Supplemental Review and Exercises Folds and Faults This is a plunging anticline Supplemental Review and Exercises

Deformation of Rocks When rocks have been deformed, folding or faulting may result How the deformation manifests its self is dependent on: Temperature Pressure Rock type Rocks under high pressure, warm, and flexible are more likely to fold Rocks under low pressure, cold, and brittle are more likely to break or fault

Driving force of deformation…… Plate Tectonics: The lithosphere is broken into “plates” free to move around on the Earth’s asthenosphere These plates are free to move with respect to each other, either slipping along one another, colliding, or pulling apart Forming three different plate boundaries which can result in various types of faulting and or folding.

The Plates http://observe.arc.nasa.gov/nasa/earth/tectonics/Tectonics2.html

The Plate Boundary Types Divergent Plate Boundary: plates move away from each other resulting in extension (tensional forces) Convergent Plate Boundary: plates move toward each other resulting in shortening (compress ional forces) Transform Plate Boundary: plates slide past each other results in lateral movement (shear forces) http://observe.arc.nasa.gov/nasa/earth/tectonics/Tectonics3.html

Convergent Boundaries Folding Anticlines: Arch shaped structure Synclines: Basin like structure Faulting Reverse Faults (shortening) Vertical movements

Divergent Boundaries Normal Faults (Extension) – vertical movements

Transform Boundaries Strike Slip Faults – lateral movement and shear forces Shear

More on folds and inclined beds…. The axis of the fold divides it in half, separating the fold into two limbs the limbs of the fold have the same strike for a non-plunging fold but opposite dips Describe orientation of the beds of rocks using strike and dip Strike: the trend of a rock with respect to north, typically parallel to bed contacts and axis of fold Dip: the inclination of the rock away from the horizontal down slope in a direction perpendicular to strike.

Strike and Dip Symbol The strike and dip symbol resembles a “T”. However the strike line is longer than the dip line. Click to insert strike and dip symbols on the below images. You may want to think of the dip symbol as having an arrow on the end pointing in the direction the rocks are dipping downward

Rules for Strike and Dip Older rocks dip in the direction of younger rocks For an anticline, dips point away from each other on opposite sides of the axis of the fold For a syncline, dips point toward each other on opposite sides of the axis of the fold

Age of the rock units play a role in interpretation…. Age is important for determining dip, and fold type. Use the geologic time scale as follows: Q – Quaternary (youngest) T – Tertiary K - Cretaceous J - Jurassic TR - Triassic P - Permian lP - Pennsylvanian M - Mississippian D - Devonian S - Silurian O - Ordovician Є - Cambrian PЄ – Pre Cambrian NOTE: the law of superposition indicates that in undeformed rocks, younger rocks are on top of older rocks at a single point in a vertical succession

Looking at the Structures Geologic Maps Cross-Sections Block Diagrams

Geologic Maps . Geologic maps of course provide geologic information as related to rock type, age, and structure Rock ages are described on the maps using the symbols previously discussed Different rock types and ages are depicted as different colors on the map with the contacts between the units represented by a line. The map symbols and configuration of the units provide structural information such as fold or fault type. Geologic maps show the features as seen in “map view” what things look like on the surface as if you were flying or walking over an area

Geologic Cross-Sections Provide information regarding the vertical configuration of the units Similar to a topographic profile, but providing subsurface information. A slice of the earth/feature to provide a side view, so what you would see if “cut” a vertical slice, like cutting a cake in half and seeing the layers inside

Block Diagrams Incorporate the information from the map view and the cross-sectional view to show both the surface and subsurface Seeing a block of the earth illustrated how it would look if you could cut a piece out like using a cookie cutter and see the top and the sides – allow you to get an idea of the feature in a three dimensional aspect

Map View and Cross Sectional View to a Block Diagram The diagrams above are all illustrating the same features just from in different views and or dimensions

The Anticline… Anticline: an arch like structure with older beds near its center that are flanked by younger rock Erosion Cross-sectional View Map View Click to add Strike And dip

The Syncline Syncline: a basin like structure with younger beds near its center that are flanked by older beds Erosion Cross-sectional View Map View Click to add Strike And dip

Plunging Folds (Map View) In map view, non plunging folds have straight contact lines while plunging folds contact lines curve around axial trace. For a plunging anticline the bed contacts converge around the fold axis in the direction the fold plunges and for a syncline the bed contacts diverge in the direction of dip around the fold axis

Three Types of Faults Normal Fault: any fault where the hanging wall has moved down with respect to its footwall. Results in elongation of rocks on the surface due to tensional forces such as along a divergent plate boundary. Reverse Fault: any fault where the hanging wall moves up with respect to its footwall. Results in shortening due to compressional forces such as along a convergent plate boundary Strike Slip Fault: any fault where adjacent fault blocks move parallel to the strike or trend of the fault surface. Results in lateral movement due to shear forces such as along transform boundaries.

Hanging VS Foot Wall If a tunnel was placed along the fault plane, a persons feet would be on the foot wall and their head by the hanging wall for a normal or reverse fault

Fault Rules 1.) The up thrown side of a normal or reverse fault will be eroded to the level of the down thrown side of the fault in time. 2.)Contact lines and associated rock formations (KEY BED) always shift in the direction of dip as the land surface is lowered by erosion. The Key Bed is any layer of rock that may be traced along the fault even though the faulting may have displaced the bed tens to hundreds of feet, pick a bed to be the key bed to determine which fault is the up thrown block

Illustrating Rules 1 and 2 For a fault cutting inclined beds: Erosion Cross-sectional View of Fault Erosion o S D Map View of Fault

Additional Fault Exercises: 1.) Place appropriate strike and dip symbols on the diagram 2.) What is the up thrown side? 3.) What type of fault is illustrated? Normal 4.) What is the dip of the fault plane? East N Up thrown Side (West)

Answer the following for the diagram below: 1.) Place appropriate strike and dip symbols. 2.) Draw in the fold axis. 3.) What type of fold is shown? Anticline 4.) What type fault is cutting the fold? Normal 5.) What is the up thrown side? Southwest is up Thrown

Answer the following for the diagram below: 1.) Place appropriate strike and dip symbols on the diagram 2.) What is the up thrown side? 3.) What type of fault is illustrated? Reverse 4.) What is the dip of the fault plane? North North Side Up Thrown N