DEFINITION OF BACK ARC BASIN ACCRETION

The formation, closure, and preservation of back arc basin

sequences has proven to be a popular model for the evolution

of some greenstone belts. Paradoxically, the dominance of

buoyant subduction styles in the Archean should have led to

dominantly compressional arc systems, but many workers

suggest back arc basins (which form in extensional arcs) as a

modern analog for Archean greenstone belts.

Turkic-type accretionary orogens are large, subcontinent

size accretionary complexes built on one or two of the colliding

continents before collision, through which magmatic

arc axes have migrated, and are later displaced by strike-slip

faulting. These accretionary wedges are typically built of

belts of flysch, disrupted flysch, and mélange, and accreted

ophiolites, plateaux, and juvenile island arcs. A. M. Celal

Sengör and Boris Natal (1996) review the geology of several

Phanerozoic and Precambrian orogens and conclude that

Turkic or accretionary-type orogeny is one of the principal

builders of continental crust with time. The record of

Archean granite-greenstone terranes typically shows important

early accretionary phases followed by intrusion by arc magmatism,

possibly related to the migration of magmatic fronts

through large accretionary complexes. In examples like the

Superior Province, many subparallel belts of accreted material

are located between continental fragments that are separated

by many hundreds of kilometers and thus may represent

large accretionary complexes that formed prior to a “Turkictype”

collision. Late-stage strike-slip faulting is important in

these Archean orogens, as in the Altaids and Nipponides, and

may be partly responsible for the complexity and repetition

of belts of similar character across these orogens.

Turkic or accretionary-type of orogeny provides a good

paradigm for continental growth. These orogenic belts possess

very large sutures (up to several hundred kilometers

wide) characterized by subduction-accretion complexes and

arc-derived granitoid intrusions, similar to the Circum-

Pacific accreted terranes (e.g., Alaska, Japan). These subduction-

accretion complexes are composed of tectonically

juxtaposed fragments of island arcs, back arc basins, ocean

islands/plateaux, trench turbidites, and microcontinents.

Turkic or accretionary-type orogens may also experience

late-stage extension associated with gravitational collapse of

the orogen, especially in association with late collisional

events that thicken the crust in the internal parts of the orogen.

In the Archean, slightly higher mantle temperatures

may have reduced the possible height that mountains would

have reached before the strength of deep-seated rocks was

exceeded, so that extensional collapse would have occurred

at crustal thickness lower than those of the younger geological

record. Another important feature of these orogens is

the common occurrence of orogen parallel strike-slip fault

systems, resulting in lateral stacking and bifurcating lithological

domains. In these respects, the accretionary-type

orogeny may be considered as a unified accretionary model

for the growth of the continental crust.

Archean cratons are ubiquitously intruded by late to postkinematic

granitoid plutons, which may play a role in or be

the result of some process that has led to the stabilization or

“cratonization” of these terranes and their preservation as

continental crust. Most cratons also have a thick mantle root

or tectosphere, characterized by a refractory composition

(depleted in a basaltic component), relatively cold temperatures,

high flexural rigidity, and high shear wave velocities.

Outward growth and accretion in granite-greenstone terranes

provides a framework for the successive underplating of

the lower parts of depleted slabs of oceanic lithosphere, particularly

if some of the upper sections of oceanic crust are offscraped

and accreted, to be preserved as greenstone belts, or

eroded to form belts of graywacke turbidites. These underplated

slabs of depleted oceanic lithosphere will be cold and compositionally

buoyant compared with surrounding asthenosphere

(providing that the basalt is offscraped and not subducted and

converted to eclogite) and may contribute to the formation of

cratonic roots. One of the major differences between Archean

and younger accretionary orogens is that Archean subducted

slabs were dominantly buoyant, whereas younger slabs were

not. This may be a result of the changing igneous stratigraphy

of oceanic lithosphere, resulting from a reduction in heat flow

with time, perhaps explaining why Archean cratons have thick

roots and are relatively undeformable compared with their

younger counterparts. Geometric aspects of underplating these

slabs predict that they will trap supra-subduction mantle

wedges of more fertile and hydrated mantle, from which later

generations of basalt can be generated.

Many granites in Archean terranes appear to be associated

with crustal thickening and anatexis during late stages of

collision. However, some late-stage granitoids may be a direct

result of decompressional melting associated with uppercrustal

extensional collapse of Archean orogens thickened

beyond their limit to support thick crustal sections, as determined

by the strength of deep-seated rocks. Decompressional

melting generates basaltic melts from the trapped wedges of

fertile mantle, which intrude and partially melt the lower

crust. The melts assimilate lower crust, become more silicic in

composition, and migrate upward to solidify in the mid to

upper crust, as the late to post-kinematic granitoid suite. In

this model, the tectosphere (or mantle root) becomes less

dense (compositionally buoyant) and colder than surrounding

asthenosphere, making it a stable cratonic root that

shields the crust from further deformation.

Late-stage strike-slip faults that cut many Archean cratons

may also play an important role in craton stabilization.

Specifically the steep shear zones may provide conduits for

massive fluid remobilization and escape from the subcontinental

lithospheric mantle, which would both stabilize the

cratonic roots of the craton and initiate large-scale granite

emplacement into the mid and upper crust.

See also CHINA’S DONGWANZI OPHIOLITE; GREENSTONE

BELTS; OROGEN.

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