Deformation and Mountain Building

Deformation

        Stress is the force applied to material that tends to change its dimensions. Strain is the effect of stress shown by the material. Strength is the limiting stress that a material can withstand without failing by rupture or continuous plastic flow. The response of a rock to stress depends on the type of stress, the amount of pressure, the temperature, the type of rock, and the length of time the rock is subjected to the stress.

        There are three types of stress:

1. Compressional stress - Forces are directed toward one another. Decreases the volume of a material. Lithostatic pressure is an all-sided confining pressure produced by burial.
2. Tensional stress - Stretching stress that tends to increase the volume of a material.
3. Shear stress - Force is parallel, but in opposite directions. resulting in displacement of adjacent layers along closely spaced planes.

        Strain on a rocks may be taken as:

1. Elastic deformation - Strain is proportional to stress. Rock will return to original volume/shape if stress is removed.
2. Plastic Deformation - Permanent deformation caused by flowing and folding at stresses above the elastic limit at high confining pressure and/or temperature. Warm rocks tend to deform plastically.
3. Brittle Deformation - Any rock will break if the applied stress is too great. Rocks at or near the surface (cold, low pressure) tend to deform by brittle rupture. Results in fracturing and faulting (rock shows differential movement on either side of the fracture surface.

        At a particular temperature and pressure, the response of a rock to stress is dependent upon the type of stress and the length of time over which the stress is applied. Slow application of stress favors plastic deformation. Rapid application of stress favors brittle deformation.

        Different types of rock respond to stress differently. Rocks have different strengths for different types of applied stress. Tensional strength is less than compressional strength.

1. Material is brittle when the difference is large. Rock behaves as a brittle material near surface where temperature and pressure are low.
2. Material is ductile when the difference is small. Rock behaves as a ductile material before it fails at high confining pressures and/or high temperatures.

        Geologists use the concept of strike and dip to describe the orientation of deformed rock layers:

• Strike - Bearing (direction) of a horizontal line on a rock bed or structure.
• Dip - The angle between the horizontal and the rock bed or structure.

        Folds are produced by plastic and elastic deformation during compressive stress. Mechanism of folding (not in book) falls in to two categories:

1. Concentric (Flexural Slip) folding - The bending of surface rock beds without change of thickness or volume (= elastic deformation).
2. Flow Folding - Occurs in plastic rocks subjected to directed stress at high pressures and temperatures (= plastic deformation). Thickness and volume of rock beds can change.

        The following terms used to describe fold parts and orientations:

1. Axial plane - Imaginary plane that intersects the crest or trough of a fold to divide it into 2 equal portions.
2. Axis - The line formed by the intersection of the axial plane and bedding plane.
3. Limbs - The sides or legs of a fold.
4. Plunge - The dip of the fold axis.
5. Symmetrical folds - Mirror image on either side of the axial plane.
6. Asymmetrical folds - One limb is steeper than the other.
7. Overturned folds - One limb has been tilted beyond the vertical, but both limbs dip in the same direction.
8. Recumbent fold - Axial plane is horizontal, so fold lies on its side.
9. Isoclinal fold - Fold limbs are parallel to one another.

• Monocline - Simple, step-like bends caused by elastic deformation of otherwise horizontal sedimentary beds.
• Anticline - Up-arched rock beds. Oldest rocks are exposed in the centers of eroded anticlines. Rocks dip away from the center of the fold. A plunging anticline has a V-shaped outcrop pattern where the V points in the direction of plunge.
• Syncline - Downarched rock beds. Youngest rocks are exposed at the centers of eroded synclines. Rocks dip toward the center of the fold. A plunging syncline has a V-shaped outcrop pattern where the V points away from the direction of plunge.
• Dome - Upwarped structure with a circular or elliptical outcrop pattern. Beds dip away from center of structure. Oldest rocks at center.
• Basin - Downwarped structure which has a circular or elliptical outcrop pattern. Beds dip toward center of structure. Youngest rocks at center of basin.

        Fracture along which no differential movement has taken place. Often occur in parallel groups called joint sets. Intersecting joint sets produce a joint system. Joints can be caused by compression or tension. Compressional stress can produce joints in the area of a fold axis. Columnar jointing is produced by tensional stress in cooling rock (volcanics). Sheet jointing is closely spaced jointing parallel to a rock's surface produced by unloading (tensional stress). Weathering and erosion of jointed rocks can produce arches.

        Rock on either side of fracture move relative to one another. Classified on the basis of the relative direction of movement. The absolute direction of movement cannot usually be determined. Produce fault scarps (cliff formed by vertical movement) and fault breccia (rock broken into angular blocks by shear stress along fault). Three basic groups of faults are recognized:

1. Dip-slip faults - Displacement is primarily vertical and parallel with the dip of the fault plane. The hanging wall (rock above the fault surface) may move up or down relative to the footwall (rock below the fault surface).
1. Normal fault - Hanging wall moves down relative to the footwall. Caused by tensional stress. Horst and graben is a series of normally faulted blocks. Down-dropped block is a graben, upraised block is a horst.
2. Reverse fault - Hanging wall moves up relative to the footwall. Caused by compressional stress. A thrust fault is a low-angle (fault plane dips <45 degrees) reverse fault.

1. Strike-slip faults - Faults having primarily horizontal displacement along the strike of the fault plane. Caused by shear stress. A transform fault is a strike-slip fault that allows lateral movement of new crust away from the mid-ocean ridge without the relative position of ridge segments changing.
1. Right-lateral - The rock on the opposite side of the fault moves to the right.
2. Left-Lateral - The rock on the opposite side of the fault moves to the left.

• Oblique-slip Faults - Fault displacement has both dip-slip and strike slip movement. Given names like left-laterial, reverse or right-lateral, normal.

        Any area of land that stands significantly higher than the surrounding country. Mountain ranges are linear associations of peaks and ridges that are related in age and origin. Mountain systems consist of several mountain ranges and represent linear zones of intense deformation and crustal thickening.

        Mountains can be produced in several different ways:

• Volcanic Mountains - Produced by hot spot activity. May be isolated or in a chain.
• Mountains Formed by Igneous Intrusions - Intrusion of batholith causes uplift and erosion. Pluton forms small mountains.
• Block-Fault Mountains - Produced by normal faulting in areas subjected to tensional stress. Horst and graben blocks are produced. Horst blocks form mountains.
• Mountains formed by compression at convergent plate margins
• Mountains formed by accretion of microplates

        Orogeny is the term used for an episode of mountain building. Mountain building is accompanied by intense deformation, metamorphism and pluton intrusion. Caused by interactions between tectonic plates at convergent plate boundaries (compressive stress).

        Most recent and present day mountain building is located in two major belts: (1) The Alpine-Himalayan Orogenic Belt and (2) the Circum-Pacific Orogenic Belt. Mountain building coincides with  plate boundaries:

• Oceanic-oceanic convergent plate boundary - Characterized by the formation of a volcanic island arc by deformation, igneous activity and metamorphism. Forms two parallel belts: 1) a seaward complex consists of deformed trench rocks (low-T, high-P metamorphism: Blueschist Facies), and 2) an island arc system consists of deformed volcanic islands (andesitic) underlain by batholiths (intermediate to felsic plutons cause high-T, low-P contact metamorphism: Amphibolite and Granulite Facies). The island arc is eventually sutured (welded) to the continent by the thrusting of back-arc basin volcanics and sediments.
• Oceanic-continental convergent plate boundary - Subduction of an oceanic plate underneath a continental plate produces a continental arc characterized by deformation, igneous activity (andesitic volcanism and granitic intrusions) and regional metamorphism. Sedimentary rocks of continental margin and trench are deformed, metamorphosed, and accreted to the continent. These accretionary wedges can contain fragments of the ocean crust and upper mantle (ophiolites). Continental sediments are deformed and regionally metamorphosed.
• Continental-continental convergent plate boundary - Begin as a continental arc. When continents collide, huge compressional forces cause crustal thickening, thrusting and uplift of coastal sediments and continental rocks. Continents become sutured together.

        More than 25% of the Pacific coast consists of about 100 accreted microplates (small lithospheric blocks of foreign origin). The microplates are carried by a larger oceanic plate and are scraped off and accreted to the continent as the larger plate becomes subducted. Consist of fragments of island arcs, oceanic ridges, seamounts and other continents. Add totally new material to the continents. Difficult to recognize in older mountain systems.

        The earliest crust was thin and composed of dense ultramafic rocks. Upwelling basalt at spreading ridges disrupted the crust and subduction destroyed it. Partial melting of basaltic crust formed andesitic island arcs, and partial melting of intermediate rocks in the lower crust produced felsic magmas. By 3.8-3.96 billion years ago, the cores (cratons) of several continents had been formed.

        The ancient rocks that compose the stable cores of continents are known as cratons. Most of the craton is buried beneath younger sedimentary rocks. The exposed portion of a craton is known as a shield. Several major orogenies are responsible for the evolution of North America:
 

1. Precambrian mountain building - Collision caused suturing of several smaller cratons into one large one (Greenland, central Canada, north-central U.S.).
2. Precambrian accretion - Built out continent along the southern and eastern margins of the craton.
3. Formation of Pangaea - Closing of ocean basins during the Paleozoic formed the Appalachian and Ouachita Mountains.
4. Breakup of Pangaea - Rifting, accompanied by widespread volcanism, resulted in extensive lava flows and intrusions in the Appalachian Mountains.
5. Development of the Cordillera - The North American Cordillera is a complex mountainous area that extends from Alaska into central Mexico. Consists of accreted island arcs, development of continental arcs, flood basalts, and block-fault mountains that developed over a long time period. Most recent episode of large-scale deformation is the Laramide Orogeny (90-40 million years ago). It occurred along an oceanic-continental convergent boundary. Uplift continues in many areas of the Cordillera today.
6. Development of the Pacific Coast - Caused by the westward drift of North America and the resulting interaction with microplates, transform faults, and oceanic ridges during subduction of the Farallon Plate under the western coast of North America.