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International Ocean Discovery Program (IODP) Expedition 360
was the first leg of Phase I of the SloMo (shorthand for “The nature
of the lower crust and Moho at slower spreading ridges”) Project, a
multiphase drilling program that proposes to drill through the out-
ermost of the global seismic velocity discontinuities, the Mohor-
ovičić seismic discontinuity (Moho). The Moho corresponds to a
compressional wave velocity increase, typically at ~7 km beneath
the oceans, and has generally been regarded as the boundary be-
tween crust and mantle. An alternative model, that the Moho is a
hydration front in the mantle, has recently gained credence upon
the discovery of abundant partially serpentinized peridotite on the
seafloor and on the walls of fracture zones, such as at Atlantis Bank,
an 11–13 My old elevated oceanic core complex massif adjacent to
the Atlantis II Transform on the Southwest Indian Ridge.
Hole U1473A was drilled on the summit of Atlantis Bank during
Expedition 360, 1–2 km away from two previous Ocean Drilling
Program (ODP) holes: Hole 735B (drilled during ODP Leg 118 in
1987 and ODP Leg 176 in 1997) and Hole 1105A (drilled during
ODP Leg 179 in 1998). A mantle peridotite/gabbro contact has been
traced by dredging and diving along the transform wall for 40 km.
The contact is located at ~4200 m depth on the transform wall be-
low the drill sites but shoals considerably 20 km to the south, where
it was observed in outcrop at 2563 m depth. Moho reflections, how-
ever, have been found at ~5–6 km beneath Atlantis Bank and <4 km
beneath the transform wall, leading to the suggestion that the seis-
mic discontinuity may not represent the crust/mantle boundary but
rather an alteration (serpentinization) front. This in turn raises the
interesting possibility that methanogenesis associated with ser-
pentinization could support a whole new planetary biosphere deep
in the oceanic basement. The SloMo Project seeks to test these hy-
potheses at Atlantis Bank and evaluate the processes of natural car-
bon sequestration in the lower crust and uppermost mantle.
A primary objective of SloMo Leg 1 was to explore the lateral
variability of the stratigraphy established in Hole 735B. Comparison
of Hole U1473A with Holes 735B and 1105A allows us to demon-
strate a continuity of process and complex interplay of magmatic ac-
cretion and steady-state detachment faulting over a time period of
~128 ky. Preliminary assessment indicates that these sections of
lower crust are constructed by repeated cycles of intrusion, repre-
sented in Hole U1473A by approximately three upwardly differenti-
ated hundreds of meter–scale bodies of olivine gabbro broadly
similar to those encountered in the deeper parts of Hole 735B.
Specific aims of Expedition 360 focused on gaining an under-
standing of how magmatism and tectonism interact in accommo-
dating seafloor spreading, how magnetic reversal boundaries are
expressed in the lower crust, assessing the role of the lower crust
and shallow mantle in the global carbon cycle, and constraining the
extent and nature of life at deep levels within the ocean lithosphere.
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