Cenozoic uplift around the North and South Atlantic
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Author||Japsen, Peter۱; Bonow, Johan M.۱; Green, Paul F.۲; Chalmers, James A.۱; Lidmar-Bergstrِm, Karna۳|
|Holding Date||11 October 2008|
There are mountains along the passive continental margins on both sides of the Atlantic - as along passive margins all over the earth. The mountains are composed of rocks that were formed at different times in the history of the earth. The crust in East Greenland is thicker than normal, but in other areas there are mountains in areas where the crust has the same thickness - or even thinner - than the crust in the adjacent lowlands. A character-istic feature of these mountains is elevated plateaux that are cut by deep river valleys that may have been enlarged by glaciers. We argue that these extensive plains that cut across rocks of different age and resistance, must have formed by fluvial erosion to well-defined base levels that most likely was palaeo-sea. Today these erosion surfaces are far from sea level due to uplift after their formation.
We have documented that the main planation surface in West Greenland was formed by Oligocene-Miocene denudation of up to 1 km as a consequence of uplift. This is in contrast to the km-scale post-rift subsidence recorded by Palaeocene-Eocene sedimentary and volcanic rocks. The present-day mountains are the result of two phases of late Neogene uplift leading to the incision of valleys below the planation surface and thus to cooling that can be dated by apatite fission-track analysis.
Elevated plains and deeply incised valleys also characterise the mountains along the Scandinavian and Brazilian Atlantic margins. In Norway, the high-level plains are distinct from a hilly, Mesozoic relief that is found as an inclined surface at lower levels along the highlands. The hilly relief is truncated by the high plains that consequently were formed later, most likely during the Cenozoic. New fission-track data from Jotunheimen indicates that the present relief was formed after the Eocene in agreement with the offshore sedi-mentary record that indicates that much of Scandinavia was transgressed in Late Creta-ceous and Eocene times. In NE Brazil, plateaux that are characterized by Palaeogene lat-erites, are being destroyed by erosion along escarpments that outline the plateaux. Conse-quently, the laterites were formed after the erosional process that shaped these planation surfaces near the Palaeogene base levels, whereas the surfaces were uplifted to their pre-sent elevations during the Neogene.
Our observations demonstrate that mountains around the Atlantic reached their present elevations during the last few million years, that they are present irrespective of previous orogenesis, and that plateaux formed during the Cenozoic are unrelated to past glaciations. The mountains are, however, located along the edges of cratons where thinner lithosphere typically is found offshore. The lateral contrasts in the properties of the lithosphere appear to make the margins of the cratons unstable long after rifting which may explain the forma-tion of present-day mountains in these regions.