1. Crustal Deformation

2. Deformation Deformation refers to all changes in the original form and/or size of a rock body. Every body of rock has a point at which it will fracture or flow, regardless of how strong it is. Deformation may also produce changes in the location and orientation of the rock. Most crustal deformation occurs along plate boundaries. Plate motions and the interactions along plate boundaries generate tectonic forces that cause rock units to deform.

3. Force and Stress Force is that which tends to put stationary objects in motion or change the motions of moving bodies. Geologists use stress to describe the forces that deform rocks. Stress is the amount of force applied to a certain area. Confining pressure refers to stress applied uniformly in all directions Differential stress refers to stress applied in a non uniform manner.

4. Types of stress Differential: stress applied unequally in different directions Compressional: differential stress that shortens a rock body. They tend to shorten and thicken the Earth’s crust by folding, faulting, and flowing Tensional: the form of stress that tries to pull apart or elongate a rock unit. This occurs as plates are being rifted apart. It tends to lengthen those rock bodies located in the upper crust by displacement along faults. Shear: a stress that causes two adjacent parts of a body to slide past one another. It causes movement along fault lines or movement occurs by ductile flow. Strain: an irreversible change in the shape and size of a rock body caused by stress (deformation – breaking or bending– due to stress).

5. How deformation occurs When rocks are subjected to stresses greater than their own strength, they begin to deform, usually by folding, flowing, or fracturing. When stress is gradually applied, rocks first respond by deforming elastically. Changes that result from elastic deformation are recoverable; the rock will return nearly to its original size and shape when the stress is removed. Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or it fractures (brittle deformation). The factors which influence the strength of a rock and thus how it will deform include temperature, confining pressure, rock type, and time.

6. Factors affecting deformation. Temperature and confining pressure: –Rocks tend to behave as brittle solids at low temperatures and confining pressures (Brittle deformation) –Rocks exhibit ductile deformation where temperatures and confining pressures are high. Ductile deformation is a type of solid-state flow that produces a change in shape and size of an object without fracturing. Rock type: the mineral composition and texture affect how a rock will deform. –Strong internal molecular bonds tend to fail by brittle fracture –Weakly cemented or metamorphic rocks with foliation or zones of weakness are more susceptible to ductile or viscous flow. Time: small stresses applied over a long time span can cause deformation where the forces were initially unable to the deform rock. Processes which rocks deform occur along a continuum that ranges from pure brittle fracture to ductile (viscous) flow; there are no sharp boundaries between these types of deformation. The patterns and folds seen in deformed rocks are generally achieved by the combined effect of distortion, sliding, and rotation of individual grains that makeup the rock.

7. Mapping Geologic Structures Mapping and reconstructing the Earth’s underlying structure is the geologist’s ernest attempt. First, Geologists identify and describe dominant structures initially. –Outcrops: sites where bedrock is exposed at the surface; most of it is covered by vegetation and sediment –Aerial photography –GPS –Reflection profiling –Satellite imaging –Seismic reflection profiling –Drilling holes provided data on the structure and composition of the underlying rocks –Strike: compass direction of the line of intersection created by a dipping bed or fault and a horizontal surface. Strike is always perpendicular to the direction of dip. –Dip: the angle at which a rock layer or fault is inclined from the horizontal. The direction of dip is at a right angle from the strike. –Sedimentary strata: sediments and deposits are deposited in horizontal layers. If the strata are still horizontal, that means that the area is still undisturbed structurally. If the strata are bent, broken, or inclined, than deformation must have resulted. Then deposition occurred thereafter. In the field, Geologists measure the strike (trend) and dip (inclination) of sedimentary rocks at as many possible outcrops as possible. These data are then plotted on a topographic map or an aerial photograph with a color coded description of the rock. Geologists can infer the orientation of the structures below and begin to interpret the region’s geologic history.

8. Folds Folds are a series of wavelike undulations which are formed by bending rock. Most folds result from horizontal compressional stress Limbs are the 2 sides of a fold. An axis is a line drawn along the points of maximum curvature in each layer. The plunge is the angle at which the fold axis is inclined. The axial plane is an imaginary surface that divides a fold as symmetrically as possible.

9. Types of Folds Anticlines are most commonly formed by the upfolding or arching of rock layers (oldest strata are found in the center). Synclines are the downfolds or troughs associated with anticlines (youngest strata are found in the center). Monoclines are large, steplike folds in horizontal sedimentary strata. These folds appear to be the reactivation of steeply dipping fault zones located in basement rocks located beneath the plateau. Symmetry –Symmetrical: limbs are mirror images of each other –Asymmetrical: limbs are non mirror images –Overturned: an asymmetrical fold where when one limb is tilted beyond the vertical –Recumbent: an overturned fold that lies on its side so that a plane extending through the axis of the fold would actually be horizontal The outcrop pattern of an anticline points in the direction it is plunging whereas the opposite is true for a syncline. Ridges and valleys result from differential weathering and erosion and are not necessarily associated with anticlines or synclines.

10. Domes and Basins Domes are broad, largely circular or elongated, upwarps in basement rock caused by deformation. The Black Hills are a large dome structure thought to be generated by upwarping. Basins are downwarped structures having a similar shape to domes. Hogbacks are narrow, sharp crested ridges formed by the upturned edge of a steeply dipping bed of resistant rock. The oldest strata in domes are found in the interior while the oldest strata are found along the edge of basins.

11. Faults Faults are fractures in the crust along which appreciable movement has taken place. Fault zones: long displacements consisting of many interconnected fault surfaces Fault gouge: loosely, coherent clayish material formed as faults move and grind against each other Slickenslides: polished, striated (grooved) surfaces that form as the crustal blocks slide past one another Fault scarps: a cliff created by movement along a fault where its surface is subject to weathering and erosion Hanging wall: rock surface immediately above the fault Foot wall: the rock surface below the fault Graben: a valley formed by the downward movement of a fault bounded block Horsts: an elongated, uplifted block of crust bounded by faults Klippe: a remnant or outlier of a thrust sheet that was isolated by erosion

12. Types of Faults Dip-slip: movement is primarily parallel to the dip (inclination) of the fault surfaces. –Normal: hanging wall moves down relative to the foot wall –Reverse: hanging wall moves up relative to the foot wall –Thrust: a reverse fault with an angle of inclination (dip) less than 45 o –Detachment: nearly horizontal that extend several miles below the surface. Here they form a major boundary between rocks below, which exhibit ductile deformation, and the rocks above, which demonstrate brittle deformation. Strike-slip: dominant displacement is horizontal and parallel to the strike of the fault surface. –Transform: these faults cut through lithosphere and accommodate motion between 2 large crustal plates

13. Fault-block mountains Fault block-mountains are parallel mountain ranges caused where tensional stresses have elongated and fractured the crusts into numerous blocks. Movement along these fractures has tilted the blocks producing these parallel mountain ranges. Tetons and Sierra Nevada are both faulted along their eastern flanks which were uplifted as the blocks tilted downward to the west.

14. Joints Joints are fractures along which no previous movement has occurred. Columnar form when igneous rocks cool and develop shrinkage fractures that produce elongated, pillar like columns. Batholiths form when sheeting produces a pattern of gently curved joints that develop more or less parallel to the surface of igneous bodies. Jointing occurs when erosion removes the overlying load. Most joints are produced when rocks in the outermost crust are deformed. Tensional and shearing stresses associated with crustal movements cause the rock to fail by brittle fracture.