Remagnetizations and postfolding oroclinal rotations in the Cantabrian/Asturian arc, northern Spain

Abstract

Devonian carbonates in the Cantabrian Arc reveal characteristic magnetizations with coherent, shallow, upward inclinations. The magnetizations appear to be carried by magnetite. Within‐site directions are very well grouped, but site‐mean declinations range from easterly to south–southwesterly in in situ as well as tilt‐corrected coordinates, as has also been observed in previous studies of other formations in the arc. The widely varying declinations of all studies roughly correlate with the overall structural trends of the arc and suggest that the sites underwent rotations in a process that involved folding about vertical axes and tightening of arc. Upon tilt correction the inclinations of our study, on the other hand, become scattered, and it is concluded that the magnetizations were acquired after Late Carboniferous folding about horizontal axes. The oroclinal rotations therefore also must have occurred after the Late Carboniferous folding phase. The previous paleomagnetic results had been interpreted mostly as primary magnetizations residing in hematite. However, inclination only fold‐tilt tests applied to these results suggest that many, if not all, of the directions were acquired during the earlier stages of the Late Carboniferous folding. Thus all paleomagnetic results from the Cantabrian Arc appear to be remagnetizations, but the ages of the remagnetizations vary from pre‐to synfolding for the mostly hematitic formations to postfolding for the Devonian carbonates. The reversed‐polarity inclinations of the hematite‐bearing formations have mean values ranging from +25° to +5°, whereas the carbonates have a mean inclination of −8°. On the basis of inclinations predicted for the area from results from stable Europe, the Pyrenees, and the Iberian Meseta, the ages of these remagnetizations can be inferred to range from about 320 Ma to 260 Ma. Because all the remagnetizations reveal rotated declination patterns, the oroclinal rotations occurred well after the main phase of Hercynian deformation (320–280 Ma). While the timing of the rotations is unconstrained at the younger end, they must have occurred during or after the Permian (best estimate is less than 260 Ma), which is much later than anticipated from other geological considerations.