Moho geometry beneath northern Algeria from gravity data inversion

Highlights

• 3D model of the crust beneath northern Algeria from Bouguer anomaly inversion.

• The first study that provides a Moho depth map for northern Algeria.

• Published wide-angle seismic profiles used to constrain the inversion parameters.

• Could help achieve a 3D velocity model for a better location of the seismicity.

Abstract

The aim of this study is to provide a Moho geometry model for northern Algeria where the few studies that have been carried out concern only restricted areas. The method we used is based on the 3D inversion of the Bouguer anomaly data computed by the Bureau Gravimétrique International (BGI) from satellite data.

The proposed Moho depths are roughly comparable to those obtained by previous studies conducted in the NE of Algeria, central part of the Saharan Atlas and according to a NNW-SSE profile from the coast to the Saharan platform. The results we obtained have been also compared to those from more detailed studies conducted in the Betics and Morocco, enabling us to validate the proposed model. The Moho depth accuracy which has been investigated through the variation of the density contrast and the initial depth shows a maximum error of ±2.8 km. The main results of this study indicate that the lowest Moho depth (~11 km) is observed in the Algerian Basin and thereby the crust thickness is of about 8 km, while the highest (~34 km) corresponds to the Saharan Atlas and the Aurès and Hodna Mountains. Along the coast, the depth of the Moho is about 25 km and increases towards the south to reach 30 km at the southern boundary of the Tell Atlas. It is noteworthy to observe that the crust does not thicken under the Kabylies that would indicate that their root is not enough to isostatically support the topography.

Introduction

The Maghreb topography is considered as the result of tectonic processes linked to the convergence between the African and European plates (Gómez et al., 2000). However, this hypothesis does not explain the elevation observed in the Moroccan Atlas located inside the African plate which is higher than that of the Tell-Rif closer to the plate margin (Frizon de Lamotte et al., 2006). According to several authors (Makris et al., 1985; Tadili et al., 1986; Wigger et al., 1992; Ramdani, 1998; Mickus and Jallouli, 1999; Ayarza et al., 2005), the topography of the Moroccan Atlas is not entirely of tectonic origin. This could be explained by a thin and hot lithosphere beneath the High Atlas that contributes to the relief by thermal doming as it has been suggested by Seber et al. (1996). Indeed, the moderate thickened crust beneath the High Atlas has been confirmed by Fullea et al. (2006) using regional elevation and geodetic data and by Diaz et al., 2016a, Diaz et al., 2016b from controlled-source and natural seismicity surveys.

The most recent studies concerning the Iberian Peninsula and Morocco were carried out by Díaz and Gallart (2009), Mancilla and Diaz (2015) and Diaz et al., 2016a, Diaz et al., 2016b who determined the Moho depth using Deep Seismic Sounding (DSS) and Receiver Function (RF) methods that use an artificial seismic source and natural seismicity, respectively. In Algeria, where few studies have been conducted, Mickus and Jallouli (1999) used ground gravity data to model the crust according to a NNW-SSE profile, 1000 km long and located in the center of Algeria. On the other hand, Meliani et al. (2016) computed the Moho depth in the central part of the Saharan Atlas from gravity and magnetic data, while Radi et al. (2017) used the inversion of the fundamental modes of Rayleigh waves generated by natural seismicity, to propose a model of the crust in the NE of Algeria.

The goal of this study based on satellite gravity data obtained from the Bureau Gravimétrique International (BGI) (Balmino et al., 2012), is to provide a 3D model of the Earth's crust that shows the Moho undulations and allows to get its depth at any location throughout the study area. This model has been obtained by inverting the Bouguer anomaly data using the 3Dinver software based on the Parker-Oldenburg's algorithm and developed by Gomez-Ortiz and Agarwal (2005). This algorithm has been already used to image the Moho shape in Antarctic (Block et al., 2009), South America (Van der Meijde et al., 2013), Africa and Arabia (Tugume et al., 2013) and northeastern Indian Ocean (Sreejith and Krishna, 2013). The average Moho depth z₀ which is an important parameter for the inversion has been chosen from five seismic profiles located at Mostaganem (Badji et al., 2015), Tipaza (Leprêtre et al., 2013), Dellys (Aidi et al., 2018), Jijel (Mihoubi et al., 2014) and Annaba (Bouyahiaoui et al., 2015).