KINEMATICS OF THE BETIC-RIF ARC FROM CRUSTAL CROSS-SECTIONS

The Betic-Rif arc comprises a tightly arcuate thin-skinned external fold and thrust belt, formed during the Miocene around the western margin of the Alboran Domain - an exotic mainly metamorphic terrane that moved relatively westwards between the converging African and Iberian plates. The complex kinematics of the arc greatly complicate the construction and restoration of balanced cross-sections. Strike-slip faulting and large vertical-axis rotations mean that material has moved in and out of the plane of section, and the orientation of the restored section line is different from the present-day. Because vertical-axis rotations occurred during thrusting along obliquely converging margins, structural trends including fault slip vectors have not necessarily rotated as much as the rocks themselves and the magnetic remanences they carry. We have approached this problem through analysis of the correlations between fold trends, slip vectors, and total rotation, to isolate the amount that slip vectors have been rotated. Sections around the external Betic-Rif arc, constructed parallel to the local present-day trend of slip vectors, have been restored to the pre-rotation trends, allowing palinspastic restoration of the Alboran Domain in early Miocene time.

During the formation of the external thrust belt, the Alboran Domain itself experienced a kinematically complex pattern of extensional deformation, which may have resulted from the removal of lithospheric mantle beneath it at about 24 Ma, following Eocene crustal thickening. Rapid NNE-directed extension and shear between 22 and 19 Ma thinned an early thrust stack as much as 50 km thick to between 5 and 10 km thickness, creating a stack of extensional allochthons separated by sub-horizontal detachment faults.

Subduction of the leading edge of the Iberian margin beneath the advancing edge of the Alboran Domain at about 19 Ma, and rapid exhumation completed by 12 Ma, created a large metamorphic dome beneath the previously extended and now overthrust Alboran Domain, bounded by an extensional detachment with predominantly W-directed shear sense. Dextral shear related to thrusting of the Alboran Domain over the Iberian margin rotated earlier mylonitic stretching lineations and normal-sense slip vectors in the Alboran Domain by variable amounts towards E.