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Transcript
Undisturbed Layers How is rock deformation discussed? (stress and strain relationships) Stress • force (pressure) acting on the rock surface Strain • a change in the shape (deformation) to the response of stress Compression Tensional undisturbed cube Shear Types of stresses and strains Confined Pressure Differential Pressure Stress is applied to the rock “uniformially” in all directions Stress is applied to the rock in various directions Uniformly “squeezed” Strained in other directions If you can see this cube “rotate”, then you can picture diagrams in 3-D. Structural Geology – Structural geologists decipher earth history by identifying and mapping deformational structures. Why understand structural geology? Decipher geologic history by looking at the subsurface Look for oil resources Look for mining resources Monitoring groundwater flow How does a geologist measure the orientation of rock layers and use these measurements to predict the geologic structure below the earth’s surface? Strike and dip Strike The compass orientation of the line of intersection between the a horizontal plane and a planer feature (a rock layer) Dip The angle between a tilted surface and a horizontal plane Measuring the “strike” of strata • represents the intersection of the tilted layer and the imaginary horizontal plane • A MAP symbol – showing the “directional trend” of strata Measuring the directional trend Map symbol imaginary plane North 315 0 W 270 90 E 180 225 45 135 S Strike is 45 degrees How would you “plot” the strike of these rock layers North Geologic Maps Strike represents the MAP orientation and “general direction” of strata Measuring the dip angle with respect to strike of strata • represents the ANGLE (tilt) of strata in reference to the imaginary horizontal plane • Dip is ALWAYS PERPENDICULAR to strike Imaginary plane Strike and dip map symbols 12 20 Dip angle 0-90o 15 47 62 Let’s add some stresses and fold these rocks Let’s add some stresses and bend the rock layers into folds Typical parts of a Fold “leg” of the fold The hinge line apex of fold Cuts the fold in two equal parts Interpreting folds Young Syncline fold Youngest rocks in the middle Map Symbols oldest Anticline fold Oldest rocks in the middle Map View Map View How can you tell the difference between an anticline and syncline on the MAP VIEW???? OLDEST found in middle YOUNGEST found in middle ANTICLINE SYNCLINE 6 5 4 3 2 1 4 5 3 4 6 5 6 5 4 3 2 2 1 Folds are tilted or “plunged” • How would the “map-view” of the plunging fold appear from horizontal to steep tilt??? horizontal Map Views Slightly steep Very Steep Direction of plunge • What happens to the map view of the anticline as the plunge steepens? Map view Direction of plunge Direction of plunge for anticlines and synclines Faults Joint – fracture within a rock body Fault – movement along a fracture or joint B A fault Joint A B Determining the type of fault using the hanging-wall (HW) and foot-wall (FW) Normal Fault • HW down relative to FW • Tensional stress HW FW FW HW HW Reverse Fault • HW up relative to FW FW • Compression stress Vertical type faults FW HW FW HW Normal faulting FW Reverse faulting HW HW FW Horizontal faulting – two blocks sliding past one another Map View Turned to Right Turned to Left What type of strike-slip fault? Right-lateral – strike slip Normal Fault • hanging wall DOWN relative to footwall • tensional forces (pulling apart) • extension of crust (“crust grows”) Reverse Fault • hanging wall UP relative to footwall • compressional forces (pushing together) • shorting of crust (“crust shrinks”) Strike-Slip fault • horizontal movement (sliding past) • moves rocks along a horizontal plane