A PRIMER ON PALEOTOPOGRAPHY

The topography of the free surface of the Earth represents a first-order feature important to many tectonic and geophysical questions. Studies of the evolution of that surface can be divided into three groups: atmospheric, gradient, and geodynamic.

The atmospheric methods are based on properties that change with elevation in the atmosphere. Temperature decreases in a predictable manner, by about 4.5° to 6°C/km within the stably stratified parts of the troposphere. One problem is that the lower 1 km of the atmosphere is commonly not stably stratified (e.g. thermal inversions), so there are problems in establishing sea-level reference sites. Paleotemperatures are estimated using leaf shape, clumped-isotopes, and organic geochemistry. Paleopressure can be estimated using vesicle size in lava flows. The most commonly used property is based on stable isotope fractionation of atmospheric water vapor as it is “orographically” lifted over mountain topography. Evaporation can "reset" the composition of the resulting meteoric water. The isotopic composition of paleo-surface water can be determined from clays and carbonate nodules in paleosols and from leaf waxes, which are a widespread detrital component in most terrestrial sediments.

The gradient methods are based on tilting of features with known initial gradients. Mountain fronts will commonly have smooth piedmont surfaces that represent the transport slope for the underlying alluvial sediments. Rivers channels commonly form at predictable gradients. Thermochronology can be used to define isochrones, which are surfaces of equal cooling age. The initial dip of such surfaces can be predicted using thermal modeling. A relative new field uses fish genetics to determine the extent of basin gradients with time.

Geodynamic modeling has started to provide credible predictions of the evolution of the Earth surface due to mantle convection. The more fundamental basis for this method is that mantle convection can be run in reverse using the surface plate velocities and mantle structure of the modern Earth as the initial condition. The features associated with this convection topography have large horizontal scales (>300 km) and low amplitudes (100’s of meters). Present calculations have a precision suitable for comparison to geologic features.