**Abstract**

The Lechmine n'Aït el Haj maar (LNH) is a mixed phreatomagmatic-strombolian vent located in the Causse of the Middle Atlas. The application of tephrostratigraphic, and geophysical studies to the volcaniclastic deposits allowed interpreting the volcanic dynamics of this volcano set up during the Quaternary. Pyroclastic deposits allow us to understand the chronology of the eruptions. These are organized in four eruptive phases. The basal sequences are phreatomagmatic, followed by a strombolian unit. The last activity of LNH is phreatomagmatic. The structural analysis revealed a localized distension signed by fracture geometry, the mixed nature of the volcanism, tectonic markers, and the mechanisms of syn-eruptive tectonics. This subsidence, controlled by the NW-SE to WNW-ESE directions tends towards a strike-slip regime fault NE–SW during the phreatomagmatic-strombolian transition. The latter is favored by the position of the LNH volcano on the path of faults of cryptokarstic origin. The LNH maar is one of numerous well preserved monogenic volcanoes of the Causse of the Middle Atlas. The appropriation of this geoheritage is very important for tourism and territorial development of the region.

**Keywords:** Lechmine n'Aït el Haj, maar, karst, monogenetic volcanoes, geoheritage, Middle Atlas

## **1. Introduction**

Monogenetic volcanic provinces have recently attracted the interest of volcanologists especially in intracontinental settings [1, 2]. The Middle Atlas Volcanic Province (MAVP) is an example of those small volcanic systems with dispersed magmatic plumbing systems that erupt predominantly basaltic magmas [3, 4].

In the last thirty years, many studies have concerned the volcanism of the Middle Atlas. A hundred of cones and maars have been listed and mapped over an area of almost 1000 km2 [5, 6]. The distribution of volcanoes is controlled by the tectonic context of the region [7, 8]. The petrological study of eruptive vents allowed distinguishing four lava types dominated by under-saturated alkali basaltic flows (68.5%). The basanites cover 22.5% of the plateau surface, the subalkaline basalts cover 7,8%, and finally the nephelinite with the smallest proportion

(1,2%). The three first types are exclusively Plio-Quaternary (3.77–0.60 Ma), the K-Ar age of the last one is Middle Miocene (16.25–5.87 Ma), and Plio-Quaternary (3.92–0.67 Ma) [9]. This magmatism results from a partial melting that occurred at around 2 GPa, i.e. near the lithosphere–asthenosphere boundary beneath the Middle Atlas (60–80 km) [10]. The analysis of the Middle Atlas aeromagnetic data allowed the characterization of the regional magnetic anomalies, their location and delimitation corresponding to the major accidents of the Middle Atlas [11, 12].

30% of MAVP volcanoes are represented by maar-diatreme-type volcanic systems [6]. They have a negative shape forming a crater that intersects the preeruptive surface. Lechmine n'Aït el Haj is the first maar ever studied in this region. After a previous work on the analysis of the eruptive sequence of the maar [13], the aim of this chapter is to understand the structural context of the formation of this maar and its eruptive dynamic.

## **2. General features of monogenic volcanism**

Magma system volumes can occur in an intraplate context devoid of a mantle plume [14]. This type of volcanism is characterized by "small" volcanoes with magmatic system often basaltic [4–6], derived mostly from a mantle that stays in the crust just the time to allow a minor fractional crystallization [15]. These monogenic volcanic fields occur in any tectonic setting [2, 16–18], not only on Earth but also on other planets such as Mars [19].

Much research has been done on monogenic volcanoes, focusing on their nature from source to surface [15, 20]. The rapid ascent of magma and the short eruptive history of volcanoes allow to understand, for example, the magmatic evolution of the systems that fed several small volcanoes over long periods of time (millions of years) [21, 22].

In many cases, the eruptive style is controlled not only by the internal properties of the magma but also by the external environmental conditions to which it has been exposed. The resulting morphology is often related to the mechanism of the dominant eruptive style, making it important criteria in the study and especially in the classification. The estimated time for the formation of these volcanoes is in the order of a few days to a decade [23].

Monogenic volcanoes are referred to as the product of a single eruption [24], however, a magma ascent is not always related to a single magma influx, which usually involves several episodes producing a geochemical evolution even over a single eruption [15, 20, 25]. This magma may come directly from the mantle or from a volume of magma trapped in a zone of contrasting density, such as the upper mantle/ crustal boundary [25]. The magma ascension begins with an inter-connectivity between the small volumes of magma in the mantle and their vertical migration forming dykes [26, 27]. These dykes generally follow pre-existing structures such as faults of the basement rocks as they move towards the surface [28].

The magma of monogenic volcanoes is often primitive. It rarely expresses itself individually; it tends to form several monogenic volcanoes in a volcanic field, where there may be tens or even thousands of individual volcanoes [29]. However, a monogenic volcanic field could experience repeated monogenic eruptions across a broad area over millions of years [30].

Once a batch (or batches) of magma begins its ascent to the surface, it faces continuous outgassing and interactions with the host environment. Once at the surface, this magma can produce a volcanic eruption that can be explosive or effusive. This is controlled by the characteristics of the eruption which are determined at a superficial depth (≤1–2 km) by the balance between internal and external factors [4].

**151**

**Figure 1.**

*Ait Oufella, 8: El-Mers, 9: Guigou.*

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj…*

The Atlas mountain chain is the result of the Oligocene compression induced by the Europe-Africa convergence and continent-continent collision. Its basement is structured during the Hercynian orogeny [307,5 ± 6,8–364,0 ± 8,2] [31]. It is continental and outcrops only at a few inlier [32]. The cover consists mainly of Jurassic deposits. It is deformed in the folded Middle Atlas, whereas it is sub-tabular in the Causse of the Middle Atlas [32]. This latter zone is distinguished with a simple structural style of inclined blocks, manifesting itself in the topography by a succession of subhorizontal layers (Tabular Middle Atlas). It is the area where the Plio-Quaternary volcanism occurred with a hundred of eruptive centers aligned in a sub-meridian direction (**Figure 1**). It is crossed by the Tizi n'Tritten Fault (TTF) and a network of faults between the North Middle Atlas Fault NMAF and TTF. All these accidents, which are at least Hercynian, from direction N45 to N70, affect the

The emission points located between Azrou and Timahdite have an NNW–SSE orientation along 50 km. The basalt flow covers the ancient Quaternary formations up to the Saïs plain in the north (**Figure 1**). They are channeled to the East by the

*Structural map of the Middle Atlas with the location of the volcanic field [33]. SMAF: South Middle Atlas fault, AOF: Ait Oufella fault, NMAF: North Middle Atlas fault, TTF: Tizi n'Tretten fault. Jurassic, cretaceous, and Paleogene synclines: 1: Bekrite, 2: Timahdite, 3: Bou-Anguer, 4: Aïn-Nokra, 5: Oudiskou, 6: Tirhboula, 7:* 

*DOI: http://dx.doi.org/10.5772/intechopen.94756*

recent Quaternary deposits [8].

**3. The Middle Atlas Volcanic Province (MAVP)**

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj… DOI: http://dx.doi.org/10.5772/intechopen.94756*
