Geografie 1966, 71, 115-141

Neotectonics in Geomorphology

Ludvík Loyda

Výzkumný ústav geodetický, topografický a kartografický, Dražického nám. 7, Praha-Malá Strana, Czechia

In geology and geomorphology vertical movements of the earth's crust have been commonly known for a long time. Horsts, faults, overthrusts, antecedences, subsiding and upheaving shores, uplifts and subsidences of the erosion base, etc. are common phenomena. There are several hypotheses trying to elucidate the origin of these movements but none of them is satisfactory. According to Penck and Davis tectonic movements of blocks, and the subsequent revival of erosion, are natural phenomena in geomorphology which, however, does not elucidate their origin. The opinion that deflation and sedimentation might change the Earth's balance was pronounced as early as in the 15th cent. by Leonardo da Vinci, and has been acknowledged ever since. Later, gravimetric measurements resulted in the framing of a hypothesis on the isostasy which has been taken for natural up to the present. Local gravimetric measurements serve the purpose of determining the underground structure of the area in question, and the inner mechanics of its development. From the computed thickness of the Earth's crust the direction and the magnitude of future tectonic movements may be forecast. For instance, the Kolchis and the Caspian Lowlands are to be uplifted in future to the altitude of 1500 m (Gzovskij, 1963). Also the comparatively rapid uplift of the Fennoscandia is in fact the confirmation of an identical glaciostatic assumption. The uplift most probably culminated at the time of the complete melting of the glacier, and has been getting smaller ever since. Under the Fennoscandia gravimetry has discovered a considerable lack of mass - 7times less than under the Ukraine (Maazik, 1960). It is presumed that this difference is going to be balanced and the Fennoscandia strongly uplifted in future. As far as the opinion of a future uplift is concerned, gravimetry strongly opposes isostasy although they ought to supplement each other. Another imperfection of the isostatic assumption is the schematic conception of the density of rocks. Continental average density makes, 2,67 g/cm3, which, of course, can vary even by 10 % in different places. Even not the assumed weight of the continental glacier at the time of the greatest Pleistocene glaciation of the Fennoscandia and North America has achieved the extend of this difference. And at the same time the whole idea of glacioisostasy is based upon the very temporary load of the continent. The hypothesis of isostatic movements and their origin assumes continuous movement of blocks going on during a balanced state of the Earth's crust. Isostatic balance would be possible only under certain conditions, i. g. in case of a symmetrical form and rotation of the Earth, a symmetrical distribution of rock mass according to the density, and symmetrical earth tides and planetary forces. None of these assumptions, however, has been fulfilled. The ignorance or overlooking of these very factors - which can influence the movements in the earth's crust before all - lead necessarily to the fact that the theory of the isostasy was based upon the assumption of fluctuating blocks loaded with sediments or ice, and upon the assumption of an uplift of the deflation area. If we admit, however, that the movements of the blocks are of endogenous or planetary origin, then deflation and sedimentation are no more the cause but the result of these movements. And just the very misplacement of the result for the cause resulted in a common belief that the origin of the movement of the blocks was thus elucidated. Also the theory of the geosynclines, and consequently the whole orogenesis is based upon the load of sediments. It is a common belief that every geosyncline must necessarily for isostatic reasons appear at a later stage of its development in the form of a mountain range. The question is whether the syncline really subsides under the load of deposited sediments. This assumption is only supported by long and deep channels existing along the margins of the continents. Also the subsidence of the Hungarian Lowlands - filled with sediments deposited by large rivers - confirms this idea. On the other hand, however, the Marianne Channel cannot be presumed to have subsided under the load of sediments deposited here from the neighbouring small islands. Or, the Romanche Channel in the middle of the Atlantic, etc. In these cases it is out of question that the load of the sediments should cause the subsidence. If it is possible in these cases to ignore the weight of the sediments, it may be presumed that also other channels, deflections, etc. have subsided for some other reasons. At the same time it is natural that sedimentation should go on in these areas - of course only as a result of their subsided position. The development of geosynclines according to Grabau (1924) - and most recently also according to Hsu (1965) has achieved the form of migration due to lateral changes in the density of rocks, and does not agree with the general idea of the development in periods of evolution and revolution (Fig. 1). A similar migration of the sedimentation basin was ascertained by Pannekoek in the Tertiary on the territory of the Netherlands (Fig. 2). Cuvier's original idea of catastrophism has achieved a milder form recently: catastrophes have become periods of revolution and the calm intervening periods are periods of evolution. This scheme of alternating periods of evolution and revolution must be based, however, upon a thorough knowledge of the whole development of the Eearth and all forces that may influence it. If also other factors than classical endogenous and exogenous forces - such as the changing position of poles, changes in rotation, earth tides, etc. - exert their influence upon the Earth then also the changes in the development of the geosyncline are not due to the increase in the thickness of sediments and the increase of pressure and temperature, but are simply due to the effects of new, periodic, non-geological factors, i. e. are due to the changed rotation of the Earth, etc. A new view of the tectonic evolution of the earth's surface and the whole earth's crust may be given by the new planetary hypothesis based upon the changes of the rotation speed of the Earth. According to this hypothesis the commonly known regularities in directions of tectonic lines (NW - SE and NE - SW) presume also a certain order of origin (Fig. 3). Stovas and Čebanenko see this order in the changes of the rotation speed of the Earth. The surface crust is connected to the main body of the earth by a plastic layer. The earth's body controls the acceleration or the slowing down of the rotation. Acceleration results in maximum deformation at the equator and the 62nd parallel. Smallest deformation occurs at the poles and the 35th parallel. A contrary situation occurs during the slowing down of the rotation speed (Fig. 4). At a rapid change in the rotation speed, the litosphere slides against the direction of acceleration or retardation of the rotation speed of the nucleus (Fig. 5). Due to this belated adaptation of the earth's crust to the changes in the rotation speed, a regular system of intersecting fault lines arises in the litosphere (Fig. 7, 6). When comparing this diagram with the known systems of faults, a surprising coincidence appears in their course (Fig. 3, 7). At the change in the rotation speed not only horizontal but also vertical movements of blocks take place. The basis for this phenomenon is given by the subsidence and the uplift of opposite margins of individual blocks. Čebanenko also quotes Chabakov (1949) and Katterfeld (1959) moth of whom studied the course of tectonic deformations and linear structures on the surface of the Mars and the Moon. On the Moon another main line N - S occurs besides the NE - SW and NW - SE directions. 98 % of all ascertained linear structure keep to these three main directions. On the Mars prevailing directions are 315°, 75° and 45-55° which correspond approximately to the principal directions of fault lines on the Earth. The development of the surface of the earth is also influenced by earth tides which reach the amplitude of several decimeters (Simonsen, 1965) and most probably cause earthquakes as well. Tamrazjan (1957) discovered the dependence of earthquakes upon individual phases of the Moon, and, at the same time (1962) a certain time coincidence between the intensification of the Sun's activity and the weakening of the activity of mud volcanos (also Lilienberg, 1963). The dependence of earthquakes upon earth tides has also been proved by Greve (1956) in Chile where the majority of large earthquakes occur in the morning or between 4-6 o'clock p. m. Consequently, it was no chance that the strongest earthquake in Czechoslovakia in 1763 - which destroyed the town of Komárno - took place at about 5 o'clock a. m. And the recent earthquake in Skoplje destroyed the town at about half past five a. m. Permanent recent vertical movements most probably take place in the whole solid surface of the Earth. They were discovered by repeated level controls. Newly compiled maps of isobases show that these movements do not adapt themselves to the form of the earth's relief or the rock structure. For instance, on the map of Bulgaria isobases run across the mountain ridge Stara Planina (Balkan Mts.) in the direction NE - SW, i. e. precisely according to the new rotation hypothesis (Fig. 8). Vertical movements of the shore line can be ascertained by the measurement of fluctuations of the sea surface. Richter (1963) introduces examples of measurements of vertical movements of blocks carried out in close neighbourhood of the coast. Measuring instruments situated in shallow water recorded the fluctuations of blocks (amplitude up to 6 cm = Stockholm, Naples, Baku). In Mäntyluoto, Finland, stream gauges placed on a single block did not record any movement (Fig. 9). The migration of geosyncline on Fig. 1 may at the present be explained in a simpler and more probable manner. Between the area of uplift and subsidence the occurrence of faults must be presumed, and the movement of blocks will be then clearly elucidated by the new Čebanenko's rotation hypothesis. According to the present knowledge the only agent modelling the surface of the earth are exogenous forces. Tectonic movements are presumed only in cases to which no other explanation may be applied. Up to the present nothing has been known about any permanent movement which would be due to the earth tides and the changes in the rotation speed. As soon as this movement was ascertained by repeated level controls, an investigation was carried out to determine whether it would be possible to ascertain present as well as recent vertical movements of the earth's surface by geomorphological methods. Gorelov (1961) compared the activity of rivers in two different territories: in the upheaving Stavropol Highlands and the subsiding Azov Cuba Lowlands. In the former area river beds are narrow and graded, incised usually some 5-7 m. The floor is either covered with a thin deposit of sand and gravel (1-5 m) or the rocky substratum is exposed. In the Azov-Cuba Lowlands wide river valleys are cut down into fine grained alluvial sediments of an average thickness of 10-15 m. In the former area sediments display the character of coarser river deposits whereas in the latter they are of the flood soil character. Besides this, graded surface of river plains has been also observed in the uplifted areas. Young and recent vertical movements of the Earth's crust have been unmistakably proved by repeated level controls and analyses of river plains. The best indicators of more recent vertical movements are the streams of rivers. By the analysis of the longitudinal profile of the river, river plain upheavals and subsidences may be discovered besides the thickness and facial structure of the terraces. Changes in the gradient of a river bed may be due to the change in hardness of the substratum or due to the change of the water volume at the confluence with another stream. If, however, the inflection point in the longitudinal profile has been due to none of the above reasons, the existence of an uplift or subsidence might be considered. Setunskaja (1962) compared on a map of the Ukraine the gradients of rivers with the rate of recent vertical movements (measured by repeated level controls - Fig. 10). The map demonstrates the relation of uplifts to the increase in the river gradient. Vertical tectonic movements may also manifest themselves in a different manner. In case of a longer uplift or subsidence of an area migration of rivers, their captures or antecedences may occur. During these movements river terraces either diverge or converge, or even may submerge under younger deposits. Besides river beds also deformations of shore lines and displacements of water-borne and wind-borne deposits are indicators of tectonic movements. Vertical movements influence the revival as well as the extinction of the karst, the origin of landslides, the disintegration of dejection cones, the origin of buried reliefs, the pediments, the enlargement or loamification of talus deposits, etc. Soviet geomorphologists characterize uplifting areas as follows: 1. by sudden changes in velocities of river streams, 2. by coarse-grained composition of slip-off slopes and shoal heads, 3. by pinching-out river terraces, 4. by the narrowing of river valleys, 5. by the development of ovrages, 6. by the origin of erosion and erosion-accumulation terraces, 7. by abnormally high terraces, 8. by a young cutting down of rivers. Characteristic features of subsiding areas are: 1. poorly dissected region, 2. flat and marshy water-divides, 3. overdeepened valleys, 4. wide valleys with law water logged fload plains, 5. intense meandering of rivers and numerous cut-aff meanders. This method of investigation may easily be used on a large scale. It serves as basis far a further study of tectonic movements and the accompanying phenomena. Geomorphology finally gets into the position when by ascertaining the manifestations of recent as well as present vertical tectonic movements, and by the study of their character, it may contribute to the elucidation of the development of the whole earth's crust.