Structural Geology
Appalachian Mountains
Pennsylvania
It has long been evident to geologists that great forces operate in Earth’s crust
These forces apparently involve bending, breaking, and overturning rock layers
Stress is a force acting upon a solid.
Strain is a change in shape or volume
of a solid as a result of stress
Differential
stress
t
1
Solid rock may respond to stress in two ways:
Joints
Ductile Behavior ((bending,
g stretching)
g)
Brittle Behavior (breaking)
Behavior determined by several factors:
Rate of strain, confining pressure, rock composition,
temperature, presence of water
2
Types of Faults
Faulting
Normal Fault
Reverse Fault
H
Hanging-wall
i
ll and
d footwall
f t ll
Strike-Slip Fault
Normal Fault
Strike-Slip Fault
Reverse Fault
3
Horst and Graben
Da Basics
• Tensional stress = normal fault
p
stress = reverse fault
• Compressive
• Shear stress = strike slip fault
Geometry of a fold
Anticline
Syncline
Plunging Anticline
4
Open
(symmetrical)
I
Isoclinal
li l
Asymmetrical
Evolution of a
fold into a
reverse fault
Overturned
Recumbent
Age relationships of folded beds
How do geologists
map complex 3-D
structures with
only surface data?
Strike and Dip
Making a Geologic Map
An eroded anticline will have older beds in the middle
An eroded syncline will have younger beds in middle
5
Plunging folds revealed by topography
Measuring
orientation of
rock outcrops
Erosion removes portions of a fold
What are the age
relationships
of these rocks?
“Law of V’s” - Plunging anticline points in direction of plunge
Plunging syncline opens in direction of plunge
Geologic
Map
Strike and Dip symbols on maps
Pay attention to age
relationships and
strike and dip symbols
6
Describe the structures and stress
7
Draw the geologic map
Three General Types of Mountains
• Fold and Thrust Belts
• Fault Block Mountains
• Volcanic Mountains
Fault Block
Fold and Thrust Mountains
8
Volcanic Mountains
So…how would you figure out the Appalachians?
A Low-Angle Reverse Fault - Thrust Fault
Fig. 15.16
9
Keystone
Thrust west of
Las Vegas
What is the geologic
structure of Michigan?
Fig. 15.17B
Ordovician
Late
Carboniferous
Geologic
Map of U.S.
Cretaceous
U.S. Orogenies
Rocky Mts, CO. Plat., Laramide
Sierra Mts. – Tectonic Rise Tertiary
Sevier
Pangea
Breakup
Pangea
Assembly
Nevadan
Jurassic
time
Western U.S.
Permian
Isostatic rise
Sonoman
Triassic
Antler
Mesozoic
Alleghenian
Pennsylvanian
Late Devonian
Ordovician
Breakup
Cenozoic
Cretaceous
Taconic
Rodinia
Eastern U.S.
Appalachians
Acadian
Devonian
Paleozoic
Grenville
Proterozoic
Hudsonian
Proterozoic
Precambrian
Rodinia
Assembly
Kenoran
Archean
10
Isostasy - equilibrium
Mountain Building
Appalachian Fold and
Thrust Belt
11