S339 Understanding the ContinentsObservations Modes of convergence The arcuate form of many arcs is a consequence of subducting a rigid plate into a sphere, with arc curvature decreasing as the subduction dip increases. Subduction dip changes from slab to slab, and within a single slab both along strike and down the dip of the Wadati-Benioff zone, as shown by seismic tomography data. Ocean basins grow and shrink due to variations in spreading and subduction rate. A number of tectonic configurations can esult in the end of a phase of subduction. Nature of the overriding lithosphere Crustal thickness varies across a single arc system and between different arcs, with active continental margins having thicker crust than oceanic island arcs. Arcs also develop thicker crust than corresponding 'normal' oceanic of continental crust, indicating that subduction leads to crustal growth. The overriding crust nearest the trench is thinner than under the arc, so this fore-arc region is commonly submerged. It may be a sedimentary basin or contain a variety of igneous rocks. The downgoing lithospheric slab in the Wadati-Benioff zone is relatively cold, while the mantle wedge and overlying arc are relatively hot regions. Accretionary prisms Sedimentary material accumulates at an accreting margin above a decollement surface that dips beneath the fore-arc. The sediments may be of oceanic or continental derivation. Just over half the world's convergent margins are currently accreting sedimentary material into prisms at oceanic trenches; the rest are non-accretionary. Structures observed in accretionary prisms are mainly compressional, although some extensional features also occur. The accreted material is weak, and further weakened by high pore fluid pressure in the prism, giving rise to chaotic internal structure. Volcanoes and magmatism Volcanoes at subduction zones form arcs with a well-defined front on the side facing the subducting plate. The volcanic front is situated about 110 km above the subducting slab. Arc volcanoes produce lava flows and voluminous pyroclastic deposits. In some cases, their growth can be punctuated by caldera collapse. Arc volcanoes erupt a range of magma compositions, forming a basalt-andesite-dactite association. The magmas are distinctive in containing more water, and therefore often erupting more explosively, than magmas from other plate tectonic settings. Back-arc regions Back-arc regions in both oceanic and continental settings are commonly extensional, basinal areas; the Andean back-arc is the main compressional example. Some oceanic back-arcs contain active spreading centres generating new crust, formed by splitting of an earlier volcanic arc. Magma generation in subduction zones Regions of reduced seismic wave speed in the mantl wedge identify partially molten zones where arc magmas are generated. Primitive arc basalts have major element and trace element properties that are inconsistent with an origin by partial melting of subducted basaltic oceanic crust, but are consistent with partial melting of peridotite in the mantle wedge. A series of metamorphic dehydration reactions transform subducted altered oceanic crust to anhydrous eclogite, with most water being driven off the slab in the first 100 km of descent. Water added to the mantle wedge from the slab lowers the solidus of peridotite, triggering the formation of basalt by partial melting. The mantle-normalized trace element pattern of arc basalt shows distinctive enrichments in elements that are readily transported by water-rich fluids (K, Rb, Ba) and strong depletions in Nb and Ta. Back-arc basalts have mantle-normalized trace element patterns that are transitional between those of MORB and arc basalts, reflecting generation by decompression melting of mantle that has been influenced by subduction. Sediment recycling Sediment addition to accretionary prisms occurs in three ways: frontal accretion by imbricate thrusting, underplating to the base of the prism, and prism-top sedimentation. Material may be extracted from the prism by subduction erosion. Limited exchange of material occurs across the basal decollement: either entrainment of oceanic basement into the base of the prism, or subduction erosion of basal prism sediments. Fluid pressure is generally high in accretionary prisms, weakening the sediment pile and influencing the shape of the prism as a whole. Fluid escape can form melanges, but these deposits may also result from subduction erosion and other processes. Superficially similar olistostromes are produced by distruption of soft sediments during slumping. Corner flow is unlikely to occur throughout the prism, though localized channel flow may give rise to melange deposits observed at the surface. Current models for prism mechanics are based on a layer of deformable (plastic) material sliding on a rigid base, and confined by a rigid buttress. The dynamic wedge theory predictions match most geological and geophysical observations of moderan and ancient accretionary prisms, notably the tapered geometry and the deformational response to changes in the prism taper. 10Be is an isotope that is produced by cosmic ray bombardment in the Earth's atmosphere and deposited on the surface in rain, so its presence in certain arc magmas signifies the subduction of surface sediemtn into those magma's generation zones. Terrane accretion at arc margins Mountain ranges and basins running parallel to the coasts in western Colombia reflect successive episodes of arc magmatism and transcurrent terrane accretion along strike-slip faults such as the Romeral Fault. This important lineament separates an ancient, essentially continental margin from younger oceanic terranes to the west. The docking ages of these terranes fall between the time their youngest crust formed, and the oldest age of plutons cutting the terrane boundary - or alternatively the oldest age of sedimentary material derived from across the terrane boundary after docking. Episodes of arc magmatism occurred during periods when convergence was at a relatively high angle (more than 25 deg), while terrane accretion was favoured during oblique convergence (less than 25 deg). Several lines of evidence, including narrow MgO range, flat chondrite-normalized REE patterns and an absence of Nb or Ta depletion, usggest the accreted terranes in western Colombia originated as oceanic plateaux rather than island arcs. California has an essentially tripartite geology, from east to west: the Sierra Nevada plutonic arc, the Great Valley fore-arc basin, and the Franciscan subduction complex. The latter includes fragments of ophiolitic ocean crist, deep marine sediments and trench sediments largely derived from the active margin. Within the Franciscan Complex, stratigraphic age, degree of deformation, and both metamorphic pressure and temperature, increase eastwards. This pattern matches the predictions of the dynamic wedge theory. Although the channel flow model may apply to restricted occurrences of melange, particularly in the Central Belt of the Franciscan Complex, it cannot explain most features of the overall prism. Metamorphic rocks in the Franciscan Complex indicate that subduction was long-lived (70 Ma), and some high-pressure lithologies remained in the prism for as much as 60 Ma. Blueschist and eclogite facies rocks were exhumed from depths of 30 to 60 km. The characteristic association of paired high-pressure/low-temperature and high-temperature/low-pressure belts has been used as a marker for ancient subduction zones. Such belts are very rare in the Precambrian, which some researchers believe indicates that different tectonic conditions prevailed at that time. Crustal growth and thickening at arc margins Plutonic rocks containing modal quartz, plagioclase and alkali feldspar are known as granitoids. They are classified on the basis of the relative proportions of quartz, alkali feldspar and plagioclase, illustrated on the QAP triangular diagram. Granitoids in arc settings are typically metaluminous (normative anorthite but no normative corundum), in contrast to those from continental rifts (commonly peralkaline: normative acmite) and continental collision zones (commonly peraluminous: normatie corundum). The roots of arc volcanoes, exposed in eroded arcs such as the Andes, form plutonic belts and batholiths running parallel to the ancient convergent plate margin. Arcs founded on continental crust generally develop calc-alkaline plutons, including the more silicic grainitoids such as granite and granodiorite. Tholeiitic granitoids, and less evolved rock types (gabbros, diorites, quartz diorites and quartz monzodiorites) are characteristic of eroded oceanic island arcs. Individual plutons are commonly zoned, becoming more silicic towards their centres. In any one arc, there is also a general progression towards more eveolved plutonic rock compositions with time. South of Ecuador in the Andes, huge calc-alkaline batholiths (eg the Coastal Batholith of Peru) have been emplaced since mid-Jurassic time into previously extended crust. This magmatic activity accounts for a significant amount of crustal growth in the Andes, though crustal thickening occurs partly by compressional deformation in the central Andes. Distinct gaps in the active magmatic arc correlate with regions where the downgoing oceanic plate subducts at a shallow angle. Although mantle melting is inhibited by this process, in rare cases there is evidence that the underplated basaltic material may melt instead to produce sodium-rich magmas. Estimates of the rate of crustal addition at arcs are obtained by dividing geophysically determined excess crustal volume by the duration of subduction. The average growth rate determined in this way is 30 km3Ma-1 per km of arc. For the entire globe, this rate is equal to a total growth rate of 1.1 km2yr-1. The most recent estimates of crustal growth in two oceanic arcs, however, is 60 km 3Ma-1 per km. Subduction is responsible for the most important crustal growth mechanism currently operating. Whether continental growth was more rapid at some stage in the geological past, as is widely accepted, or involved mechanisms other and subduction-related magmatism, are topics of debate. The debate focuses on the accuracy of estimates of the current cruatl growth rate and the geological evidence for subduction and other crustal growth processes in ancient rocks. The characteristic association of paired high-pressure/low-temperature and high-temperature/low-pressure metamorphic belts has been used as a marker for ancient subduction zones. Such belts are very rare in the Precambrian, which some believe indicates that diffferent tectonic conditions prevailed at that time. Back to S339 |