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Incoming links: Aggradation
The rate of surface modification controls how quickly a fault scarp is erased. Strong rainfall, runoff, slope failure and easily eroded lithologies can round or dissect a scarp; dry conditions and resistant rock often preserve it longer. Alluvial fan deposition and aggradation can bury a scarp even if erosion is slow. A young marker surface can preserve an offset that an older, strongly modified surface no longer records.
The balance between fault slip rate, recurrence interval and marker age controls whether renewed displacement outruns this modification. Rapid or repeated normal faulting can maintain a clear front, whereas a long interval after a small displacement permits smoothing. A very young fan deposited after the event may have no offset, while an older surface may retain cumulative rather than single-event fault throw. These two controls explain why absence of an obvious scarp is not proof of an inactive fault.
The geomorphology of active normal faulting records a competition between accumulated tectonic displacement and the ability of erosion, transport and deposition to modify it. A hanging wall moves down relative to a footwall, producing a fault scarp, mountain-front relief, and accommodation space for a sedimentary basin. Repeated earthquakes can build a large front even though each individual surface break is much smaller.[ref]
Normal-fault geomorphic features by United States Geological Survey. The primary mapping report's table of geomorphic features lists scarps, faceted spurs, oversteepened mountain fronts and rejuvenated valley floors as common normal-slip indicators.
Original sketches of tectonic, erosional and depositional controls on normal-fault landforms
. The panels distinguish cumulative block displacement, progressive scarp rounding, drainage and faceted mountain fronts, and burial of a scarp by sediment. Geometries and relative scales are illustrative. Details.
The principal controls and their observable effects are:
- Displacement, rate and timing. Larger single-event fault slip creates a higher fresh scarp; repeated slip builds cumulative offsets of fans and river terraces. The interval since the latest rupture controls degradation. For pure dip-slip faulting on a plane dipping , vertical fault throw is and horizontal fault heave is . Scarp height on an inclined eroded surface need not equal either the slip or the original vertical throw.
- Geometry and structural history. Fault dip, strike, curvature, segmentation, splays and along-strike variations in fault slip control scarp orientation and continuity. Linked faults can create a stepped front; synthetic and antithetic faults can create a graben. Cumulative deformation includes broad footwall uplift and hanging-wall tilting as well as a sharp break.
- Lithology and cover. Strong exposed bedrock can preserve steep scarps and fresh fault surfaces. Weak fractured rock, thick regolith and loose alluvium weather and slump more rapidly, spread deformation, and produce debris at the scarp base. Differential erosion can make a lithological front without recent tectonic movement, so a straight cliff alone is not sufficient evidence of activity.
- Climate and surface transport. Rainfall, runoff, frost action, vegetation and slope processes control scarp rounding and colluvium production. Steep new faces collapse towards stable slopes; later hillslope transport rounds the crest and fills the toe. A simple scarp diffusion model broadens a step over a length proportional to , but only if a suitable transport law and comparable material and climate apply.
- Drainage and sediment supply. Footwall uplift can incise channels and produce river terraces, knickpoints and triangular faceted spurs. Hanging-wall subsidence can pond drainage, deflect streams and promote alluvial fans and basin filling. Rapid aggradation can bury an active scarp, while stream incision can remove or exaggerate local relief.
- Preservation and initial landscape. Pre-existing slopes, relief, surface ages, glacial modification and human disturbance determine where offsets can be recognized. A fan abandoned long before the latest earthquake can accumulate more offset than a younger nearby surface. Compare like-age markers across a fault, and distinguish a mountain front's long history from a single event.
Useful observations therefore include scarp profiles, repeated offsets of dated surfaces, drainage organization, and continuity along strike. A high rugged front is not by itself evidence of a high present fault slip rate; conversely, a muted or buried scarp need not mark an inactive fault.