Paul F. Green, Karna Lidmar-Bergström, Peter Japsen, Johan M. Bonow and James A. Chalmers
The uplift histories of passive continental margins constitute an important area of research, first of all because of their worldwide economic importance. Uplift and exhumation control the maturation processes of petroleum in marginal sedimentary basins.
The histories of subsidence and uplift of passive continental margins are challenging to study and not least to explain in satisfactory ways, and different approaches and schools of thought have developed among different research groups over time.
The present volume applies stringent landscape analysis and state-of-the-art thermochronology to several passive continental margins around the world. The great importance of relict sedimentary covers in elevated and tilted continental margins is demonstrated, and the strengths and limitations of apatite fission track thermochronology are laid out in detail.
The authors demonstrate with a pioneering study of the margin of West Greenland and several other case studies that elevated, passive continental margins are not the results of continuous denudation and slow uplift acting on permanent highs. Instead, many margins have experienced complex histories of repeated subsidence, deposition, uplift, tilting and erosion. The nature of these processes is not yet well understood, but the regional extent of the vertical movements documented here suggests a plate-scale control.
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The continental margin of West Greenland is similar in many respects to other elevated, passive continental margins (EPCMs) around the world. These margins are characterised by extensive regions of low relief at elevations of 1–2 kilometres above sea level sloping gently inland, with a much steeper, oceanward decline, often termed a 'Great Escarpment', terminating at a coastal plain. Recent studies, based on integration of geological, geomorphological and thermochronological evidence, have shown that the high topography of West Greenland was formed by differential uplift and dissection of an Oligo-Miocene peneplain since the late Miocene, many millions of years after continental break-up between Greenland and North America. In contrast, many studies of other EPCMs have proposed a different style of development in which the high plateaux and the steep, oceanward decline are regarded as a direct result of rifting and continental separation. Some studies assume that the elevated regions have remained high since break-up, with the high topography continuously renewed by isostasy. Others identify the elevated plains as remnants of pre-rift landscapes. Key to understanding the development of the West Greenland margin is a new approach to the study of landforms, stratigraphic landscape analysis, in which the low-relief, high-elevation plateaux at EPCMs are interpreted as uplifted peneplains: low-relief surfaces of large extent, cutting across bedrock of different age and resistance, and originally graded to sea level. Identification of different generations of peneplain (re-exposed and epigene) from regional mapping, combined with geological constraints and thermochronology, allows definition of the evolution leading to the formation of the modern-day topography. This approach is founded particularly on results from the South Swedish Dome, which document former sea levels as base levels for the formation of peneplains. These results support the view that peneplains grade towards base level, and that in the absence of other options (e.g. widespread resistant lithologies), the most likely base level is sea level. This is particularly so at continental margins due to their proximity to the adjacent ocean. Studies in which EPCMs are interpreted as related to rifting or break-up commonly favour histories involving continuous denudation of margins following rifting, and interpretation of thermochronology data in terms of monotonic cooling histories. However, in several regions, including southern Africa, south-east Australia and eastern Brazil, geological constraints demonstrate that such scenarios are inappropriate, and an episodic development involving post-breakup subsidence and burial followed later by uplift and denudation is more realistic. Such development is also indicated by the presence in sedimentary basins adjacent to many EPCMs of major erosional unconformities within the post-breakup sedimentary section which correlate with onshore denudation episodes. The nature of the processes responsible is not yet understood, but it seems likely that plate-scale forces are required in order to explain the regional extent of the effects involved. New geodynamic models are required to explain the episodic development of EPCMs, accommodating post-breakup subsidence and burial as well as subsequent uplift and denudation, long after break-up which created the characteristic, modern-day EPCM landscapes.
P.F.G., Geotrack International, 37 Melville Road, Brunswick West, Victoria 3055, Australia . Corresponding author's e-mail: firstname.lastname@example.org
K.L.-B., Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden
P.J. & J.A.C., Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
J.M.B., Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark and Södertörn University, SE-141 89 Huddinge, Sweden
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