Models for landform development on passive margins. Some implications for relief development in glaciated areas

Models for pre-glacial relief along passive continental margins must reflect the style and rate of uplift, and the rates of weathering and erosion, during at least the last 100–150 Ma. Passive margins have varied elevation, patterns of rifting, uplift and flexuring. A distinction is recognised between low-lying and steeply rising margins, the latter associated with major crustal upwarps. Continuous flexuring of the crust will maintain disequilibrium in weathering systems and known or possible rates of weathering and erosion appear compatible with weathering penetration during uplift. Under hot-humid conditions a rate of weathering of 50–100 mm kat1̄ is possible and could produce 15–30 m of saprolite in 1 Ma (with 70% surface loss), while long-term denudation rates seldom exceed 40 mm ka−1. Deep, sandy and corestone profiles in zones of marginal dissection indicate effectiveness of weathering during uplift. Examples from southern Africa, southern Australia, South America and western Africa illustrate 4 morpho-tectonic settings: low elevation with minimum tilt, low domal uplift with coastal tilt, high elevation steeply arched, and high elevation faulted margin with back tilting. These examples are compared with higher latitude marginal landscapes in southeastern USA, Fennoscandia and Scotland. It is concluded that pre-glacial survival implies more than small relict features and includes: exhumed surfaces from beneath sedimentary covers; basins and troughs following zones of deeper weathering, and local planations, now warped and elevated to varying heights, above which inselbergs are locally common. While relict saprolites are found beneath duricrusts in the tropics, and strata-bound in high latitudes, most saprolite is better understood as having a dynamic history throughout the Neogene, including the Quaternary, whether in continuously warm or periodically cold climates.