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When consumption of an ocean
basin is complete the complementary continental margins of the ocean will
meet and enter into 'collision' with one another. Such has happened in
the case of the Cenozoic Himalayan and Alpine mountain systems. Orogenic
systems that were considered to be 'Geosynclinal bi-couples', such as the
Alps, are therefore composed of juxtaposed continental margins, where one
or both continental margins may include arcs terranes and even obducted
ophiolite-flysch successions emplaced earlier in the tectonic evolution
of the margin. It is of interest to note in this respect that when the
suture representing the Himalyan collisional boundary is traced
eastwards, it passes into the presently active subduction slip zone passing
beneath the Indonesian arc, and eventually into the obduction boundary
between Australia and the Pacific basin. In all three cases the subduction
polarity is from south to north.
During ocean basin closure, oceanic crust, other than the obducted ophiolites preserved in foreland basins, is virtually eliminated, and collision will cause one continental margin to asymmetrically overide the facing margin. In the case of the Himalayas, the Tethys oceanic domain that was located between the Indian and Asian continental masses is only preserved as a narrow cryptic suture zone located along the length of the Indus Valley. Most of the collisional strain related to the continued northerly migration of the Indian Plate during the Cenozoic has been taken up along two major sequentially developed north dipping, intra-continental, subduction slip zones (the older Main Central thrust, and the younger Main Boundary thrust), both located entirely within the northern continental part of the Indian Plate rather than within the Indus ocean domain or the Asian Plate. The elevation of the Himalayan mountain system, which is composed of Late Proterozoic and Paleozoic shallow water marine sediments overlying Precambrian Gondwana basement, is the direct result of the increase in thickness of the Indian Plate due to the imbricate intracontinental underthrusting. The effect of the increase in thickness is however at least partly countered by elevated rates of erosion and the natural extensional collapse of the mountain system under the influence of gravitational body forces.
When traced to the east the Himalayan system connects with the Alpine mountain system of Western and Central Europe. Along the length of the collision zone the subduction polarity changes however from south-under-north to north-under-south, that is the European foreland is subducted beneath the African Gonwana foreland rather than vice versa as in the Himalayas. Subduction of the European continental margin and its cover of obducted ophiolite material involved transfer of crust to mantle depths of the order of 50 km, as is indicated by the presence of diamonds and high pressure silica polymorphs in continental rocks of the Italian Dora Maira massif, and the common tranformation of the ophiolite rocks to eclogite and blueschist assemblages. There is no clear explanation as to how or why these rocks have made their way back to the surface. Current models involve underthrusting of continental crust beneath the diamond-bearing crust, extensional elimination of the overlying mantle wedge, and tensile extension of the colliding upper continental crustal plate.
Although the apparent mirror image symmetry of the Appalachian and Cordilleran systems of North America might suggest that the Appalachian system is a one-sided orogen similar to that of the Cordillera, in reality it is a two sided system that extends from southern Chile within the Andes to Norway and Spitsbergen within the European Caledonian system. As in the case of the Himalayan - Alpine system the subduction polarity changes from Baltica (Europe) under Laurentia (North America) in Norway to Laurentia under Gondwana (Africa) in the Southern Appalachians. Minimal collision is recorded in Britain, Newfoundland, the Canadian Maritimes, and Maine, where the Laurentian and Gondwana margins are easily discernable as being separated by a central zone of oceanic arc rocks. Geophysical data suggests that the oceanic material is entirely allochthonous, but that there is little underthrusting of the complementary continental margins. The ocean is known as the `Iapetus ocean', and in Newfoundland the southeastern 'African' continental margin is known as 'Avalonia'. Florida is also part of Avalonia, and it is likely that the whole of the eastern seaboard of North America is geologically part of Africa (Gondwana). During the opening of the present Atlantic, part of Laurentia was transferred to the British Isles, whereas part of Gondwana was tranferred to the east coast of North America. The Cambrian trilobite faunas of Avalonia are typically European, whereas the faunas of the West of Ireland and Scotland are typically North American. Geological boundaries are not political boundaries!!
Erosion. Zeitler, P.K. et al., 2001. Himalayan Geodynamics, and the Geomorphology of Metamorphism. GSAToday, 11, 1, p. 4-9
Distribution of the Alpine-Himalayan collision zone (09ophloc.gif)
Continental Pangea and the Tethys ocean (13tethys.gif)
The Himalyan collision model (13himal.gif)
Exhumation of Mountain systems (13crsthk.gif)
The Alpine suture (13alps.gif)
The Ultra-High Pressure Alpine model. (J.P. Platt)
The marginal orogens of North America (13alps.gif)
The Appalachian collisional system
The Appalachian orogen (09nfdlnd.gif)
The Iapetus ocean
The Iapetus suture relative to the present day Atlantic (13iapet.gif)
The Appalachian seismic reflection model across Newfoundland.
The Pangean supercontinent and the Tethys ocean.
The Alpine Himalayan collision zone.
The Himalayan collision model.
The exhumation of mountain belts.
The Alpine and Appalachian systems.
The Newfoundland Appalachians.
The Iapetus collisional system.
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