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  • 1.
    Green, Paul F.
    et al.
    Geotrack Int Pty Ltd, Australia.
    Duddy, Ian R.
    Geotrack Int Pty Ltd, Australia.
    Japsen, Peter
    Geol Survey Denmark & Greenland GEUS, Copenhagen, Denmark.
    Bonow, Johan M.
    Mittuniversitetet, Fakulteten för humanvetenskap, Avdelningen för turismvetenskap och geografi. Södertörn Univ, Huddinge.
    Malan, Jean A.
    New Age African Global Energy Ltd, London, England; Getech Grp PLC, England.
    Post-breakup burial and exhumation of the southern margin of Africa2017Ingår i: Basin Research, ISSN 0950-091X, E-ISSN 1365-2117, Vol. 29, nr 1, s. 96-127Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Despite many years of study, the processes involved in the development of the continental margin of southern Africa and the distinctive topography of the hinterland remain poorly understood. Previous thermochronological studies carried out within a monotonic cooling framework have failed to take into account constraints provided by Mesozoic sedimentary basins along the southern margin. We report apatite fission track analysis and vitrinite reflectance data in outcrop samples from the Late Jurassic to Early Cretaceous sedimentary fill of the Oudtshoorn, Gamtoos and Algoa Basins (Uitenhage Group), as well as isolated sedimentary remnants further west, plus underlying Paleozoic rocks (Cape Supergroup) and Permian-Triassic sandstones from the Karoo Supergroup around the Great Escarpment. Results define a series of major regional cooling episodes. Latest Triassic to Early Jurassic cooling which began between 205 and 180 Ma is seen dominantly in basement flanks to the Algoa and Gamtoos Basins. This episode may have affected a wider region but in most places any effects have been overprinted by later events. The effects of Early Cretaceous (beginning between 145 and 130 Ma) and Early to mid-Cretaceous (120-100 Ma) cooling are both delimited by major structures, while Late Cretaceous (85-75 Ma) cooling appears to have affected the whole region. These cooling events are all interpreted as dominantly reflecting exhumation. Higher Late Cretaceous paleotemperatures in samples from the core of the Swartberg Range, coupled with evidence for localised Cenozoic cooling, are interpreted as representing Cenozoic differential exhumation of the mountain range. Late Cretaceous paleotemperatures between 60 degrees C and 90 degrees C in outcropping Uitenhage Group sediments from the Oudtshoorn, Gamtoos and Algoa Basins require burial by between 1.2 and 2.2 km prior to Late Cretaceous exhumation. Because these sediments lie in depositional contact with underlying Paleozoic rocks in many places, relatively uniform Late Cretaceous paleotemperatures across most of the region, in samples of both basin fill and underlying basement, suggest the whole region may have been buried prior to Late Cretaceous exhumation. Cenozoic cooling (beginning between 30 and 20 Ma) is focussed mainly in mountainous regions and is interpreted as representing denudation which produced the modern-day relief. Features such as the Great Escarpment are not related to continental break up, as is often supposed, but are much younger (post-30 Ma). This history of post-breakup burial and subsequent episodic exhumation is very different from conventional ideas of passive margin evolution, and requires a radical re-think of models for development of continental margins.

  • 2.
    Green, Paul F.
    et al.
    Geotrack International Pty Ltd, West Brunswick, Victoria, Australia.
    Japsen, Peter
    Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark.
    Chalmers, James A.
    Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark.
    Bonow, Johan M.
    Mittuniversitetet, Fakulteten för humanvetenskap, Avdelningen för turismvetenskap och geografi. Geovisiona AB, Järfälla.
    Duddy, Ian R.
    Geotrack International Pty Ltd, West Brunswick, Victoria, Australia.
    Post-breakup burial and exhumation of passive continental margins: Seven propositions to inform geodynamic models2018Ingår i: Gondwana Research, ISSN 1342-937X, E-ISSN 1878-0571, Vol. 53, nr January, s. 58-81Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Despite many years of study, the processes involved in the post-breakup development of passive margins remain poorly understood. Integration of apatite fission track analysis (AFTA) and stratigraphic landscape analysis (SLA) at a number of margins has provided new insights into the development of elevated passive continental margins (EPCMs). In particular, by integrating evidence from the preserved rock record and landscape with information on the deposition and erosional removal of rock units which are no longer present (“missing section”) these studies have highlighted the importance of episodic positive and negative vertical km-scale crustal movements. Based on these studies we present seven propositions regarding the formation of EPCMs and the nature of the controlling processes, viz:

    1: EPCMs are not the inevitable consequence of rifting and breakup

    2: Elevated topography at present-day EPCMs developed long after breakup

    3: Similar EPCM landscapes at different margins suggest similar controlling processes

    4: EPCMs have undergone episodic km-scale burial and exhumation rather than slow monotonic denudation, both before rifting and after breakup

    5: Post-breakup km-scale exhumation at continental margins is not restricted to presently elevated onshore regions

    6: Post-breakup km-scale burial and exhumation have affected presently low lying margins as well as EPCMs

    7: Exhumation events show a broad level of synchroneity over continents and across oceans and correlate with plate boundary events and changes in plate motions.

    These propositions imply that positive and negative vertical motions at passive margins are controlled by plate-scale processes. Another key conclusion is that present-day elevation alone provides no clue to the earlier history of a margin.

    Many of the key aspects of these propositions are absent from current geodynamic models of passive margin development. Understanding the processes that control vertical movements at passive continental margins requires development of realistic geodynamic models that honour these propositions.

  • 3.
    Japsen, P.
    et al.
    Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, Copenhagen K, Denmark.
    Green, P. F.
    Geotrack International, 37 Melville Road, Brunswick West, VIC, Australia.
    Bonow, Johan M.
    Mittuniversitetet, Fakulteten för humanvetenskap, Avdelningen för turismvetenskap och geografi. Geovisiona AB, Högbyvägen 168, Järfälla, Sweden.
    Hinchey, A. M.
    Geological Survey, Department of Natural Resources, Government of Newfoundland and Labrador, P.O. Box 8700, St John’s, NL, Canada.
    Wilton, D. H. C.
    Memorial University of Newfoundland, P.O. Box 4200, St John’s, NL, Canada .
    Burial and exhumation history of the Labrador-Newfoundland margin: First observations2016Ingår i: Geological Survey of Denmark and Greenland Bulletin, ISSN 1811-4598, E-ISSN 1604-8156, Vol. 35, s. 91-94Artikel i tidskrift (Refereegranskat)
  • 4.
    Japsen, Peter
    et al.
    Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark .
    Green, Paul F.
    Geotrack International, 37 Melville Road, Brunswick West, Victoria 3055, Australia .
    Bonow, Johan M.
    Mittuniversitetet, Fakulteten för humanvetenskap, Avdelningen för turismvetenskap och geografi. Södertörn University, Alfred Nobels allé 7, SE-141 89 Huddinge, Sweden.
    Erlström, Mikael
    Geological Survey of Sweden (SGU), Kiliansgatan 10, 223 50 Lund, Sweden .
    Episodic burial and exhumation of the southern Baltic Shield: Epeirogenic uplifts during and after break-up of Pangaea2016Ingår i: Gondwana Research, ISSN 1342-937X, E-ISSN 1878-0571, Vol. 35, s. 357-377Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Abstract Cratons are conventionally assumed to be areas of long-term stability. However, whereas Precambrian basement crops out across most of the Baltic Shield, Palaeozoic and Mesozoic sediments rest on basement in southern Sweden, and thus testify to a complex history of exhumation and burial. Our synthesis of published stratigraphic landscape analysis and new apatite fission-track analysis data reveals a history involving five steps after formation of the extremely flat, Sub-Cambrian Peneplain. (1) Cambrian to Lower Triassic rocks accumulated on the peneplain, interrupted by late Carboniferous uplift and exhumation. (2) Middle Triassic uplift removed the Palaeozoic cover along the south-western margin of the shield, leading to formation of a Triassic peneplain with a predominantly flat relief followed by deposition of Upper Triassic to Lower Jurassic rocks. (3) Uplift that began during the Middle Jurassic to earliest Cretaceous caused denudation leading to deep weathering that shaped an undulating, hilly relief that was buried below Upper Cretaceous to Oligocene sediments. (4) Early Miocene uplift and erosion produced the South Småland Peneplain with scattered hills. (5) Early Pliocene uplift raised the Miocene peneplain to its present elevation leading to reexposure of the sub-Cretaceous hilly relief near the coast. Our results thus provide constraints on the magnitude and timing of episodes of deposition and removal of significant volumes of Phanerozoic rocks across the southern portion of the Baltic Shield. Late Carboniferous, Middle Triassic and mid-Jurassic events of uplift and exhumation affected wide areas beyond the Baltic Shield, and we interpret them as epeirogenic uplifts accompanying fragmentation of Pangaea, caused by accumulation of mantle heat beneath the supercontinent. Early Miocene uplift affected north-west Europe but not East Greenland, and thus likely resulted from compressive stresses from an orogeny on the Eurasian plate. Early Pliocene uplift related to changes in mantle convection and plate motion affected wide areas beyond North-East Atlantic margins.

  • 5.
    Lidmar-Bergström, Karna
    et al.
    Stockholms universitet.
    Olvmo, Mats
    Göteborgs universitet.
    Bonow, Johan M.
    Mittuniversitetet, Fakulteten för humanvetenskap, Avdelningen för turismvetenskap och geografi. Geovisiona AB, Järfälla.
    The South Swedish Dome: a key structure for identification of peneplains and conclusions on Phanerozoic tectonics of an ancient shield2017Ingår i: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 139, nr 4, s. 244-259Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The relationships between different denudation surfaces/peneplains formed across crystalline basement rocks give valuable information to the tectonic development of ancient shields. The denudation surfaces can be identified by the aid of their landforms, tilt and remnant weathering mantles in relation to cover rocks. Three types of denudation surfaces are identified across south Sweden (1) a tilted flat plain, (2) a tilted hilly surface with relative relief below 150 m and (3) stepped horizontal plains with residual hills. All three types of denudation surfaces are peneplains, denudation surfaces graded to specific base levels. The re-exposed parts of the inclined flat sub-Cambrian peneplain (SCP) extend as a landscape feature from below cover rocks in the north and east and reaches up on the highest summits of the South Swedish Uplands. The SCP (the exact unconformity) is encountered again below Cambrian covers outside the west coast. Thus south Sweden is a geological dome, the South Swedish Dome (SSD), in relation to the Cambrian cover. The southern and western low flanks of the exposed part of the dome are instead characterized by a hilly peneplain, the inclined sub-Cretaceous denudation surface, with remnants of thick, kaolinitic, clayey saprolites. This sub-Cretaceous peneplain is cut off at a distinct level in the south and west by the almost horizontal South Småland Peneplain, a never covered, epigene, peneplain. The uplift history of the SSD aids to the understanding on the development of late Tertiary drainage systems of the Baltic Basin by the Eridano River.

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