Palaeomagnetic and kinematic constraints on deformation during oblique convergence, Betic Cordillera, southern Spain

The Subbetic Zone is an external, unmetamorphosed, thin skinned fold-and-thrust belt of Alpine age, part of the Betic Cordillera, southern Spain. Miocene tectonics emplaced a 2 km pile of Mesozoic and Tertiary sediments north-westward onto Iberia. Previous palaeomagnetic studies have revealed differential vertical-axis rotations of 60° clockwise or more. Palaeomagnetic and structural studies in two study areas were undertaken with the aim of understanding how these large vertical-axis rotations accumulated in this obliquely convergent mountain belt, in the context of structures present. The El Chorro area, in the western Subbetic, is a 16 km2 structural culmination. Palaeomagnetic results show the imbricate thrust sequence has differential rotations within and between thrust sheets. It is likely folding, thrusting and rotation occurred close to the deformation front of the mountain belt. Vertical-axis rotation is attributed to a thrust sheet pinning mechanism. The Velez Blanco area covers approximately 400 km2 of the eastern Subbetic close to the Internal Zones. Structural analysis finds normal faults developed during Jurassic rifting. Rotation increases from zero to 60° clockwise from NE to SW, without abrupt changes across individual structures and correlates with a change in strike of thrust traces and stratigraphic contacts from N-S to almost E-W. This is suggestive of transcurrent shear in the more internal parts of the mountain belt. Thrust faults are mostly SSE-directed, but ESE-directed thrusting is common in the unrotated zone, suggesting that fault lineations indicative of transport directions have been rotated. Combined with strike-slip partitioning it is estimated that the local plate motion vector was N59°W, between the Alboran plate and Iberia. Two likely mechanisms were responsible for clockwise rotation in this region. 1) Early rotation during thrusting accumulated though the thrust sequence. 2) Transcurrent shear in the internal parts of the mountain belt occurred later, and led to further vertical-axis rotation