Chapter 10 Deformation Of Rocks PDF

Figure 10.1 Chapter 10 Deformation of Rocks 1  Chapter 10 Crustal Deformation Topics covered - Stress and strain Mapping strike and dip Folds Faults Joints 3  Why know...

Figure 10.1 Chapter 10 Deformation of Rocks 1  Chapter 10 Crustal Deformation Topics covered - Stress and strain Mapping strike and dip Folds Faults Joints 3  Why knowledge of folds and faults is important. orientation of folds and faults give – geologic setting – direction of forces that produces structures economic importance – oil and gas trapped by structures – must find faults so do not build structures there – ore deposits localized on faults folds faults control location of oil 4  Deformation of Rocks Deformation results in folding, flowing, fracturing Folds and faults are geologic structures in response to tectonic forces or stress on the rock Structural geology is the study of the deformation of rocks. O 9  Stress and Strain Stress (tectonic forces)- is the amount of force acting on the rock Strain (deformation) – is the shape or volume changes caused by stress 10  Three Types of Differential Stress Differential stress is force applied unequally in different directions. compressional stress – forces push compresses together – shorten the rock body reducing its volume tensional stress (extensional) – widens forces pull apart – elongate rock body twist shear stress – forces push toward but parallel to one another 11  Three Types of Deformation elastic deformation –a temporary change in shape in response to stress. This is not permanent because the elastic limit of the rock has not been exceeded so the rock rebounds when the stress is released ductile deformation - when the rock bends or flows, changes shape without breaking. This is permanent. brittle deformation (brittle failure) – when the rock fractures, breaking into separate pieces. This is permanent 12  Factors that determine if ductile or brittle deformation will occur (that is will the rock fold or fracture) depth- – brittle deformation occurs at shallower depths because the rock is fractures cooler and pressures are lower so the rock will break more easily. – ductile deformation occurs at great depth where pressure and folds temperature are higher, and the rock is more mailable. rock type – brittle behavior is favored in silicate rich rocks, such as granitic and basaltic rocks, because they are composed of interconnected silicate minerals which have strong internal bonds – ductile behavior is favored in nonsilicate rich rocks or shale because nonsilicate minerals have weaker bonds because clays composing shale are weakly cemented time – brittle deformation occurs when forces acts quickly over short time, versus ductile deformation when forces acts slowly over long time 13  Mapping rock structure s Locate outcrop - surface exposure of rock – Identify the rock type Measure strike and dip of folds, faults and rock layers Plot all the strike, dip and rock type information on a geologic map. 14  Strike Line produced by the intersection of a horizontal plane on the rock surface – if roll ball down rock surface it is the line perpendicular to the path the ball takes report as an angle relative to north – N60oE = strike is 60o east of north N N W E 60o strike S 16 809 20  Dip angle of inclination of the rock surface from the horizontal plane – the path of a ball rolling down the rock face dip is always perpendicular to the strike report angle and direction of dip dip of 30oE Horizontal plane 30o 22 Strike = N60oE Dip = 30oE 27  Geologic map plot strike and dip of each structure color code rock formations can use this map to infer the shape and orientation of structure below the ground 28 817 31  Parts of a Folded Rock limb - the 2 sides of the fold hinge line (axis) - a line drawn along the points of maximum curvature of each layer axial plane - imaginary surface that divides the fold as symmetrically as possible 33  Parts of a Folded Rock (continued) Non-plunging fold plunge - the angle of incline of the axis hogback - steeply inclined angular Plunging fold ridges – formed when folded strata are resistant to hogback weathering and form outcrop 34  Fold types anticline/syncline monocline dome/basin 35  Anticline and Syncline anticline - arching of geologic layers – older rocks on the inside syncline - trough, downfolded layers – older rocks on the outside produced by compressional forces occur together in sets 36  Features of anticlines and synclines symmetrical when both limbs are at same angle asymmetrical when the limbs are at different angle plunging when the hinge line dips into the ground – anticline outcrop points in the direction of plunge – syncline outcrop points opposite of the plunge direction these folds do not go on forever they dissipate at their ends 38 824 sumpetrical syncline syncline 40  799 Compressional forces produce anticlines and synclines Where do we get compression? – Subduction zones where plates are colliding 43 827 45 Map View of Plunging Folds 47 Before erosion After erosion of to the plane shown above 48  Axial Trace of a Plunging Anticline Landers Oil Field which occurs on the crest of an anticline 49 Kurt N. Coonstenius  Monocline A fold that has only one limb produced by vertical displacement along a fault rather than compression Overlying folded strata are folded over a vertical fault in the underlying basement rock 51  Dome and Basin dome - circular upwarping, oldest strata inside basin - circular downwarping, youngest strata inside Dome Basin 52  Faults Faults are fractures with appreciable displacement Three types of fault – Dip-slip normal fault reverse fault – Strike-slip fault 58  Dip-slip faults (either a normal or a reverse fault) Dip-slip faults have movement parallel to the dip of the fault surface, so they have vertical displacement. hanging wall - rock surface above the fault foot wall - rock surface below the fault fault scarp - line of the fault on the surface, which is generally a long low-lying cliff 60 1964 Alaska earthquake 61  Reverse Fault (A type of dip-slip fault) Hanging wall moves up relative to the footwall – older strata ends up over younger strata produced by compressional forces (pushing together) – results in crustal shortening can occur at subduction zones (convergent boundaries) Ketobe Knob, Utah https://structuralgeo.files.wordpress.com/2013/07/reverse-fault-zone- ketobe-knob-utah.jpeg 62  Normal fault (a type of dip-slip fault) In normal faults the hanging wall moves down relative to the footwall. produced by tensional forces (pulling apart) – results in lengthening and extension of the crust can occur at spreading centers 63  Fault scarp 64 Normal fault Normal dip-slip fault Hanging wall Foot wall 65  Features of normal faulted terrain horst – uplifted block bounded by two normal faults graben – down-dropped block bounded by two normal faults fault-block mountains are produced in normally faulted terrain. – Example - Basin and Range Province of the southwestern United States 66  Normal faulted terrain: Fault block mountains of the Basin and Range Province of SW United States Extensional forces lengthen the crust 67  Strike-slip fault not a dipslip fault no vert displacement Fault movement is horizontal and parallel to strike – No vertical displacement, – fault gauge erodes to produce linear trough transform fault are strike slip faults – Example: San Francisco fault system (1906 earthquake) 72  Joints- are fractures with no appreciable displacement Examples of joints columnar joints - – shrinkage fractures in cooling igneous rock creates elongate pillar-like structures Devils Tower of Wyoming ((http://blogs.agu.org/mountainbeltway/2011/08/31/columns-form- perpendicular-to-cooling-fronts/) sheeting - from unloading of overlying rock – joints parallel to the ground surface – breaks rocks into “sheets” – Creates exfoliation domes brittle failure associated with folding 79  Importance of joints Enhanced chemical weathering can influence direction of streams hydrothermal solutions deposit ore minerals risk to construction projects – Teton Dam,Idaho, failure 1976 – seepage along joints 80 Chemical weathering is enhanced along joints. 81