DEFINITION OF COLLISIONS

Collisions are the final products of subduction. There are several

general varieties of collisions. They may be between

island arcs and continents, such as the Ordovician Taconic

orogeny in eastern North America, or they may be between

passive margins on one continent and an Andean margin on

another. More rarely, collisions between two convergent margins

occur above two oppositely dipping subduction zones,

with a contemporary example extant in the Molucca Sea of

Indonesia. Finally, collisions may be between two continents,

such as the ongoing India/Asia collision that is affecting much

of Asia.

Arc/continent collisions are the simplest of collisional

events. As an arc approaches a continent, the continental

margin is flexed downward by the weight of the arc, much

like a ruler pushed down over the edge of a desk. The flexure

induces a bulge a few hundred kilometers wide in front of the

active collision zone, and this bulge migrates in front of the

collision as a few-hundred-meter-high broad topographic

swell. As the arc terrane rides up onto the continent, the thick

sediments in the continental rise are typically scraped off and

progressively added to the accretionary prism, with the oldest

thrust faults being the ones closest to the arc, and progressively

younger thrust faults along the base of the prism.

Many forearc regions have ophiolitic basement, and these

ophiolites get thrust upon the continents during collision

events and are preserved in many arc/continent collisional

orogens. The accretionary wedge grows and begins to shed

olistostromes into the foredeep basin between the arc and

continent, along with flysch and distal black shales. These

three main facies migrate in front of the moving arc/accretionary

complex at a rate equal to the convergence rate and

drown any shallow water carbonate deposition. After the arc

terrane rides up the continental rise, slope, and shelf, it grinds

to a halt when isostatic (buoyancy) effects do not allow continued

convergence. At this stage, a new subduction zone

may be initiated behind the collided arc, allowing convergence

to continue between the two plates.

Continent/continent collisions are the most dramatic of

collisional events, with the current example of the convergence

and collision of Africa, Arabia, and India with Europe

and Asia, affecting much of the continental landmass of the

world. Continental collisions are associated with thickening

of continental crust and the formation of high mountains,

and deformation distributed over wide areas. The convergence

between India and Asia dramatically slowed about 38

million years ago, probably associated with initial stages of

that collision between 25 million and 40 million years ago.

The collision has resulted in the uplift of the Himalayan

mountain chain, the Tibetan plateau, and formed a wide zone

of deformation that extends well into Siberia and includes

much of southeast Asia. Since the collision, there has been

2–2.4 inches per year (5–6 cm/yr) of convergence between

India and Asia, meaning that a minimum of 775 miles (1,250

km) has had to be accommodated in the collision zone. This

convergence has been accommodated in several ways. Two

large faults between India and Asia, the Main Central thrust

and the Main Boundary thrust are estimated to have 250 and

120 miles (400 and 200 km) of displacement on them, so

they are able to account for less than half of the displacement.

Folds of the crust and general shortening and thickening

of the lithosphere may account for some of the

convergence, but not a large amount. It appears that much of

the convergence was accommodated by underthrusting of the

Indian plate beneath Tibet, and by strike-slip faulting moving

or extruding parts of Asia out of the collision zone toward

the southwest Pacific.

The Tibetan Plateau and Himalayan mountain chain are

about 375 miles (600 km) wide, with the crust beneath the

region being about 45 miles (70 km) thick, twice that of normal

continental crust. This has led to years of scientific investigation

about the mechanism of thickening. Some models

and data suggest that India is thrust under Asia for 600 kilometers,

whereas other models and data suggest that the

region has thickened by thrusting at the edges and plane

strain in the center. In either case, the base of the Tibetan

crust has been heated to the extent that partial melts are

beginning to form, and high heat flow in some rifts on the

plateau is associated with the intrusions at depth. The intru-

sions are weakening the base of the crust which is starting to

collapse under the weight of the overlying mountains, and the

entire plateau is on the verge of undergoing extension.

The collisional process is resulting in the formation of a

layered differentiated lower continental crust in Tibet with

granitic melts forming a layer that has been extracted from a

granulitic residue, along with strong deformation. These processes

are not readily observable 30 miles (50 km) beneath

Tibet, but they are preserved in many very old (generally Precambrian)

high-grade gneiss terranes around the world that

are thought to have formed in continental collision zones.

Continent/continent collision zones tend to have major

effects on global plate motions. Convergence that used to be

accommodated between the two continents must be transferred

elsewhere on the planet, since all plate motions must

sum to zero on the planet. Therefore, continental collisions

typically cause events elsewhere, such as the formation of new

subduction zones and a global reorganization of plate motions.

See also ACCRETIONARY WEDGE; CONTINENTAL CRUST;

CONVERGENT PLATE MARGIN PROCESSES; DEFORMATION OF

ROCKS; METAMORPHISM; PLATE TECTONICS; PLUTON; STRUCTURAL

GEOLOGY; VOLCANO.

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