The Gibraltar subduction: A decade of new geophysical data

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• 作者：M.-A. Gutscher ; S. Dominguez ; G.K. Westbrook ; P. Le Roy ; F. Rosas ; J.C. Duarte ; P. Terrinha ; J.M. Mir ; a ; D. Graindorge ; A. Gailler ; V. Sallares ; R. Bartolome
• 关键词：Iberia ; Roll-back subduction ; Tethys oceanic lithosphere ; Accretionary wedge ; Active deformation ; Earthquakes
• 刊名：Tectonophysics
• 出版年：2012
• 出版时间：29 October, 2012
• 年：2012
• 卷：574-575
• 期：Complete
• 页码：72-91
• 全文大小：6181 K

文摘

The Gibraltar arc, spans a complex portion of the Africa-Eurasia plate boundary marked by slow oblique convergence and intermediate and deep focus seismicity. The seemingly contradictory observations of a young extensional marine basin surrounded by an arcuate fold-and-thrust belt, have led to competing geodynamic models (delamination and subduction). Geophysical data acquired in the past decade provide a test for these models and support a narrow east-dipping, subduction zone. Seismic refraction studies indicate oceanic crust below the western Gulf of Cadiz. Tomography of the upper mantle reveals a steep, east-dipping high P-wave velocity body, beneath Gibraltar. The anisotropic mantle fabric from SKS splitting shows arc-parallel ¡°fast directions¡±, consistent with toroidal flow around a narrow, westward retreating subducting slab. The accompanying WSW advance of the Rif-Betic mountain belt has constructed a thick pile of deformed sediments, an accretionary wedge, characterized by west-vergent thrust anticlines. Bathymetric swath©\mapping images an asymmetric embayment at the deformation front where a 2 km high basement ridge has collided. Subduction has slowed significantly since 5 Ma, but deformation of recent sediments and abundant mud volcanoes suggest ongoing activity in the accretionary wedge. Three possible origins for this deformation are discussed; gravitational spreading, overall NW-SE convergence between Africa and Iberia and finally a WSW tectonic push from slow, but ongoing roll-back subduction. In the absence of arc volcanism and shallow dipping thrust type earthquakes, evidence in favor of present-day subduction can only be indirect and remains the object of debate. Continued activity of the subduction offers a possible explanation for great (M > 8.5) earthquakes known to affect the area, like the famous 1755 Great Lisbon earthquake. Recent GPS studies show SW motion of stations in N Morocco at velocities of 3-6 mm/yr indicating the presence of an independent block, a ¡°Rif-Betic-Alboran¡± microplate, situated between Iberia and Africa.