Mechanics and mechanisms of magmatic underplating: inferences from mafic veins in deep crustal mylonite

We have sampled a sequence of 69 flows in the island of El Hierro (Canary Islands). 46 of them belong to the El Golfo formation and 23 to the most recent rift formation. Radiometric dating brackets this section between 442 ka and 134 ka with six intermediate datings. All the samples were stepwise demagnetized, partly with alternative field, partly thermally with very similar results. The large majority of the samples are characterized by a single stable component of magnetization. ChRM isolated after the first steps of demagnetization are all of normal polarity. Inclinations are on average shallower than expected from a geocentered dipole field, but consistent with results from other areas at the same latitude. Mean declinations of the rift and El Golfo sequences are both easterly deviated (about 5° and 15° respectively) and significantly different suggesting that the section has undergone a progressive clockwise rotation. VGP scatter and directional scatter calculated for the rift sequence are consistent with available data from other geographical areas, and also with recent models of paleosecular variation. On the other hand, a significantly smaller directional scatter is obtained for the lower El Golfo sequence, suggesting that a period of low secular variation occurred in the Canary Islands area between approximately 440 and 260 ka.