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Rates and Character of Opposite-Sense Small-Scale Tectonic Rotations Associated with Sinistral Transcurrent Shear Zones, Cabo de Gata Region, SE Spain

by Paul Montgomery, ChevronTexaco; Evan K. Franseen, Kansas Geological Survey; Robert H. Goldstein, University of Kansas; and Mark W. Hounslow, University of Lancaster

A series of small tectonic blocks from a 6 km2 area of the Cabo de Gata region of SE Spain underwent opposite-sense tectonic rotations during the Late Tortonian and Messinian (Miocene). Integrated paleomagnetic and chronostratigraphic analysis of upper Tortonian-Messinian strata allow for calculation of rotation rates. The upper Tortonian sequences at Cerro del Cuervo Beach and Cerro El Romeral represent a block that has undergone 1-2o/m.y. clockwise rotation that ranged between 5o-13o. The Upper Tortonian to Upper Messinian sequence at Cerro de la Molata represents a block that has undergone 2.5-8o/m.y. anticlockwise rotation of 17o. Tectonic motions observed are likely related to the crustal structure of the eastern Betics. Rotation distribution analysis reveals that clockwise rotations predominate in the south whereas anticlockwise rotations dominate in the north. A tectonic boundary between two small, oppositely rotating tectonic-blocks is proposed. The paleomagnetic data support other studies that indicate the Cabo de Gata region underwent a major period of wrench faulting beginning in the Tortonian and continuing into the Late Neogene. The nearby Carboneras and Palomares sinistral transcurrent shear zones are the physical manifestation of this tectonic deformation. Although paleomagnetic data indicate rotations between tectonic blocks, other data indicate only minor differential vertical displacement between the blocks. This is evidenced by regionally equivalent sedimentary sequences, sequence boundaries, and marine planation surfaces that occur at the same elevations. In addition, geopetal fabrics and stratal geometries indicate a lack of tilting and only meter-scale vertical offsets on intra-block faults that cut Miocene strata.

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