Cretaceous synfolding remagnetization components revealing tectonic rotation of the middle Yangtze fold belt

Abstract

To reveal the deformation process of the middle Yangtze fold belt, we conducted a paleomagnetic study on Middle Triassic limestones and Middle to Late Jurassic sandstones from Wanzhou, Chongqing.Stepwise thermal and alternating field demagnetization were used to isolate the multi-component remanent magnetizations. The Jurassic samples were overprinted by recent geomagnetic field, while three magnetization components were isolated from the Middle Triassic samples. A low temperature component (LTC) was isolated at temperatures below 200℃, an intermediate temperature component (ITC) at 200-360℃ and a high temperature component (HTC) at 400-460℃. The LTC is distributed around the present-day Earth magnetic field, probably a viscous component. Stepwise unfolding indicates that the maximum precision parameters of ITC and HTC components are achieved at 33±8% and 50±27% (with 95% confidence) unfolding, respectively. The best-clustered ITC mean direction, Dec = 11.2°, Inc = 45.2° (α 95 = 4.5°, N = 34), corresponding to a paleopole at 79.3°N, 219.5°E (d p = 3.6°, d m = 5.7°), is consistent with the Cretaceous reference direction of the South China Block (SCB). The best-clustered HTC mean direction (taking 70% unfolding), Dec = 24.2°, Inc =49.0° (α 95 = 3.6°, N = 23), corresponding to a paleopole at 69.2°N, 195.5°E (d p =3.1°, d m = 4.8°), suggests a clockwise rotation of 12.8°±3.5°. These synfolding remagnetization components clearly reveal that a clockwise rotation happened at the middle stage of folding, thus supporting that at least part of the variation in fold axis strikes is due to orocline rotation. Combined with published data, our analysis indicates that the Wanzhou-Xiangxi segment of the middle Yangtze fold belt experienced oroclinal bending. Furthermore, a published post-folding component isolated from the Middle Triassic Puqi Formation suggests a 27.5°±5.8° clockwise rotation, confirming that at least 50% of the observed clockwise rotations in the eastern middle Yangtze fold belt can be attributed to oroclinal bending. The remagnetization data and geological evidence observed in the middle Yangtze fold belt suggest that collision between SCB and North China Block (NCB) probably lasted till the early period of Early Cretaceous. paleomagnetism, remagnetization, Mesozoic, orocline, Yangtze fold belt, South China BlockOrogenic belts often show a concave and convex pattern in a map view, which was first defined as orocline by Carey [1] . Furthermore, this pattern can be subdivided into rotated and non-rotated segments, and those curvatures caused by tectonic rotations are defined strictly as orocline [2] . Paleomagnetism has been widely used to distinguish a rotated segment from a non-rotated segment in orogenic belts and to test an orocline. A salient curved pattern has been observed in the fold-thrust belts along the northern margin of the South China Block

Paleomagnetism of Jurassic Sandstones from Yangtze Fold Belt and Its Implications for the Fold Belt Curvature

“…The HTC components isolated from Jurassic samples in the Yunyang area are synfolding remagnetization components and reveal a clockwise rotation of 8 • since the late stage of folding. Triassic paleomagnetic data from this area suggested that this part of the fold belt had experienced a clockwise rotation of 13 • since the early stage of folding [20] . There is a 30 • difference in fold axis trend between the Wanzhou-Yunyang segments of the fold belt, whereas the rotation in this area is just 10 • .…”

“…Middle Triassic paleomagnetic results from the Yunyang area suggest that this part of the fold belt had experienced a clockwise rotation of 13 • ±4 • since the early stage of folding, thus supporting that at least part of clockwise rotations is due to oroclinal bending [20] . The HTC directions isolated from Jurassic samples of Yunyang area in our study are best clustered at 33.8% untilting, the mean direction is D=19.1 • , I=48.9 • (α 95 =6.3 • ), (the inclination is consistent with the reference one), corresponding to a paleopole at 73.5 • N, 198.2 • E (dp=5.5 • , dm=8.3 • ).…”

“…In either case, the remanent magnetization shows a significant (∼50 • ) clockwise rotation. Zhang et al [20] applied an incremental fold test to the Triassic remagnetization results from Puqi area, suggesting that this part of the fold belt had experienced a clockwise rotation of 27 • since the last stage of folding. So the Puqi area had experienced clockwise rotation before, during, after the folding.…”

Late Triassic Dabie–Sulu Orocline: New exhumation model of the HP–UHP rocks

“…(a) Palaeomagnetic data of the Greater South China Block (the numbers in the black circles represent the literature sources: 1—Zhang & Yang, ; 2—Kirschvink et al, ; 3—Tan, Kodama, Gilder, Courtillot, & Cogne, ; 4—Han, Xu, Zhou, & Tan, ; 5—Zhang, Tan, & Han, ) and (b) the recovery of the Greater South China Block (Guo et al, )…”

A Widespread Early Mesozoic Remagnetization in South China

“…While we are aiming to obtain a detailed paleomagnetic record from the relatively more expanded Late Cambrian succession from the Wa'ergang section to test and examine the Late Cambrian TPW in this study, we are fully aware of the extensive Mesozoic remagnetization that has been previously reported in South China (e.g., Dobson & Heller, ; Gao et al, ; Huang & Opdyke, ; Jing et al, ; Kent et al, ; Liu et al, ; Wang & Van der Voo, ; Zhang et al, ; Zhang et al, ). Although various remagnetization mechanisms have also been proposed (e.g., Gao et al, ; Huang & Opdyke, ; Jing et al, ; C. Liu, Ge, et al, ; Liu et al, ; Zhang et al, ), most previous studies focused on local processes leading to remagnetization.…”