Spatial distribution of monogenetic volcanic vents

Monogenetic basaltic volcanic fields (MVFs) occur worldwide in tectonic environments ranging from extensional to convergent. Such volcanic fields consist of numerous volcanic centers, each of which mostly represents a pathway of magma from source to surface [Valentine and Connor, 2015]. Thus analyses of spatial distribution is the principal method of study using data from fieldwork and remote sensing. Like every volcanic edifice, each volcanic field is unique. By studying individual volcanic fields, we found that the vents distribution at the surface showed alignments linked either with pre-existing crustal factures, e.g., Jurollo volcano region, Mexico [Guilbaud et al., 2011], Baja California, Mexico [Germa et al., 2013], East African Rift [Mazzarini et al., 2016]; or with the regional and local stress field, e.g., East African Rift [Muirhead et al., 2015; Mazzarini et al., 2016]. In addition, we suggest that the field shape is geometrically linked with the extent of the source in the mantle [Germa et al., 2013; Mazzarini et al., 2016].

While individual volcanic fields can highlight the important role of the local mechanism affecting their developments, understanding monogenetic volcanism as a whole requires understanding the similarities and differences between these fields. A database was compiled from the geographical coordinates of volcanic centers for 37 different volcanic fields worldwide, directly extracted from the literature, remote sensing data or kindly provided by researchers. Remote sensing analysis was performed through digital elevation models (DEM) and satellite imagery. For each field, a map of volcanic centers was generated, using a combination of DEM (SRTM or ASTER data) and Landsat images in ESRI ArcGIS, Google Earth and Google Map Terrain, and where available, data from the literature. We therefore analyzed the spatial distribution of volcanic centers in 37 monogenetic volcanic fields, and assuming that the distribution of volcanic centers relative to each other is matched by a similar source pattern within the mantle, we applied two distinct statistical methods for each: (1) the Poisson Nearest Neighbor analysis, representing the degree to which the distribution of the volcanic centers departs from a predicted Poisson distribution, and (2) a volcanic alignment to ascertain the preferred pathways, if any, used by the magma to reach the surface (Fig. 1). This was the first comprehensive global comparison of such analyses [Le Corvec et al., 2013c]. We show that independently of their tectonic environments most volcanic fields show an overall clustered distribution of their volcanic centers. Volcanic alignments in the other hand are highly influenced by the ambient tectonic environment including potential competing factors (e.g., pre-existing fracture, local stress changes).

Figure 1: Classification of the MVFs according to their geometric characteristics, progressing from the simplest to the more complex. Each rose diagram shows: black: volcanic alignment direction(s), orange: the shape of the minimum surface ellipse containing the volcanic vents, and green: any known dominant local fault system (SS: strike slip fault, NF: normal fault, subd: subduction). The filled gray circle represents the 1/3 threshold for the volcanic alignment direction(s). (a) Fields showing one alignment direction, (b) fields showing two alignment directions, and (c) fields showing more than two alignment directions. Reworked from Le Corvec et al. [2013c].