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Plate Tectonics > 1 > Flashcards

1 Flashcards

1
Q

Slow spreading rate

A

< 50 mm/yr

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2
Q

Fast spreading rate

A

up to 200 mm/yr

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3
Q

Age-Depth relationship

A

As the ocean floor moves away from the ridge, it subsides at a constant rate, regardless of the spreading rate (because the ocean floor is in isostatic equilibrium) …Constant thickness

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4
Q

Source rock for MORB (Mid Oceanic Ridge Basalt)

A

Shallow, passive mantle

Ridges are passive features - asthenospheric mantle wells up in response to plates being pulled apart

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5
Q

Magma chambers at slow-spreading ridges…

A

DO exist (as well as tiny melt lens), but not steady-state features…They must freeze solid between delivery periods

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6
Q

Ophiolites

A

Ancient fragments of oceanic lithosphere which have been uplifted and OBDUCTED on land….Models for oceanic crust generation

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7
Q

Techniques for studying internal structure of lithosphere (and MOR)

A
  1. Direct sampling - submersibles, vehicles, dredging, ocean drilling
  2. Seismic Refraction - P and S waves … time measurements
  3. Seismic tomography - Understanding thermal structure and melt distribution…3D image
  4. Ophiolites
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8
Q

Tethyan ophiolites

A

Mark closure of former ocean basin (Tethys ocean: separated gondwana and laurasia)

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9
Q

Ophiolites contain

A

BONINITES

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10
Q

Mylonitisation

A

High-temperature deformation of gabbros…common in slow-spreading ridges

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11
Q

Dunites

A

Channelways for ascending melts from mantle. Composed of OLIVINE

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12
Q

Eg of Ophiolite

A

Oman

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13
Q

Oman Ophiolite

A

Formed by spreading, and then subduction in Neotethyan Ocean, continental crust resists compression and bounces back isostatically…Leading edge obducted onto Arabian plate = Forms Oman Ophiolite Nappe

Contains metamorphic “SOLE” rock on nappe
Sedimentary rocks beneath ophiolite

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14
Q

VMS

A

Volcanogenic Massive Sulphide deposits…eg Cyprus…on seabed….later blanketed by lava flows

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15
Q

Entire Earth’s oceans cycle through crust hydrothermal circulation every

A

1Ma

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16
Q

Volcanism at convergent margins

A

Silica rich, andesitic and rhyolitic….Viscous + volatiles = EXPLOSIVE

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17
Q

Convergent plate margins?

A

Ocean/Ocean crust or Ocean/Continental crust, but Oceanic crust always subsides because it’s denser…Trenches delineate subduction zones

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18
Q

Intraoceanic over-riding plates form?

A

Island Arc volcanoes

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19
Q

Continental over-riding plates form?

A

Continental arc of volcanoes (or “volcanic arcs”)

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20
Q

Eg Island Arc

A

Aleutian Islands

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21
Q

E.g. Continental volcanic arc

A

Andean Arc…..Nazca (pacific) plate subducts beneath South American plate

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22
Q

Peru-Chile Trench, plates?

A

Nasca subducts beneath South American…..Caused Chile earthquake - largest ever…megathrust.

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23
Q

Accretionary Prism

A

Over-riding plate scrapes up some sediment from the down-going plate…gets incorporated into a wedge-shaped mass….characterised by folding and thrusting

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24
Q

Wadati Beinhoff zone

A

Band of earthquakes in down going plate

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25
Q

Back-arc basins form?

A

In intra-oceanic subduction zones….steep slab dip

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26
Q

As it ages, lithosphere becomes?

A

Thicker and denser

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27
Q

At what age is the lithosphere’s density greater than the asthenosphere?

A

around 54 million years….The lithosphere becomes denser than asthenosphere with age….so it will sink into asthenosphere

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28
Q

H (thickness of lithosphere) =

A

9.5 <- age

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29
Q

Fastest covergence =

A

Oceanic upper plate, old sub ducting lithosphere, steep slab dip + backarc basins

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30
Q

Controls on plates?

A

Trench suction, drag force, slab pull, ridge push….SLAB PULL MOST INFLUENCIAL AND IMPORTANT

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31
Q

MANTLE CONVECTION

A

Subducting plates represent downwelling convection

Upwelling convection = Hotspots (deeper) and MOR (shallower)

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32
Q

Deep earthquakes due to

A

Chase changes in down going slab

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33
Q

Phase changes in down going slab subduction zone?

A

Olivine - Spinel - Oxide (changes to higher-density crystals)…..Causes deep earthquakes

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34
Q

No earthquakes happen below

A

670km

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35
Q

Source of arc magma?

A

Slab dehydration-induced melting of Peridotite in mantle wedge….as in intra-oceanic subduction zones

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36
Q

MASH

A

basalt melt induces melting of lower continental crust….MASH zones develop in lower crust…Melting + Assimilation + Storage + Homogenisation

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37
Q

Batholiths

A

Igneous intrusion…Mix of mantle melt and melted continental crust

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38
Q

Where do accretionary prisms occur mostly

A

In margins with continental over-riding plate

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39
Q

Why does extension sometimes occur at subduction zones?

A

Due to slab roll-back force….the slab sinks more than it dips, so some extension in upper plate….eg Tonga

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40
Q

Many back arc basins found in?

A

Western Pacific basins

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41
Q

Average continental crust thickness

A

41 +/- 6.2 km

42
Q

Blueschists formation

A

Metamorphism of basalt…subduction zones only

43
Q

Eg of ultra-slow spreading ridge (<20 mm/yr)

A

South West Indian Ridge

44
Q

Caledonian Orogeny, closed which ocean?

A

Iapetus Ocean

45
Q

What happened in the caledonian orogeny?

A

Laurentia, Baltica & Avalonia collided

46
Q

What does continental rifting form?

A

Either an oceanic spreading centre or a sedimentary basin

47
Q

Metamorphic Core Complexes

A

When detachment faults have extreme extensional (normal) displacement, causing high-grade rocks to be exhumed from the crust on their footwalls…..found in Basin & Range area, USA…..Continental Rifting environment

48
Q

East african rift = ?

A

Rift valley within broad upcoming region

49
Q

Where do largest earthquakes in world occur?

A

Convergent margins…Subduction zones eg Chile - Peru/Chile trench

50
Q

East coast of South America = what margin?

A

Passive continental

51
Q

East coast of North America = what margin?

A

Passive continental

52
Q

Flood basalt = ?

A

When plume beneath rifting asthenosphere causes decompression melting.. reaches surface to create a flood basalt…eg in Ethiopia

53
Q

The Afar Triangle?

A

Comprises East African Rift Valley, Red Sea and Gulf of Aden….rifting and volcanism

54
Q

The greater the overburden (the deeper the fault), the -

A

stronger it is

55
Q

Brittle behaviour of plates

A

Strength increases with depth….Shallow

56
Q

Ductile behaviour of plates

A

Strength decreases with dept….At very deep levels

57
Q

Why do earthquakes only occur in the shallow, brittle layer, and not the ductile layer?

A

Because ductile flow is aseismic

58
Q

Continental crust dominated by which mineral rheology?

A

Quartz

59
Q

Mantle dominated by which mineral rheology?

A

Olivine

60
Q

Which is stronger - continental or oceanic lithosphere?

A

Oceanic is stronger

61
Q

How does a sedimentary basin form?

A

Rifting begins, but there are low strain rates, so cooling outstrips heating…so thermal contraction occurs, and the lithosphere thickens again. The hole is filled up with sediment loading……eg Lake Baikal (active sed basin)

62
Q

Depth of crust =

A

2500 + 350 √Age

63
Q

Thickness of oceanic crust =

A

up to 100km

64
Q

Typical depth of continental crust =

A

35km

65
Q

End product of continental rifting

A

New ocean basin…Seafloor spreading (and sheeted dyke complexes)

66
Q

Eg Active continental margins

A

West coast of North and South America

67
Q

Abyssal plain

A

Found in passive continental margins…very flat and featureless…may contain turbidites

68
Q

Passive continental margins formed by

A

End product continental rifting - new ocean basin…margins become inactive = passive continental margins

69
Q

Salt diaprisim

A

oil traps…passive continental margins

70
Q

Post rift sediments

A

Evaporites

71
Q

OCT

A

Ocean Continent Transition

72
Q

SDRS

A

Seaward-dipping reflector sequences…found in ocean-continent transition (OCT)…Comprised of subaerial basaltic lava flows

73
Q

Scottish Tertiary Igneous Provinces are what?

A

Onshore equivalent of SDRS (seaward-dipping reflector sequences)

74
Q

SDRS eg

A

East Greenland margin

75
Q

NW Europe-Greenland margin is a what?

A

Volcanic Rifted Margin….magmatism during rifting…hotspot

76
Q

Non-volcanic rifted margins have

A

No SDRS, but they do have Tilted Fault Blocks….as well as sin rift and post rift sediments….oil sources

77
Q

Eg Non-volcanic rifted margin

A

Iberia….listric faults

78
Q

Conjugate margin eg

A

Newfoundland (originally joined to iberia - rifted apart)

79
Q

Passive transform margin eg

A

Ghana/Cote d’Ivoire

80
Q

Active continental transform margin eg

A

San Andreas Fault, Dead Sea Transform

81
Q

Oldest continental crust =

A

Cratons/shields … at centre of plate!

82
Q

Greenstone belts include

A

BIF’s….Granites/Gneisses, Archaen OLDEST…Also Komatiites (ultramafic lavas)

83
Q

Komatiites

A

Old ultramafic lavas

84
Q

Himalayan orogeny collision of what?

A

India and Eurasia plates

85
Q

Rate of Himalayan collision?

A

50 mm/yr

86
Q

What is a foreland basin?

A

Found in continent-continent collision - orogeny…flexural up bulge then foreland basin

87
Q

Foreland basin eg

A

Ganges

88
Q

Name boundary between India and Eurasia

A

Indus-Tsangpo Suture

89
Q

Why is extension common in orogenic belts?

A

Due to excess energy due to elevation…causes lateral pressure gradient pushing outwards

90
Q

What are hotspots?

A

Upwelling of hot material from mantle feeds surface hotspots…independent of plate boundaries … thermal plumes

91
Q

Volcanoes subside as they move away from hotspot and eventually subside to sea level to form a what?

A

GUYOT

92
Q

Eg Mid-plate continental hotspots

A

Yellowstone

93
Q

Eg mid-plate oceanic

A

Hawaiian

94
Q

Ed on- or near-ridge

A

Easter Islands

95
Q

Absolute plate motions are measured

A

relative to the deep mantle

96
Q

Relative plate motions are measured

A

relative to other plates

97
Q

Volcanics at hotspots =

A

Ocean Island Basalts (OIB)

98
Q

Eg of Large Igneous Province

A

Ontong-Java

99
Q

What are LIP’s?

A

Submarine oceanic plateaus

100
Q

On-land equivalent of large igneous provinces?

A

Continental flood basalts….eg Deccan Trap

101
Q

Eg continental flood basalts

A

Bushveld complex, SA

Plate Tectonics (1 decks)

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