Abstract

Palaeomagnetic, structural and geochronological analysis of a suite of Oligocene mafic dykes intruded into the Malaguide allochthon in the Internal Zone of the Betic Cordillera demonstrate that at least 134° clockwise vertical-axis rotation of a large region occurred, probably during early Miocene thrusting onto the External Zones. Initial results from Ar/Ar whole rock analysis suggests that the dykes were probably intruded over a period of time between 23 Ma and 30 Ma. Results of demagnetization experiments demonstrate that the remanence in many of the dykes is composed of one to three components. The lowest temperature component is poorly defined. The intermediate temperature (IT) component, carried by pyrrhotite, has reversed polarity, a declination of 314° and an inclination of −48°. This component was probably acquired during cooling after greenschist-facies metamorphism. The high temperature (HT) component is carried by magnetite and was probably acquired at or shortly after intrusion. This component has both reversed and normal polarity, with an average declination of 320° and an inclination of −13°. Rotation of approximately 35° about a horizontal NNE-trending axis, in present-day coordinates, brings the inclination of the HT component into statistical agreement with both the IT component and the expected inclination for the Oligocene. This rotation leaves the dykes in a near-vertical orientation but brings the foliation in the surrounding schists to approximately horizontal.

We suggest that the original regional foliation may have been sub-horizontal and related to the ductile phase of late-orogenic extension in the Betic Cordillera. The dykes were intruded sub-vertically into the schists with an original NW–SE trend. The region was then tilted and rotated clockwise by 23°±15 before acquiring the IT component after which it underwent 134°±10° of clockwise rotation about a vertical axis.

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