Geology Lecture
Outline –
Convergent Plate Tectonism and Continental Evolution (Ch 14)
I. Lecture Content
Introduction - Makings of High Places
Origin of Mountains - The Nature of Orogeny
Tectonic Terranes - Orphaned crust
Continental Accretion - Crustal-scale Squeegee Process
Evolution of Continents - Billions of Years of Accretion and
Recycling
II. Introduction
A. Deformation and
1. Spectacular crustal upheavals are the
results of:
· · Same forces that power plate
tectonics
·
·
Interplay
between crustal rocks and the global
and
regional-scale deviatoric stresses created
by inter-plate
motions
2.
Deformation events are typically part of much larger
regional mountain building events called orogenies.
III. Origin of Mountains
A. Mountain Structures
1. Mountains defined:
·
·
Areas
of land that stand significantly taller than the
surrounding landmass
2
Several Types of Mountains
·
·
Fold
and Thrust belts
·
·
Volcanic
centers and chains
·
·
Metamorphic-Plutonic
Core complexes
·
·
Horst
and Graben Fault Block systems
B. Processes
that
1. Orogeny --- a mountain-building episode
·
·
Includes
a variety of mountain-building activities
and events
ü ü Regional deformation and
metamorphism
ü ü Magmatism
ü ü Uplift and erosion
·
·
Orogenic belts are regions where major mountain-
building
activities have taken (or are taking) place
ü
ü
Characteristic
of convergent plate boundaries
ü
ü
Compressional
tectonics major driving force
ü
ü
Accretionary
tectonics prevalent
·
·
Earth's
Recent Orogenic Belts (see Figure )
2.
Regional Deformational activities and events
·
· Compressional tectonics
ü
ü
Associated
mainly with convergent boundaries -
especially
cont-cont collision zones
ü
ü
Folding
and thrust faulting common
ü
ü
Mountain
building accompanied by major shortening,
thickening, and
uplift of crust
ü
ü
Examples:
·
· Tensional tectonics
ü
ü
Associated
mainly with divergent boundaries like
ocean spreading
ridges & continental rifts
ü
ü
Horst
and graben normal faulting common
ü
ü
Mountain
building accompanied by major extension,
thinning, and
uplift of crust
ü
ü
Best
examples are Mid-Atlantic Ridge and the
ü
ü
Regional
example is the
and
·
· Accretionary tectonics
ü
ü
Associated
with convergent and transform plate
boundaries at
continental margins
ü
ü
The
accretion of an exotic or suspect terrane onto
the edge of a
continent is often the primary trigger
for many orogenic
episodes
ü
ü
A suture zone marks the boundary where a terrane
is accreted to
the edge of a continental plate
·
· Regional metamorphism
ü
ü
Accompanies
folding and faulting activities
ü
ü
Regional
metamorphism is more prevalent in
compressional
tectonic setting like continental
collision and
subduction zones
3.
Magmatism
·
·
Building
of mountains by piling up of solidified lava
forming
volcanic mountains and mountain chains
·
·
Formation
of massive underground batholiths builds up
the crust from the
inside out
4.
Regional Uplift
·
·
Instrumental
in creating high places
·
·
Causes
of Regional uplift
ü ü Sustained regional compressional
forces
ü ü Sustained upward-directed forces
ü ü Isostatic equilibrium (rebound)
5.
Erosion
·
·
Instrumental
in causing greater topographic relief
·
·
Helps
sustain buoyant mountain root uplift
C. Nature of Orogeny at
Convergent Plate Boundaries
1.
Oceanic-Oceanic Plate Boundary
·
·
Paired set of parallel orogenic belts
ü ü Juvenile volcanic-plutonic arc
complex made up
mainly of new
andesitic crust
ü ü Uplifted subduction accretionary
prism
ü ü Little to no compressional
deformation
·
·
Illustration
of this orogenic setting (see Fig. )
2. Oceanic-Continental Plate Boundary
·
·
Pair set of parallel orogenic belts
ü ü Mature volcanic-plutonic arc
complex made up of
mainly
new/reworked granitic crust
ü ü Uplifted, subduction accretionary
prism that may
include blocks
of ophiolite
ü ü Accreted terranes may be sutured
into the paired
volcanic
arc-trench deposit belts
ü ü Moderate compressional
deformation
·
·
Illustration
of this orogenic setting (see Fig. )
3.
Continental-Continental Plate Boundaries
·
·
Broad,
massive, and complex orogenic belt
ü ü Number of Fold and Thrust
mountain systems
ü ü Massive uplift of mountain
systems and formation
of deep crustal
root
ü ü Numerous accreted terranes may be
sutured into
the collision
zone
ü ü Strong compressional deformation
·
·
Illustration
of this orogenic setting (see Fig. )
IV. Terranes and
Continental Accretion
A. Suspect or Exotic Terranes - Orphaned
Lithosphere
1.
Orogenic mountain systems typically include elongate
blocks of exotic
lithosphere
2. Terranes have geologic origins far from their present
geographic location
·
·
Terranes are "rafted" to a
continental margin from
potentially great distances (pole to pole)
ü ü Carried via the ocean crust
conveyor belt to
a continental
margin subduction zone
ü ü Carried laterally along a
continental margin
via
regional-scale transform faulting
·
·
Terranes
can have great variation in age and geology
ü ü Old slivers of exotic continental
crust
ü ü
ü ü Obducted oceanic crust
3. The terrane
accretion process is an important part of the
orogenic
cycle of continental mountain-building
·
·
Substantial
means of continental growth and evolution
·
·
Western
margin of
continental
growth in the last 200 million years due in large
part to continental
accretion events
·
·
Numerous
terranes have been recognized from
the way up to
·
·
Illustration
of accreted terranes of western
(see Fig. )
V. Evolution of Continents
A. Plate Tectonics - Cause of Continental Evolution
1. Oceanic seafloor spreading and
subduction processes
have caused the
growth and evolution of the continents
·
·
Probably
started some 4 billion years ago
·
·
Gradual
increase of granitic crust through a multi-
step tectonic
process:
ü
ü
Partial
melting of mantle peridotite at spreading
centers
generates new oceanic basalt
ü
ü
Partial
melting of oceanic basalt at island arc
subduction
zones = new transitional andesite
ü
ü
Accretion
of island arc terranes to form larger early
continental
masses (cratons), which increase in size
and crustal thickness
ü
ü
Partial
melting of andesitic crust at base of edges of
continental masses
over subduction zones generates
new granitic
continental crust
ü ü Rifting apart and reshuffling of
older continental
crust changes the
size and configuration of
continental masses and subjects continental
edges to basaltic
magmatism
ü ü Mixing and assimilation of older
continental crust
with fresh
mantle-derived basaltic and andesitic
magmas creates
"new" recycled continental crust
2. The cyclic, non-ending repetition
of the above tectonic
processes
over four billion years have created the
complex
granitic continents of today.
3. Illustration of Continental
Evolution (Figs. )
VI. Vocabulary –