P1247676902MTPBf - PowerPoint PPT Presentation

Title: P1247676902MTPBf


1
Introduction
Stress versus strain The two most important
terms used throughout this course are STRESS and
STRAIN.
What structural geologists actually observe and
directly measure is strain. Stresses, on the
other hand, are not directly measured, but in
principal they may be inferred or constrained
from the strain.
2
Introduction
Brittle versus ductile Generally, rocks respond
to stress in one of two ways they break, or they
bend.
Brittle deformation when the rock breaks.
Examples for brittle deformation include

• Faults
• Joints
• Dikes

3
Introduction
Ductile deformation when rocks bend or flow.
Examples for ductile deformation include

• Foliation and lineation
• Mylonite
• Boudinage

4
Introduction
Question Under what conditions do rocks exhibit
ductile/brittle behavior?
Rocks undergo ductile deformation when subjected
to high confining pressure and temperature. Thus,
brittle structures form near the surface and in
the upper crust, and ductile structures form at
greater depth.
5
Introduction
A fault near the surface becomes mylonite at
great depth
6
Introduction
The kink band structure shown below is an example
for brittle ductile deformation
7
Introduction
Time scales geological processes occur over a
wide range of characteristic time scales.
What are the characteristic time scales for fault
rupture?
Stresses that accumulate steadily over many
years, due to relative plate motion, may be
released abruptly within seconds to tens of
seconds.
8
Introduction
What are the characteristic time scales for dike
intrusion?
Modern dikes eruptions (e.g., in Hawaii) have
lasted between a few hours and a few days (show a
movie).
What are the characteristic time scales for
mylonite formation?
The physical processes that accompany the
formation of metamorphic fabric are slow (e.g.,
dissolution and re-precipitation of minerals).
9
Introduction
While the formation of brittle structures is
discontinuous and evolves via abrupt steps, the
formation of ductile fabric is more or less
continuous and is governed by slow processes.

• Length scales deformation is occurring
  simultaneously at a wide range of length scales.
• The different scales include
• Plate
• Regional
• Outcrop
• Hand-sample
• Grain
• Crystal

Figure from Allmendingers lecture notes
10
Introduction
A few slides back you have seen a picture of
mylonite at the scale of an outcrop. This is how
it may look at the scale of a few crystals or
grains
11
Introduction
Question How long is the coast of Britain?
It turned out that the answer to that question
depends on the ruler's length.
figure from Wikepedia
12
Introduction
On a log-log scale we get A power-law
function provides good fit to the data which
is equivalent to The b exponent is known as
the fractal dimension.
13
Introduction
Fractals are very common in nature
cauliflower
lightning
leaf
Drainage system
14
Introduction
Fault population too are fractals
15
Introduction
Fractal dimension of faults in Japan has been
determined using the box-counting algorithm from
Hirata, 1989
16
Introduction
Frequency-length distribution for normal faults
on the plain of Venus obtained from a Magellan
SAR image (from Scholz, 1997).
17
Introduction
Cumulative length distribution of sub-faults of
the San Andreas fault (from Scholz, 1998).
18
Introduction
Map of faults and joints exposed in the Yucca Mt.
(from Barton, 1995). Note that small joints are
more abundant than large ones.