EOS410PltTec > Basins > Flashcards

Basins Flashcards

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1
Q

Sedimentary Basin Types

A
  • Extensional Rift Basins
  • Strike-slip Basins
  • Fluxural foreland basins
  • Back-arc basins
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2
Q

Continental strike slip basins

A
  • Needs component of extension/transtention from relative plate motion (oblique) or bend in strike-slip system
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3
Q

Queen Charlotte Basin

A
  • Currently transpression with strike-slip and subduction

- Past Miocene transtension indicated by plate motion models, created basin

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4
Q

Queen Charlotte sound South and North Hecate Strait

A
  • 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
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5
Q

Releasing Bends and Step-Overs

A
  • Extensions

- Pull-apart basins

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6
Q

Pull-apart basins

A
  • Sag ponds
  • Normal faults
  • Subsidence/deposition
  • Crustal Thinning, possible intrusions
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7
Q

Restraining bends and step-overs

A
  • Compression

- Push-ups, ridges

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8
Q

Push-up Ridges

A
  • Folds

- Thrust faults

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9
Q

Right-stepping, right lateral

A
  • Extension

- Possible pull-apart basin

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10
Q

Right-stepping, left lateral

A
  • Compression

- Possible push-up ridge

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11
Q

Releasing bend

A
  • Subsidence

right bend, not step, right lateral

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12
Q

Restraining Bend

A
  • Uplift

left bend, not step, right lateral

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13
Q

Dead Sea, Israel

A
  • Pull-apart basin
  • Up to 8.5km deep
  • Negative gravity anomaly possibly associated w/ crustal root
  • Normal heat flow
  • Minimal volcanics
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14
Q

Pull-apart basin examples

A
  • Dead Sea, Israel

- Salton Trough, California

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15
Q

Salton Trough, California

A
  • Pull-apart basin
  • High heat flow
  • Positive gravity anomaly (dike intrusions?)
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16
Q

Strike-slip duplexes and flower structures

A
  • Fault strands form small blocks in anastomosing pattern (lens-shaped)
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17
Q

Transtensional flower structure

A
  • Blocks downthrown

- Negative flower

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18
Q

Transpressional flower structure

A
  • Blocks uplifted

- Positive flower

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19
Q

Alpine Fault, NZ

A
  • Flower structures
  • Basins and ridges
  • Transtension and transpression
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20
Q

Flexural, Foreland, basins

A
  • Depression of crust adjacent to load

- Acitve or Inactive

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21
Q

Mountains represent? vs. Basins

A
  • Mass excess in mountains

- Mass deficit in basins

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22
Q

Negative gravity anomalies, Himalayas

A
  • Maybe associated with crustal root (Airy)
  • Some underthrust support?
  • Airy model cannot entirely fit data
  • Mass supported by Airy and flexural rigidity
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23
Q

Isopachs

A

Lines of equal sediment thickness

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24
Q

Ocean-Continent Margin

A
  • 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
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25
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
26
Retro-foreland basin
- Located on upper plates at collision zones
27
Peripheral (pro) foreland basin
- Located on lower plate at continent-continent collision zones
28
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
29
Example of Active flexural basin
- Alpine-Himalayan foreland basins | - Since collision initiated early tertiary
30
Alpine-foreland basins
- Pyrenees, Alps, Carpathians | - Active since collision began early tertiary
31
Bouguer Gravity
- Topographic mass difference already accounted for
32
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
33
Long time scales, flexural basins
- Viscous deformation
34
Final stage flexural basins
- Local isostatic equilibrium | - Basin narrows with time
35
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
36
Foreland basin system components
- Back bulge - Forebulge - Foredeep - Wedge top - Topographic front (front of basin, on upper plate)
37
Sedimentary layers of foreland basin
- Deep marine sediments (flysch) overlain by shallower coarser marine/terrestrial seds (molasses)
38
Evolution of foreland basin
- Passive margin - Early convergent stage - Late convergent stage - Basin system migrates w/ fold thrust belt
39
Passive margin stage
- Stretched continental crust - Passive margin wedge - Oceanic crust
40
Early convergent stage
- Prominent forebulge - Flexural forebulge unconformity - Trench flysch basin - Submarine wedge, pushes towards basin
41
Late convergent stage
- Buried forebulge under Molasse basin | - Subaerial wedge, no above water, pushing up
42
Back-arc basins
- Spreading axis - On upper plate, 200-300km from trench - Can form behind volcanic magmatic arc
43
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
44
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
45
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
46
How does age and dip of subducting plate control back-arc?
- Old dense crust sinks more rapidly and steeply - Slab rollback - Back-arc extension
47
Where is a back-arc basin more likely to appear?
- Old, steeply dipping lower plate where the upper plate motion is away from trench
48
Where is an example of collision-induced fore-arc rotation?
- New Zealand
49
Back-arc end-members
- Continental arcs w/ thrust belts and foreland basins | - Island arcs with back-arc, or marginal, basins
50
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
51
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
52
W. Pacific back-arc marginal basins
- Seafloor spreading - Poorly developed magnetic stripes - Lava composition more variable and with higher water than MOR lavas
53
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
54
What are the 2 stages of subsidence due to extension?
- Initial tectonic subsidence | - Thermal subsidence
55
Initial tectonic subsidence
- Subsidence at time of stretching (eqn 1) (rapid) | - Reduced by thermal expansion uplift (eqn 2)
56
Thermal subsidence
- Gradual subsidence due to conductive cooling of lithosphere
57
Active rifting = ?
- Initial subsidence | - Followed by thermal subsidence
58
Shear zone extension model
- Simple shear - deformation is asymmetric - Basin and range?
59
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)
60
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
61
Thinnest crust (shear model)
- Tectonic subsidence reduced by thermal uplift - Followed by gradual thermal subsidence - Location of thinnest crust offset from thinnest lithospheric mantle -
62
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
63
Location of thinnest crust (shear model)
- offset from thinnest litho mantle
64
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)
65
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
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Uniform stretching: extension
- Original length x times beta
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Uniform stretching: uniform stretching
- Original thickness C to C/beta | - Original Lithosphere L to L/beta
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Uniform model: Isostatic effects of crustal thinning
- Accounting for sediment fill but not thermal effects - Surface subsides by Depth D - D =
69
Uniform model: Eqn 1
Depth of subsidence, D = [Crust (1-1/beta)(density C - Density M)]/(Density S - Density M)
70
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
71
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
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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
73
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
74
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)
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Coefficient of expansion
3.3 x 10^-5/degree C
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Gradual thermal subsidence
- Expansion uplift will decay with time
77
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)
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