The basic constraints on the first order geodynamic processes that have shaped the modern Earth are provided by the observed long-wavelength variations in topography, the geoid (which contains information about the distribution of potential energy), heat flow, seismicity and the in-situ stress field. The main features of each of these geophysical observables at the global scale is summarised below.
This regular pattern of oceanic bathymetry is disturbed by:
Continental landscapes provide one of the most dramatic of natural fractal surfaces. The characteristic feature of the fractal is the statitiscal scale-invariance, that is the general character of the surface looks the same independent of the scale of observation. For landscapes this is strictly true only over a limited range of scales (10-1m - 105 m). The invariance of landscapes on this range of scales suggests that the processes operating to sculpt the landscape (i.e., erosion, mass wastage and deposition) are either scale-invariant themselves or that when acting together produce the scale-invariant effect. Erosion acts to roughen landscape at a wide range of scales while both mass wastage and deposition act to smooth the landscape, at the short and long-wavelength, respectively.
Variations in the gravitational potential energy of the lithosphere, DUl, correlate with the dipole moment of the near-surface density distribution and therefore they can be directly related to the lithospheric component of the observed geoid anomalies, DNl :
Positive geoid anomalies of up to 10 - 15 m associated with a number of mid-ocean ridge segments, as well as age-correlated geoid offsets across fracture zones imply that ageing of the ocean lithosphere is accompanied by a decline in potential energy. The geoid anomaly predicted for the cooling half-space model (as well as the thermal plate model) for young ocean lithosphere is about d (DNo )/d t = - 0.15 m/Ma, which compares favourably with the observed geoid anomaly over the Mid-Atlantic Ridge at 44.5oN, and elsewhere, as well as with the geoid offsets across fracture zones. The total geoid anomaly of -12.7 m over 84 Ma corresponds decline in Ul of 2.9 x 1012 N m-1.
In comparison with the mid-ocean ridges, the geoid anomalies associated with continental margins and the interior of continents are far less clear. On the basis of averages taken over large areas there appears to be no systematic difference in the geoid height between old ocean basins (older than Cretaceous) and continental masses. Such an interpretation implies that the mean potential energy of the continental lithosphere is equivalent to old ocean basins. However, the data show very substantial differences between continents, with the mean geoid of the African continent some 40 m higher than the North American continent and 10 m higher than the mean for the Atlantic and Pacific ocean basins older than Cretaceous. This observed intercontinental variation far exceeds the plausible lithospheric contributions to geoid anomalies and therefore must reflect long-wavelength sub-lithospheric contributions. Moreover, a number of continental margins are characterized by distinct positive anomalies of the order 6 m across the transition from the ocean basin to sea-level continent and imply that a continental lithospheric column supporting sea level elevation has the potential energy equivalent to ocean lithosphere of age about 44 Ma.
Since the lithospheric contribution to the geoid anomaly reflects the dipole moment of the near-surface density distribution, the observed geoid anomalies across continental margins can also be used to constrain the continental lithospheric density structure. A lithospheric thickness of 125 km and a crustal density of 2750 kg /m3 is consistent with a continental marginal geoid anomaly of + 6 m. Moreover, such a density structure is consistent with the interpretation that an isostatically compensated continental lithospheric column supporting about 1 km of surface elevation above sea level is in potential energy balance with the mid-ocean ridges. While the generally poor resolution of the geoid in mountainous regions precludes definitive correlation between topography and potential energy within the continents, some evidence of the correlation is provided by the lithospheric contribution to the geoid anomaly of 24 - 27 m for the Andean Altiplano. Such inferences are consistent with a geoid that varies with continental topography as 6 - 7 m/km, corresponding to a potential energy variation of about 1.3 x 1012 N m-1. For a continent with an average elevation of 500 m, this correlation suggests a mean continental potential energy of 0.997 x UMOR
In the ocean lithosphere, the highest heat flow regions are associated with mid-ocean ridges, where absolute values are very variable (50-300 mW m-2) due to intense, but localised, hydrothermal activity. In older, deeper lithosphere the heat flow measurements become less variable and gradually decline. The decline in heat flow approximates the same dependence on t0.5 as bathymetry. This age-bathymetry-heatflow law for ocean lithosphere which provides one of the most profound insights into the structure of the lithosphere (see Chapter 7).