Alvin Dive 3356 - Dive Summary

 

Feb 17, 1999

Ken Macdonald (transcript)/ Eric Bergmanis (transcript)/ Matt Heintz (Pilot)
Launch: 18 12.85' S, 113 20.50' W; x=6250 y=13200
Transponder origin: 18° 20' S 113° 24' W

 

Dive Objectives:

  • Investigate east flank structure and geochemistry; and fill in gap at axis near 18 13.
  • Between #1 and #2, cross outward facing scarp, possible volcanic growth fault.
  • Between 1-2 and 2-3 cross and sample volcanic mound seen on DSL 120.
  • Between 3-4 cross and sample possible off-axis flow which drapes outward facing scarp; seen on DSL 120.
  • At 4, sample top of east wall.
  • At 4-5 try to see east wall while going down, cross axial floor where a gap exists and sample it.
  • 5-6 another floor crossing and sampling.
  • 6-7 ditto if time permits
  • 6-8 or 7-8 careful investigation of east wall while going up; sample top of east flank at 8.

    • Waypoints:
    6250, 13200
    5750, 12950
    5900, 12800
    5100, 12500
    4300, 12750
    4800, 13000
    4650, 13250
    5300, 13250

    Summary: We started the dive near 18° 13' S approximately 1.5 km east of the EPR axis. The axis here is characterized by a trough approximately 700m wide and 50m deep. Outside the axial trough, the lavas were primarily pillows of age 3-4, i.e., 50-90% sediment cover. We headed west, going uphill ~100m over a distance of ~1 km. While most of the lavas remained age 3 pillow lavas, we saw small outcrops of folded sheet flows of age 3. The top of the east wall of the axial trough is basically a horst bounded by dip slip faults. On top of the east wall near the west-facing scarp edge, a series of collapses occur in age 2-2.5 sheet lavas. We descended into the floor of the axial trough, losing sight of the bottom for awhile. When we landed in the floor, we saw age 2 pillow lavas and sheet flows. We crossed the trough floor from east wall to west wall 4 times. We encountered numerous horsts, grabens and large areas of collapsed lava lakes. Some of the collapses were as young as age 1.5, the youngest outcrop seen on this dive. Numerous old sulfide chimneys were observed, none were active. We never went higher than the first step in the west wall. Our traverse back up the east wall had excellent visual contact. Atop the first step were numerous lava pillars, collapses, dead sulfide chimneys and hybrid lava pillar/sulfide chimney structures. The pillars were all vertical indicating no significant tectonic tilting. On top of the east wall we had some extra time to go north tracing a continuous series of old collapsed lava lakes in age 2.5-3 sheets and lobates. Short lava pillars were observed which again were vertical indicating no tilting. 17 basalt samples were collected.

     

    Sample Locations:

         Lat. S   Long. W
1.  18 12.96 113 20.55
2.  18 12.97 113 20.71
3.  18 13.04 113 20.65
4.  18 13.13 113 20.85
5.  18 13.21 113 21.12
6.  18 13.14 113 21.35
7.  18 13.11 113 21.55
8.  18 13.01 113 21.43
9.  18 12.98 113 21.30
10. 18 12.80 113 21.37
11. 18 12.71 113 21.40
12. 18 12.66 113 21.31
13. 18 12.71 113 21.22
14. 18 12.76 113 21.12
15. 18 12.77 113 21.10
16. 18 12.59 113 21.02
17. 18 12.30 113 21.02