aula 4 dobras
DESCRIPTION
Classificação de Dobras. Conceitos e modelos estruturais.TRANSCRIPT
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2) ELEMENTOS GEOMÉTRICOS DAS DOBRAS
1) CONCEITUAÇÃO
- em função da geratriz
- em função da concavidade
- em função da idade relativa das camadas
- em função do ângulo interflanquial
- em função da simetria
- em função da posição da superfície axial e do eixo da dobra
3) DESCRIÇÃO / CLASSIFICAÇÃO GEOMÉTRICA DAS DOBRAS
D O B R A S
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- em função da morfologia em perfil
- em função da geometria em escala regional
- em função da variação da espessura das camadas
- em função da variação das isógonas de mergulho
- DESCRIÇÃO / CLASSIFICAÇÃO DAS DOBRAS (cont.)
D O B R A S - hoje
Ramsay (1962/1967)
- ESTILO DAS DOBRAS
Camadas competentes e incompetentes
Boudinagem – boudins
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Classificação em função da MORFOLOGIA,
em perfil
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Dobras arredondadas ou concêntricas
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Dobras arredondadas c/ flancos retos
Ramsay & Huber (1987)
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Dobras angulares
SIMÉTRICAS chevron
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Dobras angulares
ASSIMÉTRICA kink
Kink Bands
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Dobras angulares
ASSIMÉTRICA kink
kink folds
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dobra em caixa
ic.ucsc.edu/.../Lectures/Folds/I
Cretaceous turbidites,
Shumagin Is. Alaska.
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dobra em cúspide
Ramsay & Huber (1987)
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http://www.see.leeds.ac.uk/folds
Dobras harmônicas e desharmônicas
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Dobras harmônicas
Ramsay & Huber (1987)
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Dobras desharmônicas
Ramsay & Huber (1987) http://darkwing.uoregon.edu/~millerm/slide
s.html
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Dobras convolutas
gsc.nrcan.gc.ca
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Classificar:
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Dobra intrafolial = dobra sem raíz
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Dobra intrafolial =
dobra sem raíz
Ramsay & Huber (1987)
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Dobras ptigmáticas
Ramsay & Huber (1987)
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Classificação em função da geometria
em escala regional
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Monoclinal Terraço estrutural
Homoclinal
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Exemplos de monoclinais
20 m
40
m
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Exemplo de um HOMOCLINAL
30 m
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Classificação em função
da variação da ESPESSURA das CAMADAS
e
da variação das ISÓGONAS de MERGULHO
(Ramsay 1962 / 1967)
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e = espessura da camada
espessura não varia espessura varia
ESPESSURA DAS CAMADAS
D. concêntrica D. similar
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ISÓGONAS DE MERGULHO
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ISÓGONAS DE MERGULHO
tangentes de igual mergulho
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ab = isógona de 0˚
de = isógona de 40˚
θ = 40˚
Isógonas de mergulho
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Espessura da camada
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ab = isógona de 0˚
de = isógona de 40˚
θ = 40˚
Espessura da camada
Isógonas de mergulho
tangentes
CONVERGENTES
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ISÓGONAS CONVERGENTES
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ISÓGONAS
DIVERGENTES
ISÓGONAS
PARALELAS
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Classe 1A
Espessura zona da charneira < flancos
Classe 1B – PARALELA – CONCÊNTRICA - ISÓPACA
Espessura zona da charneira = flancos
Classe 1C
Espessura zona da charneira > flancos
ISÓGONAS CONVERGENTES
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Classe 2 - SIMILAR
Espessura zona da charneira >> flancos
Classe 3
Espessura zona da charneira >>> flancos
ISÓGONAS
PARALELAS
ISÓGONAS
DIVERGENTES
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Classe 1 = Dobra
paralela
Classe 2 = Dobra
similar
Classe 1 = Dobra
paralela
Classe 2 = Dobra
similar
Classe 2 = Dobra
similar
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1
2
3
4
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1
2
3
4
Classe 1B =
Dobra paralela Classe 2 =
Dobra similar
Classe 1A
Classe 3
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Estilo das dobras
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Caracterizar o ESTILO das dobras
Classificar as dobras, em perfil, segundo a proposta de
Ramsay (1967): paralelas (classe 1B) ou similares (classe
2) [ou classes 1A, 1B, 1C, 2 ou 3]
Determinar o ângulo interflanquial.
Observar se em 3D, a dobra é cilindrica ou acilindrica (simples
ou redobrada).
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Marshak & Mitra (1988)
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Marshak & Mitra (1988)
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Marshak & Mitra (1988)
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COMPETÊNCIA E INCOMPETÊNCIA de camadas
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A competência de uma camada é uma
PROPRIEDADE RELATIVA
E DENOTA
MAIOR OU MENOR RIGIDEZ, MOBILIDADE OU PLASTICIDADE.
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Rochas COMPETENTES : reagem de maneira RÍGIDA perante os
esforços deformacionais (ex: quartzitos,
calcários)
Rochas INCOMPETENTES : comportam-se PLASTICAMENTE perante os
esforços deformacionais, dobrando-se
mais intensamente.
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Relação decrescente de competência de algumas rochas
(em condições de pressão e temperatura ambiente)
QUARTZITO
GRANITO
BASALTO
CALCÁRIO
XISTO
MÁRMORE
ARGILITO
ANIDRITA
SAL
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QUARTZITO
GRANITO
BASALTO
CALCÁRIO
XISTO
MÁRMORE
ARGILITO
ANIDRITA
SAL
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Experimentos Analógicas c/ dois materiais (camada preta em matriz branca)
(Ramsay & Huber 1987)
A- Contraste de competência: alto
B- Contraste de competência: baixo
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Ramsay & Huber (1987)
Dobras em cúspide
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Veios de quartzo cortando folhelho e arenito
earth.boisestate.edu/home/cjnorth/more/photogallery.html
folhelho
arenito
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Dobras harmônicas e
desharmônicas
Experimentos Analógicas c/ dois materiais, em várias camadas (camadas pretas, de diferentes espessuras)
(Ramsay & Huber 1987)
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Photograph by R.C. Fletcher
Dobras harmônicas e
desharmônicas
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CONCLUSÃO:
quando um pacote multicamadas é submetido a deformação
passiva, a geometria das dobras
depende :
das propriedades mecânicas das litologias envolvidas,
do contraste de competência entre as camadas adjacentes,
da espessura das camadas
e
do volume de material envolvido (número de camadas presentes)
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Ramsay & Huber (1987)
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Estruturas presentes: dobras e boudins
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COMPRESSÃO horizontal
e
Camadas horizontais
Encurtamento DOBRAS
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COMPRESSÃO horizontal
e
Camadas a 90°
(verticais)
???
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Estiramento Boudins
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BOUDINAGEM BOUDINS (processo) (objeto geológico)
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Boudinagem
pangea.stanford.edu/projects
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Dobras e boudins
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Pegmatito ≥ microgranito ≥ gnaisse granítico > anfibolito