Paleomagnetism of the Khorat Group, Mesozoic, Northeast Thailand

Abstract

The Khorat Group, middle or late Triassic to Cretaceous, consists of up to 5000 m of paralic and freshwater sedimentary deposits, mainly alluvial flood-plain red-beds. Paleomagnetic measurements tested using thermal and AF demagnetization by several authors in different laboratories are generally consistent. Selection of samples requires care because of deep weathering in the monsoon climate of Thailand. All authors have reported a clockwise rotation of the paleomagnetic declinations, which we estimate to be 37 ± 7 degrees, and which occured at some time in the last 100 My. Inclinations are not significantly different from those of the present day, indicating that Northeast Thailand (and the Indochina plate) have remained at nearly the same latitude since about late Triassic. Relative southward movement of the South China plate possibly resulted from dextral displacement along the Red River Fault. Relative southward movement of Eurasia involved either ocean consumption or continental crust shortening of 1000 km or more. Magnetostratigraphy suggests an unconformity, supported by stratigraphic evidence, between Sao Khua and Phu Phan formations in the upper part of the Khorat Group, and the unconformity is explained by a simple model involving the combined effect of subsidence at an exponentially decreasing rate, and Mesozoic eustatic sea-level changes.