FOLDS AND FAULTS
(Dikes, Thrust Faults, Reverse Fault, Normal Fault, Fault Escarpment, Anticlinal Fold, Synclinal Fold, Slickensides)
DIKE CUT BY FAULTS
DESCRIPTION: A Dike is a sheet of rock that forms in a fracture of a pre-existing rocks.
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Figure 1 - An Igneous Dike intrudes the country rock and is subsequently offset in two places by near-horizontal faults. Nova Scotia. (Photo 1974)
THRUST / REVERSE FAULTS
DESCRIPTION: Reverse Faults are those in which the Hanging Wall has moved up with respect to the Foot Wall. A Thrust Fault is a low angle Reverse Fault. Due to the low angle, it is not uncommon for older rock to be shoved long distances OVER younger rock.
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PRE-DEVONIAN
DEVONIAN
CARBONIFEROUS
Figure 1 - Three Peaks of Chetamon in Jasper National Park
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K
M
NORMAL FAULTS
DESCRIPTION - Normal Faults are those in which the Hanging Wall slides down the inclined fault plane with respect to the Foot Wall. These type of faults are caused by Tensional forces pulling apart the earth.
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FAULT ESCARPMENTS
DESCRIPTION: A fault escarpment (or scarp) is a surface separating a down dropped from an upthrown block.
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Figure 1 - THE WASATCH FAULT ESCARPMENT. The Wasatch Fault runs parallel to the Wasatch Mountains to the east. It is actually a series of about 10 fault segments of which about 5 are active. The faults run about 220 miles between southeast Idaho to central Utah. The fault plane dips to the west. Normal faulting has resulted in mountains to the east of the fault and valleys of the Basin and Range Province to the west. Over the past 10,000 years, earthquakes of magnitude 7+ occur about every 1000 years.
FOLDS - ANTICLINES & SYNCLINES
DESCRIPTION: Folds are bends in the rock caused by compressional forces. Upfolds are referred to as Anticlines while downfolds are called Synclines. The very top of a fold is called its Crest. The bottom of a syncline is called a Trough. The sides of the folds are called Limbs. If the limbs dip in the same direction, the fold is said to be Overturned.
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Figure 1 - FOLDS IN THE OQUIRRH FORMATION, UT. Pennsylvanian. Found on US 89 between Provo and Ogden, Utah
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Figure 2 - FOLDS IN THE LODGEPOLE MEMBER OF THE MADISON FM. Mississippian. Laketown Canyon, UT
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Figure 3 - Overturned Anticline. Swift Fm. (UJ) over Swift Fm. (MJ). Found near Sun River Canyon in northwestern Montana. Red line marks the axis of the fold
SLICKENSIDES
DESCRIPTION: Slickensides are smooth, polished grooves on a fault surface caused by grinding of one rock surface on another during fault movement.
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Figure 1 - Slickensides of the Triassic Border Fault (Ramapo Fault Escarpment). New Jersey.