Geologic Structure

Structural geology Structural geologists study the architecture and processes responsible for deformation of Earth’s crust Rock structures matter! Oilfields are contained in structures as are ore deposits

Deformation Deformation refers to all changes in the original form and/or size of a rock body Most crustal deformation occurs along plate margins Occurs from tectonic forces

Deformation Deformation involves Stress - force divided by area Types of stress Compressional stress – shortens rocks Tensional stress – extends rocks Shear stress – pushing in different directions.

Deformation Strain – changes in the shape or size of a rock body from stress How rocks deform Rocks subjected to stresses greater than their own strength deform by fracturing, folding or flowing.

Deformation How rocks deform General characteristics Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed When a rock breaks, it is called brittle deformation. Any material that breaks into pieces exhibits brittle behavior Low temperature and pressure conditions When rocks bend or flow, like clay, it is called ductile deformation High temperature and pressure conditions Earthquakes release elastic energy

Deformation How rocks deform General characteristics of rock deformation Factors that influence the strength of a rock and how it will deform Temperature Confining pressure Rock type Time (strain rate)

Folds formed under high grade metamorphic conditions

Mapping geologic structures When conducting a study of a region, a geologist identifies and describes the dominant rock structures Work is aided by advances in aerial photography, satellite imagery, digital topography and Global Positioning Systems (GPS)

Strike and Dip Strike: Direction of the line of intersection between a tilted plane and a horizontal plane. 2-directional line

Strike and Dip Dip: Angle between a tilted plane and a horizontal plane that is perpendicular to strike. Maximum angle ( ≤ 90º ) Notation: ____ ° N, S, E, W, NE, NW, SE, SW

W E Horizontal Dip Angle (E. McBride) Cross-Section: Dipping Strata

W E Horizontal Dip Angle (E. McBride) Cross-Section: Dipping Strata

strike and dip (vertical) Map Symbols strike and dip strike and dip (vertical) strike and dip (horizontal) 45º

Faults Types of dip-slip faults Reverse and thrust faults Hanging wall moves up relative to footwall Accommodate shortening of the crust Results from compressional forces Reverse- dip of fault plane is above 45 degrees Thrust- dip of fault plane is less than 20 degrees.

On a reverse fault, the hanging wall moves up relative to the footwall

Thrust faults formed by crustal shortening

Faults Strike-slip fault Displacement is horizontal and parallel to strike of fault Types of strike-slip faults Right-lateral – as you face the fault, the block on the opposite side moves right Left-lateral – as you face the fault, the block on the opposite side moves left Because of their large size and linear nature, many strike-slip faults produce a trace that is visible over a great distance Crushed and broken rocks produced during faulting are more easily eroded, often producing linear valleys or troughs

A block diagram showing the features along a strike-slip fault

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

The San Andreas fault system is a major transform fault