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Mapping the Hales discontinuity in southwestern Spain

AutorAyarza, P.; Carbonell, Ramón ; Palomeras, Imma; Ehsan, Siddique ; Afonso, Juan Carlos ; Diaz, J.
Fecha de publicación2-jul-2015
EditorInternational Union of Geodesy and Geophysics
ResumenDuring the last 15 years, the IBERSEIS and ALCUDIA controlled source experiments have acquired vertical incidence and wide angle seismic reflection data in southwest Spain. Apart from providing detailed information of the crust, these datasets have also imaged a conspicuous sub-crustal reflector. First identified on the IBERSEIS wide-angle reflection dataset, this interface seemed to feature a positive seismic impedance contrast. A boundary located between 62-71 km depth, with a Vp increase from 8.2 km/s to 8.3 km/s allowed us to model clear wide angle reflections found at 180 km offsets. The fact that this reflector was not identified in the coincident vertical incidence dataset led us interpret it as a gradient zone. A correlation with the ‘Hales gradient zone’, i.e. the boundary between spinel and garnet peridotites was our preferred interpretation. The ALCUDIA experiment, later acquired northwards of the IBERSEIS profiles, also shows prominent sub-crustal arrivals. However, these reflections also appear locally in the vertical incidence dataset at 19 s TWT, further constraining the depth at which this feature is located. In addition, the ALCUDIA wide-angle dataset shows refractions that become first arrivals above 180 km. This suggests that this boundary features a positive Vp contrast, varying its character from a transition zone to a sharper discontinuity. Integration of the information provided by the IBERSEIS and ALCUDIA datasets with older data from the ILIHA project, where a mantle feature was identified at similar depths, allows us to map a regional scale upper mantle discontinuity in southwest Iberia. Its Vp increase, sub-horizontal geometry and tectonic context indicate that it is probably related to a phase transition, likely the Hales discontinuity.
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