Formation and dismantling of rider blocks related to low-angle normal faults in rifted margins

Rider blocks (or extensional allochthonous blocks) represent small blocks of pre-rift sediments and/or basement rocks transported over long-offset extensional fault zones. They are often related to low-angle normal faults (LANFs), with dips of less than 30°, and are commonly observed in post orogenic context and in rifted margins. However, the mechanisms controlling the formation, transport, and dismantling of rider blocks along LANFs remain poorly constrained. This raises critical questions regarding the origin of the blocks (hanging wall versus footwall), the influence of rheology, their three-dimensional geometry, and their evolution during extension. Addressing these issues is essential for improving our understanding of extensional processes, including continental crust thinning and the overall architecture, tectonic and sedimentary evolution of sedimentary basins.

Here, we present new observations from both active and fossil extensional systems: southern South China Sea rifted margin offshore Malaysia, and a fossil analogue preserved in Err and Bernina LANF systems within the lower Austroalpine nappes of the Central Alps, investigated through new fieldwork approaches.

To bridge the scale gap between seismic and field observations, we created Digital Outcrop Models (DOMs) in the Err and Bernina units, covering an area of 42.925 km². These DOMs are based on Structure-from-Motion photogrammetry and constitute the FATDOM database, which is openly available under FAIR (Findable, Accessible, Interoperable, and Reusable) conditions. The high resolution of the 12 DOMs enables centimetre- to decimetre-scale analysis, allowing detailed mapping of the spatial evolution of LANFs and the tectono-sedimentary architecture of the overlying rider blocks.

Based on these new observations, we present a new classification of rider blocks and identify three different types defined by their size, geometry, and mode of formation. The first type consists of blocks composed of pre-rift sediments that are passively transported along the LANFs and locally dismembered. The second type, either hanging-wall or footwall derived, preserves both basement and pre-rift sediments. Finally, the third type corresponds to break-away blocks in-between two LANF systems. For each type, we provide a detailed interpretation of their internal structure, including lithological variations, deformation patterns, and fault architecture across multiple scales. These field-based observations are further illustrated by examples from 3D reflection seismic data from the southern South China Sea rifted margin.

Our iterative comparison between present-day rifted margins and fossil analogues enables us to propose a conceptual model for the formation of rider blocks related to LANFs, applicable to extensional systems worldwide (e.g., Basin and range, Papua New Guinea…), and providing new insights into the dynamics of LANFs.