Practice Exam Flashcards
(135 cards)
1
Q
What is Airy’s Model?
A
Continental crust has a uniform density. Consequently, the highest mountains must be supported by deep crustal roots that reach greater depths within the ductile mantle below.
2
Q
Mantle convection
A
The temperature of the Earth increases with depth – a thermal gradient. On geological timescales the mantle is a viscous fluid and therefore convects.
3
Q
Models for mantle convection
A
- Whole mantle convection
- Two-layer convection
4
Q
The Wilson Cycle
A
A model that describes the opening and closing of ocean basins and the subduction and divergence of tectonic plates during the assembly and disassembly of supercontinents.
Stages: Continental rifting, oceanic divergence, oceanic convergence, continent-continent collision, post-collisional orogeny, and peneplanation.
Key Processes: Seafloor spreading, subduction, mountain building, and erosion.
5
Q
The Wilson Cycle
Stage A
A
The cycle begins with a stable continental craton, an old and stable part of the continental crust that has minimal tectonic activity. This craton remains relatively undeformed and inactive.
Example: Cratonic regions like the Canadian Shield or the Australian Shield.
6
Q
The Wilson Cycle
Stage B
A
Early Rifting
Description: Mantle upwelling beneath the stable craton leads to the thinning and stretching of the crust, initiating rifting. Faults form, and volcanic activity may occur as the crust begins to break apart.
Example: The East African Rift, where the continental crust is actively rifting.
7
Q
The Wilson Cycle
Stage C
A
Full Ocean Basin
Description: Continued rifting eventually forms a mid-ocean ridge, leading to seafloor spreading. The two rifted continental blocks move further apart, creating a mature ocean basin.
Example: The Atlantic Ocean, which is a fully developed ocean basin with passive margins..
8
Q
The Wilson Cycle
Stage D
A
Subduction Zone
Description: As the oceanic crust ages, it becomes denser and eventually begins to subduct beneath another plate. This subduction creates an active convergent margin with features like volcanic arcs and ocean trenches.
Example: The Pacific Ocean, where subduction zones exist along its margins.
9
Q
The Wilson Cycle
Stage E
A
Closing Remnant Ocean Basin
Description: Continued subduction causes the ocean basin to narrow. The ocean basin is in its final stages as the two continental plates are drawn closer together, leading toward collision.
Example: The Mediterranean Sea, a remnant of the Tethys Ocean, is closing as the African plate converges with the Eurasian plate.
10
Q
The Wilson Cycle
Stage F
A
Collision Orogeny
Description: The final closure of the ocean basin brings the two continental plates into collision, forming a collisional orogen (mountain belt). Intense deformation and uplift occur as the plates push against each other.
Example: The Himalayas, which formed from the collision between the Indian and Eurasian plates.
11
Q
The Wilson Cycle
Stage G
A
Peneplained Mountain
Description: Over time, the mountain belt formed by the collision undergoes extensive erosion, reducing it to a nearly flat surface or peneplain. This eroded surface eventually stabilizes, returning to a cratonic state.
Example: The Appalachian Mountains, which have been heavily eroded over millions of years.
12
Q
What drives plate tectonics?
A
THE SLABS DRIVE PLATE TECTONIC
SLAB PULL ~ 1X10 14 N/M
Mueller and Phillips (1991)
analysed force balance on
oceanic margins
They concluded that you
needed between 7x10 12 N/m to
1x10 13 N/m to initiate subduction on a passive margin.
The only force big enough to
do this is a mature subduction
zone.
FLEXURE RESISTANCE ~ 8X10 12 N/M
SHEAR RESISTANCE (OCEAN TRENCH FAULT) ~ 1X10 12 N/M
SLAB RESISTANCE ~ 8X10 12 N/M
RIDGE PUSH ~ 3X10 12 N/M
BASAL DRAG ~ 1X10 12 N/M
13
Q
What is the Transition Zone?
Give an example of a minerals journey?
A
Between 410km and 670km
Olivine transforms first to Wadsleyite
Then to Ringwoodite
It transforms in bridgmanite and ferropericlase in the
lower mantle.
Each phase is denser than the ones above.
Key point – the 670 km transition is endothermic. This
means in happens in the warm mantle before the cold
slabs – could prevent slabs from getting through.
14
Q
What is the D Layer?
A
AKA D Double Prime Layer
– Get flattening of velocity & density gradients between 200-250 km above core-lower mantle boundary
- Strong density contrast at CMB = good seismic reflector
–Steep thermal gradient across core-mantle boundary (1500°K)
–Temp. estimates at CMB are 3570 ±200°K to 4000°K
Partial melt in the D layer.
Temp change is the most important
silicate solid mantle with metallic liquid outter core. Seismic waves passing through this layer is a good reflector
- Post - perovskite phase transition: bridgmanite
converts into denser phase. But – only where it’s cold (eg. Slabs) - LLSVPs – Large, low shear-wave velocity provinces (chemically different).
- ULVZ (ultra-low velocity zones) – velocities so
low they have to be partial melt, or infiltrated
molten core metal.
15
Q
Picture the D Layer. Describe what you see
A
16
Q
Physical properties of the Earth’s interior?
Each Layer
A
17
Q
Supercontinents Throughout Earth’s History and there ages
A
Vaalbara: Formed around 3.6 billion years ago from the collision of the Kaapvaal and Pilbara cratons. It was the earliest known supercontinent.
Kenorland: Assembled around 2.7 billion years ago, consisting of parts of present-day North America, Greenland, Scandinavia, and southern Africa.
Columbia (or Nuna): Existed between 2.1 and 1.8 billion years ago, formed from the amalgamation of several smaller continents.
Rodinia: Formed around 1.1 billion years ago and broke apart around 750 million years ago. It was a massive supercontinent that included most of Earth’s landmass.
Pannotia: Assembled around 600 million years ago from the fragments of Rodinia. It was a short-lived supercontinent that existed for about 60 million years.
Pangaea: Formed around 335 million years ago and began breaking apart around 175 million years ago. It was the most recent supercontinent and is well-known for its C-shape.
18
Q
What are the driving and resisting forces acting on tectonic plates
List nine.
A
RESISTING:
TF = TRANSFORM FAULT FRICTION
DF, CD = BASAL DRAG (CD ON CONT)
CR = CONTINENTAL RESISTANCE
SR = SLAB RESISTANCE BY MANTLE
DRIVING:
RP = RIDGE PUSH
SP = SLAB PULL
SU = SLAB SUCTION
DF = DRAG FORCES CAN PULL PLATES ALONG
19
Q
Convergent Boundaries CC and OC Explain how each works
A
OC: The Oceanic plate is denser and cooler than the continental crust and therefore subducts below it. It has negative density greater than a thin hot crust and will go down preferentially.
CC: In this case both crusts are relatively light and hot from the lithosphere. Buoyancy must be balanced and mountain belts start to form.
20
Q
Divergent boundaries
A
OO: Tectonic plates pull apart, causing the mantle to rise and fill the gap. This mantle material is very hot (around 1300 °C) and low in density, making it buoyant. As it cools, it forms new oceanic crust, creating a ridge that sits higher than the surrounding ocean floor. Over time, as the crust cools and becomes denser, it sinks lower, leading to varying ocean depths—2.5 km at the ridge and around 5 km in deeper ocean basins.
CC: As the plates pull apart it forms rift valleys. Normal faults develop on both sides of the rift vally to form on both sides. This process thins and drops the crust, often resulting in significant geological features like the East African Rift Valley (graben).
21
Q
Transform boundaries
A
Transform boundaries occur where tectonic plates slide past each other horizontally. They serve to connect segments of divergent boundaries.
The mid-ocean ridges, where plates move in opposite directions on either side. In oceanic settings, transform faults between ridge segments are seismically active. Once past the ridge segments, the transform fault becomes a fracture zone with no relative motion and no seismic activity.
On continents, transform faults, like the San Andreas Fault, can be dramatic, causing significant earthquakes due to accumulated stress. This differential motion highlights the dynamic interactions of plates across various tectonic settings.
22
Q
What is the Ophiolite Suite?
What are are the layers and depths?
Mean age? Average Age?
A
It is the Oceanic crusts uniform stratigraphy
- Pelagic Sediments
- Basaltic (upper part of oceanic crust)
2A & 2B = pillow lavas, hyaloclastites
2C = sheeted dykes usually 1-3 m wide - Basaltic, mostly gabbro (lower part of oceanic crust).
Remnants of shallow axial magma chambers (feeds the dikes and basalts). - Ultramafic rocks & section of uppermost mantle.
Harzburgite and dunite (residuum of the original mantle)
Mean Thickness ~ 9km
Average Age ~ 180Ma
23
Q
List all the major tectonic plates
A
24
Q
What is Plate Tectonic Theory?
What are the key aspects?
A
Earth’s outer, cool, rigid shell is divided into fragments called “plates.”
Plates float on a viscous, weaker layer beneath them.
Plate movements include collision, sliding past each other, and breaking apart.
Two types of plates exist, differing in density and composition:
Continental: Thicker, less dense, older.
Oceanic: Thinner, denser, short-lived and recycled into Earth’s interior through subduction.
25
Topographic nature of Earth’s crust?
Picture the graph. Where is mass distributed in relation to altitude/depth?
26
Isostasy
Is the rising or settling of a portion of the Earth's crust that occurs when weight is removed or added in order to maintain equilibrium between buoyancy forces that push the crust upward, and gravity forces that pull the crust downward.
The crust neither floats nor sinks within the ductile asthenosphere beneath.
The state in which pressure from every side are equal.
27
Lithosphere-Asthenosphere Boundary (LAB)?
It is a temperature defined discontinuity (1300C). That is defined by:
Reduced seismic wave velocities
Higher electric conductivity
Sheared boundary (peridotite xenoliths)
It seperates the chemically depleted lithosphere from more fertile asthenosphere below it.
It is shallower beneath younger crust.
In continents it is highly varied. The older the crust means more deposition and more trapped mantle beneath.
Under Oceanic crust LVZ is approximately 80km.
The LAB depth varies, generally deeper under continents (up to 200 km) and shallower under oceanic crust (around 50-100 km).
28
What is Pratt's model?
The Compensation Depth was a horizontal surface beneath mountains. Consequently, the density must vary laterally across a mountain range, with the higher altitudes underlain by rocks of lower density
29
Orogeny, erosion and shield formation.
Draw the three main steps
30
Normal Fault
AKA dip-slip fault
Defined by hanging wall moving downwards relative to the footwall due to extensional forces.
45*
31
Reverse Fault
AKA Dip-slip fault
Hanging wall is forced upward driven by compressional forces.
45*
32
Strike-Slip Transverse
What are the two types?
Sidewards movement horizontally in the direction of the fault
left-lateral (sinistral)
right-lateral (dextral)
33
What type of fault is this?
Reverse fault
34
What type of fault is this?
Strike fault
35
What type of fault is this?
Normal fault
36
What type of fault is this?
Oblique
Fault move in two different directions simultaneously, combining both normal or reverse faulting with a strike-slip component.
37
What type of fault is this?
Thrust - compressional forces
38
When basalt cools and solidifies at Mid Ocean Ridge spreading centres, the newly formed oceanic crust is magnetised according to the Earth's magnetic field at that time and location. How have the seafloor magnetic anomalies helped inform Plate Tectonic theory?
Magnetic Stripes and Symmetry: When basalt cools and solidifies at mid-ocean ridges, it records the Earth's magnetic field at that time. Due to periodic reversals in Earth's magnetic field, the seafloor shows a pattern of alternating magnetic "stripes" of normal and reversed polarity. These magnetic anomalies are symmetrically distributed on either side of the mid-ocean ridge, indicating that new crust is being continuously created at the ridge and pushed outward, preserving a record of past magnetic reversals.
Proof of Seafloor Spreading: The discovery of these symmetrical magnetic stripes provided direct evidence for seafloor spreading, a central idea in Plate Tectonic theory. It showed that oceanic crust is generated at mid-ocean ridges, moves laterally away from the ridge, and is eventually recycled back into the mantle at subduction zones. This mechanism of seafloor spreading explains how tectonic plates move over time.
Determining Plate Movement Rates: By measuring the width of the magnetic stripes and knowing the timeline of Earth's magnetic reversals, scientists can calculate the rate of seafloor spreading. These rates match observed plate velocities, reinforcing the concept of moving tectonic plates and quantifying plate movement over geological time.
Global Correlation of Magnetic Patterns: Similar magnetic patterns are observed at mid-ocean ridges worldwide, confirming that the process of seafloor spreading and plate tectonics operates on a global scale. This coherence between different ocean basins provides consistent evidence that plate tectonics is a planetary-scale phenomenon.
39
How does Orogeny differ from an Orogen?
??
Orogeny
Definition: Orogeny refers to the process of mountain building that occurs due to tectonic forces, particularly through the collision and convergence of tectonic plates. This process involves intense deformation, folding, faulting, metamorphism, and often magmatism, which reshape the Earth’s crust.
Process: Orogeny includes various geological activities such as subduction, continental collision, crustal thickening, and uplift. These processes can create mountain belts and are driven by compressional tectonic forces.
Examples of Orogenies: Specific orogenies are named after the regions where they occurred and the geological period when they happened, such as the Himalayan Orogeny (responsible for the Himalayas), Alleghenian Orogeny (Appalachians), and Laramide Orogeny (Rockies).
2. Orogen
Definition: An orogen is the physical product or result of an orogeny. It is a mountain belt or range that has been formed through the orogenic process. Orogens are typically elongated regions of deformed rock that show evidence of past tectonic activity and mountain building.
Structure: An orogen includes geological features like fold-and-thrust belts, metamorphic cores, fault zones, and, in some cases, volcanic arcs. The rocks in an orogen are usually highly deformed and may show a history of intense compression and metamorphism.
Examples of Orogens: The Himalayan Orogen, Appalachian Orogen, and Alpine Orogen are examples of mountain belts formed by orogenic events.
40
In this unit, we discussed Earth's "heat engine" and heat transfer within the planet. What are the two main sources of heat within the Earth today?
??
Radioactive Decay: Decay of isotopes like uranium-238, thorium-232, and potassium-40 in the crust and mantle generates ongoing heat, fueling mantle convection and plate tectonics.
Residual Primordial Heat: Heat from Earth's formation and differentiation, particularly from early accretion and core formation, still remains, contributing to mantle dynamics and volcanic activity.
41
Describe stagnant lid and mobile lid mantle convection, and identify which mode occurs within planet Earth today.
For mobile lid convection, cold surficial material continuously recirculates into the mantle.
In mobile lid convection, the lithosphere is broken into separate, moving tectonic plates that are able to subduct and recycle into the mantle. This type of convection drives plate tectonics, where cold, dense sections of the lithosphere sink into the mantle at subduction zones, while new lithosphere forms at mid-ocean ridges. The continuous motion and recycling of the lithosphere make it "mobile." Mobile lid convection facilitates heat transfer, mantle mixing, and dynamic geological processes at the surface, such as earthquakes, volcanic activity, and mountain building.
Stagnant lid convection results is a thick, stable viscous crust (no PT!).
stagnant lid convection, the outer shell of the planet (the lithosphere) is thick, rigid, and stable, with little to no large-scale horizontal movement or recycling of surface material into the mantle. The lithosphere effectively acts as an insulating "lid" that prevents material from actively recirculating back into the mantle. Instead, convection is confined beneath this stagnant lid, within the underlying mantle, where heat and material circulate in a closed system. This form of convection leads to limited surface geological activity and is typical of planets or moons without active plate tectonics, such as Venus and Mars.
42
Approximate depth of the Moho beneath continental AND oceanic crust.
The crust has some variation in the MOHO depth. Average 35km. 30 to 50 km. Can reach 70km in areas of thick crust e.g Himalayas.
Oceanic crust has a more consistent thickness. Average MOHO depth of 7km. Varies from 5 to 10 km.
43
What is the base of the athenosphere?
How do seismic waves behave in it?
LAB:
100km below continents
60km below cold oceanic crus
Temperature and Ductility:
1300C isotherm
The base of the asthenosphere is defined by a temperature threshold below which mantle rocks become more rigid and less ductile, transitioning into the solid, more stable part of the mantle.
Seismic Properties:
Low seismic velocity zone
Seismic waves speed up below the asthenosphere due to the increased rigidity of the mantle rocks, which is often used to detect this boundary.
44
Plate boundaries have synonymous terms. What do these correspond to?
Convergent
Divergent
Transform
Convergent Boundary
Synonyms: Destructive boundary, subduction zone, collision zone
Divergent Boundary
Synonyms: Constructive boundary, spreading center, mid-ocean ridge
Transform Boundary
Synonyms: Conservative boundary, strike-slip fault
45
In terms of composition, is an island arc basalt the same as an oceanic island basalt?
No, island arc basalts and oceanic island basalts are not the same in terms of composition. While both are basaltic in nature, they have distinct geochemical signatures due to their different origins.
Island Arc Basalts:
Origin: Formed at convergent plate boundaries, where oceanic crust subducts beneath another tectonic plate.
Composition: Enriched in incompatible elements like potassium (K), rubidium (Rb), and cesium (Cs), as well as large ion lithophile elements (LILEs) like barium (Ba) and strontium (Sr). They often exhibit a depleted pattern in high field strength elements (HFSEs) like niobium (Nb) and tantalum (Ta).
Oceanic Island Basalts (OIBs):
Origin: Formed at intraplate settings, often associated with mantle plumes or hotspots.
Composition: Characterized by enrichment in incompatible trace elements and isotopes, including those of helium (He) and neon (Ne), suggesting a deep mantle source. They often show distinct isotopic signatures compared to mid-ocean ridge basalts (MORBs).
In summary, while both island arc basalts and oceanic island basalts are basaltic, their distinct geochemical compositions reflect their different origins and the processes involved in their formation.
46
The temperature at the lithosphere-asthenosphere boundary is approximately
1300
47
The base of a tectonic plate is defined by which boundary?
Moho
Transition Zone
Brittle-ductile transition
Lithosphere-asthenosphere boundary (LAB)
Lithosphere-asthenosphere boundary (LAB)
48
An important way to illustrate the spatial relationships of seismicity and tectonics is through the use of focal mechanisms ("beach balls"), which can tell you whether an individual earthquake is a thrust event (compressional), a normal event (extensional), or strike-slip. Connect each beach ball with its corresponding block diagram of a fault.
49
Explain why there is a negative correlation between the length of the arc-trench gap and the dip of the downgoing slab at subduction zones.
50
The cross-sectional diagram below shows the trace of the downgoing slab for a number of modern subduction zones with the slab profiles normalised to the location of the volcanic arc (Volcanic Front). With the aid of this diagram, explain why there is greater seismic activity and a much higher seismic hazard (large magnitude earthquakes and tsunamis) for the Chile and Peru margins compared to the other margins.
51
Identify which is Airy's and Pratt's model of isostasy.
A. Pratt
B. Airy
52
Describe stagnant lid and mobile lid mantle convection, and identify which mode occurs within planet Earth today.
Stagnant lid mantle convection is when the outer layer of a planet (the lithosphere) is rigid and does not move, while convection happens only in the mantle below. This mode creates a stable, unmoving outer "lid."
Mobile lid mantle convection is when the outer layer is broken into plates that can move and interact. Convection in the mantle drives these plates, causing tectonic activity like earthquakes, volcanoes, and continental drift.
On Earth today, mobile lid convection occurs, as seen in our tectonic plate movement and active plate boundaries.
53
Indicate which subducted slab geometry is indicative of trench advance versus trench retreat.
54
What causes the slab pull force and how does this differ from the slab anchor force?
55
What is a metamorphic core complex? (2 marks)
In what tectonic setting (ie., contractional, transform, extensional, stable cratonic) would you most likely find such a complex forming? (1 mark)
56
What is a Large Igneous Province? There are a number of specific criteria that distinguish Large Igneous Provinces from other igneous events or settings of magmatism. Therefore provide detail in your answer by identifying the 6 distinguishing criteria.
57
What is the difference between a Terrane and Craton?
58
What is the D" layer relevance to whole mantle convection models? (3 marks)
59
Provide one line of evidence, with a brief explanation, that indicates the Yilgarn craton contains an older crustal component than the Pilbara craton.
60
What defines a tectonic plate?
Crust & Lithosphere
61
Explain this image
Lithosphere-asthenosphere boundary (LAB) –Can be a Low Velocity Zone (LVZ) with variable depth & lateral persistence
*200-300 km depth beneath cratons *Top of LVZ is ~80 km beneath oceans
*May be non-existent or not detectable beneath most cratons
62
Explain seismic changes throughout the Earth
63
P wave
Primary: Compressional or longitudinal motion parallel to the direction of wave propagation. The waves slow down and bend at the core boundary towards the center of the Earth.
Shadow zone 105-140 degrees.
Passes through solids, liquids and gases.
64
S wave
Secondary: shear or transverse motion 90 degrees to direction of wave propagation pass through solids only.
No S waves beyond 105 degrees because of the core
65
Depth to the center of the Earth
6370km
66
Mohorovici discontinuity AKA The Moho
Boundary between the Earth's crust and the mantle. It is characterised by a sudden increase in seismic wave velocities, which indicates a transition from the less dense materials of the crust to the denser materials of the mantle.
Continents 30-70 km think
Oceanic crust 5-10 km think
The Earth's crust is composed of lighter rocks like granite and basalt, while the mantle below consists of denser rocks such as peridotite
67
Lithosphere
The crust and the solid uppermost portion of the mantle.
Contributing to tectonic activity, earthquakes, formation of mountains and ocean basins ect.
up to 75 km beneath ocean basins
up to 125 km beneath continents
68
Asthenosphere
Low viscosity. 1-2% molten. 400km depth
A semi-fluid, ductile layer of the Earth's upper mantle. It is solid rock that is so hot that it behaves like a viscous fluid over long periods. This plasticity allows it to flow slowly.
The temperature causes partial melting of rocks, reducing strength and allows for the flow. The pressure is not high enough to prevent this flow.
69
Core
Shadow zone for direct P & S
waves 105-140° seismic
discontinuities.
- outer core (2900-5150 km)
- inner core (5150-6370 km)
S waves shadow zone >105° liquid iron and nickel inner core solid iron and nickel.
70
Craton
the stable interior portion of a continent characteristically composed of ancient crystalline basement rock.
71
What is thermomanent magnetization (TRM)
Magnetite (Fe3O4) and other Fe-bearing minerals at T> the Curie point (580C for magnetite) no magnetisation.
When cooled through the Curie Point in the presence of an external magnetic field, the domains line up to the direction of the magnetic field.
This is one of the ways we date the oceanic crust.
72
Explain metamorphic core complexes?
Draw it
Metamorphic core complexes form in tectonic settings that undergo significant extension. Through low-angle faulting and tectonic exhumation, high-grade metamorphic basement rocks are uplifted and juxtaposed against an unmetamorphosed or lightly metamorphosed cover. These complexes showcase a combination of ductile and brittle deformation due to the differing conditions at depth and in the upper crust, illustrating a complex interplay of geological processes that reveal insights into the history of crustal deformation and extension.
73
What are the four key features of Metamorphic Core Complexes?
Metamorphic Basement and Unmetamorphosed Cover
MCCs feature high-grade metamorphic basement rocks uplifted and exposed beneath a layer of unmetamorphosed or low-grade cover rocks, separated by a distinct tectonic boundary.
Tectonic Exhumation via Low-Angle Faulting
Exhumation occurs along low-angle detachment faults that facilitate significant displacement, bringing deep-seated rocks to shallower crustal levels through crustal extension.
Brittle-Ductile Transition
MCCs display a brittle-ductile deformation gradient, with ductile deformation dominating at depth (high-temperature and pressure conditions) and transitioning to brittle deformation near the surface.
High-Magnitude Crustal Extension
MCCs form in regions undergoing extreme crustal extension, which causes significant thinning and allows deep-seated rocks to rise through the crust, often in rift or back-arc settings.
74
Describe each stage of the Wilson Cycle to the supercontinent Pangea
Stage A: Continental Rifting
Initiation: Mantle upwelling and plume activity beneath the ancient supercontinent Rodinia, which preceded Pangaea, started to cause significant heating and extension of the continental crust.
Magmatism and Extension: As mantle heat increased, Rodinia’s crust began to thin and stretch, forming rift zones. Magmatism led to basaltic volcanic activity along these rift zones, which initiated the breakup of Rodinia and began separating future continental blocks, such as Laurentia and Gondwana.
Stage B: Oceanic Divergence
Seafloor Spreading: The rifting led to the formation of narrow ocean basins like the Iapetus Ocean, with seafloor spreading creating new oceanic crust between the diverging continental blocks.
Passive Margins: As Laurentia, Gondwana, and other landmasses moved away from each other, the newly formed continental edges became passive margins. Thick wedges of sediment began to accumulate along these margins, eroding from the land and building up as the continents drifted apart.
Stage C: Ocean Basin Maturation
Continued Spreading: Seafloor spreading continued to widen the Iapetus and Rheic Oceans, further separating continents and developing mature ocean basins.
Sedimentation: Over time, sedimentation rates on the passive margins decreased as these margins moved further from continental sources, creating well-developed continental shelves and slope deposits along the stable ocean basin edges.
Stage D: Oceanic Convergence
Subduction Initiation: Subduction zones began to develop along the edges of these oceanic basins. The Iapetus Ocean started to close as oceanic crust began subducting beneath the continents.
Oceanic-Continental Subduction: As the Iapetus Ocean narrowed, its oceanic crust subducted beneath the approaching continental masses, creating volcanic arcs and early mountain-building activity along the convergent boundaries. This convergence signaled the beginning of Pangaea’s assembly.
Stage E: Continent-Continent Collision
Mountain Building: The final stage of oceanic basin closure brought continents such as Laurentia, Gondwana, and Baltica into collision. This collision led to intense deformation, metamorphism, and the formation of large collisional mountain ranges, such as the Appalachian and Ural Mountains, which marked the assembly of Pangaea.
Magmatism: Subduction-related magmatism occurred, forming volcanic arcs and intrusions that added igneous rocks to the growing orogenic belts within Pangaea.
Stage F: Post-Collisional Orogeny
Erosion and Isostasy: Once Pangaea was assembled, the newly formed mountain belts experienced rapid erosion. Isostatic rebound occurred as crustal material was removed, allowing the crust to rise and adjust to the new mass distribution.
Sedimentation: Eroded material from the mountain ranges was transported to surrounding basins and deposited, forming extensive foreland basins around the supercontinent.
Stage G: Peneplanation
Long-term Erosion: Over millions of years, continuous erosion reduced the mountain ranges formed during the collision to nearly flat surfaces, or peneplains. This process stabilized the topography of Pangaea, setting the stage for a new supercontinent cycle.
Eventually, the Wilson Cycle repeated, and mantle plume activity beneath Pangaea initiated new rifting. This led to the formation of the Atlantic Ocean and the breakup of Pangaea, continuing the ongoing supercontinent cycle that shapes Earth's geological history.
75
Wilson Cycle Stage A example
Western Australia (Yilgarn Craton)
The Yilgarn Craton in Western Australia is an example of a stable continental craton, an ancient and tectonically inactive area with no significant tectonic activity. It has undergone extensive erosion and weathering over billions of years, leaving a stable, flat surface.
76
Wilson Cycle Stage B example
East African Rift
In the East African Rift, a mantle plume is upwelling beneath the continental crust, causing it to thin, stretch, and form a rift system. Active volcanism and basaltic magmatism accompany this extension, and the rift is in the early stages of creating a new ocean basin.
77
Wilson Cycle Stage C example
Atlantic Ocean
The Atlantic Ocean is a mature ocean basin formed by the breakup of Pangaea. With well-developed mid-ocean ridges, the Atlantic continues to widen as seafloor spreading progresses. Thick sedimentary sequences accumulate along the passive margins, forming broad continental shelves as the continents drift further apart.
78
Wilson Cycle Stage D example
Lesser Antilles (Caribbean)
The Atlantic Ocean crust is actively subducting beneath the Caribbean Plate along the eastern boundary of the Caribbean Sea. This subduction zone has led to the formation of the Lesser Antilles volcanic island arc, a classic example of oceanic crust subducting beneath a continental or oceanic arc. This subduction is ongoing and results in both volcanic activity and seismicity, making it a well-documented example of Atlantic crust convergence.
79
Wilson Cycle Stage E example
Mediterranean Sea
The Mediterranean Sea is a remnant of the ancient Tethys Ocean, now in the final stages of closure due to the convergence of the African and Eurasian plates. Subduction zones around the Mediterranean are leading to the shrinking of the basin, setting the stage for future continent-continent collision and mountain building as Africa moves closer to Europe.
80
Wilson Cycle Stage F example
Himalayas
Following the closure of the Tethys Ocean, the Indian Plate collided with the Eurasian Plate, forming the Himalayas. This continent-continent collision created extensive orogenic belts through intense compression and uplift.
81
Wilson Cycle Stage G example
Appalachian Mountains
The Appalachian Mountains in eastern North America are a classic example of peneplanation. This ancient mountain range, once as tall as the modern Himalayas, has been heavily eroded over hundreds of millions of years, reducing it to a series of low, rounded hills and a nearly flat landscape in some areas.
82
List five mantle source components
1. Depleted Mantle (DM)
2. HIMU
3. Enriched Mantle (EM1, EM2)
4. Primitive Mantle (PM)
5. FOZO
83
Depleted Mantle (DM)
– Source of N-MORB
– Depleted isotopic character (low 87/86 Sr, 143/144 206/204 Pb, high Nd), low LILE abundances
– Undergone 1 or more episodes of melt extraction
– Depletion due to formation of continental crust
84
HIMU
U (high-u, u = 238 U/ 204 Pb)– Defined by extreme enrichment in radiogenic 206,208 Pb
– Mantle source enriched in U+Th relative to Pb
– Also has high Nb, Ta– Pbisotope isochrons indicate source age of 1.5-2 Ga
– Interpreted to be residual subducted oceanic crust in mantle
* Devolatilised slabs will lose Pb-> into crust
* Relatively enriched in Nb, Ta
85
Enriched Mantle (EM1, EM2)
– 2 mantle reservoirs enriched in incompatible elements (eg, Rb, Sm, U, Th)
EM1 generally considered to originate from
* reincorporated old oceanic/subcontinentalmantle lithosphere
– EM1 = moderate 87/86 Sr & low 206/204
EM2 generally considered to originate from *Subducted continental sediments
*Sediments contain much higher concentrations of incompatible elements than mantle
– EM2 = high 87/86 Sr & moderate Pb 206/204 Pb
* Similar isotopic ratios to av. continental crust, sediments at subduction zones
* High radiogenic Sr
86
Primitive Mantle
(PM, PRIMA)–Defined as:
*Estimated average composition of crust + mantle (= Bulk Silicate Earth)
*Same as in chondritic meteorites–Inferred mantle composition after early Earth differentiation into core, mantle & atmosphere BUT before differentiation of the first crust
–Considered to reside in lower mantle
–Debated as to whether LM is entirely or substantially primitive and untouched
87
FOZO
–Definition based on Sr-Nd-Pbisotopic compositions
–Trying to define common component to OIB magmas–Considers OIBs to be mixtures of a few components (ie, DM, HIMU etc.)
88
The MOHO or (Mohorovičić discontinuity) is the boundary between
Define what the Mohorovičić Discontinuity (Moho) is. (2 marks)
89
How long does the wilson cycle take?
What are the end members of the cyle
500myr based on Pangea
25-30 million years to break a continent apart
End members
Passive rifting:
Active rifting: Relies on the mantle plume to rift from mantle upwelling convection cell or mantle plume
90
Breakdown of chlorite in the subducted slab plays a crucial role in the formation of arc magmas, which leads to the characteristic positioning of volcanic arcs about 100-110 km above the downgoing slab
HNW = hottest, shallowest nose of mantle wedge
SWI = Slab-mantle wedge interface
Chlorite Stability and Breakdown:
In subduction zones, chlorite, a hydrous mineral, is stable within the specific pressure-temperature (P-T) range of about 2–3.6 GPa and 800–820°C, which corresponds to conditions near the base of the upper mantle wedge. As the subducting oceanic slab reaches these depths and temperatures, chlorite becomes unstable and undergoes breakdown.
Water Release During Chlorite Breakdown:
When chlorite breaks down, it releases structural water into the surrounding mantle wedge. This released water significantly lowers the melting point of the mantle material above the slab. The presence of water allows melting to occur at much lower temperatures than it would under dry conditions, a process known as hydration-induced flux melting.
Melting in the Overlying Mantle Wedge:
The water released by the breakdown of chlorite causes partial melting of the mantle wedge. This melting occurs directly above the point in the slab where chlorite decomposes, typically at depths of 100-110 km above the subducting slab. The melts generated in this hydrated mantle wedge are then buoyant and begin to ascend toward the Earth's surface, eventually forming volcanic arcs.
Positioning of Arc Volcanoes Above the Benioff Zone:
The consistent position of volcanic arcs 100-110 km above the Benioff Zone (the zone of earthquake activity marking the descending slab) is due to this specific P-T condition for chlorite breakdown. This depth defines where water is released in significant amounts to trigger mantle melting, controlling the location of arc volcanism in relation to the subducting slab.
91
Arc-Trench Gap and Slab Dip:
Arc-Trench Gap of 100-300 km: The typical arc-trench gap (distance between the trench and the volcanic arc) ranges from about 100 to 300 km. This gap reflects the dip angle of the subducting slab, which typically ranges from 40° to 70° as it descends into the upper mantle. The depth and angle of the slab beneath the arc directly influence the positioning of volcanic arcs.
92
Why arc volcanoes are sited ~110 km above downgoing slab?
Consistent Depth of ~100-110 km: The top of the downgoing slab beneath volcanic arcs is consistently located at a depth of approximately 100-110 km. This is a globally observed feature in subduction zones. For instance, in northern Japan, the slab sits just beneath the volcanic arc at this depth, as shown by seismic cross-sections.
Example of Japan: The "JJ" section through northern Japan shows that the Pacific Plate is located about 100-110 km beneath the surface directly under the volcanic arc. This consistency suggests a key depth at which subduction-related melting occurs, releasing fluids into the overlying mantle wedge and generating arc magmas.
93
The material is moving from the white bit in the middle, to the black sections at the side, so it is extensional (a stretching earthquake)
The possible fault planes dip towards the middle of the circle so this would be either a SW dipping plane or a NE dipping plane. There’s no way to tell which plane actually generated the earthquake without other external information.
94
The material is moving from the white sections at the side, to the coloured section (in this case red) in the middle, so it is compressional (a squishing earthquake). These are the kind of beach balls which you’ll see with big megathrust earthquakes.
95
We know that thrusts like to dip at a shallow angle, in some cases very shallow (<5°), so the shallow plane is a definite possibility. We also know that a vertical thrust would be geometrically unusual, so the very steep plane is pretty unlikely. If you see a beach ball like this in a subduction zone setting, its generally a pretty safe bet that the earthquake has occurred on the shallow subduction zone megathrust (with implications for tsunami hazards etc.)
96
Is this NW-SE fault or SW-NE fault?
The best way to know which is right – is to look at a map!
97
Andean margins
Andean-Rocky Mtn-type orogenesis generally related to “flat slab” subduction
What are the three steps to make this occur?
1. Positive slab buoyancy (eg, aseismic ridges, oceanic plateau [impactor] or young age of subductingplate < 10 Ma)
2. Trenchward motion of overriding plate (trench roll forward)
3. Mantle wedge suction force (lowered viscosity in mantle wedge due to hydration)
98
What is the definition for a mantle wedge?
Draw it
Mantle wedge = triangular section of mantle in the hanging wall of subduction zones. It is the mantle source region for the magmatic arc, being fluxed by fluids delivered by dehydration of the descending slab.
99
LIP
Intraplate geochemical affinities
– Many/most LIPs comprise >90% by volume of basaltic/gabbroic composition igneous rock
– Continental LIPs compositionally bimodal, some Silicic-dominant
– Basalts mostly tholeiitic in composition * Some more mafic (high-Mg) compositions
- Picrites (MgO >12 wt%) * Komatiites (MgO >18 wt%; TiO 2
– NB for Precambrian examples * Kimberlites & carbonatites.
100
What defines a LIP
Anomalously rapid emplacements of huge volumes of magma over large areas unrelated to ‘normal’ plate boundary processes
Key attributes are:
1. Areal extent >0.1 Mkm2
2. Igneous volumes >0.1 Mkm3
3. Duration max 50 Myrs
4. Pulsed character of the LIP events (1–5 Myrs). Pulses emplacing a large proportion (>75%) of the total igneous volume.
5. Tectonic (intraplate) setting
they form on continents, in the oceans, usually wholly within plates, occasionally at or near plate boundaries.
6. Composition
intraplate tectonic settings and/or geochemical affinities
Large thermal energy inputs required to produce large volumes (>1 Mkm3) of magma over a geologically short period of time (<5 myrs)
101
What are the largest known LIPs?
Oceanic plateaus
Example - Ontong Java Plateau OJP
Areal extent of ~2 Mkm2
*larger than Alaska
*comparable to western Europe
Maximum plateau thickness of 30–35 km
Igneous volume up to 70-80 Mkm3
Split apart by seafloor spreading
– The OJ, Manihiki & Hikurangi plateaus
= one mega oceanic plateau
102
Why are LIPs Important?
Linked to processes at all depths in the Earth
* Core: magnetic superchronsor subtle variations in magnetic reversal frequency
* Mantle: mafic magma source region (deep and/or shallow), plume transport, mantle overturn?
* Lithosphere: plate tectonic rifting and ocean basin formation
Economic Resources
* Ni-Cu-PGE in mafic LIPs (eg, Norilsk, Siberia);
* Epithermal Au-Ag (bonanza) in Silicic LIPs (e.g., Sierra Madre Occidental)
Temporal coincidence with mass extinctions, environmental change/destruction, climate shifts
103
Lip Types?
104
LIPs major feature of Earth history since Pangea assembly
Picture the map.
105
Composition and Rheology of the Earth's Layers
106
Yilgarn and Pilbara Craton
Age and Composition: Both cratons are Archean 3.7b in age and are composed of granite-greenstone terranes, making them important for studying early crustal processes.
Mineralization: The Pilbara Craton is well-known for its iron ore, while the Yilgarn Craton is famous for its extensive gold deposits. Both cratons have contributed greatly to Australia’s mining industry.
Geological Insights: These cratons help geologists understand early Earth conditions, crust formation, tectonics, and the development of early life (notably stromatolites in the Pilbara).
107
Cratons
* Cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time.
* They are generally tabular bodies with great areal extent (millions of square kilometres) and lithospheric thickness (200-400 km).
108
Accretionary vs Collisional Orogenies
Accretionary orogen
* oceanic slab subduction
* continental arc formation
* accretion of island arc terranes
Collisional orogen
* collision of two continental blocks
109
The Hadean - Earth’s initial config
Zircons
Zircons indicate that
* there must have been some felsic crust in the Hadean
* There was water in the crust and at the Earth’s surface
* The Hadean crust was stable for 100s millions of years from Hf isotopes
Hf isotopes in early Archean zircons suggests that there must be Hadean mafic crust that melted in the Archean
110
The Hadean - Earth’s initial config
Nd
142Nd anomalies suggest long term stable Hadean felsic crust and reciprocal depleted mantle must have existed in the Hadean
That no definitive Hadean lithologies are preserved and that 142Nd anomalies disappear in rocks younger that 3Ga indicates that Hadean crust must have been efficiently mixed back into the mantle.
111
The architecture of Archean crust?
Most of the volume of the continental crust formed in the Archean
1. Composite granitic domes
2. Mafic-ultramafic dominant greenstone belts
3. Late stage sediment dominant basins
4. Granitic gneiss terranes
112
Rifting of oceanic arcs in 3 phases
Phase 1: intermediate to silicic volcanism, grabens & calderas, high heat flow
Phase 2: mafic effusive (submarine) volcanism, dyke swarms with MORB-like compositions, fining upward sedimentation
Phase 3: seafloor-spreading, rift propagation, arc reestablishment
113
Continental BA Regions tend to be broad (up to 1000 km) mobile belts characterised by:
–Thin, hot, weak lithosphere
–High surface heat flow +-elevated topography
–Long history of deformation (extensional & contractional)
–Rigid microplates/terranes decoupled from deformation
114
Individual extensional events in continental mobile belts similar in tectonic & magmatic style to phases 1 & 2 of oceanic backarcs
–Voluminous silicic magmatism, calderas, grabens
–Rift basalts, MORB compositions, dyke swarms
–True ocean floor spreading may never be achieved
* Thicker crust
* Extension distributed over much broader region
115
BABs Summary
*Many subduction zones characterised by long-lived extension
*Extension driven by upper plate retreat
*Extension of continental and oceanic upper plates
*Extension ultimately leads to seafloor spreading and opening of new ocean basin
* Formation of new ocean basin more likely in oceanic settings
*BAB rifts either narrow rifts (oceanic) or broad mobile belts (continents)
116
General characteristics
–Eruptive volume ~5 -19 km3/year
–Originally considered to be extremely uniform in composition, interpreted as a simple petrogenesis
–Extensive studies reveal subtle variations in MOR composition
117
Ocean Basins and Mid-Ocean Ridge
Summary
* MORs and ocean basins = major consequence of LIP events and crustal rupture
* Spreading rate influences thermal structure, physical structure, crustal thickness and amount of melting
* At moderate to fast spreading ridges - 4-layer structure to oceanic crust
* Mantle exhumation can occur at v slow spreading ridges
* Typical thickness of oceanic crust is ~7 km
* Relatively homogenous global composition implies similar source and melting conditions/processes
118
Rifting process inherently asymmetric
–Produces contrasting conjugate margins
Upper plate margins
* Are relatively unextended
* Have large volumes of igneous underplate
* Have passive margin mountains and permanent topography
Lower plate margins
* Are highly extended with rift systems, marginal plateaux
* Have subdued topography and subsided since rifting
* Have thick sedimentary cov
119
Volcanic Rifted Margin??
VRMs characteristic features: List 6
1. LIP Magmatism
2. Often abrupt continent-ocean boundary (eg, Greenland, East Coast USA, Namibia)
3. Prior to VRM formation, continental land masses close to sea level with a variety of sedimentary environments (eg, fluvial, aeolian, etc)
4. Bordered by eroded mountain ranges and plateaus up to 4 km above sea level; gradual decrease in elevation away from rift axis
5. But Km-scale uplift prior to (LIP) magmatism not demonstrable on most VRMs
6. Unroofing of mantle rocks not observed (cf. Iberia)
120
passive margin mountains causes and models
*Various models have been proposed for passive margin mountains:
–Progressive denudation of older mountain belt
–Thermal uplift (eg, mantle plumes)
–Lithospheric flexure (& scarp retreat)
–Magmatic underplating
*Permanent uplift mechanisms most likely
* Large role for magmatic underplating
121
Orogenic mountains vs Passive margin mountains
Passive margin mountains usually comprise uplifted flat-lying rocks forming plateaus 1-4 km high
Orogenic mountains comprise folded and faulted rocks up to >8 km high
122
Continental Margins
Passive Rifting
–Direct application of opposing forces to the lithosphere to create extension
–Lithospheric failure by tensional stresses
–Mantle upwelling as a consequence of lithospheric thinning (ie. passive response)
–NB: igneous activity a secondary process
123
Continental Margins
Active Rifting
–Associated with active (hot) mantle upwelling (eg, mantle plume) and impinging on lithosphere
–Upwelling/plume triggers decompression melting, conductive heating and heat transfer–+/-thermal erosion (thinning) of lithosphere
–Dynamic uplift /thermal buoyancy creates higher gravitational potential for overlying lithosphere
–Tensional stresses & rifting in response to gravitational forces
–NB: Igneous activity onset is pre-to early syn-rift
124
Steeper Slab Dip (Shorter Arc-Trench Gap)
Geometry: When the subducting slab has a steep dip angle (usually around 60-70°), it descends more vertically into the mantle.
Impact on Arc Location: Because the slab reaches the depth needed for dehydration (around 100-110 km) more directly beneath the trench, the resulting mantle melting (which gives rise to arc volcanism) occurs closer to the trench.
Resulting Arc-Trench Gap: This results in a shorter arc-trench gap since the volcanic arc forms relatively close to the trench due to the steep angle of the slab.
125
Draw Fore-arc Basin
Name key features
* Up to ~166 ±60 km wide region
* Dominated by volcanogenic sediment fill
* Can have eroded accretionary wedge material
126
Draw the three types of ophiolite obduction
A. Overthrusting of oceanic lithosphere onto passive-margin or arc rocks during continental collision
B. Splitting off & overthrusting of the upper section of the subducting slab, or
C. Underthrusting of oceanic lithosphere into an accretionary prism.
127
Ocean Basins and Mid-Ocean Ridge
Summary
* MORs and ocean basins = major consequence of LIP events and crustal rupture
* Spreading rate influences thermal structure, physical structure, crustal thickness and amount of melting
* At moderate to fast spreading ridges - 4-layer structure to oceanic crust
* Mantle exhumation can occur at v slow spreading ridges
* Typical thickness of oceanic crust is ~7 km
* Relatively homogenous global composition implies similar source and melting conditions/processes
128
Draw an young slab retreating
129
Draw an old slab advancing
130
Draw Back-arc Basin
Name key features
131
Shallow Slab Dip (Longer Arc-Trench Gap)
Geometry: When the subducting slab has a shallower dip angle (typically 30-45° or less), it descends more gradually into the mantle.
Impact on Arc Location: With a shallow dip, the slab reaches the dehydration depth farther away from the trench. As the slab travels further horizontally before reaching 100-110 km depth, the mantle melting and resulting volcanic arc are positioned further inland.
Resulting Arc-Trench Gap: This creates a longer arc-trench gap because the slab’s gradual descent positions the volcanic arc at a greater distance from the trench.
132
Rifting of oceanic arcs in 3 phases
Phase 1: intermediate to silicic volcanism, grabens & calderas, high heat flow
Phase 2: mafic effusive (submarine) volcanism, dyke swarms with MORB-like compositions, fining upward sedimentation
Phase 3: seafloor-spreading, rift propagation, arc reestablishment
133
Draw a Continental BA
List key features
Broad regions (up to 1000 km) mobile belts characterised by:
Thin, hot, weak lithosphere
High surface heat flow +-elevated topography
Long history of deformation (extensional & contractional)
Rigid microplates/terranes decoupled from deformation
134
Earth's Layers are minerals
Upper Mantle
– Includes lithospheric & asthenospheric mantle
– Extends from Moho to 660 km discontinuity
135
Constraints on mantle composition
–IODP/dredge samples of oceanic crust (<1 km depth)
–Ophiolites (0 -~10 km depth)
–Xenoliths/xenocrysts in igneous rocks (crust & mantle to
~200 km, possibly to 660 km depth)
–Melting products (basalts/gabbros; UM, possible LM component)
–Gases (possible LM origin)
→all provide information on composition of upper mantle
→Xenoliths/xenocrysts also provide record of mantle metasomatism (melt/fluid addition & migration)
