Download Primary structures

School of Earth Sciences, Leeds University Text navigation menu at bottom of page.
Research in Earth Sciences
The School of Earth Sciences at Leeds has a fine record for innovative research and for
training research students. Our interests range across a broad spectrum of the Earth
Sciences, from the Earth's core to the oceans, and from the Precambrian to modern
environments. Many of our most successful projects bridge between conventional
disciplines, and often bring together unexpected partners. But we have set up research
groups under more or less conventional headings to explain our research interests, and to
allow you to focus on areas of interest.
Basic principles - rock organisation
Mountain belts formed by continental collision, rifting basins - the repositories of natural
resources, active faults and their earthquake hazards, slow-flowing rocks and plate tectonics
- all issues for structural geology. But it can be a bewildering as well as exciting subject.
There are lots of complicated rock geometries that require a three dimensional picture - well
4D really because we need to think about how geology works through time. In this web
resource we build up some basic concepts.
Styles of deformation
A simple account of the different ways a material might deform.
Primary structures
Notes on the organisation of rocks without tectonics - because if we want to understand
deformed rocks we should have an idea of what they looked like before.
New shapes
The record of distortions in grains
Measuring
How to record the orientation of structures - using a compass-clinometer
Time - the fourth dimension
Some notes on building up a geological history.--September 2003
Styles of deformation
Click on an icon to see a larger image.
When strata are caught in the vice of converging tectonic plates they
respond by deforming. Rocks can deform in many different ways. They
can break with the formation and movement along breaks - called faults. Or they can distort
in various ways.
Faults
Active faulting is the main way earthquakes are generated but for
every active fault there are hundreds (or thousands) of faults which
have moved in the geological past and now lie dormant (the thrust
faults of the Moine Thrust Belt are examples of dormant faults). The
chief thing about faulting is that all the deformation is taken up in a
narrow zone with the surrounding rocks only disturbed by being
moved.
Folds
Another common way that layered rocks deform is by
folding. You can do this yourself with a thin plank of
wood. If you push the ends of the plank together it will
warp - or fold (of course if you push hard enough then
the plank can break - congratulations - you've made a
fault!). To make folds in this way (called buckling) the
role of layering is critical - no layering and something
else happens.
Distortion (strain)
If a rock has no layering and is simply homogeneous
(rather like a pat of butter), if it's squeezed it will just
thicken up. If a pat of butter is warm enough you can
do this. You can quantify the amount of strain by
tracking how the butter pat becomes thick or less
wide. If you stamp a circle on the surface of the butter
and then distort it, notice that the circle becomes an
ellipse. In strata originally circular markers will become ellipses as the rock strains too. This
is also a way in which the amount of strain can be charted.
The various types of rock response to deformation can be found in NW Scotland. but why
the differences?
One of the most important things governing how a rock will
respond is its composition - not only what it is made of (types
of grains) but also how they are arranged (layers or random).
Then there are the conditions under which it is deformed. The
most important of these are pressure (i.e. how deep they were
when they were being deformed) and how hot. Depth is
important because breaking rocks creates new volume (voids).
The further down you go the harder it is to do this - so most
faulting in geology is limited to the top 10 km or so in the Earth.
There's still another 25 km of crust beneath this in most
continents, and over 6000km to the centre of the Earth. So
what influences how these deeper rocks deform?
Temperature. The hotter a material in general, the weaker it becomes (take common household materials like chocolate or butter). In geology the a critical
point for becoming weaker is passed when a rock reaches 70%
of its melting point (measured from absolute zero). As this varies
from material to material, the temperature dependence on rock
strength is strongly linked to rock composition. Minerals like halite
(rock salt) can distort easily under geological conditions at
temperatures of less than 100C. In contrast granite or quartzite
will need to be at temperatures in excess of 400C to be anything
like as weak.
And the final controls on how a rock will deform depend on how
the deformation is applied (or how the sample is loaded in
material science terms). Rocks respond differently if they are
extended or compressed. Further, the amount of load can control
the response. High stress (lots of force per unit area) tends to
break a rock while a low stress may result in a slower distortion .
The other chief control is how quickly the rock is distorted (strain rate). Do it quickly and the
rock is more likely to break, do it slow and the rock will distort. Again there are plenty of
household materials that respond in the same way (e.g. a plastic ruler).
The study of all these responses is called rheology - a relatively easy thing to quantify for
household objects but very difficult for rocks under geological conditions - when mountain
building events can take place over millions or years rather than minutes and seconds.
Primary structures
Click on images to see a larger version.
In order to understand deformed rocks we need to
have some idea of their organisation - their
architecture - before they were deformed. We can
start on a grain scale.
Many rocks show layering. This might be defined
by systematic variations in grain size. One layer could have predominantly large grains
while other layers might be dominated by smaller grains. Another type of layering might
result from systematic variations in the distribution of the composition (mineralogy) of grains.
This type of organisation has a general term: LOCATION FABRIC.
In sedimentary rocks the systematic development of
location fabric is
known
as
BEDDING.
For
example,
a
sandstone
may
display bedding that
is
defined
by
systematic
variations in grain size through a section. In a rock
sequence bedding might also be defined by alternations
of layers that are predominantly composed of grains of
the mineral calcite - in which case the rock layer would be called a limestone, and layers
composed of clay minerals - forming a layer of clay or mudstone. The bedding then would
be an example of a location fabric defined by variations in composition.
Location fabric can be displayed by igneous rocks - especially those that crystallised
gradually on the sides of magma chambers. In these situations subtle variations in the
chemistry or physical conditions in the magma chamber can cause particular minerals to
crystallise from the melt. Changing these conditions may cause a different mineral to
crystallise out. So variations in crystallisation can be
manifest by layering defined by alternations of different
composition (different minerals). The general term
COMPOSITIONAL BANDING is used to describe the
layering. Igneous rocks can also show location fabrics
defined by systematic variations in grain size particularly when a volume of magma cools at different
rates. Fine-grained, chilled margins on the flanks of
dykes and sills intruded at shallow crustal levels are
examples of such location fabrics.
For both sedimentary and igneous rocks, the location
fabrics are formed
by
the
same
processes that make
the original rocks
themselves. So in
this sense they can be considered as primary structures. This is useful for structural
geologists because we have a marker of the
organisation of rocks from before deformation. For
sedimentary rocks bedding is especially useful.
Sedimentary layers are commonly laid down
horizontally with bedding level, planar and continuous in
the original undeformed state. So if we find beds that
are strongly inclined, folded and cut and offset we are
able to recognise deformation and begin to analyse it. In
general the location fabrics in igneous rocks are less
useful simply because we can be less certain about the
original organisation and scale of the features.
Compositional banding will reflect some aspect of the geometry of the magma chamber
within which the igneous rock crystallised - we can assume that it was originally level, planar
or even continuous. But there are still uses to igneous rocks in structural geology - as we’ll
see later.
An aside: bedding in sedimentary rocks
Metamorphic rocks can show location fabrics too. In general these
are defined by the distribution of different minerals forming
compositional banding. In coarse-grained (grains readily visible with
the naked eye) metamorphic rocks such structures are generally
termed GNEISSIC BANDING. But this is not a primary structure because metamorphic rocks themselves originate as either
sedimentary or igneous rocks. Gneissic banding is generally formed
by the re-organisation of the original rock’s chemical constituents into
layers within which particularly minerals are concentrated. Only
rarely will this banding mimic any primary layering (igneous
compositional banding or sedimentary bedding). In general gneissic
banding is new. Nevertheless in some regional metamorphic belts
gneissic banding can have remarkably uniform orientations (leading
early geologists to mis-interpret as mimicking bedding). So if we
have reason to believe that gneissic banding had a reasonably simple, planar and parallel
form prior to a particular deformation episode we can use it to recognise later folds and
folds.
An aside: bedding in sedimentary rocks
In the main text we've told a very simple story - that sedimentary rocks are laid down in
continuous, parallel horizontal layers. But it's actually more complicated than that. Consider
a scree slope. The layers of debris making up the slope are laid down on the surface of the
scree - inclined at tens of degrees down the slope. So a sequence of sedimentary rocks
deposited as scree may contain layering that was inclined at the outset. It would be said to
have a depositional dip.
Scree deposits are rather rare in the geological record.
But other depositional environments can also break the
rule of sediments accumulating horizontally. Take a
sand dune. With each blow of wind sand grains are
dislodged down the slope of the dune, forming layers
that are inclined - another example of a depositional dip.
The same thing can happen under water (sub-aqueous
dunes). As the dune continues to form and advance
sand from the top is moved by down the slope, eroding
the old top of the dune slope. So over time a sedimentary deposit is created with inclined
surfaces bounded on the top by an erosion surface. We can recognise this in the rock
record as cross-bedding. The erosion surfaces represent the principal bedding planes and
these are continuous. In contrast the planes of cross bedding terminate at the bedding
planes.
The imaginary scree slope and dune we've used so
far
create
sedimentary
successions
with
depositional dips on the scale of each of these
landforms. But on much longer length-scales we
can also take issue with the notion of bedding as
being
originally
flat.
Imagine
sediments
accumulating today across a continental margin,
say on the Atlantic edge of Africa. The sea bed is
not horizontal - it has shallow areas (the shelf and
the abyssal plain) and an inclined portion (the
continental slope). So a batch of sediment raining down on this sea bed, perhaps blanketing
the sea floor will pick up the geometry of the sea bed itself. In other words - our imaginary
bed will have a complex shape moulded onto its substrate. And there's far more to this too but we can resist opening the Pandora's box of stratigraphic complexity any further for now!
Our simple assumptions about bedding look to be seriously flawed - but fear not. Crossbedding can be readily distinguished in the rock record as can scree slopes (characterised
by large angular blocks). And it's not often that we are concerned with understanding the
structural geology of a deformed continental margin all in one go. At the local scale the
assumption of bedding being originally flat is a fair starting point.
New shapes
Many rocks show more than just location fabric. Individual grains,
rather than having near spherical shapes, can be strongly
flattened ellipsoids. Commonly the orientations of the strongly
ellipsoidal grains line up creating a new rock structure. In this case
the fabric is defined by the shapes of grains and so this type of
organisation has the general name: SHAPE FABRIC.
An aside: lines and planes; lineation and foliation.
In general the intensity of the shape fabric increases as the rock
deforms. In some rocks this can happen to such an extent that the
original location fabric is impossible to see. Shape fabrics are
important to structural geologists because they allow some qualitative
measure on the amount of deformation. But they can also be used to
understand geometrically how the rock has
been distorted.
Consider a ball of something soft - like
plasticene. It has an original spherical
shape. But this can be distorted into
ellipsoids of different shapes. The shapes
of ellipsoids can be described in terms of
the relative lengths of the long axis, short
axis and an intermediate axis (creating three dimensions with
the axes at 90º to each other). If you push straight down on it the
plasticene becomes pancake-shaped. The technical adjective
for this is oblate. Here the short axis is reduced (of course)
while the intermediate and long axes are about the same (and
increased). To a squash in one direction results in a stretch in
two directions. If a rock is made up of grains of this shape that
are all aligned it will have a strong PLANAR fabric.
Now let's imagine another experiment with our
plasticene ball. Rather than squashing it we can draw it
out into a cigar shape. The technical adjective for this
is prolate. Now the lump has a single long axis and
two short axes that are more or less equal. If a rock is
made up of grains that are all aligned it will have a
strong LINEAR fabric. The rock itself would have the
structure of a bunch of pencils (or drinking straws).
Finally we can perform another experiments with
our plasticene ball smearing it by moving our hand
across it on a table top. The ball will deform into an
ellipsoid that is both flattened (increasingly parallel
to the table-top) and elongate in the direction we
smear in. The ellipsoid now has three axes of
different length - rather like a pitta bread! In an
ideal example the squashing in the direction of the
short axis is matched by an elongation in the
direction of the long axis, with the intermediate axis
remaining unchanged. The technical term for this type of distortion is PLANE STRAIN.
An aside: tectonites
The various types of shape fabric in rocks represent important information for understanding
deformed rocks. These uses are described elsewhere.
Measuring
Rocks contain structures that are planar and structures that
are linear. It is important to record the orientation of these
features, using a COMPASS-CLINOMETER. Lines and planes are measured in different
ways.
Lines (1D features in a 3D world)
A line has a direction (relative to North) - called an AZIMUTH. This is generally recorded in
the direction pointing downwards. The amount the line points down (between 0 and 90º) is
called the PLUNGE.
Examples of lines:
l-tectonites, fold hinge lines, lines of intersection between two planes (e.g. bedding and
cleavage).
Planes (2D features in a 3D world).
This is recorded with a STRIKE and DIP with dip direction.
A strike is a bearing (relative to North) of an imaginary horizontal line on the plane.
The dip is the amount down hill the plane is inclined at (so has a value between 0 and 90º).
The direction of dip is 90º from the strike
Notice that we also need to record an approximate direction of dip.
This information is measured for any plane - so if you're collecting data it becomes crucial to
state what type of structure it is that you are measuring.
Examples of planes:
Bedding, cleavage, joint surfaces, fault planes, fold axial planes, quarry faces.
Lines on planes
Some line features exist on planes, and can be measured with reference to that plane. The
measurement is called a pitch. First measure the plane. Then place a protractor flat on the
plane aligned so that the base-line of the protractor is parallel to strike of the plane. The dip
direction is 90º to this. Now identify the line you are trying to measure. Measure the angle
this line makes from the strike of the plane using the protractor. If the line runs in the dip
direction it will have a pitch of 90º. If the line runs parallel with the strike it has a pitch of 0º.
So the value of pitch varies from 0 to 90º.
But consider a line pitching at 45º on a plane that strikes north-south (and dips gently
eastwards). There are actually two line-directions that satisfies this description, one inclined
towards the NE and one to the SE. So to remove this ambiguity we specify the direction of
the strike (either north of south in our hypothetical example) from which the pitch was
measured (i.e. N or S).
Examples of lines on planes
Striation or mineral fibres on fault planes, glacial striations on glacial pavements, sole marks
on bedding planes, lines of intersection between two planes, l-s tectonites.
Time - the fourth dimension
In almost all understanding of geological processes it is impossible to divorce the issue of
time. And fundamental to understanding time in the geological record is the idea that the
geometric relationships between rocks betray the sequence of geological processes that
formed (and deformed) them. Building up a relative geological time scale is a fundamental
skill.
The diagram below gives you a few tools of the trade. Click on it to view the animation.
There is also an accepted Geologic Timescale,
originally based on the fossil content of
sedimentary rocks but calibrated (and extended
into
unfossiliferous
sequences)
using
radiometric dating. A current version is
displayed
to
the
left.
A key to understanding rock sequences and dividing
them into managable packages is the recognition of
unconformities. In their simplest form these represent
breaks or time-gaps
in the geological
record
at
a
particular location.
One
bed
may
overlie another but
the
boundary
between represents
a period of time
(sometimes many
tens of millions of years) when no rocks were deposited.
Alternatively rocks may have been deposited and then
eroded away before the younger series were deposited.
In general it can be quite tricky recognising these forms
of unconformity. But in other situations an older series
of rocks might be deposited, deformed and then eroded
so that the younger rocks overlie strata that are not parallel to them. This is termed an
angular unconformity and is much easier to spot. Another pattern can be found when
older rocks are folded or tilted and then the younger strata simply bank in filling the lowerlying areas first then gradually burying the rest. The younger strata are said to onlap the
older. Another way of recognising unconformities is when sediments have depositional
contacts with underlying rocks that weren't formed at the Earth's surface, such as
metamorphics or intrusive igneous rocks.
Introduction
Describing folds
Fold mechanisms
Polyphase folds
Picture Gallery
This site provides a collection of material on folds - one of the most common form of tectonic
structures. You can gain access to topics by clicking on the icons on the right. If you are
new to geology then start at the introduction. The sections on describing folds and fold
mechanisms are particularly suited to first year undergraduates (especially those taking the
module EARS1053 in Leeds!) as these cover much of the key material. The section on
polyphase folding is more advanced. If you would like a tour of the wonderful range of fold
structures then why not flick through the picture gallery?
Folds are amongst the most common tectonic structures found in rocks, and can make
some of the most spectacular features. They can occur on all scales and in all
environments.
In areas of active mountain
building, as for example here in
the front ranges of the Himalayas
in Pakistan (right), folds bulge up
the landscape. The upward
moving parts (called antiforms)
create hills while the areas that
have gone down relative to the
hills (called synforms) collect
detritus
eroded
from
the
surrounding
countryside.
A
common misconception is that rocks can only fold when
they are nearly molten. These outcrops tell a different
story. Rocks like these limestones can fold even at the
Earth's surface - provided they are given enough time. A
fold like this could take a hundred thousand years to grow.
But if the rocks were pushed together more quickly - they
could break - making faults.
Folds can occur on all scales. These folds from the island
of Syros only have a wavelength of a few millimetres. To
make folds the rocks must have a mechanical layering. If
they are homogeneous they will just squash together without folding. A stiff layer (more
viscous) will fold if embedded in weaker (less viscous) material. You can try this yourself using different types of plasticene. You can find out more about how rocks fold by clicking
here. Or you can return to the front menu.
Folds are wonderful things and highly variable. There's a plethora of jargon for describing
them. More usefully, there are many ways of measuring them so that we can quantify their
shape and orientation. From these careful descriptions we can learn more about how folds
form and what they might be telling us about the larger-scale tectonics. Use the icons to
explore or return to the main folds menu by clicking here.
Basic geometry of folds as wave-forms. Covers hinges and axial surfaces too.
How to classify folds - going into more detail.
Quantitative approaches to deducing fold geometries
from map-patterns, using structure contours. A real 3D insight!
and amplitude.
What to measure on folds - orientations, wavelength
How to use small-scale observations of folded rocks to
gain insight on the larger-scale structure.
structure.
How cleavage can be used to deduce larger-scale
Types of folding
How do they do it?
There are lots of different types of folds. How do they all form? One thing is clear - there's
no single mechanism that explains all types of folds. They form in different ways. The pages
linked from the icons take you through the different types of folds and how they might form.
Or you can link back to the main folds menu by clicking here.
Folds can form in different ways and need not invove compression along layers. Use the
icons to explore the types of folds and how they form. Or return to the mechanisms front
page.
d.
"Forced folds" is an unusual term for
those folds that are required to form
geometrically as the result of another
process. A common type of fold
results because of movement along
irregularly-shaped faults. These are
termed "fault-bend folds" and you can see an example by clicking here. Other folds can
develop as a result of sediments draping over pre-existing basement topography. Others
again can form as a consequence of magma being intruded - inflating a magma chamber.
You can return to the menu on the types of folds by clicking here.
Buckle folds are extremely common. They form when layers are compressed along their
length. These structures have been studied for many years. You can even make your own by squeezing plasticene layers in a vice or between your hands. To follow the mechanics of
buckle folds - start here. This sequence looks are simple buckles of single layers. But rocks
contain many layers and these can interact. To see these effects, including how parasitic
folds may form, click here. However, if layering is very intense, folds develop with highly
angular hinges - kinks. You can see this by clicking here. Alternatively you can return to the
mechanics front page.
Buckle folds are extremely common. They form when layers are compressed along their
length. These structures have been studied for many years. You can even make your own by squeezing plasticene layers in a vice or between your hands. To follow the mechanics of
buckle folds - start here. This sequence looks are simple buckles of single layers. But rocks
contain many layers and these can interact. To see these effects, including how parasitic
folds may form, click here. However, if layering is very intense, folds develop with highly
angular hinges - kinks. You can see this by clicking here. Alternatively you can return to the
mechanics front page.
Flexural flow--Flexural slip--Tangential Longitudinal Strain-
Ramsay 1
Ramsay 2
Ramsay 3
In many areas, particularly within the internal
parts of mountain belts, folds can be
developed more than once. Consequently
outcrops and regions can show patterns that
indicate fold interference, generated by a
later set of folds refolding another. You can
see some of the spectacular patterns that
result from fold interference by clicking on the icons. All examples come from the classic site
originally described by John Ramsay in the 1950s.
The analysis of polyphase fold patterns appears complex but is straightforward so long as
you are systematic. By clicking here you can follow a strategy for an outcrop in the western
Alps.
In this site you can find information on geological faults. To find out what these things are
and to learn something of their importance, visit the introduction first. You can also choose
topics from the column of icons. Most of this material relates to the first year course in
structural geology in Leeds (EARS 1053). However, more advanced materials lie behind if
you keep exploring. Alternatively you can choose to scroll through the picture gallery.
The outer part of the Earth is relatively cold. So when it is stressed it tends to break,
particularly if pushed quickly! These breaks, across which slip has occurred, are called
faults. The most obvious manifestations of active faulting are earthquakes. Because these
tend to happen along the boundaries between plates that is where most of the active
faulting occurs today. But faulting can occur in the middle of the plates too, particularly in
the continents. In general, faulting is restricted to the top 10-15 km of the Earth's crust.
Below this other things happen.
There is a wide range of faulting and the faults themselves can form surprisingly complex
patterns. Different types of faults tend to form in different settings - the faults at active rifts
are different from those along the edges of mountain ranges. So understanding the types
and patterns of ancient fault can help geologists to predict and reconstruct the forms of
ancient rifts and mountain ranges. The faulting patterns can have enormous economic
importance. Faults can control the movement of groundwater, they can exert a strong
influence on the distribution of mineralisation and the subsurface accumulations of
hydrocarbons. And they can have a major influence on the shaping of the landscape.
Movement on faults, with earthquakes, shatters rocks. In some places these new materials
are economically important as ready-made aggregate! In other places they can be a
problem for engineers, making hillsides unstable.
Return to the front menu and chose from the topics to find out more about faults!
Thrust fault surface in limestones from the Alps. Notice the sharp fault plane and the
shattered
rocks
below.
There are different types of fault. These can be classified on the basis of the direction of
displacement relative to the orientation of the fault plane at the time of displacement. To find
out about these different types of fault and how you can deduce which type of fault you're
looking at, click here.
Normal faults generally occur in places where the lithosphere is being stretched.
Consequently they are the chief structural components of many sedimentary rift basins (e.g.
the North Sea) where they have major significance for hydrocarbon exploration. They can
also be found in deltas, at the rear edges of huge gravitation slumps and slides. Normal
faults can show diffeent geometries - and a few are shown here. In some situations the
faults can become gently dipping at depth so that they have a spoon (or listric) shape. Other
normal faults are found in batches, dipping in the same direction, with rotated fault blocks
between. These are termed domino faults. Although most active normal faults can be shown
to dip at angles steeper than 50 degrees, there are examples of very low-angle normal
faults. These are often termed "detachments" - although this is a pretty vague term!
Detachments show gentle dips and often expose high grade metamorphic rocks in their
footwalls. These footwalls can be termed metamorphic core complexes. Normal faulting is
now thought to be an important way in which metamorphic rocks come to be at the earth's
surface today.
Thrusts are reverse faults and
commonly dominate the structure of
collision mountain belts. Some thrusts
have moved a long way - many
mountain belts have thrusts that have
moved many tens of kilometres. The
photograph above shows one such
structure from the Alps - which carries
basement of the Mont Blanc massif
onto Jurassic sediments. Many
thrusts can be shown to follow socalled staircase trajectories - you can
find out about this by clicking here. Otherwise, explore the nature of thrust systems by
selecting from the icons. The material introduces concepts used in the Leeds first year
structure course but some aspects are suited to higher level studies.
Thrusts cut upsection through previously undeformed rocks but they rarely do so in a
smooth way. The consequences of moving up a staircase of ramps and flats can be
explored by clicking here. The run of images links to field examples of the resulting
structural geometry. Alternatively, if you just wish to recap the terminology, click here.
An example from Loch Eriboll,
developed in quartz sandstones.
NW
Scotland
Classical fault-bend folds produce
very simple structures where the
bed dips simply reflect the angle
of the ramps through the
stratigraphy.
If
beds
were
horizontal before thrusting then
ramp angles are commonly less
than 30 degrees to bedding.
Consequently fault bend folds
should be open antiforms.
Furthermore, the footwall should
remain undeformed.
An example of this classical
behaviour is here - from this road section in the appalachians of New York State. However,
in many situations the relationship between bedding and thrusts is not explained by the
classical fault bend model.
In this second example, from
Hope's Nose, Devon, the bedding
in the hangingwall defines a tight
fold with over-steepened bedding
cut-offs. Clearly there is another
process going on here.
Click here to see how these types
of fold-thrust relationships might be
explained.
Return
to
menu
Example of a footwall syncline from the
Himalayan foothills of Pakistan.
Example of an asymmetric fold pair cut through
in its forelimb by a thrust. This is one of the
world classic examples of a supposed tip-line
fold, from the Broadhaven site in west Pembrokeshire.
Thrust belts commonly mark the outer edge of collision mountain ranges such as the Alps
and Himalayas. In these situations the thrusts don't just appear on their own but in herds!
The thrusts can interact to make wonderfully complicated cross-sections. To see some of
the features characteristic of thrust belts, click here. In general thrusts are directed outward,
away from mountain ranges towards the foreland. But sometimes thrusts can be found,
directed back towards the mountain ranges - these are termed "back-thrusts".
A small-scale example of a duplex structure, developed in quartz sandstones from
the north coast of Scotland. This ancient structure shows how repeated stacking
can accommodate significant displacement.
A large-scale example of a duplex structure. Notice that the duplex roof has been
largely eroded but a fragment remains, preserved as an erosional outlier of the thrust
sheet - called a klippe.
The edge of the Himalayan mountain range in Pakistan. Here a thrust at the bottom of the
scarp caried up sediments (as old as Cambrian in age) onto detritus deposited by rivers.
The thrust emerges directly to the earth's surface. Consequently it is possible to use
stratigraphic data from the sediments that are involved in the deformation to directly date the
thrusting.
A pop-up structure carried on a back-thrust/fore-thrust couplet. The result is a hanging slice
of rock. The image here shows pop-up stacking of the Cretaceous Urgonian limestone
(seen in the Bornes gorge, NW French Subalps), which forms cliffs about 200m high (the
view is rather foreshortened from the valley bottom). Notice that both thrusts "obey" the
rules in that they repeat stratigraphy. Click here for a reminder. You can understand these
types of back-thrust/fore-thrust pairs in terms of stresses and conjugate faults.
In three dimensions, thrust belts can be rather complex. Thrust sheets don't go on for ever
but can form local stacks. Individual thrust surfaces can change stratigraphic level. Laterally
restriucted culminations can be built up. To understand these structures in 3D, a critical
issue if you want to understand how to project information onto cross-section planes, there
are two approaches. The first of these is to describe the 3D geometry of individual thrust
surfaces - using cut-off line maps. The second is to understand how thrusts relate to each
other - using branch line maps. If you would like to see a real example of these
approaches, visit the 3D Assynt site.
3D Thrust sheet continuity in Assynt
You will need:
1. Geological map of Assynt
2. Simplified map of Assynt. (click here for pdf)
This exercise is designed to allow you to use 3D aspects of thrust geometry, particularly
branch lines and cut-off lines, to examine structural interpretations. The base map is the
geological 'tourist' sheet to Assynt, part of the Moine thrust belt of NW Scotland, drawn from
geological surveys at the end of the 19th century. In common with all maps, it is an
interpretation of the geology. Treat correlations of different thrust segments (e.g. the Ben
More thrust) with caution. Thrust aren't labelled in the field! You are provided with a
simplified tracing which will allow you to mark on branch lines and cut-off lines. However, to
complete the exercise and to find the thrusts as interpreted by the original Survey
geologists, you will need to use the original map. (Moine thrust = T'''; Ben More thrust = T'';
Glencoul Thrust = T' on this map).
A web-solution for this exercise is available.
The following analytical strategy is proposed:
1. Construct the branch line for the Ben More and Moine thrusts. You should first
identify all the branch points and then join them, testing whether the emergentburied behaviour is maintained around the complete loop.
2. Construct the hanging-wall cut-off lines for the base of the Cambrian against
the Ben More thrust.
3. Sketch a stratigraphic separation diagram for the Ben More thrust between a
and b. In theory this should involve first drawing all the cut-off maps for
footwall and hanging-wall. But it can be sketched now by projecting information
(fw
and
hw)
from
the
main
trace
of
the
thrust.
Comment on the different stratigraphic relationships along the Ben More thrust.
4. Now armed with the geometric interpretation, check the geometry of the Ben
More sheet. Does it behave like a thrust sheet (older on younger, thrust cutting
simply up-section in transport)? If not, suggest modifications in the map
interpretation or suggest a structural history that explains the relationships
that you have found.
The original map, published by the British Geological Survey, is the Special Sheet to the
Assynt district (1:63360), originally published in 1923 but reprinted in 1986.
This exercise is discussed, in part, by the following articles:
Elliott, D. & Johnson, M.R.W. 1980. Structural evolution in the northern part of the Moine
thrust belt, NW Scotland. Trans. R. Soc. Edinburgh 71, 69-96.
Coward, M.P. 1985. The thrust structures of southern Assynt, Moine thrust zone. Geological
Magazine 122, 595-607.
Strike-slip faults include some of the world's most famous - or infamous structures, including
the San Andreas Fault system and the North Anatolian Fault system. Both of these are
renowned for devastating earthquakes. Strike-slip faults are those where the relative
displacement is parallel to the strike of the fault. Strike-slip fault zones are commonly, but by
no means exclusively, steep and can be rather difficult to recognise on cross-sections.
However, active strike-slip faults are commonly associated with spectacular tectonic
landforms, such as the narrow basin and abrupt range-edge seen in the photograph above
(the type area for the Yammouneh Fault, part of the Dead Sea fault system in Lebanon).
Strike-slip faults need not be pefectly planar and any irregularities in shape can cause other
structures to form. You can see some of the basic ones by clicking here. Strike-slip zones in
basement at depth can also generate folds in the overlying cover.
Commentary
This way of forming basins along strike-slip faults is widely established. Some researchers
use the length of basins to infer the displacement on strike-slip fault zones because, as the
preceeding diagrams show, pull-apart basins grow in length as the movement increases.
However, this relationship assumes the fault geometry illustrated on the diagrams and
should open holes through the entire lithosphere!. An alternative view of basins on strikeslip faults is now becoming more accepted. The basins still form on what are efectively
releasing bends in the strike-slip zone, but they can be made shallower. To see how, visit
the "soft-linked" part of the fault site by clicking here - but beware of getting lost in the site
navigation.
Many classic analyses of faults generally consider the faults to cut continuously through a
rock volume. However, in detail, faults can appear as series of segments that are distinct
entities. If on a cross-section you think that different beds are offset by a single, continuous
fault surface, this fault can be termed "hard-linked". However, if bed offsets occur on faults
that do not link directly the geometry is referred to as being "soft linked". As a general rule of
thumb, the length of a fault is approximately ten times greater (or more) than the
displacement on the fault. So a fault 10 km long can only have moved 1 km. So soft linked
faults cannot accommodate very great displacements - or the unfaulted gaps in the fault
zone network must become very highly strained. This set of web pages, primarily directed at
intermediate-level structural geology courses, shows some of the basic features of softlinked faults. You can examine soft-linked normal faults, soft-linked thrust systems and softlinked strike-slip zones.
Return to faults menu.
Soft-linked
normal faults
in
Carboniferou
s sandstones
and shales
from
Saundersfoot
,
Pembrokeshi
re. The white
material
is
new vein fill.
To find out
more about
veins
and
extending
layers, visit
the
minor
stuctures
website - but beware navigation difficulties getting back here!
Example of relay ramp basins on a strike-slip faults zone. This example shows
topography created along an active branch (the Roum Fault Zone, RFZ) of the Dead
Sea fault system in southern Lebanon.
All structures form in response to forces acting on rocks - and these give rise to stresses. In
almost all geological situations stresses are always compressive but vary in different
directions. We can evaluate the stress state in terms of the orientation and magnitudes of
the so-called three principal stresses - which each act at 90 degrees to each other (i.e. they
are orthogonal). Conventionally these are denoted using the greek letter "sigma". Patterns
of conjugate faults - provided they formed together - can be related to the orientations of the
principal stress axes. To find out how, click here.
In many respects, shear zones are the deep-level equivalents to faults. They should
accommodate relative displacement of the surrounding rocks just as faults do but rather
than be surfaces, they consistute bands of rock that have undergone deformation. Some
shear zones can be narrow - rather like faults. Others can be tens of kilometres wide - the
deep-lithosphere equivalents of fault-dominated plate boundaries seen at the Earth's
surface today. This site provides a brief introduction to shear zones, illustrated by outcrop
scale examples from the field.
Picture gallery - shear zones
Click on thumbnail to go to full size image. To get back to this catalogue use the "back"
button on your browser.
Sheared aplite veins in a
deformed granitoid, Ticino,
Switzerland
A shear zone developed in
granodiorite with xenoliths
Shear zone developed in
Classic shear zone from
sandstones from Marloes
Castel Odhair in the outer
bay in Pembrokeshire. To
Hebrides. To find out how
find
out
how
these
these
structures
are
structures are formed click here.
formed click here.
Shear zone developed in
metagabbro in the Kohistan
terrane, Pakistan
Shear zone developed within
metaigneous material seen
in a lose boulder, Unst,
Shetland
Very narrow shear zone
developed in metagabbros
from the Kohistan terrane,
Pakistan
Tension gashes are a special type of vein
that can form rather spectacular patterns as you can see from the photograph next
door. By clicking here you can discover how
tension gashes are believed to form and
what you can deduce from their patterns. In
special situations, as in the photograph,
tension gashes form so-called conjugate
sets. You can investigate these by clicking
here - or you can return to the minor
structures menu.
Reprecipitation sites
Reprecipitation is an important part of the
deformation process. There are several
different sorts of site such as veins... which
can occur on all scales.
They can often form across large parts of an
outcrop
Photomicrograph of a calcite vein developed in a
limestone, from the french Alps (Chartreuse
area). This vein is about 100 microns across.
Vein arrays seen on a bedding surface in
Cretaceous limestones, Sdanetsch, Swiss Alps.
These veins are filled by new, bright white
calcite.
and be found in thinsection
.
Veins are simply voids formed by dilatation
created along fractures (commonly tensile fractures) into which has been precipitated new
minerals - such as calcite or quartz.
But there are other forms of
precipitation site such as pressure
shadows
.
There are also special vein arrays
called tension gashes.
Photomicrograph of fibrous reprecipitation of calcite around a
"rigid" pyrite grain. These features are called pressure shadows
or pressure fringes. The calcite comes from the host rock
which has dissolved from the plane of vein. This material was
collected from strongly deformed Jurassic limestones,
Beaufortain, NW French Alps.
Simple tension gash
array developed in
Carboniferous
limestones
at
the
Mumbles,
South
Wales.
This relationship between conjugate
tension gash arrays and the inferred
stress directions is directly analogous to
determining the stress orientations from
conjugate faults. Click here to visit this
site - but beware navigational difficulties
returning to this page.
Return to diagram
Sheared aplite veins in a deformed granitoid, Ticino, Switzerland. The two aplite veins had
different original orientations and therefore show different deflections into the shear zone.
Nevertheless the sense of deflection is the same and may be used to deduce the sense of
shear in the shear zone. To find out how these structures are formed click here.
Return to picture gallery
There are several ways of quantifying shear strains across shear zones. The principal aim is
to use these data to establish the displacement across the shear zone. We can use
deflected markers that were originally oriented at 90 degrees to the shear plane - this is the
easiest case to follow. Alternatively we can use the general case of any pre-existing marker
- as long as we can establish its original orientation. Finally we can determine the shear
strain from new foliations. In all case these estimations should be done on a plane (e.g. an
outcrop surface) perpendicular to the shear plane and parallel to the shear direction. It is
important then to be able to evaluate the orientation of the shear plane. You can follow the
method by clicking here.
Return to shear zones front page
This page contains material on strain, how it can be seen in rocks and how it can be
quantified. For an idea of strain in rocks, browse the picture gallery. For an introduction to
the concept of strain and some definitions, review the Introduction. The section Strain
Ellipse explores the properties of strain and the section Translation introduces the
relationship of strain to translation.
A collection of fossil brachiopds lying in various orientations on a surface. Also
indicated is a unit surface. Go to the next picture to see what happens when the fossils
are shortened in the horizontal direction and extended in the vertical direction. Note
first the coloured ones.
The fossils change shape, and the changes depend on the oriientation of the fossils.
The yellow and blue fossils keep their symmetry, the others including the green one
become distorted or sheared. The lines on the green fossil which were at right angles
are now oblique. The circle becomes an ellipse. We distinguish two sorts of strain,
which are defined in the folowing frames.
The longitudinal strain, e, is measured in terms of the changes in the lengths of lines.
The red rectangles represent pieces of a boudinaged belemnite (see picture gallery).
The length over all pieces is the length in the deformed state the length of the
belemnite found by fitting the pieces together is the undeformed length.
The shear strain is defined in terms of the changes in angle of lines that were at right
angles before deformation. The change of angle is psi and the shear strain is gamma.
A simple rigid translation occurs when every point in a body, here represented by a
grid, undergoes the same displacement. The grid suffers no distortion.
A simple rigid rotation occurs when points in
a body, here represented by a grid, undergo displacements which are simply organised
as a swivelling about an axis. The grid suffers no distortion, as for simple rigid
translation.
When the body undergoes strain as well as translation the displacement vectors form a
complex varying field. In the example shown the strain is everywhere the same as can
be seen from the fact that the initial squares of the grid are changed into
parallelograms of the same shape.
When the body is distorted by different amounts in different parts the grid squares
no longer have all the same shape.
A heterogeneous strain in a body may be such that there are no breaks. It is then said
to be compatible
A regular 4x4 grid of squares is deformed heterogeneously to produce the final grid
shown. The strain is such that there are breaks, which show up as a faults. The strain
is then said to be incompatible
Picture gallery - Strain markers
Click on thumbnail to go to full size image. To get back to this catalogue use the "back"
button on your browser.
Boudinaged dyke
Balanced
cross
section of
the
Himalaya
Stretched belemnite
with infill in gap
A
stretched
and
boudinaged belemnite
Elliptical reduction
spots in a slate
Sheared worm tubes
that were originally
perpendicular
to
bedding
Pressure shadow
Pressure shadow with
pyrite
Deformed
conglomerate
Folds
from
the
Vaniose, French Alps
Deformation structures that can be observed directly in individual outcrops (of say a few
metres across) or with the naked eye in hand specimen are commonly referred to (rather
disparagingly) as "minor structures". They nevertheless underpin many structural
interpretations - they are the building blocks that allow understanding of larger scale
structures. The set of icons opposite provide links to collections of structures.
Alternatively you can click here to visit the main site for structural teaching resources.
A circle of radius one is deformed by vertical shortening horizontal extension. The
circle becomes an ellipse,
Lines in various orientations shown colour-coded in the top right quadrant of the
original circle are deformed to the orientations and lengths as shown in the bottom
left quadrant of the ellipse. (The deformed lines should be in the top left quadrant of
the ellipse, but have been moved to the symmetrically equivalent position for clarity).
Note that some lines are shortened, some lines are extended, all lines except the
vertical and horizontal are rotated so as to be less steep.
The long and short axes of the ellipse are related to the greatest and least
extensional strains. The value of the original length, l0, is the radius of the circle, that
is 1. The change in length is given by the distance between the circle and the ellipse, e 1.
The same applies to e2, except that the difference between the original length and the
final length is now negative.
The extensional strain in a general direction can be calculated in the same way as for
the greatest and least values. Note that as the direction of measurement changes the
values of the extensional strain changes.
There are two directions in which the extension strain is zero. That is, the lines
corresponding to the points where the circle and ellipse cross one another. These lines
separate orientations that are shortened from those that are extended.
The black line through the centre of the original circle and the tangent to the point
where this line crosses the circle are at 90o to one another. After deformation these
lines have the orientations given by the red lines, from which the change in their
relative orientation is the angle psi, from which the shear strain gamma can be
calculated from gamma = tan(psi).
For directions parallel to the long and short axis of the strain ellipse the strain does
not change the right angle between the radius and te tangent to the unit circle, so the
angle psi is zero and consequently the shear strain is zero
The value of the shear strain varies with orientation.
The strain ellipse represents the values and variation of the extension and shear
strains.
Dissolution and reprecipitation of minerals are important mechanisms by which rocks can
change shape (i.e. deform). The process is generally termed Diffusional Mass Transfer
(DMT) - although some people refer to the dissolution part as "pressure solution". The key
idea is that materials tend to dissolve when compressed (like carbon dioxide gas in a fizzy
drink with the cap on) and come out of solution when the pressure is released. So the
direction of maximum compression in rocks is normal to planes of maximum solution with
precipitation in the direction of minimum compressive stress. You can judge this by clicking
here. In fine grained rocks the surfaces of dissolution are commonly wavy. These features
are called stilolites. Pressure solution is also an important way to form cleavage. There are
different types of reprecipitation sites.
Looking down on a bedding plane in deformed conglomerates, Haut Giffre area of
French Alps. The clasts are all different sorts of limestone with subtly different
solubilities. You can use these features to infer how this rock was strained.
Seen here in Carboniferous limestones from south Wales, this is a classic example of a
stilolite. These features can be mistaken for folds, with in this case the direction of maximum
compression aligned horizontally. This is wrong because the stilolite forms by solution - with,
in this case, the direction of maximum compression aligned vertically. Notice the veins too.
Fault surfaces have many structures associated with them. Commonly the fault surface can
be grooved - rather like a glaciated pavement. This analogue is highly pertinent because
glacier ice can be considered in some ways to be a rock (a solid earth material) which, in
flowing across the bedrock, abraids the surface. And the striations and grooves on fault
surfaces can have the same significance - they document the direction of relative fault
motion!
Fault surfaces can also open up small voids into which new minerals can be precipitated
(click here to see an example). If precipitation keeps up with the rate of fault slip then the
new mineral can be fibrous. The imprecise term for these features is slickenside. A better
term is shear fibre. The long directions of the fibres tell you about the relsative slip direction
of the fault and the sense of fibre growth off the fault wall rocks can give you the sense of
faulting - just like a pull-apart basin on a strike-slip fault (you can link to strike-slip faults or to
the general pages on faults by clicking here but beware losing navigational control in these
websites).
Grooves on a gently overhanging fault surface cut
into limestones in Sicily. The grooves trend close to
parallel to the strike of the fault surface, indicating
that the fault acted as strike slip.
Looking down on a fault plane developed in Cretaceous limestones from the Chartreuse
massif, French Alps. The fault surface is abraided by fine straie and decorated by newly
precipitated calcite (white patches). These have opened on the "lee side" of steps on the
fault surfaces. So we can infer that the eroded upper side of the fault has moved in the
direction the pencil points.
Cleavage is a new fabric the develops in rocks during deformation. There are several ways
in which cleavage can form. Probably the most famous type is "slatey cleavage" - so called
because it is characteristic of slates. In slates the cleavage can come to dominate the rock
so that you have to look really hard to find features like bedding. In these situations the
cleavage is said to be "penetrative". Slatey cleavage results from the mechanical
realignment or growth of platey minerals such as clays and micas, essentially so that they
are flattened perpendicular to the direction of maximum compression. In folded sequence
slatey cleavage is commonly found to be parallel to fold axial surfaces (this type of
arrangement is termed "axial planar cleavage" and is useful for determining vergence.
When rock layers with different mechanical properties are interlayered cleavage is often
found to refract through the layers. Cleavage planes can also form by dissolution of material
- although in these situations it is commonly spaced. This type is called "pressure solution
cleavage".
Example of slatey cleavage formed in late Precambrian siltstones from the Hazara hills,
Pakistan (lens cap scale). You should be able to recognise bedding (gently inclined left) and
use the bedding-cleavage relationship to define vergence.
Cleavage often goes hand in hand with folding. You can find out more about the different
sorts of cleavage elsewhere [link to go in later]. However, the neat thing about cleavage is
that in ideal cases its orientation is parallel to the axial surface of local folds - when it is
refered to as "axial planar cleavage". In this ideal situation, and even if it fans a bit, the
intersection of cleavage planes with the bedding planes (the intersection of two planes is a
line of course) will be parallel to the hinge line of the fold. You can study these relationships
by clicking here. Cleavage-bedding relationships can also be used to help unravel largescale structure using the vergence concept.
Cleavage refraction in a limestone-shale sequence, Gondran, French Alps.
Tension gashes are a special type of vein
that can form rather spectacular patterns as you can see from the photograph next
door. By clicking here you can discover how
tension gashes are believed to form and
what you can deduce from their patterns. In
special situations, as in the photograph,
tension gashes form so-called conjugate
sets. You can investigate these by clicking
here - or you can return to the minor
structures menu.
Reprecipitation sites
Reprecipitation is an important part of the
deformation process. There are several
different sorts of site such as veins... which
can occur on all scales.
They can often form across large parts of an outcrop
and
be
found
in
thinsection
.
Veins
are
simply voids
formed by
dilatation
created
along
fractures
(commonly
tensile
fractures)
into which
has
been
precipitated
new
minerals such
as
calcite
or
quartz.
Vein arrays seen on a bedding surface in
Cretaceous
limestones,
Sdanetsch,
Swiss Alps. These veins are filled by
new, bright white calcite.
Photomicrograph of a calcite vein developed in
a limestone, from the french Alps (Chartreuse
area). This vein is about 100 microns across.
Photomicrograph of fibrous reprecipitation of calcite around a "rigid"
pyrite grain. These features are called pressure shadows or pressure
But there are other fringes. The calcite comes from the host rock which has dissolved from
forms of precipitation the plane of vein. This material was collected from strongly deformed
site such as pressure Jurassic limestones, Beaufortain, NW French Alps.
shadows
.
There are also special vein arrays called tension gashes.
Simple tension gash
array developed in
Carboniferous
limestones
at
the
Mumbles,
South
Wales.
This relationship between conjugate
tension gash arrays and the inferred
stress directions is directly analogous to
determining the stress orientations from
conjugate faults. Click here to visit this
site - but beware navigational difficulties
returning to this page.
Detailed observations on a small scale
can be invaluable for building up a large-scale picture of rock structure. In many parts of the
world this is the only way of deducing how rocks are organised beyond an individual
outcrop. There are two key concepts, vergence and facing.
Deformation when rocks are warm or when it happens slowly commonly results in nearcontinuous strain, without breaking. This type of deformation is termed ductile. It
characterises deformation that happened deep in the crust, when rocks were under
metamorphic conditions. Consequently rocks that have undergone ductile deformation very
commonly are also metamorphosed.
Deformation under metamorphic conditions is commonly cyclic and polyphase. This is
sometimes manifest by refolded folds. Where strain is localised, shear zones develop.
To find out about the ways of describing rocks that have undergone ductile deformation or
to go and see some deformation - choose from the adjacent icons.
Scroll down these notes or select from the icons opposite. When rocks deform in a ductile
fashion they can generate beautifully complex structures that can bamboozle even the most
experienced structural geologist. So we need some strategies to describe these rocks and
analyse them.
To start with, we need to establish the difference between different types of rock structure.
Part of the problem is to establish how the features in a rock are organised - their
dimensionality. Even this isn't enough. For example, a rock can contain several planar
structures - e.g. bedding and cleavage. We need a way of describing these in terms of what
is defining the fabric. A good way is to use language that is non-genetic. Is the fabric
defined by aligned grain shapes or by the organisation of the location of different grain types
(shape vs location fabrics). Shape fabrics are almost always created by deformation.
Location fabrics can be produced by sedimentary (bedding), metamorphic (gneissic
banding) or igneous processes (cumulate layering), even working in tandem with
deformation (e.g. pressure solution seams).
Once we've established that deformation has occurred in a rock, it is useful to describe it in
3D. An important consideration is the 3D shape of the strain ellipsoid - the 3D extension of
the strain ellipse concept (link to strain ellipsoid in 2D strain). Another, rather classic way, of
understanding complex deformation is to unravel distinct phases of deformation.
A feature of some high strain shear zones is the development of sheath folds. These
structures form by amplification of originally weakly curvilinear folds into very strongly
curvilinear ones. The hinge lines rotate with intense deformation to align sub-parallel to the
stretching lineation.
A circle of radius one is deformed by vertical shortening horizontal extension. The
circle becomes an ellipse,
Example of a sheath fold developed in Moine
psammites. This fold has a test-tube like shape in 3D,
visible in the outcrop. An intense stretching lineation
plunges parallel to the
length of the "tube" - the
main hinge line of the
sheath fold. Folds like this
are thought to require very
high values of simple shear
in order to form.
Return to text
Picture gallery Portvasgo
ductile
structures -
Click on thumbnail to go to full size image and full description. To get back to this catalogue
use the "back" button on your browser.
General view of sheared
psammites at Portvasgo.
Foliation and lineation.
Highly
flattened
conglomerate within the
Moine
metasedimentary
sequence.
Apparently
recumbent
tight fold in highly sheared
Moine psammite.
Fold pair in highly sheared
Moine psammite.
Rodded
Moine
metasediment - example of
an L-tectonite.
Example of a shear fold
developed
in
Moine
psammites.
Highly sheared
Lewisian gneiss.
former
Picture gallery - ductile structures
Click on thumbnail to go to full size image and full description. To get back to this catalogue
use the "back" button on your browser.
Example of mylonite, from
Knockan, Scotland
Example
of
gradient
in
orthogneiss
a
strain
deformed
Looking onto the foliation
surface of highly sheared
calcschists
Looking onto a 3D outcrop
of folded quartzites, with
an
intense
stretching
lineation plunging down the
fold hinge line.
Picture gallery - minor structures
Click on thumbnail to go to full size image and full description. To get back to this catalogue
use the "back" button on your browser.
An
extended
beleminite
fossil seen on a cleavage
plane in Liassic-age shales
at Leytron, Swiss Alps
Boudinaged
amphibolite
layer
within
gneisses.
Laxfordian complex, near
Durness, Scotland
Tension gashes
Tension
gash
array
developed in limestones
Grooves
on
a
gently
overhanging fault surface
cut into limestones in Sicily
Vein arrays seen on a
bedding
surface
in
Cretaceous limestones
Pitted pebbles
Example
of
pressure
solution cleavage in silts
from south Devon
Photomicrograph of fibrous
A classic example of a
reprecipitation of calcite
stilolite seen here in
around a "rigid" pyrite
Carboniferous limestones
grain
from south Wales
Looking down on a fault
Simple tension gash array
plane
developed
in
developed in Carboniferous
Cretaceous
limestones
limestones at the Mumbles,
from the Chartreuse massif, French Alps South Wales
Photomicrograph
of
a
calcite vein developed in a
limestone
hills, Pakistan
Example of slatey cleavage
formed in late Precambrian
siltstones from the Hazara
When planar layers or linear objects are extended along their lengths they can break or
neck. This can create a characteristic segmented outcrop pattern, rather like strung-out links
of sausage. And this gives these segments their name - boudins (after the French bloodsausage) and the process is called boudinage. Under this section you can see some
examples. Pencil-like fossils called belemnites are commonly extended in the Alps - there's
an example here. The white material between the dark segments of the once-continuous
fossil is newly precipitated calcite. The other image shows a boudinaged amphibolite dyke
embedded in gniesses. In all cases it is the strong material that breaks (or boudinages) with
the less strong (lower viscosity) material flowing around.
An extended beleminite fossil seen on a cleavage
plane in Liassic-age shales at Leytron, Swiss Alps.
The boudinaged segments of the fossil are darkcoloured while newly-precipitated calcite (white) fills
the gaps. The belemnite fossil was more competent
(higher viscosity) than the shales around it at the time
of deformation.
the spaces between the amphibolite
amphibolite was more competent
viscosity) than the gneisses at the
deformation.
Boudinaged
amphibolite
(dark)
layer
within
gneisses.
Laxfordian
complex, near
Durness,
Scotland.
Notice how the
gneiss
has
"flowed"
into
boudins. The
(higher
time
of
The mighty south (Rupal) face of Nanga Parbat, with over 4 km of vertical relief
Welcome to this site which looks at aspects of the geology and tectonic evolution of the Nanga
Parbat area of the Himalayas. This material is designed primarily to support teaching in Earth
Sciences at Leeds. However, we welcome external users. Additional information will be added
to
this
site
from
time
to
time.
Rob Butler
HTML created by C. Gordon
About Nanga Parbat.
The mountain of Nanga Parbat is the westernmost 8000m peak of the Himalayan chain.
Geologically the mountain gives its name to a massif of rocks derived from the Indian
continent. These rocks were originally thrust beneath the over-riding Kohistan island arc
terrane (the southern margin of the Asian landmass prior to India-Asia collision). The
Kohistan arc rims the Nanga Parbat massif on three sides. Early workers considered the
massif to occupy the core of a north-south trending antiform. As a consequence of erosion
through this antiform, today we can see levels within the collision belt that would otherwise
be buried.
This web site provides a flavour of the geology of the Nanga Parbat massif, particularly its
tectonic history. You can follow a virtual field excursion, examine the once deeply-buried
continental crust in its heart, get additional information on the topography, exhumation and
cooling history of the massif and a list of references to follow up.




Heart of the massif
Western Margin field excursion
Other Information
Nanga Parbat front page
What rocks are in the Nanga Parbat
massif?
The Nanga Parbat massif is made up of ancient continental crust, caught up within the
young Himalayan mountain belt. In common with old basement terranes, it records a long
history of metamorphic, igneous and tectonic events. Because of the intensity of Himalayan
processes, it is difficult to see through to the early, pre-Himalayan events. Much of the
present outcrop is dominated by migmatites. And it is difficult to separate these magmatic
events out.
The youngest parts of the history are readily identified fortunately because they include
important magmatic and melting phenomena.
After the high-temperature metamorphism and melting, rocks exposed within the massif
have cooled quickly to reach the surface.
Cross-cutting relationships point to a long history
Migmatitic gneisses of the Nanga Parbat massif cross-cut
by an amphibolite sheet. Field relationships like this point to a period of metamorphism
and migmatisation that preceeds a episode of basaltic dyke intrusion.Since the basic
magmatism probably long preceeds Himalayan collision, the adjacent gneisses
presumably also experienced migmatisation long before Himalayan orogeny. These
outcrops come from the Indus gorge.
Migmatites
Melt extraction textures in migmatites from the Tato area, Nanga Parbat massif. Partial
melting of continental crust was clearly important during the history of the gneisses of the
Nanga Parbat massif. But did this feature form over the past few millions of years, during the
exhumation of the massif, or is it inherited from a period of anatexis long before Himalayan
orogeny?
Phase
relationships
petrogenesis
and
granite
.....for young melts within the Nanga Parbat massif. Further details are provided by Butler et
al. (1997).
The leucogranites are characterised by very high 87-86 strontium isotopic ratios (values of
up to 0.88 in the main bodies and over 1.0 in some pegmatitic sheets). The Rb-Sr ratio and
rare earth geochemistry argues for these being very small-volume batch-melts generated
from the breakdown of muscovite without the presence of appreciable volumes of fluid.
Low-vapour melting is supported by field relations - because the leucogranites have
migrated away (?up several km) from their source migmatites. In contrast, wet melts tend to
crystallise more readily as they rise in the crust.
Cordierite granites
Cordierite-bearing seams of granite-like material, closs-cutting migmatitic banding.
Tato area. These seams are the latest magmatic features within this part of the
massif.
Leucogranite
Part of a large leucogranite intrusion from the Fairy Meadows area of the upper
Raikhot valley. This and other intrusions within the massif form amongst the World's
very youngest granites derived from melting continental crust in collision mountain
belts.
Nanga Parbat virtual field trip
The Western Margin is of critical importance for understanding the recent and
active tectonics of the Nanga Parbat Massif. There are a variety of structures
present, including rare segments of an early ductile contact between the Kohistan arc
terrane and rocks of the Indian continental crust (which are indicated on the adjacent
map). However, the margin is dominated by later structures. Some of these are
associated with metamorphic fabrics and formed under ductile conditions while others
formed under shallower burial conditions. These are marked by extensive fracturing
and cataclasis, presumably associated with seismogenic faulting in the past. Taken
together, these structures record the progressive uplift and unroofing of the area.
You can visit a number of sites by navigating from the list below or the adjacent map,
building up a picture of this polyphase deformation history and collecting kinematic
data for your own structural analysis.





Sassi Indus
Confluence area
Ramghat
Raikhot Bridge Site
back to the Nanga Parbat contents page
The north part of the western margin of the Nanga Parbat Massif is exposed as
spectacular natural transect provided by the Indus gorge. The Sassi area is
particularly interesting. Here there are a variety of tectonic contacts that juxtapose
the two terranes. There is an early ductile contact - possibly part of the Main Mantle
Thrust - and later faults and shears of which the Shahbatot Fault is the most
continuous. Use the map and section to explore these outcrops. Deformation within
the Kohistan arc that relates to the early contact may be identified by tracking the
behaviour of a suite of intrusions - the Confluence granite sheets. To see these in
undeformed state, leave the Sassi area and return to their type area, at the
confluence between the Indus and Gilgit rivers.
Shahbatot Fault - Map
Shear structures in the Kohistan arc
Shear band developed in strongly sheared Kohistan arc rocks (with deformed
Confluence granite sheets). These may be used to deduce the sense of shear along the
early ductile contact between Kohistan and Nanga Parbat rocks.
Looking west along the Indus River. The boulders on the left are up to 2m across.
Shear structures in the Kohistan arc
Subsidiary shears which deform foliation within the Kohistan arc. These may be used
to deduce the sense of shear along the early ductile contact between Kohistan and
Nanga Parbat rocks. Looking west across the Indus river. The visible cliff height is
about 15m. Compare with the shear sense deduced from the shear band structure.
Sketch cross-section/view
Text index

Shear band
Shear zone
Cliff Close up of cliff

Asymmetric boudinage
Shahbatot fault
Another late fault
Looking north along the western margin.This view shows steeply foliated rocks of the
Nanga Parbat massif (on the east side of the Shahbatot Fault) with the near outcrops
being highly sheared rocks of the Kohistan arc (with streaked out Confluence granite
sheets), on the west side of the Shahbatot Fault.
Shear structures in the Kohistan arc
Sheared Kohistan units with confluence sheets (note folds). Looking East. Cliff height
approximately 300m (strongly foreshortened). Click here for close up of base of cliff.
Close up of base of cliff
Detail of strongly sheared Kohistan arc rocks with highly attenuated Confluence
granite sheets. Looking East. This photograph comes from a few metres above the
"early ductile contact" between Kohistan and the marbles. In these outcrops the
foliation is oriented 202-33(NW) with mineral stretching lineation plunging 32 degrees
towards 332.
Structures in the metasediments
Marbles with amphibolite sheets (former sills within limestones?). These
metasediments structurally underlie the highly sheared marginal rocks of the Kohistan
arc. Note the asymmetric boudinage. Looking East in road cutting. Outcrop height
about 8m. Compare the inferred shear sense with that for the adjacent Kohistan
rocks.
The Shahbatot Fault
Looking onto the Shahbatot Fault from the south. The fault zone here is represented a
vertical 1-2m zone of gouge, seen in this 10m high cliff section. The walls of this gouge
zone contains striations pitching 48 degrees North, on a fault surface (180-86W).
There are indications of dextral shear developed under greenschist facies conditions
in the wall rocks.
Another late cataclastic fault
A cataclastic fault cutting steep foliation (354-90) in high-grade metasediments of
the Nanga Parbat massif. Viewed looking North. The fault surface is oriented 012-40E
with striations plunging 22 degrees towards 054. Presumably this and similar faults
were moving at the same time as the major Shahbatot Fault.
The Confluence area
The upper part of the Kohistan island arc contains a suite of granite sheets. These are
termed the Parri and Confluence granite sheets. When undeformed these sheets have a
distinct geochemistry and yield Rb-Sr ages of 50-30 Ma (for the earlier, biotite-bearing
Confluence granites) and c. 26 Ma (for the muscovite-bearing Parri sheets). These sheets
form a distinctive set of markers which may be used to chart the strain as one approaches
the edge of the Kohistan arc and its contact with the Nanga Parbat massif.
From a distance the two sets of granites are indistinguishable - forming a prominent swarm.
Although the sheet network is aligned, this is not due to ductile shearing. In close up the
granite sheets form an undeformed net vein array, until within about 500m of the base of the
arc.
Looking up the Indus Valley towards the mountain massif of Haramosh.
Confluence granite sheet swarm is evident.
Part of the
Detail of the cross-cutting net vein structure of the Confluence granite sheets, seen in
a 50m cliff section, Indus gorge at Hanuchal village. Note that the sheets cut a preexisting fabric within the rocks of the Kohistan arc.
Geochemistry of the granite sheets of the
upper Kohistan arc
The upper part of the Kohistan arc terrane contains a swarm of granite sheets, well exposed
on the confluence between the Gilgit and Indus rivers.
Looking east across the confluence of the Indus and Gilgit rivers.
Confluence granites swarm is evident on the 600m cliff opposite.
Part of the
The oldest set of these sheets are called the Confluence Granites (50-30 Ma). The younger
set are considered to form the distinct Parri sheets (c. 26 Ma). These two sets are separated
on the type of mica they contain. Both swarms have high Sr and Ba abundances with 87-86
Sr isotopic ratios of 0.7045-0.7054. These, together with Nd isotopic data, suggest that the
granites had a juvenile arc source without any significant contribution from old continental
crust. Consequently these data suggest that significant underthrusting of northern Kohistan
by the Indian continental crust did not occur until after 26 Ma.
The Ramghat Outcrops
Looking East across the Indus valley to the outfalls of the Astor and Ramghat valleys.
The Ramghat outcrops lie on the extreme Left (north) of view.
Structures related to the Shabatot Fault rejoin the Indus valley just north of its confluence
with the Astor valley at Ramghat. Exposed along an irrigation canal, these faults provide
key kinematic information. Representative plan views of these fault zones (each oriented
with North up the screen) are shown below.
Plan view of fault zone cutting metagabbro of the Kohistan arc. The main fault zones
are oriented N-S - along the length of photograph.
Plan view of subsidiary structures within a 1m gouge zone developed in metagabbro of
the Kohistan arc. The main fault plane (parallel to long edge of view) is oriented 01082E, with striations plunge <10 degrees N. The Riedel shear fractures seen here may
be used to infer the kinematics in this fault zone.
Raikhot Bridge area
This area, on the western margin of the Nanga Parbat,
contains important locations where the main tectonic
contacts are exposed. Use the map to navigate to
these sites.
Raikhot Bridge area
Lower Raikhot valley
The hillsides immediately above the Indus contain important outcrops for determining the
structure of the margin of the Nanga Parbat massif. This view , looking SSW, is along strike
of the prominent lithological banding and fabric. The viewpoint looks across the Raikhot
valley onto a cliff section (400m visible height) which is crossed by the vehicle track up to
Tato village.
This viewpoint locality also has exposures of the augen gneisses of the Nanga Parbat
massif.
Use the photograph to navigate to detailed views - CLICK here for key.
Augen gneisses of the Nanga Parbat area
Deformed augen gneiss with prominent s-c fabric. Seen here looking towards SW. The
main shear fabric is oriented 056-36(SE) with the mineral alignment lineation plunging
34 degrees towards 126. These features may be used to deduce the sense of shear.
Nanga Parbat excursion: Raikhot Bridge area
Lower Raikhot valley
The hillsides immediately above the Indus contain important outcrops for determining the
structure of the margin of the Nanga Parbat massif. This view , looking SSW, is along strike
of the prominent lithological banding and fabric. The viewpoint looks across the Raikhot
valley onto a cliff section (400m visible height) which is crossed by the vehicle track up to
Tato village.
This viewpoint locality also has exposures of the augen gneisses of the Nanga Parbat
massif.
Click on boxes for detailed photographs. Note that all detailed views are in the same
approximate orientation as this viewpoint
Kinematics
Minor structures may be used to infer the direction and sense of past displacements in
shear zones. In the upper photograph we see a view of asymmetric feldspar augen and
associated s-c fabrics in gneisses. This view may be combined with the measured
orientation of strongly lineated rocks. The augen are viewed on a vertical face, looking
towards SW.
The lower photograph (looking NW)shows a view onto the plane of foliation to see stretched
melanocratic pods (former autoliths within the original meta-igneous rock). The foliation here
is oriented 045-56SE with the stretching lineation plunging 54 degrees towards 122.
The Liachar outcrops
The outcrops at Liachar village show a faulted contected of the gneisses of the Nanga
Parbat massif onto poorly consolidated fluvio-glacial deposits of the Indus Valley. looking
NNE, the photograph below shows the side of the Indus valley, with over 2000m of vertical
relief. The lower part of the slope contains the critical young tectonic contact, with the
overlying material being exclusively made up of rocks of the massif.
Details of contact.
Liachar outcrops
The lower portion of the Liachar cliff section. Use the field sketch below to visit areas in
more detail.
The Indus Outcrops
The water-washed outcrops along the Indus river are accessible under low-flow conditions
in the winter. Here there are clean sections through the contact between the Kohistan
Island Arc Terrane and the Nanga Parbat massif.
The Kohistan material consists of banded gneisses of basic composition.
Part of the banded basic gneisses. The shiny metallic mineral is sheared chromite.
Notice the deformed aggregate of garnet and plagioclase.
Most of the rock is
composed of amphibole, plagioclase and garnet. The probable protolith was a layered
basic igneous body. The orientation of this view is the same as the wide shot.
Folded metasediments (psammites, calc pelites, pelites) of the Nanga Parbat massif,
about 30m from the sheared basic gneisses. View looking along strike, with the dip
towards the SE. The fold hinges plunge steeply SW.
Looking North across the Lower Raikhot gorge to the Indus. The fault zone that is
clearly seen at Liachar can also be seen on the opposite side of the Raikhot valley, on
the near outcrops. Here too the fault is marked by extensive fracturing.
The central part of the view shows the Indus river bending around the landslip material
(shown on map). The Karakorum Highway may be seen on the far side of the river.