Fold geometries and kinematics within foreland basins is a major issue for understanding the late evolution of thrust fronts. In the foreland of the southwestern Alps, the Vélodrome complex fold involves the whole Tertiary series which have recorded the evolution of the alpine front. From a geometrical and kinematic point of view, the Vélodrome is classically described as a recumbent Mio-Pliocene syncline with a strongly curved axis; interpreted either as a classic growth-fold, a post-deposit fold, or a result of coeval salt activity. The debate reveals a lack of consensus on the internal structure of the Vélodrome and the potential role of salt motion. By a detailed field analysis coupled with an implicit 3D geometrical modelling approach, we test the different hypotheses and provide a new 3D illustrated interpretation of the Vélodrome series. The Vélodrome consists in a complex non-cylindrical structure composed by several folds with different orientations together with inter- and intra-formations unconformities. Folding is partly -depositional and began earlier in the south, near Esclangon village, than in the north. In the north, deformation started with the marly and sandy molasse (m2, middle Burdigalian), attested by the transition from the conformable conglomeratic marine molasse (m1–2, Aquitanian and early Burdigalian) to the unconformable m2 to the Valensole formation. Deformation began earlier in the south, during the deposition of the m1–2, as shown by internal unconformities. We discuss the ingredients controlling the deformation in the Vélodrome, regional tectonics (mainly since middle Burdigalian) versus salt tectonics. This study further brings new constraints on the timing and pattern of deformation of the southwestern orogenic front of the Alps. It also highlights the power of the 3D geometrical modelling approach for testing different hypotheses and better understanding 3D complex structures.

中文翻译：

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阿尔卑斯山西南部非圆柱形 Vélodrome 中新世褶皱的 3D 几何建模

前陆盆地内的褶皱几何形状和运动学是理解逆冲锋晚期演化的一个主要问题。在阿尔卑斯山西南部的前陆，Vélodrome复杂褶皱涉及整个第三纪系列，记录了高山锋面的演化。从几何和运动学的角度来看，自行车馆被经典地描述为具有强烈弯曲轴的卧式上新世向斜。解释为经典的生长折叠、沉积后折叠或同时代盐活动的结果。这场争论表明，人们对自行车馆的内部结构和盐运动的潜在作用缺乏共识。通过详细的现场分析与隐式 3D 几何建模方法相结合，我们测试了不同的假设，并为 Vélodrome 系列提供了新的 3D 插图解释。 Vélodrome 是一个复杂的非圆柱形结构，由多个方向不同的褶皱以及地层间和地层内的不整合面组成。褶皱作用是部分沉积作用，并且在靠近埃斯兰贡村的南部比北部开始得更早。在北部，变形始于泥灰质和砂质磨拉石（m2，布尔迪加期中期），从整合性砾岩海相磨拉石（m1-2，阿基坦期和布尔迪加期早期）到不整合m2再到Valensole地层的过渡证明了这一点。正如内部不整合面所示，在 m1-2 沉积期间，南方较早开始变形。我们讨论了控制赛车场变形的因素、区域构造（主要自布尔迪加利亚中期以来）与盐构造。这项研究进一步对阿尔卑斯山西南造山锋变形的时间和模式带来了新的约束。它还强调了 3D 几何建模方法在测试不同假设和更好地理解 3D 复杂结构方面的强大功能。