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EOS410PltTec > Basins > Flashcards

Basins Flashcards

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
Q

Continent-Continent Margin

A
  • 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
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26
Q

Retro-foreland basin

A
  • Located on upper plates at collision zones
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27
Q

Peripheral (pro) foreland basin

A
  • Located on lower plate at continent-continent collision zones
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28
Q

Example of Inactive flexural basin

A
  • 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
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29
Q

Example of Active flexural basin

A
  • Alpine-Himalayan foreland basins

- Since collision initiated early tertiary

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30
Q

Alpine-foreland basins

A
  • Pyrenees, Alps, Carpathians

- Active since collision began early tertiary

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31
Q

Bouguer Gravity

A
  • Topographic mass difference already accounted for
32
Q

Collision and basin formation timeline, elastic flexure

A
  • Crustal thickening
  • Load on elastic plate
  • Depression below and near mountains, flexural forebulge inland
  • Sediment influx and erosion of bulge infills basin
33
Q

Long time scales, flexural basins

A
  • Viscous deformation
34
Q

Final stage flexural basins

A
  • Local isostatic equilibrium

- Basin narrows with time

35
Q

Viscoelastic

A
  • 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
Q

Foreland basin system components

A
  • Back bulge
  • Forebulge
  • Foredeep
  • Wedge top
  • Topographic front (front of basin, on upper plate)
37
Q

Sedimentary layers of foreland basin

A
  • Deep marine sediments (flysch) overlain by shallower coarser marine/terrestrial seds (molasses)
38
Q

Evolution of foreland basin

A
  • Passive margin
  • Early convergent stage
  • Late convergent stage
  • Basin system migrates w/ fold thrust belt
39
Q

Passive margin stage

A
  • Stretched continental crust
  • Passive margin wedge
  • Oceanic crust
40
Q

Early convergent stage

A
  • Prominent forebulge
  • Flexural forebulge unconformity
  • Trench flysch basin
  • Submarine wedge, pushes towards basin
41
Q

Late convergent stage

A
  • Buried forebulge under Molasse basin

- Subaerial wedge, no above water, pushing up

42
Q

Back-arc basins

A
  • Spreading axis
  • On upper plate, 200-300km from trench
  • Can form behind volcanic magmatic arc
43
Q

Controlling factors on formation of Back-arc basins

A
  • Absolute motion of upper plate relative to trench
  • Age and dip of subducting plate
  • Collision induced fore-arc rotation
44
Q

How does absolute plate motion control back-arc?

A
  • 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
Q

Where is an example of flat slab subduction and what does it do to the back-arc basin?

A
  • East Pacific where S. America moves west towards trench
  • Slab subducts flattly under continent
  • Back-arc compresses
46
Q

How does age and dip of subducting plate control back-arc?

A
  • Old dense crust sinks more rapidly and steeply
  • Slab rollback
  • Back-arc extension
47
Q

Where is a back-arc basin more likely to appear?

A
  • Old, steeply dipping lower plate where the upper plate motion is away from trench
48
Q

Where is an example of collision-induced fore-arc rotation?

A
  • New Zealand
49
Q

Back-arc end-members

A
  • Continental arcs w/ thrust belts and foreland basins

- Island arcs with back-arc, or marginal, basins

50
Q

Continental arcs w/ thrust belts and foreland basins

A
  • 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
Q

Island arcs w/ back-arc or marginal basins

A
  • 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
Q

W. Pacific back-arc marginal basins

A
  • Seafloor spreading
  • Poorly developed magnetic stripes
  • Lava composition more variable and with higher water than MOR lavas
53
Q

Lau back-arc spreading

A
  • 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
Q

What are the 2 stages of subsidence due to extension?

A
  • Initial tectonic subsidence

- Thermal subsidence

55
Q

Initial tectonic subsidence

A
  • Subsidence at time of stretching (eqn 1) (rapid)

- Reduced by thermal expansion uplift (eqn 2)

56
Q

Thermal subsidence

A
  • Gradual subsidence due to conductive cooling of lithosphere
57
Q

Active rifting = ?

A
  • Initial subsidence

- Followed by thermal subsidence

58
Q

Shear zone extension model

A
  • Simple shear
  • deformation is asymmetric
  • Basin and range?
59
Q

Basin and Range

A
  • 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
Q

Basin and range formation over time

A

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
Q

Thinnest crust (shear model)

A
  • Tectonic subsidence reduced by thermal uplift
  • Followed by gradual thermal subsidence
  • ## Location of thinnest crust offset from thinnest lithospheric mantle
62
Q

Thinnest lithospheric mantle (shear model)

A
  • Replace higher density lith. mantle w/ lower density asthenosphere
  • Tectonic uplift increase by thermal uplift followed by gradual thermal subsidence
63
Q

Location of thinnest crust (shear model)

A
  • offset from thinnest litho mantle
64
Q

Lateral variation of surface uplift

A
  • Amount of surface uplift varies laterally
  • Crustal stretching dominant = subsidence (density crust < density asthenosphere
  • Mantle stretching dominant = uplift (density lith. mantle > density asthenosphere)
65
Q

Uniform stretching

A
  • 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
66
Q

Uniform stretching: extension

A
  • Original length x times beta
67
Q

Uniform stretching: uniform stretching

A
  • Original thickness C to C/beta

- Original Lithosphere L to L/beta

68
Q

Uniform model: Isostatic effects of crustal thinning

A
  • Accounting for sediment fill but not thermal effects
  • Surface subsides by Depth D
  • D =
69
Q

Uniform model: Eqn 1

A

Depth of subsidence, D = [Crust (1-1/beta)(density C - Density M)]/(Density S - Density M)

70
Q

Reasonable values for Eqn 1 for depth of subsidence

A
  • 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
Q

Reasonable values for Eqn 1 for depth of subsidence

A
  • 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
72
Q

Uniform model: Thermal effects, graph

A
  • Geotherm becomes steeper as depth to 1200 degrees C becomes shallower
  • Average temperature goes from 600C to 900C
  • Returns to equilibrium over time
73
Q

Uniform model thermal effects

A
  • 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
Q

Thermal effects, Eqn 2

A

Vertical expansion = (depth to asthenosphere)(coefficient of expansion)(heating of region)

  • Ex.
    (100km) (3.3x10^-5/degree C)(300C) = 1.0km thermal uplift (initial)
75
Q

Coefficient of expansion

A

3.3 x 10^-5/degree C

76
Q

Gradual thermal subsidence

A
  • Expansion uplift will decay with time
77
Q

How much time will it take for gradual thermal subsidence?

A
  • 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)

EOS410PltTec (12 decks)

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