what is a geologic structure? Plate Tectonics STRUCTURAL GEOLOGY & GEOTECTONICS Kinds of Folds Fold Classification Faults Dip-slip Faults

1. what is a geologic structure?
The term probably brings to mind images of folds and faults. Perhaps
you had the opportunity to take a field trip where you saw some of
these structures in the wild. These features are formed in
response to pushes and pulls associated with the forces that
arise from the movement of tectonic plates or as a consequence
of differential buoyancy between parts of the Lithosphere. But
what about bedding in a sedimentary rock and flow banding in a
rhyolite flow; are these structures? And what about slump folds in a
debris flow; are they structures? Well . . . yes, but the link between
their formation and plate motion is less obvious. So, maybe we need
to have a more general concept of a geologic structure.
The most fundamental definition of a geologic structure is a
geometric feature in rock whose shape, form, and distribution
can be described.
Geologic Structure

2. Plate tectonics is a scientific theory which describes the large scale motions of
Earth's lithosphere. The theory builds on the older concepts of continental drift,
developed during the first decades of the 20th century (one of the most famous
advocates was Alfred Wegener), and was accepted by the majority of the
geoscientific community when the concepts of seafloor spreading were
developed in the late 1950 s and early 1960s. The lithosphere is broken up into
what are called tectonic plates. In the case of the Earth, there are currently
seven or eight major (depending on how they are defined) and many minor
plates. The lithospheric plates ride on the asthenosphere. These plates move in
relation to one another at one of three types of plate boundaries: convergent, or
collisional boundaries; divergent boundaries, also called spreading centers; and
conservative transform boundaries. Earthquakes, volcanic activity, mountain-
building, and oceanic trench formation occur along these plate boundaries. The
lateral relative movement of the plates varies, though it is typically 0–100 mm
annually.
Plate Tectonics

3. Nicholas Steno (1631–1686) examined outcrops where the bedding of rock
was not horizontal, and speculated that strata that do not presently lie in
horizontal layers must have in some way been dislocated (the term he used for
deformed). Perhaps Steno’s establishment of the principle of original
horizontality can be viewed as the birth of structural geology.
Deformation refers to the structural changes that take place in the original
location, orientation, shape, and volume of the body of rock. It includes
physical and chemical processes that produce the structural changes. Any body
of rock, no matter how strong, will deform if the conditions are right.
Structural deformation results from stresses that exceed rock strength. When
strength is exceeded, the rock will fail by brittle or ductile deformation,
depending on how physical environment has affected the ability of a rock to
withstand stresses which decreases with temperature. The stresses can be
created in nature in a number of ways.
STRUCTURAL GEOLOGY & GEOTECTONICS
DEFORMATION

4. Figure showing buckled fence (A) and broken fence (B). Fences like rocks, respond in
different ways to shortening. (photo: J.R. Stacy)

5. Stresses that cause deformation generally build slowly but persistently, but in
some situations high stresses also can generate like in meteor fall.
Structural geology can be defined as the study of the architecture of the
Earth’s crust, resulted from deformation. Structural geology addresses the
form, symmetry, geometry and other structural components of the earth’
crust. It also focuses on strength and mechanical properties of crustal
materials. Although architecture and structural geology have much in common,
the challenges of the architect and the structural geologists are quite different.
Plate tectonics provide an essential backdrop for understanding the
significance of structures, especially regional structures.
Plate Tectonics forms the basis for understanding the dynamic circumstances
producing deformational movement. Plate interactions create the rock forming
processes generating fundamental, original properties of rock assemblages.
Continued plate motion generate the stresses that impart to rocks their chief
deformational characteristics.
Evolution of Orogenic Belts: Plate movements and their interactions.
STRUCTURAL GEOLOGY & GEOTECTONICS
Deformation Contd.

7. Fold Definition
• Folds are wave-like structures that result from
deformation of bedding, foliation, or other
originally planar surfaces in rocks.
• Fold can be very broad to tightly compressed.
• They may occur as isolated folds or extensive
fold trains of different sizes.

8. Scale of Folds
• Microscopic – Need magnification
• Mesoscopic – Occur from hand specimens to
outcrop scales
• Macroscopic – Occur on the map scale or larger

9. Anatomy of Folds
• Crest – The highest point on the cross section of
a fold
• Trough – The lowest point on the cross section
of a fold
• Limbs – The straighter portion of the fold
connecting the hinge zones
• Hinge Zones – Exhibit the greatest curvature of a
fold

11. Anatomy of Folds

13. Anatomy of Folds
• Hinge Line – The line joining
points of greatest curvature on a
folded surface
• Axial Surface – When many
hinge lines (from multiple folded
surfaces) connect forming a
plane
• Plunge – When the fold hinge is
inclined to the horizontal

14. Anatomy of Folds
• Wavelength – The distance
between subsequent crests
or troughs
• Amplitude – Half the
distance from a crest to the
subsequent trough
• Vergence – The direction of
leaning of the axial surface

15. Anatomy of Folds
• Enveloping Surface –
Enables the relation of
small to large scale folds
• First-Order Folds – The
master (largest) fold
• Second-Order Folds
– Smaller folds on the
flanks of first-order
folds

16. Kinds of Folds
• Anticline – A fold that is concave towards older
rocks in its center
• Antiform – A fold that is concave downward
• Syncline – A folds that is concave towards younger
rocks in its center
• Synform – A folds that is concave upward
• Dome – An antiform where bedding dips away from
the central point
• Basin – A synform where bedding dips towards a
central point

17. Kinds of Folds

18. Kinds of Folds
• Cylindrical Folds – Folds where a cylinder can be
inscribed inside the hinge
• Non-cylindrical Folds – Folds where the hinge is
not parallel
• Sheath Folds – Where the fold hinge curves
within the axial surface; Normally occur in shear
zones

19. Fold Classification

20. Fold Classification

21. Fold Classification

22. Fold Classification
• Parallel Folds – Maintain constant layer
thickness
• Concentric Folds – Parallel folds in which the
folded surfaces define circular arcs
• Ptygmatic Folds – Have a lobate shape and
look like intestines
• Similar Folds – Maintain their shape
throughout the section and do not die out
vertically

23. Fold Classification
• Chevron and Kink Folds – Have straight limbs
and sharp angular hinges
• Disharmonic Folds – Shape or wavelength
changes from one layer to another
• Supratenuous Folds – The synclines are
thickened and the anticlines are thinned

24. Fold Classification

25. Fold Classification

26. Beds dip away symmetrically from
the axial plane
Beds on one side of the axial plane dip
steeper than those on the other side

27. Faults
• Fractures in rock along which movement has occurred
• Different styles reflect different stresses

28. Faults
Fractures in rocks created by earthquakes.
Faults are characterized by the direction of relative
movement: slip
• Dip-slip faults
 normal
 reverse
• Strike-slip faults
• Oblique-slip faults
•Hanging Wall: Term used by miners. They could “hang”
their light on this side of the fault because it was above
them.
•Footwall: Also from the miners, this side of the wall upon
which they could stand below the hanging wall.

30. Dip-slip Faults
Motion of the fault blocks is parallel to the dip direction.

31. Reverse Fault
footwall
hanging wall
cross section

33. Thrust Fault
footwall
hanging wall
cross section
Thrust faults are low-angle reverse faults.

35. Strike-slip Faults
Motion of the fault blocks is
parallel to the strike
direction.

38. • Unconformities in sequences of strata represent times
of non-deposition and/or erosion that encompass
long periods of geologic time, perhaps millions or tens
of millions of years
• The rock record is incomplete.
– The interval of time not represented by strata is a
hiatus.
UNCONFORMITIES

39. • Three types of surfaces can be unconformities:
– A disconformity is a surface separating younger from older
rocks, both of which are parallel to one another
– A nonconformity is an erosional surface cut into
metamorphic or intrusive rocks and covered by sedimentary
rocks
– An angular unconformity is an erosional surface on tilted or
folded strata over which younger rocks were deposited.
TYPES OF UNCONFORMITIES

40. • Unconformities of regional extent may change from one
type to another
• They may not represent the same amount of geologic time
everywhere
Types of Unconformities

41. • A disconformity between sedimentary rocks in California, with
conglomerate deposited upon an erosion surface in the
underlying rocks
A Disconformity

42. An Angular Unconformity
• An angular unconformity, Santa Rosa

43. • A nonconformity in South Dakota separating Precambrian
metamorphic rocks from the overlying Cambrian-aged
Deadwood Formation
A Nonconformity

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