IITK NPTEL Structural Geology_Lecture 02: Introduction II [Prof. Santanu Misra]
Summary
TLDRThis NPTEL Structural Geology course lecture delves into the methodologies of structural geologists, who interpret rock structures through field observation, laboratory experiments, and analytical modeling. It emphasizes the importance of scale in geological structures, the challenges of reconstructing non-reversible processes, and the significance of geometric, kinematic, and mechanical models in understanding rock deformations. The lecture also highlights the interdisciplinary nature of structural geology and the value of fieldwork, concluding with a look forward to studying structural elements in future sessions.
Takeaways
- 🔍 A structural geologist interprets rock structures by observing deformed rocks and explaining their present state through scientific observations and evidences.
- 🌋 The main job of a structural geologist includes dealing with complex interactions of natural elements, understanding large-scale geologic structures, and interpreting non-repeatable or non-reversible processes.
- 🧩 Geological reconstruction is a challenging task for structural geologists due to the limitations in time scales, missing data, and multiple overlapping deformations.
- 📏 There are three primary ways a structural geologist analyzes structures: geometric models, kinematic models, and mechanical models, which are considered direct methods of observation.
- 🔄 Kinematic models are an advancement from geometric models, focusing on the relative displacements and motions within a structure.
- ⚙️ Mechanical models, also known as dynamic analysis, are the most advanced and involve the interaction of material properties and forces, often studied through laboratory experiments.
- 🧪 Laboratory experiments, such as sandbox experiments and real rock deformation experiments, help structural geologists mimic and understand geological processes that occur over long time scales.
- 🌐 Geometric models are essential not only for understanding rock structures but also for practical applications like hydrocarbon reservoir exploration.
- 🏔️ The Himalayas are an example of a large-scale geological structure where kinematic models help understand the relative movements of tectonic plates and the formation of thrust faults.
- 📉 Dynamic models, including analogue and real rock deformation experiments, provide insights into the evolution of geological processes and the development of structures like folds and faults.
Q & A
What are the primary methods a structural geologist uses to interpret rock structures?
-A structural geologist primarily uses field observation, laboratory experiments, and analytical and numerical modeling to interpret rock structures.
How do field observations, laboratory experiments, and analytical modeling complement each other in structural geology?
-Field observations provide real-world data, laboratory experiments allow for controlled testing of hypotheses, and analytical modeling offers a way to simulate and understand complex geological processes that are not observable directly.
What is the significance of scale in structural geology?
-Scale is crucial in structural geology as it influences the interpretation of structures. It helps in understanding the relative size and extent of geological features and processes, which can range from microscopic to continental.
How does a structural geologist approach the study of deformed rocks?
-A structural geologist studies deformed rocks by observing their structures, understanding the processes that led to deformation, and using scientific evidence to explain their current state.
What is a geometric model in the context of structural geology?
-A geometric model in structural geology is a qualitative or quantitative representation of the 2D or 3D orientation and arrangement of rock structures, which helps in understanding the deformation without involving material properties or forces.
Can you explain the concept of kinematic models in structural geology?
-Kinematic models in structural geology focus on the relative movements and displacements within a geological structure. They help in understanding the direction and sense of movement without considering the forces or material properties involved.
What role do mechanical models play in structural geology?
-Mechanical models, also known as dynamic models, involve the interaction of material properties and forces to understand the deformation processes. They are highly quantitative and often involve laboratory experiments or numerical simulations.
How are sandbox experiments used in structural geology?
-Sandbox experiments are scaled-down models of natural processes used to simulate geological deformations like folding and faulting. They help in understanding the dynamics of mountain building and the evolution of geological structures.
What is the importance of real rock deformation experiments in structural geology?
-Real rock deformation experiments, conducted under high pressure and temperature, mimic the conditions under which most geological deformations occur. They provide insights into the microstructural changes and the development of geological features.
How do structural geologists validate their interpretations and models?
-Structural geologists validate their interpretations and models by comparing them with field observations and natural outcrops. This ensures that the models accurately represent the complexities and processes observed in nature.
What is the significance of understanding the non-repeatable or non-reversible nature of geological processes?
-Understanding the non-repeatable or non-reversible nature of geological processes is significant because it emphasizes the importance of studying the end products of these processes. It also highlights the challenge of reconstructing past geological events and the need for careful interpretation of the available evidence.
Outlines
🔬 Introduction to Structural Geology
The speaker begins by welcoming the audience to the second lecture of the NPTEL Structural Geology course. They recap the previous lecture's discussion on how to examine rocks, focusing on whether they are deformed, homogeneous, heterogeneous, isotropic, anisotropic, continuous, or discontinuous, and the importance of scale. The lecture aims to explain the methodologies of a structural geologist, who observes deformed rocks and uses scientific evidence to explain their current state. The geologist's approaches are categorized into field observation, laboratory experiments (including rock deformation and analogue experiments), and analytical and numerical modeling. These tools, though limited, complement each other, allowing for a comprehensive understanding of rock structures. The lecture also touches on the challenges of geological reconstruction, the non-repeatable nature of geological processes, and the vast spatial and temporal scales involved.
🌋 The Role of Structural Geologists
This section delves into the role of structural geologists, who interpret and define structures based on scientific data. They deal with complex interactions of natural elements and the vast scales of geological structures, which can span millions to billions of years. The speaker emphasizes the challenge of interpreting these structures given their large scale in space and time, and the fact that most geological processes are non-repeatable or non-reversible. The lecture introduces three basic ways to observe structures: geometric models, kinematic models, and mechanical models. These are considered direct methods of observation. Geometric models are qualitative or sometimes quantitative, focusing on the 2D or 3D representation of structures. Kinematic models build on geometric models to understand the different motions within a structure. Mechanical models, the most advanced, involve the interaction of material properties and forces. The lecture also mentions indirect methods, where hypotheses about geological features are tested through models and experiments.
📏 Geometric Models in Structural Geology
The speaker explains geometric models, which are fundamental to structural geology. These models involve tracing layers in rocks to understand their deformation and continuity. Examples are given, including a fold with discontinuities and a syn-sedimentary deformation affected by faults. The speaker demonstrates how geometric models are constructed through photographs and field data, and how they can be applied at various scales, from small folds to large seismic reflection images of subduction zones. The importance of geometric models in understanding structures and aiding in fields like hydrocarbon exploration is highlighted.
🔄 Kinematic Models and Shear Sense Indicators
Kinematic models are discussed as a step beyond geometric models, focusing on the relative displacements and motions within structures. The speaker describes classic structures like delta structures and bookshelf structures, which are indicative of shear sense in structural geology. These models help in understanding the movement and interaction of different parts of a geological structure. The lecture also covers the application of kinematic models on a large scale, such as in the understanding of plate tectonics and the formation of the Himalayas.
🧪 Dynamic Models and Experiments in Structural Geology
This section introduces dynamic models, which involve the study of material properties and forces in geological processes. The speaker describes sandbox experiments that mimic the deformation of sedimentary layers, such as those in the Himalayas. The lecture shows a video of a sandbox experiment, demonstrating the formation of thrusts and the evolution of geological structures. The speaker also discusses analogue experiments with putty or plasticine layers to mimic folding processes and real rock deformation experiments under high pressure and temperature to study microstructures. The importance of validating interpretations from experiments with natural observations is emphasized.
📚 Conclusion and Future Topics
The speaker concludes the lecture by reviewing the key points discussed, emphasizing that structural geology is an interdisciplinary and quantitative science. The importance of field observation, understanding scale, and validating experimental models with natural data is highlighted. The speaker also previews the next lecture, which will focus on measuring structural elements such as lines and planes and their relationships.
Mindmap
Keywords
💡Structural Geology
💡Deformed Rocks
💡Homogeneous vs Heterogeneous
💡Isotropic vs Anisotropic
💡Geometric Models
💡Kinematic Models
💡Mechanical Models
💡Field Observation
💡Laboratory Experiments
💡Analytical and Numerical Modelling
💡Geologic Reconstruction
Highlights
Introduction to the methods of observing and interpreting rock structures in Structural Geology.
The importance of scale in understanding rock structures and the challenges it presents.
The role of a structural geologist in interpreting deformed rocks and their processes.
Categorization of approaches in structural geology: field observation, laboratory experiments, and analytical and numerical modeling.
The limitations and complementarity of field, laboratory, and analytical tools in structural geology.
Geological reconstruction as a challenge due to the non-repeatable nature of geological processes.
The significance of geometric models in structural geology for qualitative and quantitative analysis.
Kinematic models as an advancement over geometric models to understand relative displacements and motions.
Mechanical models, the most advanced, involving material properties and forces for dynamic analysis.
The application of geometric models in various fields such as seismology and hydrocarbon reservoir exploration.
The use of sandbox experiments to mimic natural processes and understand mountain building.
Real rock deformation experiments at high pressure and temperature to simulate natural conditions.
The evolution of structural geology from a descriptive to a quantitative science.
The necessity of field geology for structural geologists to observe and interpret rock features.
The validation of experimental and numerical model interpretations with natural observations.
Upcoming topics on different structural elements and their measurements in the next lecture.
Transcripts
Hello everyone. Welcome to this NPTEL Structural Geology course. We are at our
lecture number 2 and we will continue today with this introduction.
So where we stopped at our last lecture that what are the different ways to look at the rocks,
particularly, when you see a piece of rock or a photograph the questions you should ask were,
whether you are looking at a deformed rock, whether it is homogeneous, heterogeneous,
isotropic, anisotropic, whether the structure is continuous or discontinuous
and most importantly what is the scale? Now in this lecture we will mostly cover what
are the different ways to observe and interpret a rock structure? The question we would ask
mostly today is what does a structural geologist do or what are the different
processes that a structural geologist follows to interpret a rock structure? To answer this
question what does the structural geologist do, is essentially to observe deformed rocks in very,
very intelligent way and explain with some scientific observations and evidences why and
how they ended up at their present state. And to do so, a structural geologist has many
approaches. And one can categorize it in three different sections. The first one
is field observation, second one laboratory experiments, it can be real rock deformation
experiment or analogue experiment and of course analog, and the third one is the
analytical and numerical modelling. Now these three basic tools have their
own limitations. We will learn it later but at the same time these three tools are also
complimentary to each other, that what you cannot observe or understand from the field,
you can understand if you perform an experiment in the laboratory. So to continue this question again
that the main job of the structural geologist is to interpret and define on the basis of scientific
data and at the same time a structural geologist also does geologic reconstruction.
So, in the first part. to interpret and define structures, a structural geologist
has to deal with very complex interaction of natural elements. We will look at it,
all these processes soon. Then most of the geologic structures as you can imagine,
these are extremely large in their extent. For example, you can think of Himalaya or
the complexity of South India and so on. And these structures have formed with time scales
that ranges from million to billion years. And few processes are still happening.
And with our human life span, we cannot see it happening, right? So,
we see the end product and almost fossilized. A structural geologist has a challenge to interpret
this large scale in space and time. And one more important thing that structural geology has,
which is somehow advantageous to us is that most of the processes that happen in nature
are non-repeatable or non-reversible in nature. If something has happened then you cannot go
back with time, in most of the cases. Geological reconstruction is another type
of challenges because it has several limitations and structural geologists have to solve a series
of jig-saw puzzles. For example, as I was talking about the, that it takes a lot of time to produce
a structure. The rate of deformation in earth is extremely slow in almost every case. There are
of course few deviations of this. For example, if you have an earthquake, or if you have an impact
crater formation so that, these, there are few things that happen very quickly but most of the
other cases, the rate of deformation in earth is extremely slow and it is almost impossible
to track through a human life. And then you do not have any initial picture. That how it was,
how it looked before? You don't have the entire picture as well, because some pieces are missing.
You also cannot access all the areas, even if you can access, maybe there is no exposure.
And on the top of that, the deformation that we look at on the surface, it is not a single
deformation. That means one deformation has happened sometime ago and then later some
other deformation which is occurring from other directions or in a completely different nature,
it is either demolishing or overprinting the previous deformation. So this is a challenge
of a structural geologist to reconstruct and interpret, separate out these deformations and
what has happened at the first deformation, what has happened in the second deformation
and so on. So at one point, at one time this is really challenging, sometimes frustrating
for a structural geologist, but at the same time it is extremely fun. And this is why
structural geologist, most of the structural geologists like their job very much.
So there are three basic ways a structural geologist looks at the structures in earth,
or in nature. So these are geometric models, kinematic models and mechanical models and you
can classify them together as direct methods of observations. To understand that geometric models,
these are mostly qualitative and sometimes quantitative. It does not involve any other
detailed scientific, what it does, it simply tries to interpret from what we are observing,
its 2D or 3D representation, its orientation, how it is, in a more comprehensive way. We will look
at some examples later and these are mostly done using some data obtained
from field studies or some experiments. Kinematic model on the other hand is the next
stage of the geometrical model where a structural geologist after constructing the geometric models
try to understand what are the different motions at different parts of the structure? That which
part has moved which way relative to the other parts. That helps you in the next stage to
understand the deformation, displacement, motion of things. You can imagine plate
tectonics which actually deals with the motion of the plates, is a kind of kinematic model.
The mechanical model which is the most developed or the most advanced model among these three,
or we call it dynamic analysis which is highly quantitative;
this model involves interaction of material properties and forces and so on. So, these
are mostly done by performing some experiments, reconstructing the features in the laboratory and
so on. These three geometric models, kinematic models and mechanical models are very important
direct methods of structural geology study. Then there is another method. We call it indirect
method. In indirect method a structural geologist may hypothesize a feature that Himalaya has formed
by this, this and this and then he or she tries to understand it by performing some models,
experiments or numerical models. The structural geologist collects data from the field and then
either prove this hypothesis or disapprove this hypothesis. So, this is some sort of models or
ideas that most of the structural geologists nowadays do and these are done mostly for
the processes that we do not see. For example, we do not see the plumes,
we do not see the mantle convections, so there are many hypothesis and models by
which a structural geologist or tectonics expert or a geo-dynamic expert, they try
to work on some indirect methods and these are known as analytical models. In the following
slides we will mostly focus on these geometric, kinematic and mechanical models and I will try
to give you a very basic idea that what are these, how these are done and most importantly how these
are being applied to the real field. Okay so let us start with the geometric model.
If you remember in the last lecture, I talked about fold and this is a fold. So what we see
here? These light yellow and dark brown alternate layers, these are folded, and the thickness of
these layers did vary during the deposition and it remained intact while it got folded. Now if
it was a simple fold then no problem, we can simply trace one layer and figure out how did
this fold happen, which layer is what. But this structure is somehow discontinued
with some slips, for example this is one. We can see from this relative displacements
that it moved from one to another. But this is part of the kinematic model which we are
not looking at. But the challenge is to trace, that if I look at this layer, this brown layer
here then where it is in the other side. In this case, it is simple, it comes here.
Now similarly if I look at this brown layer and again, I have a discontinuity here then
my challenge is to find out whether it is this one or the other one. Now whatever interpretation you
make out of this, that whether this is this layer or the other layer, you have to come
up with some sort of logics and this logic is the fundamentals of geometric model.
For example, with this model, with this photograph I prepared this geometric model.
I did not cover all these layers but few of the layers are color-coded and they are represented
in this way. So this is a very simple example how geometric models in structural geology are
constructed. Can we apply it to a little large scale? The answer is yes.
Here we have one syn-sedimentary deformation and you can see that this alternate black,
white and few light brown layers got disturbed by a series of faults. To understand it better what
I try to do? I tried; I made a geometric model of it which looks like this. And here you can better
understand what are the different geometries of the layers, which layer moved where, which layer
is continued to where and so on. Now talking about the large scale,
this is a seismic reflection image of a subduction plate, a plate which is subducting under the sea
floor. Now looking at it, apparently it looks very homogeneous with some sort of topography
and some other little features. Now creating a geometric model of this reflection seismic
data is a daily routine of a seismologist and a structural geologist helps in this
process significantly. So if someone who is an expert of constructing geometric models
apparently which is look extremely homogeneous and most likely there is nothing but this person
would interpret this structure in this way. So what you can see, it has a basal decollement
and then series of thrusts are imagined from this basal decollement with the reflection data
and some velocity calculations, one can also find if there are some different lithologies
and so on. Again I repeat, this model does not include any of the material parameters.
I had some data and with this data I had to interpret what has happened there. That is it.
This is the geometric model. Can it be applied in other fields? The answer is yes.
What we see here, this is integration of different sets of data. This is a
computer-generated image. It involves millions and billions of data. This is hydrocarbon reservoir
somewhere in the North Sea. I took it from the internet so dynamic graphics is the website,
you can go and check. What we see here with this geometric model, these red patches that we see
at different areas, these are oil-rich areas. So once the geometric model comes up properly with
the interpretation of different geophysical, structural and other petrological data it is
possible for an oil engineer to design the wells and then it can actually intrude different parts
of this, hydrocarbon reservoir and extractor. So geometric model is essentially important
not only for understanding the structures but at the same time, after understanding
the structures one can also explore simply by constructing a perfect geometric model.
Let's look at the kinematic models. As I said kinematic models are one step advanced from the
geometric models because after constructing the geometry, the kinematic model includes
the relative displacements, motions of the observation we are making at. Now what we
see here at the top photograph is a very classic structure that we commonly observe
in ductile shear zone. We will learn about it later. This is known as delta structure.
Now looking at it, if we try to construct this geometric model without all these arrows and
so on, this is how it should look like. Now it is possible to look at this structure and with
some other observations that this little ball which is inside, maybe it is rotating this way,
and therefore you have some sort of tail-like features at the both ends. You can also construct
this foliations or little layers outside that concave embayment that also proves
that while this little ball is rotating, it was also dragging the other layers.
Now again this model doesn't consider what is this material of this ball, what is the material
outside, and so on. It simply constructs first the geometric model and then it considers that what
was the relative motion and this is very important as shear sense indicators in structural geology.
There are many other shear sense indicators but this is one of the most important ones.
Now here is another one. This is microstructure observed under microscope. You can see the scale
here this, this length here is about 500 microns. What we see here, a mineral grain
is broken along this lens and then there are some sense of steps. Now these steps indicate
that one grain has moved past another grain. And just looking at this we can construct that
this grain has moved down relative to this and similarly others. Now this is a classic
structure of structural geology in ductile shear zone as well, in brittle ductile shear zone and
this is known as bookshelf structure. So this is how it happens. So if you have a series of books
you tilt the books then they rotate past each other. And this is how they slip individually.
So this is a kinematic model again. This is a field photograph and looking at it,
I color-coded the two layers with green and orange and this displacement plane that has
happened. So you can quickly, just looking at this geometric model you can say that this entire block
has moved this direction relative to the other block. So once you interpret this, this you are
actually working on a kinematic model. Now in large scale as I said, you know in the
past lectures the tectonic model, the plate tectonics is some kind of a kinematic model. Why?
Because it doesn't involve the individual material properties of the plates, what are their different
properties and so on. It simply constructs that how one plate moved relative to other,
what was the mutual interaction between them and so on. So this is how Himalaya is conceived,
the development of Himalaya. So on the south side you can see,
this is south, you have this Indian plate and then here you have Tibetan plate and then Indian plate
moved with time and then we have formed series of thrust faults and then this is the present
day configuration of Himalaya where we have lot of faults, major faults are known as MCT,
MBT, MFT, STD and so on and there is also certainly a basal decollement which is MHT,
Main Himalayan Thrust. Now this is a kinematic model. It does not involve any sort of forces. It
does not involve any sort of material properties, the dynamics of these plates and so on. But if
we have to involve that then actually we are approaching to develop a dynamic model.
Let us have a look. So this is again the cross-section of Himalaya and what we see
here that, in this side you have India, the southern part, so here you have India and here
you have Tibet. So in-between you had this Tethyan sediments that got squeezed out from
series of crusts, from series of thrusts and we have this present configuration of Himalaya. Now
to understand this entire process how did it happen, one can perform a dynamic model
by conducting sandbox experiment. So a sandbox experiment is a very simple
experiment which is scaled down from the nature. So this is a typical sandbox. What it does that,
in this space you actually add series of layers of sands. You add different colors to distinguish
the different layers which represent or which are analogous to the sedimentary layers and
then you drag this belt so that these sand bodies, entire sand bodies move from one
side to another side and this is a block which is analogous to the rigid Tibetan plate where
the entire Tethyan sediments are hitting and we would like to see what is happening here.
Now what you see here, that we are involving the materials. We would like to see what is
happening with them. So this is a dynamic model. We are involving some sort of
forces as well. So let us have a look of this movie and see how it works.
So this alternate yellowish dark blue, then again yellowish green, yellowish and the dark blue,
these are different sand layers representing or analogous to the sediments, sedimentary layers
and you see deformation has already started. The belt started moving from this side.
What we see here with more displacements a series of thrust folds are being formed. So
these type of models not only discuss the dynamics of the processes but it also tells
you the evolution of the process, that how from one setup, one geologic setup you actually
achieved a different geologic setup just by adding some sort of deformation.
So this is a quick view of this entire experiment. You can see series of thrusts being formed. They
formed at different angles initially and while they rotated they achieved a different angle.
It also achieved some sort of elevation and these are the very basic or key ideas of
mountain building processes and one can observe all these processes using a dynamic model.
Now if we try to look at to another geological process which is folding
and we try to understand how one fold interferes with another fold in nature,
then one can perform the analogue experiments. So here is an example that we have performed
in the laboratory. What we see in Stage 1, we have prepared one fold, one set of folds here where you
can see the folds are here and these are the hinge lines of the fold marked by the black lines.
Now the folds are at an angle and then if I compress it, this is what we have done,
then we see some tiny folds are coming up. And these experiments were performed to understand
whether the new folds are generating are as big as the previous one or they are small of
the previous one or even larger than the previous one and we figured out that no, these are small,
at least these are very small in this case. But this is a dynamic model. We tried to,
why? Because we tried to see, we tried to mimic the layers of the geological features like the
layers when we form folds. These are ductile so we used some putty or plasticine layers which
are ductile in nature. The scales and other things are also maintained with respect to the geological
scales. So this is again a dynamic model. Now, one can also do real rock deformation
experiments at high pressure and temperature because most of the deformation processes
in nature, that happen at depth and there we have pressure and temperature. So here is an
example. A series of experiments performed by one of my colleagues OK, in ETH Zurich
with his famous Color marble experiments. So the first image what we see here is an intact,
un-deformed color marble. You can see the grain size is in equilibrium,
more or less similar grain size. It has nice triple junctions and so on. And then OK,
sheared this at high pressure and temperature and you can see, if this is the shear direction,
he applied and with evolution of the strain, this is shear strain 1, this is shear strain 2 and this
is shear strain 11, you see first what you observe is that the initial grain size of Color marble
has reduced to a very fine grain size rock. And with time you can also see that here we are
trying to develop some sort of directional layers, okay in the rock and this is exactly what happens
in nature and in experiments you can produce that with real rocks. So this is a dynamic model.
And based on that, with the stress-strain curve one can actually understand that at which stage
of this deformation what kind of features you see in your microstructure.
So we are at the end of this lecture so some review comments from this introduction
section that we had in this lecture and the previous lecture. So structural geology
is highly inter-disciplinary subject as you have understood and it was initially
little bit of descriptive subject but now it is very much quantitative science.
The key skill of a structural geologist is essentially how you observe a particular
feature in a piece of rock, or in an outcrop and then based on your observation you try to
answer these questions why, how and when. One of the most important conclusions that
we would like to make from this introductory lecture that it is the rock that contains the
information. You have to go and see the rock in the field. So therefore field geology is
very important for structural geologists. The scale is one of the very important
parameter to study structural geology. Whatever interpretation you do from a structure,
it is important that you remember what is the scale. Experiments, numerical models etc have
their own limitations but they are excellent to mimic the complexity of the nature and they also
complement each other. Whatever interpretation you do out of your experiments or numerical models,
it is important that you validate them with the nature. So I conclude this lecture.
And in the next lecture we will start a new topic that what are the different structural
elements that we measured lines, planes and their mutual relationships. Thank you and stay tuned.
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