Tangled up in folds: tectonic significance of superimposed folding at the core of the Central Iberian curve (West Iberia)

The amalgamation of Pangea during the Carboniferous produced a winding mountain belt: the Variscan orogen of West Europe. In the Iberian Peninsula, this tortuous geometry is dominated by two major structures: the Cantabrian Orocline, to the north, and the Central Iberian curve (CIC) to the south. Here, we perform a detailed structural analysis of an area within the core of the CIC. This core was intensively deformed resulting in a corrugated superimposed folding pattern. We have identified three different phases of deformation that can be linked to regional Variscan deformation phases. The main collisional event produced upright to moderately inclined cylindrical folds with an associated axial planar cleavage. These folds were subsequently folded during extensional collapse, in which a second fold system with subhorizontal axes and an intense subhorizontal cleavage formed. Finally, during the formation of the Cantabrian Orocline, a third folding event refolded the two previous fold systems. This later phase formed upright open folds with fold axis trending 100° to 130°, a crenulation cleavage and brittle–ductile transcurrent conjugated shearing. Our results show that the first and last deformation phases are close to coaxial, which does not allow the CIC to be formed as a product of vertical axis rotations, i.e. an orocline. The origin of the curvature in Central Iberia, if a single process, had to be coeval or previous to the first deformation phase.

石炭纪时期泛大陆（Pangea）的聚合形成了蜿蜒的造山带——西欧华力西造山带（Variscan orogen）。在伊比利亚半岛，该蜿蜒构造以两大构造主体为核心：北部的坎塔布连构造弧（Cantabrian Orocline）与南部的伊比利亚中部弧形构造（Central Iberian curve，简称CIC）。本文对伊比利亚中部弧形构造核部的一处区域开展了详细构造解析。该核部经历了强烈变形，形成了波纹状叠加褶皱样式。研究识别出三期变形，均可与区域华力西变形期次相对应：主碰撞事件形成了直立至缓倾的圆柱状褶皱，并伴生轴面劈理；随后在伸展垮塌过程中，第二期褶皱系统得以形成，其褶皱轴近水平，并发育强烈的近水平劈理；最终在坎塔布连构造弧形成过程中，第三期褶皱事件对前两期褶皱系统进行了重褶皱，该期形成了褶皱轴走向100°至130°的直立开阔褶皱，同时发育皱纹劈理与脆韧性走滑共轭剪切构造。研究结果表明，第一期与第三期变形近乎共轴，这说明伊比利亚中部弧形构造并非通过垂直轴旋转形成，即构造弧（orocline）。若伊比利亚中部的弯曲形态仅由单一过程形成，则该过程需与第一期变形同期或早于第一期变形发生。