Recent investigations of modern oceanic crust
along the rifted walls of the Hess Deep Rift [Francheteau
et al., 1990; Francheteau et al., 1992; Karson
et al., 1992; Karson et al., 2002a] have provided
a new perspective on the internal structure of
fast-spread crust and by inference the processes
attending accretion at spreading centers like
the East Pacific Rise.
 These
studies have shown that the structural architecture
and geochemistry of fast-spread oceanic crust
is far more complex than current tectonic models
imply. Most recently, a 30-km section of the north
wall of the Hess Deep Rift was surveyed with a
combination of DSL-120 side-scan sonar, digital
mosaic surveys with the ROV Argo II, and Alvin
dives [Karson et al., 2002a]. The program demonstrated
the feasibility of this multi-scale mapping approach
in a single cruise and that major escarpments
on the seafloor provide important “tectonic windows”
into the structure and composition of the upper
oceanic crust.
Pito Deep
In January 2005, we plan to use a similar approach
of nested scale sampling and analyses to investigate
the structure and composition of EPR-generated
crust exposed at Pito Deep. The crust at Pito
Deep accreted at a faster spreading rate than
that exposed at Hess Deep. Previous studies of
Pito Deep, using the French submersible Nautile,
showed a section of upper ocean crust extending
from gabbro at the base, passing upward into sheeted
dikes, and overlain by lavas. We plan to map these
exposures to define the structural geometry, igneous
composition, hydrothermal alteration and magnetic
properties of these upper crustal units in order
to provide important constraints on the nature
of accretion along superfast-spreading ridges
like the southern EPR. In addition to the effects
of differing spreading rates, we expect the results
of this study of oceanic crust generated along
a segment center will differ from those observed
at the relatively cool, segment-end setting represented
by Hess Deep. These data and information will
be used to test current models of oceanic crust
generation and refine our understanding of ocean
floor processes in space and time.
Topography of the Pito Deep
(from Hey et al., 2002)
Lauching submersible Alvin from the R/V
Atlantis
(courtesy Woods Hole Oceanographic
Institution)
Working inside Alvin
Collecting orientation data on the ocean floor
using the Geocompass. Note Alvin's two mechanical
arms (left and right) and sample basket (bottom).
|
