Recent high-resolution mapping of Guaymas hydrothermal fields (Southern Trough)

Abstract

Insights into the factors controlling fluid circulation through the crust and the nature of fluid venting at the seafloor are first steps in understanding their effect on ocean properties and climate change. New data on the seafloor morphology, sub-surface sedimentary stratification, and water column of the sedimented Southern Trough hydrothermal field (Guaymas basin) were acquired during the BIG cruise in 2010. These data provide accurate and high-resolution information on the geological context of the vents, on the distribution of acoustic anomalies in the water column, and on the possible nature of the fluid generating these echoes. More than 40 hydrothermal edifices were observed. The southern zone of the study area hosts hydrothermal sites that differ from the northern area. The southern vents are located inside or at the edge of small sub-circular depressions and the relationship between active edifices and collapsed areas involves different steps in the continous hydrothermal setting. Sub-bottom data show surface and sub-surface events, with some reflection layers possibly indicating subsurface hydrothermal precipitates or lithification with an estimated age of approximately 10 000 to 20 000 years. Based on the position and maximum altitude of the acoustic anomalies above the seafloor, two types of fluid emission echoes are distinguished: 1) anomalies reaching a maximal altitude of ∼350 m above the seafloor and seen both at the northern and southern fields and 2) strong, narrow and straight anomalies reaching 1334 or 1702 m above the seafloor that are only present in the southern hydrothermal fields of the studied area. We suggest that there are two types of echoes reflecting different fluid escapes based on the physical conditions of fluid venting and degassing and their relationship to geologic features: hydrothermal fluids or hydrothermal fluid mixed with hydrocarbon gas, oil or condensates rising through the water column. The collapsed depressions observed in the southern part facilitate the release of light hydrocarbon (gas, oil, and condensates) soluble at a high temperature and transported by hydrothermal fluids towards the shallow sedimentary levels where they accumulate. These light hydrocarbons rapidly migrate at high levels in the water column. This contrasts with the northern fields where hydrothermal circulation linked to deeper faults, re-mobilize heavier non-soluble hydrocarbons which do not migrate at high levels in the water column.