6000 Plate tectonics - 3 · Earthquake Repeat Time Mean time interval between large earthquakes on...
Transcript of 6000 Plate tectonics - 3 · Earthquake Repeat Time Mean time interval between large earthquakes on...
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EESC 2200The Solid Earth System
Plate tectonics - 317 Sep 08
Hot SpotsMagnetic ReversalsIsostasyContinental Tectonics
Dep
th (m
)
2500
6000
1550
9 36 100
Homework 1:Due Monday
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Fracture zone
Transform Fault
Review:
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RIDGE
RIDGETransform Fault
Fracture zone
Pitman Fracture Zone Fly-by
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ES101-Lect 2B
• Transform
• San Andreas Fault
• Big Earthquakes, No Volcanoes
• Offsets
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ES101-Lect 2B
• Transform
• San Andreas Fault
• Big Earthquakes, No Volcanoes
• Offsets
right lateral
slides-general
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SAF 35 mm/year
Pacific-N. American50 mm/year
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1906 EarthquakeMagnitude 7.8
6 meters of slip on afault 500 kmlong
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earthquake cycle
Fault at boundary between plates is “locked”Stress builds up on fault as plates moveStress exceeds strength of faultFault suddenly slips in an earthquakePlate boundary moves
Fault locks again
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Earthquake !
fault locked
loading
more loading
faul
t
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Earthquake Repeat TimeMean time interval betweenlarge earthquakes on aparticular fault
For San Andread6 meters = 6000 mm
6000 mm / 35 mm/year= 170 years
170 years of plate motionwas released by thisearthquake
Basis for believing thatThe repeat time forSuch earthquakes isAbout 200 years.
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Aftershocks of Sumatra Dec 2004
Fig. 2. Map showing aftershock locations for the first 13weeks after the 26 December 2004 earthquake from theNEIC (yellow dots, with radii proportional to seismicmagnitude). Moment-tensor solutions from the HarvardCMT catalog (21) are shown for the 26 December 2004and 28 March 2005 mainshocks (large solutions atbottom, with associated centroid locations) andaftershocks. Star indicates the epicenter for the 2004rupture obtained by the NEIC. Dashed line shows theboundary between the aftershock zones for the twoevents.
Lay et al, Science (2005)
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Chapter 4: The Way the Earth Works: Plate Tectonics Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Locations on Earth where tectonic plates meet.Locations on Earth where tectonic plates meet. Identified by concentrations of earthquakes.Identified by concentrations of earthquakes. Associated with many other dynamic phenomena.Associated with many other dynamic phenomena.
Plate interiors are almost earthquake-free.Plate interiors are almost earthquake-free.
Plate BoundariesPlate Boundaries
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Hot spot
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Hot Spots - independent of plates
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Iceland
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Hawaii
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0 Ma
1 Ma
2 Ma
Hot spot records plate motion
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0 Ma2 Ma 200 km
200 km2 Ma
= 10 cm/yr
Hot spot demo
Hot spot records plate motion
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ES101-Lect 3
N
S
The Earth's Magnetic Field
Magnets line up with North magnetic pole
-- generated in core
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ES101-Lect 3
N
S
The Earth's Magnetic Field REVERSES sometimes
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ES101-Lect 3
Rock MagnetismSome minerals (magnetite) can be magnetized
N
As magmas cool, magnetite crystalsorient and point to north pole
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ES101-Lect 3
Magnetic Reversal Time Scale
Normal
Reversed
0
0.72
2.5
Brunes
Matuyama
3.4GaussGilbert
1
2
3
4
Ma
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ES101-Lect 3
Basalt crustForms at Ridge, cools, freezes Mag. Field
Plates Spread, New Basalt Crust Forms
animation
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ES101-Lect 3
Magnetic Field Flips "recorded"
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ES101-Lect 3
... leaving magnetic "stripes" on sea floorStripes are • parallel to ridge • youngest at ridge
A Plate Tectonic strip chart
Vine-Mathews (1963): Magnetic stripes give ages
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Chapter 3: Drifting Continents and Spreading Seas Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Magnetic AnomaliesMagnetic Anomalies Towed magnetometers measure ocean crust.Towed magnetometers measure ocean crust. Magnetism oscillates perpendicular to the MOR.Magnetism oscillates perpendicular to the MOR. These variations are + and - magnetic anomalies.These variations are + and - magnetic anomalies. Anomalies are linear belts that parallel MOR.Anomalies are linear belts that parallel MOR.
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ES101-Lect 3
Pac
Magnetic Stripes "Date" the Seafloor
Global
Atl
Atlantic animation
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Chapter 3: Drifting Continents and Spreading Seas Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Sea-Floor SpreadingSea-Floor SpreadingAges increase away from the MOR.Ages are “mirror images” across the MOR.
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Spreading Rates vary from 3 to 16 cm/yr
Slow spreading: Atlantic
Fast spreading: East Pacific Rise
how do we know?
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Magnetic Stripes!
100 km
2 Ma
= 100 kmper 2 Ma
= 50 km/Ma
= 5 cm/yr
half-spreading rate; full = 10 cm/yr
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ES101-Lect 3
Paleomagnetism: Evidence for continental drift
Some rocks retain magnetic field at formation
(cooling, sedimentation, ...)
Old rocks: field does not point to North pole
N
why not?
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ES101-Lect 3
200 Ma
N
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ES101-Lect 3
100 Ma
N
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ES101-Lect 3
Today
200 Mapaleo-pole
N
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ES101-Lect 3
"paleo-pole"
--> Plates moved!
N
-> A method for reconstructing continental drift
200 Ma
200 Ma
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! Plates with slabs are fast! ! (Pacific, Nazca)
! Plates with continents are slow! ! (Africa, Eurasia, Americas)
Why are some slow, some fast?
slow fast
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What are possible Driving Forces?
Drivingflow
Flowresistance
ridge pushboundaryresistance
cold, denseslab sinks
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plates with slabs are fast:
dense slab drives motion
plate with continents are slow:
continents resist flow
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Continental Tectonics
continent = a passenger
bump and grind
Continent
oldest oceanic crust = 170 Maoldest continental crust = ??
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Density, buoyancy, and isostasy
• Crust is less dense than mantle
• Lithospheric mantle is colder and denser than asthenospheric mantle
• As the lithosphere is rafted away from the ridge axis, it thickens and the seafloor deepens because of isostasy
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Density and temperature
• Coefficient of thermal expansion "
# = #0 (1 $ "T)
• For mantle rocks, " = 3 x 10-5
• Density of mantle rocks #0 at low T: 3.3 g/cm3
• Density of mantle asthenosphere #A (T = 1300°C): 3.17 g/cm3
• Average density of mantle lithosphere #L: 3.24 g/cm3
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Cooling lithosphere
#C = 2.8 g/cm3
#A = 3.17 g/cm3
Pressure atthis depth =
constant
Sea level
hL
hC
#L = 3.24 g/cm3
#W = 1 g/cm3
Ridge axis (t = 0)
Ridge flank (t = 60 Ma)
hW
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Continental Margins
1. passive margin
Continent
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Passive Margins: NOT a plate boundary
•!Can hold high volumes of sediment" (up to 15 km thick!!)
Passive Margin
sediments
Continent
Atlantic Coast
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Passive Margins are created by Rifting
Atlantic OpeningRift System
~ 225 Ma
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faults
sediment
basin
Volcanism
East Africa
W. US
Continental Rifting
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Continental Margins
2. convergent margin
Continent
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Pacific
Plate
Juan
de Fuca
Cascades
Continental
Volcanic
Arc
Shasta
Hood
MSH
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Continental Margins
3. collision margin
Continent
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Parts of the Collision
suturedeformed arc
Orogeny = Mountain Building
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India
Sediments
Fault,Earthquakes
limestone!
Tibet
Himalayan Orogeny
suture
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Mountain Belts
Appalachian Orogenies
Folds and Faults
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Valley and Ridge Province
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cratons: old
cores of ancient
rock
Continents built outward...
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shield: The stable center
of continent
Continents built outward...
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Continents built outward...
Old Interior
Younger margins
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passive margins: !
Old Rifting sites...
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Passive and active margins: the Wilson Cycle
Rifting leads to Ocean Basin formation(Red Sea, Atlantic)
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...later, plates converge and subduct ocean...
arc terranes
arc-con.collision
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Finally, continents collide..
continent-continentcollision...
..later Rifting event can start process over
The Wilson Cycle
Orogeny!
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Net Growth of Continents:
Accretion of volcanic arcs!!
Arcs = new crust, born from the mantle
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Cratonsorogenic belts
OLD MOUNTAINBELTS
the seams between cratons
(accreted arcs)
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N. AmEurasia
AfricaS. Am
Ant.Aust.
Ind
200 Ma: Pangaea
Supercontinent!
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N. AmEurasia
AfricaS. Am
Ant.Aust.
Ind
200 Ma: Pangaea
Laurasia
Gondwanalandplate recon demo