Complex monogenetic volcano in karst setting: Lechmine N'kettane volcano (Middle Atlas, Morocco)

Abstract

The Lechmine N'kettane is a Quaternary volcano, located within the Middle Atlas Volcanic Field (MAVF) in central Morocco. It is built on the faulted contact between Liassic limestone and Plio-Quaternary fluvio-lacustrine deposits. In map-view it consists of an elliptical maar crater, surrounded by a tephra ring, within which a scoria cone is nested in its northern crater zone.The Lechmine N'Kettane volcano is monogenetic in the sense of its small eruptive product volume and lack of evidence of significant time breakthrough it grows. The volcano formed from an eruptive locus that migrated laterally and vertically within the short duration of the eruption in a zigzagging pattern, along a complex set of generally NE-SW and NW-SE-trending faults. It represents a perfect example of how a volcano form and evolve under the influence of a combination of specific factors such as the lithological characteristics of the substrate, its hydrogeological parameters, magma flux and the local structural framework of the country rocks.The petrographic, granulometric and morphological (including terrain modelling) analyses of the Lechmine N'kettane pyroclastic deposits show that it was constructed in four eruptive phases with variable eruptive styles. The first, relatively dry, phreatomagmatic phase, that took place on a NE-SW fault in the northeastern part of the crater, was generated by the interaction between the ascending basaltic magma with meteoric water in the karst aquifer hosted by the Liassic limestone. The second phase is represented by a magmatic scoria fallout deposit whose explosion locus moved westward, along the same NE-SW fault. During the third phase, the explosion center migrated southward, along a NNW-SSE fault, and produced the last phreatomagmatic event by interaction of magma and water-saturated Plio-Quaternary sediment. The fourth eruptive phase is a purely scoria event, corresponding to the construction of the nephelinitic scoria cone in the northwestern part of the tephra ring. Between eruptive products formed in respective eruptive phases no evidence was recognized to establish significant time gaps between their formation.