Remagnetization of Pre‐Fan Sediments Offshore Sumatra: Alteration Associated With Seismogenic Diagenetic Strengthening

Abstract Increases in temperature and pressure caused by rapid burial of sediments seaward of the Sumatra subduction zone have been hypothesized to trigger dehydration reactions that diagenetically strengthen sediments and contribute to the formation of an over‐pressured pre‐décollement, which together facilitate the occurrence of large shallow earthquakes. We present paleomagnetic, rock magnetic, and electron microscopic analyses from drill cores collected offshore Sumatra at Site U1480 during IODP Expedition 362 that support this hypothesis. The older pre‐fan units (Late Cretaceous to early Paleocene) were deposited when Site U1480 was moving rapidly northward with the Indian plate from a paleolatitude of 50° to 30°S, which would equate to expected absolute paleomagnetic inclinations of 70°–43°. Most of the older pre‐fan sediments, however, have shallow observed inclinations (shallower than ±20°), indicating that the sediments were overprinted when Site U1480 was located near the paleoequator, as it has been since the early Oligocene. Electron microscopic observations reveal that the pre‐existing detrital magnetite grains have undergone pervasive dissolution and alteration by hydrothermal fluids. The diagenesis observed is consistent with mineral dehydration, possibly driven by rapid burial of pelagic sediments by the ∼1250 m thick Nicobar Fan sequence. In addition, the elevated burial temperature also facilitated the smectite to illite conversion reaction. We hypothesize that chemical reactions resulted in the formation of fine‐grained magnetite that records a chemical remanent magnetization overprint. This overprint is consistent with the alteration occurring after burial by the thick Nicobar Fan sequence sometime in the past few million years.