The fallacy of interpreting SSDS with different types of breccias as seismites amid the multifarious origins of earthquakes: Implications

Abstract

At present, there are no criteria to distinguish soft-sediment deformation structures (SSDS) formed by earthquakes from SSDS formed by the other 20 triggering mechanisms (see a companion paper in Vol. 5, No. 4 of this journal by Shanmugam, 2016). Even if one believes that earthquakes are the true triggering mechanism of SSDS in a given case, the story is still incomplete. This is because earthquakes (seismic shocks) are induced by a variety of causes: 1) global tectonics and associated faults (i.e., mid-ocean ridges, trenches, and transform faults); 2) meteorite-impact events; 3) volcanic eruptions; 4) post-glacial uplift; 5) tsunami impact; 6) cyclonic impact; 7) landslides (mass-transport deposits); 8) tidal activity; 9) sea-level rise; 10) erosion; and 11) fluid pumping. These different causes are important for developing SSDS.Breccias are an important group of SSDS. Although there are many types of breccias classified on the basis of their origin, five types are discussed here (fault, volcanic, meteorite impact, sedimentary-depositional, sedimentary-collapse). Although different breccia types may resemble each other, distinguishing one type (e.g., meteorite breccias) from the other types (e.g., fault, volcanic, and sedimentary breccias) has important implications. 1) Meteorite breccias are characterized by shock features (e.g., planar deformation features in mineral grains, planar fractures, high-pressure polymorphs, shock melts, etc.), whereas sedimentary-depositional breccias (e.g., debrites) do not. 2) Meteorite breccias imply a confined sediment distribution in the vicinity of craters, whereas sedimentary-depositional breccias imply an unconfined sediment distribution, variable sediment transport, and variable sediment provenance. 3) Meteorite, volcanic, and fault breccias are invariably subjected to diagenesis and hydrothermal mineralization with altered reservoir quality, whereas sedimentary-depositional breccias exhibit primary (unaltered) reservoir quality. And finally, 4) sedimentary-collapse breccias are associated with economic mineralization (e.g., uranium ore), whereas sedimentary-depositional breccias are associated with petroleum reservoirs. Based on this important group of SSDS with breccias, the current practice of interpreting all SSDS as “seismites” is inappropriate. Ending this practice is necessary for enhancing conceptual clarity and for advancing this research domain.