Folds, Faults, and Geologic Maps

Folds, Faults, and Geologic Maps Chapter 9 Geology Today Barbara W. Murck & Brian J. Skinner Folds, South Georgia Island N. Lindsley-Griffin, 1999

Stress Stress - the force acting on a surface, per unit area Fig. 9.2, p. 243 N. Lindsley-Griffin, 1999

Stress and Strain Strain - the change in shape or volume of a rock in response to stress. (Fig. 9.3, p. 244) N. Lindsley-Griffin, 1999

Types of Deformation Elastic - nonpermanent; rock returns to original shape when stress is removed Ductile (plastic) - permanent; rock flows or bends when stressed beyond its elastic limit Brittle - rock breaks or cracks Fig. 9.5 A, p. 245 N. Lindsley-Griffin, 1999

Controlling Factors Factors that influence how rocks deform: Temperature - increase in T makes solids more ductile Confining Pressure - increase in P inhibits formation of fractures; solids resist breaking (Fig. 9.6, p. 246) Undeformed Ductile Brittle N. Lindsley-Griffin, 1999

Controlling Factors Factors that influence how rocks deform: Rate of deformation - stress applied quickly enhances brittle behavior. Stress applied slowly over a long period of time promotes ductile behavior. (Fig. 9.6, p. 246) Undeformed Ductile Brittle N. Lindsley-Griffin, 1999

Controlling Factors Factors that influence how rocks deform Composition - some minerals are very brittle (quartz, garnet, olivine) as are rocks that contain them. Minerals that behave ductilely: mica, clay, calcite, gypsum Rocks that behave ductilely: shale, slate, limestone, marble (Fig. 9.5, p. 245) N. Lindsley-Griffin, 1999

Strike and Dip Strike - orientation of a horizontal line in a plane. (Expressed as a compass direction) Dip - angle between a tilted surface and a horizontal plane. (Expressed as an angle and direction) N. Lindsley-Griffin, 1999

Faults Hanging Wall Footwall Footwall Fault - a fracture in the crust along which movement has occurred. Normal Fault(Fig. 9.4, p. 245) Faults are classified by how steeply they dip and relative movement of the blocks: Hanging wall - block on top of the fault Footwall - block below the fault N. Lindsley-Griffin, 1999

Faults Normal Fault - hanging wall down relative to footwall. Tensional stress (Fig. 9.9, p. 250) N. Lindsley-Griffin, 1999

Faults Horst - two normal faults dip away from each other, block between them is elevated. Graben - two normal faults dip towards each other, block between them drops down. Tensional stress (Fig. 9.10, p. 250) N. Lindsley-Griffin, 1999

Faults Reverse Fault - hanging wall moves up relative to footwall Dip is steep Compressional stress (Fig. 9.11, p. 251) N. Lindsley-Griffin, 1999

Faults Thrust Fault - hanging wall moves up relative to footwall Dip is < 30 degrees (Fig. 9.12, p. 251) Reverse fault for comparison N. Lindsley-Griffin, 1999

Faults Strike-slip fault - movement is mostly horizontal and parallel to strike of fault. Left-lateral - block opposite you moves left. Right-lateral - block opposite you moves right. (Fig. 9.13, p. 251) N. Lindsley-Griffin, 1999

Folds How do you describe something as complex as a fold? Fold axis - the line along which the axial plane intersects a single rock layer Axial plane (axial surface) - the plane that divides the fold in half as symmetrically as possible Fig. 9.17 B, p. 255 N. Lindsley-Griffin, 1999

Folds How do you describe something as complex as a fold? Fold limbs - the two sides of the fold that are separated by the axial plane Anticline - limbs point down Syncline - limbs point up Fig. 9.17 B, p. 255 N. Lindsley-Griffin, 1999

Folds How do you describe something as complex as a fold? Orientation of both axial plane and fold axis are used. Horizontal fold axis with vertical axial plane = upright fold Plunging fold - fold axis is not horizontal Fig. 9.17 B, p. 255 N. Lindsley-Griffin, 1999

Folds Five common types of fold geometries (Fig. 9.20, p. 257) N. Lindsley-Griffin, 1999

Folds To describe a fold, first determine orientation of axial planes, limbs, axes Here, axial planes are not quite vertical, axes point straight out, limbs dip at different angles: open, asymmetrical, anticline - syncline Fig. 9.17 A, p. 255 N. Lindsley-Griffin, 1999

Folds Here, both limbs dip in the same direction so that one limb is actually upside down: overturned, symmetrical folds N. Lindsley-Griffin, 1999 Fig. 9.1, p. 242

Folds Open, asymmetrical folds (Fig. 9.4 A, p. 245) N. Lindsley-Griffin, 1999

Folds Special fold types Domes and Basins(Fig. 9.18, p. 256) N. Lindsley-Griffin, 1999

Folds Special fold types Monocline - upper and lower limbs are horizontal, only the central limb is inclined (Fig. 9.16, p. 254) N. Lindsley-Griffin, 1999

Showing strata on maps(Fig. 9.21, p. 258) Geologic Maps Contacts - boundaries between distinct rock types Block diagram shows eroded strata, cross section Geologic map shows contacts with strike and dip symbols. N. Lindsley-Griffin, 1999

Folds Showing folds on maps(Fig. 9.19, p. 256) Plunging folds make horseshoe patterns on surface Block diagram shows pattern projected to horizontal surface, as if strata were eroded flat Map shows how folds are depicted on geologic maps. N. Lindsley-Griffin, 1999

Cross Sections Cross sections show how structures and rock layers behave at depth. Appalachian Mtns. (Fig. 9.26, p. 266) N. Lindsley-Griffin, 1999

Geologic Maps Geologic maps - show topograpic contours, major folds and faults, contacts between rock units, age and type of rock.(Fig. 9.22, p. 259) N. Lindsley-Griffin, 1999

Topographic Maps Topographic maps use contour lines to depict topography. (Fig. 9.23, p. 263) N. Lindsley-Griffin, 1999

Folds, Faults, and Geologic Maps