RECONSTRUCTING PALEOELEVATION USING A DECONVOLUTION OF ERODED SEDIMENTS

Despite recent accomplishments linking the development of topography with tectonics in active orogens, reconstructing paleoelevation from an understanding of the process that both uplift and erode rocks has proven to be elusive. We approach the problem with the premise that for well drained landscapes mean erosion rate and local relief as well as local relief and mean elevation are highly correlated. Our approach defines a crude measure of the erodibility of the landscape at a regional scale, called k_d, where the mean rate of mechanical erosion is proportional to the mean elevation above base level. This erosion constant must be locally calibrated for prevailing rock type and climate. In an orogen, the rate of change of mean elevation can be cast as the isostatically compensated balance between a constructive crustal source term, a destructive erosion term, and a eustatic term. We define the source term as any geologic process, such as crustal thickening, that could change the mean elevation of a landscape with respect to base level, exclusive of changes in mean elevation caused by erosion and the isostatic response to erosion. We propose a simple landscape evolution model that employs an exponential unit response function to erode the landscape with k_d and create new topography with the crustal source flux for 5 m.y. time steps. We solve for the general relationship between the source flux into the orogen and erosional flux out where offshore sediment volumes are used to determine the erosional flux and global sea level curves account for changes in our elevation datum, removing the eustatic component of the source term. A particular limitation of our model is that it cannot uniquely solve for both changes in source or climate. The model is applied to several orogens where the unroofing history is well known. Significant growth and decay of paleoelevation occurs when k_d is taken to be a constant. In contrast, the initial mean elevation of an orogen at the point when it passes into the decay phase is determined when the source is taken to be zero, and k_d is allowed to vary. The results of our approach are consistent with other geological and paleobotanical reconstructions of orogenesis and growth of topography for the mountain belts studied.