Colisional

Dobras Falhas e Dobras Falhas e MontanhasMontanhas

Dobras e EmpurrõesDobras e Empurrões

• Enormes cadeias de montanhas se formam quando placas convergem.

• Rochas Contorcidas mostram a força da tectônica de placas.

Limites de Placas Convergentes Limites de Placas Convergentes e Dobramentoe Dobramento

Colisão Oceano-Oceano

Island Arc: Japan,Aleutians, Cent. Am.

Colisão Continente-ContinenteBelt:Alps, Himalayans, Appalachians

Evidência de Compressão LateralEvidência de Compressão Lateral

• Camadas inicialmente horizontais são dobradas, quebradas e deslocadas.

• Muitas rochas dobradas são colocadas lado a lado ou uma sobre as outras.

Arenito DobradoArenito Dobrado

Source: Martin Bond/Science Photo Library/Photo Researchers, Inc.

Estudando Falhas e DobrasEstudando Falhas e Dobras

• O ramo da geologia que estuda a deformação crustal é chamada Geologia Estrutural.

• Estruturas Geologicas.

StressStress

Unidade são Pressão: Força/Área

Três tipos de stress

a) Compressão causa dobras

b) Tensão causa afinamento

c) Cisalhamento causa falhas

Compression, Tension, Compression, Tension, and Shearing Stressand Shearing Stress

Convergente Divergente Transformante

Tipos de deformaçãoTipos de deformação

• Deformação Elastica

• Feições Rúpteis

• Deformação Plástica

Relação Relação entre entre Stress e Stress e StrainStrain

Baixa Baixa Temperatura e Temperatura e Pressão e Pressão e Sudden StressSudden Stress

Alta Temp ou Alta Temp ou PressãoPressão

Fatores afetando a deformação de Fatores afetando a deformação de rochasrochas

• Intensidade do stress aplicado

• Calor –Temperatura da rocha

• Quantidade do tempo de Stress aplicado

• Composição da Rocha

Interpretando Rochas DeformadasInterpretando Rochas Deformadas

• Mais penetrativo em rochas sedimentares

• Importância da deformação– Indica o passado do movimento das placas– Indica antigos eventos geológicos– Localização de recursos naturais

• O MAIS IMPORTANTE

• Mapeamento • : Orientação das rochas: direção e mergulho

Strike and DipStrike and Dip

DobrasDobras

• Dobras definição: Bends em camadas de rochas• Tipos: sinclinais e anticlinais Sinclinal (dobrada para baixo) Parte mais

interna são rochas mais jovens Anticlinal (Dobrada para cima) Parte mais interna dado por rochas mais antigas

Partes de uma dobras (flacos, Plano axial, eixo)

Nota: Anticlinais e sinclinais são estruturas em rochas e não superfícies da paisagem

Rochas DobradasRochas Dobradas

Source: Breck P. Kent

Anticlinais e Sinclinais

Source: Tom Bean

Lucky we have ways of recognizing right side upWhat are they?

OlderYounger

OverturnedArea

Older

Younger

Rochas dobradas antes da erosãoRochas dobradas antes da erosão

Após ErosãoApós Erosão

Topography may be opposite of Structure Topography may be opposite of Structure

AnticlineAnticline Before/After Erosion Before/After Erosion

Notice center rock oldest

Topography may be opposite of Structure Topography may be opposite of Structure

Syncline Before/After ErosionSyncline Before/After Erosion

Notice center rock youngest

Simetria de DobrasSimetria de Dobras

a) Dobras Abertas ou Simétricas

b) Dobras Assimétricas

c) Dobras Inclinadas

d) Dobras Recumbentes

e) Dobras Reclinadas

Not a good drawing, axial plane should be horizontal

Plunging FoldsPlunging Folds

Nose of anticline points direction of plunge, syncline nose in opposite direction

UpEnd Down

End

Caimento de Dobras Caimento de Dobras

Source: GEOPIC©, Earth Satellite Corporation

Interpretando DobrasInterpretando Dobras

• Determine if center rocks are older or younger than flanks: fossils, right side up clues (graded bedding and mudcracks)

• Are limbs parallel or “Nosed”?

• Determine limb dips from measurements, stream V’s. Strike and Dip

• Use nose rules for anticlines and synclines

Again: Strike and DipAgain: Strike and Dip

3-D: Domos e Bacias3-D: Domos e Bacias

FraturasFraturas

Fraturas

- Juntas: fraturas SEM movimento relativo

- Falhas: fraturas com movimento relativo

Joints: Fractures – with no movementJoints: Fractures – with no movement

Source: Martin G. Miller/Visuals Unlimited

Tipos de Falhas – Falhas de Tipos de Falhas – Falhas de deslizamento (Dip-slip faults)deslizamento (Dip-slip faults)

1) Termos: Hanging wall (capa) and footwall (lapa)

2) Falha Normal(a) Grabens(b) Horsts

3) Falha Reversaa) Baixo ângulo chamadas de empurrão ( Thrust faults)

4) Falhas Obliquas

Dip-Slip Dip-Slip FaultsFaults

Source: John S. Shelton

Normal Fault: Hanging Wall Down

KeyBed

Hanging wall overhangs the fault plane

Normal Fault (Hanging Wall down)

Reverse Fault Reverse Fault (chamada de “Thrust Fault” SE for de baixo ângulo)(chamada de “Thrust Fault” SE for de baixo ângulo)

Younger

(Hanging wall Up)

Evidências de falhasEvidências de falhas

a) Deslocamento visivel de rochas

b) Rochas Pulverizadas e “Slickensides”

c) Camadas-chaves cortadas por falhas reaparecem em qualquer lugar

Fracture Zones and SlickensidesFracture Zones and Slickensideshttp://pangea.stanford.edu/~laurent/english/research/Slickensides.gif

• Falhas transcorrentes Srike-slip faults

1) Exemplo: San Andreas Transform fault

2) Paisagem Distintiva (vales lineares, cadeias de lagos, saltos topográficos)

3) Rocha fresca pulverizada

San San Andreas Andreas FaultFault

Source: Georg Gerster/Wingstock/Comstock

Movimentos Horizontal ao longo Movimentos Horizontal ao longo Strike-Slip FaultStrike-Slip Fault

Oblique SlipOblique Slip

Also seen in Transform Faults such as San Andreas

• Strike-slip faults 1) Exemplo: Falhas transformantes de Mid-

Ocean Ridge

2) Pequeno recobrimento da cadeia

2) Falha de San Andreas é também uma cadeia de recobrimento, mas numa escala diferente

Falhas & Placas TectônicasFalhas & Placas Tectônicas

Divergence

Convergence

Transform

• Falha Normal : mid-ocean ridges e rift continental rifts são a mesma coisa.

• Margens Divergentes

– Superfície da rochas é empurrada

– Lapa das falhas é puxada para baixo

Formação de Horst e Formação de Horst e GrabenGraben

Horst and Horst and Graben FormationGraben Formation

Graben na Graben na IslândiaIslândia

Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc.

• Falhas Reversa e de empurrão : Limites de placas convergentes

• Capa é jogada para cima.

Lewis Thrust FaultLewis Thrust Fault

Lewis Thrust Fault (cont'd)Lewis Thrust Fault (cont'd)

Lewis Thrust Fault (cont'd)Lewis Thrust Fault (cont'd)

Source: Breck P. Kent

PreCambrian Limestone over Cretaceous Shales

Placas tectônicas e falhasPlacas tectônicas e falhas

• c) Strike-slip faults: Limites Transformantes

San San Andreas Andreas FaultFault

Tipos e processos de construção Tipos e processos de construção de Montanhas (Orogênese)de Montanhas (Orogênese)

1. Montanhas de Vulcões

2. Montanhas dobras e empurradas (Fold-and-thrust)

3. Montanhas de blocos falhados

4. Montanhas soerguidas

Tipos de MontanhasTipos de Montanhas

• 2. Fold-and-thrust– Formadas por colisão Continente-

Continente

Appalachian Appalachian Mountain Mountain SystemSystem

Modelo para evolução dos Modelo para evolução dos AppalachesAppalaches

Supercontinent breaks up, rifts apart.

Another rift starts moving Africa west. The ocean floor breaks and one side subducts, starting a new island arc.

AnotherRift

The ocean floor breaks again, new subduction adds volcanics to an existing microcontinent

Net westward movement pushes the ridge, subduction zone and fragment into N.AmericaRifting restarts to the East

Arc and subduction zone collide w/ N.Am., westward subduction starts

The continents collide

Rifting Restarts

Montanhas Colisionais Montanhas Colisionais (The Grand Tetons in Wyoming)(The Grand Tetons in Wyoming)

Source: Peter French/DRK Photo

Montanhas em RiftsMontanhas em Rifts

• Rift Valleys, Mid Ocean Ridges

• Provincia Basin and Range???• Blocos de falhas normais como no

Este da Africa

• Margens Divergentes?

Origem da Basin and Range Origem da Basin and Range Southwestern North AmericaSouthwestern North America

Montanhas soerguidasMontanhas soerguidas

a) Encurvamento suave sem muita deformação

b) Material do manto ascendentes

c) Distante dos limites das placas

d) Adirondack Mountains: soerguimento de rochas profundas do PreCambriano.

The Adirondack Mountains The Adirondack Mountains of Northern New Yorkof Northern New York

Source: Clyde H. Smith/Allstock/Tony Stone Images

Tectônica de Placas Tectônica de Placas Construção de Construção de

MontanhasMontanhas

• Orogênese – O processo que coletivamente produz um cinturão montanhoso

• Inclue dobramento, falhas de empurrões, metamorfismo e atividade ígnea

• Crescimento de Montanhas têm ocorrido em um passado recente da Terra

• Cadeia Alpine-Himalayan

Earth’s major mountain beltsEarth’s major mountain belts

Crescimento de Montanhas em Crescimento de Montanhas em margens convergentesmargens convergentes

• Placas tectônicas definem o modelo para orogêneses

• Zonas de subducção ativas– Arcos Vulcânicos são exemplificados pelas Ilhas

Aleutian Islands e o Arco Andino da America do Sul

Mountain Belts & Continental CrustMountain Belts & Continental CrustCinturões Orogênicos – Grupos de

cadeias de Montanhas– Tamanho e distância – 1000’s km x 100’s

km

Mountain Belts & Continental CrustMountain Belts & Continental CrustAge – younger-higher, older-lower, cores of

continents are Cratons, N.America/ Greenland = Precambrian Shield

Rock type – sedimentary, igneous, metamorphic

Mountain BeltsMountain Belts

• Mountain Belts-– Folding & Faulting – Fold and Thrust Belts

CinCinturões Orogênicosturões Orogênicos

– Metamorfismo e plutonismo (migmatitos)

– Falhamento Normal Faulting

– Espessamento de Rochas e densidade

– Atividade Tectônica

EvoluçãEvoluçãoo• Mountain Belt evolution – 3 stages

(Accumulation, Orogenic, Uplift & Block Faulting)– Accumulation stage – accumulation of thick

sequences of sedimentary or volcanic rock along passive and active continental margins

– Orogenic stage – intense deformation, & intrusion of plutons (gravitational collapse & spreading), Wilson cycle of repeating events

– Uplift & block-faulting stage – isostatic adjustment, development of fault-block mountain ranges, lithospheric delamination


• Mountain Belt evolution– Estágio Orogênico

Mountain BeltsMountain Belts• Mountain Belt evolution

– Uplift & block-faulting stage


- Acresção de terrenos Tectonostratigraficos

- (terranos suspeitos )

WHAT IS OROGENY?WHAT IS OROGENY?Processo de construção de Processo de construção de

montanhasmontanhas

Deformação Dobramento e falhas de empurrão

Metamorfismo Intrusões : batólitos etcAtividade Vulcânica

Mauna Kea

Shield volcanoHot SpotBasalt

Mauna Loa inBackground

Kilaeua is Behind MaunaLoa

KilaeuaNewest ground inThe world

Asthenosphere comingTo the surface

Composite VolcanoMt Rainier

Compressive forcesSubduction zonesAndesitic composition

Guagua Pichincha, EcuadorQuito in foregroundComposite volcanoes explosive

Normal fault

Footwall moves Up relative toHanging wall

Tension forces

FOOTWALL

HANGING WALL

Tilted fault-block range: Sierra Nevada from east,Steep side of block fault; Ansel Adams photo

Tilted Fault-blockSierra Nevada from westSide, low angle

Yosemite valley the result Of glaciation on low-anglerelief

Wasatch RangeFrom Salt Lake City

Typically fault-Block system

Grand Tetons: Another fault-block system

Alternating normal faults lead to a characteristic pattern called aHorst and Graben system. An area under tension will often haveMultiple mountain ranges as a result.

Horst and Graben Horst and Graben Landscapes (paisagem)Landscapes (paisagem)

Figure 12.14

Basin and range province: tilted fault-block mountains in Nevada.The results of a horst and graben system. Nevada is under tension Because of rising magma which is unzipping the system, all the wayFrom Baja California

Sierra Nevada and Wasatch Ranges part of this system

REVERSE FAULTS: Hanging wall moves up relative to footwallResult of compression: plates collidingTwo types: low-angle or thrust faults, and high-angle reverse faults

Individual layers can move 100’s of kilometersAlps are a great example

Thrust faults main cause Of folded mountains

Appalachian Mountains of the US

Atlas Mountains, Northern Africa

Classic folded terrain: well-developed anticline

ZAGROS MTSPERSIAN GULF

AlternatingAnticlines andSynclines

High-angle reverse faultsForms “Sawtooth Mtns”

Flatirons classic example

Sawtooth effect result ofDifferential erosion

White Cloud peak

SAWTOOTH RANGE,IDAHO

Alice Lake

COMPLEX MOUNTAINS

Tend to have a little ofEverything: volcanoes,Folds, thrust faults, normalfaults

ALPS

HIMALAYAS

ANDES:ANDES:

Classic example Classic example

ANDES: CLASSIC EXAMPLE OF GENERIC MTNS

A) Compression causes expansionB) Layered rock formedC) Thrust-faultingD) Igneous intrusions: PlutonsE) UnderplatingF) Regional metamorphism

Nazca Plate

South American Plate

ANATOMY OF AN OROGENGIC BELT

• Tipo Andino• Montanhas crescem ao longo de margens

continentais

• Estágios de desenvolviment• Margem passiva

– Margem Continental faz parte da mesma placa adjacente a crosta oceânica

– Deposição de sedimentos ao longo da plataforma continental e produzindo uma espessa cunha de sedimentos de água rasa

• Andean-type mountain building• Estágios de desenvolvimento – Margem

continental ativa– Forma em zonas de Subducção

– Inicio de processos de Deformação

– Convergência do bloco continental block e a subducção da placa oceânica leva a deformação e metamorfismo da margem continental

– Desenvolimento de Arco Vulcânico Continental

– Formação de Prisma Acrescionáio

• Acumulção caótica de rochas sedimentares e metamorficas com ocasional pedaços de crosta oceânica

• Composto de duas zonas aparentemente paralelas

• Segmento marinho

• Consiste de sedimentos dobrados, falhados e metamorfisados e fluxo vulcânico

– Arco Vulcânico

• Desenvolve sobre o Bloco continental

• Consiste de grandes corpos intrusivos misturados juntos com rochas metamórficas de alta temperatura

– Sierra Nevada batholith é um exemplo de um resto de arco vulcânico continental

• Colisão Continental• Duas placas litosféricas, ambas compostas de

crosta continental• Os Himalaias são as montanhas mais jovens

formadas pela colisão da India com a Eurasia à 45 Ma.

• Continental collisions• Os Appalaches formados entre 250 à 300

Ma resulta na colisão da América do Norte, Europa, e Africa.

• Orogenêse aqui é complexa incluindo subducção, atividade ígnea, colisão de blocos continentais, dobramento e soerguimento da crosta

• Acresção Continental e cresciemento de montanhas

• Terceiro mecanismo de orogênese• Pequenos fragmentos crustais colidem e

junta-se com a margem continental• Responsável por regiões montanhosas na

borda do Pacífico • Blocos crustais Acrescidos são chamados

TERRENOS

Colisão Continente-continenteColisão Continente-continente

Colisão inicia-se ~20 milhões de anosHimalayas são levantados a razões de 1cm/yearMovie

Le Pichon et al., 1993

Exact estimates of material present in the orogen

ESTIMATES OF MISSING CONTINENTAL MATERIALTopography and erosional levels are taken into consideration:

Dewey et al. (1986) ca 1,2 x 104 km2

Le Pichon et al (1993)Linear shortening between 1850 - 2600 kmSurface loss during the past 45 myr from 57 to 62 x 105 km2

Rate of surface loss: ≈ 1,1 x 10 km2 x 10-6yrArial deficit in sections ≈ 33 - 52 x 105 km2 (max)

18 - 30 x 105 km2 (min)(Depends on estimates of original surface elevation)

WHAT IS THE EXPLANATIONS FOR THE DEFICIT?

India Tarim

erosion

Present continentalcrust

1) LATERAL TRANSPORT OF MATERIAL

Tapponnier et al., 1982, 1986

The lateral extrusion modelFor SE Asia

FournierJolivet et al.

2) VERTICAL TRANSPORT OF MATERIAL(SUBDUCTION / EDUCTION)

LATE- TO POST-OROGENIC TECTONIC PROCESSES

THE PRESENT DEFORMATION PATTERN HAS A STRONG CORRELATION WITH TOPOGRAPHY, AND CANNOT BE EXPLAINED ONLY FROM THE PLATE-MOTIONS AND AMBIENT FORCES AFFECTING THE REGION

collision

subduction

STABLE EURASIA NORTH AMERICA

PLATE

N. CHINA

OK

PACIFIC PLATE

PHSP

S. CHINA

INDIA

AUSTRALIA

AF

Syn- to post-orogenic extension

Regional extension

Jolivet et al, 1999

Movimentos Verticais da crostaMovimentos Verticais da crosta

• Ajustamento Isostático

• Crosta menos densa flutua sobre rochas deformadas e densas do manto

• Conceito de crosta suspensa em balanço graviatacional é chamado de isostasia

O principio da of isostasiaO principio da of isostasia

Mountain building away from Mountain building away from plate marginsplate margins

• Example: the American West, extending from the Front Range of the southern Rocky Mountains across the Colorado Plateau and through the Basin and Range province

• Espessamento Crustal sugere que a diferença de elevação onde as cadeias de montanhas se encontra deve ser o resultado do fluxo do manto

• Manto quente provoca o soerguimento da cadeia e como resultado gera-se platôs e bacias.

• Colorado Plateau and the Basin and Range province

Cadeias de Montanhas geradas a Cadeias de Montanhas geradas a distância da margens de placasdistância da margens de placas

• Soerguimento associado com a Província Basin and Range province iniciou a 50 Ma e permanece até hoje.

• Nem todos os geológos que estudam na região concordam com o modelo.

• Outra hipotese sugere que os terrenos da América do Norte produz o soerguimento visto no oeste americano

TerrenosTerrenos

• Regiões da Terra Geologicamente distintos, cada qual se comporta como um bloco crustal coerente

• 10 Ma– Foreland basin connected to Atlantic along thin seaway

• Infilling of foreland basin led to formation of Amazon River from seaway

Convergent Margins: Oceanic-ContinentalConvergent Margins: Oceanic-ContinentalAndes MountainsAndes Mountains

Margens Convergentes:Margens Convergentes: Modelos ideais Modelos ideais

Two TypesA. Subduction Type

Subduction involves only one land mass

B. Collision TypeSubduction involves collision

of two land masses

Modes of Interaction1. Island arc-oceanic2. Oceanic- continental3. Island arc- continental4. Continental- continental

• Hinterland– Overriding continent

• Foreland– Continent being

overridden• Suture Zone

– Area of severe deformation and metamorphism

– The subducting lithosphere detaches, due to continental buoyancy

Historia de uma Bacia ForelandHistoria de uma Bacia Foreland

Pode esta sequência pode ser descrita como transgressiva or regressiva?

Variousstages of orogenic maturityalong strike

Andean margins

Forelandflexure

Forelandbasin

Suture(s)

Hinterland orogenic plateau

Commoninternalstructureof orogenicbelts (inspace and time)

5) Remnant stageContinental collision, suture zones, deform-ation and metamorphism, mountain buildingExtensional collapse, faulting and collapsebasins4) Terminal stageNear closure of ocean, mature arcs andback-arc, accreationary wedges, HP-LTmetamorphic complexes(Mediterranean See area)

3) Vaning stage: Intra-oceanic subductionand island arcs transition to Andean margins. (SE Asia and Western Passific)

2) Mature stage Passive margins with largeshelf-areas (Atlantic Ocean)

1) Embryonic to Young stage.Rifts to small ocean basin with sea-floor spreading. (East African rift and Red Sea)

Schematic view of stages in a classical Wilson cycle

Late - to post-orogenic tectonic processes and exhumation mechanisms (ROCKS APPROACHING THE SURFACE)

1) EROSION (MINOR ON A REGIONAL SCALE)

2) THRUST STACKING + EXTENSION AND/OR EROSION(IMPORTANT FOR BRINGING HP AND UHP ROCKS NEXT TO EACH OTHER?

3) VERTICAL CO-AXIAL SHORTENING/HORISONTALSTRETCHING

(IMPORTANT FOR MID AND LOWER CRUST AFTER EXHUMATION TO AMPHIBOLITE FACIES)

4) HINTERLAND EXTENSION FORLAND SHORTENING(IMPORTANT AT AN EARLY STAGE OF COLLISION)

5) WHOLE-SALE EXTENSION BY PLATE-DIVERGENCEand/or TRANS-TENSION (IMPORTANT)

(from: Molnar and Lyon-Caen)

Normalfault-planesolution

Reversefault-planesolution

Strike-slipfault-planesolution

Map showing major earthquake fault plane solutions and the topography in the Himalayan-TibetanRegion. Notice the strong correlation betwen altitude and contractional earthquakes. Notice also theDominant NW-SE of the principal tension axes as shown by the normal fault.plane solutions.

(Molnar and Lyon-Caen)

Horizontal projections of principal stress axes directions derived from fault-plane solutions (pink-reverse, blue-normal, green-strike-slip) in the previous figure.

AMBIENT FORCEFROM PLATE MOTION

BODY FORCE FROMTOPOGRAPHY ONTHE SURFACE AND ON LITHSPHERE

THE THERMAL EFFECT OF REMOVAL OF THICK MANTLE LITHOSPHERE

Vertical stretching/lithospheric thickening

Horizontal stretching/lithospheric thinning

Modified from: England & Platt, 1994

Crust

Lithospheric mantle

Higher geotherm leads topartial melting in the lithosphere

Convective removalof thermal boundary layer

re-equilibration and extension

Partial melting in astenosphere during decompression

Adiabatic geotherm

Conductive geotherm

From late to post orogenic tectonics incontinental collision zones to rifts

The end of a Wilson cycle does not mark the end of the tectonicThe end of a Wilson cycle does not mark the end of the tectonicactivity in a mountainbelt. In many orogenic belts high-gradeactivity in a mountainbelt. In many orogenic belts high-graderocks formed by the crustal-thickening during collision getrocks formed by the crustal-thickening during collision getquickly exhumed.quickly exhumed.

In many instances the exhumation processes are too fast to be In many instances the exhumation processes are too fast to be accounted for by erosion alone. We have to resort to tectonicaccounted for by erosion alone. We have to resort to tectonicprocesses to explain the exhumation.processes to explain the exhumation.

The geology and seismic ativity in several modern orogenic beltsThe geology and seismic ativity in several modern orogenic beltshave an intimate relationship between shortening and extension. have an intimate relationship between shortening and extension.

Some definitionsSome definitions::

ExhumationExhumation --> rocks approaching the surface. --> rocks approaching the surface.UpliftUplift --> rise of the earth´s surface with respect to --> rise of the earth´s surface with respect to

reference levelreference levelSubsidence Subsidence --> lowering of the earth´s surface with respect to --> lowering of the earth´s surface with respect to

reference levelreference level

Extension gives some easily recognizable features:Extension gives some easily recognizable features:

1)1) Thermal: Narrowing of isotherms; steep geothermThermal: Narrowing of isotherms; steep geotherm2)2) Structural: Normal faults and detachmentsStructural: Normal faults and detachments3)3) Metamorphic: Metamorphic hiatus exision across structural Metamorphic: Metamorphic hiatus exision across structural

featuresfeatures4)4) Sedimentary: Creation of accomodation space for sediments Sedimentary: Creation of accomodation space for sediments

Some definitionsSome definitions::

ExhumationExhumation --> rocks approaching the surface. --> rocks approaching the surface.UpliftUplift --> rise of the earth´s surface with respect to --> rise of the earth´s surface with respect to

reference levelreference levelSubsidence Subsidence --> lowering of the earth´s surface with respect to --> lowering of the earth´s surface with respect to

reference levelreference level

Extension gives some easily recognizable features:Extension gives some easily recognizable features:

1)1) Thermal: Narrowing of isotherms; steep geothermThermal: Narrowing of isotherms; steep geotherm2)2) Structural: Normal faults and detachmentsStructural: Normal faults and detachments3)3) Metamorphic: Metamorphic hiatus exision across structural Metamorphic: Metamorphic hiatus exision across structural

featuresfeatures4)4) Sedimentary: Creation of accomodation space for sediments Sedimentary: Creation of accomodation space for sediments

An orogenic crust will, however, not go on thickening forever An orogenic crust will, however, not go on thickening forever and the topographic elevation will reach a threshold value that and the topographic elevation will reach a threshold value that depends on the rate of convergence, the strength and density depends on the rate of convergence, the strength and density structure of the orogenic lithsophere. structure of the orogenic lithsophere.

Plateau height h ≈ 3.5 km for a convergence rate of ca 5 cm/year

If convergence continues at this rate the plateau will rise to the threshold value, and then grow in width (spread laterally as indicated by pink boxes).For the avereage height (h) to increase, we either have to • increase the rate of convergence, • increase the strength of the rocks • introduce a vertical force lifting the rocks higher, by reducing their average density so that they will float higher.

Increased topography will enhance the rate of Increased topography will enhance the rate of exhumation within the thickened crust by:exhumation within the thickened crust by:

EROSIONAL PROCESSESEROSIONAL PROCESSES• Increased topography will increase the precipitation, henceIncreased topography will increase the precipitation, hence

increase the rate of erosionincrease the rate of erosion• Increased topography will increase the slope instability, henceIncreased topography will increase the slope instability, hence

enhance landsliding and mass transportenhance landsliding and mass transport

TECTONIC PROCESSESTECTONIC PROCESSES• Extensonal and strike-slip faulting to transport material away Extensonal and strike-slip faulting to transport material away

from toptgraphically elevated areasfrom toptgraphically elevated areas

Increased topography will enhance the rate of Increased topography will enhance the rate of exhumation within the thickened crust by:exhumation within the thickened crust by:

EROSIONAL PROCESSESEROSIONAL PROCESSES• Increased topography will increase the precipitation, henceIncreased topography will increase the precipitation, hence

increase the rate of erosionincrease the rate of erosion• Increased topography will increase the slope instability, henceIncreased topography will increase the slope instability, hence

enhance landsliding and mass transportenhance landsliding and mass transport

TECTONIC PROCESSESTECTONIC PROCESSES• Extensonal and strike-slip faulting to transport material away Extensonal and strike-slip faulting to transport material away

from toptgraphically elevated areasfrom toptgraphically elevated areas

Mechanism resulting in extensional exhumation:Mechanism resulting in extensional exhumation:

1)1) Underplating and extension (critical taper)Underplating and extension (critical taper)

2)2) Slab-breakoff and orogenic collapseSlab-breakoff and orogenic collapse

3)3) Diapiric rise along density contrastsDiapiric rise along density contrasts

4)4) Subduction roll-backSubduction roll-back

5)5) Plate divergence (including transtension)Plate divergence (including transtension)

Mechanism resulting in extensional exhumation:Mechanism resulting in extensional exhumation:

1)1) Underplating and extension (critical taper)Underplating and extension (critical taper)

2)2) Slab-breakoff and orogenic collapseSlab-breakoff and orogenic collapse

3)3) Diapiric rise along density contrastsDiapiric rise along density contrasts

4)4) Subduction roll-backSubduction roll-back

5)5) Plate divergence (including transtension)Plate divergence (including transtension)

Some good actualistic examples:Some good actualistic examples:

Himalaya - Tibet plateau RegionHimalaya - Tibet plateau RegionMediterranean RegionMediterranean Region

-->Agean Sea-->Agean Sea-->Italy - Corsica section-->Italy - Corsica section-->Alboran Sea (Spain - Morocco)-->Alboran Sea (Spain - Morocco)

Some good actualistic examples:Some good actualistic examples:

Himalaya - Tibet plateau RegionHimalaya - Tibet plateau RegionMediterranean RegionMediterranean Region

-->Agean Sea-->Agean Sea-->Italy - Corsica section-->Italy - Corsica section-->Alboran Sea (Spain - Morocco)-->Alboran Sea (Spain - Morocco)

EXTENSION AT THE EXTENSION AT THE SAME TIME AS SAME TIME AS CONVERGENCE,CONVERGENCE,

SUBDUCTION ROLL-BACKSUBDUCTION ROLL-BACK

EXTENSION CHASES AFTEREXTENSION CHASES AFTERCONTRACTIONCONTRACTION

EASTWARD MIGRATION OFEASTWARD MIGRATION OFTHE EXTENSION AND THE EXTENSION AND COMPRESSION SINCE THECOMPRESSION SINCE THEEARLY TERTIARYEARLY TERTIARY

EXTENSION AT THE EXTENSION AT THE SAME TIME AS SAME TIME AS CONVERGENCE,CONVERGENCE,

SUBDUCTION ROLL-BACKSUBDUCTION ROLL-BACK

EXTENSION CHASES AFTEREXTENSION CHASES AFTERCONTRACTIONCONTRACTION

EASTWARD MIGRATION OFEASTWARD MIGRATION OFTHE EXTENSION AND THE EXTENSION AND COMPRESSION SINCE THECOMPRESSION SINCE THEEARLY TERTIARYEARLY TERTIARY

From Jolivet et al. 2004

From Jolivet et al. 2004

Late-to post Orogenic collapse

Formação de um arco de ilhas Formação de um arco de ilhas VulcânicoVulcânico

The origin and evolution of The origin and evolution of continental crustcontinental crust• There is a lack of agreement among

geologists as to the origin and evolution of continents

• Early evolution of the continents model • One proposal is that continental crust

formed early in Earth’s history

The origin and evolution of The origin and evolution of continental crustcontinental crust

• Early evolution of the continents model• Total volume of continental crust has not

changed appreciably since its origin

• Gradual evolution of the continents model • Continents have grown larger through

geologic time by the gradual accretion of material derived from the upper mantle

The origin and evolution of The origin and evolution of continental crustcontinental crust• Gradual evolution of the continents model

• Earliest continental rocks came into existence at a few isolated island arcs

• Evidence supporting the gradual evolution of the continents comes from research in regions of plate subduction, such as Japan and the western flanks of the Americas

Continents and OrogenyContinents and Orogeny• To a certain extent, the distinction between craton and mobile belt

is arbitrary, and relates only to the age since the last deformation event. It is nevertheless useful because once a mobile belt is stabilized, it can preserve details of geologic history for very long times.

Note this triple-junction here

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Continents and OrogenyContinents and Orogeny• The rocks making up orogenic belts are a combination of juvenile materials (continental arcs

have a major mantle-derived component of new crust) and reworked rocks from older terranes (either by deformation in situ or by erosion and redeposition). One can think of major continental provinces in terms of the age of deformation, rather than the age of the rocks as such (though this will often be the same). Since not all the material in a new mobile belt is new, young mobile belts can be seen to truncate and incorporate parts of older mobile belts.

Here it is again

168

Continents and OrogenyContinents and Orogeny

• Why do continents deform in a distributed fashion over wide zones? Because continental crust and lithosphere are relatively weak. And why is that? We’ll go through the long answer…

• Orogenic belts can be thousands of kilometers wide (examples: Himalaya-Tibet-Altyn Tagh system; North American cordillera), which shows that the simple plate tectonic axiom of rigid plates with sharply defined boundaries is not that useful in describing continental dynamics.– Really, rigid plate dynamics applies best to oceanic lithosphere only.

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Rheology at Plate ScaleRheology at Plate Scale

• This requires us to go into continuum mechanics, which describes how materials deform (strain) in response to applied forces (stress).

• It is possible to find clear examples where obviously weak mechanical properties of crust contribute directly to distributed deformation, as in this picture of the Zagros fold-and-thrust belt, which is full of salt (the dark spots are where the salt layers have risen as buoyant, effectively fluid blobs called diapirs or salt domes (the image is 175 km across).

• Broadly speaking, we can understand the difference between continents and oceans in this regard by considering the strength of granitic (quartz-dominated) and ultramafic (olivine-dominated) rock as functions of pressure and temperature…

170

The origin and evolution of The origin and evolution of continental crustcontinental crust

• Explanations describing the origin and evolution of the continents are highly speculative

Modern Mountain RangesModern Mountain Ranges

Major Mountain BeltsN. American Cordillera (A)Appalachians (B)Caledonian Belt (C) Andes (D) Urals (E)Himalaya (F) Alps (G)Tasman Belt (H).

http://www.physicalgeography.net/fundamentals/10k.html

Relevant Stages in the Plate Relevant Stages in the Plate Tectonic CycleTectonic Cycle

Stage A: Stable cratonStage A: Stable cratonStage B: RiftingStage B: RiftingStage D: Passive MarginStage D: Passive MarginStage F: Island Arc CollisionStage F: Island Arc CollisionStage H: Continental CollisionStage H: Continental Collision

J.T. Wilson: 1960’sJ.T. Wilson: 1960’s

- Devised this simple ideal model for - Devised this simple ideal model for categorizing plate movement.categorizing plate movement.

- Also developed concept of “hot - Also developed concept of “hot spots”, which helped resolve an spots”, which helped resolve an apparently conflicting phenomenon to apparently conflicting phenomenon to plate tectonics.plate tectonics.

Most of the following is adapted from LS Fichter’s Geol 230 course at JMU.

Divergent Margins- RiftingDivergent Margins- Rifting

• Convection cell development beneath continental crust results in thermal doming and crustal foundering.

• Axial rift graben forms, with horst terraces to either side.

GrabenHorstNormal Fault


• Sedimentary Record:– Records a transgressive sequence

A) Thick siliciclastic sediment accumulations from alluvial fansB) Quartz sand and shales from transitional environments (beach, estuaries, lagoons)C) Carbonates develop as the continental margin moves away from the heat source and tectonic stability is established.

Sediment deposition records increasingly higher sea levels.Q: In this case, is sea level transgression due to eustasy or due to

regional causes??


• Passive versus Active Margin– Passive: No tectonic activity

• Example: East Coast, US (mountain building ended 250 mya)

– Active: Tectonic activity (rifting, convergence, transform)• Example: West Coast, US


• Modern Example: Gulf of Aden, Red Sea

• movie

Guiding QuestionsGuiding Questions

• What is the Wilson Cycle?• How are passive and active margins differentiated?• What environments of deposition does rifting produce?• Does a rift sedimentary sequence indicate transgressing or regressing

sea level?• What are the two major types of convergent boundaries?• What are the four ways that lithospheric plates may interact at a

convergent boundary?• What are examples of:

– An island arc-oceanic crust type boundary?– An oceanic crustcontinental boundary?– A continent-continent boundary?

• What is a foreland basin, and how does it form?

Convergent Margins:Convergent Margins:Ideal ModelsIdeal Models




of two land masses


Convergent Margins: Island Arc- Oceanic TypeConvergent Margins: Island Arc- Oceanic Type

• Tectonic Components of a Volcanic Arc System

– Backarc

– Forearc• Zone of active

subduction

– Ocean Basin

• Normal Ocean floor= 5km• Trench=6-7km• Fractional (partial) melting

at 120km• Results in formation of

volcanic front• Approximate angle of

subduction ~25 degrees

Sedimentary Processes• Melange: A mixture of

metamorphosed sediments scraped from a subducting plate

• Immature lithic rich sediments shed from the volcanic highlands into the forearc and backarc troughs

• Sedimentary Basin:– FOREARC BASIN

• Sedimentary Review:– Lithic?

– Immature??

– Short or Long system?


• Modern Example:– Japan

Barujari Volcano,August 1994,Lombok Island,Indonesia

Pacific Plate

Eurasian Plate

Phillipine Plate

Marianas Trench: 36,000 feet below sea level


Convergent Margins:Convergent Margins:Ideal ModelsIdeal Models




of two land masses


• The previous example involved convergence of two slabs of oceanic crust.

• This example involves convergence of oceanic with continental crust.

Convergent Margins: Oceanic-ContinentalConvergent Margins: Oceanic-Continental

• Igneous Core– Plutons result from partial

melting of subducted lithosphere

– Volcanoes form, elevate crust

• Fold and Thrust Belt– Compressional forces

result in rocks that are folded and thrust over top of one another.

• Metamorphic Belt– Rocks on either side of

core are deformed by core’s heat and other processes

Convergent Margins: Oceanic-ContinentalConvergent Margins: Oceanic-ContinentalMountain Building ProcessesMountain Building Processes

• Accretionary Wedge– Marine sediments that are

pulled into the subduction trench by the downgoing plate. Includes melange.

• Forearc Basin– Describes the basin that

forms at the leading edge of subduction.

• Foreland Basin– Forms inland of the

developing mountain range, as a result of overburden from the fold and thrust belt.

– Process is called Lithospheric Flexure.

Processos de Construção de Cadeia de Processos de Construção de Cadeia de MontanhasMontanhas

• Sedimentos de uma Bacia Foreland– Molassa

• Derivada do cinturrão de dobras e empurrões

– Flysch• Filitos, turbiditos

– Inundação rápida

– Acumula Turbiditos

Deformation ProcessesDeformation Processes

• Syncline– Rocks folded concave up– Vertices at bottom

• Anticline– Rocks folded concave down– Vertices at top– “A” makes an anticline


• Folds and faulting– Increase folding– Develop overturned fold– In an overturned fold, one

limb is greater than 90 degrees from horizontal.

– Overturned fold can break– Thrusting of overlying strata

results.


• Dip– Angle that the bed forms

with the horizontal plane

• Strike– Compass direction that lies

at right angles to the dip– Always horizontal– Regional strike

• Overall trend of fold axes

• Modern Example: Andes Mountains in South America

• Longest continuous mountain chain in the world

• Subduction began during the Mesozoic (~200 mya)

• Mountain belt moving progressively inland

Pachapaqui mining area in Peru

Convergent Margins: Oceanic-ContinentalConvergent Margins: Oceanic-Continental

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