Geologic Structures By Dr S A Isiorho Copyright © by Isiorho Structural Geology/Rock Deformation Structural Geology is that part of geology that looks at shape, arrangement, interrelationship between rock units and the forces behind them Rocks are subjected to great forces- particularly at plate margins Stress- force that tends to change shape or size of a rock Stress could be Compressive- converging plate boundary Tensional- diverging plate boundary shearing- transform plate boundary Strain- rocks response to stress through change in shape and volume Copyright © by Isior Types of Deformation (Strain) Plastic strain- body does not return to its original size/shape i.e the rock is ductile Elastic strain- body recovers to its original size/shape after stress is removed Stress is applied gradually to deep warm rocks deformation not permanent (minor amount of stress) Fracture (Brittle)- body breaks or cracks stress amount exceeds the yield point or elastic limit Copyright © by Isior Factors affecting Rock deformation Heat leads to stretching of rocks at near earth’s surface without breaking at depth- plastic deformation Time Composition different minerals have different strength Copyright © by Isior General Structural description Structures gives clues to geologic history of the past Principles of horizontality- what is that? How are structures depicted in a geologic map? In describing folds, need to know the orientation of the rock in space- STRIKE and the angle at which rock is inclined to the horizontal- DIP Sketch of a fold Copyright © by Isior Folds Folds- rocks deform plastically (most occur at convergent plates) Basin Syncline- trough-like Anticline- arch-like Types of folds: symmetrical, broad, open, isocline, overturn,recumbent, plunging rock deformation that is bowl shaped Domes rock deformation that is oval-shaped bulges Copyright © by Isior Faults A fracture is a break in rock (could be a joint or a fault) Fault is when there is a relative movement along the break Fault Types Strike-Slip faults horizontal movement (transform plate boundary) Left-lateral & right-lateral Dip-Slip faults Normal (tensional stress) Reserve (compressional stress) Thrust (low angle reserve fault) Oblique (combination of strike-slip & dip-slip) Copyright © by Isior Unconformities Unconformities represent a break (missing time/period) in rock record Three types of unconformity Dis-conformity Angular unconformity Non-conformity Copyright © by Isior Mountain Creation Most dramatic effect of stress Fold and Thrust Mountains Fault Block Mountains- tensional Unwarped Mountains- broad regional uplift Copyright © by Isior