GEOS 1004 - Convergent Boundaries

The Composition of North America: ?

mountain belts, orogens, cratons

Orogens = ?

old mountain belts

Shields = ?

orogens and cratons (very old mountain belts)

What are Platforms?

veneers overlying shields

Hadean Period: ?

4.6 to 4.1 billion years ago

Subduction produces…

melting and arc of volcanic islands

Weathering of islands produces…

Si & Al rich sediments

What is a Zircon?

dense, hard, low diffusion; preserves history

Oldest examples of Hadean Zircons are from…

Western Australia

Zircons weathered out of original rock —> ?

older than the rock (due to the principal of inclusion)

Implications of a > 3 billion year old zircon?

granite formation > 3 billion years old

Implications of a > 3 billion year old conglomerate?

weathering, erosion, transportation, and deposition of sediments in liquid water > 3 billion years old

Implications of a > 3 billion year old metamorphic rock?

plate tectonics are at least that old

Earth in the Archean Period: ?

rise of the continents; continental rocks drastically increase; fragments coalesce into protocontinents

What are Protocontinents?

relatively small masses of continental rock; end up combining to form small continents

Mountains generated by subduction: ?

volcanic island arcs (produced by melting) and continental volcanic arcs

Mountains generated by collision (most obvious kind of mountain building): ?

two fold-thrust belts; distinct suture (boundary between rocks of one continent and rocks of another continent)

Accretionary Wedges / Prisms: ?

as a plate subducts, overlying sediments can be scraped off; near continents because of more sediments

Paired Mountain Ranges: ?

forearc sediments compress into coast ranges (when subduction ceases, erosion uncovers arc batholiths)

As the subduction of oceanic lithosphere comes to a halt…

there is a period of uplift

How can Convergence create Divergence?

old, cold plate subducts and sinks —> trench retreats seaward; the other plate is pulled along —> basin forms behind the arc as the spreading continues

Collisional Mountains - Large: ?

entire continents colliding (Himalayas); distinct boundary (suture)

Collisional Mountains - Small: ?

smaller masses (islands, microcontinents) can accrete to continents; series of unrelated pieces (suspect terranes) piled up at the edge

Mountains From Divergence: ?

generated by normal faulting; resulting basin and range topography (basins are low areas that fill with sediment; ranges are exposed fault-bounded blocks; rift margins rise to form mountains)

Fault Block Mountains: ?

the valley has dropped below the level of the mountain range

True or False: Crustal root is less dense than the mantle.

true

Isostasy: ?

the crust floats on the mantle in gravitational balance (higher peaks = deeper roots; lower peaks = more shallow roots)

As mountains erode…

the crust rebounds (the mass is shifted to one side or another or both) and the root of the mountain starts moving up

Epeirogeny: ?

vertical crustal movement without broad deformation (no mountain building - contrasts with orogeny)

Cratons (former mountains): ?

older continental crust where orogeny has ceased; not necessarily flat or level; strata thickens in the basin; thin sediments in the dome region

The Wilson Cycle (cyclicity): ?

periodic rifting and collision of individual continents

Stages of the Wilson Cycle: ?

Stage A: stable craton; Stage B: early rifting; Stage C: full ocean basin; Stage D: subduction zone; Stage E: closing remnant ocean basin; Stage F: collision orogeny; Stage G: peneplained mountain

North America in the Proterozoic Era: ?

rise of the supercontinent

What are Supercontinents?

the majority of continental masses in one assemblage (current definition: at least 75% of then existing continental crust)

First Consensus of a Supercontinent: ?

Rodinia (began assembly about 1.3 billion years ago) - last major addition: Grenville Orogeny

Breakup of Rodinia began about…

700 million years ago

Midcontinent Rift (Keewanawan Rift): ?

began about 1.1 billion years ago; hypothesized that the Grenville Orogeny to the east acted as a brake

Reelfoot Rift & Rough Creek Graben: ?

began about 700 million years ago; best known for New Madrid Seismic Zone

Supercontinent Cycle: ?

analogous to Wilson Cycle for continents rifting and reforming

North America in the Phanerozoic Era: ?

the appearance of the modern

Paleozoic Supercontinent: ?

Pangaea (the most recent, so the best current understanding)

Taconic Orogeny (Appalachian Story): ?

Around 450 mya, the marginal sea between the volcanic island arc and North America closed. The collision, called the Taconic Orogeny, thrust the island arc over the eastern margin of North America.

Acadian Orogeny (Appalachian Story): ?

A second episode of mountain building that occurred about 350 mya and involved the collision of a microcontinent with North America.

Alleghanian Orogeny (Appalachian Story): ?

The final event, which occurred between 250 and 300 mya, when Africa collided with North America. The result was the formation of the Appalachian Mountains.

About 40 mya… (Rocky Mountain Story)

began with subduction - Pacific plate was subducted underneath North America; because there was compression, there was a series of thrust faults; because the plate was warm, it stayed relatively close to the surface, which helped create mountain ranges in the west

About 30 mya… (Rocky Mountain Story)

it changes from subduction to a transform boundary

About 20 mya… (Rocky Mountain Story)

San Andreas Fault is still moving, and there is now uplift and extension between the Sierra Nevada Mountains and the Rocky Mountains; this uplift and extension created the topography of the Basin and Range Province