Seismic Velocity Under the Ridge-Crest
While direct correlation of seismic velocity profiles with
petrologic stratification is problematic and controversial,
there is burgeoning evidence that velocity layering is
correlated with porosity and alteration/metmorphic fronts associated with
hydrothermal ciculation (Jacobson,
1992). The increasing ability to relate seismic data
to parameters of biological
importance (porosity, permeability, and/or alteration) means that both
reflection and refraction studies of the upper oceanic crust offer
possible constraints on the subsurface microbial habitat.
I critically review a recent seismic refraction survey conducted at the
Endeavour segment of the Juan de Fuca Ridge, and summarize the
confirmational results from an additional refractive, as well as
a reflective survey in the same region. After
this synopsis of recent seismic research on near-axis,
upper-crustal structure and evolution, I follow
efforts
(Wilkens et al, 1991;
Jacobson, 1992;
Rohr, 1994; and
Davis et al, 1996)
to synthesize seismic and heat flux data, and consider the
implications for potential subsurface habitat.
Critical review of Endeavour seismic velocity profiles
The geophysical work of Curdak and Clowes, and Rohr,
promises a method with which to remotely determine the spatial
distribution of porosity
and permeability in the axial upper crust, as well as the change in
that distribution off-axis. Both studies provide ranges of
velocities for particular depth ranges within the axial crust, and
Rohr (1994)
documents an unexpectedly rapid increase in layer 2A velocity in 0.6 Ma
crust -- an increase correlated with a fundamental change in
hydrothermal circulation (from open to closed convection) observed
by Davis et al (1996), and postulated
to be caused by by sudden capping of the upper crust where Cascadia
Basin sediments are dammed by a 400 m step in basement topography.
While comparison of velocity profiles can be ground-truthed to certain
extents through laboratory experiments on materials of variable
(fine-scale porosity), observations from the deeper ODP cores (like
hole 504b), and ophiolite stratigraphy, which parameters affect
the propagation velocity of seismic waves in not established. Recent
debate has focussed upon the relationship between seismic structures and
the aspect ratio of voids in crustal rocks
(Wilkens et al, 1991), as well as
Poisson's ratio (Shaw, 1994).
Outline
