Week 15 Flashcards
McKenzie model overview
Pure shear
Non-coaxial deformation
Stretching and thinning
Wernicke model overview
Simple shear
Low angle and high angle faults
Overall thinning
Asymmetry
Different distribution of thinnning
McKenzie model process
RIFT
- deep crust thins by ductile deformation
- upper extended by planar faults = strong seismogenic layer
SYN RIFT SEDIMENTS
-20Ma
LITHOSPHERE THINNED AND REPLACED
- by asthenosphere
ASTHENOSPHERE COOLS
POST RIFT SUBSIDENCE/SEDIMENTS
50-100Myrs
What is good about the McKenzie model?
It is predictive
N.B. Graph = water-filled basin
If sediment filled = more loading = more subsidence
Steer’s head geometry =
when post rift phase occurs over a wider area than syn
Measuring beta
- Crustal thickness = stretching profile
- Sum the heaves
Original length = deformed length - sum of heaves
Beta = original/deformed
Wernicke model process
Low angle extensional detachment faults
Thermal subsidence region offset from area of rifting
EXPLANATION FOR ASYMMETRIC PASSIVE MARGINS
How do continental rifts evolve into ocean spreading?
- Detachment = hyper-extended continental domain
- no crust left just mantle+sediment - Volcanic seamount forms
- Emplacement of lava flows
- see in continental dipping reflectors (CDR) - Spreading on oceanward side
= HYBRID CRUST
What makes up the hybrid crust?
Mantle peridotite
Gabbros
Basalts
Serpentinised peridotites
Syn-rift sediments
Controls on extensional faulting
- Basin architecture
2. Sediment distribution/facies type etc
Growth fault =
active at same time as sedimentation
Leads to thickening of sequence into hanging wall of fault
(Syn rift sediments in seismic sections thicken towards the fault)
Continental model for basin fill evolution
Gawthorpe and Leeder 2000
- Winds/structural topography = migration and deposition of aeolian sands
- Shallow lakes in hanging wall depocentres
–>
- Fluvial lake developments/alluvial fans sourced from footwall catchments
Coastal marine model for basin fill evolution
Gawthorpe and Leeder 2000
- High stand/low stand interplay
High stand = islands where footwall uplift
Low stands = shoreline terraces form
Evidence for focused subsidence and strain localisation in grabens
Greatest sin and post rift subsidence in area of maximum crustal thinning
Consistent with McKenzie
Random distribution at start then POSITIVE FEEDBACK
- strain localisation controlled by faulting?
- elastic interactions between seismogenic faults
- faults in stress shadow less likely to slip = die
= no need to invoke large-scale ductile flow in upper mantle like McKenzie
Example of strain localisation
Corinth rift v rapid extension 10mm/yr
Underfilled (i.e. sedimentation not fast enough = water) to 800m
= strain localised onto one fault system through basin
