Cronología de los procesos de remagnetización química por enterramiento

Abstract

The widespread chemical remagnetizations are common in sedimentary rocks, particularly if they are caused by burial. This is because the heating linked to burial generates the growth of ferromagnetic (s.l.) grains. The growth of magnetite in limestones as responsible of widespread remagnetizations has been documented in several thick sedimentary basins. A case that has been studied in depth by our research group is that of the Mesozoic Atlasic Basin subsequently inverted during the Cenozoic. The lower Jurassic limestone recorded a systematically normal polarity overprint carried by magnetite. This remagnetization has been dated at 100 Ma, within the so called Cretaceous Normal Polarity Superchron (CNS). The magne- tic mineralogy of these rocks is dominated by fine grained magnetite with domain state between superparamagnetic (SP) and stable singledomain (SSD). This remagnetization shows a very homogeneous paleomagnetic direction and age throughout the wide studied area (~10.000 km2), suggesting an acquisition during a short event. However, the extensional stage of the basin lasts tens of millions years, keeping the necessary burial conditions for growth of magnetite grains across several polarity chrons, including the CNS. In this study we address the question of timing of growth of magnetic particles and blocking of magnetization in magnetite, as well as the record of the remagnetization. We analyze the hypothesis that the remagnetization direction is just the average of magnetic moments of the entire SSD magnetite population that grew from the Middle Jurassic up to the Cenozoic, resulting in the expected direction at the middle of the CNS. In order to test this hypothesis, simulations of the record of magnetization as well as rock magnetism experiments to demonstrate the existence of SSD magnetite grains blocked with reversed and normal polarities in the same sample have been developed. These results can explain the frequent occurrence of widespread remagnetizations during superchrons.