Paleomagnetism of the Late Cretaceous Padana Fm in Xigaze forearc basin and its paleogeographic constraints on the southern margin of Asia prior to the India-Asia Collision

Abstract

The Xigaze forearc basin was formed along the southern margin of Asia as the Neo-Tethys Ocean subducted beneath Asia. Among the strata within the Xigaze forearc basin, the Padana Formation (Fm) comprises purple-red and gray shale interbedded with sandstone, and likely holds crucial evolution information of the southern Asian margin during the Late Cretaceous prior to the India-Asia collision. However, the chronology of the Padana Fm has not been well constrained. To refine the chronology of the Padana Fm and better constrain the paleopositions of the southern margin of Asia, we conducted a paleomagnetic study of the strata in the Xigaze forearc basin, with a particular focus on the Padana Fm. A total of 263 paleomagnetic samples were collected and subjected to stepwise thermal demagnetization, revealing two-component magnetizations. The low temperature component (LTC) was removed by ~300℃, representing overprints of the present geomagnetic field. The high-temperature component (HTC) was typically unblocked at 580∼680 °C, indicating magnetite and hematite as major remanence carriers. A total of 110 reliable HTCs were isolated from the sandstones in the Padana Fm and passed reversal tests, representing primary remanence. Changes in polarities of the HTCs with stratigraphic heights define four polarity zones, which, together with results of previous detrital zircon U-Pb ages and biostratigraphic zones, allow to constrain the chronology of the Padana Fm to 83.6 Ma-69.2 Ma. With the significantly refined chronology, the paleomagnetic data of the Padana Fm can be used to constrain the tectonic evolution of the Xigaze forearc basin in the Late Cretaceous. Tentative interpretations of the coeval paleogeography of the southern Asian margin prior to the India-Asia collision will be discussed.