Seamount Expedition (CASM) in 1983. The CASM vent is found at 

 the northern end of the caldera, where fissures associated with 

 the northern Juan de Fuca Rift Valley intersect the caldera wall 

 (Chase et al . , 1985). Rather than with fissures generated by 

 rifting processes, Ashes Vent Field is linked to normal faulting 

 that created the caldera, probably following lava drainback from 

 a shallow magma chamber (Fig. 2). 



Samples and photographs necessary for geologic mapping and a 

 detailed study of the biogeography of the area were obtained 

 during four exploratory Alvin dives (1411 through 1414) to Ashes 

 Vent Field in 1984. Towed camera photographs were also used. 

 The dive site in the vent field was selected on the basis of 

 available SeaMarc I and SeaBeam data for the area. Following the 

 first dive, a transponder was deployed allowing short baseline 

 ranging from the R/V Atlantis II during subsequent dives. A 

 suite of sediment samples surrounding the active hot vents was 

 collected in 1986 using the Canadian submersible Pisces IV . The 

 database consists of approximately 14,000 bottom photographs, as 

 well as mineralogical ( XRD and SEM) and elemental (instrumental 

 neutron activation, ICP-AES and AAS ) analyses on the chimney and 

 seven sediment samples. 



The focus of research has been on both active smoker 

 chimneys and the transition zone between vent and nonvent 

 environments within the caldera. Two sulfide chimneys about 

 5 m high were observed precipitating on top of a field of broken 

 lobate lava flows in 1984. In the immediate vicinity of the 

 chimneys, clumps of vestimentif eran worms and patches of clams 

 were noted. The clams were concentrated in depressions between 

 lobate flows and along cracks in sheet flows. Surrounding the 

 locus of observed high temperature venting is a low-temperature 

 field of hydrothermal precipitates that forms a zone about 150 m 

 wide, extending 1,300 m along the strike of the caldera boundary 

 fault. Chemical analyses of hydrothermal deposits, including 

 both chimney and hydrothermal sediments of the transition zone, 

 show a range of elemental assemblages that can be explained by 

 variable thermo-chemical environments. Photographic data 

 indicate that physical environmental gradients also affect the 

 biogeography of epibenthic organisms. 



STRUCTURE AND GEOLOGY OF THE AXIAL CALDERA HYDROTHERMAL VENTS 



Structural analyses of the SeaMarc I image are shown in 

 Figure 3. Analysis of this image previously described by Crane 

 et al. (1985) are shown in the form of identifiable fissures, 

 faults, and structural lineaments. SeaMarc I was towed at 200 m 

 above the ocean floor in an earlier study; the image extends 

 beyond the walls of the caldera as mapped by SeaBeam (Fig. 1), 

 providing good sidescan imagery of the caldera walls, but poor 

 coverage of the floor of the caldera directly beneath the tow 

 track of SeaMarc I. The SeaMarc I imagery and its tectonic 



237 



