Timing of magnetization and vertical-axis rotations of the Cotiella massif (Late Cretaceous, South Central Pyrenees)

The Cotiella thrust sheet in the South Central Pyrenees is a well-studied example of an extensional basin that formed at the northern Iberian rift margin during the Late Cretaceous, and was inverted in the Late Cretaceous and Cenozoic on top of the South Pyrenean Central Unit. A palaeomagnetic study of the thick sequence of shallow marine syn-extensional carbonates filling the Cotiella Basin reveals the presence of a widespread secondary magnetization of both normal and reversed polarities. Neither in situ nor bedding tilt correction provides a satisfactory grouping of the palaeomagnetic directions, thus suggesting syn-deformational magnetization. Best clustering of palaeomagnetic vectors is obtained after unfolding into a post-extensional pre-compressional stage, coherent with an acquisition age between Santonian and Paleocene. Strong magnetic enhancement and very distinct magnetic properties with depth suggest a link with burial diagenesis of organic matter, precipitation of iron sulphides and subsequent oxidation to SD fine-grained magnetite. The secondary magnetizations of the Cotiella record significant vertical axis rotations ranging from 14° to 52° clockwise, coherent with the scenario of the Cotiella thrust sheet transported piggy-back and bent during the emplacement of the lower Gavarnie–Sierras Exteriores thrust sheet.