Paleomagnetic and rock magnetic study of combustion metamorphic rocks in Israel

Abstract

Paleomagnetic and rock magnetic studies were combined with the mineralogy and metamorphic petrology of a sequence of combustion metamorphic (CM) rocks in Israel. Samples from 12 sites from the three major outcrops of the Hatrurim Formation (“Mottled Zone”) were analyzed. These are originally Cretaceous bituminous chalks and marls which have undergone spontaneous combustion during the Late Neogene. The analyses included natural remanent magnetization (NRM) intensity, alternating field (AF) and thermal demagnetization, and isothermal remanent magnetization (IRM) measurements. Mineralogical phases were identified by X‐ray diffraction and electron microprobe. All metamorphosed rocks have a NRM intensity far above normal sedimentary rocks. This increase in intensity is observed even in rocks where no mineralogical indications of metamorphism are observed. The major contributors to the rock magnetism in these rocks are maghemite, hematite and rare magnesioferrite. The magnetic properties were acquired during the burning process as a consequence of oxidation of Fe‐sulfides. The in situ direction of the mean magnetic vector is Dec = 8.2°, Inc = 49.2°, K = 40 and α95 = 5.2 whereas the stratigraphically corrected mean is Dec = 25.1°, Inc = 46.9°, K = 26 and α95 = 6.5. The stratigraphic mean direction is in accordance with the Pliocene expected field direction for this area as derived from stable Africa poles. Therefore combustion must postdate tectonic tilt or vertical axis rotation associated with Late Cretaceous major folding and faulting and rifting associated with the Dead Sea Transform. Normal and reverse polarity and paleosecular variation indicate that combustion at the different sites was not simultaneous.