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Introduction to Structural Geology

• Plate Tectonics and Crustal Stress
• Faults Rock Failure Modes
• Principal Structural Styles
• Thrust Belts
• Strike-Slip Zones
• Gravitational Systems
• Rifts
• Seismic Expression of Structural Styles

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Introduction to Structural Geology

• Plate Tectonics and Crustal Stress
• Faults Rock Failure Modes
• Principal Structural Styles
• Thrust Belts
• Strike-Slip Zones
• Gravitational Systems
• Rifts
• Seismic Expression of Structural Styles

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Introduction to Structural Geology

• Plate Tectonics and Crustal Stress
• The Layered Earth
• Isostacy
• Seismicity and definition of crustal plates
• Plate margin types
• Caribbean plate tectonic evolution model
• Intraplate stresses and the World Stress Map
  project
• Heat Flow
• Anomalous structures

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The Layered Earth
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The Layered Earth

• What is the difference between the crust and
  lithosphere?
• Geologists use two distinct classifications for
  the earths layering
• Compositional
• Rheological (mechanical)

SiO2 rich rocks
Rigid
Approx. 1300ºC
SiO2 poor rocks
Flow

• The crust (whether continental or oceanic) is the
  thin layer of distinctive chemical composition
  overlying the ultramafic upper mantle. The base
  of the crust is defined seismologically by the
  Mohorovicic discontinuity, or Moho. Oceanic and
  continental crust are formed by entirely
  different geological processes the former is
  typically 6 - 7 km thick, the latter about 35 -
  40 km.

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The Layered Earth

• The lithosphere is the rigid outer layer of the
  Earth required by plate tectonic theory. It
  differs from the underlying asthenosphere in
  terms of its mechanical (or rheological, ie,
  'flow') properties rather than its chemical
  composition. Under the influence of the
  low-intensity, long-term stresses that drive
  plate tectonic motions, the lithosphere responds
  essentially as a rigid shell whilst the
  asthenosphere behaves as a highly viscous fluid.
• The weaker mechanical properties of the
  asthenosphere are attributable to the fact that,
  within this part of the upper mantle,
  temperatures lie close to the melting temperature
  (with localised partial melting giving rise to
  magma generation). The base of the lithosphere is
  conventionally defined as the 1300 ºC isotherm
  since mantle rocks below this temperature are
  sufficiently cool to behave in a rigid manner.

The lithosphere includes the crust (whether
continental or oceanic) and the uppermost part of
the upper mantle. It thins to a few kilometres at
ocean spreading centres, thickens to about 100 -
150 km under the older parts of ocean basins, and
is up to 250 - 300 km thick under continental
shield areas. Lithospheric Plates the basis
for plate tectonic theory
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Definition of the Principal Lithospheric Plates
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PLATE MARGINS

• Three types of plate margins have been defined
• CONVERGENT
• DIVERGENT
• CONSERVATIVE (Transform)

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CONVERGENT PLATE MARGIN
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BENIOFF ZONES
BENIOFF ZONE Dipping zone of earthquake
epicentres characterized by gradually deepening
events up to 500km depth. May be located beneath
a continental margin or an oceanic margin
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AGE OF THE OCEANIC CRUST
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DIVERGENT PLATE MARGINS CONTINENTAL
EXTENSIONAL FAILURE
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DIVERGENT PLATE MARGINS TRANSITION FROM
CONTINENTAL RIFTING TO OCEANIC SPREADING
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DIVERGENT PLATE MARGINS TRANSITION FROM
CONTINENTAL RIFTING TO OCEANIC SPREADING
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Plate Tectonics and Crustal Stress World
seismicity from 1975-1995
Shallow earthquakes Continental areas and
oceanic ridges Deep earthquakes Oceanic
trenches
from USGS website
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Plate Tectonics and Crustal Stress Pacific
plate seismicity from 1975-1995
Shallow earthquakes Continental areas and
oceanic ridges Deep earthquakes Oceanic
trenches
from USGS website
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Plate Tectonics and Crustal Stress European
plate seismicity from 1975-1995
Shallow earthquakes Continental areas and
oceanic ridges Deep earthquakes Oceanic
trenches
from USGS website
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Plate Tectonics and Crustal Stress Caribbean
plate seismicity from 1975-1995
Shallow earthquakes Continental areas and
oceanic ridges Deep earthquakes Oceanic
trenches
from USGS website
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Plate Tectonics and Crustal Stress Antarctic
plate seismicity from 1975-1995
Shallow earthquakes Continental areas and
oceanic ridges Deep earthquakes Oceanic
trenches
from USGS website
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Plate Tectonic Setting 180Ma
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Plate Tectonic Setting 170Ma
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Plate Tectonic Setting 160Ma
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Plate Tectonic Setting 150Ma
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Plate Tectonic Setting 140Ma
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Plate Tectonic Setting 130Ma
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Plate Tectonic Setting 120Ma
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Plate Tectonic Setting 110ma
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Plate Tectonic Setting 100Ma
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Plate Tectonic Setting 90Ma
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Plate Tectonic Setting 80Ma
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Plate Tectonic Setting 70Ma
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Plate Tectonic Setting 60Ma
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Plate Tectonic Setting 50Ma
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Plate Tectonic Setting 40Ma
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Plate Tectonic Setting 30Ma
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Plate Tectonic Setting 20Ma
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Plate Tectonic Setting 10Ma
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Plate Tectonic Setting 0Ma
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Plate Tectonics and Crustal Stress Intraplate
Stresses
Black Blue Compressional stresses Red
Extensional stresses Green Strike-Slip stresses
from WSM website
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Plate Tectonics and Crustal Stress North
American plate intraplate stresses
Black Blue Compressional stresses Red
Extensional stresses Green Strike-Slip stresses
from WSM website
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Plate Tectonics and Crustal Stress European
plate intraplate stresses
Black Blue Compressional stresses Red
Extensional stresses Green Strike-Slip stresses
from WSM website
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Plate Tectonics and Crustal Stress European
plate intraplate stresses (detail)
Black Blue Compressional stresses Red
Extensional stresses Green Strike-Slip stresses
from WSM website
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Plate Tectonics and Crustal Stress Heat Flow
Heat Loss is the driver for Global Plate Tectonics
COLD HOT
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Plate Tectonics and Crustal Stress Heat Flow
2
km
4
6
300
200
mW m-2
100
40-80 mW m-2 range for continental crust
0
100
200
0
Age of oceanic crust (million years)
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Isostacy
Isostacy Isostasy forms the basis of the theory
of isostatic rebound. Isostasy itself is based
on the opposing influences of two main forces
buoyancy and gravity. For the earth, it is the
reason that the relatively rigid lithospheric
plates float at certain levels in the underlying
ductile asthenosphere. Blocks, or plates, will
adjust themselves vertically until the forces of
buoyancy and gravity are balanced. When the
forces are balanced, the blocks are considered to
be in isostatic equilibrium and there will be no
vertical movement. The level at which both
continental and oceanic blocks begin to float in
the mantle is called the level of compensation
and generally corresponds to the top of the
asthenosphere. At this location, the mantle will
flow in response to stress (e.g. a surficial
load) therefore the principal stresses at this
level are all equal to the lithostatic pressure
(P). If this system is in isostatic equilibrium,
the lithostatic pressure under both the
continental and oceanic blocks must be equal at
the level of compensation. This pressure is equal
to the weight of a column of material that
extends from the level of compensation to the
surface. P h.?.g where P
pressure h height or thickness ?
density g gravity
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Isostacy
Isostacy At any location the pressure exerted at
the level of compensation is approximated by
P hc.?c.g hm.?m.g where P
pressure hc crust thickness
(30km) ?c crust density (2.6-2.7kg/m3)
hm mantle thickness (100km) ?m mantle
density (3.3kg/m3) g gravity
Crust (hc.?c)
Mantle (hm.?m)
level of compensation
asthenosphere
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Isostacy
Isostacy It is assumed that the steady state
condition is that of isostatic equilibrium.
Tectonic processes such as crustal extension or
contraction produce short term isostatic
anomalies that are compensated by isostatic
uplift and/or subsidence EXTENSION
Before rifting hci.?c.g hmi.?m.g
After rifting hcf.?c.g hmf.?m.g
Excess load above Level of Compensation
causes isostatic subsidence after rifting
c
m
a
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Isostacy
Isostacy It is assumed that the steady state
condition is that of isostatic equilibrium.
Tectonic processes such as crustal extension or
contraction produce short term isostatic
anomalies that are compensated by isostatic
uplift and/or subsidence CONTRACTION
Before thrusting hci.?c.g hmi.?m.g
After thrusting hcf.?c.g hmf.?m.g
Deficit load above Level of
Compensation causes isostatic uplift after
thrusting
c
m
a
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Plate Tectonics and Crustal Stress Anomalous
structures
Gravity image of northern Yucatan
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Plate Tectonics and Crustal Stress Anomalous
structures
Cenote distribution
Gravity image (horizontal gradient) of northern
Yucatan
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Plate Tectonics and Crustal Stress Anomalous
structures
Meteor impacts
Gravity image (horizontal gradient) plus seismic
profile of northern Yucatan
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