Palaeomagnetic evidence and tectonic origin of clockwise rotations in the Yangtze fold belt, South China Block

Fold axis strikes in the Yangtze fold belt of the South China Block (SCB) undergo significant changes over distances of > 1000 km. This large-scale variation provides an ideal opportunity to test the oroclinal-bending hypothesis using palaeomagnetic methods, which we have attempted by drilling the Lower Triassic Daye Formation limestones in western Hubei Province. Thermal demagnetization isolated two components in most samples. A low unblocking temperature component (< 350 degrees C) that does not decay to the origin is interpreted as a drilling induced magnetization. The second, high temperature component (HTC), isolated at temperatures > 400 degrees C, unblocks univectorially towards the origin. The HTC passes the McFadden-fold test with an overall mean tilt-corrected direction of Dec = 255 degrees, Inc = -24 degrees (N = 7, alpha(95) = 9 degrees). Scanning electron microscopy (SEM) observations suggest an early diagenetic, possible (bio)chemical origin for the magnetic extracts dominated by Ti-poor magnetite. Rock magnetic data show no evidence that the HTC has been affected by tectonic or compaction strain. Our data together with previously published results suggest a general clockwise rotation pattern in the Middle Yangtze fold belt, which is probably related to the collision between the North and SCBs. Comparison of palaeomagnetic rotations with fold axis trends in the fold belt suggests that about 30 degrees clockwise rotation occurred in the Middle reaches of the Yangtze River, while a 15 degrees difference in fold axis trends would be due to initial variation within the fold belt. However, since little is known about the timing of the clockwise rotation, whether the Middle Yangtze fold belt is an orocline awaits further studies.