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One dimensional geometric analysis has been carried out in several scan lines from 885 measures of twins in calcite grains to determine grain width (in microns) and twin density (number of twins.mm-1 ) distributions. Grain width and twin density have a good fit to the log-normal frequency distribution. Twinning in calcite implies intracrystaline deformation mechanism with low shear stress. When the process begins low grain width and calcite twins are developed with a probably random distribution what could be supported by a negative exponential distribution tendency. The twinning process continues until a "critical" value of grain width and density which is going to influence in the scaling process, and becoming the distribution to log-normal type. But some data also conform to a power-law (fractal) frequency distribution from determined range or sizes (300 to WOO mm) and density (2 to W twins.mm-1) with some superimposed random (negative-exponential) elements, possible due to the irregularities at grain scale, but also because this systems show multifractal behavior.

Pleistocene;karst¡c deposits developed into Upper Cretaceous limestones of the Prebetic zone, in the . boundary region between the Iberian and Betic chains, were sampled in order to investigate the calcite e-tyyin.occurrence in these modern sediments. Amazingly, sparry calcite grains from the karstic coating have mechanical twins, and they are very abundant in these grains close to recent faults (twin density of 5.7 twins/mm). Analysis of calcite e-twining by inverse methods reveal that the region has undergone an extensional regime with maximum shortening direction NE-SW. Similar results were obtained from fault population analysis. Which points out that calcite e-twin analysis are useful to establish the neotectonic principal stresses orientation in this region. The e-twin analysis techniques based on strain determinations also yield good results

Calcite e-twin density (twins/mm) and percentage of calcite grains with one and two twin sets from sparry grains that fill microveins were used to precise the differential stress related with the inversion of the Cameros Basin, a Mesozoic sedimentary basin located in the northwestern part of Iberian Chain (Spain). The results obtained using techniques based on the percentage of twined grains yield better results that those techniques based on twin density. The deduced stress magnitudes fit well with the differential stress interval proposed for fold-thrust belt domains. Combination of twin data with rock mechanics data provides adequate estimations of differential stresses.

Pleistocene;karst¡c deposits developed into Upper Cretaceous limestones of the Prebetic zone, in the . boundary region between the Iberian and Betic chains, were sampled in order to investigate the calcite e-tyyin.occurrence in these modern sediments. Amazingly, sparry calcite grains from the karstic coating have mechanical twins, and they are very abundant in these grains close to recent faults (twin density of 5.7 twins/mm). Analysis of calcite e-twining by inverse methods reveal that the region has undergone an extensional regime with maximum shortening direction NE-SW. Similar results were obtained from fault population analysis. Which points out that calcite e-twin analysis are useful to establish the neotectonic principal stresses orientation in this region. The e-twin analysis techniques based on strain determinations also yield good results

The study of c-axes distribution from sparry grains that fill microveins were used to define maximum horizontal extension directions (

The study of c-axes distribution from sparry grains that fill microveins were used to define maximum horizontal extension directions (

In Hurd Peninsula (Livingston Island, Western Antarctica) has been analysed the orientation of calcite c-axes and e-twin from the sparitic grains that filled the hydrothermal veins and lodes in this region. The c-axes orientations allow to establish that during the Upper-Cretaceous tectonic episode, when these veins were developed, took place an E-W extension, although other extension direction, e.g. NW-SE, also are active. The analysis of the calcite e-twins show the existence of compressional strain ellipsoids with two different orientations. A first group has an orientation that is compatible with a N-S compression direction, i.e. the same direction associated with the development of the veins. While the other is close to E-W and It is probably related with local tectonic structures. The presence of type I and II e-twins allow to deduce that the deformation temperature ranges between 150 to 300°C. Both, the strain intensity and the temperature increases towards the south part of Hurd Peninsula

In Hurd Peninsula (Livingston Island, Western Antarctica) has been analysed the orientation of calcite c-axes and e-twin from the sparitic grains that filled the hydrothermal veins and lodes in this region. The c-axes orientations allow to establish that during the Upper-Cretaceous tectonic episode, when these veins were developed, took place an E-W extension, although other extension direction, e.g. NW-SE, also are active. The analysis of the calcite e-twins show the existence of compressional strain ellipsoids with two different orientations. A first group has an orientation that is compatible with a N-S compression direction, i.e. the same direction associated with the development of the veins. While the other is close to E-W and It is probably related with local tectonic structures. The presence of type I and II e-twins allow to deduce that the deformation temperature ranges between 150 to 300°C. Both, the strain intensity and the temperature increases towards the south part of Hurd Peninsula

In the central part of the Cameros basin (NW Iberian Chain) has been analysed the orientation of calcite c-axes and e-twin from the sparitic grains that filled extension veins in this region. The c-axls orientations allow to establish that during the mesozoic extensional episode, when was developed this basin, besides of an extension NE-SW also existed another with a NNW-SSE trend, this last extension direction probably has a regional character. The calcite e-twins show the existence of two different orientations for strain ellipsoids. A first group has orientations that would be compatible with structures associated with the development of the basin inversion and with the NE-SW schistosity. While the other is almost perpendicular to the former and it is compatible with a NW-SE compression not described until the moment. This last deformation episode probably has a regional meaning (Cuadarrama compression? from Capote et al., 1990), and it likely also has a relative importance in the development of structures that show this region

In the central part of the Cameros basin (NW Iberian Chain) has been analysed the orientation of calcite c-axes and e-twin from the sparitic grains that filled extension veins in this region. The c-axls orientations allow to establish that during the mesozoic extensional episode, when was developed this basin, besides of an extension NE-SW also existed another with a NNW-SSE trend, this last extension direction probably has a regional character. The calcite e-twins show the existence of two different orientations for strain ellipsoids. A first group has orientations that would be compatible with structures associated with the development of the basin inversion and with the NE-SW schistosity. While the other is almost perpendicular to the former and it is compatible with a NW-SE compression not described until the moment. This last deformation episode probably has a regional meaning (Cuadarrama compression? from Capote et al., 1990), and it likely also has a relative importance in the development of structures that show this region

The twinned-calcite strain-gauge technique allows determination of main axis strain magnitudes and orientations in limestones rocks midly deformed (Groshong, 1972), during Alpine orogeny in the Iberian Chain. This estudy combinates calcite with analysis. The calculated paleostrain and paleostress tensor have similar characteristics and a good correlation have been established between these two procedures.

The twinned-calcite strain-gauge technique allows determination of main axis strain magnitudes and orientations in limestones rocks midly deformed (Groshong, 1972), during Alpine orogeny in the Iberian Chain. This estudy combinates calcite with analysis. The calculated paleostrain and paleostress tensor have similar characteristics and a good correlation have been established between these two procedures.

The strain tensor in limestones has been calculated using the strain-gauge technique (Croshong, 1972). This method is based in the analysis of the mechanical twinning in calcite. In the same outcrops, these-results have been compared with the data from the fracturation analysis. The calculated paleostrain and paleostress tensors have similar characteristics, and a good correlation could be established between these two procedures. The twin density has also been used for estimating the paleostress levels

The strain tensor in limestones has been calculated using the strain-gauge technique (Croshong, 1972). This method is based in the analysis of the mechanical twinning in calcite. In the same outcrops, these-results have been compared with the data from the fracturation analysis. The calculated paleostrain and paleostress tensors have similar characteristics, and a good correlation could be established between these two procedures. The twin density has also been used for estimating the paleostress levels