Marathon Expedition Leg 6

Thomas Washington Weekly Report 281615z Aug 84. Surveyed axis of Tonga Trench
16S to 26S with a single longitudinal Seabeam swath 5 to 7 km wide plus a dozen
transverse profiles. Special survey of Horizon Deep and of interection of Tonga
and Kerrmadec trenches with Louisville Ridge. Seabeam performed well at world's
2nd deepest spot, with maximum soundings of 14.00 seconds, just 60m deeper than
previous max. The important bottom-water passage at the 26S trench intersection
found to have a surprisingly shallow seamount-clogged sill at 5625m. Used
profiler and Seabeam to map violent deformation of oldest Louisville Ridge
guyots as they enter trench. Now proceeding to survey and sampling sites along
the younger more remote parts of the hot-spot chain. Already running out of
names for newly discovered guyots and seamounts. So far making good time in fine
weather. (Lonsdale)


Thomas Washington 091840Z September ’84.  Rolling along Heezen Fracture Zone
near 55ºS through mountainous seas and snowstorms.  Neither Seabeam system nor
its operators appreciate the 45 degree rolls, but still collecting useful though
noisy data.  Completed work on Louisville Ridge wit successful dredges of fresh
but not glassy lava from the two youngest volcanoes in the chain.  One is a
small submerged island low at 480 M (Yohe Guyot).  Further southeast at 139ºW
Hylas Seamount has a sharp crest at 540M and may be growing at present location
of the hot spot.  Neither of these huge volcanoes had been sounded before, but
both show up plainly in seasat altimetry.  Most effective dredging tool proved
to be a 55 gallon drum lavishly reinforced by the ship’s engineers.  Lonsdale


Thomas Washington 171445Z September ’84.  At 59ºS speeding toward Drake Passage
and Cape Horn.  Work in Eltanin Fracture Zone included survey of the single 50km
long spreading segment linking Tharp and Heezen Transforms, fairly complete
Seabeam coverage of Heezen Transform and its deep rift valley (with wall up to
5km high), and mapping the junction with the southernmost East Pacific Rise. 
This last survey slightly marred by decision to avoid a large iceberg drifting
obstinately across the transform/spreading center intersection.  Collected a
nice sequence of magnetic anomalies 1 through 21 down east flank of rise, and
now profiling section of curst created by ultra fast spreading (half-rate
110mm/yr) between Pacific and Aluk plates.  Weather continues as appalling as
you would expect for this latitude in winter.  Bruised and battered scientific
party suffering roll burn-out but eagerly anticipating 1 day R and R during
refueling stop at Tierra del Fuego.  Lonsdale


Thomas Washington 241439Z September ’84.  On last lap of this 7,599 mile leg of
Marathon, profiling north along outer continental shelf of Patagonia from Tierra
del Fuego to plate estuary, with a dogleg to avoid Falklands waters.  Lonsdale