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HISTORICAL GEOLOGY
LECTURE 5. STRUCTURE OF THE EARTH, OROGENESIS,
CONTINENTAL GROWTH BY ACCRETION.
Denser elements, such as
IRON AND NICKEL,
form the CORE; lighter
minerals such as the
FELDSPARS (which
have a low melting point
and low density) form
the CRUST; minerals
having intermediate
density, such as the iron
and magnesium-rich
silicates OLIVINE and
PYROXENE form the
MANTLE.
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Even after 4 billion years of
cooling, the outer core remains
molten and part of the upper
mantle is partially molten and
capable of flowing very slowly.
Elements Of The Crust And
Upper Mantle
Continental crust: averages
about 35 km thick. Rich in
feldspars and "granitic" rocks.
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Oceanic crust: 5-12 km thick.
Rich in iron and magnesium basalt rock.
Upper mantle: the uppermost
part of the mantle (to approx. 100
km deep) is solid. This part of the
mantle together with the
overlying crust is the
LITHOSPHERE.
The Asthenosphere: this part of
the mantle, between about 100
and 700 km deep, is partially
molten and capable of flowing
very slowly. Plate motion may be
driven by convection here.
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Continental crust can be
divided into 4 regions:
a) Shields: old, precambrian
metamorphic and igneous rocks
at the "core" of continents.
b) Platforms: adjacent to shields,
similar rocks; covered by newer
sedimentary rocks formed
during transgressions (shield +
platform = craton).
c) Orogenic belts, usually around
the edge of the continents,
severely deformed due to plate
collision. Orogenic = "mountainbuilding".
d) Passive margins, the trailing
edge of continents; little or no
tectonic activity. Coastal plains.
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The craton tends to be very stable, but has undergone broad warping
in the past - especially in platform areas, creating basins, domes and
tilted strata.
View of the Canadian Shield
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Platforms Platforms are parts of the craton which are covered by
relatively undeformed layers of sedimentary rock, formed when seas
occupied these areas (due
to tilting/sinking of the
craton or rise in sea level).
A large part of the interior
of North America consists
of platform areas. Structure
within the platform rocks is
usually quite subdued:
however, many parts of
platforms are not
completely flat; instead
broad warping of the
underlying craton has
produced gently tilted beds
(< 5o), domes (e.g. the Black
Hills of Dakota; Ozark
Mountains of Missouri)
and basins (e.g. Michigan Harry Williams, Historical Geology
Basin).
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An exception to the generally
subdued structure of platforms,
resulting from plate movements, is
intracratonic rifting which forms
rift valleys, seas and aulacogens in
platform regions.
Red Sea rift
(view to the south)
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A relatively new idea in plate
tectonics suggests that rifting
may actually be initiated by
hot spots which cause the
continental crust to dome up
and split into a triple rift.
Commonly, one of the rift arms fails,
while the other two become active
and contribute to an opening linear
ocean basin. So the convection cell
that drives sea-floor spreading may
actually be a line of "hot spots".
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The rift valley forms a
large linear trough,
bounded by fault scarps
and floored by volcanic
rocks. If the valley extends
below sea level, it becomes
the site of sedimentation.
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The failed arm, represented by a deep linear trough into the adjacent
continent, may become a natural location for a major river valley within
platform areas - probable examples include the Mississippi and the
Amazon.
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It is also possible for a failed arm of
a rift to become "squeezed shut" by
later plate movements - forming an
aulacogen. The Arbuckle
Mountains lie on the northeastern
margin of the southern Oklahoma
aulacogen.
Closure of the aulacogen
resulted in severe folding and
faulting of marine
sedimentary rocks which had
accumulated in the trough one of the few places that
platform strata are so
deformed.
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PLATE TECTONICS
We are concerned primarily with plate collision
and orogenesis:
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Orogenic Belts In the case of North America there are three major orogenic belts the older Appalachian and Ouachita chains in the east and south, and the much
younger Rockies in the west. All were formed by plate convergence and have
similar structural features. All three areas underwent uplift, metamorphism,
volcanism, folding, thrust faulting and later erosion.
Appalachians
Rockies
Ouachitas
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OROGENESIS AT CONVERGENT BOUNDARIES
1. Passive Margins: prior to orogenesis, the continental boundary is a PASSIVE margin.
Sedimentation at passive margins reflects the progressive increase in water depth.
Nearshore deposits are coarser - sand grading to silt and clay; further out on the
continental shelf in clean shallow water, carbonates form in tropical regions. A common
feature of continental margins undergoing extensive sedimentation is SUBSIDENCE,
due to the weight of sediment ISOSTATICALLY DEPRESSING the crust; in this way
shallow water deposits (e.g. 100’ depth) can build up thicknesses of 1000's of feet.
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Andean-Type Orogenesis
1. Passive stage (pre-convergence) -> marginal deposits form.
hills
mountains
cliffs
ridges
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valleys Folds
2. early subduction -> marginal deposits are deformed by compression.
and thrust faults are formed.
3. Volcanic arc forms.
4. Lateral growth by accretion; emplacement of igneous masses;
metamorphism; further deformation of marginal deposits.
canyons
deltas
beaches
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5. Continued uplift and deformation results from continuing plate
convergence.
Examples = Andes of western south America; Rockies of North America.
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The Sierra Nevada and the Coast
Range are good examples of
Andean-type orogenesis. The
Sierra Nevada batholith is a
remnant of a continental volcanic
arc. The Coast Range consists of
accretionary wedge sediments
that have been deformed and
uplifted by plate convergence
(Note: the subduction zone
between the Pacific and North
American plates that formed
these features has since changed
into a transform fault – the San
Andreas).
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Continental Collision-Type Orogenesis
About 20 million years ago - India was separated from Asia by a progressively
narrowing ocean basin - the Tethys Sea. The “collision” begun with the
subduction of the oceanic plate beneath the Tethys Sea. This caused the onset
of orogenesis in Tibet (uplift, folding, faulting, metamorphism, volcanism).
Initial growth was similar to Cordilleran-type orogenesis; however, when the
continents collide one of them can not be subducted (too thick and buoyant),
therefore the plates are welded together forming a SUTURE ZONE and
producing a large mountain chain, containing sedimentary, igneous and
metamorphic rock.
17.23a
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The period of mountain building is termed an
OROGENY. The Himalayas are very high
because they are very young (geologically).
Uplift continues and erosion hasn’t had long to
wear the mountains down.
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Suture zones usually have symmetrical patterns of rock types (folded
sedimentary, metamorphic, granite) and the high rugged mountains
resulting from orogenesis are subject to intense erosion, shedding
sediment to both sides of the mountain range.
sediment
sediment
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The present-day relief of mountain belts is a result of erosion: in North
America, the Appalachians being much older than the Rockies are more
subdued and lower and consist mainly of exposed and eroded folds.
Appalachians
Rockies
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folds
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Young mountain belts like the Rockies are higher and more rugged.
Fault block mountain ranges are also common
UPLIFT
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The Grand Tetons of Wyoming are an example of a block of crust
uplifted along a fault.
faults
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All orogenies have common features: 1. compression, buckling and
uplift at the edge of the continent. 2. emplacement of igneous rock
masses 3. folding, faulting and metamorphism 4. erosion of the
growing mountains and deposition adjacent to the mountains (including
the back-arc basin).
5. scraping off,
folding and uplift of
sediment
sea floor rock and
sediments onto the
edge of the continent
- resulting in:
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Continental Growth By Accretion
Caused by the "plastering on" of
material at the edge of a continental
plate adjacent to a subduction zone.
The material that accumulates in
this fashion can be small continental
masses MICROCONTINENTS or
oceanic features such as volcanic
arcs and seamounts (submarine
volcanoes). The Seychelles Bank in
the Indian Ocean is an example of a
microcontinent, which has
apparently become detached from Africa. These larger masses
of rock are scraped off the subducting plate and plastered onto
the continent to form ALLOCHTHONOUS or EXOTIC
TERRANES (meaning they originated elsewhere).
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Example: Wrangellia consists
of basaltic island arc volcanic
rocks, deep marine shales
and shallow marine
limestones. This material
originated in the area
presently around New
Guinea in Triassic time (225
million years ago) - travelling
about 10,000 km to its
present location. The
collision of these smaller
masses with the larger
continent also causes
orogenic activity, resulting in
coastal mountain ranges.
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