The
so-called “typical” values for the Earth’s topography are often used in the literature, such
as the mean continental altitude (MCA), the Moho depth for “normal” continental
crust, or “typical” depth of mid-oceanic ridges. However, the statistical
relevance of those values is hardly discussed. Focussed on data for the global
topography, this paper presents statistical analyses regarding various
environments. It is shown in particular that the definition of the mid-oceanic
ridge is not straightforward, and varies considerably according to what is
actually considered: the ridge “inner-rift”, the ridge “crest”, or the “virtual
ridge” at spreading centre. This definition is also a function of the spreading
rate and has strong implications for the rationale on the age-depth relationship of the sea-floor. In addition,
the latter relationship is highly dependent on how the topographic data are
corrected from sediment load. The correction itself implies numerous aspects
that relies on the precision and associated uncertainties of, in particular,
the sediment thickness, sediment porosity, and the mantle, water and sediment
densities. In this respect, the analysis carried out here favours a plate
cooling model (PCM) for the age-depth dependence of the
sea-floor. The topographic elevation at trench proves also to be related to the
age of the sea-floor through a different PCM equation. Away from the trench,
the oceanic lithosphere is affected by flexuration, for which equations can be defined
assuming that the end-load position is not located at trench. On the other
hand, the elevation of magmatic arc does not appear to be related to sea-floor
age or spreading rate. However, the correlation between the arc-trench distance and the topographic elevation of arc
for continental crust seems to be an indicator of slab dip and therefore the
existence of slab roll-back processes. Along intra-oceanic magmatic arc, a
periodicity in topographic elevation suggests a periodicity in the occurrence
of magma chambers, and therefore magmatic processes that need to be further
studied. At passive margin, the transition between continental and oceanic
crust seems to be relatively sharp in average. Subdivision of the datasets
according to the age of the continent-ocean boundary (COB)
indicates that rift and passive margin shoulders are found within a couple of degree
away from the COB and for ages younger than ca.
20 Ma. Finally, the statistical analysis of continental data assumed to be free
of thinning or thickening effects suggests that the MCA should rather be
considered in terms of “lowlands” and “highlands”. Relying on model of Moho
depth, the “normal” crustal depth might be thinner than commonly accepted. In
any case, the filtering of reduced topography can help to determine the impact
of dynamic topography.
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