Magnetostratigraphy of the Middle Miocene continental sedimentary sequences of the Mae Moh Basin in northern Thailand: evidence for counterclockwise block rotation

A magnetostratigraphic study has been performed on the Mae Moh basin, well known for its lignite mining in the Lampang province. Paleomagnetic samples were collected from 66 stratigraphic levels. Rock magnetic investigations indicate the presence of low coercivity minerals. Specimens subjected to progressive thermal (or alternating field) demagnetization procedures show that nearly all of them exhibit a low temperature magnetization component, and a high temperature characteristic component (ChRM), with either positive or negative virtual geomagnetic pole latitudes and opposite polarity, considered as the characteristic Miocene magnetization. The overall mean paleomagnetic direction (incl.=22.2°, decl.=358.3°, κ=15, α95=4) documents a counterclockwise vertical axis rotation of about 13°±1.32 with respect to the expected Miocene direction derived from the Eurasian polar wander curve (incl.=42°, decl.=11°). Our paleomagnetic results are not consistent with the previously reported paleomagnetic data. The rotation observed in the Mae Moh basin can be the response to local tectonics. A section sampled for magnetostratigraphy reveals a polarity sequence of nine magnetozones that can reliably be correlated to the geomagnetic polarity time scale. According to biochronological constraints, the magnetostratigraphic results from the mammal-bearing succession correlate with chron C5ABn–C5An2n, between 13.5 and 12.1 Ma. According to the mean sedimentation rate of about 17.5 cm/ka, ages of 12.5 and 12.8 Ma are proposed for the fossiliferous levels (J5 and K1, K2 lignite zones) were the mammal remains were found.