ELEVATED, PASSIVE CONTINENTAL MARGINS: NOT RIFT SHOULDERS BUT EXPRESSIONS OF EPISODIC, POST-RIFT BURIAL AND EXHUMATION DRIVEN BY CHANGES IN PLATE MOTION AND/OR MANTLE FLOW

Many studies of elevated, passive continental margins (EPCMs) assume that their characteristic, large-scale morphology with high-level plateaux at 1 to 2 km or more above sea level (a.s.l.) and deeply incised valleys has persisted since rifting and crustal separation, and that the absence of post-rift sediments is evidence of non-deposition.

The high mountains in West Greenland, however, expose evidence of km-scale, post-rift subsidence, and recent studies showed that typical EPCM morphology with elevated plateaux formed c. 50 Myr after breakup through a process of uplift and dissection of a regional, post-rift erosion surface. Since the West Greenland margin shares all the morphological characteristics of EPCMs, the results from West Greenland lead us to question the common assumption that EPCMs have remained high since the onset of continental separation. We present published evidence of post-rift burial followed by uplift and exhumation from a number of EPCMs and their adjacent basins to support the notion that EPCMs are not permanent highs and that their morphology is unrelated to rifting and continental breakup.

We suggest that EPCMs represent anticlinal, lithospheric folds formed under compression where an abrupt change in crustal or lithospheric thickness occurs between cratons and rift basins. We propose that EPCMs are expressions of episodic, compression-induced uplift episodes; one episode of uplift results in erosion of the region to produce a low-relief surface near the level of the adjacent, opening ocean, and a second (or more) episode(s) raises the plateau to its present elevation, after which the plateau is dissected by fluvial and possibly glacial erosion.

In special cases, EPCMs and their hinterland may also be affected by mantle upwelling. Along the SE Greenland margin, the regional post-rift plateau extends to elevations higher than 3 km a.s.l., and it is likely that these high elevations are related to the presence of the near-by Iceland plume.

Reference: Japsen, P., Chalmers, J.A., Green, P.F., and Bonow, J.M., 2011, Elevated, passive continental margins: Not rift shoulders but expressions of episodic, post-rift burial and exhumation: Global and Planetary Change (in press).