Seismic structure of the crust beneath the Rif Cordillera

Abstract

In this thesis I present a geophysical study that aims to define and characterize the crustal structure in the northern part of Morocco, especially beneath the Rif Cordillera. The geophysical data used in this thesis was acquired during the RIFSIS (2011) survey within the framework of the RIFSIS project, which was designated to overcome the lack of results on crustal structure in the northern part of Morocco. The RIFSIS project is mainly based on the acquisition of new wide-angle seismic profiles and on the integration of these data with the various seismic data sets currently available, such as the GASSIS, PICASSO, Topo-Iberia projects.

The present work is based on the processing, modeling and interpretation of Wide-Angle seismic (WA) profiles, Receiver Functions (RFs) data projects, and has been focused on the crust below Morocco. The WA data was acquired during the RIFSIS survey in the Rif Cordillera from two profiles, the north-south profile (~430 km long) and the east-west profile (~330 km long). Both profiles extend across the Rif orogen, from the Middle Atlas to the Betic Range, and from the Gharb Basin to the Algerian border, respectively. The RFs data was acquired from the temporary deployment of seismological broadband stations from the Topo-Iberia and PICASSO projects. These stations were deployed along all the north Morocco territory. In this thesis I also have worked with Pn tomography of the uppermost mantle seismic velocity and anisotropy in the Euro-Mediterranean region.

The results of the Pn tomography show significant features well correlated with the geological structures and evidence the heterogeneous character of the Euro-Mediterranean lithosphere. The processing of RIFSIS seismic data provides us the tectonic crustal structure model of the Rif Cordillera, whereas the modeling of WA data from travel-time forward modeling provides 2-D seismic velocity distribution of the crust and uppermost mantle. Furthermore, from the results that these projects provide, it becomes possible to infer the geometry of the crustal-mantle boundary, also presented in this thesis. These velocity-depth models hold major variations in the crustal thickness, especially the EW profile that shows a rapid change of 15-20 km in Moho depths within 30 km horizontal distances. Maximum depths around 50 km are found below the external Rif domain, while thinnest values of about 29 km are located eastwards, in the foreland and Atlasic terrances up to the Algerian border. The model along NS profile displays also marked differences in crustal thickness, ranging from 40 km beneath the Betics and internal Rif sampled domains, to 47-48 km beneath external Rif, and a progressive thinning southwards till Middle Atlas domain where the Moho is found at 32 km depth.

In this thesis I also processed the WA data as low-fold WA seismic stack images. The deployment logistics during the RIFSIS project allowed that all the stations recorded all shots, not just aligned with the two profiles. From these profiles we can obtain a deep crustal image in­between the profiles and the shots. Although some uncertainties may be inherent to those approaches, a large crustal root, reaching more than 50 km, is well documented in the central part of the Rif Cordillera. At the same time I performed a RFs study in the Rif domain area, to determine the crustal thickness beneath the northern Morocco. The results obtained from the different methodologies used in this thesis reveal a crustal root well-documented in the central part of the Rif Cordillera, which is supported by the area large negative Bouguer anomaly values.

Aquesta tesi es centra en l’estudi de l’escorça cortical en el nord del Marroc, especialment sota de la Serralada del Rif, amb el principal objectiu de cobrir la falta de dades i resultats sobre l’estructura cortical de la zona. Per això, es realitza el processat, modelat i interpretació geològica de dos perfils geofísics de sísmica de gran angle adquirits durant la campanya sísmica RIFSIS (2011) en la Serralada del Rif, que s’han processat posteriorment com a com perfils low­fold. S’ha treballat amb dades de funcions receptores dels projectes Topo-Iberia i PICASSO. També s’ha treballat amb imatges tomografies del mantell superior sota la regió Euro-Mediterrània a partir d’ones Pn. A partir d’aquestes dades s’han pogut obtenir els resultats següents: 1. Dos models de velocitats de propagació d’ones P (Vp) i de la geometria de la Moho. 2. Obtenció d’imatges de l’escorça profunda al llarg dels perfils sísmics i entre els perfils i els dispars a partir dels perfils low-fold. 3. Obtenció d’un mapa de la topografia de la Moho del nord del Marroc a partir de les funcions receptores. 4. Els resultats obtinguts de les tomografies de Pn són altament coincidents amb les imatges tomogràfiques 3-D i mostren significants característiques ben correlacionades amb les estructures geològiques de l’Euro-Mediterrani. Amb aquestes metodologies s’ha pogut documentar una arrel cortical en la part central de la Serralada del Rif, resultats que coincideixen amb l’anomalia negativa de Bouguer observada en la regió. Finalment es proposa que la convergència de les plaques Ibèrica i Africana, juntament amb la possibilitat d’un slab sota el mar d’Alboran adjunt a la litosfera a la part central del Rif, podria ser l’explicació de la formació de l’arrel cortical sota el mateix Rif.