Soft-sediment deformation structures interpreted as seismites in lacustrine sediments of the Prebetic Zone, SE Spain, and their potential use as indicators of earthquake magnitudes during the Late Miocene

Lacustrine formations of Late Miocene age in the Prebetic area, SE Spain, show several types of deformational structures that are interpreted as seismites. They are present in both marginal and deep lacustrine facies. Seismites formed in marginal lake environments comprise sand dikes, pillows and intruded and fractured gravels. In deep lacustrine facies, the seismites are represented by pseudonodules, mushroom-like silts protruding into laminites, mixed layers, disturbed varved lamination and loop bedding. The measured orientations of these structures are consistent with the orientations of the main faults limiting the basins, showing that their origin is clearly related to the tectonic stress field that prevailed in the region during the Late Miocene. The magnitudes of the earthquakes that deformed the sediments have been estimated after published data from both ancient and recent lake deposits accumulated in tectonic active regions elsewhere. A rank of earthquake magnitudes with two end-members, i.e. the lowest magnitudes recorded by loop bedding in laminites and the highest magnitudes represented by intruded and fractured gravels, is proposed. (C) 2000 Elsevier Science B.V. All rights reserved.