Jean Chorowicz

A combined algorithm for automated drainage network extraction

Based on existing principles of automated drainage network extraction we have developed two methodological algorithms, the “profile scan” and “hydrological flow modeling,” and used them to extract networks from digital elevation models (DEMs). The “hydrological flow modeling” algorithm specializes in the extraction of well-connected hierarchically arranged skeletal channel networks. On the other hand, the channels extracted by the “profile scan” algorithm lack adequate connectivity, but this algorithm is suitable for the extraction of wide valley bottoms and other flat areas. A combination of the two algorithms gives a more versatile algorithm capable of yielding networks which are not only well connected but also portray the surface character of the drainage network thus generated. The good functioning of our algorithm is not inhibited by the presence of pits in the DEM. There is therefore no preprocessing of the DEM prior to drainage extraction. Rather, at the end of extraction, isolated spots are eliminated since these are the ones most probably representing artifacts.

East African rift system: offset, age and tectonic significance of the Tanganyika-Rukwa-Malawi intracontinental transcurrent fault zone

Abstract The 1000 × 200 km Tanganyika-Rukwa-Malawi fractures area is shown to be a right lateral intracontinental transform zone belonging to the East African Rift System. Using the results carried out from complementary techniques (remote sensing, field structural analysis and K-Ar isotopic dating) it appears that tectonics in this area have acted at different times, the oldest being at 2000-1400 m.y., the youngest being still active today, and are mainly due to the reactivation of NW-SE structures. The area consists mainly of large NW-SE “en echelon” dextral transcurrent faults, along which the Southern Tanganyika, Rukwa and Northern Malawi troughs are opening. NE-SW transverse faults showing a curved horizontal shape end these troughs axially. Eocene to recent volcanic activity is associated with the intersection of the northeastern main NW-SE transcurrent fracture (the Livingstone fault).

Fault tectonics of the Baja California Peninsula and the opening of the Sea of Cortez, Mexico

Abstract A detailed field analysis of Neogene and Quaternary faults in Baja California has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula. The deformation, which is related to the opening of the gulf, affects the whole peninsula, but decreases from east to west. Most observed faults, normal and/or dextral, strike NNW-SSE to WNW-ESE; their mechanisms include both strike-slip and dip-slip, as well as intermediate motions. Compressional events have occurred since Late Neogene times, but were probably of minor quantitative importance because reverse faults are rare and small. The principal fault pattern includes dextral NNW-SSE Riedel shears and N-S tension faults induced by dextral strike-slip along two main NW-SE fault zones bordering the peninsula: the Gulf of California to the east, which is the most important, and the Tosco-Abreojos fault to the west. The resulting pattern of deformation shows that the eastern part has been a complex transform-extensional zone since Late Miocene-Early Pliocene times.

Principles of drainage basin analysis from multisource data: Application to the structural analysis of the Zaire Basin

Abstract Classifications which attempt to combine descriptive and genetic aspects of types of drainage basins must take into account the application of new techniques (satellite imagery, high-precision topographic maps and digital terrain models). In addition, the use of new concepts such as neotectonics and drainage basin history will lead to a reassessment of classification schemes. After a review of current classifications, we propose a genetic scheme that is based on the development of drainage networks and which includes integration of local and regional-scale perturbations. State-of-the-art methods of drainage basin analysis are applied to the Zaire Basin to provide new information on deep structures. The influence of basin and block tectonics is revealed in the context of major Late Proterozoic shear faults which have been rejuvenated up to the present day. This is demonstrated from an analysis of the drainage network in the light of geological and geophysical observations.

The Tana basin, Ethiopia: intra-plateau uplift, rifting and subsidence

The Tana basin is situated on the northwestern plateau of Ethiopia, west of the Afar depression. The basin is perched on a topographic high. New data from digital elevation modelling and satellite imagery analysis confirm the basins location at the junction of three grabens: the Dengel Ber (buried), Gondar (exposed by erosion) and Debre Tabor (reactivated). This structural complex was notably active during the build-up of the mid-Tertiary flood basalt pile, into which the Tana basin is impressed. Fault reactivation occurred in the Late Miocene–Quaternary, accompanied locally by predominantly basaltic volcanism. Fault-slip indicators are consistent with crustal subsidence centred on the present morphologic basin. Concentric and radial dike patterns in the Tana region indicate that diking and basin formation were contemporary. Tana rifting and magmatism occurred above the inferred western side of the Afar mantle plume-head.

Volcanic vents rooted on extension fractures and their geodynamic implications in the Ethiopian Rift

Abstract Most of the extension fractures located in the Ethiopian rift are related to rift dynamics, and they have consistently similar orientations for hundreds of kilometers. The orientation of extension fracture gives the local extension direction which is perpendicular to its horns (end segments of an extension fracture). We have established clear geometrical relationships between tectonics and volcanism in the Main Ethiopian Rift by interpreting high-resolution images obtained from Landsat-TM and SPOT satellites and by quantitatively analyzing the geometry of extension fractures, elongated vents and linear volcanic clusters. Applying the relationships obtained, we show that extension fractures in the rift served as channels for magma rising to the surface, and that extension fractures underlie most of the elongated volcanic vents and linear volcanic clusters. The geometry of extension fractures beneath volcanic edifices can be deduced from the shape of elongated vents and pattern of linear volcanic clusters rooted on them. Utilizing the orientations of extension fractures directly observed and those inferred from elongated volcanic vents and linear clusters, we found that the extension direction of the Main Ethiopian Rift is northwest–southeast and that the direction has been rotated clockwise for about 20° in the time interval 2.83 to 0.023 Ma. The Recent axis of rift opening is oriented N40° and located closer to the southeastern escarpment giving an asymmetric geometry to the Main Ethiopian Rift (MER).

Northwest to north-northwest extension direction in the Ethiopian rift deduced from the orientation of extension structures and fault-slip analysis

Northeast to north-northeast trends of eruptive centers, some of which formed at the termination of faults, and en echelon extension fractures—the largest one of which is 0.1 km wide, up to 1 km long, and at least 50 m deep—exist between lat. 7°N and 9°N in the Ethiopian rift on Systeme Probatoire d9Observation de la Terre (SPOT) imagery. The extension fractures form scars in the rift floor. Border faults of the rift are generally oriented northeast to north-northeast. Two fault systems observed on Land-sat Multi-spectral Scanner (MSS) cross the rift, the Bilate-Weyira fault with a north-northwest strike and the Wonji fault belt with a north-northeast trend. North-northwest-striking faults cutting across the rift are interpreted to be transfer faults that cut the rift into rhomboid-shaped basins. Extension fractures and rows of eruptive centers—collectively referred to as extension structures—are used in the paper to infer the direction of plate motion in the Ethiopian rift. Northwest to north-northwest extension of the rift is deduced by assuming that the direction of opening of extension structures is also the orientation of the minimum principal stress σ3. Fault-slip inversion on faults that cut post-Miocene volcanic rocks confirms this interpretation.

A new kinematic approach for the Danakil block using a Digital Elevation Model representation

Abstract Data from the literature are integrated in a regional Digital Elevation Model (DEM) in order to analyse the motion of the Danakil block with regard to the Arabian and Somalian plates. The application of the poles and angles of rotation taken from the literature, induces a superposition of the Danakil block on the Arabian plate, and the formation of a gap in the Afar region. The determination of new poles of rotation using a best-fitting procedure allows one to avoid these drawbacks. In all cases, calculations are carried out keeping the Nubian plate stationary. The present approach shows that the Danakil block is an independent entity and is not related to the Nubian and Arabian plates. Between the Oligocene and the Miocene, it has been submitted to a N20°E sinistral strike-slip motion anterior to the rotation itself that was started in the middle Miocene. This rotation is directed by a mechanical couple due to the combination of the Red Sea and North Afar extensions.

A new technique for recognition of geological and geomorphological patterns in digital terrain models

Abstract We have developed a technique that recognizes monoclinal sedimentary strata showing strike ridge geomorphology and fluvial deposits from Digital Terrain Models (DTMs). This technique is efficient and reliable and may be expanded to analysis of other landforms. Techniques for rapid geomorphic analysis of DTMs are becoming increasingly useful since digital stereoscopic SPOT data will soon be available worldwide. Our technique recognizes geomorphic elements in two steps. First, elementary objects including crests, thalwegs, and various slope types are classified from slope changes along N - S, E - W, NW - SE, and NE- SW profiles through pixels in the DTM image. Second, grammatical pattern recognition is used to identify and map geomorphic forms from the succession of elementary objects.

A numerical approach to the analysis and classification of channel network patterns

A large number of samples of visually classified channel networks are taken from some published works. The samples are representative of the following pattern types: dendritic, parallel, rectangular, trellis, and pinnate. By means of computer techniques, essential parameters of elements of these pattern samples are determined. These include lengths, directions, and degrees of curvedness and meandering of channel segments, as well as confluence angles. The parameters are used to determine pattern attributes such as density, texture, parallelism, rectangularity, and bifurcation ratios. Thresholds are generated for the attributes and used in the construction of classification models for the five pattern types studied. The processes and models are coded in a computer program for use in the automated classification of numerically valued channel networks. After classification the image is displayed with each individual network bearing a color which shows the pattern type to which it has been assigned.