the corresponding chemical event