Pre-existing faults in the mechanical basement are believed to play an important role in controlling deformation of the thin-skinned Jura Mountains fold-and-thrust belt, which constitutes the northernmost extension of the European Alps. We use brittle-viscous analogue models to investigate the influence of frontal and oblique basement steps on the subsequent evolution of structures during thin-skinned shortening. Vertical offset between two rigid baseplates (simulating the mechanical basement) causes the formation of reverse faults and grabens in the overlying brittle layers that are not reactivated during subsequent thin-skinned shortening. However, baseplate steps localise deformation, causing a temporary frontward propagation of deformation in an early stage and inhibiting propagation afterwards. Downward baseplate steps induce very strong deformation localisation and foster the formation of fault-bend folds. Models featuring upward steps develop step-controlled pop-up structures with imbricated fronts and viscous ramps that shorten dynamically with progressive contraction. We find that deformation localisation increases both with higher step-throws and lower obliquity (α) of the strike of the step (e.g. frontal step α = 0°). With increasing step-throws, α = 30° and α = 45° oblique upward-steps lead to a characteristic imbrication of the brittle cover with laterally confined thrust-slices and step-parallel oblique-thrusts, which rotate up to 15° about a vertical axis over time. Step-controlled backthrusts preceding the formation of thrust-slices do not show notable rotation and hence constitute excellent indicators for the orientation of oblique upward-steps. The topographic patterns of oblique-step models resemble individual thin-skinned structures of the Internal Jura (i.e. Pontarlier and Vuache fault zones, the nappe system SE of Oyonnax and the Chasseral anticline), strongly suggesting that pre-existing NNE-SSW and NW-SE striking oblique upward-steps in the basement controlled deformation in the overlying cover. Our model results may be applied to other thin-skinned fold-and-thrust belts worldwide that formed above pre-existing basement structures.

机械基底中预先存在的断层被认为在控制薄皮侏罗山脉褶皱冲断带的变形方面起着重要作用，该带构成了欧洲阿尔卑斯山的最北端。我们使用脆性粘性模拟模型来研究正面和倾斜基底台阶对薄皮缩短过程中结构随后演变的影响。两个刚性底板之间的垂直偏移（模拟机械基底）导致在上覆的脆性层中形成反向断层和地堑，在随后的薄皮缩短过程中不会重新激活。然而，底板台阶使变形局部化，在早期引起变形的临时向前传播并在之后抑制变形。向下的底板台阶引起非常强烈的变形局部化并促进断层弯曲褶皱的形成。具有向上台阶的模型开发了带有叠瓦前和粘性坡道的台阶控制弹出式结构，这些坡道随着逐渐收缩而动态缩短。我们发现变形局部化随着更高的步距和更低的台阶撞击倾角（α）而增加（例如正面台阶α = 0°）。随着步距的增加，α = 30° 和 α = 45° 的斜向上步导致脆性盖层的特征叠瓦状，具有横向限制的推力切片和阶梯平行斜推力，围绕 a 旋转高达 15°纵轴随时间变化。推力切片形成之前的阶梯控制反推力没有显示出明显的旋转，因此构成了倾斜向上阶梯方向的极好指标。斜阶梯模型的地形模式类似于内侏罗的单个薄皮结构（即 Pontarlier 和 Vuache 断层带、Oyonnax 推覆系统 SE 和 Chasseral 背斜），强烈表明预先存在的 NNE-SSW 和 NW-地下室中的 SE 显着的倾斜向上台阶控制了上覆盖层的变形。我们的模型结果可应用于世界范围内形成于预先存在的基底结构之上的其他薄皮褶皱冲断带。Pontarlier 和 Vuache 断层带、Oyonnax 的推覆系统 SE 和 Chasseral 背斜），强烈表明基底中预先存在的 NNE-SSW 和 NW-SE 显着倾斜向上台阶控制了上覆盖层的变形。我们的模型结果可应用于世界范围内形成于预先存在的基底结构之上的其他薄皮褶皱冲断带。Pontarlier 和 Vuache 断层带、Oyonnax 的推覆系统 SE 和 Chasseral 背斜），强烈表明基底中预先存在的 NNE-SSW 和 NW-SE 显着倾斜向上台阶控制了上覆盖层的变形。我们的模型结果可应用于世界范围内形成于预先存在的基底结构之上的其他薄皮褶皱冲断带。