2.1.2. Passive margins 1
From rift to passive margin Stage «!rift!» Lias (180 Ma) Aptian (120 Ma) U. Cretaceous. (80 Ma) Stage «!break-up!» Stage «young margin!» Stage «mature margin!» 2
Croûte Continentale Manteau lithosphérique Asthénosphère État initial Failles (séismes) résistance cassant ductile cassant ductile visqueux profondeur moho 1300 C 3
Graben (failles normales & sédiments continentaux) Stade rift précoce 1300 C 4
Graben Déformation cassante Stade rift Déformation ductile 1300 C Amincissement de la lithosphère 5
Cisaillement asymmétrique Amincissement lithosphérique Stade rift mature Remontée des isothermes & Asthénosphère Fusion partielle réchauffement Diminution de densité Soulèvement 1300 C Fusion partielle (limitée) 6
Dénudation du manteau Faille de Détachement asymmétrique Stade dénudation manteau Élimination de la croûte Dénudation du manteau 1300 C Altération marine 7
Rupture lithosphère Stade «!break-up!» Asthénosphère en surface => accrétion croûte océanique arrêt transmission de contraintes Asthénosphère en surface arrêt activité des failles Rebond isostatique Accrétion croûte océan. 1300 C Séparation des marges 8
Stade marge jeune Refroidissement de la lithosphère Refroidissement = f(amincissement) Sédimentation post-rift (envir. marin) Discordance post-rift Failles inactives Subsidence Densification Subsidence différentielle Subsidence thermique Basculement de la marge 9
Stade marge mature Subsidence décroissante en f(temps) Deep-sea-fan Sédimentation post-rift Accommodation décroît sur plateforme Subsidence progradation Progradation + deep sea fan sur cr. océan. 10
Two types of continental margins Argentina Namibia Seaward-dipping reflectors = basaltic lava flows W. Greenland 11
Structure of non-volcanic vs volcanic margins 12
Structure of non-volcanic vs volcanic margins Ocean. cr. C.O.B. Stretched Cont. cr. Ocean. cr. igneous crust. Cont. cr. 13
Formation of volcanic margins Mantle fusion (lithosph + asthenosph) Upwelling of asthenosphere Large lava flows (traps) Dike intrusion in Cont. Cr. Emplacement of grabbro bodies at Moho = underplating Break-up : accretion of oceanic crust After break-up : differential subsidence => Seaward dipping lava flow Active rifting => volcanic margin subsidence 14
Volcanic vs non-volcanic margins mid-atlantic ridge NOAA! Parana Basaltic Traps Tristan da Cunha Hot-spot Angola Namibie 15
LIP s: basalt plateaus = traps Associated with volcanic margins (e.g. Atlantic volcanic M.) Plume = convection => decompression => mantle fusion 16 Large Igneous Provinces
Non-volcanic margin : Stretched lithosphere West iberian margin Reston Manatschal & al, 2001 17
Stretched lithosphere & mantle denudation Non-volcanic Margins Ex: Tethysian margin Ex: Albian Pyrenean basin Lagabrielle & Bodinier 2008 Froitzheim & al 18
Continent-Ocean Boundary Conjugate margins Upper plate (Newfoundland) PERON-PINVIDIC, Gwenn (2006) lower plate (Iberia) Oc.crust accretion Drifting Break-up: 1st accretion Formation of 1st magnetic anomalies, Mantle exhumation & serpentinisation End of rifting (no more faulting) 19
Continent-Ocean Boundary : S. Atlantic Unternher & al 2009 20
Break-up = end of rifting résistance profondeur lithosphere structure => depth of necking => isostatic flexural response Break-up unconformity : signature of stress release 21
Non-volcanic passive margin structure : Gabon Ocean. Cr. 10km 50km Moho post-rift Syn-rift Cont. Cr. thinned Moho from Meyers et al. 1996 22
Non-volcanic passive margin structure : Angola 23 Unternher & al 2009
Non-volcanic Passive margin structure : Gulf of Lion N S ECORS Lower Cont.crust Intermediate crust Lithosphere mantle Low angle faults, extreme crust thinning, antithetic lower crust/mantle detachment = Lower crust and/or mantle denudation from Séranne, 1999 Cr.Oc. 24
Continental rift-> passive margin : subsidence time syn-rift sediment post-rift sediment Sediment thickness Prerift Synrift fast subsidence = synrift postrift subsidence: cooling of the lithosphere Previously thinned Postrift 25
Subsidence of passive margins Subsidence due to geodynamics Total Subsidence = geodynamics + load theoritical curve backstripping from Gabon Margin theoritical vs computed = geodynamic events to integrate & interpret 26
Subsidence of passive margins Time (Ma) Depth(km) backstripping from East Coast of USA Backstripped curve = load effect removed (error bars : bathymetry, eustacy, ) Theoritical curve of thermal subsidence Increase in subsidence=> phase of deformation? 27
South Gabon Postrift Synrift Postrift sequence affected by faulting => Margin deformation during postrift? 28 Post-rift tectonics on passive margins
Gravity-driven tectonics over decollements 0 10km 0 10km Raccourcissement Chevauchements, diapirs Shales decollement : Niger delta Extension Failles de croissance grès argiles evaporites décollement : Gabon margin Raccourcissement Chevauchements évaporites 100 km Extension Failles de croissance 100 km 29
Salt tectonics : extension upslope 30
Salt tectonics : shortening downslope waterdepth:3500m Thrust front Mini-basin 5 km 1 SecT.D. 31
Salt tectonics at margin scale: section view shortening Evaporites = décollement Salt tectonics = fonction (slope & load) extension Gabon margin 32
Salt tectonics at margin scale: map view Escarpement = Thrust Front Platform slope + mini-basins Abyssal Plain Gulf of Mexico More info about salt tectonics on: http://homepage.ufp.pt/biblioteca/salttectonicsnovo/pages/page1.htm#universidade 33
Gravity tectonics at margin scale: section view Decollement on shale interbeds shortening domain Extensional domain Oceanic crust Niger Delta SW 5km Namibia Margin Imbricated thrusts at margin toe (shortening domain) NE 34
Post-rift tectonics : basement-involved deformation 85Ma Postrift activation of faults initiated during rifting (affecting the basement) => intra-plate stress => plate-tectonic-driven deformation 35
Basement-involved tectonics : ridge-push 85Ma Decrease in oceanic accretion rate => Decrease of ridge-push 36
Basement-involved tectonics : inversion Inversion of old passive margin into subduction Whitjack Future inversion? 37