Geological Structuresare the strains or deformations that rocks undergo when subject to different stresses.

Stress and strain: Read this report on the geology of Sideling Hill Road Cut by David Brezinski (Maryland Geological

Survey) at the URL: http://mgs.dnr.md.gov/esic/brochures/sideling.

html― Stress can be (a) compressive

(or forcing shortening of space,as at the folded mountain belts), (b) tensional (stretching or elongation, such as during sea floor spread), and (c) shear (e.g., San Andreas fault).

– Strain can be (a) plastic, (b) elastic (e.g., postglacial rebound) or (c) brittle.

Measuring the strain:Axis of Anticline

Axial Plane

Axis of Syncline

Limb

Axis of Anticline

Axial Plane

Axis of Syncline

Limb

― Based on the principle of original horizontality and the law of superposition of strata, undeformed layers lie horizontally and in the exact stratigraphic order of their formation.

― Strike and dip (or inclination) measure any departures from horizontality.

Folds:

Oil is lighter than water, and therefore rises towards

the surface, e.g., an anticlinal trap structure.

― Plastic deformation (i.e., rocks permanently bend under stress)typically creates folds. Anticlines (limbs inclined oppositely), synclines (limbs inclined towards one another), isoclines (limbs inclined alike), recumbent (horizontal fold-axis) and monoclines (one limb inclined) are the common fold structures: they can be regional (anticlinorium and synclinorium) or local, and may plunge doubly as well. Open and overturned folds also occur.

Fractures, joints and faults:Columnar jointing in the granites

at Joshua Tree National Park

― Brittle strain (i.e., when strain rate is too great to be accommodated by plastic strain) produces fractures, joints and faults.

― Joints are fractures without displacements: tensional stress tends to produce single joint sets perpendicular to stress direction, compressive stress usually produces two intersecting joint sets.

― Faults are defined by direction of slippage: (a) dip-slip faults have dominant slippage along the dip and can be normal (caused by tensional stress) and reverse (caused by compressive stress); (b) strike-slip faults have dominant slippage along the strike and can be right-lateral and left-lateral.

Wasatch fault in Utah is a typical normal fault whereas, as for reverse faulting, the magnitude 6.4 Northridge earthquake of January 17, 1994, ruptured a reverse fault that resulted from compression.

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― San Andreas Fault is a typical strike-slip (transform) fault. Notice the right offset of the stream channel, shown by the double arrow, in this USGS aerial photograph taken by R.D. Borchert looking WSW into San Luis Obispo County near Fellows, California.

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San Andreas Fault is a typical strike-slip (trans-form) fault. Notice the right offset of

the stream channel, shown by the double arrow, in this USGS aerial photograph taken

by R.D. Borchert looking WSW into San Luis Obispo County near Fellows,California.

Unconformities: denote hiatus in geological time; and can be (a) disconformities, (b) angular unconformities and (c) nonconformities.