CHAP 11: Crustal Deformation + Mountain Building *

confining pressure - In situations such as deep burial, confining pressure exerts uniform stress in all directions. - Uniform stress can be thought of as many equal forces all acting in the same direction (all inward, all outward, etc).

differential stress - The influence of stress in a single direction via tension, compression, or shearing produces the most deformation of crustal rocks.

aa - Aa is a feature that occurs in lava when it is cooling and the surface is brittle, while it is in the process of flowing. - Rocks often undergo brittle and/or ductile deformation to produce a wide variety of structural features.

shear - Where two plates are sliding horizontally past each other at a transform boundary, you would observe shear stress being exerted on the rocks. Because there is no vertical motion of the plates, the crustal thickness is unchanged. - Transform boundaries are "conservative" with respect to crustal thickness, meaning the crust is neither thickened nor thinned at a transform boundary.

tension - Where two plates are moving away from each other at a divergent boundary, you would observe tensional stress being exerted on the rocks. This would result in relatively thinner crust along the boundary. - Crustal thinning occurs at divergent boundaries.

dome - Because the limbs of the structure associated with the Black Hills all dip away from the center of the hills, the feature is known as a dome.

ductile - As stress is applied to rocks at depth, ductile deformation induces folds.

overturned - When compressional stress is applied to rocks and one of the limbs is pushed far beyond the vertical, it results in the formation of an overturned fold.

compressional - When compressional stress is exerted on rocks at depth, they can be bent (ductile deformation) and folds are formed.

divergent boundaries - Because rocks at divergent boundaries experience tensional stresses, the likelihood of folds forming there is low.

tensional - When crustal rocks near the surface are subjected to tensional stress, they undergo brittle deformation and fault-block mountains can be produced. - Fault-block mountains often consist of alternating upthrown and down-dropped blocks.

normal - Normal faults are generally found where plates are being separated and tensional stresses are exerted on rocks at divergent boundaries, such as the Great Rift Valley of East Africa.

The hanging wall moves down relative to the footwall. - When tensional stress is applied to rocks and normal faulting occurs, the hanging wall moves downward relative to the footwall. - If you were to walk down the fault plane, the footwall is where your feet would make contact and the hanging wall is on the block where you'd hang your lantern (or hat, etc).

The rocks on the footwall side of the fault would be older than those on the hanging wall side. - Following the erosion of a normal dip-slip fault, older rocks would be exposed on the surface of the footwall side of the fault.

convergent boundaries - Because compressional stress is exerted on rocks at convergent boundaries, reverse, thrust, and overthrust faults are the common types associated with this type of boundary. - Overthrust faults involve both folds and large-scale thrust faults.

Cascades - The Cascade Mountains in the northwestern United States were produced by the subduction of the Juan de Fuca Plate beneath the North American Plate, creating a volcanic arc.

Europe and Asia - Uplift of the Ural Mountains took place at the end of the Paleozoic, when the continental masses of Asia and Europe were welded together by collision.

Caledonian Mountains - The Caledonian and Appalachian Mountains were formed by the collision of North America, Europe, and Africa during the formation of Pangaea and then were separated during the breakup of this supercontinent.

reverse faults - As portions of continental plates collide, associated compressional stress produces reverse faults in brittle surface rocks.

formation of a volcanic arc - As the oceanic crust of the Nazca Plate is forced under the South American Plate in a subduction zone, compression and volcanism has led to the creation of the Andes.

terranes - Small accreted crustal fragments such as those occurring in Alaska and British Columbia that possess unique geologic history are known as terranes.

the British Isles - The British Isles are indeed a small landmass, but they are part of the larger continental landmass of Europe rather than an isolated crustal fragment.

Small crustal fragments have been accreted on the northwestern margin of the North American Plate. - The mountain ranges that dominate the coasts of Alaska and British Columbia were formed as a sequence of terranes was smeared onto the margin of North America when the intervening oceanic crust was subducted.

The island arc is peeled off the oceanic plate and a volcanic arc is produced by subduction. - As the oceanic plate is subducted, any island arc system that has been developed on it will be peeled off and smeared onto the edge of the continent, producing a terrane.

The terrane cannot be subducted, which causes the oceanic plate on which it has been traveling to break off and continue downward beneath the continental crust. - The density of the terrane is such that it is more buoyant than the oceanic plate on which it is riding. As a result, the oceanic plate breaks off and continues to travel beneath the continental lithosphere and is subducted and recycled.

uplift and erosion - The batholiths would have been uplifted as the continent rose and exposed through erosion.

Isostasy

The crust would move up in elevation. - The crust will bounce back after glaciation just like it would if large mountains would erode.

The roots would rebound upward. - The crust will uplift as weight is removed, which will also make the roots move upward.

two landmasses collide - When two landmasses collide, such as at a convergent boundary, the edges will fold and buckle to create mountains.

intrusive igneous rock, folds, thrust faults - Partial melting creates magma, which is emplaced near the plate boundary. Rocks that are compressed become "wrinkled." Thrust faults result from compressional force.

schist - Metamorphic schist forms in high-pressure environments such as subduction zones.

a mass of rock that formed elsewhere and was added to a continent - Terranes are added to continents where subduction zones develop.

convergent boundary - Terranes are added to a continent through subduction.

accretion - Accretion occurs as a result of a subducting plate.

Iapetus Ocean Island Arc and sediments, the Avalon Terrane, and a remnant of Africa.

1) Compressional Stress - squeezes rocks together as if in a vise (convergent boundaries) 2) Tensional Stress - pulls apart or elongates rocks (divergent boundaries) 3) Shear Stress - causes one part of rock body to move past another (transform boundaries)

The term strain refers to the relative deformation of a body. In two dimensions, it would refer to the relative change in length, which is (new length/old length). It is dimensionless. Stress has to do with how much force is being applied to an object.

Brittle Deformation - Stress breaks the chemical bonds that hold a material together. Ductile Deformation - Stress changes the shape of an object without breaking bonds (like clay or metals).

1) Anticline - upfolded, or arched, rock layers 2) Syncline - downfolded rock layers 3) Where folds tilt at an angle and die out they are said to be Plunging. 4) Dome - circular, or slightly elongated, upwarped displacement of rocks, oldest rocks are in the core. 5) Basin - circular, or slightly elongated, downwarped displacement of rocks, youngest rocks are in the core.

1) Normal fault - hanging wall block moves down 2 & 3) Reverse and thrust faults - Hanging wall block moves up, caused by strong compressional stresses. Reverse fault - dips greater than 45°. Thrust fault - dips less than 45° 4) Strike-slip faults - Dominant displacement is horizontal and parallel to the trend, or strike.

(e.g., Andes Mountains ) • Oceanic-continental crust convergence • Types related to the overriding plate - Passive margins » Prior to the formation of a subduction zone » e.g., east coast of North America - Active continental margins » Subduction zone forms » Deformation process begins » Continental volcanic arc forms » Accretionary wedge forms

Isostasy - Concept of a floating crust in gravitational balance • When weight is removed from the crust, crustal uplifting occurs • Process is called isostatic adjustment

1) Elastic Deformation - Stress is applied gradually so rocks can respond slowly. Chemical bonds are stretch but don't break. When stress is removed, bonds go back to original length. 2) Brittle Deformation - Stress breaks the chemical bonds that hold a material together. 3) Ductile Deformation - Stress changes the shape of an object without breaking bonds (like clay or metals).

Folds, Anticlines "up", Synclines "down"

1) Dip-slip fault - Movement along the inclination (dip) of fault plane. Parts of a dip-slip fault. Hanging wall - the rock above the fault surface. Footwall - the rock below the fault surface. 2 types of dip-slip faults: Normal fault • Hanging wall block moves down • Associated with fault-block mountains • Prevalent at spreading centers • Caused by tensional forces Reverse and thrust faults • Hanging wall block moves up • Caused by strong compressional stresses • Reverse fault - dips greater than 45° • Thrust fault - dips less than 45° 2) Strike-slip faults - Dominant displacement is horizontal and parallel to the trend, or strike. Transform fault. Large strike-slip fault that cuts through the lithosphere. Often associated with plate boundaries. 3) Joints - Fractures along which no appreciable displacement has occurred. Most are formed when rocks in the outer-most crust are deformed.

Orogenesis refers to processes that collectively produce a mountain belt. Most mountain building occurs at convergent plate boundaries. 1) Andean-type mountain building (e.g., Andes Mountains ) • Oceanic-continental crust convergence • Types related to the overriding plate 2) Continental collisions • Where two plates with continental crust converge 3) Continental accretion • Third mechanism of mountain building • Small crustal fragments collide with and accrete to continental margins • Accreted crustal blocks are called terranes • Occurred along the Pacific Coast

Isostasy is the concept of a floating crust in gravitational balance. When weight is removed from the crust, crustal uplifting occurs. Process is called isostatic adjustment.

1) Temperature - Warm temps make rock more ductile. Cold temps make rock more brittle. 2) Confining Pressure - Confining pressure squeezes the rock together, making it harder to break. 3) Rock type - Composition affects the strength of internal bonds. 4) Time - Slow deformation tends to make rocks bend; fast deformation tends to make them break.