Kinematics of a twisted core complex: Oblique axis rotation in an extended terrane (Betic Cordillera, southern Spain)

Abstract

The Sierra de las Estancias is a metamorphic core complex that developed during the late Oligocene to early Miocene extensional collapse of the Betic Cordillera. Structural and paleomagnetic analysis shows that the northern half of the complex has been strongly modified by rotation about an inclined axis, requiring reassessment of the kinematics of the extensional phase of deformation. The extensional tectonic event produced two major detachment faults, associated mylonitic foliation and lineation, and pervasive sets of shear bands and brittle normal faults with a top-to-NE shear sense. An older foliation was largely transposed and reoriented during this event. The regional foliation now defines a broad antiform with a steep north dipping limb; and the trend of shear-related lineations swings from NNE to ENE from the southern gently dipping limb into the north dipping limb. The remanent magnetization in a suite of near-vertical early Miocene mafic dikes intruded into the north dipping limb has a declination of 243° and an inclination of -27°. Assuming that the dikes were originally vertical, a rotation of approximately 50° about an inclined SSW plunging axis preserves the near-vertical orientation of the dikes and brings the remanence into agreement with the expected orientation for an early Miocene age. Correction for this rotation brings the foliation and lineation in the surrounding schists into orientations subparallel to those on the present southern limb of the antiform, and the dikes into a vertical E-W orientation. The corrected orientation of the shear-related lineation (NNE), normal faults, and dikes suggests that the original direction of extension during the late orogenic extensional event in the Internal Betic Cordillera may therefore have been around NNE/SSW, rather than E/W as currently accepted. The observed net tectonic rotation can be decomposed into a series of geologically plausible rotations involving domino-style rotation due to slip on the late brittle normal faults, tilting about the regional fold axis related to thrust-ramping onto the Iberian margin during Miocene collision, and a 30° clockwise rotation about a vertical axis related to oblique convergence with the Iberian margin. Copyright 2004 by the American Geophysical Union.