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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
Indus
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!!
Recent paper:
Erosion. Zeitler, P.K. et al., 2001. Himalayan
Geodynamics, and the Geomorphology of Metamorphism. GSAToday, 11, 1, p.
4-9
Overhead
sequence:
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.
FIGURES
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.
