--> Abstract: Post-rift Uplift of Passive Continental Margins is Caused by Their Response to Moderate Compression in Continental Crust, by James A. Chalmers; #90130 (2011)

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Post-rift Uplift of Passive Continental Margins is Caused by Their Response to Moderate Compression in Continental Crust

James A. Chalmers
Geophysics, GEUS, 1350 Copenhagen K, Denmark.

Many passive continental margins are flanked by a mountain range up to more than 2 km high (Elevated Passive Continental Margins; EPCMs), e.g. Norway, east and west Greenland and Baffin Island in the Arctic and other margins elsewhere, that have been uplifted long after continental break-up. Explanations for these uplifted margins have been ad hoc, but there has hitherto been no explanation that accounts for their presence at both volcanic and non-volcanic margins and in both polar and tropical climatic environments.

A continent breaks up by extension and thinning of the continental crust. Thinning varies from small amounts in the proximal rift to perhaps a factor of 5 or more adjacent to oceanic crust.
Continental crust > ca. 20 km thick contains two weak layers, one between strong upper (quartz-rich) and lower (dioritic) crust and the other between strong lower crust and strong mantle.
Continental crust < ca. 20 km thick is too thin for there to be weak layers and the strong layers are effectively annealed to one another and to the underlying strong mantle.

Rifting of a passive continental margin must take place under tension. After rifting ceases, however, the margin can come under compression from forces originating elsewhere on or below its plate, e.g. collision between continental plates or from the excess lithostatic load of any mountain range formed from such a collision. The World Stress Map (www.world-stress-mp.org) shows that, where data exists, all EPCMs are currently under compression.

It is now known that continental crust responds to moderate compression stress by forming gentle folds with a wavelength of 200-400 km and an amplitude of ca. 1 km. The geometry of the lower crust and lithospheric mantle under the distal basin causes this part of the basin to flex downwards under compression and a corresponding upwards flexure develops under the proximal basin and its margin, uplifting it by several hundred meters.

Under moderate compression, material in the crust’s weak layers starts to flow towards the rift from under the adjacent continent. The lack of weak layers under the thinned, distal rift basin means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material therefore accumulates under the proximal rift, thickening the crust there and lifting it by isostatic response to the thickening. Material flows into the rift until the crust under it is once more as thick as it was prior to extension, but no thicker. This thickened layer extends gradually further and further below the rift, at speeds of a few tens of thousands of years per km, uplifting it and exposing post-rift sediments.

Both the thickening and folding continues until there is a reduction in imposed far-field compressive stress, after which the thickened crust ‘freezes’ in place. Erosion of the uplifted area leads to further uplift of the remaining material because of the isostatic response to the erosion.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

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