Preliminary results on the application of the aminoacid racemization technique in the Murcia Region (SE Iberian Peninsula) and their interest in paleoseismological research

Geochronology is a critical issue in paleoseismological research. The aminoacid racemization technique shows important advantages respect to more traditional dating methods; not just for the lower costs and promptness, also because the object to analyze is relatively frequent, in this study: terrestrial gastropods. Furthermore, the costs of the analysis are by far faster and cheaper compared to other dating techniques. Racemization results allow comparing the relative age from different sedimentary units from one trench to another.Additionally, the racemization technique can also be used as a geochronological tool, provided a calibration curve has been first obtained for the particular climate for the area and, ideally, for a particular genus. In this study we show the results obtained from the analysis of 40 samples of terrestrial gastropods from 7 different trenches located in the Murcia Region (SE Spain). Making use of the D/L ratio of aspartic acid we show the coherence found between relative stratigraphic ages and the racemization age. Finally, we show a provisional conversion equation between age of racemization, obtained from Torres et al. (1997) algorithm, and the likely age of the samEl control geocronológico es una cuestión crítica en los estudios de paleosismología. La técnica de racemización de aminoácidos ofrece importantes ventajas respecto a otros métodos de datación, tanto en los costes y rapidez, como en la facilidad de encontrar el objeto de análisis; en este estudio, gasterópodos terrestres. Los resultados permiten comparar la edad relativa entre unidades sedimentarias diferentes de unas zanjas a otras. La técnica de racemización también es una herramienta geocronológica, si bien es necesario primero establecer una curva de calibración para el ambiente climático de la zona e, idealmente, para un género concreto. En este estudio se muestran los resultados obtenidos en 40 muestras de gasterópodos terrestres recogidas en 7 zanjas de investigación paleosismológica en la Región de Murcia. Haciendo uso de la relación D/L del ácido aspártico mostramos la coherencia entre las edades relativas estratigráficas y su edad de racemización. Finalmente, proponemos una relación provisional de conversión entre las edades de racemización obtenidas por el algoritmo de Torres et al. (1997) para gasterópodos de la zona central de la Península Ibérica y la edad probable de las muestra

Variability of the aminoacid racemization relation in a group of Quaternary continental gastropods in Murcia (Spain)

La aplicación de los resultados de un análisis de racemización de aminoácidos en geocronología ofrece importantes ventajas respecto a otras técnicas, al ser esta más rápida y económica. Sin embargo, como con cualquier otra técnica, la racemización no está exenta de fuentes de incertidumbres que deben ser analizadas previamente a la interpretación. En este trabajo se exponen los resultados obtenidos del análisis de racemización de un grupo de 25 gasterópodos continentales fósiles hallados en una excavación de investigación paleosismológica en el municipio de Alhama de Murcia. En particular se compara la ratio entre aminoácidos con estructura dextrógira y levógira (relación D/L) de los ácidos aspártico y glutámico. En primer lugar, se revisan las principales fuentes de incertidumbre que afectan a la racemización. Seguidamente se muestra la metodología empleada para descartar muestras y, finalmente, se aborda una interpretación preliminar de los resultados en relación con la estratigrafía local y la paleosismologíaThe use of the amino acid racemization technique in geochronology offers a great advantage compared to other techniques, as racemization analyses are usually much faster and cheaper. However, as in any other technique, amino acid racemization is subject to a number of uncertainties, which have to be thoroughly analysed previously to any interpretation of the data. In this work, we present the results obtained in a group of 25 continental gastropods found in a paleoseismological trench in Alhama de Murcia (Betic Range, Spain). We carry out the comparison of dextrorotatory and levorotatory configuration (relation D/L) of the aspartic and glutamic acids. First, we review the different sources of uncertainty in the racemization process. Subsequently we show the methodology followed for discarding samples and, finally, we interpret in preliminary terms the results in relation to the local stratigraphy and paleoseismolog

Sierra de Cártama: Rcccnt activity fold in the Western Betic Chain (Hoya de Málaga)

In this work is shown how an active fold (the Sierra de Cartama fault) is marked by the drainage pattern and topography, besides of structural data. The Sierra de Cartama fold is an asymmetric fold with double plunge and determines the shape of an elongated saw in the Western Betic Chains (South of Spain). Watching the drainage patterns, it can be noticed how streams and creeks round the saw. The uplift of the saw raise the valleys steeply, even leaving a hanging valley ("wind gap ") at the western part of the structure. Regarding to these observations, the lateral propagation of the fold seems to be toward the west. The proposed structure implies the existence of a compressive neotectonic activity in the zone. The compressive horizontal direction is N10e-30eW, perpendicular to the fold axis. In this tectonic environment it could be possible to interpret a reverse fault in deep associated with the fold growt

Morphotectonic evidences of recent activity of the El Salvador Fault Zone in the Ilopango – San Vicente segment

El Salvador Fault Zone is one of the main strike-slip structures of the Central American volcanic arc. The February 2001 earthquake took place in the Ilopango – San Vicente segment of this fault zone. In this work we show new morphotectonical evidences, that support the hypothesis that this segment is highly active, and accommodates an important amount of deformation. The pattern of the fluvial network, and the features that the Jiboa river shows along its trace, allows us to estimate minimum accommodated dipslip and strike-slip motions. The amounts estimated are 100 m and 1500 m respectively for dip-slip and strike-slip, which confirms that the characteristic event type for this fault is an strike-slip event with a little proportion of dip-slip. Precise datations are needed in order to obtain better seismic hazard assesses for this zon

Geotechnical and geomechanical characterization of the fault gouge of the active Alhama de Murcia fault

En este trabajo se presenta el estudio de los parámetros mecánicos de la roca de falla (fault gouge) de la falla de Alhama de Murcia (FAM) próxima al bloque levantado. Esta falla es una de las más activas de la Península Ibérica y destaca por tener segmentos con un espesor de roca de falla (FG) exhumada de 50-80 m de espesor en régimen frágil. Su composición mineralógica se compone mayoritariamente por minerales del grupo de la mica (moscovita y paragonita) y cuarzo; en la fracción arcillosa predomina la ¡Hita acompañada por caolinita. Muestra una marcada fábrica tectónica caracterizada por la orientación preferente de los minerales laminares según una textura anastomosada que controla la resistencia mecánica de la falla. Para este estudio se han tomado muestras inalteradas procedentes de dos sondeos sobre los que se han realizado distintos ensayos de esfuerzo-deformación considerando diferentes orientaciones de los esfuerzos respecto a la fábrica. El ángulo de fricción (50°) para los planos orientados desfavorablemente. Estos elevados valores fueron obtenidos también en muestras remoldeadas de FG sin orientaciones preferentesWe present the results of the mechanical parameters of natural clay rich fault-rock (fault gouge) from one of the most active faults in the Iberian Peninsula, Alhama de Murcia Fault (FAM). FAM shows segments partially formed by exhumed fine grained fault gouge (FG) with a 50-80 m thickness developed mainly in a brittle regime. It is composed mainly by mica (muscovite and paragonite) and quartz, the clay minerals are mainly illite with some caolinite. The FG shows a notable tectonic fabric characterized by the preferred orientation of plate -like minerals arranged in an anastom osing texture which controls the mechanical behavior of the fault. Undisturbed samples, taken from two fault observatory boreholes, have been used for laboratory tests. The result of the study shows that there is important variability in strength properties depending on the stress orientation in relation to fabric. The friction angle (rp) varies between very low values (16°-26°) for planes oriented favorably to the tectonic fabric to very high values (>50°) for planes unfavorably oriented. This high value of friction angle was a lso obtained in reconstructed and not oriented FG sample