Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 38

FIGURE 2.12 Idealized and schematic interpretation of events showing the tectonic evolution of the
Appalachians. The different locations and dip directions of all the subduction zones are not known
precisely. Although mountains do form in rifting events, geologists generally only assign names to
convergent or collisional events when significant igneous and metamorphic rocks are produced.
(A) A continent-to-continent collision in the Late Proterozoic produced the crystalline rocks of the
Grenville Orogeny. (B and C) A rift subsequently developed between the two continents and the
ancient Atlantic Ocean of the Paleozoic was formed, complete with continental shelves (geoclines)
and offshore island arcs. (D) The Taconic Orogeny resulted when island arcs collided and were
sutured onto the continent, together with geoclinal sediments. (E) The continental fragment of
Avalonia and parts of northern Europe next collided and were sutured onto the continent in the
(F) Acadian Orogeny. This was followed in the Alleghenian Orogeny (G) by collision with Africa.
(H and I) In Mesozoic time rifting took place once again, leaving part of Africa behind under what
is now Florida, and a new complex of geoclinal sediments was deposited as a continental shelf in
the newest Atlantic Ocean. That this process is continuing is demonstrated by the fact that the
Atlantic is still growing wider at the rate of ∼3 cm/yr (1.2 in./yr). (Adapted from various sources.)
Andean-Type Mountain Belt
The first stage in the development of ocean-continent convergence and formation of
an idealized Andean-type mountain cordillera occurs prior to the formation of a later
subduction zone. During this earliest stage, the continental edge exists as a passive
margin which accumulates sandstones, limestones, and shales in the shelf miogeocline
and the deeper water eugeocline. At some point the passive continental margin can be-
come active. A subduction zone then develops beneath the continental plate and gives
rise to a new magmatic arc together with a suite of associated features similar to the
Aleutian-type of island arc. This includes andesitic volcanoes of intermediate silica com-
position on the overriding plate margin. Partial melting occurs in the mantle wedge
located above the subducting plate, magma is emplaced at depth as batholiths, high-
temperature metamorphism affects the deeper surrounding rock, and an accretionary
wedge accumulates. Prolonged subduction can lead to an accretionary wedge that is
large enough to rise above sea level (Strahler 1998; Brandon et al. 1998).
Another reason for the growth of the cordilleran mountain belt is the stacking up
of fold and thrust-fault sheets on the continental or backarc side of the magmatic arc.
Slices of the mountain belt are moved landward over the continental interior or craton,
in some cases as the cold, rigid craton is pushed beneath the hot, mobile core of the
mountain belt. These active continental margins, with the landward chain of volcanoes
and the seaward accretionary wedge, are subject to extensive, long-term deformation
and metamorphism. Both Aleutian-type island arcs and Andean-type mountain belts de-
velop progressively larger size through long-continued subduction and associated sedi-
mentary, igneous, and metamorphic activities.
Collisional Mountain Ranges
Collisional mountain ranges develop from Andean-type orogenesis, and are complex be-
cause of the variety of low-density lithosphere that can be brought into the subduction
trench. Two kinds of island arc-continental collisions are possible: one where the sub-
duction dips beneath the continent and one where it dips away. Where the subduction