Abstract

Map-scale geologic faults with a conical shape are apparent in a public-domain 3D seismic image of block F3 in the Dutch sector of the North Sea. These cone-shaped normal faults exist among many polygonal normal faults in fine-grained clay sediments. The conical faults were discovered using new algorithms for seismic image processing, but they are apparent without that processing. They appear circular in horizontal image slices and have an inverted-V shape in vertical sections for any azimuth through their vertical axes of symmetry. They form horsts bounded by planar strata above and below, with maximum throws of about 35 m roughly halfway between their bases and tops. The bases are up to 1 km wide, and dips of the conical faults are in the range 35º–38º. Because polygonal faults exist elsewhere at or near the seafloor with dips of about 60º, most such faults likely formed close to the depositional interface; the much lower dips of deeply buried faults, such as the faults observed here, are likely due to vertical compaction. We inferred that polygonal and conical fault geometries result from a uniform radial extension stress caused by volumetric contraction, analogous to the formation of mudcracks. Cone-shaped faults have been observed in laboratory experiments on rocks and ceramics, where the conical geometry results from uniaxial compression. In the subsurface, the formation of conical faults implies that stresses and sediments were exceptionally homogeneous. The cone shape suggests a pointlike heterogeneity near the depositional surface, which served as a trigger for downward propagation of the faults, rather unlike the random, in-plane propagation and linking inferred by others for polygonal faults.

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