Alboran Sea igneous intrusions revealed by magnetic anomalies and related to extensional opening constrain the ongoing continental collision

Abstract

The Alboran Sea is a Neogene basin formed by the extension of a continental crustal domain (Alboran Domain) during its westwards displacement. This process involved the collision of its margins with the African and Iberian palaeomargins, which led to the formation of the Betic and Rif cordilleras. The Alboran Sea is characterized by the presence of several highs that correspond to volcanic edifices and/or submarine ranges. The distribution of the magnetic anomalies on this basin shows some dipoles centered in these volcanic edifices. The most intense dipoles are aligned in two groups in the center of the basin: a NE-SW elongated group starts in the Ibn-Batouta bank, runs eastwards centered in the Alboran Channel along the northern boundary of the Alboran Ridge and continues eastwards, changing to a NW-SE group of aligned dipoles, along the Yusuf fault. Since these anomalies are not related to surface volcanic highs, several profiles have been modeled across these dipoles. The models show that the sources are probably crustal scale, basic igneous intrusions located at depths from 8 to 14 km. It is remarkable that these intrusions are northwards displaced with respect to the Alboran Ridge, which seems to be the main volcanic high of the Alboran Sea. The orientation of these groups of magnetic dipoles and the absence of a clear relation with the Neogene calc-alkaline volcanism of the main highs support the idea that these intrusions may be related to the rifting of meso-Mediterranean terrains that formed the Alboran Domain (like the AlKaPeCa Domain) during Oligocene-Early Miocene. This rifting process led to the spreading of the Algerian basin and the individualization of the Alboran Domain from other domains. Furthermore, that orientation shows a similar trend to the rift axis proposed for that period, so the intrusions could represent the western tip of that rift. Later, these intrusions could be affected by the STEP fault (Subduction Tear Edge Propagator fault) that accommodated the westward displacement of the Alboran Domain along its southern limit and that is related with the formation of the Yusuf fault. Since Tortonian, the tectonic inversion of the Alboran basin was characterized by the prevalence of the NNW-SSE compression over the ESE-WSW extension, which continues today. During Pliocene and Quaternary, this stress led to the formation of a tectonic indentation, whose front, the Alboran Ridge, is located next to the main intrusions. Thus, it is likely that the intrusions act as a backstop that have favored the folding and uplift of the Alboran Ridge in the front of the indenter. This constitutes an excellent example of how intrusions originated during the extensional, initial stages of a basin can condition and control the style of the later tectonic inversion of this basin in the context of the peri-Mediterranean chains.