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Analysis of lineaments from satellite images normally includes the determination of their orientation and density. The spatial variation in the orientation and/or number of lineaments must be obtained by means of a network of cells, the lineaments included in each cell being analysed separately. The program presented in this work, LINDENS, allows the density of lineaments (number of lineaments per km 2 and length of lineaments per km 2 ) to be estimated. It also provides a tool for classifying the lineaments contained in dierent cells, so that their orientation can be represented in frequency histograms and/or rose diagrams. The input ®le must contain the planar coordinates of the beginning and end of each lineament. The density analysis is done by creating a network of square cells, and counting the number of lineaments that are contained within each cell, that have one of their ends within the cell or that cross-cut the cell boundary. The lengths of lineaments are then calculated. To obtain a representative density map the cell size must be ®xed according to: (1) the average lineament length; (2) the distance between the lineaments; and (3) the boundaries of zones with low densities due to lithology or outcrop features. An example from the Neogene Duero Basin (Northern Spain) is provided to test the reliability of the density maps obtained with dierent cell sizes. 7

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A lineament analysis of the Duero Basin (north Spain) suggests that cover rocks have been influenced by a previously fractured basement in sediment cover with little deformation. The Duero Basin is covered with horizontal Neogene rocks (mainly sandstones, shales and limestones) with a total outcropping area of about 50 000 km 2 and a maximum thickness of 300 m. The only structures found within the Neogene are map-scale monoclines near the basin margins, and joints and faults, most of them without significant displacement. From the analysis of a Landsat Thematic Mapper (TM) scene, lineaments were mapped at the eastern half of the Duero Basin. The orientation frequency of lineaments shows an absolute maximum NE-SW to ENE-WSW, with several sub-maxima oriented E-W, NW-SE and WNW-ESE. These fracture directions controlled most of the present-day fluvial network. Within the Palaeozoic and Mesozoic rocks cropping out at the basin margins the orientation of lineaments corresponds with that of mappable faults, particularly in the Palaeozoic basement. The zones with maximum density of lineaments are associated with map-scale WNW-ESE thrusts and folds located below the horizontal Neogene. The origin of the main fracture systems in the Neogene rocks of the Duero Basin appears to be controlled by older structures, namely the NE-SW faults that cross-cut the granitic and gneissic basement of the Duero Basin and its southern and western margins. These faults are late Variscan (probably Permian) in origin and were reactivated during the Mesozoic and the Cenozoic. Their activity in Miocene and post-Miocene times is related to strikeslip and extensional movements linked to the recent intraplate stress field in the Iberian Peninsula.

The Almazán Basin (northern Spain) is a Tertiary basin, 90 × 45 km wide, located in the western part of the Iberian Chain. It is filled with folded Palaeogene (2500-3000 m thick) and horizontal Neogene (500 m) beds. From the study of a satellite image, more than 2300 lineaments have been identified in the Neogene horizontal rocks. Their absolute orientation maximum is 060-070, and relative maxima are 100-110 and 120-140. Some spatial variations in the regional trends can be observed: (1) Throughout the central part of the basin, the east-northeast trending lines are the most representative.(2) Near the eastern border of the Basin the northwest-southeast set (parallel to the trend of thrust and folds in the older rocks) is dominant. (3) In the northwestern part of the basin the northwest-southeast and WNW-ESE sets coexist with the main east-northeast set. The maximum density of lineaments is found along a WNW-ESE line that corresponds with the axis of maximum thickness for the Tertiary units. A detailed field study of fracturing at the outcrop scale (more than 2300 fractures measured) was carried out. The orientation distribution of lineaments coincides with the orientation of left-lateral faults at the outcrop scale, and differences are greater when considering all kinds of fractures in different sectors. The origin of fracturing at different scales in the Almazán Basin can be related to the Neogene stress field and the influence of previous structures: (1) The northwest-southeast set of fractures formed as a result of reactivation of older faults, inherited from the late Variscan fracturing. (2) The main ENE-WSW set of fractures corresponds to neoformed left-lateral joints and faults under the northeast-southwest Neogene compression, the dextral conjugate set of joints within this system (NNE-SSW) being much less developed.

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