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GEOL 451-2010
Geology of North America
Review of some Lithotectonic Principles
Updated January 2011
University of Regina
GEOL 451-2011
R. Macdonald, Instructor
Coverage in this presentation
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Uniqueness and interactive nature of the
Earth system
Basic Earth Structure
Lithotectonic entities
Largely from Condie p.14 onwards
An Approach to Earth Processes
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1. PETROCENTRIC
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Processes concerning only rocks of the earth’s crust and mantle,
e.g. sedimentation, metamorphism, even diagenesis
But rocks react with the biosphere, oceans and atmosphere
Climate factor, asteroids, flood, tsunamis, etc
Earth physiology - Jim Lovelock’s GAIA hypothesis
The earth system maintains itself through positive feedbacks
2. TIME-CENTRIC
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Geologists tend to think in very long periods of time
But some earth processes can occur very rapidly
A return to CATASTROPHISM?
Uniqueness of the Earth and interaction of the
Earth Elements
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Need to consider the entire Earth system: earth-ocean-atmosphere
Earth physiology: James Lovelock’s Gaia Hypothesis
Feedback loops (+) and (-)
Recycling lithosphere
Knowledge explosion of the past 15 or so years
Tuzo Wilson (1968):
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Data collecting
Hypotheses (transient)
New unifying theories
The whole earth system
Crustal recycling Crustal evolution
Thermal history
ET impacts
Metamorphism
Life history
Life
Crust
Tectonism and
tectonic history
Mantle hotspots
Earth cooling
Oceans
Atmosphere
Solar radiation
Climate
Magmatism
Earth’s axial tilt
Earth’s
core-mantle
Magnetic
fields
Weathering
1
Fundamental Earth Structure
1. Rigid lithosphere
rests on weaker
asthenosphere
2. Lithosphere is
fragmented into
segments and
plates in relative
motion which
continually
change shape
and size
What are some of the major lithotectonic
features of the Earth?
Intraplate - Continental
Cratons: Shields and Platforms
Precambrian Shields
Relatively stable older cratons, generally Precambrian and without a cover of
Phanerozoic rocks.
Continental platforms
Relatively stable older cratons overlain by oval shaped Phanerozoic sedimentary,
shallow water, ssts, lsts, shales, deltaic and fluvial, commonly not much more than
1000 m thick
Intraplate - Continental
Buried Precambrian
Shields, cored with
older cratons
aka Platforms
Relatively stable older cratons,
generally Precambrian but with a
cover of Phanerozoic rocks.
Intraplate - Continental
(Intra)cratonic basins, aka ENSIALIC basins (2)
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Deep, sometimes formed over failed rifts
Other causes (see Kent)
Epicontinental seas, some evaporites (e.g. Prairie Evaporite)
Examples:Williston, Hudson Bay and Michigan basins, Amadeus
and Carpentaria basins of Australia, Paris Basin, Parana Basin,
Chad basin.
Sedimentary and volcanic loading produces crustal densification
on cratons and continental platforms. Interior sag basins
Diverse origins, extension, thermal effects, higher density of
underlying crust
Typically have the longest timeframe
Intraplate - Continental
(Intra)cratonic basins, aka ENSIALIC basins
Intraplate - Continental
Inland-sea basins
Major I style, typically dormant
Overlie continental crust,
connected intermittently to open
seas, or cut off with extensive
saline de[posits
e.g. Black Sea
Caspian Sea
Gulf of Mexico
Regional Crustal Subsidence due to local sediment loading
Example: Gulf of Mexico
and Mississippi River
Sediments delivered by
major river systems
eventually deposit a nonnegligible load on the crust,
resulting in some
subsidence. This provides
accommodation (space) for
further sediment loading.
(positive feedback).
NOTE: Some
reinforcement by petroleum
extraction
Basin Formation
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Due to sags produced in the
crust by diverse
mechanisms:
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Magma depletion
Isostatic compensation: melting
of ice caps
Deep crustal/mantle
underthrusting
Magma accession:
emplacement of higher
temperature melts in the crust
Basement block movements by
a variety of causes
Load deepening
etc.
Some basin
subsidence
mechanisms
Robert
Macdonald:
Why do Continents
Break Up?
The Earth's interior is
hot. The heat comes
from the heat of
formation of the Earth
that has not yet
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dissipated and heat
generated by
of
 decay
Largely
recognized today as formed over Mantle
unstable isotopes
distributed through
the
hotspots/plumes
mantle and crust.
While the lithosphere
 May be a sign of incipient plate movements, marking
cools primarily by
conduction, thebeginning
mantle
of continental break-up
cools by convection.
Most of theconvective
Why do continents break-up?
heat from the mantle is
dissipated at the
midocean ridges and
through cooling
seafloor. Beneath large
continents, however,
heat builds up in the
mantle. This excess
heat should weaken
the continental
lithosphere making it
easier to rift.
Intraplate - Continental
Continental Rifts
the
Robert
Macdonald:
Why do Continents
Break Up?
The Earth's interior is
hot. The heat comes
from the heat of
formation of the Earth
that has not yet
dissipated and heat
generated by decay of
unstable isotopes
 Earth’s
distributed
through the interior contains formational and isotope-generated heat
mantleandLithospheric
crust.
crust cools by conduction, but the
While the lithosphere
 Mantle
cools by convection dissipated at MORS and ocean floors
cools primarily
by
conduction,
the mantle large continents heat builds up in the Mantle, weakening
 Beneath
cools by convection.
Crust
Most of thethe
convective
heat from
mantle is
 the
Relatively
higher membrane stress in equatorial regions due to
dissipated at the
higher amount of earth curvature
midocean ridges and
through
 cooling
Trench rollback at subduction zones
seafloor. Beneath large
 Hotspots/plumes
(randomly formed)
continents,
however,
heat builds up in the
mantle. This excess
heat should weaken
the continental
lithosphere making it
easier to rift.
Intraplate - Continental
Some causes of continental rifts
Intraplate - Continental
The East African rift system showing
the Afar Triangle as a triple-junction at
the intersection of the Red Sea, Aden
and East African rifts. Possibly the
expression of a mantle plume.
Diverging rifts starts a new round of
continental drifting and ultimately
“creates” new ocean floor. Dots
indicate young volcanoes.
Intraplate - Continental
The East African rift system showing
the Afar Triangle as a triple-junction at
the intersection of the Red Sea, Aden
and East African rifts. Possibly the
expression of a mantle plume.
Diverging rifts starts a new round of
continental drifting and ultimately
“creates” new ocean floor. Dots
indicate young volcanoes.
But not so simple
Intraplate - Continental
1.
2.
3.
4.
5.
6.
Initial doming and normal faulting.
As lower crust & lithosphere thins by ductile
shear, heat flow increases and normal
faulting occurs in the brittle upper crust.
Increased heat flow produces bimodal
(basaltic and rhyolitic) volcanism
Subsiding rift basins collect infill sediments .
If rifting continues the crust/lithosphere thins
to zero and seafloor spreading is initiated
Sediments on continental passive margins
drape drape over normal faulted basement
After the initial thinning, margins continue
to subside for tens of millions of years by
continued cooling and loading subsidence
Intraplate - Continental
RRR Triple Junctions and Aulocogens
If rifting stops before complete continental breakup, the failed rift or aulocogen
infills with sediments and be buried in the subsurface, perhaps to be reexposed by some later episode of erosion or be discovered by seismic
exploration.
Aulocogens are commonly associated with continental breakup. Continental
rifts seem to start as a number of rift-rift-rift triple junctions. Two of the rift arms
become a new ocean basin and the third becomes a failed rift, although it may
still be active as a continental rift system. The East African rift (EAR) appears
to be a modern example, as ti is the failing arm from the triple junction including
the Red Sea and Gulf of Aden.
See also
Basin and Range
Half grabens
East African Rift
Transcurrent rifting
Intraplate - Continental
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Rift-related igneous activity:
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bimodal volcanic signature
distinctive trace element geochemistry
continental rift basalts are enriched in alkalis (K, Ba, Rb), and
incompatible elements, LIL.
deep mantle-plume contribution
mantle fluids and metasomatism.
lithospheric mantle contribution
Other features:
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distinctive trace element geochemistry
with sediment traps, accommodation space
arkoses, immature sediments
half grabens
fault driven sedimentation: alluvial fans and debris flows
Along-strike changes = segmentation and depocentres
every rift basin is unique
Intraplate - Continental
The failed third arm
(called an aulocogen)
is a topographic low.
 Many major rivers
in the world flow
down aulocogens
e.g. Amazon,
Mississippi, Niger, St.
Lawrence, Rhine, and
parts of the Nile
Intraplate – Oceanic Crust
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Oceanic plateaux
Ocean basins - sag basins
pelagic clays, oozes,
turbidites
Volcanic islands/
seamounts/guyots
Produced by Mantle plume
hotspots - long-lived
structures fixed within the
mantle.
Lithospheric plates move
over them, typically in a
datable track.
e.g. Hawaii, Yellowstone, Galapagos
Intraplate Oceanic
Mantle plume hotspot tracks
Ages in million years
Intraplate Oceanic
Intraplate Oceanic
Long lived global hotspots
Divergent - Continental
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Proto-oceanic troughs
Red Sea <5 Ma oceanic crust in centre, thick salt
deposits due to ocean cut off
 Passive margins
Continental rises and terraces (prisms/wedges,
continental crust thinned, transitory and oceanic
crust, can include pelagic turbidite. May be
caused by densification by metamorphism
e.g. Eastern N. America seaboard. Stable EA coast
Divergent - Continental
Detailed Cross-section of a Passive Margin
Atlantic Margin
What is the relative
age of the basalt?
Jurassic salt
Cretaceous &
Cenozoic sediments
Triassic rift valley sediments
Divergent - Oceanic
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MORs (Mid-oceanic rifts)
Divergent - Oceanic
Oceanic
Crustal
Age
revealed
against
passive
margins
Convergent - Intraoceanic
Oceanic volcanic arcs
with intra-arc basins
Deep sea trenches – arc-trench gaps (containing
fore-arc basins) – active volcanic (island arc) arc –
back-arc
Convergent - Intraoceanic
Two oceanic slabs
converge; one subducts
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The subducted slab
produces melting in the
overlying mantle wedge
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Magma Is less dense
than overlying crust /
lithosphere and rises as
volcanoes.
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If the volcanoes emerge
as islands, a volcanic
island arc (or archipelago)
is formed
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e.g. Japan, Aleutian
islands, Tonga islands
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Oceanic Back-Arc Basins
1.
2.
3.
4.
Back-arc basins (or retro-arc
basins) are submarine basins
associated with island arcs and
subduction zones
Found at some convergent
plate boundaries, presently
concentrated in the Western
Pacific Ocean
Most result from tensional
forces caused by oceanic
trench rollback rollback and the
collapse of the edge of the
continent
Back-arc basins were not
predicted by plate tectonic
theory, but are consistent with
the dominant model for how
Earth loses heat
Ocean ic Back-Arc Basins
Convergent - Continental
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Continent:Continent with subduction
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North-south profile across the eastern Alps. Subsurface profile from
seismic reflection data. After Adrian Pfiffner
Common when two
continents collide and the
buoyant continental
lithosphere does not
subduct
Any original trenches are
eliminated
Collision then thickens the
crust, along the suture
separating the original
continents
Crustal thickening then
responds isostatically,
producing a large mass of
buoyant continental crust
e.g. Himalayas, Alps,
Appalachians
Convergent Continental
Continent:Continent
with subduction
Example from the Himalayas
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Part of Africa breaks away ca. 50
Ma ago
Travelled to the north at ca. 10
cm/annum
Is subducted under continental
Asia, cause it to rise in elevation
Plate movements continue today,
so Hilary had it a few centimetres
easier to climb Everest than
today’s climbers
Cause of the Indonesian tsunami
Convergent - Continental
Convergent - Continental
Head 0n
with
obduction
Convergent - Continental
Obduction
styles
Convergent – Continental Margin
Products:
 Deep sea trenches
 Trench slope
subduction basins
 Accretionary
complexes
 Mélange
 Foreland arcs
 Fore-arc basins
 Intra arc basins
 Back-arc basins
 Foreland fold-thrust
belts
Crustal melting occurs above the descending slab
producing batholithic rocks surmounted by
volcanic. Sediments are derived mainly from the
arc and are siliclastic Sediments are subducted or
scraped off into the accretionary complexes
e.g. Sunda, Aleutian, Peru-Chili, and Japan.
Convergent – Continental Margin
Vertical sequence:
Volcanic arc
Crust (sub-arc
lithosphere
TTG)
2. Upper Mantle wedge
1. Subducting slab
Convergent – Continental Margin
Convergent – Continental Margin
Convergent – Continental Margin
and Oceanic
Transcurrent (strike –slip & transform)
Transtensional
Transpressional
Transrotational
Intracontinental
wedge basins
Transcurrent (strike –slip & transform)
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Transform faults
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Most transforms are prominent linear breaks associated with midocean ridge segments.
Known as fracture zones these occur between offsets in the spreading
ridge.
Fracture zones are a geometrical necessity due to the fact that seafloor genesis occurs on a SPHERE.
Suspect terranes
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This term applies to a terranes which have been brought in from a long
distances, exotic in nature to the terranes they now abut.
With accurate age-dating and other methods of establishing
provenance it may be possible where the suspect terranes come from,
and how far they have travelled
Analysis of such terranes is the main basis for constructing paleo maps
Plate Tectonic Mechanisms
 No one mechanism accounts for
all major facets of plate tectonics
 Convective flow in the plastic
2,900 km-thick mantle is the best
option
 Other mechanisms generate
forces that contribute to plate
motion.
 Slab-pull on cold plate in
subduction zone
 Ocean ridge-push
 Gravitational sliding on
oceanic ridges
The Six Major Types of Sedimentary Basin
(with examples)
Indonesia
Nevada
E. Africa
Offshore Calif.
Michigan Basin E. Coast NA
Six major types of sedimentary basins are shown in their platetectonic settings. The major physical cause or causes of
subsidence for each case are shown below on above the diagram.
Seismicity related to Subduction
A scheme relating igneous rocks to
plate tectonics