Abstract. Orogenic belts formed by collision are impressive manifestations of plate tectonics. Observations from orogenic belts, like the Western Alps, indicate an important involvement of the mantle lithosphere, significant burial and exhumation of continental and oceanic crustal rocks and the importance of the plate interface strength that can be modified, for example, by the presence of serpentinites. A popular model for the formation of such belts is the so-called orogenic wedge model. However, most wedge models consider crustal deformation only and do, hence, not consider subduction, the impact of related buoyancy forces arising from density differences between subducted crust and surrounding mantle and the effects of different plate interface strength. Here, we quantify the relative importance of buoyancy and shear forces in building collisional orogenic wedges. We leverage two-dimensional (2D) petrological-thermo-mechanical numerical simulations of a long-term (ca. 170 Myr) lithosphere deformation cycle involving subsequent hyperextension, cooling, convergence, subduction and collision. We compare simulations employing density fields calculated with linearized equations of state with simulations employing density fields calculated by phase equilibria models including metamorphic reactions. Further, we consider serpentinisation of the mantle material, exhumed in the hyperextended basin. Our models show that differences in density structure and in shear strength of serpentinites or upper crust have a strong impact on the evolution of orogenic wedges. Higher serpentinite strength causes a dominance of shear over buoyancy forces, resulting in either thrust-sheet dominated orogenic wedges, involving some diapiric exhumation at their base, or relamination of crustal material below the overriding plate. Lower serpentinite strength (equal importance of shear and buoyancy forces) generates orogenic wedges that are dominated by diapiric or channel-flow exhumation. Deep subduction (> 80 km) and subsequent surface exhumation of continental crust along the subduction interface occurs in these models. Employing phase equilibria density models decreases the average buoyancy contrasts, allows for deeper subduction of continental crust and reduces the average topography of the wedge by several kilometers. A decrease of upper crustal shear strength causes smaller maximal crustal burial depths. Progressive subduction of continental crust increases upward-directed buoyancy forces of the growing wedge and in turn increases horizontal driving forces. These driving forces eventually reach magnitudes (≈ 18 TN m⁻¹) which were required to initiate subduction during convergence. We suggest that the evolving relation between shear and buoyancy forces and the increase of horizontal driving force related to the growing Alpine orogenic wedge has significantly slowed down (or choked) subduction of the European plate below the Adriatic one between 35 and 25 Ma. This buoyancy-related choking could have caused the reorganization of plate motion and the initiation of subduction of the Adriatic plate. We discuss potential applications and implications of our model results to the Pyrenean and Alpine orogenies.

中文翻译：

---

建筑造山楔的浮力与剪切力

摘要。碰撞形成的造山带是板块构造的令人印象深刻的表现。从像西阿尔卑斯山这样的造山带观察到，表明地幔岩石圈的重要参与，大陆和海洋地壳岩石的大量埋葬和掘出以及板状界面强度的重要性，例如，可以通过蛇纹岩的存在来改变。形成这种带的一种流行模型是所谓的造山楔形模型。但是，大多数楔形模型仅考虑地壳变形，因此不考虑俯冲，俯冲的地壳与周围地幔之间的密度差异引起的相关浮力的影响以及不同板界面强度的影响。这里，我们量化了浮力和剪切力在建筑碰撞造山楔中的相对重要性。我们利用长期（约170 Myr）岩石圈变形周期的二维（2D）岩石热力学数值模拟，包括随后的超伸，冷却，收敛，俯冲和碰撞。我们比较了采用由线性状态方程计算的密度场的模拟与采用由包括变质反应的相平衡模型计算的密度场的模拟。此外，我们考虑在超伸展盆地中挖掘出的地幔物质的蛇形化。我们的模型表明，蛇纹石或上地壳的密度结构和抗剪强度的差异对造山楔的演化有很大影响。较高的蛇纹岩强度会导致剪切力超过浮力，从而导致以冲断层为主的造山楔，在其基部会产生一些腐熟的腐烂物，或者使地壳物质在上覆板之下分层。较低的蛇纹岩强度（剪切力和浮力的重要性同等）会生成造山楔，这些楔由二尖瓣或河道泄洪为主。在这些模型中，发生了深俯冲（> 80 km）和沿俯冲界面的大陆壳表层挖掘。采用相平衡密度模型可以降低平均浮力对比，允许更深地俯冲大陆壳并将楔​​形的平均地形减小几公里。上地壳剪切强度的降低会导致较小的最大地壳埋藏深度。陆壳的逐步俯冲作用增加了楔形物向上的浮力，反过来又增加了水平驱动力。这些驱动力最终达到幅度（≈18 TN m^-1）在收敛过程中引发俯冲所需。我们认为，剪切力和浮力之间的演变关系以及与不断增长的高山造山楔有关的水平驱动力的增加已大大减慢了亚德里亚纪以下35至25 Ma之间的欧洲板块的俯冲（或or陷）。这种与浮力有关的窒息可能引起板块运动的重组和亚得里亚海板块的俯冲作用的开始。我们讨论了潜在的应用以及我们的模型结果对比利牛斯山和高山造山带的影响。