Magnetic constraints on the thermal evolution of a collapsing orogen

The thermal evolution of an orogen undergoing late-stage extension was investigated using rock magnetic properties of a suite of mafic dyke rocks affected by greenschist facies metamorphism in the internal zones of the Betic Cordillera, southern Spain. The natural remanent magnetization (NRM) in the dykes intruded into the lowest geological unit is made up of up to three components. The lowest temperature component (LT) is in the direction of the present day magnetic field and is believed to be a chemical remanent magnetization (CRM) or viscous remanent magnetization (VRM) acquired in the recent magnetic field. The intermediate temperature (IT) component unblocked between 200°C and 450°C is thought to be largely a thermoviscous overprint acquired during metamorphism. This component is carried by either primary or authogenic sulfides and low-unblocking temperature magnetite. The component of magnetization with the highest blocking temperature (HT) is isolated above 450°C and is interpreted as the primary component of remanent magnetization. It is most likely that this component is carried by magnetite that resides in the plagioclase and has been shielded from the metamorphism, which transformed most of the original magnetite to metamorphic amphibole, chlorite and biotite. Thermal demagnetization of these dykes separates the IT overprint from the HT primary remanence at a sharp junction occurring at 450°C. For single domain grains this translates to a peak palaeotemperature in the natural sample of approximately 370°C, which is close to the estimated temperature experienced by the greenschist facies country rocks (400°C). In contrast, results obtained from a dyke that is intruded into an overlying weakly metamorphosed geological unit, indicates that temperatures only reached about 175°C in this unit. These results are consistent with temperatures deduced from geological constraints and they imply that between 4 and 6 km of section has been removed between the two units during late orogenic extension. These new data are then combined with previously published Ar–Ar wholerock geochronology and information on vertical axis rotations to put constraints on the thermotectonic evolution of this collapsing orogen.