Structural GeologyTectonic Stresses Large Scale Strain of the Crust i.e., Geologic Structures

Inner core: Solid iron Outer core: Liquid iron,

convecting (magnetic field)

Mantle (Asthenosphere) : Solid iron-magnesium silicate, plastic, convecting

Crust (Lithosphere): Rigid, thin5-30km

Crust: Rigid, Thin

Mantle: Plastic, Convecting

Tectonics and Structural Geology

Tectonic Stresses resulting fromInternal Energy (heat driving convection) Strains (deforms) the Mantle and Crust

Bends Rocks, i.e., ductile strain (Folds)Breaks Rock, i.e., brittle strain (Joints) and Moves large blocks along Faults andReleases energy Earthquakes

Folds and Faults (Palmdale, Ca)

Eastern PennsylvaniaEastern Pennsylvania

NorthwesternAfrica

MetamorphicGrade

High

Low

NortheasternNorth America

NorthwesternAfrica

Strike and Dip of Planar Feature

Two methods of reporting planar orientation Quadrant: N15oE, 45oS (geologists) Azmuthal: 195o/45o (engineers)

Stikes and Dips are used to identify geologic structures

Brunton Pocket Transit

Clinometer Azimuthal Compass Transit

Stresses at Plate

Boundaries Divergent (Tensional)

| Convergent (Compressional) | Transform (Shear)

e.g., Pacific NWKehew, Fig. 1-20Lab. Man., Fig. 21-2

Geologic Structures Different stresses result in

various forms of strain (geologic structures) Folds (compressive

stresses may cause ductile strain)

Faults (Any type of stress may cause brittle strain. The type of fault depends on the type of stress)

Anticline (fold)

Syncline (fold)

Plunging Anticline

Eastern PennsylvaniaEastern Pennsylvania Folds and faults resulting from

compressive stresses Anticlines (many plunging) Synclines (many plunging) Reverse faults Thrust faults

Domes and Basins

Bedrock Geology of the Michigan Basin During and after

the deposition of Michigan’s sedimentary rocks

The crust warped downward

Exposing younger rocks in the center and

Older rocks on the rim (e.g. Toledo)

When shallow crust is strained rocks tend to exhibit brittle strain

Brittle Strain Joints

Sheet Joints (due to Expansion and Exfoliation)

Fault: Movement occurring along a discontinuity Brittle strain and subsequent movement as a

result of stress Fault

terminology

Faults

Fault: When movement occurs along a discontinuity

Fault type depends on the type of stress

Normal Faults

Normal Faults, Horsts and Grabens

Structures at Divergent Boundaries

Tensional Stresses cause brittle strain and formation of sets of normal faults

i.e., Horsts and Grabens

Horsts and Grabens Older Rocks are exposed along the ridges

formed by the horsts

Younger rocks lie beneath the grabens Sediment fills in the linear valleys

Nevada

“Washboard topography” is the result of Horsts and Grabens

A.k.a, Basin and Range

Structural Oil Traps

Reverse and Thrust Faults

Compressive stress causes the hanging wall to move upward relative to the foot wall Reverse Fault

At convergent plate boundaries ancient rocks can be thrust over younger rocks Thrust Fault

Thrust Fault: Glacier NP, Montana

Old

Younger

Structures at a Convergent Boundary

Structures within Mountain Belts

Compressional and TensionalStructures

E.g., The Apls

Intense folding and thrusting of sedimentary rocks

Strike Slip Faults

Physiographic Features

San Andreas Fault

What type of fault is this? What other features are

associated with the fault?