A Study On The Geomorphologic Indexes In Active Tectonics Based On DEM Data

Tectonic geomorphology is the landscape that is controlled by tectonic processes and established by endogenic along with exogenic processes. Tectonic geomorphology has experienced two stages: the static tectonic geomorphology in the early period and the dynamic tectonic geomorphology after 1960s. After 1980s, many researchers emphasize that tectonic geomorphology mainly is the one controlled by neotectonics. With the study on active tectonics in recent twenty years, active tectonic geomorphology, especially tectonic geomorphology related with the active faults, has already attracted much attention of researchers and has become a new research field. Tectonic geomorphology, in essence, is to study the deformation and dislocation of a variety of landscapes with the aim to reveal the dynamics in the earth. Among the main analytical methods of tectonic geomorphology, the morphological analysis is the most basic and the most commonly used one. And the morphology of the landscape can be described quantitatively by many geomophologic indexes, thus, more and more studies on tectonic geomorphology are made through a series of geomophologic indexes as the high-resolution DEM data has been developed in recent years. Therefore, this progress makes the tectonic geomorphology to a new stage.This thesis attempts to carry out a study on tectonic geomorphology on the large-scale region -- the Ordos Block and surrounding areas and the small-scale region– the Huoshan Mts. piedmont fault area with geomophologic indexes. Two types of geomophologic indexes are selected in this work. One is the regional indexes, and the other is the watershed indexes. Fractal parameters which are the regional indexes can be used to describe all kinds of landscapes with different scales. This thesis adopts the fractal parameters to characterize the fractal features of the Ordos Block and surrounding areas as well as the Huoshan Mts. piedmont fault area. On the basis of the fractal features of the two study areas above, the topographic significance, geological significance and tectonic significance of the fractal parameters are discussed. According to the dip-slip movement of the Huoshan Mts. piedmont fault and the characteristics of gullies across the Huoshan Mts. piedmont fault, this thesis adopts the watershed indexes to characterize the responses of the gullies to the tectonic movement. The watershed indexes using in this thesis include the mathematical functions fitting the longitudinal profiles of the gullies, the slope-area (S-A) plot, the concavity index and the steepness index from the stream-power incision model, the stream length-gradient index (SL index) and Hack profile.1. Fractal parameters of the landscape and the geomophologic zonationFractal theory provides a quantitatively descriptive way and a mathematical model to the phenomenon, of which morphology, distribution or structure is extremely irregular but self-similar. Fractal geometry describes the overall characteristics of the earth surface in a comprehensive and scientific way. From many methods available for calculating the fractal parameters, the variogram method is selected in this thesis. The variogram method has proved to be a more intuitive and better way to measure fractal dimensions of landscapes which are self-affine and can be used with the grid-based DEM data. The cellular model which is carried out in a moving window operation can be used to reveal the variation of the fractal feature over landscapes. The DEM data with the spatial resolution of 90 m and 2.5 m are employed to the Ordos Block and surrounding areas, and the Huoshan Mts. piedmont fault area, respectively. Such fractal parameters as the fractal dimension (D) and the ordinate-intercept (γ) of the Ordos Block and surrounding areas and the Huoshan Mts. piedmont fault area are obtained using the variogram method combined with the cellular model. The results show that in both the Ordos Block and surrounding areas and the Huoshan Mts. piedmont fault area, the distributions of the fractal parameter are region-dependent apparently. The zonation results achieved by K-Means clustering algorithm reveal that the quantitative geomorphic zones defined by the fractal parameters are consistent well with the landscape types, lithology and structural features in the both study areas, and each zone has characteristic fractal values.There are five fractal zones defined by the fractal dimension and the ordinate-intercept comprehensively in the Ordos Block and surrounding areas. The Hetao Basin and the Yinchuan Basin (Zone Aa) are characterized by high fractal dimensions and low ordinate-intercepts (D = 2.8-3.0,γ= 0-1) in contrast to the mountainous areas including the Lvliang Mts., the Helan Mts. and Wulaer Mts. (Zone Cc) where low fractal dimensions and high ordinate-intercepts (D = 2.4-2.6,γ= 2.5-3.5) are found. The other zones are defined by the following fractal parameters, respectively: the North Shaanxi Loess Plateau in the southern part of the Ordos Block (Zone Bc), D = 2.5-2.7 andγ= 2.5-3.5; the Kubuqi Desert and the Mu Us Desert in the northern part of the Ordos Block (Zone Db), D = 2.2-2.4 andγ= 0-1; the Shanxi Graben System and the Weihe Basin (Zone Dd), D = 2.2-2.4 and no predominant ordinate-intercept values.Similarly, there are five fractal zones defined by the fractal dimension in the Huoshan Mts. piedmont fault area. The northern and central sections of the Huoshan Mts. (Zone A) are characterized by low fractal dimensions of 2.1-2.3. The fractal dimensions in the southern section of the Huoshan Mts. (Zone B) are higher than that in the northern and central sections and mainly in the range of 2.2-2.4. The loess mesas along the Fenhe River (Zone C) have the intermediate fractal dimensions of 2.3 -2.5. The fractal dimensions increase from the back part to the frontal part of the alluvial fans. The fractal dimensions in the back parts of alluvial fans (Zone D) fall mainly in the range of 2.4-2.6. The frontal parts of alluvial fans and the flat area of the Fenhe River have the highest fractal dimensions of 2.6-2.7.Based on the fractal features of the Ordos Block and surrounding areas and the Huoshan Mts. piedmont fault area, the topographic significance of the fractal parameters are discussed with the perspective of signal waves. The fractal dimension reveals the frequency of variation in elevation of the landscape and the ordinate-intercept value reflects the amplitude of the relief. High fractal dimension implies a high frequency of variation in elevation, while a low fractal dimension implies a low frequency of variation in elevation. The high ordinate-intercept value reflects the high amplitude of the relief, while the slow one reflects the small amplitude of the relief. The relationships between the fractal dimension and the strata and geological processes are analyzed to reveal the geological significance and tectonic significance of the fractal dimension. The tectonics can bring the information into the topography at long wavelengths, thus tending to make the fractal dimension low. Erosional process roughens the earth surface and can make the fractal dimension high. The lithology is an important factor affecting the erosional roughening, therefore, the fractal dimension is related to lithology closely. The diffusive process and depositional process make the earth surface smooth and the fractal dimension trends to be low.2. Morphological indexes of longitudinal profiles of gullies and their responses to the tectonic movementThis thesis analyzes several morphological indexes about the longitudinal profiles of 64 gullies across the Huoshan Mts. piedmont fault to characterize the responses of the gullies the tectonic movement. These indexes include the mathematical functions fitting to the longitudinal profiles of the gullies, the slope-area (S-A) plot, the concavity index and the steepness index from the stream-power incision model, the stream length-gradient index (SL index) and Hack profile.Longitudinal profiles of the gullies in different tectonic settings and different developmental stages show different morphological forms which can be fitted by such mathematical functions as the linear function, the exponential function, the logarithmic function and the power function. The longitudinal profiles of the gullies across the Huoshan Mts. piedmont fault are fitted best by the linear forms or the exponential forms with slightly concave shape. After analyzing such main factors affecting the morphological forms of the gully longitudinal profiles as the time, the climate, the rock of the riverbed and the tectonic movement, this thesis concludes that the linear forms or the exponential forms with very small concavity on the gully longitudinal profiles show the very young gullies are caused mainly by the strong activities of the Huoshan Mts. piedmont fault since Pliocene. The characteristics of the gully longitudinal profiles above exhibit the response of the gullies to the uplift of the fault.The stream-power incision model describes the relationship between the tectonic uplift and fluvial incision. When the bedrock uplift rate and the fluvial incision rate are kept in equilibrium, the S-A plot obtained from the stream-power incision model presents a straight line. When the bedrock uplift rate is higher than the fluvial incision rate, the S-A plot exhibits a convex form. On the contrary, the S-A plot shows a concave form in the situation that the bedrock uplift rate is smaller than the fluvial incision rate. All of the S-A plots of 64 gullies across the Huoshan Mts. piedmont fault present a convex form, which is the response of the gullies to the strong uplift of the fault. The average value of the concavity index (θ) of 64 gullies is 0.223, which is much smaller than the empirical value (0.49) of the rivers in steady state. The very small value of the concavity index indicates that the response of the uplift of the fault is stronger than the fluvial incision. The steepness values of the gullies rising from the Laoyeding Mt. are highest, while they are lower in the gullies rising from the northern mountains and southern mountains. Moreover, the steepness values of the gullies in the northern mountains are higher than that of the gullies in the southern mountains. The distribution of the steepness values described above corresponds to the difference in the uplift rates of the Huoshan Mts. The Hack profile which is a semi-logarithmic plot is usually used to depict the overall adjustment of the river longitudinal profile to tectonic movement, while the stream length-gradient index (SL index) is used to describe the changes on the local slope of the river longitudinal profile. All of Hack profiles of 64 gullies are featured by the convex curves, which are the characteristic response to the uplift of the fault. In addition, some of Hack profiles have an inflexion in the position of the fault. On these Hack profiles, the upstream sections have the convex shapes, while the downstream sections are presented as the early stage of re-establishment of grade equilibrium after the fault displacement. In the analysis of SL index, there are extremely high SL / k' values appearing in the reaches near to the fault, which implies the knickpoints caused by the fault movement exist.All the features of the morphological indexes of the gulliy longitudinal profiles described above show the responses of the gullies to the fault movement. Each fault movement in vertical direction can be recorded by the knickpoints, therefore, it is a good way to know the fault movement event through the knickpoints. The knickpoints are identified in 23 gullies. Some knickpoints are testified by the field data. There are 7 gullies with one knickpoint. This kind of knickpoints, of which the heights range from 6-9 m lie on the fault, which implies that there are several knickpoints superposed together and the knickpoints have not migrated upstream. There are 16 gullies with 2-3 knickpoints whose heights range between 2 and 6 m. The latest knickpoints have been migrated upstream to the distance of 40-70 m from the fault. The knickpoints of intermediate ages are at a distance of 150-150 m upstream from the fault and the oldest ones at a distance of 300-500 m. The rates of headward erosion for the gullies have a distinctively positive trend with the upstream areas. Under the conditions that the latest knickpoints are associated with the 1303 Mw8.0 Hongdong earthquake (EventⅢ) and that the gullies keep the same rate of headward erosion during the Holocene, EventⅡis estimated to take place during 3336-2269 a B.P. and EventⅠis estimated to take place during is estimated to take place during 3336-2269a B.P., respectively. The recurrence of events is about 1500-2600 a. These results are consistent with that obtained through the trench investigations.