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branches, defining the Cocos and Nazca oceanic plates, are much nearer the American
continental subduction zone. Convex arc shapes can be seen in the coastlines of British
Columbia, Central America and Ecuador-Peru, completing the Pacific Ring of Fire, with
the trench a short distance offshore. The Sunda arc forming the Indonesian islands
between Malaysia and northern Australia represents a transitional phase, present on a
smaller scale in South Island, New Zealand.
During convergence, B-subduction magmas erupt through continental crust as
terrestrial volcanoes. Recent major eruptions along the Pacific coast of the Americas
include Katmaï (erupted 1912) and Mount St Helens (1980) in North America and El
Chichón (1982) and Nevado del Ruiz (1985) in Central and South America. Magma
depleted of its more volatile components also crystallizes as huge granitic batholiths ,
intruded at depth from deep magma reservoirs. More than 2 million km 3 of granite
intruded in the roots of the Andes now lie exposed by erosion over a surface area of
nearly 500,000 km 2 .
In due course, subduction of remnant oceanic crust marks the death of the ocean and
leads eventually to collision between converging continental plates. Despite the
acquisition of denser rocks, lighter continental crust remains buoyant and influences the
development of different continental subduction processes. Convergence rates fall
sharply along the intracontinental suture but driving forces are still sufficient to cause
crustal shortening. Since neither slab of lightweight continental crust is capable of
significant B-subduction, crustal shortening must be compensated by crustal thickening .
This is achieved by complex thrusting of slivers of crust into each other which does not
proceed to sufficient depths/temperatures for crustal recycling (Figure 10.10). Instead,
downward displacement of light crust, known as Ampferer - or A - subduction , is
compensated by isostatic elevation of the developing pile to form thick continental plate.
Basal material may extend deep enough to experience remagmatization at moderate
temperatures to form granitic cores. Intercontinental collision tectonics in the suture zone
are active along almost the entire Alpine- Himalayan systems, also known collectively as
the Tethyan orogen , after the ocean which spawned them.
Transform faulting is a variation of general subduction. Different rates and directions
of ocean spreading are transmitted onshore as strike - slip faulting, with horizontal
displacement along the strike or fault axis. Plates may move past each other with nothing
more than seismic effects at their conservative margin, or with oblique subduction or
spreading. This occurs at the plate boundary itself or,
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