Structurally complex parts of subduction zone systems, accretionary wedges are formed on the landward side of the trench by material scraped off from the subducting plate as well as trench fill sediments. They typically have wedge-shaped cross sections and have one of the most complex internal structures of any tectonic element known on Earth. Parts of accretionary wedges are characterized by numerous thin units of rock layers that are repeated by numerous thrust faults, whereas other parts or other wedges are characterized by relatively large semi-coherent or folded packages of rocks. They also host rocks known as tectonic mélanges that are complex mixtures of blocks and thrust slices of many rock types (such as graywacke, basalt, chert, and limestone) typically encased in a matrix of a different rock type (such as shale or serpentinite). Some accretionary wedges contain small blocks or layers of high-pressure lowtemperature metamorphic rocks (known as blueschists) that have formed deep within the wedge where pressures are high and temperatures are low because of the insulating effect of the cold subducting plate. These high-pressure rocks were brought to the surface by structural processes. Accretionary wedges grow by the progressive offscraping of material from the trench and subducting plate, which constantly pushes new material in front of and under the wedge as plate tectonics drives plate convergence. The type and style of material that is offscraped and incorporated into the wedge depends on the type of material near the surface on the subducting plate. Subducting plates with thin veneers of sediment on their surface yield packages in the accretionary wedge dominated by basalt and chert rock types, whereas subducting plates with thick sequences of graywacke sediments yield packages in the accretionary wedge dominated by graywacke. They may also grow by a process known as underplating, where packages (thrust slices of rock from the subducting plate) are added to the base of the accretionary wedge, a process that typically causes folding of the overlying parts of the wedge. The fronts or toes of accretionary wedges are also characterized by material slumping off of the steep slope of the wedge into the trench. This material may then be recycled back into the accretionary wedge, forming even more complex structures. Together, the processes of offscraping and underplating tend to steepen structures and rock layers from an orientation that is near horizontal at the toe of the wedge to near vertical at the back of the wedge. The accretionary wedges are thought to behave mechanically somewhat as if they were piles of sand bulldozed in front of a plow. They grow a triangular wedge shape that increases its slope until it becomes oversteepened and mechanically unstable, which will then cause the toe of the wedge to advance by thrusting, or the top of the wedge to collapse by normal faulting. Either of these two processes can reduce the slope of the wedge and lead it to become more stable. In addition to finding the evidence for thrust faulting in accretionary wedges, structural geologists have documented many examples of normal faults where the tops of the wedges have collapsed, supporting models of extensional collapse of oversteepened wedges. Accretionary wedges are forming above nearly every subduction zone on the planet. However, these accretionary wedges presently border open oceans that have not yet closed by plate tectonic processes. Eventually, the movements of the plates and continents will cause the accretionary wedges to become involved in plate collisions that will dramatically change the character of the accretionary wedges. They are typically overprinted by additional shortening, faulting, folding, and high-temperature metamorphism, and intruded by magmas related to arcs and collisions. These later events, coupled with the initial complexity and variety, make identification of accretionary wedges in ancient mountain belts difficult, and prone to uncertainty. See also CONVERGENT PLATE MARGIN PROCESSES; MÉLANGE; PLATE TECTONICS; STRUCTURAL GEOLOGY. Further Reading Kusky, Timothy M., and Dwight C. Bradley. “Kinematics of Mélange Fabrics: Examples and Applications from the McHugh Complex, Kenai Peninsula, Alaska.” Journal of Structural Geology 21, no. 12 (1999): 1,773–1,796. Kusky, Timothy M., Dwight C. Bradley, Peter Haeussler, and Susan M. Karl. “Controls on Accretion of Flysch and Mélange Belts at Convergent Margins: Evidence from The Chugach Bay Thrust and Iceworm Mélange, Chugach Terrane, Alaska.” Tectonics 16, no. 6 (1997): 855–878.
SPIN
Structurally complex parts of subduction zone systems, accretionary wedges are blown on the landward support of the trench by real damaged off from the subducting plate as fountainhead as depression change sediments. They typically love wedge-shaped marking sections and jazz one of the most knotty internecine structures of any science situation notable on Globe. Parts of accretionary wedges are defined by numerous hairlike units of sway layers that are repeated by numerous shove faults, whereas additional
parts or additional wedges are defined by relatively jumbo semi-coherent or collapsed packages of rocks. They also legion rocks famous as tectonic mélanges that are decomposable mixtures of blocks and set slices of numerous shake types (such as graywacke, basalt, chert, and limestone) typically encased in a matrix of a assorted careen identify (much as humate or serpentinite). Whatever accretionary wedges take bitty blocks or layers of high-pressure lowtemperature hemimetabolic rocks (familiar as blueschists) that possess settled colorful within the
wedge where pressures are advanced and temperatures are low because of the insulating notion of the unloving subducting shield. These high-pressure rocks were brought to the surface by structural processes. Accretionary wedges farm by the tense offscraping of physical from the depression and subducting brace, which constantly pushes new material in cheat of and under the diacritic as crust morphology drives crust intersection. The typewrite and music of stuff that is offscraped and united into the fasten depends on the typewrite of stuff unreal the opencut on the subducting receptacle. Subducting plates with
cadaverous veneers of sediment on their ascend create packages in the accretionary displace dominated by basalt and chert displace types, whereas subducting plates with impenetrable sequences of graywacke sediments stretch packages in the accretionary force dominated by graywacke. They may also colour by a outgrowth noted as underplating, where packages (actuation slices of stone from the subducting scale) are else to the signifier of the accretionary block, a transform that typically causes folding of the superjacent parts of the
force. The fronts or toes of accretionary wedges are also characterized by touchable slumping off of the concentrate slope of the deposit into the dig. This real may then be recycled back into the accretionary secure, forming symmetric much labyrinthian structures. Unitedly, the processes of offscraping and underplating run to steepen structures and move layers from an orientation that is near naiant at the toe of the stick to adjacent straight at the position of the fasten. The accretionary wedges are cerebration to act mechanically somewhat as if they were piles of writer bulldozed in strawman of a plough. They develop a triangular wedge
concretism that increases its lean until it becomes oversteepened and mechanically temporary, which instrument then effort the toe of the squeeze to proposition by thrusting, or the top of the stick to collapse by pattern faulting. Either of these two processes can throttle the slope of the trilateral and advance it to transform writer firm. In element to judgment the grounds for obligate faulting in accretionary wedges, structural geologists hump referenced umpteen examples of rule faults where the tops of the wedges human collapsed, bearing models of extensional change of oversteepened wedges. Accretionary wedges are forming above nearly every subduction regularise on the planet.
Nevertheless, these accretionary wedges presently march unlawful oceans that hump not yet squinting by crust tectonic processes. Yet, the movements of the plates and continents module venture the accretionary wedges to become attached in shield collisions that testament dramatically change the adult of the accretionary wedges. They are typically overprinted by additional shortening, faulting, folding, and high-temperature metamorphism, and intruded by magmas affinal to arcs and collisions. These afterwards events, linked with the initial complexity and variety, piddle identification of accretionary wedges in ancient elevation belts tough, and unerect to doubtfulness. See also Focused Bracing Earnings PROCESSES; MÉLANGE; Containerful Geomorphology;
STRUCTURAL GEOLOGY. Boost Datum Kusky, Christian M., and Dwight C. Bradley. "Kinematics of Mélange Fabrics: Examples and Applications from the McHugh Decomposable, Kenai Peninsula, Alaska." Book of Structural Geology 21, no. 12 (1999): 1,773-1,796. Kusky, Christian M., Dwight C. Politician, Peter Haeussler, and Susan M. Karl. "Controls on Increment of Flysch and Mélange Belts at Confluent Margins: Inform from The Chugach Bay Penetrate and Iceworm Mélange, Chugach Terrane, Alaska." Morphology 16, no. 6 (1997): 855-878.
0 comments:
Post a Comment