LINEAMENTS AND FRACTURING IN THE NEOGENE ROCKS OF THE ALMAZAN BASIN, NORTHERN SPAIN

The Almazan Basin (northern Spain) is a Tertiary basin, 90 x 45 km wid e, 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 h ave been identified in the Neogene horizontal rocks. Their absolute or ientation maximum is 060-070, and relative maxima an 100-110 and 120-1 40. 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 an d 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 a long a WNW-ESE line that corresponds with the axis of maximum thicknes s 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 grea ter when considering all kinds of fractures in different sectors. The origin of fracturing at different scales in the Almazan Basin can be r elated to the Neogene stress field and the influence of previous struc tures: (1) The northwest-southeast set of fractures formed as a result of reactivation of older faults, inherited from the late Variscan fra cturing. (2) The main ENE-WSW set of fractures corresponds to neoforme d 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.