Insights into extensional events in the Betic Cordilleras, southern Spain: New fission-track and U–Pb SHRIMP analyses

Abstract

A combination of the U–Pb SHRIMP and zircon fission-track analyses has been used to constrain the age and evolution of the extensional tectonics in the Betic Cordilleras, southern Spain. The studied rocks are dolerite dykes intruded in the Malaguide Complex and in the underlying Los Reales nappe and their country rocks. The dykes are grouped in two nearly perpendicular systems, characterized by N–S strikes in the Los Reales nappe and E–W strikes in the Malaguide Complex. Two periods of extension have been identified. The age of the older event, ranging from ~ 33 to 25 Ma, is constrained by the intrusion of the dykes (33 Ma) and the joint cooling of the Los Reales–Malaguide domain from 25 Ma onwards. This event is associated with the structural emplacement of the Malaguide Complex over the Los Reales nappe, along a brittle–ductile extensional detachment and is tentatively ascribed to the tectonic collapse of the orogenic belt. A minimum time span of 10 m.y. is established between the onset of the extensional collapse and the widespread Aquitanian HT-LP metamorphism. In the present geographic frame, the kinematics of this event is characterized by a change from an E–W extension at lower levels (Los Reales nappe) to a N–S extension in upper levels (Malaguide Complex). A younger extensional event of Miocene age (~ 18 Ma) has been deduced based on the fission-track analysis within the “calizas alabeadas” member of the Malaguide Complex.