Paleomagnetism and cross‐folding in a key area of the Asturian Arc (Spain)

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Abstract
In the northwest corner of Spain, Paleozoic and older rocks outcrop in an arcuate pattern (Asturian Arc), concave toward the east. The arc may be primary (before sedimentation) or secondary (after sedimentation). Supporting a secondary origin are many structural features, including two sets of folds, one radial and one tangential to the arc. The Villasecino area was selected for a pilot study of the relationships between paleomagnetism and folding. Detailed structural mapping has shown that radial folds postdate tangential folds. Interference of the two sets has produced the doubly plunging Villasecino anticline. Forty two oriented paleomagnetic cores were drilled at six sites distributed around the anticline, all within red limestones (griotte) of the Alba formation ( lower Carboniferous). The typical sample contains 95% by volume of fine‐grained calcite; each grain being rimmed by very fine‐ grained goethite and other iron‐bearing minerals. The natural remanent magnetism (NRM) is of moderate intensity (about 3 × 10−3 A m−1) readily measured and very homogeneous for each site. Within‐site scatter of vectors is low, but the site mean varies significantly from site to site. The angle between bedding and magnetization vector is nearly constant. These data suggest that magnetization predated folding. Thirteen samples from six sites were subjected to alternating field demagnetization up to 50 mT, and this showed the NRM to be very stable. Thermal demagnetization indicated a unique stable component with blocking temperature in the range 300°–600°C. Chemical demagnetization was attempted using sodium sulphite. Results suggest that the carriers of magnetization are not iron hydroxides but probably hematite. Even after demagnetization, a simple bedding correction does not yield a good grouping of magnetization vectors. We have therefore tested five other, more complex models of unfolding, based on structural observations. The statistically best grouping is produced by a model involving two successive fold phases, where first folds have horizontal E‐W axes and second folds have vertical axes. This is compatible with structural evidence. After unfolding, we obtain D = 101.8°, I = 13.1° (α95° = 1.5°) for the orientation of the earth's magnetic field in the area. If we compare with the direction for the Carboniferous of ‘stable Europe’ we get a difference in declination of 50° to 65° according to the model used for closing the Bay of Biscay, possibly in association with tectonic evolution of the Asturian Arc.