Anomalously shallow inclination in middle-northern part of the South China block: palaeomagnetic study of Late Cretaceous red beds from Yichang area

Abstract

SUMMARY We present new palaeomagnetic results from Late Cretaceous red beds of the Paomagang Formation collected at 42 sites in the Yichang area (30.7°N, 111.7°E), middle–northern part of the South China Block. A high unblocking temperature component around 680°C was isolated from 26 sites by stepwise thermal demagnetization. Fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the Paomagang and Wangdian areas, respectively. Normal and reversed polarity sequence found in the Wangdian area passed a reversal test at the 95 per cent confidence limit. The tilt-corrected mean direction for Yichang area is D= 3.8°, I= 23.3° (α95= 5.1°, N= 26), corresponding to a palaeopole at 71.5°N, 280.0°E with A95= 4.3°. Comparison with the expected inclination from the 80 Ma Eurasian APWP pole indicates a inclination flattening of 26.3°± 6.4°. Middle Cretaceous to Cenozoic NW–SE extension tectonics within the SCB contributes only 2.0° to the inclination flattening. While the massive samples of the Paomagang Formation show a moderate degree of anisotropy (1.017 < Pj < 1.086) and steeper inclination (I= 29.3°), the laminated samples, which form more than 75 per cent of the studied samples, show a higher degree of anisotropy (1.052 < Pj < 1.227) and shallower inclination value (I= 22.7°). A good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the Yichang area.