Download Evolution of the Ocean Basins

Evolution of the Ocean Basins
Chapter 3
Major Concept (VI)
• Geological time is often difficult to grasp
because of how long it is compared to all
of human history (to say nothing of the
incredibly brief time span of a single
human life).
• The subdivision of geological time and the
record of important events is called the
Geologic Time Scale.
Dinosaurs and many other spp die
50% of all species die
96% of all species die
• The divisions of geologic time were
originally decided upon on the basis of
the appearance and disappearance of
specific fossils.
– So these are relative dates
• Just as we divide time into segments like
years, months, and days to identify both
duration and some sense of position, we
divide geologic time as well.
• The longest division is called an eon.
Eons are divided into eras, eras are
divided into periods, and periods into
epochs.
• The eras, in order of most distant past to
most recent time, are:
– Precambrian,
– Paleozoic (ancient life),
– Mesozoic (middle life, or the Age of Reptiles),
and
– Cenozoic (recent life, or the Age of
Mammals).
• The development of radiometric dating
techniques allowed geologists to assign
absolute dates to the different time
divisions
• Because of the difficulty we typically have
in comprehending time spans as great as
billions of years, it is often easier to
imagine that the earth is much younger,
say tens of years old, and then consider
what major events occurred in that period
of time.
• Suppose, for example, that rather than the
earth being 4.6 billion years old, it is only
46 years old.
• Then we can recognize the following
important events in this order:
– 0 years of age: the earth formed
– the first 6 years of the planet’s history have
not been preserved in the record
– 7 years of age: the oldest discovered rocks
were formed
– 12 years of age: the first living cells appeared
– 22—23 years of age: photosynthesis began
producing oxygen
– 31 years of age: there was sufficient oxygen in the
atmosphere to allow the growth of complex cells that
require oxygen
– 40 years of age: the first hard-shelled organisms
appear, preserved later as fossils
– 41 years of age: the first animals with backbones,
called vertebrates, appeared
– 41 years and 8.5 months: land plants appear and are
shortly followed by a period of time when fish are the
dominant animal form
– 43 years of age: the appearance of reptiles
– 44 years of age: dinosaurs are abundant
– 45 years of age: the dinosaurs disappear
– about 45 yrs: flowers can be found
– 45 yr and 6 months: mammals, birds, and
insects are the dominant lifeforms
– 25 days ago: human ancestors appeared
– 11 days ago: the first member of the genus
Homo appears
Oldest Rocks
• 3.85 billion years old
• Oceanic crust is less than 200 million yrs
– Oceanic crust has been renewed at least 1520 times
• Tectonic cycle may have been faster earlier
– Earth was much hotter
Fig. 3.19
• Recent evidences and calculations of
thermal energy in the earth suggest a 500
million year cycle in major plate
movement.
• There are several stages starting with
rifting and ending in a declining ocean
basin.
Stage
Examples
Embryonic
Mature
E. Africa rift Crustal
extension
Red Sea,
Subsidence
Gulf of CA spreading
Atlantic
Spreading
Decline
Pacific
Terminal
Mediterran
ean Sea
Himalayas
Young
Relict scar
Motion
Shrinking
Features
Rift valleys
Narrow
seas
Active ridge
Ridges and
trenches
Shrinking & Young
uplift
mountains
Shrinking & Young
uplift
mountains
• North America appears to have a core
that was assembled 1.8 billion years
ago from several large pieces of
continental crust more than 3 billion years
old.
• These large crustal pieces are called
cratons.
• Smaller crustal fragments, called
terranes, with a history distinct from the
cratons can be found along their margins
(see fig. 2.30).
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Terranes
Major Concept (IV)
• The configuration of the continents
changes through geologic time.
• This movement of the continents across
the face of the earth is called continental
drift.
• Early observations of the apparent fit of
South America and Africa were made
by:
– Francis Bacon, English (1561—1626),
– George Buffon, French (1707—88),
– Alexander von Humboldt, German (1769—
1859), and
– Antonio Snyder, American, in the 1850s,
said that the Atlantic Ocean had formed
when a single great landmass was split by
volcanic activity.
• Between 1885 and 1909 Edward Suess,
Austria, proposed that the southern
continents had been joined in a single
great continent he called Gondwanaland.
• Suess believed that parts of the continents
had sunk to form the oceans due to
isostacy.
• Around the turn of the century, Alfred
Wegener and Frank Taylor
simultaneously proposed the theory of
continental drift.
– Taylor later stopped pursuing the idea.
• Wegener suggested that roughly 200
million years ago the continents were
joined in a single great landmass called
Pangaea.
© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pangaea
• Pangaea later separated into a northern
continent called Laurasia and a southern
continent called Gondwanaland.
• Laurasia included what we now recognize
as North America and Eurasia while
Gondwanaland was comprised of Africa,
South America, India, Australia, and
Antarctica .
© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pangaea Series
From R.S. Dietz and J.C. Holden, Reconstruction of Pangea: Breakup and Dispersion of Continents, Permian to Present in Journal of Geophysical
Research, 75:26 (September 1976). American Geophyscial Union, Washington, DC.
© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pangaea
Series
From R.S. Dietz and J.C. Holden, Reconstruction of Pangea: Breakup and Dispersion of Continents, Permian to Present in Journal of Geophysical
Research, 75:26 (September 1976). American Geophyscial Union, Washington, DC.
• Panthalassa Ocean – the major ocean
– Later becomes the Pacific
• Tethys Sea gets smaller
Birth of an Ocean
• Begins with crustal extension
– Ductile lower part of crust is stretched
– Brittle upper part if rifted
• When separation occurs, gap is filled with
basalt
– Thin and dense oceanic crust is below sea
level
© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Rifting
• The Red Sea Rift began during the Miocene
Epoch (about 25 million years ago) and
continues today.
– Its formation is related to the formation of the Aden
Rift.
– The two rifts have now effectively separated Africa
from Arabia, although the two were once part of the
same landmass, the Afro-Arabian craton.
• It has been suggested that the rifting was
caused by Africa being displaced 200 kilometers
south and west of its original position.
• Both Rifts attain oceanic depths (i.e. 2,000
meters below sea level) over most of their
extent.
• It is possible that Africa is actually pivoting away
from Arabia.
• Evaporites are 4 km thick on flanks
– Date to Miocene 20 million years ago
• Ended only 5 million years ago
– Obscure the underlying crust
• Formed when connections to the ocean
were intermittent
– Finally broke open to Indian ocean
A 200-foot-wide (60-meter-wide) explosive vent opened in Ethiopia in September of
2005 after two days of nearly continuous seismic activity—a visible sign of a 37-milelong (60-kilometer-long) tectonic split. This recent tear in the Earth's continental crust
near the Red Sea is the largest single rip seen since satellite monitoring began
• Only in axial zone is there true oceanic
crust produced by seafloor spreading
• Red sea can be subdivided into 4 zones
– Rift Valley – true ridge system
– Multi-deeps – contain brines and metal
deposits
– Transitional – deeps become narrower
– Northern – isolated deeps, no magnetic
anomalies
Major Ocean Basins
• Spreading histrory is least complicated in
the Atlantic
• Pacific and Indian more so, because of
subduction zones and adjustments in
spreading direction
Fig. 3.19
• Hawaiian Island chain shows the
complexity
• It is possible that the ridge systems are
migrating relative to the deep earth.
– Spreading center could be destroyed in a
subduction zone
Indian Ocean
• Northern boundary is a complex
subduction zone
– Himalayas and Java Trench system
– Spreading south of India has changed
direction from north-south to north-east-southwest about 50 Ma ago
– 90-east ridge is aseismic
• On old transform fault
– Crust age changes in opposite directions on either side
Fig. 3.19
• Collisions don’t have to happen head-on
– Many examples of oblique collisions
• Major transcurrent faulting may occur
– Strike-slip faults
• Age of ocean basin is not correlated to
decline in spreading activity
– Pacific is oldest, but has fastest spreading
rate
Mediterranean
• Ocean in its final stages of life
– Only major remnant of the Tethys Sea
– Shrinking as Africa moves northward
• Crust is unique
– Broken into many small plates – not as old as
would be expected
– Sediments include evaporites
Messinian Salinity Crisis
• In the early 1960s, seismic soundings in
the Mediterranean found ubiquitous
evidence of an acoustic reflector a few
hundred meters below the sea floor and
closely simulating its topography –
christened the M layer.
• A decade later drillings in 3000m depth
brought sediment cores of anhydrites
(precipitated only from saline ground-water
above 30° C) and stromatolites (organic
fossils of algae mats in an intertidal
environment) to the surface, and also
gypsum.
• These sediments indicated that six million
years ago the Mediterranean, which had
much the same topography as today and
had been deep sea for many millions of
years, had been a series of brackish lakes
and dessicated tidal flats, (Hsu, 1983).
• Research since then has built up the
picture of one of the greatest evaporitic
basins in Earth history, not only in shallow
marine basins, but also widespread in the
deeper Mediterranean.
Eastern Med.
• The major feature of the deep basin is a large
arcuate swell, the Mediterranean Ridge
– Extends over 1500 km between the southwest of
Peloponisos and southern Turkey.
• Other prominent topographic features are:
– the Nile cone (off Egypt),
– the Anaximander mountains (off southern Turkey),
– the Eratosthenes seamount and Florence rise (south
and west of Cyprus, respectively),
– the Calabrian external arc, south of Sicily.
• The Mediterranean Ridge is a wide (100 to
200 km) and voluminous sedimentary
construction (up to 10 km thick) which
results from the accretion and deformation
of thick piles of sediment as a
consequence of the subduction during at
least the last 20 Ma of Africa beneath
southern Europe.
• Today the rate of convergence between
Libya and Crete, as determined by satellite
methods, is about 30 mm/year.
• The Mediterranean Ridge is one of the
very few accretionary prisms progressively
incorporated into direct collisional
processes.
Western Med.
• The western Mediterranean basins are
almost totally surrounded by mountain
belts that for the most part remain
tectonically and seismically active.
• The three subbasins of different ages
reflect different stages in the development
of the western Mediterranean by back arc
spreading in response to subduction of the
African lithosphere beneath southern
Europe.
• Though less active than the eastern
Mediterranean, the western Mediterranean
includes significant geologically active
segments as well as passive ones.
– At present, two margin segments are
becoming progressively inverted in response
to the ongoing convergence between Africa
and Europe:
• the Algerian margin,
• to a lesser extent, the Ligurian margin off France
and Italy.
• The Algero-Provençal basin, which
extends between the coasts of Spain, the
Baleares, France, Corsica, Sardinia and
Algeria started forming some 25/18 Ma
ago.
• Today's basin continues northwards to the
Ligurian Sea, with some of its margins
showing noticeable tectonic activity,
namely those off North Africa.
• The basin is being progressively infilled by
terrigenous sediments derived from
erosion of the surrounding reliefs.
• The Rhone deep-sea fan, built after the
accumulation of eroded material from
Massif Central and western Alps
represents the main sedimentary edifice in
the deep western Mediterranean.
• The much shallower Alboran Sea, southwest of
the Algero-Provençal basin, is a composite
basin.
– Resulting from the interactions between the eastwarddipping subduction of the nearby western Atlantic
oceanic crust and the relative compressive plate
motions between Africa and the Iberian Peninsula,
presently anchored to Europe.
– A complex pattern of active tectonics, also recorded in
the nearby onshore tectonic belts, results from this
setting.
• The Tyrrhenian Sea is the youngest of the
western Mediterrranean sub-basins.
– Its formation that started 8 My ago and continues
today disrupted the European continental border and
led to the emplacement of small, oceanic crust floored
deeps in which submarine volcanoes are observed.
– Its southern margin, along Sicily and southern Italy,
shows seismically active lineaments and large active
volcanoes like Stromboli, Vulcano and others,
• reflecting the Ionian/African oceanic lithosphere subduction
beneath Calabria.