Abstract. Seismic reflection interpretation at magma-poor rifted margins shows that crustal thinning within the hyper-extended domain occurs by in-sequence oceanward extensional faulting which terminates in a sub-horizontal reflector in the topmost mantle immediately beneath tilted crustal fault blocks. This sub-horizontal reflector is interpreted to be a detachment surface that develops sequentially with oceanward in-sequence crustal faulting. We investigate the geometry and evolution of active and inactive extensional faulting due to flexural isostatic rotation during magma-poor margin hyper-extension using a recursive adaptation of the rolling-hinge model of Buck (1988) and compare modelling results with published seismic interpretation. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on published seismic reflection profiles can be generated by the flexural isostatic rotation of faults with initially high-angle geometry. Our modelling supports the hypothesis of Lymer et al. (2019) that the S reflector on the Galician margin is a sub-horizontal detachment generated by the in-sequence incremental addition of the isostatically rotated soles of block-bounding extensional faults. Flexural isostatic rotation produces shallowing of emergent fault angles, fault locking, and the development of new high-angle shortcut fault segments within the hanging wall. This results in the transfer and isostatic rotation of triangular pieces of hanging wall onto exhumed fault footwall, forming extensional allochthons which our modelling predicts are typically limited to a few kilometres in lateral extent and thickness. The initial geometry of basement extensional faults is a long-standing question. Our modelling results show that a sequence of extensional listric or planar faults with otherwise identical tectonic parameters produce very similar seabed bathymetric relief but distinct Moho and allochthon shapes. Our preferred interpretation of our modelling results and seismic observations is that faults are initially planar in geometry but are isostatically rotated and coalesce at depth to form the seismically observed sub-horizontal detachment in the topmost mantle. In-sequence extensional faulting of hyper-extended continental crust results in a smooth bathymetric transition from thinned continental crust to exhumed mantle. In contrast, out-of-sequence faulting results in a transition to exhumed mantle with bathymetric relief.

中文翻译：

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裂谷边缘超伸展大陆地壳内的伸展断层几何形状和演化导致地幔折返和异地岩形成

摘要。贫岩浆裂谷边缘的地震反射解释表明，超伸展域内的地壳减薄是由连续的向海伸展断层作用发生的，该断层作用终止于紧邻倾斜地壳断块下方的最顶部地幔中的亚水平反射体。该次水平反射体被解释为随着向海依次发生的地壳断层而依次发育的脱离面。我们使用 Buck (1988) 滚动铰链模型的递归适应，研究了由于岩浆贫瘠边缘超伸展期间的弯曲均衡旋转而导致的活动和非活动伸展断层的几何形状和演化，并将建模结果与已发表的地震解释进行了比较。在渐进式顺序断层作用的情况下，我们表明，在已发表的地震反射剖面上成像的次水平反射体可以通过具有最初高角度几何形状的断层的弯曲等静压旋转来生成。我们的模型支持莱默等人的假设。 (2019) 认为加利西亚边缘的 S 反射体是由块体边界伸展断层的等静压旋转底部按顺序增量添加而产生的次水平脱离。弯曲等静压旋转导致突现断层角度变浅、断层锁定以及上盘内新的高角度捷径断层段的发展。这导致三角形上盘片转移并均衡旋转到挖出的断层下盘上，形成伸展异地，我们的模型预测其横向范围和厚度通常限于几公里。基底伸展断层的初始几何形状是一个长期存在的问题。我们的建模结果表明，一系列具有相同构造参数的伸展性断层或平面断层产生非常相似的海底测深地形，但具有不同的莫霍面和异地断层形状。我们对建模结果和地震观测的首选解释是，断层最初在几何形状上是平面的，但在深度上均衡旋转并合并，形成地震观测到的最顶部地幔中的次水平脱离。超伸展大陆地壳的连续伸展断层作用导致从变薄的大陆地壳到挖出的地幔的平滑的测深过渡。相反，无序断层导致向具有测深起伏的挖出地幔过渡。