Pacific-Type (= Active) Margins
53
assemblages are found on land, they are referred to as ophiolites and are mapped in
the hope of finding ancient subduction zones. It is like hunting for lost oceans on
land. Occasionally the reward is discovery of massive copper sulfides and other ores.
The mechanism allowing these ophiolites to escape subduction by vertical displacement of several kilometers ("obduction") is a matter of speculation.
The steep slopes leading into the trench are favorable for larg-scale gravitational
transport of rock masses from the land side into the subduction zone. The jumbled
masses (melange) thus generated are then sheared, and metamorphosed (i. e., baked
and cooked) under pressure (but at relatively low temperatures). Blue schists, and
subsequently amphibolites can form under these conditions.
In classic geologic literature, the sediments of trailing edges are known as miogeosynclinal, and those of collision edges as eugeosynclinal. The geosyncline part of
terms, of course, stems from the observation that the Earth's crust must have subsided
in order to accumulate the thick masses of sediment found in the mountains.
The nature of the active margins is subject of ongoing research and still holds
many surprises. The early simple concept of an origin from scraped off material left
by the downgoing slabs had to be modified. There is little transfer of material in
places and, in fact, there is "tectonic erosion", whereby portions of the margin are
swallowed by the subduction zone. This may be initiated by massive slumping into
the trench (e. g., Japan Trench), which delivers materials for building continental
roots, and for metasomatic processes within an island arc.
The role of fluids has received increased attention. Both tectonic motions (faulting, overthrusting) and chemical reactions within the accretionary prism are generally
influenced by the presence and composition of such fluids expulsed by tectonic
compaction and from dehydration reactions of commonly very high pore fluid pressures (Fig. 2.8). Gases are important, too. In the Caribbean Barbados Ridge Complex,
for example, the low-angle fault between the accretionary wedge and the underthrusting oceanic crust is greased by methane bearing fluids, which keep the wedge detached from the downgoing slab.
Special complexity is added to the subduction system by the phenomenon of
"back-arc spreading" (Fig. 2.7b). This is localized sea floor spreading, which occurs
landward of volcanic arcs, as in the Philippines or west of Guam. More than 75 % of
these marginal basins are concentrated in the Western Pacific. That extension
(necessary to let magma rise) should be associated with collision is surprising. Are
the island arcs drifting oceanward, pulled to the east by subduction?
Of course active, Pacific-type margins also are sediment traps. However, here
sediments are piled up into chaotic mixtures of various types of rocks. In addition,
one must keep in mind that enormous masses of material simply disappear deep into
the mantle. The scale of the subduction activity is difficult to imagine - the lithospheric slab now entering the Japan Trench is more than 10 000 km long! At present
rates, it will vanish in about 100 million years.
53
assemblages are found on land, they are referred to as ophiolites and are mapped in
the hope of finding ancient subduction zones. It is like hunting for lost oceans on
land. Occasionally the reward is discovery of massive copper sulfides and other ores.
The mechanism allowing these ophiolites to escape subduction by vertical displacement of several kilometers ("obduction") is a matter of speculation.
The steep slopes leading into the trench are favorable for larg-scale gravitational
transport of rock masses from the land side into the subduction zone. The jumbled
masses (melange) thus generated are then sheared, and metamorphosed (i. e., baked
and cooked) under pressure (but at relatively low temperatures). Blue schists, and
subsequently amphibolites can form under these conditions.
In classic geologic literature, the sediments of trailing edges are known as miogeosynclinal, and those of collision edges as eugeosynclinal. The geosyncline part of
terms, of course, stems from the observation that the Earth's crust must have subsided
in order to accumulate the thick masses of sediment found in the mountains.
The nature of the active margins is subject of ongoing research and still holds
many surprises. The early simple concept of an origin from scraped off material left
by the downgoing slabs had to be modified. There is little transfer of material in
places and, in fact, there is "tectonic erosion", whereby portions of the margin are
swallowed by the subduction zone. This may be initiated by massive slumping into
the trench (e. g., Japan Trench), which delivers materials for building continental
roots, and for metasomatic processes within an island arc.
The role of fluids has received increased attention. Both tectonic motions (faulting, overthrusting) and chemical reactions within the accretionary prism are generally
influenced by the presence and composition of such fluids expulsed by tectonic
compaction and from dehydration reactions of commonly very high pore fluid pressures (Fig. 2.8). Gases are important, too. In the Caribbean Barbados Ridge Complex,
for example, the low-angle fault between the accretionary wedge and the underthrusting oceanic crust is greased by methane bearing fluids, which keep the wedge detached from the downgoing slab.
Special complexity is added to the subduction system by the phenomenon of
"back-arc spreading" (Fig. 2.7b). This is localized sea floor spreading, which occurs
landward of volcanic arcs, as in the Philippines or west of Guam. More than 75 % of
these marginal basins are concentrated in the Western Pacific. That extension
(necessary to let magma rise) should be associated with collision is surprising. Are
the island arcs drifting oceanward, pulled to the east by subduction?
Of course active, Pacific-type margins also are sediment traps. However, here
sediments are piled up into chaotic mixtures of various types of rocks. In addition,
one must keep in mind that enormous masses of material simply disappear deep into
the mantle. The scale of the subduction activity is difficult to imagine - the lithospheric slab now entering the Japan Trench is more than 10 000 km long! At present
rates, it will vanish in about 100 million years.