Late Cenozoic deformation in the South Caspian region:

effects of a rigid basement block within a collision zone

Mark B. Allena, Stephen J. Vincenta, G. Ian Alsopb, Arif Ismail-zadehc, Rachel Flecker d,e

aCASP, Department of Earth Sciences, University of Cambridge, West Building,
181A Huntingdon Road, Cambridge, CB3 0DH, England
bCrustal Geodynamics Group, School of Geography and Geosciences, University of St.Andrews, Fife, KY16 9ST, Scotland

cGeology Institute, Azerbaijan Academy of Sciences, 29A H. Javid Prospect, Baku, Azerbaijan

dDepartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge, England

eSchool of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, and Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, Scotland

mark.allen@casp.cam.ac.uk

Active deformation in the South Caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone. Rigid basement to the South Caspian Basin moves with a westward component relative both to stable Eurasia and Iran, and is beginning to subduct at its northern and western margins. This motion is oblique to the approximately north-south Arabia-Eurasia convergence, and causes oblique shortening to the south and northeast of the South Caspian Basin: thrusting in the Alborz and Kopet Dagh is accompanied by range-parallel strike-slip faults, which are respectively left- and right-lateral. There are also arcuate fold and thrust belts in the region, for two principal reasons. Firstly, weaker regions deform and wrap around the rigid block. This occurs at the curved transition zone between the Alborz and Talysh ranges, where thrust traces are concave towards the foreland. Secondly, a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence: west-to-east thrusts in the eastern Talysh represent underthrusting of the South Caspian basement, but pass via an arcuate fan of fold trains into SSW-directed thrusts in the eastern Greater Caucasus, which accommodates part of the Arabia-Eurasia convergence. Each part of the South Caspian region contains one or more detachment levels, which vary dependent on the pre-Pliocene geology. Buckle folds in the South Caspian Basin are detached from older rocks on thick mid-Tertiary mudrocks, whereas thrust sheets in the eastern Greater Caucasus detach on Mesozoic horizons. In future, the South Caspian basement may be largely eliminated by subduction, leading to a situation similar to Archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust.