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What Is a Fault Scarp？

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What Is a Fault Scarp？ 2 5 6 勿 lain S . Stew art and Paul L . H ancock W hat is a fault scarp? slip . The term "fault scarp" is used by som e w orkers to describe only fresh scarps, w hereas other w orkers, particularly those in tectonically inactive terranes such as the U K , apply it to degraded scarps or even faultline scarps. This article review s the contrasting structural and m orphological characteristics of fault scarps that w ere form ed during recent tectonic activity, and w e suggest som e defi nitions of key term s. F ault-scarp studies in。study of young normal-fault scarps cutting Quaternary sediments in the B asin and R ange province of the w estern U SA , W allace (1977) proposed that fault scarps possessing a steep, angular m orphology w ere generally younger than scarps having a gentle. rounded m orphol- ogy. This assessm ent w as form alized later by B ucknam and A nderson (1979), w ho dem onstrated em pirically that the m axim um scarp angle decreases w ith age and increases w ith scarp height. T he recognition that certain elem ents of scarp profiles, such as scarp angle and scarp height, represent tim e-dependent criteria has allow ed contrasting fault- scarp populations to be discrim inated and to have broad age categories assigned to them (for exam ple, H olocene, late Pleistocene). M ore recently, fault-scarp studies have attem pted to sim ulate m athem atically the degradation of fault scarps in order to provide absolute age esti- m ates (N ash, 1986). In addition to p r o v iding inform ation on the tim ing of tectonic activity w ithin a region, fault-scarp studies have revealed m uch infor- m ation concerning the spatial and temporal distribution of the faulting itselt. L ateral changes in fault-scarp m orphology along a fault zone, for exam ple, are interpreted com m only as the surface expression of the deeper level segm entation of the fault zone (see C rone and H aller, 1991）．D etailed investigations of historical fault breaks and fault-scarp stratigraphy have show n that m any active faults rupture at relatively regular intervals, that they have sim ilar m agnitudes, and that they rupture along traces roughly coincident w ith those of previous events. Such studies form the basis of existing m odels of fault behavior. Fault scarps and fault-generated la n d fo rm s In tro d u ctio n A fault scarp is defined here。、。tectonic landform coincident. or roughly coincident, w ith a fault plane that has dislocated the ground 葬surface. Figure I illustrates exam ples of sim ple scarps related to single T he need：。augment the instrumental and historical record of tectonic increm ents of slip. D ifferent styles of fault m otion create contrasting activity has, in the last decade, focused attention on palcoseism olo- types of scarps, although som e fault-generated landform s are m orpho- gy一the identifi cation and study of prehistoric earthquakes as deter- logically indistinguishable from one another. N orm al-fault scarps are m ined from the geological and geom orphological record. O f great generally the easiest to detect, even w here they are not related to the im portance has been the recognition that the characteristics of fault- prim ary fault on w hich m otion occurred . For exam ple, during the 1980 generated landforins, such as fault scarps, can be sensitive indicators El A snam (A lgeria) m agnitude 7.3 earthquake, the m ost prom inent of the style and tim ing of tectonic activity w ithin a region (W allace, fault-generated landform s w ere norm al-fault scarps that form ed as a 1977). In tectonic geom orphology, the term "fault scarp" has been result of secondary stretching above a thrust-related topographic flex- applied to a w ide range of scarps that are m ore or less coincident w ith ure (fig. 2) (K ing and V ita-Finzi, 198 1）．C onversely. reverse-fault a fault plane on w hich recent m otion has taken place. Such scarps scarps form ed during the sam e event w ere variable in m orphology and range from sm all, ephem eral slopes created by a single increm ent of laterally discontinuous. In contrast to norm al- and reverse-fault scarps displacem ent to high bedrock escarpm ents that form ed during repeated that are best displayed w here a nearly level surface is offset, scarps D ecem b er 1990 伪 ‘ 井 5 E N W norm al-fault scarp te nsion a l fi ssu re reverse-fault scarp }*- - Zi5 妊之李乡 土 泛主兰甲州 涉5夕 夔理贫李纂拜、、 从二户扭三裕呀 ～共丈于了二士一几牛梦纷、、 、 ，～ 0 m 500 L es － I F igure 2一 Structure of the south-central p art of the fault zone active during the 1980 E l A snam (A lg eria) earthquake. A lthough H IG H -A N G L E the m ain thrust w as expressed as a series of discontinuous, poorly develop ed reverse一ult scarps, stretching (open arrows) in the R E V E R S E hanging w all of the thrust resulted in the form ation of clearly exp ressed norm al-fault scarp s and tensional fi ssures. R edraw n F A U L T from P hilip and M eg hraoui (1983j ig . 15). S C A R P . scarps (fi g. 3C ). The term inology of these features, how ever, is con- fused by references to m ultiple scarps as "branched fault scarps" (C otton, 1949) and by references to com posite scarps as "m ultiple- event scarps" (for exam ple, M ayer. 1984 ), "m ultiple offset scarps" (Petersen, 1985), and "rejuvenated scarps" (M cC alpin, 1987). In order to describe the situation w here displacem ent is distributed across a num ber of overlapping en echelon fault strands. C otton (1949) coined the term "splintered scarp" (fi g. 3D ). Scarps form ed by norm al faultin g a re located com m only along preexisting faults that are coincident w ith the contact betw een bedrock in the footw all and Q uaternary colluvial or alluvial sedim ent in the hanging w all (C rone and others, 1987). In som e faulting events how ever, the upw ard-propagating fault steepens tow ard the surface (H ancock and B arka, 1987) and form s fresh, steeply inclined to over- hanging scarps in colluvium or alluvium . Such a tendency m ay lead to reverse faults being expressed as apparent norm al-fault scarps and vice versa (Philip and M eghraoui, 1983). For exam ple, the directions of opening vectors that w ere constructed from apparent reverse-fault scarps form ed during the 1978 T hessaloniki (northern G reece) m agni- tude 6.1 earthquake show ed extension. T hese directions indicated a slip vector com parable to that displayed by striae on bedrock norm al faults reactivated during the sam e faulting event (fig . 4) (M ercier and F 讼are I一 Sim p le scarp s related to single increm ents others, 1983) of slip on a new ly prop agated fault. M odified from 刀ancock (1988）欢．刃． T he shap e s of fault scarps also are strongly infl uenced by the properties of the displaced rocks. G ordon （197 1），for exam ple, docu- m ented eight m ain fault-scarp types that accom panied reverse faulting associated w ith strike-slip faulting are detected m ost readily w here the during the 1968 M eckering (A ustralia) m agnitude 6.9 earthquake. fault trace crosses sloping ground (fig. 1). W here the near-surface m aterial w as strong, the reverse faulting oc- The initial m orphology of a fault scarp a ls o is strongly influenced curred on a single slip plane to form a sim ple overhanging scarp that by the style of near-surface deform ation. Studies of norm al-fault scarps collapsed or w as eroded back (fig. 1). W here the surface m aterial w as in the w estern U SA have focused prim arily on piedm ont scarps一sm all w eak, how ever. internal w arping resulted in the form ation of a topo- faults cutting poorly consolidated Q uaternary sedim ent deposited at the graphic flexure, here called a fold-lim b scarp (tig. 5). Such scarps also base of range fronts (G ilbert, 1928, p. 33). W here a piedm ont scarp is have been referred to as "m onoclinal scarps" (C otton , 1950) and "fold the product of a single increm ent of m otion, it com m only displays a scarps' (M atsuda and others, 1989). A lthough these scarps are com - characteristic m orphology (fi g. 3A ) (W allace, 1977) com prising a parable in m orphology to norm al-fault scarps. they differ from true steep (> 50') free face, a m oderately inclined (30'-40') debris slope, fault scarps in that the scarp face corresponds to a dip slope (that is, a and a gently inclined (5'- 10') w ash slope. M ore com m only, how ever, tilted or rotated erosional or depositional slope) (T hom bury, 1964). O n the m orphology of piedm ont scarps is com plicated either by near- a larger scale, fold-lim b scarps are interpreted as the surface expression surface splay faults. w hich produce m ultiple scarps (fig. 3B ), or by of blind thrusts, ahead of w hich deform ation is accom m odated bN repeated displacem ents along the sam e fault, w hich form com posite fault-propagation folds (Stein and Y eats, 1989 ).
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