IODP Expedition 340T:
Atlantis Massif Oceanic Core Complex APL
Site U1309 Summary
PDF file is available for download.
Background and objectives
The primary scientific objective of IODP Expedition 340T was to investigate hydration of the crust within the domal core of Atlantis Massif, located just west of the spreading axis of the Mid-Atlantic Ridge at 30°N. To
accomplish this, the expedition completed the wireline logging of existing Hole
U1309D on the Central Dome of Atlantis Massif, focusing on temperature and
acoustic velocity measurements, and a zero-offset vertical seismic profile
(VSP). The study aimed to provide insight into potential sources of seismic
reflectivity in the footwall of this oceanic core complex, which could be
attributed to variations in alteration between lithologic units or to narrow fault zones with higher porosity and potentially pore fluids.
All components of the
planned logging program at Hole U1309D were successfully conducted during
Expedition 340T. The Triple Combo, Sonic, and Magnetic Susceptibility wireline
logging runs produced high quality data. The majority of the
VSP data are noisy and will require substantial post-processing.
However, a few stations in the upper 150 m of the section recorded clear,
strong seismic arrivals, thus providing in-situ constraints on average
properties across the zone inferred to be most strongly
affected by detachment processes at the Central Dome.
The first tool string deployed in Hole U1309D was the Triple Combo, measuring hole size, density, gamma ray, resistivity and borehole fluid temperature. This was the first time Hole U1309D
had been entered since the conclusion of Expedition 305 operations
in February 2005, so the borehole fluid had seven years to reach equilibrium
with the surrounding rock formation. The dominant trend in the downhole
temperature profile is quasi linear increase with depth, with modest deviations focused at two distinct intervals. The maximum temperature recorded in the hole was ~145°C at 1404 mbsf, more than 20°C hotter
than the maximum temperature recorded at the end of Expedition 305, when
drilling and flushing had altered conditions considerably. Gamma ray and
resistivity data confirmed repeatability of Expedition 305 data.
The Versatile Seismic
Imager (VSI), essentially a downhole three-axis geophone accelerometer, was
deployed three times as part of the VSP experiment. Vertical seismic profile
station coverage at zero offset now extends the full length of the hole,
including the uppermost 150 mbsf where fault detachment processes are expected
to have left their strongest imprint. Recorded waveforms are noisy; the exact
cause is unknown but likely is due to a combination of factors such as sea
conditions, wireline tension, and possibly ship-generated noise. First arrivals
from 93 out of the total 659 shots, corresponding to 33 of the 55 station
depths, were graded as excellent to very good on at least one channel,
primarily the vertical component. The good VSP stations for Expedition 340T
thus range from 86 to 1360 mbsf extending the Expedition 305 data that covered
the interval from 272 to 792 mbsf.
The sonic tool string,
including the Dipole Sonic Imager (DSI) and General Purpose Inclinometer Tool
(GPIT), was deployed twice in Hole U1309D. A full set of sonic data was
obtained, including borehole compressional, shear, and Stoneley
wave slowness (the reciprocal of velocity) from 700 to 1384 mbsf, an interval
where no velocities were measured at the end of Expedition 305. These data are
the first in situ measurements of the velocity of gabbros typical of oceanic
lower crust, with compressional velocity (Vp)
values higher than 7000 m/s.
The final tool string
run in Hole U1309D included the deep-reading sensor of the Magnetic
Susceptibility Sonde (MSS). This expedition marked the first sea trial of the
deep-reading sensor of the newly rebuilt LDEO MSS, providing a measure of the
magnetic susceptibility of the borehole, which depends on the concentration and
composition of magnetic minerals in the wallrock.
This tool string is temperature-limited, so the logging run sampled only the
cooler, upper interval of the hole down to 2419 mbrf
(763 mbsf). Magnetic susceptibility was measured on core recovered from Site
U1309 during Expeditions 304/305; the MSS log shows similar features,
suggesting the measurement is a reliable one, and corresponds well with
fine-scale (meter to submeter scale) variations in lithology.
conclusion of logging operations, a short camera survey was carried out in the
vicinity of Hole U1309D. The survey was designed as a 5-m expanding spiral
centered at Hole U1309D, in order to document the local seafloor environment.
Following the survey, the ship navigated back to a distinctive mound-like
feature that was imaged a few meters from Hole U1309D during re-entry and again
in the camera survey.