TROPICS

processes, is a necessary antecedent of these processes, or is largely unrelated.
For example, the generation of continental crust is commonly thought to be
associated with the removal of dense lower crustal residue, and that this
delamination requires isostatic adjustment and uplift of the remaining more
buoyant crust. This regional epeirogenic uplift and consequent erosional
unroofing would leave a recognizable signal on the landscape that can be
differentiated from localized horizontally-directed tectonic forcing (e.g., Cocos
Ridge collision).
5. SPECIFIC PLANS, METHODS, TECHNIQUES
5.1 Seismic properties of the crust and the upper mantle.
The primitive basaltic oceanic plateaus and arcs and a silica-rich continent are very
different in mean crustal Vp, with the low value of 6.45 being characteristic of continents
(Christensen and Mooney, 1995) and a much higher value of 6.8-7.0 being observed in
areas of thick basaltic crust (e.g. in Iceland (Menke et al., 1998) and the Aleutians
(Shillington et. al, 2004)). Poisson's ratio varies significantly too, owing to the extremely
low value for the mineral quartz, with values of 0.24 (corresponding to a velocity ratio of
1.71) being characteristics of a silicic crust, while higher values, often exceeding 0.27
(velocity ratio of 1.78) are typical of basaltic terrains (Zandt and Ammon, 2002).
Seismology thus provides an extremely effective way to gauge the degree to which the
bulk crust beneath Costa Rica has become "continental", and the geographical extent of
this change.
Seismology will also contribute to understanding elevation gradients in Costa Rica,
and especially in discriminating between three end-members, in which the highland
topography is alternatively compensated by: anomalously low crustal density due to its
continental nature; anomalously high mantle buoyancy due to either temperature of
chemical depletion; or anomalously thick crust, due to mechanical thickening of a still
largely basaltic material (Figure 2). In the crust, the correlation of seismic velocity with
density is mainly due to petrology, with the faster, more mafic minerals being denser
than the slower, more felsic ones. In the mantle, seismic velocities are most strongly
controlled by temperature, especially in areas (such as southern Costa Rica) where no
present-day volcanism is occurring, and hence where the complicating effects of partial
melt are likely to be absent. A seismically-derived density structure (e.g. applying
Christensen and Mooney’s (1995) formula for inferring density from crustal velocity)
that includes crustal petrology, crustal thickness and mantle thermal buoyancy effects
will provide a means for discriminating the alternative models.
Mantle flow-induced fabric, determined via seismic anisotropy, can also be used to
constrain models of crustal evolution. We would expect delamination of a dense crustal
component to be associated with large-scale vertical flow in the mantle. Vertical flow
implies a vertical fast direction, and a negative transverse anisotropy parameter (i.e.
Gaherty’s (2001) parameter, #Vs, which is a straightforward seismic observable).
However, should this flow have a strong horizontal component (e.g., Behn et al., 2007),
its azimuth will also be diagnostic. Models that involve a slab widow in southern Costa
Rica, and call for the transport of hot Pacific mantle northward beneath Costa Rica (e.g.,
Hornle et al., 2008) imply a flow direction roughly perpendicular to that envisaged by
models in which Cocos Ridge is subductig, and high topography is achieved by purestrain
shortening and thickening of the highlands.
The hypotheses that we are testing all require knowledge of ‘bulk earth properties’:
the thickness of the crust and the velocity structure of the crust and mantle, and their
geographical variation, especially at wavelength of tens of kilometers. Developing such
a seismic model is more challenging than first appears, since uniformity of coverage,
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TPI 6838742