Lineaments and fracturing in the Neogene rocks of the Almazán Basin, northern Spain

$: Lineaments and fracturing in the Neogene rocks of the Almazán Basin, northern Spain$
1. Introduction

In some geological environments, as occur with horizon-tal beds of shales and clays, it is difficult to infer fracturepatterns from the study of aerial photographs, since muchof the photogeological appearance of rocks is conditionedby landscape evolution. In these areas analysis of satellitelineaments can give a vision of the structures present inthese rocks. In spite of lower spatial resolution relative toaerial photographs, satellite images have other advan-tages (Chuvieco, 1990): higher spectral resolution, digitalformat, homogeneity in data recollection, synoptic viewand temporal periodicity. For these reasons the use ofsatellite imagery in different branches of science has beenincreasing in recent decades.

In geology, and especially in structural geology, satel-lite imagery helps to identify materials and their struc-ture, independently of their age and type of deformation(Jutz & Chorowicz, 1993; Drury & Berhe, 1993; Nash etal. 1996). These images are especially useful for recog-nizing linear structures and lineaments, defined by Sabins(1987) as straight linear features, presumably expressinga subsurface phenomenon, that differ distinctly from thepatterns of adjacent features. The surface features makingup a lineament may be geomorphic (caused by relief) ortonal (caused by contrast differences). In folded and/or

thrusted areas, lineaments can correspond to severalkinds of structures or bed contours (Sanderson & Chinn,1989; Heddi & Eastaff, 1995), whereas in flat-lying beds,linear structures are usually linked to fracturing (Arlegui& Soriano, 1996).

The Tertiary Almazán Basin, located in the westernend of the Iberian Chain, close to the Duero Basin, showsa widespread cover of Neogene horizontal beds (up to500 m thick), which only show brittle deformation (faultsand fractures). The rock types present in the Neogenebeds show great variety (conglomerates, sandstones,shales and limestones), which makes them suitable forlineament analysis and the comparison of geomorphicexpression of fractures in different lithologies. Thefavourable outcrop conditions in the greatest part of thebasin (only the northern and western parts show a densevegetation cover) also allow detailed study of fracturingat the outcrop scale.

The main objective of this work is the study of satelliteimages, to determine the fracture pattern in the Neogenerocks of the Almazán Basin at a medium scale (from hun-dreds to thousands of metres). We utilized a TM satelliteimage from which a false colour composition wasobtained. After the visual analysis of this image a map oflineaments was made. The high number of lineamentsidentified in the studied region called for use of an auto-matic program for line exploration to determine theirlength and orientation. Statistical analysis was applied to

Geol. Mag. 135 (2), 1998, pp. 255–268. Printed in the United Kingdom © 1998 Cambridge University Press 255

Lineaments and fracturing in the Neogene rocks of the AlmazánBasin, northern Spain

A. L. CORTÉS*, A. MAESTRO, M. A. SORIANO & A. M. CASAS

Departamento de Geología, Universidad de Zaragoza, 50009 Zaragoza, Spain

(Received 28 January 1997; accepted 9 October 1997)

Abstract – The Almazán Basin (northern Spain) is a Tertiary basin, 90 × 45 km wide, located in the westernpart 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 theNeogene horizontal rocks. Their absolute orientation maximum is 060–070, and relative maxima are100–110 and 120–140. Some spatial variations in the regional trends can be observed: (1) Throughout thecentral part of the basin, the east–northeast trending lines are the most representative. (2) Near the easternborder of the Basin the northwest–southeast set (parallel to the trend of thrust and folds in the older rocks) isdominant. (3) In the northwestern part of the basin the northwest–southeast and WNW–ESE sets coexistwith the main east–northeast set. The maximum density of lineaments is found along a WNW–ESE line thatcorresponds with the axis of maximum thickness for the Tertiary units. A detailed field study of fracturing atthe outcrop scale (more than 2300 fractures measured) was carried out. The orientation distribution of linea-ments coincides with the orientation of left-lateral faults at the outcrop scale, and differences are greaterwhen considering all kinds of fractures in different sectors. The origin of fracturing at different scales in theAlmazán Basin can be related to the Neogene stress field and the influence of previous structures: (1) Thenorthwest–southeast set of fractures formed as a result of reactivation of older faults, inherited from the lateVariscan fracturing. (2) The main ENE–WSW set of fractures corresponds to neoformed left-lateral jointsand faults under the northeast–southwest Neogene compression, the dextral conjugate set of joints withinthis system (NNE–SSW) being much less developed.

* Author for correspondence: [email protected]

these data, to check differences in spatial distribution oflineaments, relationship with the age and lithology of therocks and their main trends. A detailed field study was alsocarried out, to finally establish the relations of fractures tothe main structural trends of the studied region, and to therecent stress field responsible for their formation.

2. Geological setting

The Almazán Basin is a Tertiary basin located betweenthe Iberian Chain and the Duero Basin (Fig. 1). TheIberian Chain is an intraplate mountain range deformedduring the Variscan and Alpine orogenies. The lateVariscan fracturing formed two main sets of faults distrib-uted throughout the chain and surrounding areas(Arthaud & Matte, 1975): northeast–southwest andnorthwest–southeast. These faults influenced the evolu-tion of the Iberian Basin during the Mesozoic rifting(reactivated as normal faults) and the Tertiary compres-sion, reactivated either as wrench or reverse faults,according to their orientation (Alvaro, Capote & Vegas,1979; Salas & Casas, 1993).

The Almazán Basin occupies an area of more than4000 km2, with an average elevation of 1100 m above sealevel. It is located between three main units within theIberian Chain (Fig. 1):

(1) The Cameros Massif, located to the north, consistsof Jurassic marine limestones and Upper Jurassic–LowerCretaceous terrestrial sandstones, shales and limestones.They overthrust the Tertiary deposits of the AlmazánBasin to the south, with a horizontal displacementbetween 2 and 5 km (Platt, 1986; Guimerà, Alonso &Mas, 1995). The strike of these thrusts changes from aWNW–ESE direction in the western part to east–westand northeast–southwest near Soria.

(2) The northern (or Aragonian) branch of the IberianChain bounds the Almazán Basin to the east. It consists ofCambrian and Ordovician shales and sandstones uncon-formably overlain by Jurassic and Cretaceous sandstonesand limestones, folded with a northwest–southeast trend.To the north, the northwest–southeast folds of theAragonian branch are in continuity with the east–westfolds of the Cameros Massif. Most of these folds wereformed during the Palaeogene compression.

(3) The southern (or Castilian) branch, forms thesouthern limit of the Almazán Basin. It consists mainly ofTriassic sandstones and limestones and Jurassic lime-stones, with very gentle folds trending WNW–ESE. Itscontact with the Almazán Basin is an unconformity of theNeogene strata upon the Mesozoic, the latter being gentlytilted northward.

The western limit of the Almazán Basin is marked by

256 A. L. C O RT É S A N D OT H E R S

Figure 1. Geological map of the Almazán Basin, with its location within the Iberian Peninsula.

outcrops of Upper Cretaceous folded deposits, that indicate a minimum in the thickness of the Tertiarydeposits. It is in continuity with the Duero Basin, filledwith Tertiary deposits, that occupies practically all thenorthern Iberian Meseta.

The infill of the Almazán Basin consists of Palaeogeneand Neogene sediments. Palaeogene materials crop out inthe northeastern part of the basin; in the rest of the areathe Neogene strata overlie the older rocks. ThePalaeogene reaches a thickness of 2500–3000 m (Bond,1996), whereas the Neogene is about 500 m. The age ofthe Palaeogene rocks ranges from Palaeocene to LateOligocene and the age of the Neogene rocks from EarlyMiocene (Agenian) to Pliocene (Armenteros et al. 1989;Carballeira & Pol, 1989; Lendínez, 1991a,b,c). TheTertiary units are composed of alluvial deposits, withconglomerates and sandstones near the borders and lacus-trine limestones and gypsum toward the basin centre.Clastic deposits dominate in the Palaeogene units, whilein the Neogene units lutites and lacustrine limestones arethe most widespread rocks.

The Palaeogene is divided into four tectonosedimen-tary units (P-1, P-2, P-3 and P-4), bounded by uncomfor-mities or sedimentary breaks (Armenteros et al. 1989;Carballeira & Pol, 1989; Bond, 1996; Mediavilla et al.1996; Santisteban et al. 1996; Santisteban, Mediavilla &Martín-Serrano, 1996). The Neogene series is dividedinto five tectonosedimentary units: N-1 (LowerMiocene), N-2 (Lower–Middle Miocene), N-3(Middle–Upper Miocene), N-4 (Upper Miocene) and N-5(Pliocene). The main unconformity within the basin,clearly recognized in its eastern border, separates thePalaeogene from the Neogene. The Quaternary depositsare mainly gravels and sands distributed in fluvial terraces along the Duero river and some of its tributaries.

The Almazán Basin shows an overall simple synclinalgeometry, with a maximum thickness of preservedTertiary deposits located along an ENE–WSW line(ITGE, 1990). Two large monoclines involving thePalaeozoic rocks below the basin, Palaeogene in age, arelocated south of the syncline axis (Fig. 1). To the northand east, the Palaeogene beds show several folds trendingnorthwest–southeast to east–west and verging northward.Fold limbs dip between 30° and 90°. These folds can befollowed at depth by means of seismic profiles (ITGE,1990) to the Palaeozoic basement. The Neogene beds arepractically horizontal all over the Almazán Basin andshow an onlap geometry on the Mesozoic rocks of its bor-ders. Near the northern border they show very low dips,probably depositional, toward the basin centre. Faults atseveral scales, usually not larger than a few hundredmetres, are also found in the Neogene units (Lendínez &Ruiz Fernández, 1991).

The main Tertiary stress field is a regional north–southcompression that lasted throughout the Palaeogene. Thisnorth–south compression was responsible for mostmacro- and meso-structures (faults, stylolites andimpressed pebbles) found in Palaeogene rocks (Casas-

Sainz & Maestro-González, 1996). Three minor com-pression directions are also present: northeast–southwest,east–west and northwest–southeast. The northeast–south-west compression is interpreted as a deviation from thenorth–south compression at a regional scale, and thenorthwest–southeast stress field as the main compressiondirection in the Spanish Central System (Casas-Sainz &Maestro-González, 1996). The Neogene rocks record amain northeast–southwest compression, a minor north-west–southeast compression, and extensional roundeast–west stress states (A. Maestro, unpub. 3rd cycle the-sis, Univ. Zaragoza, 1994; Cortés et al. 1996; Cortés &Maestro, 1997).

3. Methodology

3.a. Study of remote sensing image

In this study, we use a Thematic Mapper image (200-31)from the satellite Landsat, including the Almazán Basinand the main ranges that limit it. It was acquired at 10.19local solar time on September 12th, 1984. The coordinatesof the centre of the image are 41.78157 N for the latitudeand 1.93449 W for the longitude. As is well known theThematic Mapper images are very useful because of theirhigh spatial and spectral resolution. The TM sensorrecords seven bands from the region of the visible to theshort-wave infrared electromagnetic spectrum. Theirinstantaneous field of view is 30 m for the six reflectivebands and 120 m for the thermal band (number 6).

From the image a region of 4772 × 2847 pixels (140 ×84 km) was selected. We tried different combinations ofthree bands and, in general, the best visual quality wasobtained with a false colour image using the bands 2, 4and 5 (in blue, green and red, respectively). Because ofthe large size of the image we prepared 24 subscenes(four rows and six columns) for an easier and moredetailed manipulation. To improve the visual quality ofthe image we enhanced it with a linear contrast stretchassigning 1 % of the pixels to black and the upper 1 % towhite (digital numbers 0 and 255). The remaining pixelvalues are distributed linearly between the two ends. Tostudy the zone in more detail, print sublimation copies ofsubscenes at a scale of 1:100 000 were obtained. Thevisual analysis of all the images was made both directlyon the screen of the computer (using a computer-assisteddesign program) and on the paper prints.

3.b. Analysis of lineaments

The high number of lineaments drawn required the use ofan automatic exploration program to determine differentparameters for each line. The exploration program usedreads vectorial format files, scans first along the X-axis,then along the Y-axis, following the sequence in which thelines were drawn. The program generates a file includingfor each line the (X, Y) coordinates of its ends, its lengthand its angle in degrees reckoned clockwise positive fromnorth. From this file the lengths and orientations of the

Lineaments and fracturing, Almazán Basin 257

studied lineaments are represented and statistical analysisof the data can be carried out.

To determine the spatial variation and density of linea-ments, the studied area was divided into 47 cells, each10.6 km on the side. Lines belonging to different cellswere separated using the XY coordinates of the first andlast point of each line. These cells were analysed andstudied separately to establish comparisons betweenthem. For calculation of densities of lineaments in cellslocated at the boundaries of the Neogene, only the surfaceof the Neogene within the cell was considered. Likewise,when lineaments cut across the Neogene boundary, onlythe length of the segment located on the Neogene wastaken into account.

3.c. Study of fracturing at the outcrop scale

The structures studied at the exposure scale were joints(including fractures with no apparent offset between bothwalls) and strike-slip, normal and reverse faults with littleoffset. On striated faults several measurements weretaken: strike, dip, pitch of the striation and relative senseof movement. For joints, only strike and dip were consid-ered. The methodology in the outcrop data collection forthe quantitative study of orientations is based on the defi-nition of different fracture sets, and determination of thedominant direction in each exposure (55 sites in total).The fracture orientation data were processed using

frequency histograms, rose diagrams and smoothed frequency curves. The last method is based on a tech-nique of Wise & McCrory (1982). This technique allowsthe elimination of the statistical noise introduced in thedata collection, by means of the rolling windows averageor mobile means (the window size is 15°: Wise &McCrory, 1982). In this work 42 data on average werecollected in each site.

4. Results

4.a. Analysis of lineaments

From the study of the image obtained with the combina-tion of bands 2, 4 and 5, up to 2331 lineaments that affectthe Neogene layers were mapped (Fig. 2). The number ofsatellite lineaments is very high, in general, in the centralpart of the Almazán Basin, and decreases toward thenorthern and southern margins. The largest number of lin-eaments is located near the contact with the Aragonianbranch, in the eastern part of the basin. In the proximityof the Duero river the number of lineaments is low,possibly due to the presence of Quaternary terraces inwhich lineaments cannot be identified. Similar conditionsoccur north of El Burgo de Osma-Almazán, where thepresence of dense vegetation canopy prevents the identifi-cation of lineaments.

The distribution of lineaments (Fig. 2) is apparently


Figure 2. Map of lineaments determined from satellite image.

independent of lithology, because their number is veryhigh in areas where shales (northeast of Santa María deHuerta) and sandstones (south and north of the same vil-lage) dominate, and low where there are limestones(south of Almazán), sandstones (north of Almazán) andconglomerates (around Morón de Almazán). If we con-sider the age of the sedimentary rocks, the number of lin-eaments decreases toward the Pliocene and the maximumconcentration appears clearly in the Lower–MiddleMiocene rocks (tectonosedimentary units N1 to N3).

A quantitative analysis of lineaments was carried out todetermine the size and orientation patterns of lineamentsets at the regional scale. The minimum and maximumrecorded lengths are 95 m and 7098 m, respectively. Thehistogram of lineament lengths shows a log-normal dis-tribution, with a mode about 600 m (Fig. 3a). The direc-tion of all the data was analysed (1) considering thenumber of lineaments in each direction and (2) weightingthe lineament trace length. The results of these analysesdo not show significant differences in the distribution oforientations (Fig. 3b); the main direction is ENE–WSW(065–070°), and subsidiary lineament directions are100–105°, 125–130°, 010–015°, 020–025°, 155–160°and 165–170°. This diversity of lineament directions wassimplified by means of the statistical method of Wise &McCrory (1982). The smoothed frequency curve shows

an absolute maximum at about 066° and two relativemaxima at about 100° and 132° (Fig. 3c). The other sub-sidiary directions are not well defined on this graph.

The relationship between azimuth and length (Fig. 3d)shows that the highest density of lineaments correspondsto ENE–WSW orientations and lengths between 400 and1000 m. East–northeast lineaments are dominant in the0–3000 m interval. Lineaments longer than 3000 m showseveral maxima (Fig. 3d). A significant aspect oflength–orientation relationships is that east–southeast tosoutheast lineaments are increasingly important as lengthincreases.

4.b. Spatial variations and density of lineaments

Sectorial analysis by means of square cells shows that theeast–northeast lineament set is the dominant one in thecentral part of the Almazán Basin. The other relativemaxima appear, more or less represented, in practicallyall the cells. The strongest variations with respect to themain tendency appear near the Aragonian branch, wherethe dominant set is northwest–southeast, with a relativemaximum oriented northeast–southwest. The southernborder of the basin does not show great variation com-pared to the central part (Fig. 4). Near the northern borderthe main east–northeast set divides into two maxima at


Figure 3. (a) Histogram showing the length distribution of the mapped lineaments. (b) Histogram indicating the relative frequency of orientation of lineaments. (c) Smoothed frequency curve of the orientation of lineaments. (d) Relationship between orientation oflineaments and their length. The rose diagrams to the right indicate the orientation of lineaments by length intervals (the outer circlesrepresent 20 %).

55° and 85°, whose bisector corresponds to the mainmaximum in the central part.

The map derived from this analysis shows the densityof fracturing in the studied area (Fig. 5). The lineamentdensity is represented using the cumulative length of linesby square kilometre. Density contours are elongated in aneast–southeasterly direction. This preferred directioncoincides with the main monoclines in the Palaeogenebeds underlying the Neogene of the Almazán Basin (Fig.5, see also Fig. 1). The highest values of lineament den-sity are located along the main east–southeast directedaxis of the basin: (1) in the western part, south of Burgode Osma; (2) in the central sector, south of Almazán,coinciding approximately with the uplifted horizontallimb of the main monocline located south of Almazán(see Figs 1, 5); and (3) in the eastern part, near theAragonian branch. There is a slight west to east increaseof the lineament density with east–southeast elongatedcontour maxima.

4.c. Fracturing at the outcrop scale

In the Almazán Basin, satellite lineaments correspond tolinear erosion gullies and vertical scarps or steep slopes atboth sides of infilled valleys. Since the Neogene of the

Almazán Basin consists of flat-lying beds, these lineargeomorphic features must correspond with fault-linescarps (in the sense of Cotton, 1950), or aligned small-scale faults and joints, as can be observed in otherNeogene basins (Arlegui & Soriano, 1996).

To check the relationship between lineaments and frac-turing, a field study of fractures (faults and joints) in theNeogene rocks was carried out. Although measurementof fracture orientation at the outcrop scale cannot be sosystematic and homogeneous as satellite lineament deter-mination, due to the characteristics of sampling and thelocation of exposures (limited by lithology, road-cuts,etc.), an effort was made to obtain homogeneously distributed data sites throughout the studied area (Fig. 6).

Directions of fractures were obtained from 55 sites,with 2343 planes measured (Fig. 6). Most of them corre-spond to joints (2057), and the rest (286) to striated faults(58 dextral faults, 49 sinistral faults, 26 reverse faults and153 normal faults). Most joints and strike-slip faults havevertical planes, whereas normal and reverse faults showdips between 45° and 80°. Overall, fractures at the out-crop scale show northwest–southeast and northeast–southwest orientation maxima, although their directionaldistribution is not so clustered as the orientation of linea-ments, and the local variability of the dominant orientation


Figure 4. Rose diagrams showing the orientation in each cell of the 300 × 300 arbitrary units (10.6 × 10.6 km) square grid. The outercircles represent 20 % of the lineaments for each cell. The number of lineaments within each cell is also indicated.

in each site is higher. Analysis of total data shows severalrelative maxima striking 020–030°, 040–050°, 070–080°,140–150° and 160–170° (Fig. 6).

The studied region was divided into three sectorsshowing different fracture patterns (Fig. 6). The westernarea shows a dominant northeast striking fracture set,with subsidiary north–northeast, southeast and north–northwest to north striking sets. In the central part, thepattern is very similar to the general fracture pattern at thebasin scale, since the majority of data come from this sector. The north–northwest striking set shows a higherfrequency, but most data show strike between north–northeast and east–northeast. Many sites show a clearlydominant east–northeast orientation. The eastern area,close to the Aragonian branch, shows a strongly dominantnorthwest striking set and subsidiary east–northeast andnortheast striking sets.

5. Interpretation and discussion

5.a. Comparison between lineaments and fracturing

The Neogene rocks of the Almazán Basin show a definitepattern of lineaments represented throughout the studiedarea. The density and orientation of lineaments do notdepend on the lithology of sedimentary units but on their

spatial position within the basin. Furthermore, densitycontours cross-cut the limits of lithological formations.On the other hand, outcrop observation of fractures isseverely limited by the rock types, since good exposuresare found in cemented conglomerates, sandstones andlimestones, but not in argillaceous units. However, thedetermination of satellite lineaments is easier in lutites,probably due to the greater influence of large fractures inerosion processes.

It is not easy to establish a direct relationship betweenfractures and satellite lineaments from field observationsand photogeological analysis, since the identification oflineaments in the field requires special outcrop and expo-sure conditions, and the study of aerial photographs atintermediate scales in Almazán Basin (1:33 000) does notallow identification of either faults or lineaments, becauseof the low relief conditions, farming and landscape mor-phology.

From our study it can be deduced that, both in the over-all and cell analyses the maxima of lineament orientationare also relative maxima of orientation of fractures, withsome differences, that can be appreciated from the com-parison of frequency curves (see Appendix for qualitativeand quantitative comparison of orientation of lineamentsand fractures in each cell). These differences in statisticalorientation probably arise from the discontinuous character


Figure 5. Density map of lineaments. Density expressed as the cumulate length of lineaments per km2 in each cell.

of brittle deformation and scale changes, and can be sum-marized in the following points:

(1) Fracture orientations at the outcrop scale showmore statistical ‘noise’, in which all intervals show atleast frequencies from 0.01 to 0.1 (see Appendix). This isespecially evident when considering data from the entirebasin (Fig. 6). At the same time fracturing at the outcropscale is less homogeneous, more data being necessary toobtain well-defined maxima. Statistical orientation at theoutcrop scale is also strongly affected by the direction ofroad cuts and natural slopes, and in those sites where thedirection is parallel to the main lineament trend (north-east to east–northeast), this set of fractures is not repre-sented.

(2) There is an inhomogeneous distribution ofnorth–south lineaments throughout the basin, whereas thenorth–south fracture set is well represented in manyexposures.

(3) Climatic and overall drainage conditions within thebasin (that is, the general slope of the basin toward the west)may control erosion processes and the greater developmentof lineaments over defined sets of fractures, probablyinhibiting the development of erosion on other sets.

The difference in orientation between lineaments and

fractures is lower when considering only strike-slip faults(Fig. 7), whereas the relationship with joints and normaland reverse faults is not so clear. This leads to the hypoth-esis that most lineaments found should correspond tostrike-slip faults with little offset.

5.b. The origin of Neogene fracturing in the Almazán Basin

A problem arising when considering lineaments as faultsor fractures is the length/displacement relationship infaults (see, for example, Cowie & Scholz, 1992); linea-ments up to 7 km long are found in Neogene rocks, whereno mappable faults exist. Faults at the outcrop scale showmaximum displacements of less than 1 m (Cortés &Maestro, 1997), and many of the fractures measured correspond to joints with no offset. The answer to theproblem of the lack of correlation between fault lengthand fault displacement can be explained in two ways: (1)Mapped lineaments consist of several, more or lessaligned, different fractures of smaller length, that cannotbe distinguished at the scale of a satellite image. (2) Pre-existing faults in the Mesozoic and Palaeozoic rocksunder the Tertiary of the Almazán Basin influenced thedevelopment of fractures in the Neogene deposits under


Figure 6. Rose diagrams showing the orientation of fractures (joints and faults) at the outcrop scale (outer circle represents 20 %). Thenumbers in parentheses indicate the number of fractures measured in each site. The total frequency and the orientation of fractures inthe three sectors considered is represented by comprehensive rose diagrams. The rose diagram in the lower part indicates the orienta-tion frequency for all data.

the more recent stress field. The main late-Variscan faultdirections, re-activated as normal faults during theMesozoic and as strike-slip or reverse faults during thePalaeogene compression, strike northeast–southwest inthe western part of the basin and northwest–southeast nearthe Aragonian branch. Inherited fractures could be reacti-vated with little displacements under the Neogene stressfield which was not able to form large macro-structures.This can also explain the variations in orientation of longlineaments (re-activated fractures) with respect to shortlineaments (possibly neoformed fractures, see Fig. 3).

The origin of some of the Neogene fracture sets byreactivation of older faults is supported by the variationsof their orientation in different sectors within theAlmazán Basin. Near the Aragonian branch, northwest–southeast fractures and lineaments are dominant, whereasin the western part of the basin, where severalnortheast–southwest late-Variscan faults are located (G.De Vicente, unpub. Ph.D. thesis, Univ. Complutense deMadrid, 1988; Clemente & Pérez-Arlucea, 1993; Casas-Sainz & Maestro-González, 1996; Bond, 1996), the dom-inant set is oriented northeast–southwest to ENE–WSW.The northeast–southwest set of fractures is also found inthe eastern part of the Duero basin (Mediavilla et al.1996), where they partly controlled the deposition of theNeogene tectonosedimentary units, and in the central partof the Spanish Central System (Vegas et al. 1990). In thiscase the orientation maxima of fractures and lineamentsare 010–030.

In the central part of the basin the dominant set of frac-tures is ENE–WSW. Here, the Neogene is separated fromthe underlying Palaeozoic rocks by at least a 1000-m-thick cover of Mesozoic and Palaeogene strata, and there-fore the role of pre-existing fractures in this sector is notas clear. This ENE–WSW set of lineaments could corre-spond to neoformed structures, probably controlled bythe recent stress field. This is characterized by a north-east–southwest compression axis under a wrench regime(Casas-Sainz & Maestro-González, 1996) and aneast–northeast to east–southeast extension under anextensional regime (Cortés & Maestro, 1997).

The main ENE–WSW set of lineaments could be left-lateral shear joints and faults associated with the Neogenenortheast–southwest compression. This hypothesis issupported by the east–northeast oriented maximum ofleft-lateral faults at the outcrop scale (Fig. 7a). The conju-gate set (NNW–SSE to north–south dextral fractures) ismuch less developed, both at the satellite and outcropscales. Vegas et al. (1990) interpreted this situation, witha single set of northeast–southwest left-lateral faults, tobe the result of re-activation during the Middle–LateMiocene of Late Cretaceous faults. The latter resultedfrom the east–northeast dextral shear in the central part ofthe Iberian peninsula, and the clockwise rotation ofblocks bounded by northeast–southwest faults.Nevertheless, the unequal development of the two sets offractures defining a conjugate system is common inwrench regimes (Freund, 1974).


Figure 7. Smoothed frequency curves showing the orientationof different types of striated faults (with determined sense ofmovement) at the outcrop scale. (a) Relationships betweenlineaments and strike-slip faults. (b) Relationships between left lateral and right lateral faults and their compatibility with northeast and southeast compressional stress fields. (c) Relationships between normal and reverse faults.

The north–south set of sinistral faults and theENE–WSW set of dextral faults (Fig. 7b) cannot beexplained by an east–northeast compression, and theymust be related to the northwest–southeast compression,late with respect to the northeast–southwest compressionin the Almazán Basin (Casas-Sainz & Maestro-González,1996). Finally, normal faults with orientation maximanorthwest–southeast and northeast–southwest could alsoform as conjugate systems associated with an extensionaxis parallel to the bisector of the obtuse angle betweenthese direction maxima (see Fig. 7c).

6. Conclusions

The results obtained in this work prove the necessity ofintegrating analysis at different scales to obtain a repre-sentative picture of fracturing. Due to the special condi-tions of the studied rocks (argillaceous lithology,horizontal position, low relief and the existence of a verythin cover of Quaternary soils), it is very difficult to deter-mine the directions of fracture in the Neogene materialsof the Almazán Basin from conventional aerial pho-tographs. The study of satellite lineaments has proved tobe a useful tool in this case, since it provides the system-atic pattern of fracturing affecting the horizontal Neogenerocks.

Lineaments in the Neogene rocks of the AlmazánBasin correspond to morphological features conditionedby fractures with little or no offset (sets of joints andfaults), whose orientation maxima coincide with those oflineaments in different parts of the basin.

There is a clearly defined pattern of lineaments, withan ENE–WSW mode. This main direction is alsoreflected, although not so clearly, by the orientationanalysis of fractures (joints and faults) at the outcropscale. In the eastern sector of the basin, near theAragonian branch, the main orientation of lineaments andfractures is northwest–southeast, probably due to theinfluence of pre-existing fractures in the Mesozoic andPalaeozoic underlying the Tertiary rocks.

The density of lineaments in the Almazán Basin isgreatest at the proximities of several large-scale struc-tures: the non-outcropping monocline located south ofAlmazán, and the northwest–southeast folds of theAragonian branch.

The fracture pattern in the Almazán Basin, with a dom-inant east–northeast direction, can be linked to the north-east–southwest Neogene compression under a wrenchregime, with a higher development of east–northeastsinistral fractures and faults, and northwest to northeastnormal faults.

Acknowledgements. The authors are very grateful to MaríaJosé Marín and María Pilar Sancho, from the Centro deTratamiento Digital de Imágenes of the Universidad deZaragoza, who developed the automatic exploration program.We also thank Richard J. Lisle and J. Cheeney for their carefulreviews of the manuscript and their discussion about some ofour hypotheses, and F. Plo and A. Cebrián-Guajardo for their

help with statistical methods. This work was supported by pro-jects PB93-1218 of the D.G.I.C.Y.T. and 221-85 of theUniversidad de Zaragoza.

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Appendix. Statistical comparison between fracturingand lineament orientation data

For a more accurate comparison between lineament and fractureorientation, we present here the frequency curves of orientation


Figure A.1. Location of fracture sites within the cell frame used for the sectorial analysis of lineaments. Cell numbers refer to thoseused in Figure A.2 and Table A.1.


Figure A.2. Frequency curves of lineaments and outcrop fractures for each cell (only cells with a minimum number of ten data of eachkind are presented).


Figure A.2. (cont …).

of both types of structures in each cell. Fractures include joints,strike-slip, normal and reverse faults. Fracturing data from outcrops are grouped according to their location (Fig. A.1).Only cells with a minimum number of ten data of each type (lineaments and fractures) are analysed.

Frequency curves allow a visual comparison of both types ofdata and estimation of the correlation between the orientationmaxima of the two curves in each cell (Fig. A.2). Quantitativeanalysis of directional data is not easy to perform when severalorientation maxima exist, and they do not fit a von Mises distri-bution (see for example Gaile & Burt, 1980; Davis, 1986). Thetests commonly used in statistics rely more on the general shapeof the cumulative curves than on the coincidence between orien-tation maxima. We have chosen a non-parametric test, theKolmogorov-Smirnov test, considering that the null hypothesisis that the orientation of fractures at the outcrop scale in eachcell is the same as the orientation of lineaments (that show amore regular pattern).

The results (Table A.1) show that, although the Kolmogorov-Smirnov test gives an idea about the similarity between the pairs of frequency curves compared, the results are not satisfac-tory when comparing curves which have different shape butshow coincident direction maxima (cells 18, 20, 27). In these cells results from the Kolmogorov-Smirnov test indicatethat the null hypothesis can be rejected, whereas the visual comparison indicates a perfect correlation in the orientationmaxima for the two kind of data. In other cases, results comingfrom the Kolmogorov-Smirnov test agree with visual compari-son, both with good (cells 9, 11, 22, 33, 36, 37, 38, 40, 45) andbad (cells 8, 10, 13) correlation. Finally, some cases remainambiguous because of the coincidence of only one or two orien-tation maxima of the two curves (cells 29, 30, 31, 32, 39, 46). In an overall view we find that the Kolmogorov-Smirnov test istoo restrictive, and we consider that critical values larger than 95(significance levels α < 5 %) could be used in this case (seeTable A.1)

268 Lineaments and fracturing, Almazán Basin

Table A.1. Application of the Kolmogorov-Smirnov test to the comparison between frequency curves of fractures at the outcrop scale and lineaments;critical values for the maximum difference (K–S) for different confidence cones (α = 90 to α = 99.5)

Critical values Critical values Critical values Critical values Critical valuesCell number K–S (α = 90) (α = 95) (α = 97.5) (α = 99) (α = 99.5)

3 0.302 0.221 0.252 0.280 0.314 0.3364 0.438 0.330 0.376 0.419 0.469 0.5035 0.600 0.394 0.450 0.501 0.560 0.6016 0.371 0.643 0.733 0.818 0.914 0.9808 0.346 0.207 0.236 0.263 0.294 0.3159 0.236 0.216 0.246 0.274 0.307 0.329

10 0.550 0.253 0.289 0.322 0.360 0.38611 0.194 0.239 0.273 0.304 0.340 0.36513 0.441 0.183 0.208 0.232 0.260 0.27814 0.280 0.158 0.180 0.201 0.224 0.24017 0.321 0.173 0.198 0.221 0.247 0.26418 0.388 0.172 0.196 0.219 0.244 0.26220 0.473 0.296 0.338 0.377 0.421 0.45221 0.231 0.165 0.189 0.210 0.235 0.25222 0.295 0.233 0.266 0.296 0.331 0.35527 0.250 0.185 0.211 0.236 0.263 0.28228 0.183 0.125 0.142 0.159 0.177 0.19029 0.287 0.137 0.157 0.175 0.195 0.20130 0.253 0.197 0.224 0.250 0.279 0.30031 0.236 0.170 0.194 0.216 0.241 0.25932 0.319 0.248 0.283 0.315 0.352 0.37833 0.313 0.251 0.287 0.320 0.357 0.38336 0.269 0.217 0.247 0.276 0.308 0.33137 0.188 0.181 0.206 0.230 0.257 0.27638 0.197 0.164 0.187 0.208 0.233 0.25039 0.555 0.153 0.175 0.195 0.218 0.23440 0.167 0.217 0.248 0.276 0.309 0.33143 0.367 0.186 0.212 0.236 0.264 0.28345 0.095 0.150 0.171 0.191 0.213 0.22946 0.627 0.326 0.372 0.415 0.463 0.497

The numbers in bold indicate the critical values for which the null hypothesis cannot be rejected.

Lineaments and fracturing in the Neogene rocks of the Almazán Basin, northern Spain

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