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convincing evidence that great tracts of Earth's highest mountain ranges achieved their
present altitude and appearance only during the later Tertiary and Quaternary periods.
Older, tectonically quiet and more subdued orogens, such as the Palaeozoic mountains of
Appalachia-Scandinavia (the Caledonian orogeny) and the Atlas, Urals and Tien Shan
(the Hercynian orogeny), can still be recognized in continental interiors, and remnants of
ancient Archaean orogens contribute now to stable continental cratons , or shields .
Subduction processes absorb collision by converting crustal shortening into thickening
along a narrow front. Occasionally, sustained thickening and isostatic uplift elevate broad
plateaux rather than narrow orogens. The Tibetan plateau is the most celebrated example,
covering over 2 million km 2 at a mean altitude of some 5 km and with a crustal thickness
of 70-80 km. It reached its present height during the past 3-5 Ma and is instrumental in
Quaternary changes in global atmospheric circulation and the south-east Asian monsoon.
The Bolivian Altiplano,
Figure 10.11 The endogenic (tectonic) and exogenic
(geomorphic) processes which shape an orogen. A volcanic
arc has 'gone onshore' and the resultant orogen is formed by
isostatic uplift of mélange, flysch and continental slivers
around a granitic batholith. Lithospheric creep, which
reinforces uplift, creates subsidence elsewhere, and subsiding
basins trap siliciclastic sediment eroded from the orogen. B-
subduction magmatic eruptions and alpine glaciation
ornament the orogen; key as for Figure 10.8.
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