"Oceanic upper mantle dynamics" may be defined as the ensemble
of dynamical processes which by virtue of their relatively short
length scales (few tens to a few hundred km) are largely
confined to the upper mantle. The central thrust of the OMD
initiative is to provide new constraints on models of such
processes by exploiting emerging possibilities of
high-resolution/large-aperture array observations.
The field of oceanic upper mantle dynamics is vast and diverse.
Rather than attempt the hopeless task of surveying it all, I have
decided to concentrate on three themes that encompass a substantial
proportion of current work in the field and are likely to continue
to do so in the future:
1. Plumes in the upper mantle. While we are all but certain that
mantle plumes exist, our observational images of them are still
extraordinarily poor due to the small (ca. 100 km) length scales
involved. OMD-style array observations thus promise to yield
enormously improved understanding of the geometries of plumes
in the upper mantle and their interaction with the oceanic
lithosphere. By reason of their accessibility, size, and
spectacular surface manifestations, the Hawaii and Iceland
plumes necessarily hold a preeminent place. Detailed 3D
fluid dynamical models exist for both Hawaii (Ribe and Christensen
1994, JGR; Morgan et al. 1995, JGR; Moore et al. 1998, Science)
and Iceland (Ribe et al. 1995, EPSL; Ito et al. 1996, 1999,
EPSL). However, these models have not yet been tested against
tomographic images derived from wide-aperture arrays, and
the available seismic evidence suggests that they may be
unrealistic in significant ways (e.g., tilting of the plume
conduit: Li et al. 2000, Nature; Shen et al. 2002, EPSL.)
While Hawaii and Iceland are important, other significant
hotspots should not be forgotten. One important configuration
to study is that of a hotspot near (but not on) a ridge,
such as the Galapagos hotspot. Perhaps the crucial outstanding
question concerning such hotspots is how they communicate
with the ridge: many lines of evidence seem to suggest
the existence of a sublithospheric "channel" (Morgan 1978, JGR)
but dynamical models have not been able to predict these.
Wide-aperture array data could help resolve the question
decisively. Another important group of hotspots are those
above the Pacific "superswell". Courtillot et al. (2002,
EPSL, submitted) have recently suggested that these hotspots
may originate at transition-zone depths on top of a hot but
chemically distinct "dome" that has risen from the deep mantle.
This hypothesis may be testable with wide-aperture array seismic
data.
2. Seismic anisotropy.
Seismic anisotropy is ubiquitous in the oceanic upper mantle,
and provides unique information on the state of deformation
as a function of position. The link between the two is
deformation-induced "lattice preferred orientation" (LPO) of
olivine crystals, which exhibit strong anisotropy in both P and
S wave velocities. During the past 10 years or so, considerable
progress has been made in modeling the relation between LPO and
deformation path, taking into account the effects of both
intracrystalline slip and dynamic recrystallization
(Wenk and Tome 1999, JGR; Kaminski and Ribe 2001, EPSL.)
As a result, it is now possible to calculate directly from
mantle flow models the seismic anisotropy that would be
expected to develop at each point. Such calculations suggest
that the relation between LPO and the local flow can
be quite complex, even in relatively simple flow fields
(Kaminski and Ribe 2002, G-Cubed, in press; (Blackman and
Kendall 2002, G-Cubed, in press.)
To date, detailed geodynamical modeling of seismic anisotropy has
focussed on ridges (Blackman et al. 1996, Geophys. J. Int.;
Toomey et al. 2002, EPSL) and subduction zones/backarc regions
(Fischer et al. 1998, PAGEOPH; Hall et al. 2000, JGR) for which
relatively simple 2D or 2.5D flow models can be constructed.
A promising direction for the future is modeling of more realistic,
fully 3D flows. Preliminary calculations by Browaeys and Ribe (in
preparation) for a 3D convection model of plume-lithosphere
interaction beneath Hawaii suggest that the resulting anisotropic
structure is too complex to be represented by traditional layered
models for which simple inversion schemes exist. Direct forward
modeling will therefore be an indispensable tool for interpreting
future array observations of seismic anisotropy.
3. Plate boundary formation and convection. The theory of plate
tectonics, for all its extraordinary success, has had the unintended
effect of encouraging a conception of lithospheric plates as distinct
from the underlying convective circulation. An exciting direction of
current research attempts to correct this problem by showing how
plates themselves can be generated by thermal convection in fluids
with complex rheology. The key issue in here is the formation and
dynamics of the boundaries (convergent, divergent, and strike-slip)
between plates. It has long been known that neither
temperature-dependent nor power-law viscosity is adequate for
generating the sorts of narrow "platelike" boundaries seen on earth
(e.g., Christensen and Harder 1991, Geophys. J. Int.) Recent
work has focussed on the role of more exotic rheological features,
both singly and in combination: pre-existing weak zones (Zhong and
Gurnis 1995, Science), stick-slip self-lubrication (Bercovici 1995,
JGR), plastic yielding with melt-induced viscosity reduction (Tackley
2000, G-Cubed), and viscoplasticity with a low-viscosity asthenosphere
(Richards et al. 2001, G-Cubed.) While the surface motions predicted
by these simulations are strikingly "platelike", none has yet
been able to reproduce all the features of the earth's surface
tectonics, including isolated strike-slip motion and boundary
longevity.
A particularly important "subquestion" in this domain is that of
subduction initiation. Here again there is no consensus, although
numerous models have been proposed: slip on a preexisting fault zone
(Toth and Gurnis 1998, JGR), failure under tensile stress (Kemp and
Stevenson 1996, Geophys. J. Int.), thermo-mechanical feedback of
low-temperature plasticity (Branlund et al. 2001, EPSL), and sediment
loading combined with hydrolitic weakening (Regenauer-Lieb et al.
2001, Science).
While the above models are promising, most are based on ad hoc
assumptions about rheology that are not yet well constrained. A sounder
physical basis for many of the underlying ideas has been provided by
the two-phase damage theory of Bercovici et al. (2001, JGR), but
large-scale computations using it are still in the future. Another
critical need is to find suitable regions on earth where OMD-type
array data might provide much-needed observational constraints on
the mechanisms involved in plate boundary formation.