Basins Flashcards
Sedimentary Basin Types
- Extensional Rift Basins
- Strike-slip Basins
- Fluxural foreland basins
- Back-arc basins
Continental strike slip basins
- Needs component of extension/transtention from relative plate motion (oblique) or bend in strike-slip system
Queen Charlotte Basin
- Currently transpression with strike-slip and subduction
- Past Miocene transtension indicated by plate motion models, created basin
Queen Charlotte sound South and North Hecate Strait
- South = extenstional faulting in miocene with half grabens, syn-rift seds and volcanics, overlain by flat seds
- North: extensional faults reactivated 5Ma by pliocene compression, thrusting and folding results in basin inversion
Releasing Bends and Step-Overs
- Extensions
- Pull-apart basins
Pull-apart basins
- Sag ponds
- Normal faults
- Subsidence/deposition
- Crustal Thinning, possible intrusions
Restraining bends and step-overs
- Compression
- Push-ups, ridges
Push-up Ridges
- Folds
- Thrust faults
Right-stepping, right lateral
- Extension
- Possible pull-apart basin
Right-stepping, left lateral
- Compression
- Possible push-up ridge
Releasing bend
- Subsidence
right bend, not step, right lateral
Restraining Bend
- Uplift
left bend, not step, right lateral
Dead Sea, Israel
- Pull-apart basin
- Up to 8.5km deep
- Negative gravity anomaly possibly associated w/ crustal root
- Normal heat flow
- Minimal volcanics
Pull-apart basin examples
- Dead Sea, Israel
- Salton Trough, California
Salton Trough, California
- Pull-apart basin
- High heat flow
- Positive gravity anomaly (dike intrusions?)
Strike-slip duplexes and flower structures
- Fault strands form small blocks in anastomosing pattern (lens-shaped)
Transtensional flower structure
- Blocks downthrown
- Negative flower
Transpressional flower structure
- Blocks uplifted
- Positive flower
Alpine Fault, NZ
- Flower structures
- Basins and ridges
- Transtension and transpression
Flexural, Foreland, basins
- Depression of crust adjacent to load
- Acitve or Inactive
Mountains represent? vs. Basins
- Mass excess in mountains
- Mass deficit in basins
Negative gravity anomalies, Himalayas
- Maybe associated with crustal root (Airy)
- Some underthrust support?
- Airy model cannot entirely fit data
- Mass supported by Airy and flexural rigidity
Isopachs
Lines of equal sediment thickness
Ocean-Continent Margin
- Retro foreland basin on upper plate
- Andean-type
- Shallow wadati-benioff zone dip
- Shallow ocean trench
- Magmatic arc to backarc compression to retro-foreland basin
Continent-Continent Margin
- Pro vs. retro foreland basins
- Retro-foreland Basin on upper plate
- Bivergent wedge between plates
- Thrust faults on either side of wedge
- Peripheral pro foreland basin on lower plate behind wedge
Retro-foreland basin
- Located on upper plates at collision zones
Peripheral (pro) foreland basin
- Located on lower plate at continent-continent collision zones
Example of Inactive flexural basin
- Alberta Basin
- Due to stacking of Rocky mnts. thrust sheets
- 35Ma, end of thrust faulting
- Then, Erosion of 10km thickness from thrust pile which led to uplift
Example of Active flexural basin
- Alpine-Himalayan foreland basins
- Since collision initiated early tertiary
Alpine-foreland basins
- Pyrenees, Alps, Carpathians
- Active since collision began early tertiary
Bouguer Gravity
- Topographic mass difference already accounted for
Collision and basin formation timeline, elastic flexure
- Crustal thickening
- Load on elastic plate
- Depression below and near mountains, flexural forebulge inland
- Sediment influx and erosion of bulge infills basin
Long time scales, flexural basins
- Viscous deformation
Final stage flexural basins
- Local isostatic equilibrium
- Basin narrows with time
Viscoelastic
- Long time scales = viscous deformation
- Load continues to depress land and push up bulge inland (isostatic equilibrium)
- Bulge moves towards load and basin narrows
Foreland basin system components
- Back bulge
- Forebulge
- Foredeep
- Wedge top
- Topographic front (front of basin, on upper plate)
Sedimentary layers of foreland basin
- Deep marine sediments (flysch) overlain by shallower coarser marine/terrestrial seds (molasses)
Evolution of foreland basin
- Passive margin
- Early convergent stage
- Late convergent stage
- Basin system migrates w/ fold thrust belt
Passive margin stage
- Stretched continental crust
- Passive margin wedge
- Oceanic crust
Early convergent stage
- Prominent forebulge
- Flexural forebulge unconformity
- Trench flysch basin
- Submarine wedge, pushes towards basin
Late convergent stage
- Buried forebulge under Molasse basin
- Subaerial wedge, no above water, pushing up
Back-arc basins
- Spreading axis
- On upper plate, 200-300km from trench
- Can form behind volcanic magmatic arc
Controlling factors on formation of Back-arc basins
- Absolute motion of upper plate relative to trench
- Age and dip of subducting plate
- Collision induced fore-arc rotation
How does absolute plate motion control back-arc?
- Assumes lower plate rooted in mantle
- Plate motion towards trench compresses back-arc
- Plate motion away from trench extends back-arc and gap filled by mantle upwelling
Where is an example of flat slab subduction and what does it do to the back-arc basin?
- East Pacific where S. America moves west towards trench
- Slab subducts flattly under continent
- Back-arc compresses
How does age and dip of subducting plate control back-arc?
- Old dense crust sinks more rapidly and steeply
- Slab rollback
- Back-arc extension
Where is a back-arc basin more likely to appear?
- Old, steeply dipping lower plate where the upper plate motion is away from trench
Where is an example of collision-induced fore-arc rotation?
- New Zealand
Back-arc end-members
- Continental arcs w/ thrust belts and foreland basins
- Island arcs with back-arc, or marginal, basins
Continental arcs w/ thrust belts and foreland basins
- Shallow trench (6km)
- Young plate, shallow dip
- Thick overlying crust (precambrian) (70km)
- Leads to compression and back-arc fold and thrust belt
- Ex. Chilean type E. Pacific arc under compression
Island arcs w/ back-arc or marginal basins
- Deep trench (11km)
- Older plate, steep dip
- Thin overlying crust (mafic, intermediate)
- Leads to extension, back-arc basin, (mantle partial melts?)
- Ex. Mariana type W. Pacific arc under extension
W. Pacific back-arc marginal basins
- Seafloor spreading
- Poorly developed magnetic stripes
- Lava composition more variable and with higher water than MOR lavas
Lau back-arc spreading
- Low-velocity zone linked w/ arc LVZ below 100km
- Slab rollback
- Convection in mantle wedge
- Partial melt of convecting mantle due to slab dehydration
- Spreading
What are the 2 stages of subsidence due to extension?
- Initial tectonic subsidence
- Thermal subsidence
Initial tectonic subsidence
- Subsidence at time of stretching (eqn 1) (rapid)
- Reduced by thermal expansion uplift (eqn 2)
Thermal subsidence
- Gradual subsidence due to conductive cooling of lithosphere
Active rifting = ?
- Initial subsidence
- Followed by thermal subsidence
Shear zone extension model
- Simple shear
- deformation is asymmetric
- Basin and range?
Basin and Range
- Region of ductile deformation (core complex) adjacent to region of brittle deformation
- Each w/ different timing
- Extension by large-scale low-angle detachment from upper crust (brittle deformation) to lower lithosphere (ductile shear)
Basin and range formation over time
0Ma- Wedge shaped hanging wall slides down fault (thickness at a given point decreases)
3Ma- Asthenosphere rises, uplift
8Ma- Brittle deform and rotated fault blocks in sed basin and basement
14Ma- Large fault-block ranges (little internal deformation) occur adjacent to core complex (mid/lower crust material) uplifted on ductile shear zone
Thinnest crust (shear model)
- Tectonic subsidence reduced by thermal uplift
- Followed by gradual thermal subsidence
- ## Location of thinnest crust offset from thinnest lithospheric mantle
Thinnest lithospheric mantle (shear model)
- Replace higher density lith. mantle w/ lower density asthenosphere
- Tectonic uplift increase by thermal uplift followed by gradual thermal subsidence
Location of thinnest crust (shear model)
- offset from thinnest litho mantle
Lateral variation of surface uplift
- Amount of surface uplift varies laterally
- Crustal stretching dominant = subsidence (density crust < density asthenosphere
- Mantle stretching dominant = uplift (density lith. mantle > density asthenosphere)
Uniform stretching
- Lithosphere stretched and thinned
- Surface subsides and moho rises to maintain isostatic equilibrium
- Beta = Stretch factor
- Extension = Original length x times Beta
- Uniform thinning = original thickness C to C/Beta; Lithosphere L to L/Beta
Uniform stretching: extension
- Original length x times beta
Uniform stretching: uniform stretching
- Original thickness C to C/beta
- Original Lithosphere L to L/beta
Uniform model: Isostatic effects of crustal thinning
- Accounting for sediment fill but not thermal effects
- Surface subsides by Depth D
- D =
Uniform model: Eqn 1
Depth of subsidence, D = [Crust (1-1/beta)(density C - Density M)]/(Density S - Density M)
Reasonable values for Eqn 1 for depth of subsidence
- Crust, C = 40km
- Final crust thickness of 20km
- Beta = 2
- Density of crust = 2.8g/cm^3
- Density of mantle = 3.2g/cm^3
- Density of sediment = 2.1g/cm^3
Reasonable values for Eqn 1 for depth of subsidence
- Crust, C = 40km
- Final crust thickness of 20km
- Beta = 2
- Density of crust = 2.8g/cm^3
- Density of mantle = 3.2g/cm^3
- Density of sediment = 2.1g/cm^3
- D = around 7.3km of tectonic sediment
Uniform model: Thermal effects, graph
- Geotherm becomes steeper as depth to 1200 degrees C becomes shallower
- Average temperature goes from 600C to 900C
- Returns to equilibrium over time
Uniform model thermal effects
- Due to passive asthenospheric upwelling
- Thermal expansion, region heated by 300C
- Coefficient of expansion is 3.3x10^-5/degree C
- Vertical expansion, eqn 2
- Expansion uplift will decay w/ time, gradual thermal subsidence
Thermal effects, Eqn 2
Vertical expansion = (depth to asthenosphere)(coefficient of expansion)(heating of region)
- Ex.
(100km) (3.3x10^-5/degree C)(300C) = 1.0km thermal uplift (initial)
Coefficient of expansion
3.3 x 10^-5/degree C
Gradual thermal subsidence
- Expansion uplift will decay with time
How much time will it take for gradual thermal subsidence?
- Thermal time scale To = L^2/(pi^2 x k)
- Where L = lithospheric thickness and k = thermal diffusivity
- Ex. (125km^2)/(3.14^2 x 10^-6m^2s^-1)
To = 50Ma (lithospheric time constant)
