a) Location of the Guaymas Basin. b) EM122 bathymetry of the Guaymas spreading axis; NT, Northern Trough; ST, Southern Trough. c) Composite bathymetry of the Southern Trough with EM122 data (in color) and AUV high-resolution mapping (in gray). The location of the nearest plume described in Merewether et al. (1985) is indicated by a white plus sign. d) High-resolution AUV mapping in the central part of the Southern Trough; black circles and numbers indicate the location and the fluid emission echoes, respectively. Letter-number codes refer to the hydrothermal sites described in Lonsdale and Becker (1985); dotted lines represent the limits of sills (Lonsdale and Becker, 1985) and black barb-lines are the main faults redrawn from Lonsdale and Becker (1985) and from high-resolution bathymetry. The black lines labeled P4 and P5 correspond to the AUV sub-bottom profiles over the hydrothermal area.