aula 4 - Tectonica de Placas -rifting

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divergente: CadeiasMeso Ocenicas

II. Tipos de limites de placas

convergente: Zonascolisionais Arcosvulcnicos Transformante:FalhadeSan Andreas FalhaAlpine,N.Z.

A. Limites Divergentes Usualmenteiniciasedentrode

continentescomohotspot atravessandoacrostae eventualmentecresceparaformar umabaciaocenica MidOceanRidges(MOR) ContinentalRiftValleys

Doistiposdemargensdivergentes

Feies de Dorsais Meso-OcenicasValedeaberturacentral(tamanho inversamenteproporcionalarazode expanso) Regiodealtofluxodecalor Terremotosrasos Vulcanismobasalticoquaseexclusivo

Terremotos associados com margens Divergentes

Rifting e expanso do fundo ocenico ao longo da cadeia Meso AtlnticaPeter W. Sloss, NOAA-NESDIS-NGDC

Rifting and Seafloor Spreading

EastAfrica,RioGranderift

Feies de Rift Continental

Inciodaformaodeumoceano(pode noevoluirtanto) Riftinginiciasecomumajunotrplice (Doiscentrosdeexpanspsejuntam paraformarumabaciaocenica). TiposdeRochas:basaltosearenitos

Hot Spot induz rifting Hot spot atravessa a crosta -> 3 braos de um rift Divergncia inicia-se 2 braos so ativos 1 brao no ativo Oceano Linear se desenvolve

Insero de um Rifting alongo do East African Rift System

Peter W. Sloss, NOAA-NESDIS-NGDC

Insero de um Rifting dentro de um Continente

Nile Delta

Gulf of Suez

Gulf of Aqaba

Red SeaEarth Satellite Corp.

O Golfo da California Formado pelo Rifting da Baja California para o MexicoWorldsat International/Photo Researchers

Rifte Falhas normais que delimitam bloco central abatido em relao aos blocos laterais Tectnica extensional

Mar Vermelho proto oceano

rea da Terra tem superfcie constante

Cadeia meso-ocenica Rifte central Cadeia de vulces Formao de crosta ocenica Descoberto pelas anomalias magnticas Terremotos rasos

Lavas almofadadas

Suite de Ofiolito Idealizado- Crosta ocenicaSedimentos de Mar Profundo

Pillow basaltos

Gabbro

Peridotito

Modelo para Formao de crosta ocenica em Cadeias Meso-Ocenicas

A crescimento de bacia ocenica 1

Idade de Bacias Ocenicas

After map by Sclater & Meinke

Cadeias Meso-Ocenicas Characterizadas por; Uma cadeia de montanhas Falhamentos normais espaados Fluxo Mantlico abaixo da crosta Basalto de fundo ocenico so formados preenchendo as fraturashttp://rblewis.net/technology/EDU506/WebQuests/quake/normalfault.gif

Bathymetry of the Atlantic Ocean

Abyssal Plain Passive Margin MOR

Abyssal Plain Passive Margin

Fundo Ocenico formado nas Dorsais Formando por camadas de rochas basalticas saindo do meio da cadeia. A expanso do fundo ocenico ocorre em zonas estreitas, chamadas de rift zones, localizadas na crista de cadeias ocenicas. Percolao fludos ao longo fraturas geram nova litosfera ocenica (crosta jovem). Sedimentos so mais espessos no eixo da cadeia

Nova litosfera se forma na crista da cadeia. Nova crosta criada elevada por que quente e ocupa um volume maior do qye as rochas frias da bacia ocenica profunda . Existe soerguimento do manto.

A estrutura da crosta ocenica

Black Smoker on cracks near magma

Black Smokers rico em sulfetos macios

http://collections.ic.gc.ca/geoscience/images/detail/F92S0220.jpg Circulation of hot water in cracks at mid-ocean ridge dissolves metals which are re-precipitated as sulphide ores

Ophiolite Suite Estrutura de crosta ocenica. Trs camadas na crosta Camada Superior consiste de sedimentos sobre uma pilha de pillow lavas Camada Mdia nmerosas intercalaes de diques chamado de enxame de diques Camada Inferior gabbro formados em cmaras magmticas basalticas

Camada do manto tambm faz parte do complexo Ofioltico. - Magma que gera novo fundo coenido originado derochas mantlicas parcialmente fundidas (peridotitos) na astenosfera.

Ophiolite SuiteFormao de Serpentinitos devido a circulao de gua2 km 3-6km 3-6km

Outcrop of pillow basalt

Margens divergentes: estrutura dos riftes continentais e das dorsais ocenicas

Tipos de rifts continentales

Tipos de riftes continentais: riftes ativos

Tipos de riftes continentais: riftes passivos

Tipos de riftes continentais: cisalhamento puro e simples

Rift por cisalhamento simples

Modelos analgicos de Withjack & Schlische (GSSP, in press)

Segmentao de rifts e zonas de transferncias

Elmohandes (1981)

Sand models by Acocella et al. (1999)

Os modelos reproduzem discontinuidades pr-existentes dentro do embasamento

A Geometria da zona de transferncia em funo da geometria do rifte

Rift de lago Baikal

Rift de Africa oriental

Extenso continental e magmatismoDistribuip do vulcanismoVulcanismo fora do eixo Pliocene

East African Rift System

Main Ethiopian Rift

Corti, 2005

Extenso continental e magmatismo

Vulcanismo ao longo de falhas obliquas e enechelon faults Quaternrio

East African Rift System

Main Ethiopian Rift

Corti, 2005

Extenso continental e magmatismo

Vulcanismo localizado em zonas de transfernciaWestern Branch

East African Rift System

Corti, 2005

Arquitetura do rift

: offset angle S: overlap L: overstep

Corti, 2005

Evoluo do Rift da EtiopiaEast African Rift SystemNE-SW trending border faults system N20E to N-S trending fault system, affecting the rift floor (Wonji Fault Belt)

Ethiopian RiftSo ma Pla lian te

ica

nP late

Afr

Ind Oc ian ea n

WFB segments

The WFB costitutes a system of oblique en-echelon faults formed at about 1.8-1.6 Ma

Corti, 2005

Ejemplos de fallamiento extensional en la parte meridional del rift etiopico. Tomado de Ebinger et al., 2000

Proposed evolutionary sequence of volcanism and faulting. (A) 4535 Ma. Eruption of as much as 1-km-thick sequence of basaltic lavas and later, more minor rhyolites and rhyolitic tuffs onto a relatively flat, low-lying surface. Extension and subsidence occurred in some JurassicCretaceous basins west and south of the floodbasalt province. (B) 3225 Ma. Initial volcanism and faulting within the Turkana depression, to the west of present-day Lake Turkana. Volcanism was restricted to the area of rift basins. Initial extension occurred within the Red Sea and Gulf of Aden during this period. (C) 1810 Ma. Basaltic volcanism recommences in the rifted zone as border fault segments begin to develop. Rifting migrates eastward to the Lake Turkana region, and faulting propagates southward. (D) 50 Ma. After a 5 m.y. hiatus in volcanism, new faults and eruptive centers develop in the southern parts of the broadly rifted zone, and faulting and volcanism in the Eastern rift also migrate eastward.

Figure 2. A: Cross section of Main Ethiopian rift along line A-A9 (Fig. 1). Seismic velocities (km/s) are from Berckhemer et al. (1975). Dot is crustal thickness from receiver function study (Hebert and Langston, 1985). Bold line above is geodetically determined extensional velocity along line of section (Bilham et al., 1999). B: Stretching and heating lead to adiabatic decompression melting beneath central rift valley, and strain is accommodated by diking in magmatic segments. Inset: Crustal structure within magmatic segment (see Fig. 3). C: As thinning continues, broad zone of transitional crust is created between conti-nental and new oceanic crust. Asal rift (Fig. 1) is model for oceanic rift structure (e.g., Ruegg, 1975). Tomado de Ebinger and Casey, 2001

Dorsais ocenicas e ofiolitos

It is the longest mountain chain, the most active volcanic area and until recently, the least accessible region on the earth.

Magma seeps up from deep within the mantle to form the East Pacific Rise (shown in cross section along the crest of the ridge). Investigators speculate that partially melted rock from depths of 30,000 to 60,000 meters percolates upward and is produced in greater quantities in some areas (dark red) than in others (light red). They propose that the molten rock fills and expands magma chambers. Seismic measurements suggest that the tops of the chambers are at the depth indicated by the broken red line. Molten rock ascends from the magma chamber through cracks in the crust and then solidifies or erupts onto the ocean floor. The depth of the ridge (black line at top) was determined from sonar measurements.

Dorsais de espalhamento lento: Dorsal medio-atlntico (MAR)

Dorsais de espalhamento rpido: Dorsal do Pacifico oriental (EPR)

http://imager.ldeo.columbia.edu/rid gembs/ne_pac/html/home.html

Imagen de um rift valley ocenica

Detalhe de un rift oceanico (Juan de Fuca)

Junta triple (R-R-R)

Segmentos de dorsais superimpostas(EPR)

Topography (left) and magnetization (right) of the East Pacific Rise near 12N. The topographic map shows that the overlapping spreading center offsets the Rise by 8 km. Colors indicate depths of from 2350 (pink) to 3500 (dark blue). The two arms of the discontinuity overlap by 27 km. The arms narrow and deepen near the discontinuity, presumably because the supply of magma to the region is low. The ocean floor near the discontinuity is unusually deep and is littered with ridge tips, especially on the west side. It turns out that regions that are not well supplied with magma are highly magnetized. In the map at the right, magnetization decreases in strength from red to yellow regions. The map reveals the wake (red) of the overlapping spreading center. The green-blue troughs were created 700,000 years ago when the earth's magnetic field reversed polarity. The wake shows that the overlappi