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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
Back-arc basins form?
In intra-oceanic subduction zones....steep slab dip
26
As it ages, lithosphere becomes?
Thicker and denser
27
At what age is the lithosphere's density greater than the asthenosphere?
around 54 million years....The lithosphere becomes denser than asthenosphere with age....so it will sink into asthenosphere
28
H (thickness of lithosphere) =
9.5 <- age
29
Fastest covergence =
Oceanic upper plate, old sub ducting lithosphere, steep slab dip + backarc basins
30
Controls on plates?
Trench suction, drag force, slab pull, ridge push....SLAB PULL MOST INFLUENCIAL AND IMPORTANT
31
MANTLE CONVECTION
Subducting plates represent downwelling convection | Upwelling convection = Hotspots (deeper) and MOR (shallower)
32
Deep earthquakes due to
Chase changes in down going slab
33
Phase changes in down going slab subduction zone?
Olivine - Spinel - Oxide (changes to higher-density crystals).....Causes deep earthquakes
34
No earthquakes happen below
670km
35
Source of arc magma?
Slab dehydration-induced melting of Peridotite in mantle wedge....as in intra-oceanic subduction zones
36
MASH
basalt melt induces melting of lower continental crust....MASH zones develop in lower crust...Melting + Assimilation + Storage + Homogenisation
37
Batholiths
Igneous intrusion...Mix of mantle melt and melted continental crust
38
Where do accretionary prisms occur mostly
In margins with continental over-riding plate
39
Why does extension sometimes occur at subduction zones?
Due to slab roll-back force....the slab sinks more than it dips, so some extension in upper plate....eg Tonga
40
Many back arc basins found in?
Western Pacific basins
41
Average continental crust thickness
41 +/- 6.2 km
42
Blueschists formation
Metamorphism of basalt...subduction zones only
43
Eg of ultra-slow spreading ridge (<20 mm/yr)
South West Indian Ridge
44
Caledonian Orogeny, closed which ocean?
Iapetus Ocean
45
What happened in the caledonian orogeny?
Laurentia, Baltica & Avalonia collided
46
What does continental rifting form?
Either an oceanic spreading centre or a sedimentary basin
47
Metamorphic Core Complexes
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
East african rift = ?
Rift valley within broad upcoming region
49
Where do largest earthquakes in world occur?
Convergent margins...Subduction zones eg Chile - Peru/Chile trench
50
East coast of South America = what margin?
Passive continental
51
East coast of North America = what margin?
Passive continental
52
Flood basalt = ?
When plume beneath rifting asthenosphere causes decompression melting.. reaches surface to create a flood basalt...eg in Ethiopia
53
The Afar Triangle?
Comprises East African Rift Valley, Red Sea and Gulf of Aden....rifting and volcanism
54
The greater the overburden (the deeper the fault), the -
stronger it is
55
Brittle behaviour of plates
Strength increases with depth....Shallow
56
Ductile behaviour of plates
Strength decreases with dept....At very deep levels
57
Why do earthquakes only occur in the shallow, brittle layer, and not the ductile layer?
Because ductile flow is aseismic
58
Continental crust dominated by which mineral rheology?
Quartz
59
Mantle dominated by which mineral rheology?
Olivine
60
Which is stronger - continental or oceanic lithosphere?
Oceanic is stronger
61
How does a sedimentary basin form?
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
Depth of crust =
2500 + 350 √Age
63
Thickness of oceanic crust =
up to 100km
64
Typical depth of continental crust =
35km
65
End product of continental rifting
New ocean basin...Seafloor spreading (and sheeted dyke complexes)
66
Eg Active continental margins
West coast of North and South America
67
Abyssal plain
Found in passive continental margins...very flat and featureless...may contain turbidites
68
Passive continental margins formed by
End product continental rifting - new ocean basin...margins become inactive = passive continental margins
69
Salt diaprisim
oil traps...passive continental margins
70
Post rift sediments
Evaporites
71
OCT
Ocean Continent Transition
72
SDRS
Seaward-dipping reflector sequences...found in ocean-continent transition (OCT)...Comprised of subaerial basaltic lava flows
73
Scottish Tertiary Igneous Provinces are what?
Onshore equivalent of SDRS (seaward-dipping reflector sequences)
74
SDRS eg
East Greenland margin
75
NW Europe-Greenland margin is a what?
Volcanic Rifted Margin....magmatism during rifting...hotspot
76
Non-volcanic rifted margins have
No SDRS, but they do have Tilted Fault Blocks....as well as sin rift and post rift sediments....oil sources
77
Eg Non-volcanic rifted margin
Iberia....listric faults
78
Conjugate margin eg
Newfoundland (originally joined to iberia - rifted apart)
79
Passive transform margin eg
Ghana/Cote d'Ivoire
80
Active continental transform margin eg
San Andreas Fault, Dead Sea Transform
81
Oldest continental crust =
Cratons/shields ... at centre of plate!
82
Greenstone belts include
BIF's....Granites/Gneisses, Archaen OLDEST...Also Komatiites (ultramafic lavas)
83
Komatiites
Old ultramafic lavas
84
Himalayan orogeny collision of what?
India and Eurasia plates
85
Rate of Himalayan collision?
50 mm/yr
86
What is a foreland basin?
Found in continent-continent collision - orogeny...flexural up bulge then foreland basin
87
Foreland basin eg
Ganges
88
Name boundary between India and Eurasia
Indus-Tsangpo Suture
89
Why is extension common in orogenic belts?
Due to excess energy due to elevation...causes lateral pressure gradient pushing outwards
90
What are hotspots?
Upwelling of hot material from mantle feeds surface hotspots...independent of plate boundaries ... thermal plumes
91
Volcanoes subside as they move away from hotspot and eventually subside to sea level to form a what?
GUYOT
92
Eg Mid-plate continental hotspots
Yellowstone
93
Eg mid-plate oceanic
Hawaiian
94
Ed on- or near-ridge
Easter Islands
95
Absolute plate motions are measured
relative to the deep mantle
96
Relative plate motions are measured
relative to other plates
97
Volcanics at hotspots =
Ocean Island Basalts (OIB)
98
Eg of Large Igneous Province
Ontong-Java
99
What are LIP's?
Submarine oceanic plateaus
100
On-land equivalent of large igneous provinces?
Continental flood basalts....eg Deccan Trap
101
Eg continental flood basalts
Bushveld complex, SA

Plate Tectonics (1 decks)

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