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Unravelling the Influence of Throw and Stratigraphy in Controlling Sub-Seismic Fault Architecture of Fold-Thrust Belts: An Example From the Qaidam Basin, NE Tibetan Plateau

Abstract

Understanding fault architecture at an outcrop scale is fundamental as it controls the fault zone compartmentalization which plays important control on fluid flow properties across fault zones. Although previous studies have investigated the relationship between faults and folds developed within compressional systems, the detailed fault architecture of thrust faults remain poorly constrained, particularly at meso-scale. Not only are sub-seismic faults poorly imaged but even seismically resolvable structures are poorly imaged because of steep dipping nature of reflections, which makes it difficult to predict the fault zone geometry in detail. The detailed fault architecture analysis of an exceptionally well-exposed fold-thrust belt in the Qaidam Basin is reported. The Lenghu5 fold-thrust belt, with well-exposed outcrops, is selected for high-resolution fieldwork to collect multiple scale data, including stratigraphy, dips and cross sections. The detailed stratigraphic logging and high-resolution cross sections allow us to quantitatively evaluate the fault throw distribution along the fault zone, both in section view and plan view. Integrating the high-resolution fault system mapping, it is identified that the main thrust fault accounts for between 80 and 85% of the cumulative throw of the fault zone, while the fault splays (15-20% of the cumulative throw) lead to the development of small-scale structures, e.g., small pair of syncline/anticline in the footwall and small accommodation faults in the hanging-wall. The 3D geospatial model, integrating multi-scale data based on high-resolution fieldwork, enables the investigation on the detailed fault zone architecture of a natural fold-thrust belt. The non-uniform fault throw along the fault zone leads to a high degree of lateral structural variation at regional scale, whereas the local stratigraphy may control the development of minor structures in both the hanging wall and footwall at smaller scale. The Lenghu5 geometry is directly comparable with some well-studied subsurface examples, such as the Niger Delta, and therefore provides invaluable insights into the geometry of faults in poorly imaged fold-thrust systems.