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Faulting and deformation of fan-delta deposits located near active tectonic zones can be attributed either to failure due to basin-ward gravitational mass movement, or to tectonic activity. Distinguishing between these two failure mechanisms is sometimes crucial when attempting to assess the seismic risk and earthquake recurrence time of such terrains. In a case study described here, close spatial association between subsurface faults, determined in seismic profiles, and overlying surface deformation features mapped in the field, would imply tectonic activity as the cause for deformation rather than basin-ward gravitational mass movement. The geologic and seismic study of the 400 m long Nahal Darga fan-delta of Holocene age, located on the western margins of the tectonically active Dead Sea transform, is used here as a test case for this debate. The studied fan-delta is comprised of two deformation zones. The Western deformation zone consists of NNE trending normal faults with up to 2 m offsets, mostly crossing part of the section and terminating below the surface. Only a few faults cross the entire Holocene section. The Eastern zone is characterized by many small normal faults, which are restricted to only a few beds and NNE trending allochthonous bodies (slumps) that developed due to tectonic activity. A shallow, high-resolution seismic study carried out in the Nahal Darga fan-delta, reveals the existence of deep-seated faults extending up to about ten meters below the surface. The location of the subsurface faults below the surface deformation zones suggests association between the two and a tectonic origin for both. It is concluded that detailed geological and structural mapping of fan-deltas coupled with a high resolution seismic survey is an essential and reliable tool for the analysis of tectonostratigraphy in fan-deltas. Correspondence to: Y. Eyal (eyal@bgumail.bgu.ac.il)

High-resolution seismic study of the Nahal Darga fan-delta, Dead Sea, Israel, with the aim to relate the surface and subsurface tectonic structures