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orogen
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An orogenic experience?
(or - A Tale of Two Orogens)
*
Synopsis
mountain ranges: orogensmountain building = orogenesis
mountain building event(s) = orogeny (orogenies)
subduction-related, Andean-type mountainscollision-related, Himalayan-type mountainswhat drives plate motion?Andean-type mountain ranges
Orogenic belts
subduction of oceanic lithosphere gives rise to 2 different features dependent on type of overriding plate:oceanic island arcs
continental linear mountain chain
formerly known as Cordilleran-type, now called Andean-type mountain ranges1
2
The awesome Andes
subduction of Nazca Plate beneath S. American Platenow 70-120 mm yr-1Colombia to Tierra del Fuego>8,000 km longAconcagua, Torres del PainePeruvian Andes
2 parallel mountain belts Eastern Cordillera = Palaeozoic: metamorphicsWestern Cordillera = Mesozoic-Cenozoic: sedimentary/igneousseparated to S. by Altiplano (plateau)subduction since at least late Triassic?compressional tectonicsIntermediate eruptions, acidic intrusions
Precambrian basement + Palaeozoic sedimentsTrias-Jur.: calc-alkaline volcanism (cf. andesite)Cretaceous-Cenozoic: Andean Coastal Batholith intruded>1,000 granite plutons injected into volcanicsform Western CordilleraPeru
Pacific Ocean
Subduction trench
Columbia
Venezuela
8.unknown9.unknown10.unknown11.unknown12.unknownRising in the east
subduction and the intrusion of batholiths:compressional tectonics causing crustal thickening
Eastern Cordillera riseto E. rocks thrust eastward to form Sub-Andean fold-thrust beltNazca Plate
S. American
Plate
Western Cordillera
Altiplano
Eastern
Cordillera
Sub-Andean
Fold-Thrust Belt
How exotic!
North American Cordillera (incl. Cascades/Rockies)differentmore complex mosaic of terranesexotic, suspect or displaced terranesvolcanic arcs, oceanic plateaux, fragments of continental crust, aseismic ridgesBit by bit
too buoyant to subductaccrete (weld) onto continental marginorogen grows laterallynumerous exotic terranes accrete over time (>200My)thus an accretionary orogenHazardous
Cascades, Wn. USAmounts Baker, Hood, Adams, Rainier, Shasta, Crater Lake, etc.subduction-related, explosive volcanismstratovolcanoesmajor volcanic hazardsash falls, lava and pyroclastic flowslahars (volcanic mud flows)8.32a.m., May 18th, 1980
eruption of Mount St. Helenslandslidelateral blastblew 540m tons of ash (~1km3) settled over 60,000km2reduced summit by 440mkilled 61 peoplelahars flowed 40kmEruption of Mount St. Helens
The ash cloud
Forests flattened by the lateral blast
The dead: car caught in lahar down Toutle River
All quiet now?
Himalayan-type mountain ranges
Closure?
subducting plate may be bordered by continental crustcontinued subduction eventual closure of oceanocean crust subducts completely: brings continental crust to trenchcontinental crust too buoyant to subductCrash, bang, wallop
continent-continent collisionrapid relative motion haltedcollisional mountain range formed by crustal shorteningfold-thrust beltssurface along which continents collided is known as a suturesuture may contain ophiolite sliversSuture
Himalaya
youngest collisional mountain rangeHimalaya usually taken as analogy for formation of all collisional mountains250-350 km wide x 3000 km long (from Afghanistan to Burma)older: Appalachians, Caledonides, Urals, AlpsThe Roof of the World
lithologic/ tectonic units parallel to mountain beltincl. oceanic crust, passive continental margin, island arc & granite batholithsevidence of complex collisional historymost of Himalayan range is part of Indian Platestill rising ~0.5-4 mm yr-1Ta, ta, Tethys
accretion of small crustal blocks to Asian margin since Palaeozoicclosure of Tethyan OceanIndia collides w. Laurasia/Eurasia ~50Ma~2000km of crustal shortening due to southward thrustingTethys
N
S
Himalayan history: detail
micro-continents of N. & S. Tibet accreted to Laurasia ~140 Ma & ~100 Ma, respectivelyGondwana breaks up, India drifts northIndia collided w. Eurasia ~50 Ma as Tethys closedN. Tibet
S. Tibet
~200 Ma
~100 Ma
Sutures and thrusts
Indus-Zangbo Suture (IZS, below) separates India & what is now Eurasia continued convergence formed Main Central Thrust (MCT) & Main Boundary Thrust (MBT) to S.Eurasian Plate
Indian Plate
Indentations
India still drifting N. at ~45 mm yr-1continued deformationIndian Plate: old, strongEurasian Plate: younger, warmer, softerindentation tectonicsmodelling using rigid block indenting plasticenemodels deformation wellincl. strike-slip faults where China/E.Asia being squeezed to E.Himalayan geology: 1
Himalayan geology: 2
Eurasian Plate:
Northern Trans-Himalaya: Cretaceous-Eocene granite batholiths intruding Palaeozoic sedimentsIndian Plate:
central Higher Himalaya: Precambrian gneiss & Mesozoic sediments intruded by Miocene granite batholiths which overthrust theTrans-Himalaya
Higher
Himalaya
Lower
Himalaya
Sub-Himalaya
Himalayan geology: 3
Lower Himalaya: Precambrian-Mesozoic metasediments which overthrustSub-Himalaya: conglomerates, etc., from erosion of mountainsTrans-Himalaya
Higher
Himalaya
Lower
Himalaya
Sub-Himalaya
8.50a.m., 8th October, 2005
Kashmir earthquake7.6 on Richter ScaleIndian-Eurasian plate boundaryhypocentre 26km depth19km NE of Muzaffarabad, Pakistani Kashmir~100,000 dead>1,000 aftershocks magnitude 4.0 & aboveChile: 3.34a.m., 27th February, 2010
Nazca-South American plate boundary, convergence 80 mm yr-1focus 115km NNE of Chile's 2nd city, Concepcin, depth ~35 km700 km rupture, slip ~10m8.8 on Richter Scale (=5th strongest ever)~300 deadtsunami of 2.5m struck along 700km of coast, killed 15 peopleimmediately N of the largest quake ever recorded: magnitude 9.5 (6,000 dead, tsunami 25 m/82 ft high even 10m high 104 km away)What drives plate motion?
Driving forces
mantle convection cellsare they mantle-wide?or in paired cells above & below MTZ?viscous drag on base of plates driven by lateral motion of mantle?this exists but not believed to be a major driving forceP-uuuuuush!
edge-force mechanismsridge-pushMORs are hot, buoyant and elevated wrt older ocean crustgravity causes elevated lithosphere to push on lithosphere away from ridgeCause or consequence?
ridge-push moves new lithosphere away from ridge axisnew asthenosphere rises up to fill gapupward movement of asthenosphere at MORs is a consequence of spreading, not the cause of it!Slab pull
slab more dense than asthenospheresinks and pulls rest of plate along behind ita more minor force is trench suctionIt all adds up
ridge-push + slab-pull (+ trench suction)+ shear force (drag) on base of plate from mantle convectionif shear is in same direction as plate movement, plate speeds upif shear in opposite direction, plate is slowed14.unknownThe End
of my bit!
So you want to be a vulcanologist?