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In this study, we focused on the formation and preservation processes of colluvial sedimentary breccias within an extensional context. The breccias studied in this work (from Crete and the Pyrenees) are characterized by poorly sorted polygenic deposits of highly angular carbonate clasts whose size ranges from pebbles to blocks measuring several millimeters to several meters. This study shows that the colluvial sedimentary breccias were formed during extensional tectonics and are spatially associated with large-throw normal fault. They are related to the creation of a substantial topography in the footwall of the normal fault followed by its collapse on the hanging wall, leading to fast sediment accumulation and preservation. These breccias are organized, with pebble-to cobble-sized clasts near the slope of the fault while the mega-clasts, such as boulders and blocks, are preserved away from the slope. Such a clast size organization is indicative of a dry rockfall process and is opposite to that of alluvial fan systems dominated by gravity process involving water. This study also revealed that the breccias from the NE of the Pyrenees, encountered on both flanks of the Bas Agly syncline and previously attributed to the Lower Cretaceous, are related to a first phase of extension during the Upper Jurassic.

<p>A breccia is a rock made up of angular clasts. Its formation can be the result of several types of geological processes (sedimentary, tectonic, hydraulic, magmatic, etc.). The aim of this study is to understand the formation and the preservation of sedimentary breccias with a significant thickness (several tens to hundreds of meters) in extensional environments, by comparing the Bas-Agly Basin, in the eastern French Pyrenees, to a recent analogue (Pleistocene-Holocene deposits of the Chora Sfakion region, SW Crete). In both cases, the breccias mainly consist of carbonate elements.</p><p>Our preliminary results show that:</p><p>- Along the coast of the Chora Sfakion region, sedimentary breccias are preserved at the front of a major normal fault scarp over a distance of ca. 20 km. Their formation results from a destabilization of the topographic slope triggered by the activity of the fault. The breccias were preferentially developed at the expense of dolomitic layers that underwent intense fracturation during an older deformation phase. Breccia deposition was related to processes of aerial or sub-aquatic landslides. Relatively fine-grained unsorted breccias are found close to the main fault whereas larger blocks and olistoliths are found farther away and down-slope, attesting for large mass slides. Preservation of the breccias has been favoured by subsidence at the front of the fault.</p><p>- The sedimentary breccias of the Bas-Agly Basin bear characteristics that are broadly comparable to those of Crete. With respect to the Cretan case, the Bas-Agly deposits, which consist of breccias alternating with fine sediments, represent a more distal part of the system. However, the age of the breccias in the Bas-Agly Basin is widely debated, with estimates ranging from the Late Jurassic to the Eocene. Depending on the actual age, the tectonic environment could have been quite different, e.g. extensional or compressional. Thus, it is crucial to know the real age of these rocks. In order to solve this issue, we initiated a geochronological study, using in-situ U-Pb dating by LA-ICP-MS and focusing on the carbonate matrix of the breccias. This approach has proven successful and yielded ages consistent with the proposed extensional environment.</p><p>In summary, extensional tectonics appear to favour both the production and the preservation of large volumes of sedimentary breccias, which, therefore, may be considered as a marker of this tectonic regime. Whether compressional tectonics could produce a similar situation is a topic of ongoing research.</p><p>Keywords: breccias, sedimentary, syntectonic, extension, Crete, Pyrenees, U-Pb dating</p>

<p>Sedimentary breccias formed during extensional tectonics are spatially associated with large-throw normal faults. They result from the creation of a steep topography that becomes unstable, producing major rockfalls. The studied breccias, in Crete and in the Pyrenees, are up to 300 meters thick and are characterized by poorly sorted polygenic deposits of pebbles to boulders composed of highly angular plurimillimetric to plurimetric carbonate clasts. A lateral evolution is observed, with pebble-size clasts found near the normal fault and boulder-size clasts away from the fault. This evolution is related to the rockfall process as the total kinetic energy acquired by the small clasts during the fall is lower than that acquired by the bigger ones; as a result, the latter are able to travel farther. Interestingly, the fact that the smallest clasts are proximal while the bigger ones are more distal is contrary to the distribution found in alluvial fan systems, making it possible to differentiate from one another. The studied breccias commonly show disorganized layers and/or no noticeable layering across large distances. We interpret this feature as related to the movement on the normal fault, which progressively tilts the breccia layers and favours their gliding along the slope. Gliding is an important internal process to take into account in rockfall systems because it may disorganize the layering, create specific geometries like onlap around olistoliths, and produce deformation inside the breccia layers; the latter feature could be mistakenly interpreted as resulting from post-deposition regional deformation.</p><p>According to our observations, active normal faults with large throws provide the conditions for the formation and preservation of great volumes of sedimentary breccias through the following processes: i) footwall uplift, creating a pronounced topography with steep slopes, giving rise to major rockfalls, ii) hangingwall rapid subsidence, which allows the accumulation and preservation of the breccias without clast reworking by drainage systems. The latter is reinforced by the fact that, during the early stages of extension, the main watersheds point in a direction opposite to the fault slope whereas only small, discontinuously distributed watersheds flow in the direction of the fault slope. Upon ongoing extension, the size of these small watersheds increase. At one point, the sedimentary flow coming from these watersheds becomes more important than rockfall processes. Part of the breccia body is then eroded, reworked, and replaced by conglomerates of an alluvial fan deposited unconformably above the breccias.</p><p>Summing up, sedimentary breccias are readily formed as thick syn-tectonic deposits during early stages of extensional basin development. Thus, they may be considered as a typical lithology, and a marker, of continental extension.</p>

Modélisation analogique de brèches sédimentaires dans un talus d'éboulis avec ou sans extension -comparaison avec un cas