FOLDS, FAULTS AND GEOLOGIC MAPS Objectives Define three types of stress. Describe the differences between elastic, brittle, and ductile deformation. Define cratons and orogens. Define strike and dip. Define and describe synclines, anticlines, and other types of folds. Distinguish between a topographic map and a geologic map. Stress The force acting on a surface, per unit area May be greater in certain directions than in others Pressure A kind of stress in which the forces acting on a body are the same in all directions Rock Deformation 1
Rock Deformation Tension A stress that acts in a direction perpendicular to and away from a surface Compression Stress that acts in a direction perpendicular to and toward surface Rock Deformation Shear A stress that acts in a direction parallel to a surface Strain A change in shape or volume of a rock in response to stress Uniform stress causes change in volume only Differential stress may causes change in shape Rock deformation Elastic deformation Temporary change in shape or volume from which a material rebounds after the stress is removed Paper clip permanently deformed: stress was greater than elastic limit Paper clip returns to original shape 2
Ductile deformation Permanent but gradual change in shape or volume of a material, caused by flowing or bending Brittle deformation Permanent change in shape or volume, in which a material breaks or cracks Brittle at room temperature Ductile at high temperature Ductile Rock deformation Brittle Rock Deformation Main factors that affect rock deformation Temperature Confining pressure: uniform pressure due to depth Rate of deformation Composition High confining pressure Low confining pressure 3
Rate of deformation Rock Deformation Where rock deformation occurs Craton A region of continental crust that has remained tectonically stable for a very long time Orogen An elongated region of crust that has been deformed and metamorphosed through a continental collision Isostasy Similar to an ice berg floating in water; it floats at a level determined by the density contrast The flotational balance of the lithosphere on the asthenosphere Rock Deformation 4
Rock Deformation Structural geology - the study of: Stress and strain Processes causing stress and strain Deformation and rock structures that result from stress and strain Strike The compass orientation of the line of intersection between a horizontal plane and a planar feature (e.g. rock layer or fault) Dip The angle between the tilted surface and a horizontal plane 5
Faults and fractures Fault; fracture with movement along the fracture surface Normal fault The block of rock above the fault surface moves downward relative to the block below 6
Faults and fractures Reverse fault The block on top of the fault surface moves up and over the block on the bottom Thrust fault A reverse fault with a shallow angle of dip Strike-slip fault A fault in which the direction of the movement is mostly horizontal and parallel to the strike of the fault San Andreas Fault Strike-slip fault 7
Offset drainages San Andreas San Jacinto fault The fault on the left is a right-lateral fault because the offset stream is to your right as you face the fault. 8
Folds A bend or warp in a layered rock Monocline A local steepening in otherwise uniformly dipping strata Anticline A fold in the form of an arch, with the rock strata convex upward and the older rocks in the core Monocline Syncline A fold in the form of a trough, with the rock strata concave upward and the younger rocks in the core The geometry and the orientation of a fold is described by the: Axial plane Axial trace Limbs Axis Plunge Parts of a fold Limb 9
Domes and basins Dome Michigan Basin 10
Geologic Maps Geologic map Shows the locations, kinds, and orientation of rock units Shows structural features such as faults and folds Topographic maps Show the shape of a ground surface, as well as the location and elevation of surface features, usually by means of contour lines Topographic Map Geologic Maps 11
Geologic Maps 12
Geologic Maps The Canadian Rockies 13