Description: Underway temperature and salinity data was collected along the cruise track with two autonomous measurement systems. Usually, the systems are changed after 6 hours. While temperature is taken at the water inlet in about 6.5 m depth, salinity is estimated within the interior measurement container from conductivity and interior temperature. Salinity was calibrated independently for both measurement containers (MCs). No correction was done for temperature. For details to all processing steps see Data Processing Report.

Description: Highlights • Subaqueous spreading occurs on gently inclined surfaces (〈3°). • Gliding planes could be clays or sandy materials undergoing loss of strength. • It is documented on some of the largest marine landslides. • SubSpread Database includes 32 case studies. • Contourite and glaciogenic deposits represent often the slipping surfaces. Abstract Subaqueous spreading, a type of extensional mass transport that is characterized by a ridge and trough morphology, has been documented globally but is poorly understood. Subaqueous spreading is observed on gently inclined surfaces (typically 〈3°) when sediment bodies experience a sudden reduction of shear strength along their basal plane during clay softening or liquefaction of sands or silty sand sediment. Historically, spreading has been associated with very large landslides, but many unknown aspects of these mass movements have yet to be clarified. Does spreading influences the large catastrophic failure? What are the sedimentological and morphological aspects that contribute in initiating this process? These are some of the research questions that spurred the present work. Here, we introduce a database that incorporates information from thirty-two case studies, and use this to provide key insights into the sedimentary and morphological aspects of subaqueous spreading that will assist in the identification of spreading elsewhere. We find that subaqueous spreading is most common along passive glacial margins, but is also observed along active margins. The occurrence of contourites interlayered with glaciogenic deposits is, in most cases, associated with landslides (or landslide complexes) with spreading morphology. The database shows that seismic loading is commonly suggested to be the dominant trigger mechanism, although more geotechnical observations and modelling analysis would be needed to support this conclusion. We compare subaqueous spreading with terrestrial spreading, in particular to earthquake-related lateral spreading and clay landslides. We find that subaqueous spreading shares the same driving processes and potentially also some of the trigger mechanisms that are associated with the terrestrial spreading cases. Future work will be required to address the association between spreading and its occurrence on some of the largest landslides on Earth, its development mechanism, and its potential hazard implications.