Journal of the Geological Society, London, Vol. 143, 1986, pp. 335-342, 4 figs, 4 tables. Printed in Northern Ireland

Geomorphological approaches to the study of neotectonics

J. C. DOORNKAMP Department of Geography, The University, Nottingham NG7 2RD, UK

Abstract: The study of morphotectonics is concerned with the analysis of whose form or origins have been affected by neotectonic activity. Traditional morphotectonic studies have been used as a basis for more refined (e.g. statistical) analyses. After the 1960s, however, there emerged new techniques and new approaches to the study of morphotectonics. These have made more precise not only the recognition of morphotectonic features, but have also improved their dating. The time has come to integrate morphotectonic studies more fully both with the approaches used by other disciplines and with modern geomorphological theory.

Neotectonics is frequently associated with morphotectonics, traditional period but which pursued more subtle and more which is concerned with the of landforms elusive data, and used more refined analytical techniques. whose character is related to recent . Morphotec- Typical of these were the studies in Uganda where the early tonics can be sub-divided into two parts. One part centres models of rift formationand drainage reversal (to on structural activity resulting from isostatic adjustment formLake Victoria) defined by Wayland (1929, 1934a,b) since the Quaternary, and the other is more concerned with were elaborated by Doornkamp & Temple (1966). neotectonics which is not itself responsea to post- Statistical analysis of the warped rift valley shoulders Pleistoceneisostatic effects. Thisaccount is restricted to (Doornkamp 1972) revealed zones of warping much more neotectonics; topics relating to isostatic effects are discussed precisely than had hitherto been the case. Careful surveying in studies such as those by Andrews (1970) and by Morner of the raised beaches of Lake Victoria (Temple 1964) (1980). showed how early deformation was replaced by incision of Morphotectonics was a significant topic in many of the the Nile outlet as the controlling factor in determining the older (traditional)text books on geomorphologyand altitude of raised beaches and lake strandlines. (e.g. Holmes 1944; Cotton 1948; King 1951, 1962; This phase of morphotectonic studies may be exemplified Monkhouse 1954). Thereafter it was a somewhat neglected by reference to studies of warping onthe flanks of the topic, though some recent text-books have returned to the Western Rift Valley in Uganda. The simplistic model which subject(e.g. Ollier 1981) or include accounts of specific hadbeen established forthe area by the early 1960sis countries which are tectonically active, such as New Zealand showninFig. 1. Theplanated ‘landscapes’ had been (Soons & Selby 1982). recognized and mappedbetween the Western Rift Valley Despitethe many case studiesreferred to below, and Lake Victoria. Warping had been defined (as shown in morphotectonicsresearch is still grossly under-subscribed, Fig. 1) and affected both ‘landscapes’. Trend-surface and its full potential as aresearch topic remains to be analysis, along selected east-west sections, confirmed this realised.Evidence of this lies in the new directions of model and helped to refine it (Fig. 2). In particular this research which are beginning toemerge, including new analysis allowed a fairly precise definition of the nature and approaches tothe study of river systems,slopes, and position of the axes of warping (see map on Fig. 2). Such history in tectonically active areas. studies sought to elaborate and test earlier well known ideas on morphotectonics. Traditional morphotectonic studies An additional trend in following earlier ideas has been to quantify landform characteristics in the hope that these may The traditional, and some would say simplistic, morphotec- add to morphotectonic knowledge. Such was the approach tonic studies were largely concerned with the more obvious used by Bull & McFadden (1977) intheir analysis of the associations between landformsand geological structure. ages of tectonic activity along the Garlock in These are identified in Table 1. In many cases the landforms California. Theseauthors used amountain-front sinuosity themselves were taken as indicators of their structural origin index which is defined by: (as for examplein the case of rift valleys) evenbefore geophysical data were available to confirm the assumed link Lmf betweenform andstructure. Similarly, studies (typically S=-- Ls those in East Africa) associated drainage development with neotectonicswhenever river behaviour could not be explainedin any other way (e.g. Teale 1950). These where: Lmfis the length along the mountain-piedmont traditionalstudies were openly perceived as being junction; physiographically based (Hills 1961). Ls is the overall length of the mountain front. S approaches1.0 with increasing straightness,and straightness is used as an indication of recent uplift. Advances based on the traditional approach Increased sinuosity is seen to reflect the work of streams During the 1960s and 1970s studies appeared which relied which cross the mountain-plains boundary. upon themorphotectonic models defined during the Using this index Bull & McFadden (1977) provided the 335

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Table 1. Traditional style morphotectonic studies Class 3 Tectonically inactive S = 2.0-7.0 -associated landforms include pedimented MorphotectonicExamples criteria mountain fronts and embayments, steep slopes only on resistant strata, a few large and Fault scarps (incl. rift valleys) Cotton (1948) integrated valley systems in the mountains. Spur end facets Thornbury (1954) By using this approach Bull & McFadden recognized spatial Shutter ridges Cotton (1948) variationsin tectonic activity within the region of their Separation of river terraces Lensen (1968) Deformation of alluvial fans Bull (1964, 1977) study. Changes in strandline elevation Matsuda er al. (1978) Warping of planation surfaces Doornkamp &Temple (1966) Recent approaches to morphotectonics Formation of lakes Cotton (1948) Inrecent years there have emerged a number of new River reversal Wayland (1929) Changes in river pattern Teale (1950) approaches to the study of morphotectonics. These fall into Emerged coral reefs Bloom et al. (1974), three categories: Bender et al. (1979) 1. use of new techniques; 2. search for new types of data; 3. application to earthquake prediction.

following classification: New techniques Class 1 Active tectonism S = 1.2-1.6 The new techniques which are being used in morphotectonic -associated landformsinclude unentrenched (and neotectonic) studies are essentially more sophisticated alluvial fans, elongated drainage basins, narrow techniques of recordingcrustal deformation and surface valley floors, steep hill slopes. movements. Geodeticdata and repeated levelling were Class 2 Moderate to slightly active tectonism S = 1.8-3.4 described by many workers including Thurm et al. (1971), -associated landforms include entrenched allu- Vanicek & Nagy (1980), Kahle et al. (1980) and Brown et al. vial fans, large drainage basins, steep hillslopes, (1980). Leary et al. (1981) took a different approachand valley floors wider than their floodplains. used data based on the measurement of water levels along

A B Western Greaorv-~ - Rift Lake Rift

Westward-flowlng drainage on undeformedupland landscape ‘Back tilt ’ I comDonent

D Deviatlons reglster the warp component

Linear trend-surface / approximatesback-tllt component

Fig. 1. A simplistic model of rift deformations in south-west Uganda. A Pre-rift drainage to the Congo basin, B Rifting and surface warping, C Differential warpingof two planation surfaces, D Warping in relation to a linear trend-surface calculated throughthe height values of a deformed planation surface.

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Deviations Elevation feet A *.----. feet 1000 .. -. A’ - 6000 500 - --5500 0- -500 -

A - A’ Upper surface ( ‘landscape‘) B - B’ Lowersurface ( ‘landscape’ ) - Profile of deviations from the linear trendsurface ------Generalisedelevation profile of the‘landscape’ 4 v Inferredlocation of warping

UPLAND LOWLAND LANDSCAPE LANDSCAPE

UPWARP I - - - DOWNWARP o o m o

STEEPLY DEFORMED e 0 km 50 SURFACE

Fig. 2. The location of axes of warping in south-west Uganda through a trend-surface analysis of two planation surfaces (the “upper” and ‘‘lower’’ landscapes).

aqueducts. These new techniques are not providing data New types of data which are directly comparable with those produced by the earlier (traditional) studies. These techniques provide data New types of data being used in morphotectonic studies are about present-day neotectonics while the traditional studies listed in Table 2. They extend the range of morphological are concerned with the sum of effects over a longer period responses to tectonic activity beyondthose used in most of time(e.g. since the Miocene). Indeed, it is sometimes traditionalstudies. Singular amongstthese are studies of very difficult to match the rates of crustal movement that are river channel sinuosity and of slope morphology. being measured by these techniques with theamount of River sinuosity studies (such as those of Adams (1980) in elevation (or depression) of the landformsconcerned. the United States) have been used to identify surface tilting Measured rates of (presentday) uplift (e.g.in New by currentand recenttectonics. The conceptual link England) imply mountain building on the scale of the Alps. between sinuosity and tilting involves the analysis of the This clearly is notthe case, and requires much more river sloperequired to carry the sedimentload. Since theoretical analysis to show how current rates of meandering rivers tendto establish and maintain the deformation fit longer term (neotectonic) evolution. equilibrium gradient required to carry their sediment load,

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Table 2. New types of data used in morphotectonic studies illustratedwhat is possible by this approach. Wallace examined a series of slopes repeatedly affected by faulting, Displacement of man-made Rogers & Nason, 1971 and associated with datable features (such as 14C dated structures shorelines, volcanic ash of known age, dendrochronological Fractured cave structures Lange 1970 evidence), and examined changes in slopesteepness with Response of stream channels Cooke & Mortimer, 1971 age (Table 3). In addition, Wallace (1977) established Adams 1975 associated morphological criteria that can assist in the Downstream changes in river Adams 1980 recognition of multiple scarp displacement (by repeated sinuosity faulting), these include: Rates of sedimentation Lofgren & Rubin, 1975 1. pronounced breaks in slope on the scarp profile; Fluvio-glacial gravel Sharma et al., 1980 disposition 2. benches or terraces along channels that cut across the Vigour of plant growth Babcock 1971 scarp; Soil morphology and mineral Douglas 1980 3. changes in the steepness of channels that cut across alteration the scarp; Morphology of scarp slopes Palmer & Henyey 1971 4. scarprelative relief which exceeds the vertical Wallace 1977 movement associated with a single event; Analysis of tide gauge records Balling 1980 5. progressive displacement of older deposits more than younger ones. Some of these criteria are similar to those currently used in China for morphotectonicstudies (Doornkamp & Han any change in the gradient of the valley floor (e.g. induced Mukang 1985). by tilting) can be accommodated by the river througha This theme of slopedevelopment after faulting was change in sinuosity. Adams (1980) showed that an uplift of taken further by Bucknam & Anderson (1979) who carried 1 m at the upstream end of a 30 km river reach of slope out a quantitative analysis of fault scarps in western Utah. 10-4 rad (0.095 m/km) would increase the fall from 3 to 4 m In particular they established a set of relationships between and the sinuosity by 33%. Thus, where valleys are being scarp height and steepnessdepending on the age of the steepened by tilting the river can maintain a constant slope scarp (Fig. 3). From this it can be shown that (under the by becoming more sinuous and, convers,Ay, if tilting denudational conditions prevailing in western Utah) a scarp decreases the slope the channel tends to straighten. Adams 3 m high decreases in steepness from 28 to 10" in a period of (1980) carefully eliminatedman-induced channel changes 1000 years between 103 BP and 105 BP. Such observations from his study of the rivers Missouri, Kansas, Illinois and providea local model for predicting earlier (Quaternary) Mississippi. Hethen comparedmeandering changes with movements on otherwise un-dated slopes. These particular geodetic dataand showed that in most cases observationscannot be transferred toother environments behaviour (sinuosity) can be solely related to tilt with different rates of denudation. measurements. New types of data are also being acquired by appealing Thisstudy by Adams (1980) establisheda theoretical to other disciplines. Lichenometry, for example, features in model which may be applicable in other alluvial landscapes the work of Nikonov & Shebalina (1979), and biostratig- such as those of the Ganges in Bangladesh, or the Yangtze raphy in that of Baranova & Biske (1971). and Yellow Rivers in China. A study of sinuosity changes may provide significant ideas on tectonic deformation that could greatlv assist in hvdrocarbon exdoration. esueciallv earthquakeprediction v, ,I where solid rock outcrops or seismic data sources are poor Earthquake prediction has drawn on a range of sciences and and evidence of tectonic behaviour is scant. approaches from to a study of animal behaviour. Another new approach uses slope morphology as the basis Todate the role of morphotectonics in earthquake for elucidating tectonic events. The study by Wallace (1977) predictionhas been small. Morphotectonicstudies have

Table 3. Criteriafor recognizing fault generated slope characteristics (after Wallace 1977). (Fault scarps in Great Basin, USA).

Slope Unit Comment Unit Slope

Crest Produced by faulting. Rounded by weatheringand mass wasting. Becomes increasingly rounded especially after free face disappears. Very rounded after 10,000 years. Freeface Produced by faulting.Dominant for 100 years or so, but buried from below by weathering products. Disappears in about 2000 years. Debris slope Formed by weathering of free face. Slope of30-38". Dominant after 100 years and remains so for 100,000 years. Disappears after about 1 million years. Washslope Below debrisslope (3-W), may carry alluvial fans. Developed by 100 years, significant by 1000 years and dominant by 100,000 years.

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v , 11.9, I I ' I ""I 0.: 1 3 5 10 Scarp height (metres)

Fig. 3. A relationship between scarp height and scarp-slope steepness for slopes of different ages in western Utah (after Bucknam and Anderson, 1979).

concentrated on postdiction not prediction. They have been concerned with the study of landformsresulting from tectonics, not the prediction of tectonics. In China, however, Han Mukang and other workers (see Doornkamp & HanMukang 1985) have used morphotec- tonics as a basis for recognizing structural stress fields. For

an assessment of the morphotectonics east Of the Fig. 4. Structural relationships in theTangyin earthquake region, TaihangShan showed distinct faulting in Pliocene and China(after Doornkamp Han, 1985), younger-- deposits. These faults are believed to have formed as a result-of compressionalstress from theNE and SW in morphotectonicresearch is very complex. There are at (Fig. 4). South of Tangyin this has produced an incomplete least three reasons for this: graben, for although the eastern fault is complete that on 1. new data allow greater precision; the western side is not; though well expressed in the south it geomorphological theory has changed; fades away northwards, towards Tangyin, as a hinge fault. 2. 3. new approaches are emerging. Within the Tangyin areathere are deformationsand displacements apparent in the river terraces. Inaddition, there is variability in the elevation of the sedimentary New data boundary betweenHolocene floodplain and channel Since the coming of new datingtechniques (e.g. 14C, deposits, and in the elevation of synchronous archaeological U-series,amino acid racemization) more precise evidence features.Repeated levelling also reveals changes in has been gathering on the ages of landforms that have been landformaltitudes overrecent years. All of this involved in morphotectonics.Some examples are given in morphotectonicevidence indicates recent andcontinuing Table 4, and the topic is extensively discussed in Keller & structural activity. Indeed Han Mukang isof the view that Rockwell (1984). Precise dating techniques are beginning to this evidence indicates that there is residual structural stress reveal local variations in features such as scarp displacement in the western flank of the half graben, especially at its or terrace altitudes which indicate much more variability in northernend, and that this is boundto lead tofurther thedates of scarpdisplacement than is apparent from earthquake activity. traditional studies. On 1st October 1978, anearthquake (M = 3.9) took place along this line of stress. Further earthquakes here are likely. As with all morphotectonicstudies, however, Geomorphological theory Chinese scientists are able to predict the 'where', as in this Some of the more significant advances in geomorphological case at Tangyin, butnot the 'when'.Nevertheless, such theory are not yet reflected in morphotectonic studies. To analyses are being used in a positive sense, namely to help take but one example, river terraces are accepted, by most to decide where to place monitoring stations. workers in the neotectonic-morphotectonic field, as an indication that base level has fallen or that river incision is a response to or tilting. It has beenshown, Future research however, that river terraces can be created by changes in Whereas many would perceive morphotectonicstudies as sediment load without any external influence such as tilting simplistic recent structural geology, more recent research in or a fall in base level (Schumm 1979; Lewis 1944). Terrace geomorphology is in fact beginning to demonstrate that the formation (or, more precisely, cut and fill processes) may opposite is true. Far from being simplistic, the way forward also occur inresponse to humanintervention (Cooke &

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Table 4. Morphotectonic studies using precise dating techniques

Dating Technique Location Author(s) Feature

14c S California Fault scarp Clark et al. 1972 California San Andreas Fault Sieh 1978, 1981 Nevada, USA Lake shorelines Wallace 1977 California River terrace Rockwell et al. 1984 Japan River terraces Yoshikawa et al. 1981 California Fault scarps Lofgren & Rubin 1975 Uranium series New Guinea, Raised coral reefs Bloom et al. 1974 Barbados Barbados Raised coral reefs Bender et al. 1979 New Guinea Raised coral reefs Chappel 1974 California Marine terrace Lajoie et al. 1982 New Hebrides Neef & Veeh, 1977 Fission track Japan Faults Yasukawa et al. 1971 Kakuta 1976 Dendrochronology S. California Fault scarp Clark et al. 1972 Nevada Lake shorelines Wallace 1977 Palynology Venezuela Off-set lateral moraines Schubert 1982 Tephrochronology S California Fault scarp Sharp 1967, 1981 Sedimentation California, USA San Andreas Fault Sieh 1978 Venezuela Fault movement Schubert 1982 S Brazil Vertical movements Fulfaro & Suguio 1980 W eathering California Weathering San Andreas Fault Keller et al. 1982

Reeves 1976), or to variationsin rainfall, especially in InDenmark, lineamentstudies (Lykke-Andersen 1981) semi-arid lands (Cooke 1974). have shown that similar controls may exist on the character It is clearly necessary, before using terracedata in of glacial deposits and their subsequent faulting. morphotectonicstudies, tomake certain thatthe terraces An almost neglected topic within morphotectonic studies are in fact a direct result of neotectonic activity. is that of the persistence of neotectoniclandforms in a diagnostically distinguishable form.Despite the studies by Wallace (1977) on the changes in fault-scarpslope angles New approaches with age,and the recentadvent of more precise dating Recent geomorphologicalresearch on rates of denudation techniques, there has beenno integration within mor- show very high rates in areas of intensehuman pressure photectonicstudies of recenttheoretical developments in (e.g. Yellow River,China) and in areas of rapidrecent geomorphology. uplift (e.g. Himalayas). In the latter case such rates In particular, the ideasdeveloped by Brunsden & are only partly a response to that uplift for factors such as Thornes inseriesa of publications (e.g. Brunsden & intensity of rainfall and land use changes (especially forest Thornes 1979; Brunsden 1980) need to betested in the clearance)have a considerable influence on erosion rates context of morphotectonicresearch. For example, fault and river sediment loads. It is unlikely, therefore, that the scarp formation may be relatively rapid and, in the terms of neotectonic influence on erosion rates can be successfully Brunsden & Thornes, produce a ‘ramp input’ which disturbs disentangled from other causes. However, this approach can the general trend towards the development of a denudation be turned round the other way. Present day erosion rates controlled set of characteristiclandforms. However, the may be so high as to suggest that, man’s influence presence of the fault scarp may lead to changes from one notwithstanding, certain areas of hilly country would by now processdomain to another (e.g. landslides on steep scarp be laid low, unless tectonic uplift had been taking place. It is slopes, river incision back from the scarp edge). Landscape conceivable that the southern part of England falls into this response, and the persistence time of such geomorphological category. events will dependon the sensitivity of that terrain to Recent studies of satellite imagery have shown that change. Inthe case of river incision into and backwards major lineaments, probably of deep-seated origin, traverse fromthe faultscarp this is afunction of river network areas not normally thought of as being tectonically active at density, climate (i.e. runoff), and resistance to erosion. This the present time. Once again southern England comes into varies from areato area, andmeans that characteristic this category. These lineaments tendto suggest thatthe landforms of fault-scarp formation will persist for different geological crust consists of fault-bounded blocks, and that lengths of time in different places. This being so it would be these blocks may periodically ‘shuffle’ around inminor wrong to use steepness or straightness of the scarp slope as a adjustments to global conditions. If this is indeed the case basis for long-distance comparisons of the (relative) ages of thenthe relationshipbetween such blocks and the recent faulting. behaviour of river systems shouldbe carefully examined. Inareas of surface warping (such asin the Uganda Through river studies it may be possible to advance further example given above) the control on landform evolution has ideas on the neo-tectonic behaviour of the crust. continuedfor a long time as asteady (perhaps pulsed)

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perturbation. Until the period of river reversal, which may BUCKNAM,R. C. & ANDERSON,R. E. 1979. Estimation of fault-scarp ages have beenno earlier than the MiddlePleistocene, river from a scarp-height-slope-angle relationship. Geology, 7, 11-14. incision at least kept pace with uplift. This led to waves of BULL,W. B. 1964. Geomorphology of segmented alluvial fans in western Fresno County, California. U.S. GeologicalSurvey Professional Paper, landform modification transmitted along the drainage 352-3, 89-129. network,for in essence, warping doesmore to river - 1977. The alluvial fan environment. Progress in , 1, gradients than it does in any direct sense to hill slopes. An 222-70. examination of the down-valley variations in river channel -& MCFADDEN,L. D. 1977. Tectonic geomorphology north and south of the Garlockthe Fault, California. In: DOCHRING,D. 0. (ed.), slopes throughout south-west Uganda may therefore reveal Geomorphology in Arid Regions George Allen & Unwin,London, a greatdeal about the nature of warping,its magnitude, 115-38. frequency and spatial variability within the area. Signs of the CHAPPEL,J. 1974. Geology of coral terraces, Huon Peninsula, New Guinea: older events will be preserved in the least geomorphologi- Astudy of Quaternarytectonic movements and sea-level changes, cally sensitive parts of Uganda (e.g. Buhweju County, Bulletin of the Geological Society of America, 85, 553-70. SW CLARK,M. C., GANTZ, A. & RUBIN,M. 1972. Holocene activity of the Ankole) and not in the Inselberg lowlands (see Doornkamp CoyoteCreek fault as recorded in sediments of Lake Cahuilla, U.S. 1968). Geological Survey Professional Paper, 787, 112-30. A further topic of morphotectonics that may need new COOKE,R. U. 1974. The rainfall context of arroyos initiation in southern thinking is that of lake level changes (e.g. in Africa). While Arizona. Zeitschrift fur Geomorphologie, Suppl. 21, 63-75. - & MORTIMER, C., 1971. Geomorphological evidence of faulting in the such changes may indeed be related in partto climate southernAtacama Desert, Chile. Revue de Geomorphologie Dyna- (Street 198l),there is a close coincidence between the mique 20, 71-8. distribution of lakes and recently active tectonic areas. COOKE,R. U. & REEVES, R. W.1976. Arroyos and Environmental change in Tectonic influences needto bedisentangled from the the American Southwest, Oxford University Press. COTTON, C.A. 1948. Landscape 2nd Edn.,Cambridge University Press climatic, and amorphotectonic approach may help todo 509 PP. this. DOORNKAMP,J. C. 1968. Therole of inselbergs in the geomorphology of southern Uganda. Transactions of the Institute of British Geographers 44, 151-62. Conclusion DOORNKAMP,J. C. 1972. Trend-surface analysis of planation surfaces, with an Morphotectonic studies have received new stimulus from the EastAfrican case study. 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Received 1 May 1985; revised typescript accepted 30 August 1985

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