Conceptual view of the Earth evolution (Stevenson 2007)

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Geodynamics M1 2021 Cinzia G. Farnetani

[email protected]

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Conceptual view of the Earth evolution (Stevenson 2007)

Initial condition 'the cosmic heritage' : nature and origin of Earth's

constitutive material ; physics of the formation processes

Evolutionary path processes like core/mantle convection ; plate tectonics

operating on long time scales

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Hadéen 4500-3800 Ma (durée 600Ma) formation du noyau terrestre, formation de la lune, formation de la proto-croute continentale,

formation des océans d'eau liquide ....

Archéen 3800-2500 Ma (durée 1300Ma) formation de la croûte continentale, début de la tectonique des plaques

apparition de la vie, la photosynthèse libère O₂ grande oxygénation

Protérozoique 2500-550 Ma (durée 1950Ma)

Phanérozoique 550-0 Ma

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Geological activity at the Earth's surface is a consequence of convective motions in the mantle.

Mantle convection provides the energy required to move and to deform the plates at the Earth's surface.

We will see that plates are not 'passively' driven by convection but are an integral part of convection

Plate tectonics

(e.g., Hess, 1962; Runcorn, 1962 ; Vine & Matthews, 1963 ; McKenzie & Parker, 1967; Le Pichon, 1968; Morgan, 1968...)

is a successful theory, given its predictive capacity regarding : --the distribution and magnitude of earthquakes,

--seafloor ages and seafloor spreading

--the distribution of volcanic/magmatic activity

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Tectonic plates, today

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Earthquake Depth (km)

(year 2004-2005)

Earthquakes : depth and distribution

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How do we know ...

about EARTHQUAKES

Seismographic stations record the arrival of seismic waves.

Seismologists can thus calculate the

earthquake's hypocenter, its magnitude....

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The earthquake magnitude : M = 0.66 logM_sis - 6

M_sis= μ A d is the seismic moment,

μ is the shear modulus, A is the area of the rupture along the fault, d is the displacement offset between the two sides of the fault.

When magnitude M increases by 1 the energy released is multiplied by 30

Earthquakes magnitude

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World heat flow map

Average heat flow for oceans ~100 (

milli

W m

^-2

), for continents ~63 (

milli

W m

^-2

) Total heat flow : Q ~29 TW > Q ~14 TW

(milli W m^-2)

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HEAT FLUX

By measuring the temperature at different depths below the Earth's surface one can calculate the temperature gradient.

Fourier law relates the surface heat flow (W m

^-2

) to the tempetature gradient (K m

^-1

)

q = - K dT/dz

The rock thermal conductivity (W m

^-1

K

^-1

) is measured in the laboratory.

on continents (Canada)

in oceans

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Age of the ocean floor (Ma)

Present day distribution of area vs. age of the ocean floor (Coltice et al., 2012)

...we will talk again about it...

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DRILLING (forages) THE OCEAN FLOOR How do we know ...

1966-1983: Deep Sea Drilling Project (DSDP)

1983-2003: Ocean Drilling Program (ODP) 2000 drill holes throughout the world's ocean basins 2003-2013: Integrated Ocean Drilling Program (IODP) 2013-now : International Ocean Discovery Program:

Exploring the Earth Under the Sea.

Drilling enables to date and analyze rocks and also to measure the thickness of the sediments (yellow), which increases with distance from the ridge

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Continental rocks with age 0-250 Ma

continental crust continental crust below the water below the water

Age of continents

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Continental rocks with age 250-700 Ma Continental rocks with age 250-700 Ma

700 Ma 700 Ma

Age of continents

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Continental rocks with age 700-1700 Ma Continental rocks with age 700-1700 Ma

Age of continents

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Continental rocks with age 1700-2500 Ma Continental rocks with age 1700-2500 Ma

Age of continents

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Continental rocks with age 1700-3800 Ma Continental rocks with age 1700-3800 Ma Continental rocks with age 2500-3800 Ma Continental rocks with age 2500-3800 Ma

Cratons : ancient mountain belts which have been eroded Cratons : ancient mountain belts which have been eroded and form the core of a continent. Craton's name is in white and form the core of a continent. Craton's name is in white

Age of continents

SLAVE SLAVE

SUPERIOR SUPERIOR

GREENLAND

GREENLAND FENNO-FENNO- SCANDIAN SCANDIAN

SIBERIAN SIBERIAN

INDIAN INDIAN

W. AUSTRALIAN W. AUSTRALIAN TANZA-

TANZA- NIANNIAN KALAHARI KALAHARI CONGO

CONGO W. AFRICAN W. AFRICAN AMAZONIAN

AMAZONIAN WYOMING

WYOMING

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Continents and oceans :

the Earth's topography and bathymetry

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The hypsometric curbe

sea level

(a) (b)

More frequent (20.9%) altitude for continents : 0.8km More frequent (23.2%) depth for oceans : 3.7km

Mariana trench

sea level

Mt. Everest

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CRUSTAL THICKNESS (km)

[W. Mooney]

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How do we know ... Moho depth

Moho seismic discontinuity marks the base of the crust.

The underlying mantle has higher seismic velocity.

In 1909 Andrija Mohorovicic was the first to detect Pn refracted phase.

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How do we know ... ANdean COntinental Research Project

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₂

O~1%.

ρ

Cont. crust

~ 2400-2700 kg/m

³

(rhyolite-granite)

ρ

Ocenic crust

~ 2800-3200 kg/m

³

(basalt-gabbro)

THE CRUST : its composition

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Volcanic arc Ocean basin

Continent

Granite : 70-75 wt% SiO₂ ; 14 wt% Al₂O₃ ; 5-12 wt% alkalis (Na₂O, K₂O) ; 2 wt% oxydes of Fe, Mg, Ca Basalt : 45-50 wt% SiO₂ ; 14 wt% Al₂O₃

7-11 wt% FeO ; 7-10 wt% MgO 6-8 wt% CaO ;

2-5 wt% alkalis (Na₂O, K₂O) 50%plag + 25-40% pyx + 10-25%Ol At ~60 km depth basalt transforms

into eclogite, (e.g., pyroxene + garnet).

Eclogite is denser than mantle peridotite

Dunite

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Peridotite mineral assemblage

~57wt% Olivine (Mg

_0.9

Fe

_0.1

The generally accepted composition is Ringwood (1977) pyrolite

The thermal gradient is high (10-30°C/km) compared with the underlying convecting mantle (~0.3°C/km).

Mechanical perspective: 'part' of the mantle which behaves as rigid and can fracture (brittle failure), in contrast to the underlying ductile astenospheric mantle.

The thickness of the mechanical lithosphere is 1.5-2 times less than the thickness of the thermal lithosphere, and corresponds to isotherms between 400-600°C.

Seismological perspective: the bottom of the lithosphere is marked by a reduction of the seismic velocities (LVZ, low velocity zone) possibly caused by partial melts in the astenosphere.

THE LITHOSPHERE

Depth of the lithosphere-astenosphere boundary

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Thickness of the continental lithosphere

TOP : Figure from Cottrell et al., 2004

LEFT : Figure from McKenzie & Priestley, 2008 a=West African Craton, b=Angolan Craton, c=Tanzanian Craton, d=Kalahari Craton

The thickness can reach 250-280 km This ''mantle keel'' is attached to the continent and it translates with it.

Variable composition, low-density highly-refractory, depleted peridotite.

Compositional buoyancy

(Jordan,1981)

180 280

80

Depth to base of the lithosphere (km)

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Fundamental differences between continental and oceanic lithosphere

Continental lithosphere

- is compositionally heterogeneous and thick ; - its internal structure is highly variable ;

- it has evolved over Ga through repeated episodes of thermal and/or tectonic perturbations ; - over large continental areas no LVZ can be detected.

Oceanic lithosphere

- its composition is quite homogeneous ;

- few thermal perturbations and tectonic events affect the oceanic lithosphere during its short (max 200 Ma) residence time at the Erath's surface ;

- as the lithosphere moves away from the ridge, it cools and thickens by thermal diffusion ; - its thermal structure and thickness are essentially function of only one variable : age

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Thickness of the oceanic lithosphere

If cooling of the lithosphere is governed by thermal conduction, then the depth of the base of the lithosphere (y) depends on the age of the lithosphere (t) as :

y = 2.32 (κt)

^1/2

κ ⁼¹⁰

^-6

^m

²

/s thermal diffusivity. t = d/v = distance from the ridge / plate velocity

Circles : lithospheric thickness measured in the Pacific.

Lines : calculated isotherms

Leeds et al., 1974

The thickness of the oceanic lithosphere is thus variable,

a maximum value is ~ 100 km

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Depth relative to the ridge axis as the lithosphere thickens, it weights more and pulls down the surface of the ocean floor, deflecting it downward.

The subsidence of the ocean floor causes an increase of the ocean depth, proportional to

√

age (Turcotte & Oxburgh, 1967 ; Parsons & Sclater 1977 ; Stein & Stein 1992)

The heat flow out of the ocean floor is predicted to obey a 1/

√

age law.

Some discrepancies between predictions from the theory (half-space model) and observations, mainly for age > 80 Ma.

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A tectonic plate is often ''made'' of both continental and oceanic lithosphere A recent plate model : besides the 14 largest plates (Pacific, Africa,

Eurasia, Australia, Antarctica, NorthAmerica, SouthAmerica, Nazca, Arabia, Philippine, Caribbean, Cocos, India, Juan de Fuca) there are 38 small plates

Paul Bird, 2003

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Plate boundaries

1 : Spreading ridges

*

croûte océanique lithosphère océanique manteau

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GLOBAL MID-OCEAN RIDGE SYSTEM

The ridge system is ~65000 km long. Global rate of ocean crust production 3.4 km² /year.

MORB= Mid-Ocean Ridge Basalts. Oceanic crust makes up ~60% of the Earth's surface The oceanic crust : once created is transported off-axis to each side of the spreading center as it ages, it becomes progressively altered ; sediments accumulate

it is ultimately consumed at subduction zones

White & Klein, 2014

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Olive et al., 2015 White & Klein, 2014

6.2 cm/yr

A rift valley (size

~Grand Canyon) is typical of slow spreading ridges

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Solidus : for a given composition (peridotite) it gives P-T conditions at which partial melting starts.

Dashed lines (15%, 25% ....) give the % of partial melt in the rock

Liquidus : the rock is totally molten Mantle rocks rising toward the ridge undergo a pressure decrease

(decompression) and an adiabatic(*) temperature decrease.

Orange: mantle abiabat for T_p=1280°C

~appropriate for normal upper mantle Red: mantle abiabat for T_p=1480°C

~appropriate for mantle plumes Star marks the onset of melting

Decompression melting beneath a ridge

~30km

~90km

(*) adiabatic means that no energy is lost/gained during compression/decompression.

Adiabatic gradient in the mantle ~ 0.3 °C/km Adiabatic gradient in the mantle ~ 0.3 °C/km

see my next course

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Transform faults offset the ridge by hundreds of km

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The upper mantle below the ridges,

what does seismic tomography tell us ?

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The model PREM (see next lecture) provides the seismic velocity of body waves (V_P and V_S) as a function of depth

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Le temps de trajet de l'onde est identique au temps théorique : les vitesses sismiques sont identiques à PREM, tout au long du trajet.

Notion de base de tomographie sismique

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Anomalie POSITIVE

Anomalie POSITIVE Δ Δ V > 0 pour V > 0 pour Δ Δ T < 0 (zones froides) T < 0 (zones froides)

Le temps de trajet de l'onde est inferieur au temps théorique: les vitesses sismiques, quelque part le long du trajet, sont plus rapides par rapport à PREM. Anomalie Positive

Notion de base de tomographie sismique

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Anomalie POSITIVE

Anomalie POSITIVE ΔV > 0 pour Δ V > 0 pour Δ Δ T < 0 (zones froides) T < 0 (zones froides) Anomalie NEGATIVE

Anomalie NEGATIVE Δ Δ V < 0 pour V < 0 pour Δ Δ T > 0 (zones chaudes) T > 0 (zones chaudes)

Le temps de trajet de l'onde est inferieur au temps théorique: les vitesses sismiques, quelque part le long du trajet, sont plus rapides par rapport à PREM. Anomalie Positive Le temps de trajet de l'onde est superieur au temps théorique: les vitesses sismiques, quelque part le long du trajet, sont plus lentes par rapport à PREM. Anomalie Negative

Notion de base de tomographie sismique

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Shear-wave tomography by S. Grand (2002)

§ Seismic velocity anomalies below spreading ridges are a relatively shallow feature :

(a) From the surface to 100 km depth.

Spreading ridges are associated to slow shear wave velocity anomalies (b) From 250 to 325 km depth the

velocity anomaly below ridges has generally disappeared.

§ Oceanic ridges migrate and often collide with tranches (Atwater 1970).

These observations (§§) support the idea that mantle upwelling beneath ridges occurs as a 'passive' response to plate divergence.

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*

subduction de la lithosphère océanique SOUS la

lithosphère continentale

lithosphère continentale croûte

océanique croûte

continentale

Plate boundaries

2 : Subduction zones

SLAB

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GLOBAL DISTRIBUTION OF SUBDUCTION ZONES

Modified from Syracuse et al., 2010

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410 660 1000

P wave tomographic model

Profiles across the middle part of central America

Fukao and Obayashi, 2013

Fast seismic velocity anomalies are associated with zones that are colder than the surrounding

mantle.

ep D

(k th

) m

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Profiles across the southern Bonin arc

410 660 1000

ep D

(k th

) m

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Profiles across the Tonga arc

ep D (k th ) m

410

660 1000

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GLOBAL DISTRIBUTION OF ACTIVE VOLCANOES GLOBAL DISTRIBUTION OF ACTIVE VOLCANOES

ISLAND ARCS : when oceanic lithosphere subducts beneath oceanic lithosphere.

CONTINENTAL ARCS (or active continental margins) when oceanic lithosphere subducts beneath

continental lithosphere.

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Subduction zones are characterized by explosive volcanoes

1980 Mt St. Helens altitude changed from 2950 to 2549 m

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Subduction zone magmatism

§ Dehydratation of slab's crustal rocks causes hydratation of the mantle wedge

§ Increasing the fluid content (H₂O, CO₂) in the mantle wedge decreases the solidus temperature, and causes partial melting.

mantle wedge

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Hydrous melting

Melt fraction F vs. T, at P = 1 GPa

Katz et al. 2003

Colors correspond to different bulk water contents. 0 bulk wt% = anhydrous melting Possible water content in the uppermost mantle : 200 - 700 ppm = 0.02 - 0.07 wt %

Note : water content might be higher (1-2 wt %) in the transition zone (>400 km depth)

slope change marks cpx-out

5 wt% melt 10 wt% melt 15 wt% melt

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Manteau Arc magmatique Avant-arc

Magma

‘

Lithosphère

Les zones de

subduction

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Manteau Arc magmatique Avant-arc

Magma

‘

Les zones de

subduction

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Arc magmatique

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Magmatisme d’arc

5 Ma

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Magmatisme d’arc

Present

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1.6 - 0 Ma ^Quaternary 5.5 - 1.6 Ma Pliocene

11.5 - 5.5 Ma Late Miocene

16.5 - 11.5 Ma Mid. Miocene

33.0 - 16.5 Ma Early Miocene

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