FAULTS

Faults crop out as linear features along the Earth's surface and usually provide an easily eroded zone between harder, unfaulted rock.  Thus they can localize narrow linear valleys such as the San Andreas "Rift" Zone, separating Pt. Reyes Peninsula to the west from mainland California to the east in the photo below.  At the northwest end this fault valley is flooded by the Pacific to form Tomales Bay, but fluvial drainage is obvious at the southeast end and a delta is apparent just south of the center of the valley.  (Photo courtesy of the USGS.)

  

 

Dip-slip faults involve vertical offset and so create an escarpment that generally persists even after much erosion.  The scarp resulting immediately from the offset is called a fault scarp as shown in the upper diagram.  The slope is identical with the dip direction and angle of the fault.  Minor erosion of the scarp begins immediately because the equilibrium profiles of streams that cross it have been disrupted by the offset.  They respond, of course, by downcutting V-shaped valleys that intersect the scarp in V-shaped notches, and by depositing the eroded material as fans at the toe of the scarp.  As long as the valleys have not been downcut to the level of the downthrown block there will be a vertical fall between the bed of the valley and the top of the fan.  Viewed straight on this looks like a stemmed glass and is called wineglass structure (though they actually look more like martini glasses).  Eventually downcutting reaches the level of the downthrown block and the uneroded fault scarp between pairs of V-shaped notches are called triangular facets.

After much erosional modification of the scarp its slope will no longer reflect either the dip angle or direction (or both) of the fault surface.  Such a feature is called a fault-line scarp.  In the upper diagram below the scarp still faces in the same direction as the fault dip, but the angle has been much modified.  This is a subsequent fault-line scarp.  In the lower diagram the scarp faces opposite the dip direction (this is a reverse fault) as an obsequent fault-line scarp.

The photo below (by David Carter) was taken southwestward from near the summit of Mt. Cammerer in the northwestern Great Smokey Mountains National Park.  The highlands to the left (east) are part of an extensive region of similar high rugged mountains of the "Blue Ridge"  The lowland to the right (west) is part of a similarly extensive region of low, linear ridges and intervening valleys of the "Valley and Ridge".  The scarp between is a fault-line scarp resulting from differential erosion of the two regions.  The Blue Ridge is underlain by durable crystalline rocks whereas the Valley and Ridge is underlain by comparatively softer sedimentary rocks.  The Blue Ridge has been thrust from the east (left) over the Valley and Ridge, so the fault system responsible dips in that direction.  But the scarp faces west (right), and so is obviously obsequent.  This topographic feature is easily recognizable from northwestern Georgia to southwestern Virginia as a continuous scarp of approximately this height (~1200 m or 4000 ft here).  The Front Ranges of the Rocky Mountains lie on a similar (but even higher) obsequent fault-line scarp.

   

CONTINUE

RETURN TO STRUCTURES PAGE

RETURN TO TOPICS PAGE

RETURN TO GEOMORPHOLOGY PAGE

RETURN TO DR. CARTER'S HOMEPAGE