An orogenic experience?

(or - A Tale of Two Orogens)

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Synopsis

mountain ranges: orogens

mountain building = orogenesis

mountain building event(s) = orogeny (orogenies)

subduction-related, Andean-type mountainscollision-related, Himalayan-type mountainswhat drives plate motion?

Cordilleran-Andean systemconvergent boundary tectonism (i.e. subduction-related)

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 ranges

1

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 Paine

Peruvian 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 tectonics

Intermediate 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 Cordillera

Peru

Pacific Ocean

Subduction trench

Columbia

Venezuela

8.unknown9.unknown10.unknown11.unknown12.unknown

Rising 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 belt

Nazca 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 ridges

Bit by bit

too buoyant to subductaccrete (weld) onto continental marginorogen grows laterallynumerous exotic terranes accrete over time (>200My)thus an accretionary orogen

Hazardous

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 40km

Eruption 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?

Alpine-Himalayan systemcontinent-continent collision (Gondwana & Eurasia)

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 subduct

Crash, 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 slivers

Suture

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, Alps

The 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-1

Ta, 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 thrusting

Tethys

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 closed

N. 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 sediments

Indian Plate:

central Higher Himalaya: Precambrian gneiss & Mesozoic sediments intruded by Miocene granite batholiths which overthrust the

Trans-Himalaya

Higher

Himalaya

Lower

Himalaya

Sub-Himalaya

Himalayan geology: 3

Lower Himalaya: Precambrian-Mesozoic metasediments which overthrustSub-Himalaya: conglomerates, etc., from erosion of mountains

Trans-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 & above

Chile: 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 force

P-uuuuuush!

edge-force mechanismsridge-pushMORs are hot, buoyant and elevated wrt older ocean crustgravity causes elevated lithosphere to push on lithosphere away from ridge

Cause 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 suction

It 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.unknown

The End

of my bit!

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