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Geofísica internacional
On-line version ISSN 2954-436XPrint version ISSN 0016-7169
Geofís. Intl vol.44 n.3 Ciudad de México Jul./Sep. 2005
Articles
A palaeomagnetic study of Upper Pliocene volcanic rocks in the area of the Levant Fault near Homs, western Syria
1 Department of Geology, The University of Damascus, Syria
2 School Earth Ocean & Environmental Sciences, University of Plymouth, PL4 8AA, U.K.
Thermal demagnetisation of samples from 75 sites in basaltic rocks, mostly within 5 km of the Levant Fault, identifies consistent directions of remanence in most sites. Normal, Reversed and Intermediate polarities are identified, with Reversed polarities dominating. The Normal and Reversed sites are similarly magnetised, 5.6±5.3 A/m, while those of sites of Intermediate or undefined direction are somewhat more strongly magnetised, 11.9±9.5 A/m. The low field susceptibilities are consistent, 30.9±15.2 mSI, implying the geomagnetic field may have been somewhat stronger than during Normal or Reversed polarity. When the mean directions of the Normal sites are reversed, all sites have similar mean inclination values, 48.2° (δInc. = 2.7°), but the site mean declinations show a regional pattern. Sites from the 3 regions west of the Levant Fault zone (Marmarita, Qalat El-Hossen, Tel Kalakh) have similar declinations, 186° (δDec. = 5.4°). These are consistent with those from the Sufer and E. Ain Kut regions, 197° (δDec. = 3.9°), furthest east of the Levant Fault zone. All these localities have directions that are consistent with those expected for this part of Arabia for this time. However, all sites east and close to the Levant Fault zone (Bahur, W. Ain-Kut, Mzeineh and Buqeia) have more eastern declinations, averaging 154.0±12.0°. The declination difference, 37.3°, indicates the presence of rotated blocks adjacent to the eastern side of the Levant Fault. Such a rotation is consistent with the 25 km displace-ment predicted for this location and appears consistent with the subsequent volcanological and tectonic evolution of the Levant Fault, suggesting significant rotation and seismic activity during the last 5 Ma.
KEY WORDS: Palaeomagnetism; tectonic rotations; Levant Fault; Upper Pliocene; Syria; Lebanon
La desmagnetización térmica de 75 sitios en rocas basálticas, la mayor parte a menos de 5 km de la Falla Levante, identifica direcciones de remanencia consistentes en la mayoría de los sitios. Las polaridades identificadas son Normales, Inversas e Intermedias, dominando las Inversas. Los sitios Normales e Inversos están magnetizados similarmente, 5.6±5.3 A/m, en tanto que los sitios Intermedios o direcciones indefinidas están algo más fuertemente magnetizados, 11.9±9.5 A/m. Las susceptibilidades de campo bajo son consistentes, 30.9±15.2 mSI, lo que implica que el campo geomagnético pudo haber sido algo más intenso que durante las polaridades Normales o Inversas. Cuando la media de las direcciones de los sitios Normales se invierten, todos los sitios tienen valores similares de la inclinación media, 48.2° (δInc. = 2.7°), pero la declinación media de las de los sitios muestra un patrón regional. Sitios de 3 regiones al oeste de la zona de Falla Levant (Marmarita, Qalat El-Hossen, Tel Kalakh) tienen declinaciones similares, 186° (δDec. = 5.4°), declinaciones que son consistentes con las esperadas para esta parte de Arabia para este tiempo. Sin embargo, todos los sitios al este y cercanos a la zona de Falla Levant (Bahur, W. Ain-Kut, Mzeineh y Buqeia) tienen declinaciones más al este, en promedio de 154.0±12.0°. La diferencia de declinación, 37.3°, indica la presencia de bloques rotados adyacentes al lado oriental de la Falla Levante. Tal rotación es consistente con el desplazamiento de 25 km predicho para esta localidad y parece consistente con la subsecuente evolución volcánica y tectónica de la Falla Levante, sugiriendo rotaciones significativas y actividad sísmica durante los últimos 5 Ma.
PALABRAS CLAVE: Paleomagnetismo; rotaciones tectónicas; Falla Levante; Plioceno superior; Siria; Líbano
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Received: January 13, 2004; Accepted: March 29, 2005
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