Global Platec

8/13/2019 Global Platec

1/60

Global Expressions of Plate Tectonics

Through Time


2/60

Plates = Crust + Lithospheric Mantle


3/60

Testing Plate Tectonics

This model is consistent with a large number of detailed

tests, including:

* sea floor spreading

* paleomagnetic paths

* locations and focal depths of earthquakes

* seismic tomography

* age structure of the continents* hotspot tracks


4/60

BATAS DIVERGEN


5/60

Tektonika Lempeng: Batas Divergen

* Gempabumi dangkal

* Terutama volkanisme basalt

* Sesar normalpenipisan kerak

Samodera:* ridgessecara volkanisme & kegempaan aktif, tetapi

tertutup lautan

Benua:

* rift valleys(Great Basin of W.US; East Africa)

* Danau basin tertutup (evaporasi)

* Sedimentasi klastik cepat


6/60

Batas

Lempeng

Divergen

kecepatan:

lambat ~ 1 cm/yr

cepat ~ 10 cm/yr


7/60

Continental

Extension

The north-south ridgesof the Basin and

Range Province of the

western US come from

crust-scale extension.

With continued

extension this could

develop into an ocean

basin.

Buy your beachfront

property in

central Utah now!

CANV

AZ


8/60

Extension in the Red Sea

RedSea

Nile R.

Gulf ofSuez

Gulf of

Aqaba

Afr ica

Arabian

Pen.


9/60

The East Africa Rift

A triple-junction is an unstable

plate join where one arm usually

fails, forming a single divergent

plate boundary.

In the not too distant future (geologically) the eastern part of

Africa will rift off, becoming a big brother for Madagascar.


10/60

Continental Leftovers -- Passive Margin

This is what the edge of the North American continent looks like

as it slopes into the Atlantic.

The normal faults are all that remain from the tearing apart of

Pangaea and the opening of the ocean, ~200 Myr ago.


11/60


12/60

0 180 Myr

Looking for old ocean crust? You can find it as far as

possible from divergent plate boundaries. Some of the

oldest borders the US East coast and the African northwest.

The Atlantic Record

O C t f th Gl b


13/60

Ocean Crust of the Globe

Where else on Earth is there old ocean crust?


14/60


15/60

Transforms exist to accommodate movement of other plate

margins on the globe of the Earth. Plate motion can never be

entirely convergent or divergent: somewhere there needs to be

places where plates slide past one another.

Hence the orientations of transforms will parallel the direction

of motion of the plates involved.

Plate Tectonics: Transform Margins

* most common in ocean crust

* abundant intense but shallow earthquakes

* essentially no volcanism

* strike-slip faulting


16/60

These boundaries, where plates slide horizontally past one

another, typically are not volcanically active.

Do you see why that is?

Transform

Margins

Transforms were the

last plate tectonic

margin to become

well understood.


17/60

Transforms became obvious when high-resolution views of the

sea floor showed their spectacular offsets of the mid-ocean ridges.

Transforms Under the

Sea


18/60

Transforms are rarely seenon land. Unfortunately for

Californians, the best

natural example is the

San Andreas Fault.

Motion on this fault will

eventually send southern

California as an island up

toward Alaska.

At a rate of ~1 cm/yr, how

long will this take?

Continental

Transforms

Divergent and Transform


19/60

Divergent and TransformMargin Earthquakes

The similarity betweenearthquakes at these kinds

of margins is that all of the

foci will be shallow

(in the upper


20/60

in this image

E-W = transform

N-S = divergent

Divergent and Transform Plate Margin

Earthquakes

On both the normal

(divergent margin)

and strike-slip

(transform margin)

faults, earthquakes

are shallow-focus.


21/60

Within-Plate

Earthquakes

Earthquakes occurring

away from active

convergent plate

boundaries are alsoalways shallow focus.

The devastating

earthquakes in Turkey

in the last few

decades, for instance,

all originate within the

shallow crust.


22/60

Plate Tectonics: Convergent Margins

* deep focus earthquakes

* seismic tomographic evidence for subduction

* positions of oceanic trenches

* island arcs

* nature and age of volcanic rocks

* orogenic belts (granite batholiths, high pressure

rocks, fold/thrust belts)

* ophiolites

* microplate terranes


23/60

Development of a Subduction Zone

C t M i

C t i


24/60

Convergent Margin

Earthquakes

Convergent margin

earthquakes have lots of

energy, given the forces

and masses involved, but

are there differences

between continental and

oceanic collisions?


25/60

The Wadati-Benioff Zone

The only place where there are deep-

focus earthquakes is at subduction

zones, where foci align on a plane called

the Wadati-Benioff zone.

S bd ti Z


26/60

sites of deep-focus

earthquakes

Kuril-Kamchatka,

eastern Russia

(ocean-ocean)

Subduction Zones

Subduction Zones


27/60

western

South

America

(ocean-

continent)

sites of deep-focus

earthquakes

Subduction Zones


28/60

Contrast: Divergent and Intraplate Earthquakes

v. Convergent Margins

Subduction


29/60

Most of the U.S.

west coast is a

transform margin,but subduction

goes on beneath

WA, OR and N. CA.

This has producedthe volcanoes of

the Cascade

Range, including

Mt. Saint Helens.

Subduction

in the

Pacific

Northwest


30/60

Note that the slab appears to penetrate deep

into the mantle, to the boundary of the core.

This image

shows the track

of a cold(seismically fast)

subducting slab

beneath NorthAmerica, a

process going

on for >140 Myr.

Seismic

Tomography

Convergent Margins


31/60

Convergent Margins

ocean crust v.

continental crust

ocean crust v.

ocean crust

continental crust v. continental crust


32/60

Convergent Margins

Of note:

island arc, oceanic trench, accretionary wedge,

age of volcanic rocks and intrusive equivalents

C M i


33/60

Convergent Marginsocean-continent convergence

Of note:

continental arc, oceanic trench, accretionary wedge,

age of volcanic rocks and intrusive equivalents,

granite batholiths, thrust fault belt

O i


34/60

Orogenic

Belts

and granite batholiths. These belts separate more ancient

blocks of cratonic crust.

Orogenyis the

process of

mountain

building by plate

collision.Orogenic belts

comprise high

grade

metamorphic

rocks, fold-and-

thrust provinces,fold and thrust belt of the

Appalachian orogen, central PA

Convergent Margins


35/60

Convergent Margins

What kinds of faults are these?

continent-continent convergence


36/60

Age Structure of theContinents

Continents are collections of material that get put together and

torn apart through time. The cratonsare the old, stable (interior)

portions of continents.

Cratons


37/60

Cratons

Cratons are the old, stable

interior portions of the

continents.

Like continents themselves,

cratons are frequently

agglomerations of old

terranes that have been

sutured together by the

accretion process through

geologic time.

In this figure orogenic belts

suture the older cratonic

blocks together.

Ophiolites


38/60

Ophiolites

An ophiolite is a fragment of the

ocean crust that has beentectonically obducted (accreted,

uplifted) onto continental crust.

Ophiolites occur as

conspicuous but narrow belts of

mantle rock in orogenic belts.

Part of the Appalachian

ophiolite belt (metamorphosed

and deformed) crops out in and

around Baltimore.

Microplate Terrane Accretion


39/60

Microplate Terrane Accretion* During plate convergence, material on the ocean floor can

be swept up and attached (accreted) to the leading edge

of a continent.

How are microplate terranes recognized?

differences in:

-- ages of rocks

-- igneous history

-- structural (deformation) history

-- fossil assemblages/paleoenvironment

-- paleomagnetism

Materials:

-- seamounts (oceanic volcanoes)

-- sediments

-- small continents & island arcs

Large Microplates


40/60

Continents grow by

adding real estate :

some chunks are big

(other continents)...

Large Microplates


41/60

Small Microplates

... some chunks are small

(seamounts, accreted

sediment, island arcs,

continental fragments, etc.).

Accreted Terranes of


42/60

Accreted Terranes of

Western North America

All of these elongate belts are

packages of ocean basin sediments,

seamounts, island arcs and small

continents that were at one time

adrift in the Pacific.

Millions of years of subduction on the

western continental margin lead to

their accretion.

Plate tectonics:

the natural empire-builder.

The New England Appalachians


43/60

The New England Appalachians


44/60

M tl Pl


45/60

Mantle Plumes

When plumes pierce continents they produce volcanic

eruptions calledflood basaltsfor their massive volumes.

Plumes originate deep in the mantle, some at the core-

mantle boundary.

Large Igneous Provinces


46/60

Large Igneous ProvincesAt various points in Earth history massive basaltic eruptions

have taken place, producing what are sometimes called

flood basalts, or large igneous provinces.

These eruptions have left lasting marks of the Earths surface

and may have had significant effects on global climate.

activity on Hawaii, except

that much greater volumesare involved and eruptions

last for much shorter time

periods.

Why these massive eruptions occur ispoorly understood, but they are

fundamentally like hot spot

Columbia River

16 Myr

Deccan Traps

65 Myr

Ontong-Java plateau

120 Myr

Hotspot Tracks: Trails of Mantle Plumes


47/60

Hotspot Tracks: Trails of Mantle Plumes

-- plumes are responsible for the only volcanic activity

not associated with plate margins

-- hotspot tracks provide an independent meansof determining the direction and velocity of tectonic plates

H t S t Hot spots do not occur exclusively


48/60

Hot Spots Hot spots do not occur exclusivelyat divergent plate boundaries.

Here is a hot spottrack that traces

back into the

South American

continent.

Similarly, the

source ofvolcanism in

Yellowstone, WY,

is a hot spot.

Hot Spots and Ridges


49/60

Hot Spots and RidgesMany hot spots are

found in close

proximity to divergentplate boundaries (as in

the case of Iceland, a

hot spot at the Mid-

Atlantic Ridge).

Plumes may play a

significant role ininitiating spreading, but

this remains difficult to

test rigorously.

Gl b l T t i


50/60

Global Tectonics

Gl b l T t i


51/60

Global Tectonics

Plate Reconstructions: Ancient Earth


52/60


Using the direction and inclination of magnetized rocks (that

can be preserved through at least 500 Myr), the paths

charted the continents can be charted and we can

reconstruct plate positions of the past.



53/60


Plate Reconstructions: 200 Myr


54/60




55/60




56/60




57/60


Plate Reconstructions: Future Earth?


58/60

Plate Reconstructions: Future Earth?

Notes: So. California

Australia

E. Africa

Plate Tectonics on Other Planets?


59/60

Plate Tectonics on Other Planets?

To determine if plate tectonics were active on other

planets, what would we look for?

Volcanic activity alone does not require plate tectonics,

but it is a sign of geological activity. Extinct volcanoes

abound on both Mars and Venus.

Plate Tectonics on


60/60

Other Planets?

Patterns of fractures andridges like these have

suggested past plate

tectonic activity.

Even so, Earth-style

plate tectonics on

other planets cannot

be verified.

Venus

Documents

Global Platec