Abstract
► Tectonic deformation of the southern part of the SCB is examined by paleomagnetism. ► Early Cretaceous paleomagnetic direction is obtained at Nanning (108.3°E, 22.8°N). ► CCW rotated declination gradually increases toward southwest in the SCB. ► The feature in declination is ascribed to tectonic rotation of the SCB. ► Mechanically weak crust is considered as a requisite condition for this rotation.
To examine tectonic deformation in the southern part of the South China Block, red bed samples have been collected at 37 sites from the Early Cretaceous Xinlong Formation, south side of the Nanning town (108.3°E, 22.8°N). Stepwise thermal demagnetization and rock magnetic investigations revealed magnetite and hematite as the main carriers of remanent magnetizations in the studied samples. Pre-folding origin for hematite-related magnetic component is supported by positive fold test, whereas a syn-folding origin is assigned to a component carried by magnetite. A 100% tilt-corrected mean direction for hematite-related component (Dec/Inc=3.3°/41.7°, k=43.3, α95=4.1°, N=29) is considered to have been acquired at the time of deposition during the Early Cretaceous. Comparison of this direction with previously reported Cretaceous paleomagnetic directions from the Cathaysia Block (a part of the South China Block) indicates a gradual increase in counter-clockwise deflected declination toward southwest in this block. The amount of this counter-clockwise deflection increases up to 13.4°±4.7° in the southern part of the Cathaysia Block compared to the stable part of the South China Block. This deflection is ascribed to block rotation as a result of tectonic deformation in the South China Block during the indentation of India into Asia. Geological studies and seismic tomographic interpretations suggest that a large degree of counter-clockwise deflection remained restricted to the Nanpanjiang-Youjiang basin, where low-seismic velocities for both P and S waves prevail. The presence of mechanically weak crust is considered as a requisite condition for this rotation in the southern part of the South China Block.