Revised 8 / 06 (Monroe 6th ed.)

Structural Geology - Chapter 13

Stress, Geologic Structures, and Deformation of the Crust

 

Including...

Introduction

Introduction to crustal stress

Directed stress and crustal deformation

Geologic structures

Joints and fractures

Faulting

Folding

Directed stress and plate tectonics

 

 

Introduction

Before we start, we need to review the following:

There are 2 main earth elevations

Above and below sea level

There are 2 main earth processes

Construction and Destruction

Tectonics and surface weathering

Over the course of geologic time these 2 are in balance

There are 2 main igneous rocks

Basalt and granite

All three of these are inter-related (DESCRIBE)

What we are studying tonight is the reality of tectonics

DEFINE: Tectonics

Tectonics: the study of earth processes which result in the creation and deformation of magma and rock

Constructional processes

My favorite part of geology (really should save to last!!)

Some are real flashy: get some videodisc frames

Volcanoes

Earthquakes

All involve stress at varying degrees of intensity

Mt. St. Helens - good example of stress at the surface

Volcanoes, earthquakes indicate immense stress at deeper levels

The earth is a huge rock, and it gets real hot and tight real soon with depth

Before we continue, a bit of background

Lithostatic stress vs. directed stress

Litho-static stress (Monroe; fig. 8-7, pg. 241)

The load weight of overlying rock

Equal pressure in all directions

Directed pressure (REVIEW)

Acts in a specific direction

EXAMPLE: Drop a pencil; gravity is the directed stress

Generally related to tectonic processes associated with plate motions

Results in faults, folds, and other orogenic (mountain building) processes

The 3 types of strain (Monroe; fig. 13-3, pg. 392)

Each is associated with a particular type of plate boundary

And results in different types of crustal deformation

Tension - spreading centers

Lengthens the crust

Compression - subduction zones

Crustal shortening

Shear - transform faults

REVIEW: the brittle-ductile transition zone

 

Introduction to crustal stress

There are several things we can all agree upon

The earth isn't flat

Erosion tries hard, but can't keep up with uplift

There is movement of large sections of the crust vertically & horizontally

Immense stresses at an extremely slow rate

Visualize India into Tibet

DIGRESS TO: Strickler's 2nd Law of GeoFantasy

The crust doesn't re-adjust to changing stress at a uniformly even rate

Different sections of the crust are moving at different velocities

(DEFINE: velocity)

Therefore they interact at their edges - plate margins

We all know that rocks are hard!

Usually takes considerable force to break one

And when it does break, it can do so forcefully

It's also clear that most of the rocks we see at the surface are broken

Cracked, shattered, tilted, bent

Some are folded without breaking

Indicate that, under proper conditions, rock isn't hard at all, but plastic

How rocks deform, and why, is the study of structural geology

Involves near surface to deep crustal processes

Complex interactions between temperature and directed stress

Most ultimately tied to plate interactions

All this differential motion results in deformation of the crust

DEFINE: Deformation

Like a sack full of pissed-off cats

Several things can happen when rock is stressed

Break - fractures & joints (Monroe; fig. 13-14, pg. 400)

Break & slip - faults (Monroe; fig. 13-16, pg. 402)

Fold - plastic deformation (Monroe; fig. 13-7, pg. 395)

What happens depends on 4 main factors

Rock type

Temperature

Magnitude of the force (stress)

Strain rate

 

Directed stress and crustal deformation (strain)

Rock type

Different rocks have different physical (and chemical) properties

React differently to stress and strain

Fundamental differences - igneous vs. sedimentary vs. metamorphic

More subtle differences

Sandstone more brittle than limestone

CaCO3 more reactive than silica

Temperature

Directly related to depth below surface

Temperature and pressure both increase dramatically with depth

Near surface processes - low temperatures and pressures

Rocks are brittle

Rocks undergo elastic deformation (Monroe; fig. 13-4, pg. 392)

Result of directed pressure (or stress)

If elastic limit is exceeded, a rock will rupture

Works a lot like a spring (or a meter stick)

Will deform (stretch or compress) when stress is applied

Returns to original shape when stress is removed

But if overstrained, the spring (or meter stick) will break

Rocks are like this

When hit by a hammer, the rock will elastically deform (compress)

And snap back into its original shape

This snap back is why a hammer "bounces" off a rock

If hit too hard (overstrained), the rock will rupture

Indicating its elastic limit has been exceeded

Deep seated processes - high temperatures and pressures

Rocks behave as ductile (plastic) substances

Deform elastically at first, then plastically

Results in permanent deformation

May rupture if stress exceeds elastic limit

Note that this limit is directly affected by the elevated temperature and pressure

Works like modeling clay

Does not snap back into shape when the stress is removed

Evidence of deep seated deformation is only evident after uplift and erosion

Therefore, any folded rock at the surface indicates extreme uplift and erosion

Magnitude of the force - pretty obvious

Like a hammer vs. a wrecking ball

Strain rate - the rate in which the pressure is applied

The faster rock is forced to deform, the more likely it is to break instead of fold

There are actually very few cataclysmic tectonic forces in nature

See Strickler's 2nd Law of GeoFantasy

Cataclysmic results are relatively common, but it's clear that...

Low stresses over a long period can result in intense plastic deformation

 

Geologic Structures

DIGRESS TO: Orientation of planar features

Strike and dip (Monroe; fig. 13-5, pg. 393)

Show accepted symbols

Inclined plane

Horizontal and vertical planes

 

Joints and fractures

The most common type of structure (at least at and near the surface)

Try to find an outcrop without fractures

A break in the rock along which little or no movement has occurred

Result of brittle failure due to compressional and/or tensional stress

Usually come in "sets"

Caused by regional directed stress

Common to all rocks exposed at the surface

Indicates that the causes are many and varied

Tectonic activity

Mountain building (orogenics), plate interactions

Non-tectonic stresses

Shrinkage due to cooling or drying

Columnar basalt

Expansion due to release of pressure - very common at surface

Exfoliation

 

Faults (Monroe; pg. 410)

A joint or fracture along which noticeable movement has occurred

Single breaks ­ Fault Plane

Complex zones of shearing ­ Fault Zone

Can be the result of plastic deformation at depth

With no specific fault plane

Several basic types

Related to the relative sense of displacement across the structure

Dip slip vs. strike slip

Dip slip: relative motion of the hangingwall vs. footwall

DEFINE: Hangingwall and footwall (Monroe; fig. 13-15, pg. 401)

DEFINE: Relative sense of displacement

Normal Faults; Tensional stress (Monroe; fig. 13-17, pg. 404)

Hangingwall drops relative to footwall

Results in lengthening of the crust

Horst & Graben (Monroe; fig. 13-18, pg. 406)

Actually valley building, not mountain building

Reverse Faults; Compressional stress (Monroe; fig. 13-17, pg. 404)

Hangingwall goes up relative to footwall

Results in shortening of the crust

Thrust Faults; Compression (Monroe; fig. 13-17, pg. 404)

Low-angle reverse (dip < 45°)

Horizontal displacement greater than vertical displacement

(Are there low angle normal faults, and what are they called?)

Strike Slip Fault (Monroe; fig. 13-17, pg. 405)

Example: San Andreas

Near vertical dip

Right and Left Lateral

Oblique (Monroe; fig. 13-17, pg. 405)

Some combination of the above

Most faults are probably somewhat oblique

Drag folds - relatively common

Can indicate sense of relative motion

WARNING: We'll see these in G-103 during our work with geologic maps - it will pay to remember them!

 

Folding

"Directed compression of the crust, resulting in a semi-plastic deformation"

Immense stresses at an extremely slow rate

Usually occurs at depth in the crust

Increased heat and pressure cause rocks to bend and fold instead of break

REVIEW: the brittle-ductile transition zone

Most commonly observed in sedimentary rocks (Monroe; fig. 13-7, pg. 395)

Also evident in igneous and metamorphic rocks

DIGRESS TO: Metamorphics

In any event, folding and metamorphism go hand in hand

Both occur at depth at increased temperatures and pressures

Generally related to compression of the crust

Crustal shortening (Monroe; fig. 13-10, pg. 397)

Therefore same stress environments as those which produce reverse faults

Result of regional directed pressure: DEFINE

Lots of names (Monroe; fig. 13-11, pg. 398)

Based on relationship between the axis and the limbs of the fold

Axis of the fold (Monroe; fig. 13-8, pg. 396)

Axial plane: divides fold in half

Each half is called a "limb"

Can be defined by taking strike & dip

Commonly sub-parallel in regionally folded terrain

Axial plane cleavage (Monroe; fig. 13-8, pg. 396)

Increased temp. & pressure in axis can result in local metamorphism

Anticline (Monroe; fig. 13-10, pg. 397)

More or less well defined linear axis with symmetrical limbs

Upwarping of crust

Oldest beds in center

DIGRESS TO: old/young vs. frown/smile

DO NOT always result in a topographic high! (Monroe; fig. 13-9, pg. 396)

Usually accompanied by a...

Syncline (Monroe; fig. 13-10, pg. 397)

More or less well defined linear axis with symmetrical limbs

Downwarping of the crust

Youngest beds in center

DO NOT always result in a topographic low! (Monroe; fig. 13-9, pg. 396)

Plunging folds (Monroe; fig. 13-12, pg. 399)

Fold axis is inclined to the horizontal

WARNING: We'll see these in G-103 during our work with geologic maps - it will pay to remember them!

Dome vs. Basin (Monroe; fig. 13-13, pg. 400)

Less common than anticline/syncline

No real axis & limbs dip away (or to) the center of the structure

Lots of others

Lumpers & Splitters again

Can get extremely complicated mathematically

In any event, all tell the story of directed stress within the crust

 

Directed stress and plate tectonics

Plate tectonic overview

Spreading, subduction, lateral offset

Each has a specific style of deformation due to differing types of stress

Compression of the crust

Subduction zone complexes

Reverse faults and folding

Intermediate to felsic magma

Where many major mountain ranges are formed

(Monroe; fig. 13-20, pg. 408)

(Monroe; fig. 13-21, pg. 409)

Extension of the crust

Spreading centers

Normal faults, no folding

Mafic magma

Basin and Range province

Horst and Grabben block faulting (Monroe; fig. 13-18, pg. 406)

Transform faults (Monroe; fig. 13-19, pg. 407)

Lateral side-to-side shearing

Commonly offsets spreading ridges

Not associated with magma generation

Therefore no volcanics

 


GeoMan's Home Page | RCC Index | High School Geology Index

 

You are GeoManiac number since April 1, 1997