Geomorphological Approaches to the Study of Neotectonics
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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 landforms 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 geomorphology of landforms elusive data, and used more refined analytical techniques. whose character is related to recent tectonics. 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 valley 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. geology (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 landform 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 Fault 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/335/4893096/gsjgs.143.2.0335.pdf by guest on 26 September 2021 336 DOORNKAMP J. C. 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/335/4893096/gsjgs.143.2.0335.pdf by guest on 26 September 2021 GEOMORPHOLOGICAL APPROACHES TO NEOTECTONICS 337 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