11.6 Collisional Mountain Belts

How are most major mountain belts generated?

1 or more buoyant crustal fragments collide with continental margin b/c of subduction

What happens when a crustal fragment arrives at a trench? Why?

collision between 2 continental blocks because too buoyant to subduct

Cordilleran-Type Mountain Building

collisional, subduction zones form island arcs that collide with continental crust

Cordilleran-Type mountain building is associated with which ocean? What does it mean for that ocean?

Pacific Ocean, may never close: rapid spreading balanced by high subduction rate

How are mountains formed under Cordilleran-type mountain building?

collision & accretion of small slivers of continental crust onto an active continental margin

Terranes

crustal fragments of exotic material, accreted blocks of crust

How are terranes transported?

plate tectonics

How are terranes and terrain different?

terranes = accreted foreign blocks

terrain = lay of the land

Where are terranes common in the US?

western US

What were terranes before they were accreted?

microcontinents, island arcs, submerged oceanic plateaus from basaltic lava

Microcontinents

small fragments of continental crust, like Madagascar

What happens to small structures compared to large structures when they encounter a subduction zone?

small features get subducted along descending plate (like seamounts)

large, buoyant features collide & accrete onto the continental margin

Describe how accretion of continental fragments occurs

upper crustal layers peeled from descending plate

thrust in thin sheets onto adjacent continental block

new subduction zone forms seaward of accreted terrane

each collision displaces earlier accreted terranes further inland

Does convergence/subduction end with accretion of a crustal fragment?

no, a new subduction zone forms seaward of accreted terrane

What is the effect of multiple accretions?

earlier terranes displaced further inland, building thickness of continental margin

The correlation between mountain-building and accretionary wedges was developed at the…

north american cordillera

Where were the terranes that make up the North American Cordillera before being accreted?

eastern Pacific

How did the North American Cordillera get terranes accreted to the continent?

breakup of Pangaea caused subduction of the Farallon plate beneath west coast

additional crustal fragments along the Pacific Margin

piecemeal addition

Alpine-Type Mountain Building

continental collisions

Which mountain belts were formed by the closures of major ocean basins?

himalayas, appalachians, urals, alps

How do continental collisions affect the crust?

laterally shorten, vertically thicken

Suture

zone where 2 continents collide & are welded together, or between 2 accreted terranes

Ophiolites

oceanic lithosphere trapped between colliding plates, identifies collision boundary

Why are ophiolites present in sutures?

they were oceanic lithosphere in the basin that collision closed

Fold & Thrust Belt

deformation of thick sequence of sedimentary rocks

found in compressional mountains

When did the Himalayan mountain building episode begin?

50 million years ago, alpine collision type

What collision formed the Himalayas?

India collided with Asia after the oceanic lithosphere had subducted

Describe the Himalayan collision:

India was located between Africa & Antarctica

started moving rapidly north

subduction zone: created Andean-type plate margin, continental volcanic arc & accretionary wedge

India’s north margin = passive continental

What happened before India collided with Asia?

accretion of smaller crustal fragments

Describe what occurred during the Himalayan collision:

high folding & faulting

major thrust faults, stacked Indian crust atop itself

tallest peaks capped with tropical marine limestones (from continental shelf)

Why is the Tibetan Plateau elevated yet flat?

uplift from Indian subcontinent thrusting under Tibet

After the collision, did India stop moving into Asia?

no, continued moving northward

Escape Tectonics

lateral displacement of Asian crust, parts of Asia squeezed east as India continued north

escaping the impact of collision

Why was the interior of Asia more deformed than India?

Precambrian continental shield of India mechanically strong, resists deformation

young accreted crustal fragments of Asia weaker, warmer, more deformation

What mountains do Appalachians share a similar origin with?

British Isles, Scandinavia, northwest Africa, Greenland

What generated the mountain system that the Appalachians were a part of?

3 main orogenic events that resulted in Pangae formation

What happened to the Rodinia supercontinent?

continental rifting and seafloor spreading

creation of new ocean basin

created a microcontinent

What was the stage for the 3 orogenic events to form the Appalachians?

ancient ocean basin began to close, forming multiple subduction zones

Describe the first step in Appalachian orogenies:

600 mil years ago, ocean basin closing

microcontinent and volcanic arc within the ocean basin

Taconic Orogeny of the Appalachians

volcanic arc thrusted/accreted over North America 450 million years ago

volcanic rocks & marine sedimentary rocks metamorphosed & added

Acadian Orogeny of the Appalachians

ocean basin continued to close, microcontinent collided with NA 350 mil years ago

thrust faults, metamorphism, and granite intrusions

added to width of NA

Alleghanian Orogeny of the Appalachians

Africa collided with North America 250-300 million years ago

earlier accreted material displaced 250 km inland

deformed continental shelf

folded & thrust faulted sandstones, limestones, shales

Breakup of Pangaea

began rifting 180 mil years ago

rift eastward of the suture

piece of Africa welded to NA

How did the Alps form?

Africa & crustal frags collided with Europe, closing of Tethys Sea

How did the Urals form?

assembly of Pangaea, Europe & northern Asia collided, did not break apart