**2. Geological setting**

#### **2.1 Overview of Monte Amiata volcano**

Monte Amiata (42°53<sup>0</sup> <sup>15</sup>″N, 11°37<sup>0</sup> <sup>24</sup>″E) is a middle Pleistocene polygenetic volcano culminating at 1738 m above sea level (a.s.l.) and located in southern Tuscany (Italy) within the Tuscan Magmatic Province [26]. The province formed in the inner sector of the Late Cretaceous–Early Miocene Northern Apennine thrust-and-fold belt [46, 47] and appears related to crustal thinning and asthenosphere upwelling [48, 49] associated with a Miocene-Pleistocene extensional tectonic regime [50, 51] that favored the partial melting in lower crust and mantle [26]. Magmatism in the province extended from 8 Ma to 0.2 Ma and includes plutonic and volcanic rocks. Monte Amiata is the youngest volcanic activity of the province. Its volcanic products overlay a sedimentary substratum represented by marly limestones and calcareous sandstones of Mesozoic-Cenozoic age [52].

Monte Amiata was active in a short interval of time (305–231 ka, [53, 54]). The source vents are eruptive fissures forming a 7 km-long, NE-SW-trending volcanic rift zone (**Figures 1** and **2**; [45]) related to a regional transtensional fault system (Bágnore—Bagni S. Filippo Shear Zone; [55]). Times and modes of the volcanic activity and evolution, such as the volcano-tectonic deformations of the edifice and the periodic refilling of the shallow silicic reservoir with new basaltic magmas ([56], and references therein), were probably controlled by the tectonic deformations along this shear zone [45].

The main composition of Monte Amiata lavas is trachydacite, with subordinate latite [40, 42, 44, 57]. Mafic magmatic enclaves (ME) are present within the silicic lava flows [58] and reflect the indirect mantle input to the volcanic activity and geological evolution [44, 45].

Based on a new accurate stratigraphic and structural geological survey [45, 59–62], we have innovatively proposed that the Monte Amiata geological evolution consists of two main periods of activity corresponding to two Unconformity Bounded Stratigraphic Units (UBSUs; [63]): the older Bágnore Synthem (BAS) and the younger Monte Amiata Synthem (MAS) (**Figure 1**). They are separated by a period of volcanic quiescence represented by a major geological unconformity during which a surface of intravolcanic saprolite paleo-weathering and associated tectonic deformations

#### **Figure 2.**

*Geologic sketch map of Monte Amiata showing the sheet-like lava flows (SLLFs) object of the study. The last phases of activity (Valle dell'Inferno, Piano della Contessa, and La Croce Subsynthems; see Figure 1), which not include SLLFs, are omitted. Colors of stratigraphic units are as in Figure 1. The more intense color tone indicates exposed SLLFs, and the lighter one indicates the inferred portions covered by younger units. Letters for name of SLLFs: A— Abbadia San Salvatore, C—Castel del Piano, F—Sorgente del Fiora, G—Piancastagnaio, P—Pozzaroni, Q— Quaranta, V—Vivo d'Orcia. The presence of ogive structure on some flow surface is depicted. Front and margins of the Pozzaroni lava flow show outflow lobes. Geologic mapping from your original field survey. Digital terrain model (DEM) basis from the technical map of the Tuscany Region at the 1: 10,000 scale.*

developed at expense of the BAS rocks [62]. Then, the recognition of other surfaces of discontinuity of lower rank has made it possible to divide these two synthems into seven subsynthems (**Figure 1**). Monte Amiata succession does not record pyroclastic deposits, on the contrary to common composite silicic volcanoes that show packages of intercalated tuffs and lava flows.

From its earliest stages, BAS volcanic activity is characterized by the emplacement of several extensive, individual SLLFs (Bagnólo Subsynthem-BSS; **Figure 1**) that flowed N, SE, and S for very long distances (up to 8 km) and crop out in the distal portion of the volcanic edifice (**Figure 2**; [45]). After the construction of an effusive cone formed by the successive emplacement and overlap of channelized compound lava flows (Faggia Subsynthem-FSS; **Figure 1**), more localized individual SLLFs are once again extruded (Montearioso Subsynthem-MSS; **Figures 1** and **2**; [45]). Younger units, composing MAS, comprise long individual SLLFs that reach 5–6 km in length (**Figure 2**) associated with extrusive lava domes and coulées (Valle Gelata Subsynthem-GSS; **Figure 1**), followed by numerous exogenous lava domes with associated coulées (Valle dell'- Inferno-ISS and Prato della Contessa-PSS Subsynthems) and, finally, by some smaller channelized lava flows (La Croce Subsynthem-CSS; **Figure 1**; [45]).
