TECTONIC SCARPS AND FAULT VALLEYS

Abstract

Faulted and strongly deformed land surfaces are everywhere subject to rapid dissection; but bahadas or aprons of alluvium are not as commonly developed in association with tectonic scarps in humid regions as they are in the semiarid North American Great Basin. Undercutting by rivers leads on the other hand in some cases to considerable erosional modification of tectonic forms, so that the fault scarps of humid regions may come to bear but little resemblance to type forms as generally described and figured in textbooks. The Awatere fault-scarp type, as found in New Zealand, is contrasted with the Basin Range type. Scarps of the Awatere type are not fringed by bahadas, and in some cases they have been undercut by rivers. The category of tectonic scarps includes monoclinal as well as fault scarps and also combinations of monoclinal with fault features. Many fault-line scarps also are tectonic, but only such of these as have been exposed by erosion in a first postfaulting cycle. The facets of a dissected tectonic scarp, though they may be very flat and strikingly coplanar, rarely preserve actual fault surfaces. They are land-forms of normal development, usually graded, with a straight base line. Fault valleys, as distinguished from purely erosional fault-line valleys, are common and are occupied by postfaulting consequent rivers. Asymmetric furrows formed initially between inclined block surfaces and the scarps of a fault-block complex may guide drainage, and the valleys in such major fault angles are in many cases long-lived. Relatively minor fault-angle valleys are formed in districts with considerable prefaulting relief only when fault scarps develop along the lines of pre-existing rivers; this is not unusual if some rivers are already fault-guided, flowing perhaps in fault angles of earlier formation or in fault-line erosional valleys. Other consequent valleys may have been formed concurrently with a long series of intermittent lateral (transcurrent) movements, and besides these there must be some which are walled on both sides by fault scarps related to a single fault or fault zone. Where reversal of movement has taken place in successive faultings, opposed scarps may be present. Reversal of the dip-slip movement on steeply inclined fault planes is not uncommon among ancient faults, and reversal in very recent times has taken place on active faults in New Zealand. If movement in the new sense continues, trenches with the dimensions of valleys will be formed, potential fault valleys of an opposed-scarp type.