Chapter 13

DEFORMATION, MOUNTAIN BUILDING, & EVOLUTION OF CONTINENTS

 

Rock deformation

Deformation means change in shape or size or both.  It is the results of stress.

stress- force applied to a given area of rock; commonly result of tectonic forces.

strain- deformation caused by stress; if stress > strength of rock, rock will strain (deform).

 

Types of stress (fig. 13.3):

            -compressional stress- rocks shorten by folding or faulting.

            -tensional stress- rocks lengthen by faulting or stretching.

            -shear stress- rocks deformed by sliding by one another.

 

Types of strain (fig. 13.3):

-elastic strain- rocks return to original shape after deformation; only to elastic limit.

-permanent strain- stress exceeds elastic limit; deform permanently in 2 ways:

-plastic strain- rock shows ductile behavior; flows (fig. 13.4).

-fracture- rock shows brittle behavior; breaks.

Controls on type of strain (plastic vs. fracture):

-plastic strain occurs when: high temperature, small stress over long period of time, low strain rate, soft rock (shale).

            -fracture occurs when: low temperature, large stress applied rapidly (hammer), high strain rate, hard rock (granite).

                       

Strike & Dip

Used to describe orientation of tilted rock layers (fig. 13.7).

-dip- measure of maximum steepness of an inclined layer from the horizontal.

-strike- a horizontal line perpendicular to dip; (direction of a line formed by intersection of a horizontal plane (imaginary) with inclined rock layer) (fig. 13.7).

 

Deformation & Geologic Structures

Deformation is the change in shape or volume or both.  Any features resulting from deformation is referred to as geologic structure.

Folds (figs. 13.1, 13.8 - 13.9, 13.12):

            -Anticline- up-arched fold; oldest beds in core (middle).

            -Syncline- down-arched fold (fig. 13.11, 13.12); youngest beds in core.

            -Monocline- bend in otherwise horizontal layer (fig. 13.8).

            -Domes and Basins- circular equivalents of anticlines and synclines (fig. 13.15).

 

Fractures:

-Joint- fracture along which no movement has occurred (brittle); commonly occur in parallel sets (figs. 13.16, 13.17).

 

-Fault- fracture along which there has been movement that is parallel to the fault plane; rocks on either side of fault divided into hanging wall (HW) ("hangs" over head)

and footwall (FW) (can "stand" on) (fig. 13.18)

-normal fault- HW moves down relative to FW; result of tensional stresses (divergent plate boundaries) (figs. 13.18a, 13.19a, 13.23).

-reverse fault- HW moves up relative to FW; result of compressional stresses (convergent plate boundaries) (fig. 13.19b, 13.20b).

-thrust fault- reverse fault that dips at a low angle (<45˚); common in Ouachitas and other mountain ranges (fig. 13.19c).

-strike-slip fault- rocks on opposite sides of fault slide horizontally past one another; result of shear stresses (fig. 13.19d).

            -right lateral strike-slip fault- block across from observer moved to right (fig. 13.21).

            -left lateral strike-slip fault- block across from observer moved to left (fig. 13.19d).

-oblique-slip fault- faults that show both horizontal (strike-slip) and vertical (normal or reverse) motion.

 

Deformation and Origin of Mountains

 

Mountain- any area of land that stands significantly higher than surrounding area.

Mountain range- series of mountain peaks (usually linear) related in age and origin.

Mountain system (mountain belts)- mountainous region consisting of several mountain ranges (Rocky Mountains, Appalachians, Alps, Andes, Himalayas, etc.).

 

Types of mountains:

Volcanic- an isolated or chain of volcanoes (Cascades in Pacific NW, Hawaiian islands, etc.).

Block fault- series of normal faults with uplifted (mountain) and down dropped fault blocks; Basin and Range in Nevada and surrounding states) (fig. 13.23).

Erosional- rocks more resistant to erosion topographically higher than surrounding areas with rocks less resistant to erosion (no deformation necessary) (fig. 13.22) (Ozark

mountains).

Mountain Systems- largest mountains; result of compression along convergent plate boundaries (thrust faults abundant) (Appalachians, Alps, Rockies, etc.).

 

How do Mountains Form (Orogeny)

 

Characteristics of Mountain Systems-

1)      very long compared to width;

2)       young mtns. typically higher than old mountains (eroded over time);

3)       rocks folded and thrust faulted;

4)       rocks in interior of mountain systems usually metamorphosed and intruded by plutons;

5)       active mountain systems have frequent earthquakes and volcanic activity;

6)       active mountain systems are presently at convergent plate boundaries.

 

Plate Boundaries & Mountain Building

Present day mountain building occurring in 2 areas: Alpine-Himalayan belt and circum-Pacific belt (=convergent plate boundaries) (fig. 13.24).

 

Mountain building/orogenies may occur at:

oceanic-oceanic (fig. 13.25),

oceanic-continental (fig. 13.26),

and continental-continental plate boundaries (Himalayas as an example- fig. 13.27).

 

Terranes & the Origin of Mountains

 

New continental crust comes from: 1) sediments eroded from older continental crust; 2) new volcanic and plutonic igneous rocks; 3) accretion of terranes.

 

Terranes- small accreted lithospheric blocks not native to a continent; differ from surrounding rocks in: 1) fossil content 2) stratigraphy 3) structural trends 4) paleomagnetic

 properties.

 

Examples of terranes: 1) volcanic island arcs 2) seamounts/oceanic ridges 3) small fragments of other continents.

            Much of western North America made up of collided terranes (fig. 13.28).

 

How Did the Continents form and Evolve

 

By ~3.8-4 BY ago, island arcs (andesitic) and batholiths (granitic) collided and formed several continental nuclei (fig. 13.29).

 

Shield- Precambrian igneous and metamorphic rocks exposed at the surface over a large area; occur within most continents: Canadian shield in N. America (fig. 13.30).

 

Craton- central part of continents that have remained structurally stable for long periods; includes shield and younger rocks covered by younger sedimentary rocks.