Unravelling the kinetics of burial remagnetizations

Abstract:

Burial diagenesis is one of the main mechanisms claimed for explaining pervasive remagnetizations in sedimentary basins and orogenic wedges worldwide. It involves authigenic formation (e.g. chemical remagnetization) of magnetic minerals in response to enhanced temperature conditions at deep basinal positions. Key questions about the kinetics of this process in natural cases remain unsolved because most of the best-documented cases occurred during long-lasting magnetic superchrons. The Western Pyrenees display a dual polarity record of a burial remagnetization affecting the deepest part of the Eocene Jaca turbiditic basin and its underlying units. A preliminary study of 49 sites distributed along three cross sections reveals that postfolding (remagnetized) ChRM directions are found within the lowermost part of the studied sedimentary pile, which is affected by cleavage, in such a way that ChRM unblocking temperatures increase as a function of depth. Noteworthy, this remagnetization is associated with hysteresis data typical for chemical remagnetizations (samples falling mainly within the SP+SD+MD mixing line in the Day plot), and displays up to five different polarity zones that are consistent along strike in the three studied cross sections. Prefolding ChRM directions are found, in contrast, in the overlying sequence above the cleavage domain. We intend to carry out a detailed, multidisciplinary study of this remagnetization in the framework of the recently funded UKRIA4D project, that will integrate: A) a full characterization of the remagnetization front combining paleomagnetic and rock-magnetic data; B) characterization of the burial conditions based on SEM and TEM observations of mineral assemblages and temperature estimates from RAMAN spectroscopy on carbonaceous material; and C) magneto-biostratigraphic dating of the 7 km-thick overlying sedimentary sequence. Integration of these data will allow establishment of the burial conditions that led to the observed remagnetization, on which, at least, a qualitative kinetic model for burial remagnetizations will be developed.