GY111 Physical Geology Deformation of the Earth s Crust Stress Strain Stress a force applied to an area Example tire pressure in psi Strain a change in original shape or volume produced by stress Elastic strain analogous to a steel spring or rubber band Plastic strain analogous to deforming mud or putty Types of Stress Lithostatic Stress stress due to the burial and overlying overburden of rock Lithostatic stress can only cause a change in volume referred to as dilation Directed stress stress is unequal in different directions Directed stress is generated by plate tectonic motion and will cause a change in shape referred to as distortion Stress vs Strain Diagrams Illustrate the mechanical behavior of rock materials Brittle rocks near the surface of the Earth behave as brittle materials their behavior is mainly elastic Brittle Deformation Elastic Limit Rupture Stress Distortion below E L is 100 recoverable Strain Ductile Deformation Ductile deformation requires a significant component of plastic mechanical behavior Elastic Limit plastic Rupture Stress Distortion below E L is 100 recoverable Permanent strain Strain Mechanical Behavior of Rocks Near surface rocks that are under low T P conditions behave as brittle material Fault fracture slippage Joint fracture no slippage Deep rocks under elevated T P conditions behave as ductile material Folding Examples of Deformation Experiments Lab equipment can reproduce all geological conditions except geologic time Undeformed Low T P brittle High T P ductile Mapping Geological Structures Orientation Planar strike azimuth and dip angle with dip quadrant Linear trend azimuth and plunge angle Azimuth compass direction along the horizontal map surface 0 90 northeast quadrant 90 180 southeast quadrant 180 270 southwest quadrant 270 360 northwest quadrant Strike is always read from a northern quadrant therefore it must always be 0 90 or 270 360 Dip maximum angle of inclination in a geological plane bedding fault joint fracture etc The azimuth direction of the dip is always perpendicular to the strike Geologic Period Abbreviations Quaternary Q Tertiary T Cretaceous K Jurassic J Triassic Tr Permian P Carboniferous C Devonian D Silurian S Ordovician O Cambrian C Precambrian p C These abbreviations are commonly used to indicate ages of beds on geologic maps In North America the Carboniferous period Is subdivided into the following 2 periods Pennsylvanian P Mississippian M Examples of Planar Structures Both would be measured with a strike and dip Bedding Planes Bedding Fault Planes Strike and Dip Planar Structures Strike is the azimuth direction of the horizontal line in a plane By convention strikes are read from a north quadrant so the legal values are 0 90 or 270360 Dip is the maximum angle of inclination in a planar structure This angle will always be measured in a plane perpendicular to strike The dip angle must be paired with a quadrant direction since there are 2 sides to any strike line Example 040 60NW strike 040 dip angle 60 in a 310 NW direction Note that 310 is 90 degrees from 040 Maximum possible dip angle is 90 In this case there is no dip quadrant A horizontal plane has no definable strike and 0 dip angle Strike and Dip Symbols A 0 0 B C 0 38 270 52 270 90 90 41 180 180 90 270 180 45 D E 0 90 270 0 F 90 270 0 90 270 65 180 180 G H 0 180 0 I 0 80 90 270 180 270 12 180 90 270 25 180 90 A 000 52E B 000 41W C 060 38NW D 090 65S E N A 0 F 315 90 G 300 80NE H 330 12NE OT I 030 25SE Dip Direction Relationships The dip direction of bedding is in a direction toward younger strata unless the strata is overturned overturned folds are discussed later Younger Dip Direction Schematic When beds are not overturned the dip directions points toward younger beds Tr J K 50 50 T 50 T Younger K 50 Tr P J T K Overturned Strata Note that when strata are overturned the dip direction points toward older strata 55 55 55 Older S D 55 C O C 55 O M D S O Trend and Plunge Linear Structure Trend azimuth direction of a linear structure projected up to a horizontal plane Plunge incline angle of a linear structure Note that the trend may have any azimuth value 0 360 Maximum possible plunge is 90 Trend and Plunge A 0 0 B C 0 05 90 270 65 90 270 90 270 15 180 180 D 0 0 E 90 270 180 F 90 270 0 90 270 40 180 180 G H 0 180 0 I 0 72 90 270 90 270 90 270 23 55 180 180 180 A 210 15 B 330 05 C 060 65 D 120 40 E 030 00 F N A 90 G 240 23 H 300 72 I 150 55 Faulting Faults are generated in brittle rock layers when the elastic limit is exceeded by deformation forces Because brittle behavior is confined to the lithosphere faults do not extend into the asthenosphere Fault Classification Classified by the nature of the slippage of one fault block past another block Dip Slip slippage is parallel to dip of fault Normal hanging wall down motion Reverse hanging wall up motion A special case of reverse where the fault dips 45 degrees Strike Slip slippage is parallel to strike of fault Right lateral a right hand turn must be followed to find offset features Left lateral a left hand turn must be followed to find offset features Oblique Slip has combined strike slip and dip slip motion Hanging Wall and Foot Wall To classify a dip slip fault you must correctly identify the hanging wall and footwall blocks Hanging Wall Footwall Dip Slip Fault Motion Examples Note that normal faults accommodate tensional stress whereas reverse faults accommodate compressional stress Reverse Normal Thrust Fault Offsets Some fault offsets are recognizable on the ground surface Fault Scarp Strike Slip Fault Motion Examples Movement is parallel to strike of fault therefore offset is seen in a map view Tectonic Associations of Fault Types Divergent tension tends to produce normal dip slip faults Convergent compression tends to produce thrust low dip angle reverse dipslip faults Transform shear produces strike slip faults Folding Folding is produced by the compression generated at convergent plate boundaries Folds require rocks to be under significant T and P so that the layers of rock can bend without breaking i e ductile Fold Geometry Anticline concave down arch Syncline concave up trough Fold Age Relationships Anticlines contain the oldest strata in the center of the structure Bedding dips away from the center of the structure if the fold is not overturned Synclines contain the youngest strata in the center of the structure Bedding dips toward the center of the structure if the fold is not overturned Fold Symmetry