Download Tectonics of the Precambrian

Tectonics and climate of the
Precambrian
Geology 103
When Did the Solar System Form?
• 4.56 billion years ago
• How do we know? (evidence for formation)
•Lunar samples - 4.5 to 4.6 Ga
•Meteorites - 4.56 Ga
•Earth – 3.9 (or 4.4 Ga)
Lunar meteorite at
http://meteorites.wustl.edu/lunar/stones/mac88105.htm
Meteorite photo by Carl Allen at
http://ares.jsc.nasa.gov/Education/Activities/ExpMetMys/..%5C..%5CSlideSets/ExpMetMys/Slides1-9.htm
How Did We Get a Solar System?
Image: LPI
Huge cloud of cold, thinly dispersed
interstellar gas and dust – threaded with
magnetic fields that resist collapse – solar
nebula theory of Swedenborg (1734), Kant
(1755) and Laplace (1796).
Hubble image at
http://hubblesite.org/newscenter/archive/releases/nebula/emission/2006/41/image/a/
How Did We Get a Solar System?
Image: LPI
Concentrations of dust and gas in the
cloud; material starts to collect (gravity
> magnetic forces)
Hubble image at
http://hubblesite.org/newscenter/archive/releases/nebula/emission/2005/35/image/a/
How Did We Get a Solar System?
Gravity concentrates
most stuff near center
Heat and pressure
increase
Collapses – central
proto-sun rotates
faster (probably got
initial rotation from the
cloud)
Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_1.html
How Did We Get a Solar System?
• Rotating,
flattening,
contracting disk solar nebula!
Equatorial Plane
Orbit Direction
NASA artwork at http://en.wikipedia.org/wiki/Image:Ra4-protoplanetary-disk.jpg
How Did We Get a Solar System?
• After ~10 million years, material in center of nebula hot
enough to fuse H
• “...here comes the sun…”
NASA/JPL-Caltech Image at
http://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20060724.html
How Did We Get a Solar System?
• Metallic elements (Mg, Si,
Fe) condense into solids
at high temps. Combined
with O to make tiny grains
• Lower temp (H, He, CH4,
H2O, N2, ice) - outer
edges
Planetary Compositions
Hubble photo at
http://hubblesite.org/newscenter/archive/releases/star/protoplanetary-disk/2005/10/image/a/layout/thumb/
How Did We Get a Solar System?
Inner Planets:
• Hot – Silicate minerals, metals, no light elements, ice
• Begin to stick together with dust  clumps
Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_3.html
How Did We Get a Solar System?
• Accretion - particles collide and stick together … or
break apart … gravity not involved if small pieces
• Form planetesimals, up to a few km across
Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_3.html
How Did We Get a Solar System?
• Gravitational accretion: planetesimals attract stuff
• Large protoplanets dominate, grow rapidly, clean
up area ( takes ~10 to 25 My)
Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_4.html
The Precambrian divisions are defined
broadly by atmospheric changes
• Hadean: Lots of carbon dioxide, water vapor and
methane
• Archean: Water vapor forms oceans, oxygen starts to be
made by photosynthetic organisms
• Proterozoic: Significant oxygen in atmosphere, massive
drop in carbon dioxide
Graphically…
Some boundaries
coincide with
other events
Present-day plate
tectonics “begins”
Period of heavy
bombardment
{
Period of major accretion
(~ 10-30 my)
Archaen-Proterozoic transition
To modern plate tectonics
Present-day plate
tectonics “begins”
period of
rapid crustal
growth
Period of heavy
bombardment
{
1. Early plates became bigger and thicker
2. Continued recycling of oceanic crust
formed large amounts of buoyant
continental crust
•
Continued partial melting/distillation
•
Separation of Si and other elements from
Mg and Fe
•
Conversion of mafic material to felsic
material through rock cycle
3. Decrease in heat production slowed mantle
convection
•
Drove system to larger convection cells
•
Allowed larger plates to travel farther
on the Earth’s surface and cool more
•
Led to subduction rather than collision of
plates
•
Modern plate tectonics
Period of major accretion
(~ 10-30 my)
The Witwatersrand (South Africa)
goldfields
Evidence against the theory
• Not all gold deposits are
the same age
• Clearly, some other
mechanism deposits
gold in this fashion –
anoxic inland seas?
More evidence for atmospheric
change in Archean
• Banded iron formations
(BIFs) are interlayered
alternating chert
(jasper) and iron oxide
• Mostly found in
Archean, some in
Proterozoic, almost
none in the Phanerozoic
Mechanism for generating BIFs
Since the Archaean
• Intensity of plate tectonics has varied over time
• Wilson cycles – 500 my cycles
– Evidence of a supercontinent at 600-900 my (Rodinia)
– Pangea formed ~ 300 my
• Periods of rapid sea floor spreading (and vice
versa)
– Sea level rises because large amounts of shallow basalt
form and don’t cool (and subside) much
– High CO2 release – released at spreading centers when
new crust forms and subducting crust has sediment on
it including calcite which releases CO2 when it melts
Meanwhile, plate tectonics settles down
• Archean rocks worldwide
are of only two types:
granite/gneiss complexes
(a high-grade
metamorphic rock) and
intervening greenstones
(metamorphosed basalt
and some sedimentary
rock)
• Superior province in
North America is among
the biggest in the world
What does a greenstone belt remind
you of?
But still different than today’s plate
tectonics
• Komatiites are
ultramafic
igneous rocks
that are
common in the
Precambrian
but unknown
today
• Hotter mantle?
• Wetter mantle?
• Diamonds!
First continents form and stick around
So, by the Proterozoic…
• Division between
Archean and
Proterozoic is based on
oxidizing conditions
found in surface waters
(1.8 by)
• Tectonics is more
similar to today’s;
evidence for rifting and
subduction and terrane
accretion
•T
W
J
s
A
What evidence exists for Rodinia?
• Grenville orogeny rocks
(sometimes called “mobile
belts”), originally defined to
explain Canadian shield
rocks, were found to exist
on many other continents
• All this mountain-building
implies some large-scale
tectonic event, like a
supercontinent (name was
suggested in the 1990s)
• Rodinia is constructed at 1.1
by, rifts apart by 0.85 by
The Grenville orogeny rocks
• Primarily marine
sandstones and
carbonates (limestones)
• No bioturbation
• Since then, these rocks
have been
metamorphosed, but
the original rock is
easily inferred
Conventional reconstruction
• Line up all the
Grenville orogenic
belts and create
the
supercontinent
• Note that
Antarctica and the
US (Laurentia) are
quite separated
The SWEAT hypothesis
• Rodinia joined the
southwest (SW) US
(West Texas,
specifically) with
eastern Antarctica (EAT)
• Shown through lead
isotope measurements
of similar age rocks that
were part of a rift in
both areas
• Key point: there was
not just one zone of
orogeny as in the
conventional theory
Precambrian climate
• Positions of continents, especially existence of polar
continents, determines when ice ages occur
Positive feedback
• If glaciers can build extensively
to within 30° of the equator,
the extensive ice will reflect a
large portion of the Sun’s
energy back into space, cooling
the surface and allowing more
glaciers to grow
• “Icehouse Earth” or “Snowball
Earth” hypothesis (W. Brian
Harland, Cambridge, 1964)
How to get out of the Icehouse
• Joe Kirschvink (Caltech, 1992) argued that volcanic
activity and carbon dioxide production would not
cease even during an Icehouse event, and nothing
would “scrub” the carbon dioxide out of the
atmosphere, enhancing the greenhouse effect
More detail about CO2 scrubbing
Life alters as Rodinia breaks up
• Ediacaran fauna appears
– first evidence of
multicellular life
• No hard parts, preserved
as molds
• Unclear if they are truly
related to modern phyla,
or represent extinct phyla
• Ediacaran period is a
recognized division of the
Proterozoic eon (630 –
542 my)