Clockwise rotations recorded in redbeds from the Jinggu Basin of northwestern Indochina

Abstract

Paleomagnetic data have been used to infer clockwise rotations and significant southward motion of the Indochina block during Cenozoic extrusion from the India-Asia collision zone. Because the Cenozoic of the Indochina block has been sparsely sampled to date and is key to determining the timing of this tectonic motion, we performed an extensive paleomagnetic study on Paleocene to Oligocene redbeds and middle Miocene sandy silts from the Jinggu Basin (23.5°N, 100.7°E), in northern Indochina. Paleomagnetic results from the redbeds pass fold tests, but they show exclusively normal polarity. There is controversy, however, on the age assignment to the lower part of the succession. If the age of the redbeds is indeed Paleogene, this indicates a prefolding remagnetization associated with Oligocene deformation of the Indochina block. If the age of the redbeds would be Late Cretaceous, their magnetization may be primary, and deposition could have taken place during the Cretaceous normal superchron. However, the abundance of secondary hematite in the redbeds in combination with the biostratigraphy, which indicates a Paleogene age for at least the upper two formations of the redbeds in the Jinggu Basin, implies a pervasive remagnetization. The middle Miocene sediments pass both the fold test and the reversals test and contain magnetite as well as hematite as carriers, suggesting a primary magnetization. Our large data set from the redbeds (>2000 paleomagnetic directions) demonstrates an ∼30°−35° clockwise rotation of the Jinggu Basin with respect to Eurasia, for both scenarios, i.e., when compared to a ca. 100 Ma pole (if the redbeds carry a primary natural remanent magnetization) or to a ca. 30 Ma pole (if the sequence is remagnetized). The middle Miocene results, however, indicate that the Jinggu Basin experienced no significant (2° ± 5.6°) rotation with respect to Eurasia. Since no major deformation has occurred within northern Indochina during Late Cretaceous to Eocene times, our results reflect a major clockwise rotation of the Indochina block during its Oligocene to early Miocene extrusion from the India-Asia collision zone.