Topographic control on shallow fault structure and strain partitioning near Whataroa, New Zealand demonstrates weak Alpine Fault

It is notoriously difficult to characterise the strength and stress states of major plate boundaries. By taking advantage of the well-constrained stress contribution of topography adjacent to a segmented section of the Alpine Fault, New Zealand, we have identified a mechanical mix that produces the distinct fault segmentation pattern seen in field observations. Slope-generated shear and normal stresses rotate the principal stresses relative to the regional tectonically derived stress state and under certain strength states influence the displacement pattern. Three-dimensional models show that the scale and form of the near-surface partitioning depend on both topographic relief and local fault strength relative to the bedrock. The models suggest the Alpine Fault is weak to moderately weak relative to the bedrock and is a single structure to within c. 500 m of the surface, above which segmentation occurs. Adjacent to the Alpine Fault, the stress state is highly variable. The intermediate principal stress, <i>σ</i>₂, is rotated from tectonically dominated, near-vertical beneath ridges to near-horizontal beneath large valleys. Individual segments along the Alpine Fault dominated by strike-slip faulting, oblique thrusting or thrusting, can be identified by extracting the topographic contribution to the stress state from numerical models.

表征大型板块边界的强度与应力状态向来是公认的难题。借助新西兰阿尔卑斯断层（Alpine Fault）某分段区域周边地形的约束良好的应力贡献，我们明确了一种力学混合机制，该机制可产生野外观测中所见的独特断层分段模式。由地形产生的剪应力与正应力会使主应力相对于区域构造成因的应力状态发生旋转，且在特定强度状态下会影响位移模式。三维模型显示，近地表应力分配的规模与形式，既取决于地形起伏，也取决于相对于基岩的局部断层强度。模型表明，相较于基岩，阿尔卑斯断层整体偏弱至中等偏弱；在地表以下约500米的深度范围内，该断层为单一构造结构，超过该深度则出现分段现象。紧邻阿尔卑斯断层的区域，应力状态存在高度变异性。中间主应力σ₂的方向会从脊部下方以构造主导的近垂直状态，旋转至大型河谷下方的近水平状态。沿阿尔卑斯断层分布的、以走滑断层作用、斜向逆冲作用或逆冲作用为主的各个分段，可通过从数值模型中提取地形对应力状态的贡献来加以区分。