Triassic contractional deformation and sedimentary response in the Helanshan fold–thrust belt, Northwest China: evidence from growth strata

The Helanshan fold – thrust belt is located between the Alxa and Ordos blocks and has been a deformation area of tectonic activity since the Mesozoic. During the Mesozoic tectonic deformation, the Jurassic WNW – ESE-trending contraction transformed the Triassic NNE – SSW shortening. Field mapping and structural analysis reveal that the Middle Triassic Zhifang Formation in the northern wing of the Rujigou overturned anticline is characteristic of a growth strata, which is in response to SSW-NNE contraction. Detrital zircon U – Pb age spectra of the Middle Triassic Zhifang Formation show that the Triassic sedimentary materials originated from the North China Craton to the north, the Alxa block to the north, the Qilian orogenic belt to the south, and the Qinling orogenic belt to the south. The youngest age at the bottom of the growth strata is 282 ± 11 Ma, and the youngest age at the top is 291 ± 12 Ma, indicating that the strata maximum depositional age was later than the Permian, revealing that the youngest detrital zircon of the Middle Triassic Zhifang Formation is derived from the Rujigou overturned anticline, reflecting the characteristics of the reverse sedimentary sequence of detrital zircon and suggesting an interaction between structure and deposition. We propose that with the closure of the Paleo-Tethys Ocean, under the far-field response to the collision between the North China Craton and South China Craton, the Helanshan fold – thrust belt expanded northward and formed a northward protruding tectonic node that controlled the sedimentary process of the Triassic foreland basin.

贺兰山褶皱-逆冲带（Helanshan fold – thrust belt）位于阿拉善地块（Alxa block）与鄂尔多斯地块（Ordos block）之间，自中生代（Mesozoic）以来便持续处于构造活动变形区域。中生代构造变形过程中，侏罗纪时期北西西-南东东（WNW–ESE）走向的收缩作用改造了三叠纪（Triassic）时期北北东-南南西（NNE–SSW）向的挤压作用。野外填图与构造解析结果显示，汝箕沟倒转背斜（Rujigou overturned anticline）北翼的中三叠统纸坊组（Middle Triassic Zhifang Formation）具备生长地层（growth strata）的典型特征，这一构造-沉积响应对应南南西-北北东向的挤压作用。对该组地层的碎屑锆石U-Pb年龄谱（detrital zircon U–Pb age spectra）分析表明，其沉积物质主要来源于北侧的华北克拉通（North China Craton）、阿拉善地块，以及南侧的祁连造山带（Qilian orogenic belt）与秦岭造山带（Qinling orogenic belt）。生长地层底部的最年轻碎屑锆石年龄为282±11 Ma，顶部最年轻碎屑锆石年龄为291±12 Ma，表明该地层的最大沉积时代晚于二叠系（Permian）；同时揭示出中三叠统纸坊组的最年轻碎屑锆石源自汝箕沟倒转背斜，反映出碎屑锆石的反向沉积序列特征，指示构造活动与沉积作用之间存在相互耦合关系。我们提出，随着古特提斯洋（Paleo-Tethys Ocean）的闭合，在华北克拉通与华南克拉通（South China Craton）碰撞的远场响应作用下，贺兰山褶皱-逆冲带向北扩张，并形成了向北突出的构造节点，该节点控制了三叠纪前陆盆地（foreland basin）的沉积演化过程。