On the nature of mantle heterogeneities and discontinuities: evidence from a very dense wide-angle shot record Ramon Carbonell * CSIC-Inst. Earth Sciences, Lluis Sole I Sabaris s/n, Jaume Almera, 08028 Barcelona, Spain Received 2 July 2003; received in revised form 29 January 2004; accepted 13 June 2004 Abstract A seismic survey with a receiver spacing of 50 m provided one of the most densely sampled wide-angle seismic reflection images of the lithosphere. This unique data set, recorded by an 18-km-long spread, reveals that at wide-angles the shallow subcrustal mantle features high amplitude reflectivity which contrasts with a lack of reflectivity at latter travel times. This change in the seismic signature is located at approximately 120–150 km depth, which correlates with the depth estimates of the lithosphere–asthenosphere boundary (LAB) of previous DSS studies. This seismic signature can be simulated by two-layer mantle model. Both layers with similar average velocities differ in their degree of heterogeneity. The shallow heterogeneous layer and the deeper and more homogeneous one correlate with the lithosphere and the asthenosphere, respectively. Studies involving surface outcrops of ultramafic massifs and mantle xenoliths infer that the upper mantle is a heterogeneous mixture of ultramafic rocks (lherzolites, harzburgites, pyroxenites, peridotites, dunites, and small amounts of eclogites). Laboratory measurements of physical properties of these mantle rocks indicate that compositional variations alone can account for the wide-angle reflectivity. A temperature increase would homogenize the mixture, decreasing the seismic reflection properties due to melting processes. It is proposed that this would take place below 120–150 km (1200 8C, the LAB). D 2004 Elsevier B.V. All rights reserved. Keywords: Wide-angle seismic reflection; Petro-physical model; Uralides; Lithosphere–asthenosphere boundary 1. Introduction The structure and composition of the upper mantle are still unknown and are controversial issues for the Earth sciences. The mantle bstratigraphyQ , and mantle lithology are very difficult topics to address. The only means to obtain direct knowledge on the mantle composition are the very limited surface exposures of upper mantle units and the petrological study of xenoliths. Recent advances in geothermobarometry and trace element analysis allow the determination of equilibration temperatures of individual grains of a concentrate. These data can be placed in a temperature depth context and provide a way to map mantle 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.025 * Tel.: +34 9340 95410; fax: +34 9341 10012. E-mail address: rcarbo@ija.csic.es. Tectonophysics 388 (2004) 103– 117 www.elsevier.com/locate/tecto