Evidence for weak geomagnetic field intensity prior to the Cretaceous normal superchron

Abstract

A palaeomagnetic and geochronologic investigation has been carried out on a sequence of 19 lava flows at Sihetun in northeastern China. Age determinations by K/Ar dating indicate that the sequence spans 124–133 Ma, that is, the time just prior to Cretaceous normal superchron (CNS). Rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo-single domain magnetite. Stepwise thermal demagnetization successfully isolated a well-defined direction of characteristic magnetization (ChRM) in all the lavas. The mean palaeodirection determined is D/ I=5.4°/58.7° ( α 95=2.4°) and D/ I=175.9°/−60.0° ( α 95=6.4°) for 15 normally magnetized flows and 3 reversely magnetized flows, respectively. One lava was found to possess an intermediate direction of magnetization. The inferred palaeolatitude of the sampling site is very similar to the present-day position, supporting the contention that suturing between the Mongolia and north China plates was probably completed by the Late Jurassic, and that subsequently little rotation or translation occurred in the region west of the Tanlu fault zone. Thellier–Thellier palaeointensity experiments with systematic partial thermoremanent magnetization (pTRM) checks were conducted on 66 samples from 13 lava flows. Results from the 29 samples (44%) which met strict reliability criteria indicate a range of virtual dipole moments (VDMs) from 2.83 to 4.17×10 22 A m 2 with an average of (3.53±0.02)×10 22 A m 2. This finding helps confirm that a weak geomagnetic field probably occurred just prior to the CNS.