Structural Geology PDF

Structural Geology Folds, faults, & mountain belts  There are three main types of plate boundaries. Most of the exciting stuff in geology happens at these boundaries. Transform Divergent Convergent  Plate-tectonic forces and nature of crustal deformation ◼...

Structural Geology Folds, faults, & mountain belts  There are three main types of plate boundaries. Most of the exciting stuff in geology happens at these boundaries. Transform Divergent Convergent  Plate-tectonic forces and nature of crustal deformation ◼ Plate boundaries are characterized by directed pressure ◼ Directed pressure causes the Earths crust to bend and break  Convergent plate boundaries Characterized by horizontal compression (directed pressure)  Divergent plate boundaries Characterized by vertical compression (directed pressure)—Your book uses the term tension  Transform plate boundaries Characterized by horizontal shear (directed pressure)  Plate-tectonic forces and nature of crustal deformation ◼ Bending and breaking is recorded in the rocks by folding of the layers and faults—we call these features geologic structures or structures for short Fault Fold  Elastic vs. plastic deformation Elastic deformation: Chemical bonds are stretched by directed pressure until they break Like stretching a rubber band Plastic deformation: Chemical bonds in mineral crystals rearrange in response to directed pressure Material flows, like silly putty  Elastic vs. plastic deformation Factors that choose between elastic and plastic deformation ◼ Heat: Materials must be at high temperatures to flow plastically ◼ Time: Materials must have lots of time to flow plastically Plastically deformed cobbles  Recording geologic structures Strike: Compass direction of a horizontal line on a dipping plane Dip: Angle from horizontal of a plane—Lies at a right angle to strike  Types of geologic structures: Folds Folds: Bending or warping of rock layers Considered plastic deformation, but can also occur by slip between rock layers Layers must have different viscosities—Solid is really just a material with a very high viscosity  Shapes of folds Fold axis: line or hinge around which layers are bent Fold axial plane: Surface formed connecting all of the fold axis lines  Types of folds Anticline: Convex-upward, arch-like fold Syncline: Convex-downward (concave upward), trough-like fold Is this fold an anticline or a syncline?  Types of folds Plunging folds: Fold hinge is not horizontal  Types of folds Domes and basins: Dome-shaped and basin-shaped folds Dome Basin  Types of folds Overturned folds: Folds where one side has been rotated all the way through vertical ◼ Steep side generally points away from plate boundary  Rock-age patterns produced by ◼ folds Anticline: Oldest rocks exposed in the center ◼ Syncline: Youngest rocks exposed in the center Eroded fold looks like stripes from bird’s-eye view Folding  Examples of folds Folded quartzite layer in schist in the Pilar Mountains, New Mexico. Folding took place by plastic flow  Examples of folds Folded sedimentary layers in Dorset, England. Folding likely took place by slip between the beds  Examples of folds Sheep Mountain anticline in Wyoming. Classic example of a plunging anticline  Examples of folds Folds in the Appalachian Mountains of Pennsylvania  Fractures and Faults Fractures: Rock that has broken due to too much elastic strain Faults: Fracture where two sides have moved relative to each other Fault  Elements of faults Term comes from 18th century British coal mines. Miners encountering an offset coal seam would say there is a “fault in the seam”  Elements of faults Hanging wall: Fault block that lies above a dipping fault plane Footwall: Fault block that lies below a dipping fault plane— Should be able to walk on it.  Nature of faults Types of faults in an area generally function of the plate-tectonic setting Every single fault type can be found at any plate boundary Faults typically form complicated 3-D networks Active faults in California and Nevada  Types of faults Strike-slip faults: Faults that form by horizontal slip between two adjacent blocks of rock Fault Motion  Think-pair-share activity What type of plate boundary should be dominated by strike-slip faults? 1. Answer the question 2. Compare, discuss, and refine your answer with one or two of your neighbors. 3. Write up your results and pass them in.  San Andreas fault system Transform plate boundary between Pacific Plate and North American Plate Velocity of California relative to Denver San Andreas fault system San Andreas fault system Active strike-slip faults in the San Francisco Bay area San Andreas fault system Active faults in the San Diego area  Types of faults Dip-slip faults: Blocks of rock on either side of the fault move up or down the dip of the fault plane  Dip-slip faults Normal faults: Dip-slip faults in which the footwall has moved up relative to the hanging wall Accommodate extension of the crust Dominate divergent plate boundaries  Normal faults Active normal faults have distinct topography ◼ Footwall blocks form linear ridges called horsts ◼ Hanging-wall blocks form valleys called grabens Fault Motion  Normal faults example The Teton Mountains were uplifted by a large normal fault system Cross section through the crust in Grand Teton National Park  Normal faults example The Basin and Range province of the western United States Active faults and the velocity of the Basin and Range province relative to Denver  Dip-slip faults Reverse faults: Dip-slip faults in which the hanging wall has moved up relative to the footwall Accommodate shortening of the crust Fault Motion  Reverse faults example The Rocky Mountains in Colorado formed by large reverse faults WY CO UT CO AZ NM  Reverse faults example The Rocky Mountains in Colorado formed by large reverse faults  Dip-slip faults Thrust faults: Reverse faults that are very gently dipping—can even be horizontal in places Accommodate shortening of the crust—Generally form along with folds Dominate most contractional mountain belts at convergent plate boundaries  Thrust faults example Lewis thrust fault in Glacier National Park Rocks in hanging wall have been displaced >65 miles