Hainan Island lies near the Red River Fault, a prominent tectonic feature produced by the India-Asia collision. There, we carried out a geochronologic and paleomagnetic study on Cretaceous rocks in order to better understand the kinematic history of the region. U-Pb zircon dating of tuff intercalated in red bed sedimentary rocks yielded a concordant age of 106.6 ± 0.3 Ma; a mafic dyke intruding the red beds yielded a concordant age of 104.6 ± 0.7 Ma. Stepwise demagnetization experiments on 448 sedimentary rock samples and 191 dyke samples isolate solely normal polarities. Paleomagnetic directions of the dykes cluster in two distinct populations in geographic coordinates, indicating that dyke intrusion occurred in two pulses of limited duration (secular variation was not averaged) after tilting of the sediments. Baking of the sediments from the dykes only occurred near the contacts. Together with published data, the mean directions of 104 sites most tightly group at 58.3 ± 3.2% unfolding, indicative of a synfolding remagnetization, which can be constrained to have occurred within a 2 Myr period between sedimentation and dyke intrusion. We suggest that warm (50–100°C) fluid interaction during basin development led to new mineral growth spawning chemical remagnetization. The corresponding paleomagnetic pole at 81.5°N, 145.2°E (A₉₅ = 2.4°) is indistinguishable from the coeval Eurasian reference pole, suggesting the South China Block has remained fixed to Eurasia since 105 Ma. A contour map of paleomagnetic rotations from 115 studies in the region shows that the Red River Fault roughly demarcates rotation magnitudes/signs, suggestive of a major tectonic boundary.

Article link: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2021JB023474