Geology 102

Oceanic crust about 5-8 km thick,

– Mafic (Fe and Mg) composition (basalts and gabbros)

– Average density 3 g/cm3

– Rocks approx 180 million yrs old or less.

Continental crust about 20-70 km thick (40-45 km thick average),

– Felsic (felspar and silica) composition (mixed/granitic).

– Less dense 2.7 g/cm3

– Rocks are older (4 billion yrs old - Acasta gneiss in the NWT.)

Lithosphere:

– Crust plus uppermost mantle (differ in density but firmly attached)

– Rigid and brittle behavior

Asthenosphere:

– Below the lithosphere (still belongs to upper part of the mantle)

– Low-velocity seismic waves indicate partially molten rock

– Plastic behavior

In 1570, Dutch cartographer Abraham Orelius published Theatrum Orbis Terrarum, the first modern atlas of the world.

• Based on these very early maps, Orelius postulated in Thesaurus Geographicus(1596) that the Americas were “torn away from Europe and Africa…by earthquakes and floods.”

• Several others in the 18th and 19th century postulated the movment of continents, including Antonio Snider-Pellegrini who published “La Création et ses mystères dévoilés” in 1858

• Between 1889 and 1909, Roberto Mantovani noted the similarity of geological formations on southern continents and proposed that they were once joined then separated by an expanding Earth

Wegener’s Evidence:

1. Continents fit together;

2. Geological similarities (matching rock formations of Late Paleozoic to Early Mesozoic age between South America and western Africa

3. Paleontological similarity of fossil flora and fauna between continents

4. Paleoclimatic evidence of glacial deposits across southern hemisphere and evidence for tropical paleoclimates in regions now at high-latitudes.

5. Note the very recent breakup of Pangea that Wegener proposed – much too recent.

Glossopterisflora are an extinct order of seed-bearing fern trees with “tongue”-shaped leaves that dominated the Permian Period and are found only on southern hemisphere continents

Why was the hypothesis was rejected??

1) Wegener was a meteorologist/astronomer not a geologist!!

2) Inductive (facts first) vs Deductive (theory first). Americans preferred multiple working hypotheses because science was to be anti-authoritarian. Wegener’s theory was too ambitious and autocratic.

3) Widespread belief in uniformitarianism.

4) No suitable mechanism to explain how continents could move through Earth’s crust

5) Floating masses on a rotating sphere are forced to the equator and then stay there

6) Wegener’s estimates of rate were orders of magnitude too fast

During the 1950s, geophysicists studying paleomagnetism provided two critical lines of evidence that would lead to a new theory of global tectonics:

1) apparent polar wander (continents moved)

2) seafloor spreading (ocean basins are young)

Earth’s Magnetic Field

Earth has a magnetic field generated by the movements in liquid outer core. A ‘north’ magnetic pole and a ‘south’ magnetic pole with lines of magnetic force from one pole to the other. When magnetic force flows from south to north the polarity is considered normal (compass points north). Polarity can reverse (mag. force flows north to south) or ‘flips’ (compass points south).

Vine-Matthews-Morley Hypothesis (1963)

Proposed that if seafloor spreading were correct, mid-ocean ridges should show symmetric stripes of normal and reverse paleomagnetism

Canadian geophyscisist John Tuzo Wilson (1908-1993) :

Early critics noted that the volcanic islands of Hawaii are far removed from plate boundaries. Wilson postulated (1963) that the oceanic crust must be moving over a stationary mantle “hot spot” and that the chain of islands should increase in age reflecting the movement of the plate.

• Wilson’s second contribution (1965) proposed a third type of fault to connect ocean ridges and trenches. He gave the name “transform fault.” He then reasoned that these features connected to divide the Earth into several large rigid plates. Plate Tectonics was born!

• Wilson cycle “Did the Atlantic close then re-open” (1966)

Hot spot: a relatively small geographic area where heating and igneous activity occurs within the crust (e.g., Yellowstone and Hawaii) due to a concentration of heat from a thermal plume that rises from the mantle.

Three basic types of boundaries between two plates that move relative to each other:

1. Divergent Boundary (constructive): plates move away from each other. Crust is produced at divergent boundaries.

a) Oceanic plates separate (seafloor spreading)

b) Continental plates pull apart (continental rifting)

2. Convergent Boundary (destructive): plates move toward each other. Crust is consumed in subduction zones and shortened/thickened by collision

a) Oceanic-Oceanic convergence (subduction)

b) Oceanic-Continental convergence (subduction)

c) Continental-Continental convergence (collision )

3. Transform Boundary (conservative): plates slide parallel past each other. Crust is neither produced nor consumed

a) Oceanic-Oceanic (Fault)

b) Continental-Continental (Fault)

Divergent Boundary (constructive):

• Seafloor spreading along mid-ocean ridges

• Creation of new oceanic crust leading to the formation of an ocean basin

• Tensional stress and normal faults

• High heat flow • Active volcanism (basalt)

• Shallow earthquakes

example: Mid-Atlantic Ridge

Divergent Boundary (constructive):

• High heat flow causes doming and rifting

• Tensional stress and normal faults

• Triple-Junctions, grabens and rifts

• Results in passive margins and a new ocean basin

example: East African Rift Valley

Subduction:

• Convergence forces oceanic crust (denser) beneath continental crust (less dense)

• Oceanic crust is consumed (destructive boundary)

• Continental crust does not subduct – leads to collision of continental plates

• Partial melting of mantle wedge and of descending slab results in active volcanism

• Inclined zone of shallow to deep (700 km) earthquakes = Wadati-Benioff zone

Oceanic-Oceanic Convergence:

• Either oceanic plate subducts (but not both)

• A deep-sea trench forms with the subducting plate

• An Island Arc forms on the margin of the overriding plate

Example: West Pacific

Oceanic-Continental Convergence:

• Oceanic plate subducts beneath continental crust

• A deep-sea trench forms with the subducting plate

• A Continental Volcanic Arc forms on the continental plate

Example: Volcanic Arc(Cordillera)

Continental-Continental Convergence:

• Two continental plates converge

• Continental crust does not subduct

• Collision results in mountain building

• Thickened crust rises by isostacy

Oceanic Transform Faults:

• Two oceanic plates slide past each other in opposite directions at ocean ridges

• Enormous strike-slip faults that terminate abruptly at both ends

• Offsets the mid-ocean ridges

• Seismically active

Associated with Mid-Ocean Ridges

Continental Transform Boundaries:

• Two plates of continental crust grind past each other in opposite directions

• Shear stress

• Transform (Strike-slip) faults

• Seismically active

• Low heat flow an no volcanism

Example: San Andreas

Mantle Convection – The Standard Model

• Convection in the mantle is caused by heat from Earth’s interior.

• Convection thought to drive lithospheric plates.

• Rising convection cell at divergent boundaries causes plates to spread laterally.

• Descending convection cell at subduction zones where cool lithosphere dragged into mantle.

Seismic tomography is a method for mapping temperature anomalies in the mantle using seismic waves from earthquakes

• Mantle consists of regions that are colder/hotter than normal

• Seismic waves have higher velocities in cold dense rock than hot rock of the same type

Seismic tomographic slice through the equator (Schubert et al., 2001):

• Beneath subduction zones mantle is colder than normal

• Beneath spreading ridges mantle is not always hotter than normal.

• More complicated than the classic model of cylindrical convection cells.

• Note that at Carlsberg Ridge the African plate moves in the opposite direction of the convection cell.

– Drag: convective drag on the base of the plate

– Push: elevation at ridge pushes plate ahead of it

– Pull: descending slab pulls the plate along

– Suction: Broken plate segments create suction

Age of the Ocean Basins:

• Absolute age of the paleomagnetic stripes increases away from the spreading centres.

• No part of the sea floor is older than Mesozoic age (< 200 Ma)

Hawaiian hot spot

– Mantle plume rises and creates a hot spot that produces volcanism

– Plate movement continues over the plume and the stranded volcano cools while another forms

– Produces a chain of volcanic islands that are progressively older away from the hot post

– Chain indicates direction and rate of movement

GPS (Global Positioning System) is used to measure plate velocities:

Average rate: 5 cm/year; not all plates move at the same rate, relative motion

Stages of the Tectonic Cycle:

1. Continental Rifting

2. Initiation of a new ocean basin

3. Initiation of subduction

4. Ocean closure and orogenesis

5. Continental collision

6. Erosion

Continental Rifting Continental break-up by crustal extension/divergence due to rising convection currents in the underlying mantle

Doming stage:

– High heat flow causes the crust to be elevated, stretched and thinned. Extensional forces insufficient to rift the crust.

Rifting stage

– Extension breaks crust into normal fault blocks that subside rapidly to form a grabens

– Accumulate thick sediments and lakes form

– Rift Valley, East Africa

Triple junction:

– Boundary between three tectonic plates. Three-armed grabens at plate boundaries

– Hot spot causes doming and thinning of the crust, which separates by tension

– May have multiple types of plate boundaries

Rift valleys:

– Extension breaks continental crust into fault blocks that subside rapidly and accumulate sediments in lakes, e.g. Rift Valley, East Africa.

Aulacogen: a failed arm of a triple-junction that becomes a sediment-filled graben:

– Mississippi River

– Amazon river

– Niger River

Rifting creates passive margins:

– When rifting continues, continents separate along ridge axis that becomes a mid-ocean ridge.

– No subduction along the margins

Initiation of Subduction

• Passive margins become active margins by:

1. Sediment loading can flex the lithosphere

2. Changes in plate direction and motion

3. Pre-existing zones of weakness

• Subduction consumes oceanic crust and initiates closure of the ocean basin

Closure of the Ocean Basin

• Subduction consumes oceanic crust and initiates mountain building

• As oceanic crust is consumed the ocean basin closes

• Leads to continental collision and a major episode of mountain building

Orogenesis: process of mountain building

Orogeny: mountain building event

Orogenesis

– Subduction causes volcanism, accretion of terrenes, and closure of ocean basins

– Continental crust can’t be subducted

– Subduction polarity can change

– The great thickness of continental crust and its low density relative to that of the asthenosphere lead to mountain building

– Suturing: unification of two continents along a subduction zone

– Ophiolite: remnant of seafloor pinched up along suture during collision

Terrane: geologically distinctive regions of Earth’s crust each of which has behaved as a coherent crustal block

North American Cordillera composed of numerous terranes with a complex history docking and post-accretion dispersion

Mountain belts have a characteristic structure

1) trench

2) accretionary wedge

3) forearc basin

4) igneous core

5) metamorphic belt

6) fold-and-thrust belt

7) foreland basin

Trench:

– Deep-sea depression along the trace of the subduction zone.

Accretionary wedge:

– Body of rock scraped from the subducting plate and accumulated on the margin of the overriding plate. Mélange is a chaotic deformed mixture of rocks.

Forearc Basin:

– Basin on the oceanic side of the arc that accumulates deep-sea marine muds and graywackes mixed with oceanic crust.

Volcanic Arc:

– Central igneous core of volcanoes and plutons

Metamorphic Belt:

– Deformed rocks by heat and pressure on either side of the igneous core

Fold-and-Thrust Belt:

– Folded and faulted margin of the mountain chain with large thrust sheets

Foreland Basin:

– Downwarping of the continent creates a basin that fills with sediments (flysch and molasse) from the mountain system

Development of foreland basin

– Orogenesis causes downwarping of lithosphere beyond the fold and thrust belt

– Axis is parallel to mountain chain

– Rapid formation; usually deep and often flooded

– Sedimentary basin filled with flysch and molasse