DEFINITION OF accretionary wedge Structurally complex parts of subductionzone systems, accretionary wedges are formed on thelandward side of the trench by material scraped off from thesubducting plate as well as trench fill sediments. They typicallyhave wedge-shaped cross sections and have one of the mostcomplex internal structures of any tectonic element knownon Earth. Parts of accretionary wedges are characterized bynumerous thin units of rock layers that are repeated by numerous thrust faults, whereas other parts or other wedgesare characterized by relatively large semi-coherent or foldedpackages of rocks. They also host rocks known as tectonicmélanges that are complex mixtures of blocks and thrustslices of many rock types (such as graywacke, basalt, chert,and limestone) typically encased in a matrix of a differentrock type (such as shale or serpentinite). Some accretionarywedges contain small blocks or layers of high-pressure lowtemperaturemetamorphic rocks (known as blueschists) thathave formed deep within the wedge where pressures are highand temperatures are low because of the insulating effect ofthe cold subducting plate. These high-pressure rocks werebrought to the surface by structural processes.Accretionary wedges grow by the progressive offscrapingof material from the trench and subducting plate, which constantlypushes new material in front of and under the wedgeas plate tectonics drives plate convergence. The type and styleof material that is offscraped and incorporated into thewedge depends on the type of material near the surface onthe subducting plate. Subducting plates with thin veneers ofsediment on their surface yield packages in the accretionarywedge dominated by basalt and chert rock types, whereassubducting plates with thick sequences of graywacke sedimentsyield packages in the accretionary wedge dominated bygraywacke. They may also grow by a process known asunderplating, where packages (thrust slices of rock from thesubducting plate) are added to the base of the accretionarywedge, a process that typically causes folding of the overlyingparts of the wedge. The fronts or toes of accretionary wedgesare also characterized by material slumping off of the steepslope of the wedge into the trench. This material may then berecycled back into the accretionary wedge, forming evenmore complex structures. Together, the processes of offscrapingand underplating tend to steepen structures and rock layersfrom an orientation that is near horizontal at the toe ofthe wedge to near vertical at the back of the wedge.The accretionary wedges are thought to behave mechanicallysomewhat as if they were piles of sand bulldozed infront of a plow. They grow a triangular wedge shape thatincreases its slope until it becomes oversteepened andmechanically unstable, which will then cause the toe of thewedge to advance by thrusting, or the top of the wedge tocollapse by normal faulting. Either of these two processes canreduce the slope of the wedge and lead it to become more stable.In addition to finding the evidence for thrust faulting inaccretionary wedges, structural geologists have documentedmany examples of normal faults where the tops of the wedgeshave collapsed, supporting models of extensional collapse ofoversteepened wedges.Accretionary wedges are forming above nearly everysubduction zone on the planet. However, these accretionarywedges presently border open oceans that have not yet closedby plate tectonic processes. Eventually, the movements of theplates and continents will cause the accretionary wedges tobecome involved in plate collisions that will dramaticallychange the character of the accretionary wedges. They aretypically overprinted by additional shortening, faulting, folding,and high-temperature metamorphism, and intruded bymagmas related to arcs and collisions. These later events,coupled with the initial complexity and variety, make identificationof accretionary wedges in ancient mountain belts difficult,and prone to uncertainty.See also CONVERGENT PLATE MARGIN PROCESSES;MÉLANGE; PLATE TECTONICS; STRUCTURAL GEOLOGY.
Tidak ada komentar:
Posting Komentar
Catatan: Hanya anggota dari blog ini yang dapat mengirim komentar.