This chapter provides a brief
overview of structural geology
and tectonics. A review of the
Earth’s geologic history as it
relates to structural geology
and tectonics is given. Types of
faulting and the structures that
result from these fault motions
are also discussed.
1.1 What are Structural Geology and
Tectonics
Structural geology & tectonics - motions
& processes that build Earth’s crust
Types of Motions:
Rigid body motion - transportation, no
change in size or shape; no permanent
imprint
Deformation - motion that changes size
and/or shape
Structural geology scale - submicroscopic
to regional
Tectonics Scale - regional to global
Continuum Mechanics- describes the
motion of material bodies of continuous
matter, taking into account the body’s
deformability. Includes kinematics and
mechanics.
1.2 Structural geology, tectonics and the
use of models
Geometric Models
-3D interpretations of structures in Earth
-based on: mapping, geophysical data
-examples: geological maps, vertical cross
sections
Kinematic Models
-prescribe motions that could have carried
system from undeformed→deformed state
-not concerned with why, how or physical
properties of the system
-assess validity by comparing observed
motions w/ model
Mechanical Models
- models motions as the consequence of
their relationship with forces and material
properties.
1.3 The interior of the Earth and other
terrestrial bodies
Core
-dense, predominantly iron-nickel alloy
-solid inner core, liquid outer core
Mantle
-thick, much lower density than core,
magnesium-iron silicates
1. Lithospheric mantle - crust and upper
mantle (100 km under ocean, 200-300 km
under continents)
2. Asthenosphere - closer to melting
temp→weaker
3. Mesosphere - stronger, high density,
crystalline phases of magnesium-iron
silicates or oxides
Crust
-thin layer that surrounds mantle
-low-density materials
-igneous rocks-granitic to basaltic
-sediments and sedimentary rocks
-metamorphic equivalents of above rocks
Temperature gradient - +25oC per km in
crust and mantle (this change decreases
with depth)
Convection
-moves heat out of liquid core
-carries heat transferred from core & from
radioactive decay w/in mantle to surface
Heat escapes earth by:
- Conduction through cold lithospheric
boundary layer
- Advection of heat in magmas
- Upwelling of asthenosphere at oceanic
spreading centers
1.4 The Earth’s crust and plate
tectonics: Introduction
Continental Crust - granodioritic
composition
Oceanic Crust-basaltic composition
Surface elevation - Bimodal
-continents-w/in 100’s of meters of sea
level
-ocean floor-~5 km below sea surface
Tectonics-lithosphere is divided into
plates the move (rigid body motion)
-deformation of plates concentrated in
belts ~100’s of km wide
Plate Boundaries
-Divergent
*plates move away from each other
*Material flows up from mantle
*horizontal stretching & vertical
thinning of crust
*normal faulting surface, ductile &
thinning deeper
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-Convergent - subduction zones
*thrust faults w/strike-slip faulting
*continent-continent collision-folding,
metamorphism & igneous activity
-Transform - sliding (horizontal)
*strike-slip faults
*vertical zones of ductile deformation
w/sub-horizontal direction of
displacement.
Factors influencing structures
-orientation and intensity of forces
-motions to which rocks are subjected
-physical conditions - pressure and temp
-mechanical properties of rock
Brittle deformation
-rock fracture, low temp & pressure
Ductile deformation
-high temp and pressure but below
melting temp and low intensity of applied
forces or slow imposed deformations
-flow of rock in solid crystalline state
-stratigraphic layers, stretching & thinning
of layers, parallel alignment of grains
1.5 Ocean basins
Ophiolites - on-land exposed rock
sequences thought to represent old
oceanic crust
Oceanic crust
-3-10 km (avg. of 7 km) thick
-igneous rocks of basaltic composition
-differences in elevation due to
differences in density and thickness of
underlying crust and mantle
Averaged layer model
Vp is seismic p-wave velocity
Layer 1-Vp=3-5 km/s
-unconsolidated sediment of pelagic,
hemi-pelagic or turbidic origin
Layer 2-Vp=4-6 km/s
-predominately submarine basaltic
extrusive and shallow intrusive rocks
- subdivided into 2A, 2B, 2C based on
how velocity increases in depth
Layer 3-Vp=6-7.5 km/s
-mafic, ultra-mafic plutonic rocks and/or
serpentinized mantle peridotite
-3A & 3B-reflect olivine quantityi.
Features of Oceanic Plate Margins
Midocean ridges
-divergent plate margins-high regions
-40,000 km long, 2.5 km high (above
floor), and 1000-3000 km wide
-active normal faults
Maggie Ortiz, 2011
Edited by Jenny Marion and Paul Knobel, 2013
Transform fault boundaries
-seismically active parts of fracture zones
-sharp ridge and trough topography
-steeply dipping faults
-deformed oceanic rocks
-up to 10,000 km long, up to 100 km wide
-crust tends to be less thick here
Convergent
-Chains of volcanic islands accompanied
by parallel trenches (island-arc-deep sea
trench pairs)
-extend 1000’s of km
-volcanoes 70-80 km apart-rise above
ridges-few 100 km wide
-trenches-up to 12km deep, ~100km wide
*landward side-active thrust faults
-crust- ~25 km thick
ii. Features of oceanic plate interiors
Abyssal plains
-vast areas of flat ocean floor
-deepest regions of ocean
-5 km below sea level
Oceanic plateaus
-broad elevated regions
-variety of origins
-100-1000’s 0f km2 area
-1-4 km above normal ocean floor
Aseismic ridges
-linear ridges characterized by high
elevation, thick crust, and lack of
associated seismic activity
-most cases-linear constructional ridges
formed by chains of basaltic volcanoes
1.6 The structure of continental crust
Continental crust
-older, thicker, less dense, lower velocity
-~35 km thick (average)
-velocity tends to increase with depth
-velocity inversions
Upper crust-metamorphosed rocks
intruded in places by granitic rocks
Middle crust-migmatite
Lower crust-highly folded rocks
-moho discontinuity is less sharp
1.7 Precambrian shields
-Precambrian shield-large areas where
Precambrian rock >60 Ma are exposed
-Archean rocks- >2500 Ma
-Proterozoic rocks- 2500 Ma to 540 Ma
-Archean regions-greater evidence of
crustal instability
i. Archean terranes
Archean Terranes
-divisible on basis of metamorphic grades
-high grade gneissic regions
*amphibolite or granulitic facies of
metamorphism
*form bulk of archean regions
*quartz-feldspathic gneisses derived
by metamorphism of felsic igneous
rocks
*complexly mixed with greenstone
belts over 10-100’s of km
- greenstone belts - rocks at greenschist or
lower grades of metamorphism
*Greenstone - mafic to silicic volcanic
rocks and shallow intrusive bodies
Sutures-regions of deformed oceanic
material thought to be remnants of
disappeared oceans
Structural features of Archean Terranes
-highly deformed and display more than 1
generation of folds
-contacts between gneissic and greenstone
are complex
-sedimentary rock types fall into one of
two categories:
*immature volcanogenic sediments
*quartzite-carbonate-iron-assemblages
-Similar sedimentary and tectonic
conditions occurred globally during
Archean times
During Archean:
-higher temps in earth
-thermal gradient 2-3x higher than present
-plate tectonics likely operated
-oceanic crust prob. thicker
-continents smaller and less numerous
ii. Proterozoic terranes
Terranes include both highly deformed
mobile areas and slightly deformed stable
regions
-Cratons - tectonically stable regions of
crust (became abundant in Proterozoic)
Deformed belts
-multiply deformed regions rich in
volcanic rocks
-thick sedimentary sequences deposited in
linear troughs
- include Proterozoic dike swarms
Aulacogens-series of smaller linear
sediment-filled grabens
1.8 Phanerozoic regions
i. Interior lowlands and cratonic
platforms
-most platform sediments are marine
ii. Orogenic belts
-thick sequences of shallow-water
sandstones, limestones and shales
deposited on continental crust
-crude bilateral symmetry
-form at convergent margins
iii. Continental rifts
-abundant normal faulting, shallow EQ
activity, mountainous topography
iv. Modern continental margins
Passive, rifted, or Atlantic-style margins
(horizontally lengthened and vertically
thinned continental crust)
-initiate at divergent plate boundaries
Convergent or Andean-style margins
-where consuming plate boundaries are
located along a continental margin
-abrupt topographic change from a deep
sea trench offshore to a high belt of
mountains w/in 100-200 km off the coast
-narrow or absent continental shelf
-mountains have chain of active
stratovolcanoes
Transform or California-style margins
-sharp topographic differences between
oceans and continents
- active strike-slip faulting, poorly
developed shelf, irregular ridge & basin
topography and many deep sedimentary
basins
Back-arc or Japan style margins
-composite margins - passive margin
separated by narrow oceanic region from
an active island arc
Maggie Ortiz, 2011
Edited by Jenny Marion and Paul Knobel, 2013
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