Crustal Deformation Structural Geology

Structural Geology Tectonic collision deforms crustal rocks producing geologic structures. Folds Faults Joints and Fractures

Deformation All changes in the original location, orientation or form of a crustal rock body. Deformation common at plate margins. Deformation concepts… Force Stress Strain

Stress Stress - Force applied to a given area. Determines the concentration of force. 3 major types of differential stress Compressional stress Tensional stress Shear stress

Compressional Stress “Push-together” stress. Shortens and thickens crust. Associated with mountain building

Tensional Stress “Pull-apart” stress. Thins and stretches crust. Associated with rifting.

Shear Stress Slippage of one rock mass past another.

Strain Changes in the shape or size of a rock body caused by stress. Strain occurs when stresses exceed rock strength. Strained rocks deform by folding, flowing, or fracturing.

How Rocks Deform Factors controlling rock strength and deformation style. Temperature and confining pressure Low T and P = faulting deformation High T and P = folding deformation Rock type – Mineral composition controls strength. Time – Stress applied for a long time generates change.

The Formation A mappable rock unit.

Folds Rocks are bent by crustal deformation into a series of wave-like undulations called folds. Most folds result from compressional stresses which shorten and thicken the crust. Stephen Marshak

Common Types of Folds Anticline – Upfolded or arched rock layers. Syncline – Downfolds or rock troughs. (Think “sink”)

Anticline

Syncline

Anticlines and Synclines are common in fold and thrust belts related to mountain belts.

Common Types of Folds Monoclines – Large, step-like folds in otherwise horizontal sedimentary strata. Domes -Upwarped circular or slightly elongated structure. Oldest rocks in center, younger rocks outside. Basins – Downwarped circular or slightly elongated structure. Youngest rocks are found near the center, oldest rocks on the flanks.

Faults

Faults Breaks in rock that exhibit offset. Exist at a variety of scales. Sudden movements along faults are the cause of most earthquakes. Classified by movement… Horizontal Vertical

Fault Types Dip-slip faults – Motion is up & down Strike-slip faults – Motion is left or right

Dip Slip Faults May produce long, low cliffs called fault scarps.

Dip Slip Faults Fault blocks classified as Footwall (rock mass below the fault) Hanging wall (rock mass above the fault)

Types of Dip-Slip Faults Two dominant types Normal fault Reverse Fault

Types of Dip-Slip Faults Normal fault Hanging wall moves down relative to the footwall. Accommodate lengthening or extension of the crust.

Normal Faults Larger scale normal faults are associated with fault-block mountains (Basin and Range of Nevada).

Fig. 11.17b W. W. Norton

Types of Dip-Slip Faults Reverse faults Hanging wall block moves up relative to the footwall block Reverse faults have dips greater than 45o and thrust faults have dips less then 45o Accommodate shortening of the crust Strong compressional forces

Types of Dip-Slip Faults Thrust faults - A special case of reverse fault. Hanging wall block moves up relative to the footwall block Thrust faults are characterized by a low dip angle (less then 45o). Accommodate shortening of the crust Strong compressional forces

U.S. Geological Survey

Strike-Slip Faults Displacement is horizontal Types of strike-slip faults Right-lateral – as you face the fault, the block on the opposite side of the fault moves to the right Left-lateral – as you face the fault, the block on the opposite side of the fault moves to the left

Strike-Slip Faults Strike-slip fault Transform fault Large strike-slip fault that cuts through the lithosphere Accommodates motion between two large crustal plates

Joints Joints are a very common rock structure. They are fractures with no offset. Occur in parallel groups.

The End