Relationships between topographically expressed zones of flow accumulation and sites of fault intersection: analysis by means of digital terrain modelling

Abstract

Topographically expressed zones of flow accumulation often coincide with fault intersections because of increased rock fracturing. We have conducted a study of the interrelationships between topographically expressed accumulation zones and fault intersections, and the function of these sites in landscape evolution. The investigation has been performed with the use of digital terrain models, geological and soil data for the Crimean Peninsula. First, we carried out an analysis of associations of sites of fault intersections, intensive rock fracturing and abnormally high discharges of springs and boreholes, relating to fault intersections, with three types of landform element (zones of flow accumulation, transit and dissipation). We found that all phenomena under study are closely associated with topographically expressed accumulation zones. This has demonstrated that, within these zones, the soil moisture is influenced both by upward transport of deep-seated groundwater and by accumulation of overland lateral flows. Second, we predicted the effect of topography on irrigation-induced changes in the salt regime of soils and the groundwater level, assuming that topographically expressed accumulation zones can be marked by properties of fault intersections. We found that water leaking out of the North Crimean Canal can result in secondary salinisation of soils and a considerable rise of the watertable within some accumulation zones located downslope. Salts collected in the accumulation zones, and their slow movement through rock fractures, can lead to salinisation of the aquifers. We believe that topographically expressed accumulation zones are areas of contact and substance exchange between overland lateral and deep flows.

Introduction

Relief generally affects substance movement; that is, migration and accumulation of water, mineral and organic substances over the landsurface and in the soil by gravity (Young, 1972; Gerrard, 1981). In turn, relief depends partly on geological structure (Meshcheryakov, 1965; Ollier, 1981). For example, valleys determine principal routes of surface flow. Valley networks are often connected with faults (Gerasimov and Korzhuev, 1979; Ollier, 1981) which can serve as pathways for upward transport of deep-seated substances to the landsurface (e.g., Kerrich, 1986). Therefore, some portions of surface flow routes can coincide with some discharges of deep-seated substance pathways. The study of these associations can be useful to gain a better understanding of the relationships between: (1) geological structures and landforms, and (2) endogenic and exogenic processes influencing landscape evolution.

Gravity-driven overland and intrasoil transport can be interpreted in terms of divergence or convergence and deceleration or acceleration of flow (Shary, 1995). Deceleration or acceleration of flow is determined by vertical (or profile) landsurface curvature (k_v). k_v is the curvature of a normal section of the landsurface; this section includes the gravity acceleration vector at a given point on the landsurface. Flow tends to accelerate when k_v>0, and to decelerate when k_v<0 (Speight, 1974; Shary, 1991). Divergence or convergence of flow is controlled by horizontal (or plan) landsurface curvature (k_h). k_h is the curvature of a normal section of the landsurface; this section is perpendicular to the section with k_v. Flow diverges when k_h>0, and converges when k_h<0 (Kirkby and Chorley, 1967; Shary, 1991). Notice that k_h and k_v, among other topographic attributes, are of frequent use in landscape investigations carried out with digital terrain models (Moore et al., 1991; Shary et al., 1991; Florinsky, 1998).

Flow convergence and deceleration result in the accumulation of substances at soils caused by slowing down or termination of overland and intrasoil transport. On different scales, the intensity of these processes and the spatial distribution of accumulated substances can depend on the spatial distribution of the following landform elements (Shary et al., 1991).

• Elements characterised both by convergence and by deceleration of flow; that is, by both k_h<0 and k_v<0 [topographically expressed zones of flow accumulation, or accumulation zones (Fig. 1)]. Other authors have described these elements as areas of concave plan and profile shapes (Yefremov, 1949), areas of concave radial and contour lines (Troeh, 1964), concave–concave forms (Krcho, 1983) and convergent-footslope areas (Pennock et al., 1987).

• Elements offering both divergence and acceleration of flow; that is, both k_h>0 and k_v>0 [topographically expressed zones of flow dissipation, or dissipation zones (Fig. 1)]. Other authors have described these elements as areas of convex plan and profile shapes (Yefremov, 1949), areas of convex radial and contour lines (Troeh, 1964), convex–convex forms (Krcho, 1983) and divergent-shoulder areas (Pennock et al., 1987).

• Elements that are free of a concurrent action of flow convergence and deceleration as well as flow divergence and acceleration; that is, values of k_h and k_v have different signs or are zero [topographically expressed zones of flow transit, or transit zones (Fig. 1)].

Micro-depressions (plan sizes range from several decimetres to several meters) — that is, accumulation zones at the micro-topographic scale — control the spatial distribution of the wettest parts of the soil cover and hence determine soil and plant properties (Rode, 1953; Fedoseev, 1959). In arid and semiarid regions build-up of salts and groundwater salinisation occur in meso-depressions (plan sizes range from several tens to several hundreds of meters) and macro-depressions (plan sizes reach several kilometres and more), whereas micro-depressions are not salt-affected because they are washed out with rainfall at regular intervals (Kovda, 1971). Shallow groundwaters (Ivanova, 1960), landslides (Lanyon and Hall, 1983), soil gleying, maximum thickness of humus and carbonate horizons (Pennock et al., 1987), rock fracturing, increased hydraulic conductivity, increased aquifer discharges (Lukin, 1987), source areas of overland flows (Wood et al., 1990), saturation zones (Feranec et al., 1991) and increased pollution (Gurov and Kertsman, 1991) were found in topographically expressed accumulation zones in humid regions.

Fault intersections are important features of the Earth's crust (Poletaev, 1992). Looseness, fracturing and permeability of rocks (Poletaev, 1992), increased seismicity (Gelfand et al., 1972), landslide formation (Karakhanyan, 1981), active erosion and karst development (Korobeynik et al., 1982), swamp development (Trifonov et al., 1983) and abnormally high discharges of springs and boreholes (Morozov et al., 1988) occur at fault intersections. These sites can control intensive magmatism, volcanism (Poletaev, 1992), ore fields and deposits (Kutina, 1969; Fyodorov et al., 1989), and oil and gas fields (Dolenko et al., 1967).

Trifonov et al. (1983) were probably the first to establish an association of some relief depressions with fault intersections using remotely sensed data. Poletaev (1992) found qualitatively that, as a rule, fault intersections are topographically expressed by depressions. Florinsky (1993) proved quantitatively that topographically expressed accumulation zones can coincide with fault intersections. Indeed, mapping of k_h and k_v reveals two groups of topographically expressed lineaments (Florinsky, 1992). Structures of the first group correspond to convergence areas (k_h<0), whereas lineaments of the second group relate to deceleration areas (k_v<0). These lineaments largely indicate strike–slip and dip–slip faults, respectively (Florinsky, 1996). Clearly, an intersection of two lineaments or faults of different groups is associated with a topographically expressed accumulation zone, by its definition (see above). In a similar manner, a fault segment outside fault intersections and an area between faults relate to transit and dissipation zones, respectively, by their definitions (see above). (We do not maintain that tectonic structures control all accumulation, transit and dissipation zones: certain of them can be formed by pure geomorphic mechanisms.)

The objective of this paper is to investigate comprehensively some interrelationships between fault intersections and topographically expressed accumulation zones, and the function of these sites in landscape evolution. First, we carry out an analysis of associations of sites of fault intersections, intensive rock fracturing and abnormally high discharges of springs and boreholes, relating to fault intersections, with topographically expressed accumulation, transit and dissipation zones. Second, we study the effect of topography on irrigation-induced changes in the salt regime of soils and the groundwater level, assuming that topographically expressed accumulation zones can be marked by the properties of fault intersections.