LINEAMENT ANALYSIS ON LANDSAT IMAGERY IN THE CENTRAL BADAJOZ-CORDOBA SHEAR ZONE . ARGUMENTS FOR BRITTLE STAIN PARTITIONING AND BLOCK ROTATION UNDER TRANSPRESSION

Se realiza un estudio estadístico de los tecto-lineamientos observados por tres analistas diferentes sobre una Cotografia de satélite de la porción central de la Zona de Cizalla de BadajozCórdoba (Macizo Ibérico Meridional). Los resultados alcanzados permiten establecer la presencia de dos dominios estructurales separados entre sí por una banda central que coincide con el afloramiento de las rocas más metamórficas de la Zona de Cizalla. Cada uno de estos tres dominios tiene una distribución particular de lineamientos estructurales, explicándose el conjunto en el marco de un cizallamiento sinistroso en régimen transpresivo en una banda que se ajusta bastante a la distribución conocida para ese área de fallas y accidentes tardi-hercínicos de desgarre y a la posición de una anomalía gravimétrica de Bouguer.

Introduction.Remotely-sensed Iineaments: Applications and constraints So far, the uses of remote sensing techniques (Weber, 1985) in the areas of geologic surveying, such as geological mapping, seismology, metallogeny and mineral exploration, and the search of potential sites of petroleum and gas deposits, have known large advances.From the different techniques, the recognition and analysis of linear features on satellite images has provided very useful devices for mineral exploration, mainly when combined with geochemical, geophysical and field data (Antón-Pacheco & Sander-son, 1989;Martínez-Alonso et aL, 1989;Tsombos & Kalogeropoulos, 1989;Widdowson, 1989) and groundwater exploration (Sanz de Galdeano et aL, 1985).In the same way, lineament analyses are ofien related to tectonic studies at all scales, from the microtectonic (Mekarinia et aL, 1989) through the fold-belt and plate-boundary geodynamics (Poscolieri & Salvi, 1985;  Wise et aL, 1985).As a consequence of this, the lineament tectonics rises as an independent branch of Structural Geology and Geotectonics in which consideration is given to the structural lineament analysis of ancient and modero mountain belts, platforrns, sedimentary basins, as well as to the lineament genesis and its relationships to the characteristics and annomalies known in the crust and the upper mantle (Kats el al., 1986).
The procedure of lineament analysis on remotely sensed images involves the computer-aided processing of large amounts of data sets (Koronovskii el al., 1986) as well as the performance oC areal density distributions by means oC contour line maps (which may lead to the identification of regional structural trends), the consideration of subparallel lineament swarms rather than individual lines, and the rejection of sorne linear elements while others are combined into lineament zones or lineament frameworks.The interpretation oC lineaments is based on the spatial correlation of remotely sensed images of geological objects as well as on the density of the available geological-geophysical data.This interpretation involves successive approximation to the object, generating the remote-sensing image from the better known to the less studied areas (Trofimov, 1985).
The detection of lineaments holds a series of scarcity sources, a group of image features (such as spectral range of wavelength, illumination angle or restitution) as well as environmental (point of observation, quality of lightning) or psicological conditions (optical illusions, knowledge of the studied area, prejudgments) should be considered as a cause of scattering in the studies of this type (Farrow, 1975).Moreover, every analyst holds its own way of photointerpretation, and this constitutes an additional source of scatter.As a consequence of all this, bearing in mind the influence of the so-called «human component» on photointerpretation, three different analysts have performed a statistical study of photolines on a geologically well known area of the lberian Massif: the Badajoz-Córdoba Shear Zone.This area occupies most of the central part of the picture 25DEC81 2-218-33701 1143-1800 A 06MAY85 processed by TELESPAZIO Cor ESA-EARTHNET.
The aim of this paper is to contribute to the knowledge of the Badajoz-Córdoba Shear Zone in order to ascertain either the presence oC fault-bounded blocks, tbe characteristics of the relations between latehercynian faulting and remotely sensed lineaments, and the tectonic regime involved in the variscan orogenic closing-stages.

Geological features of the study area
As it was pointed aboye, the study area constitutes (fig.1; fault distribution taken from Apalategui & Higueras, 1983;Apalategui el al., 1983;Arriola el al., 1983 andOdriozola el al., 1983) the central part of tbe Badajoz-Córdoba Shear Zone, which is located within the Hercynian lberian Massif to the SW of the lberian Peninsula.This major structure of botb tbe B. ABALOS, R. RAMON-LLUCH, L. M. MARTINEZ-TORRES lberian Massif and the European Variscan Belt separates two terranes, the Central-Iberian to the N and the Ossa-Morena to the S (Apalategui & Higueras, 1983;Apalategui & Quesada, 1987).This belt has undergone a long tectonometamorphic history since Upper Proterozoic up to Lower Carboniferous, a 300 m.y.time span (García-Casquero el al., 1985 and1988;Quesada, 1989;Shafer el al., 1989).The different episodes involved initial eclogitic metamorphism (Abalos & Gil Ibarguchi, 1989) and large scale thrusting followed by an extensional tectonic regirne during Lower Paleozoic times, and then a major transpressive sinistral evolution from ductile (Abalos, 1989) through brittle conditions.Tbe latter episode masks aH the previous structures (as shown in the Upper Paleozoic fault distribution map from fig. 1) and imprints the cbaracteristics oC a sinistral intracontinental sbear zone to the whole area (Arthaud & Matte, 1975y 1977).

Fault distribution and the Iineament anaIysis
The roses of lineament orientation distribution rnaxima shown in fig. 2  The most obvious and widespread fault system is the N12o-130E, either to tbe NE and SW of the fault boundary between the Ossa-Morena and Centrallberian Zones.The faults of this system are Lower Paleozoic thrusts separating tectonic units with associated normal or inverted metamorphic jumps.These faults are reactivated during a sinistral wrenching episode during Lower Carboniferous times, and are cross-eut by broadly E-W sinistral faults (N90E system) and N-S (systerns N20E and N170E) apparently dextral faults.The latter are common in a NW-SE-trending band at the central area of fig. 1, and cross-eut the Lower Carboniferous rnaterials of the Matachel Basin.NE-SW systerns occur as minor faults aH over the studied area but, from cartographic evidences, they do not seern to play a significative role.
The linearnent frameworks of the analysts A (R.R.LL.; label A in fig. 3 (L.M.M.T.; label C) have been eonstrueted to be compared between them with the aim of ascertaining results of geologie relevance.The orientation distribution patterns based on field geology, when eompared with the orientation diagrams of fig.3, let establish that the lineament systems observed by every analyst may be interpreted in terms of the aboye explained fault systems despite the quantitative differenees among them.

Structural-kinematic interpretation and geodynamic implications
The lineament density eontour map (fig.4) provided by lineament framework B in fig. 3 was drawn (by hand) on the basis of the areal distribution of the interseetions among different photolines.From a tectonic point of view, the kinematic inferenees supported by the information yielded by this lineament den- the boundary Ossa-Morena-Central-Iberian Zoite (fig.5) and subsidiary N-S corridors of lineament high density, which could thus represent the presence of a sinistral transpressive deformation regime for the area.The N120-130E band separates two IOnes with different lineament patterns and constitutes a domain boundary.Within each of the dornains referred aboye, the lineaments contained show a relatively constant arrangement which lets the deffinition of tectonic blocks with more or less sharp boundaries (Vegas el al., 1987).In fact, field geology may support these inferences, as large N130E sinistral faults define a group of NW-SE bands containing altemately minor E-W dextral faults or N-S-trending dextral ones.These lineament arrangements (fig.6) may explain the strain partitioning necessary to the accomodation of the deformation due to a E-W shortening between two major blocks by means of block rotations (Garfunkel & Ron, 1985) along NW-SE bands.The minor blocks contained in the NW-SE bands undergo clockwise or anticlockwise rotations as a result of the orientation of the more widespread minor faults involved respect the E-W general direction of shortening.
Bearing in mind this kinematic hypothesis, sorne lineament combinations may be searched for in networks A, B and C of fig. 2 to fit this model.As shown in fig.6, the more successful combinations involve lineaments of the systems NI20-130E, N-S and N40E.The latter, together with those of the N-S system, define a conjugate array from wich an appro--: . .-:t.~,,: ximately NNE-SSW compressional stress component rnay be deduced.Lineaments of the two first systems not only define lozenge-shaped areas or domains, they are arranged in subparallel lineament swarms as follows: the N120-130E swarms are bounded by N-S linearnen15 and occupy areas devoided of linearnents which strike is close to N-S (see Bl, B2 and A2 networks in fig.6).On tbe contrary, tbe N-S lineaments display NI20-130E-trending arrangernents and are cross-eut by N120-130E lineaments.These facts, together with the presence of lozenge-shaped dornains nearly lacking lineamen15, which boundaries fit the bands containing a great deal of N130E-trending lineaments, let us establish the presence of a central sheared domain (dotted area in the central squared graph of fig. 6) separating a northern area coinciding with the Central-Iberian Zone and a southern one which corresponds to the Ossa-Morena Zone.
In this way the presence of a clear block tectonism may be inferred from remotely sensed lineament analysiso Too block tectonism takes place at a escale larger than the reported aboye in relation with fault distributions.In fact, the central dotted band in the squared map from fig. 6 contains some of the NW-SEtrending domains involving clockwise and anticlockwise fault-bounded block rotations cited in a preceding paragraph.
This conclusion, which is not evident from field geology (but is supported by the fault distribution rnap of fig. 1) is strengthened by the additional fact that a N130E Bouguer gravity anornaly (Gaibar, 1976) fi15 such band, and separates a northern dornain of negative gravity annomalies from a southern one with positive values of the referred pararneter.The N-S trending lineaments fit with local N-S arrangemen15 of the boundary between the positive and negative areas as well (Gaibar, 1976).

Conclusions
The structural-kinematic rnodel discussed in the previous section rather agrees with the tectonic regime involved in the E-W shortening of a roughly NW-SE crustal band situated between two larger and harder blocks: the Ossa-Morena and Central-Iberian crustal blocks.This band constitutes a restrained intracontinental plate boundary and records a transpressive sinistral wrenching which involves the generation and relative rnotion/rotation of large fault-bounded domains.This deformation process took place during the Lower Carboniferous and might explain the occurrence of extensional areas fullfilled by the deposition of terrigenous and volcanic sequences then shortened as a result of the progressive readjustment of crustal blocks.The location of this weak crustal band should not be considered as a suture due to tbe collision between two plates during the Upper Paleozoico On the contrary, the tectonic, petrologic and geochronological data available support the hypothesis that the forementiond suture zone resulted from the Precambrian-Lower Paleozoic collision between tbe Ossa-Morena and a northern plateo Nevertheless, as a result of such plate convergence, a weak crustal band was created which would acomrnodate later deformations, acting during the Upper Paleozoic as a ductile-brittle intracontinental shear zone.

AKNOWLEDGEMENTS
We are thankful to P. Tsombos at the Institute of Geology and Mineral Exploration of Athens (Greece) for the stimulating encouragement which made possible the realization of this paper and to an annonimous referee for comments and suggestions to the original manuscript.
have been constructed from the fault distribution shown in fig. 1.They correspond to the SW (A) and NE (B) parts of the central Badajoz-Córdoba Shear Zone, considered as a whole in rose C. A and B are ascribable to two neighboring areas separated by the Hornachos Fault, one of the proposed boundaries between the Ossa-Morena and the Central-Iberian Zones of tbe Iberian Hercynian Massif.This figure, referred to the orientation distribution maxima of the faults and tectonic boundaries shown in fig. 1, rnay be used here to ascertain either the main fault systerns involved (rose C) and the differences between the patterns corresponding to the N (rose B) and S (rose A) of the fault separating the Ossa-Morena and Central-Iberian terranes.