DEFORMATION, MOUNTAIN BUILDING, & EVOLUTION OF CONTINENTS
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 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).
-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
Mountain Systems- largest mountains; result of compression along convergent plate boundaries (thrust faults abundant) (Appalachians, Alps, Rockies, etc.).
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.
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
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).
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.