Regional deformational features of the South China Block inferred from Middle Triassic palaeomagnetic data

Abstract

SUMMARY Middle Triassic red-purple sandstones were sampled at four localities in the northeastern part of the South China Block (SCB) to detect regional deformational features. Most of the samples gave a characteristic remanent magnetization with unblocking temperatures up to 680 ◦ C. The primary nature of this magnetization was ascertained by a positive fold test and a reversal test. The tilt-corrected mean direction from the Yueshan area (30.6 ◦ N, 116.8 ◦ E), which is located at a distance of 25 km from the eastern part of the Tan-Lu Fault, shows a counter-clockwise deflection of 67.3 ◦ with respect to north. However, mean directions from the Nanzhang (31.5 ◦ N, 111.7 ◦ E), Tongling (30.9 ◦ N, 117.9 ◦ E) and Nanjing (32.0 ◦ N, 118.9 ◦ E) areas, which are located between 75 and 90 km from the northeastern boundary of the SCB, indicate a northeasterly deflection in declination, such as D = 38.2 ◦ , I = 25.1 ◦ (α 95 = 6.4 ◦ ), D = 32.1 ◦ , I = 25.2 ◦ (α 95 = 9.7 ◦ ) and D = 34.9 ◦ , I = 27.8 ◦ (α 95 = 5.4 ◦ ), corresponding to palaeopoles of λ = 50.5 ◦ N, φ = 220.5 ◦ E,dp/dm = 3.7 ◦ /6.9 ◦ , λ = 55.6 ◦ N, φ = 231.8 ◦ E,dp/dm = 5.6 ◦ /10.4 ◦ and λ = 53.9 ◦ N, φ = 229.1 ◦ E, dp/dm = 3.2 ◦ /5.9 ◦ , respectively. Comparison with the previously reported Permo-Triassic palaeomagnetic results from the SCB suggests that very consistent palaeopoles are observed within a stable belt up to 1800 km along the northern margin of the SCB (covering the Changxing, Nanjing, Tongling, Nanzhang and Emeishan areas). This type of behaviour implies that an area located between 70 and 225 km from northern margin of the SCB forms a stable platform where very little relative rotation has been detected. However, the inner region of this block, especially the Mesozoic fold-thrust belt, experienced a clockwise rotation of 11.5 ◦ ± 5.0 ◦ to 30.3 ◦ ± 9.2 ◦ with respect to the stable outer region later than the Middle Triassic. Areas close to the Tan-Lu Fault were subjected to post-Triassic counter-clockwise rotation by 90 ◦ with respect to the stable tectonic belt. We thus conclude that the northern stable belt of the SCB suffered no oroclinal bending during its suturing with the North China Block.