**4.3 Characterization of the structure of the LNH maar by applied geophysics**

The treatment of the magnetic anomalies of the Middle Atlas allowed highlighting the existence of anomalies of short and long wavelengths. These last are linked to magnetized sources, notably the plio-quaternary basalts. Other anomalies

### **Figure 8.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

In the phreatomagmatic deposits (U1, U2) general subsidence is marked by conjugate fault-systems (**Figure 12d**) found also in the limestone basement (**Figure 11**). In the uppermost part of U2 (**Figure 12e**), there is a shift from an extension by normal fault perpendicular to the NE–SW structural direction, to a

*Strombolian pyroclastic fallout on the eastern flank of LNH; (a) succession of phreatomagmatic deposits overlain by the strombolian unit (U4); (b) air fallout from the first plume of the strombolian phase; (c) formation of spatter cone near the emission zone; (d) fallout of scoria with bombs far from the volcanic* 

The distribution of strombolian deposits have an elliptical shape in map view with a 900 m long axis (550 m short axis) oriented N60E which corresponds to the regional structural direction. The NE and SW extremities of the major axis are distinguished by markers that reflect a general northward collapse movement. This

• On the western flank, a shear has been observed at the southern limit, cutting the blocks of the massive basalt flow with a right lateral movement of 30° dip towards the foci as shown by the striation on the fault plane (**Figure 12a**). On this flank,

strike-slip system by permutation of the stress axes σ<sup>1</sup> - σ<sup>2</sup> (**Figure 13**).

distension controls the injection of basalt and its massive westward flow;

**158**

**Figure 7.**

*eruptive center.*

*Modality of expression of the eruptive activity in the maar of Lechmine n'aït el Haj in the Causse of Middle Atlas; (a) flow of basalt emitted during a collapse of the western sector of the crater (1) overlain by strombolian (2) and then phreatomagmatic fallout (3); (b) basalt prisms (1) overlain by Strombolian fallout of U3b (2); (c) corded lava flow resulting from the agglutination of lava.*

### **Figure 9.**

*Volcanic lithofacies of the last phreatomagmatic unit (U4); (a) in the northern flank; (b) in the southern flank.*

### **Figure 10.**

*The outcrop of the Maar of Lechmine n'Aït el Haj in the middle of basaltic lava flows where cryptokarstic cavities are aligned according to the major directions of the Middle Atlas.*

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**Figure 12.**

**Figure 11.**

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

*Distribution of fracturing in the Maar of Lechmine n'Aït el Haj (southern hemisphere) [38].*

*Fracturing systems in the Lechmine n'Aït el Haj maar; (a) striated fault in the basalt casting west of the maar; fracture types affecting the strombolian (b,c) and phreatomagmatic (d,e) formations of the maar;* 

*(f) geometric feature indicating a permutation of the stress parameters controlling fracturing.*

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

*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*

**Figure 11.** *Distribution of fracturing in the Maar of Lechmine n'Aït el Haj (southern hemisphere) [38].*

### **Figure 12.**

*Fracturing systems in the Lechmine n'Aït el Haj maar; (a) striated fault in the basalt casting west of the maar; fracture types affecting the strombolian (b,c) and phreatomagmatic (d,e) formations of the maar; (f) geometric feature indicating a permutation of the stress parameters controlling fracturing.*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**160**

**Figure 10.**

**Figure 9.**

*flank.*

*The outcrop of the Maar of Lechmine n'Aït el Haj in the middle of basaltic lava flows where cryptokarstic* 

*Volcanic lithofacies of the last phreatomagmatic unit (U4); (a) in the northern flank; (b) in the southern* 

*cavities are aligned according to the major directions of the Middle Atlas.*

**Figure 13.** *Stress permutation during the volcanic activity of LNH maar.*

coincide with the major accidents, such as those delimiting the Middle Atlas [39]. On the other hand, for the entire volcanic field of the Middle Causse atlas, very few studies focus on physical volcanology [40]. For example, a recent study [38] provides new information, based on geophysical prospecting combining magnetic and gravimetric methods, to the analysis of volcaniclastic deposits of LNH maar.

Gravity and magnetic data were obtained from a geophysical campaign in the LNH maar. The treatment and modelization of the collected data allowed understanding the geological features of the volcanic center and its geophysical properties [38]. Each model is limited by available geological information, including petrophysical properties, surficial geology and interpretation of geophysical data (regional and local magnetic survey data).

The 2D model was built by the GM-SYS software incorporating the geological and petrophysical properties of the study area [39] or those of comparable materials [41], and a design of the structures expected in these types of volcanoes. Approximate diatreme depths were constrained based on accessory lithic fragments observed in pyroclastic deposits. However, they represent minimum values, as phreatomagmatic explosions at deeper levels are often too small to transport material to the surface [42–44]. The gravity anomaly is modelized considering the topography. The gravity value is calculated at the surface and compared to the observed data. The reduced magnetic field to the pole (RTP) is calculated at an altitude of 2 m which corresponds to the height of the sensor. Since the magnetic susceptibility values considered represent minimum values, the susceptibility of the model has been increased to the maximum range expected for basaltic rocks [45, 46].

In the LNH model, the low gravity observed through the volcanic crater corresponds to shallow diatremes (~500 m), of lower density than their environment.

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**Figure 14.**

*pole-reduced magnetic response [38].*

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

Local positive gravity anomalies, associated with magnetic anomalies of similar wavelength, are observed in the volcanic edifice (**Figure 14**). These anomalies express the presence of intrusive dykes or vents that have a higher density and magnetic susceptibility than those of the diatremes and surrounding host rocks [38]. The low magnetic signal around the diatreme fits with the pyroclastic nature of the volcanic deposits. Model adjustments suggest the involvement of a karst component to minimize the gaps between the calculated and observed anomalies. These adjustments take into consideration the density and magnetic susceptibility

Wide and shallow diatremes indicate abundant water supply and/or poorly lithified sediments. Deeper diatremes suggest a downward propagation of watermagma interaction due to the drying of water in the deep levels [44, 47, 48]. This suggests that explosive magma-water interactions in LNH initially occurred with shallow, poorly lithified, and water-saturated sediments, before propagating

*Simulated model of the maar Lechmine n'Aït el Haj from observed and measured values of gravity and* 

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

values of the volcanic materials.

downward.

### *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*

Local positive gravity anomalies, associated with magnetic anomalies of similar wavelength, are observed in the volcanic edifice (**Figure 14**). These anomalies express the presence of intrusive dykes or vents that have a higher density and magnetic susceptibility than those of the diatremes and surrounding host rocks [38]. The low magnetic signal around the diatreme fits with the pyroclastic nature of the volcanic deposits. Model adjustments suggest the involvement of a karst component to minimize the gaps between the calculated and observed anomalies. These adjustments take into consideration the density and magnetic susceptibility values of the volcanic materials.

Wide and shallow diatremes indicate abundant water supply and/or poorly lithified sediments. Deeper diatremes suggest a downward propagation of watermagma interaction due to the drying of water in the deep levels [44, 47, 48]. This suggests that explosive magma-water interactions in LNH initially occurred with shallow, poorly lithified, and water-saturated sediments, before propagating downward.

### **Figure 14.**

*Simulated model of the maar Lechmine n'Aït el Haj from observed and measured values of gravity and pole-reduced magnetic response [38].*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

coincide with the major accidents, such as those delimiting the Middle Atlas [39]. On the other hand, for the entire volcanic field of the Middle Causse atlas, very few studies focus on physical volcanology [40]. For example, a recent study [38] provides new information, based on geophysical prospecting combining magnetic and gravimetric methods, to the analysis of volcaniclastic deposits of LNH maar. Gravity and magnetic data were obtained from a geophysical campaign in the LNH maar. The treatment and modelization of the collected data allowed understanding the geological features of the volcanic center and its geophysical properties [38]. Each model is limited by available geological information, including petrophysical properties, surficial geology and interpretation of geophysical data

The 2D model was built by the GM-SYS software incorporating the geological and petrophysical properties of the study area [39] or those of comparable materials [41], and a design of the structures expected in these types of volcanoes. Approximate diatreme depths were constrained based on accessory lithic fragments observed in pyroclastic deposits. However, they represent minimum values, as phreatomagmatic explosions at deeper levels are often too small to transport material to the surface [42–44]. The gravity anomaly is modelized considering the topography. The gravity value is calculated at the surface and compared to the observed data. The reduced magnetic field to the pole (RTP) is calculated at an altitude of 2 m which corresponds to the height of the sensor. Since the magnetic susceptibility values considered represent minimum values, the susceptibility of the model has been

In the LNH model, the low gravity observed through the volcanic crater corresponds to shallow diatremes (~500 m), of lower density than their environment.

increased to the maximum range expected for basaltic rocks [45, 46].

(regional and local magnetic survey data).

*Stress permutation during the volcanic activity of LNH maar.*

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**Figure 13.**
