Monogenetic vent self-similar clustering in extending continental crust: Examples from the East African Rift System Francesco Mazzarini* and Ilaria Isola Istituto Nazionale di Geofi sica e Vulcanologia, Sezione di Pisa, Via della Faggiola, 32, 56126, Pisa, Italy ABSTRACT 2004). It is widely accepted that the transport and are formed if basaltic magmas stop at inter- of magma from lower crustal-upper mantle mediate to shallow crustal magma chambers, The spatial clustering of fracture networks source/ reservoir regions to its fi nal level of whereas monogenetic volcanoes erupt only once and vents in basaltic volcanic fi elds has been emplacement mainly occurs through dykes and (e.g., MacDonald, 1972) and are constructed analyzed in three sectors of the East Afri- sills (Lister and Kerr, 1991; Petford et al., 1993; when magma directly erupts from feeders. can Rift System, the classical example of an Rubin, 1995; Petford et al., 2000). These fl uid- According to Connor and Conway (2000) vol- active continental rift. Fracture trace maps driven fractures or hydro-fractures are generally canic fi elds are dominantly basaltic in composi- and monogenetic basaltic vents have been opening-mode fractures (e.g., Gudmundsson, tion and are formed by monogenetic vents each thus collected in the Afar Depression, in the 2002; Gudmundsson and Brenner, 2004). The produced by a single episode of volcanic activ- Main Ethiopian Rift, and in the Virunga Belt magma ascent rate depends on magma prop- ity and associated to feeder dykes. The occur- (Western Rift). The mapped vents are gen- erties (including viscosity, density, tempera- rence and spatial distribution of monogenetic erally younger than 2 Ma, and most are of ture, and heat content), country rock properties eruptive structures within volcanic areas are Holocene age. (including temperature, density, thermal con- linked to fracture systems and associated stress All the analyzed fracture networks have ductivity, and permeability), and the stress fi eld. fi elds (Takada, 1994a). Moreover, morphomet- self-similar clustering with fractal exponents Several lines of evidence, e.g., high-density ric parameters of monogenetic cones, such as − (Df) varying in the 1.54 1.85 range. Also, xenolith settling in basalt magmas (Basu, 1977; cone elongation, breaching direction, and cone vents show a self-similar clustering with frac- Spera, 1980; Petford et al., 2000), numerical alignment, indicate the direction of fractures − tal exponents (Dv) in the 1.17 1.50 range. For analysis (Dahm, 2000a, 2000b), and magma acting as magma feeders (Tibaldi, 1995). all the studied sectors, the relationship Df > cooling rates (Maaloe, 1973), indicate veloci- It has been proposed that fractures fi lled by −2 −1 −1 Dv has been observed. The fractal exponents ties of magma ascent of 10 ms up to 1 ms , magma (i.e., dikes) tend to coalesce during for vents (Dv) of power-law distributions which imply high bulk permeability of the crust. their ascent to the surface, thereby controlling are computed in a range of lengths with a Rock-fracturing processes enhance the bulk per- the fi nal level of magma emplacement. The lower and an upper cutoff. The upper cutoff meability of the crust and allow the ascent of actual distribution of volcanic vents at the sur- (Uco) for the fractal clustering of vents in the magma at rates that are akin to the time-scale face, i.e., the formation of monogenetic and/or studied sectors of the East African Rift Sys- characterizing magmatic activity (Rubin, 1993; polygenetic volcanoes, is mainly controlled by tem are compared with the respective crust Petford et al., 1993; Petford et al., 2000; Canon- the magma input rate and the crustal strain rate thickness derived by independent geophysi- Tapia and Walker, 2004). Fluid-driven fracturing (e.g., Fedotov, 1981; Takada, 1994a, 1994b). cal data. The computed Ucos for the studied (fractures fi lled by magma, i.e., dyke formation) High-strain rate or small magma input rate pro- sectors well match the crust thickness in the is thus the viable mechanism for emplacing mote the formation of monogenetic volcanoes volcanic fi elds. A preliminary conceptual magma within the crust (e.g., Turcotte, 1982; whereas low-strain rate and high magma input model to explain the relationships between Hutton, 1996; Petford et al., 2000). Depending rate mainly generate polygenetic volcanoes. the upper cutoffs of the fractal distribution on magma buoyancy relative magnitude, crust’s It is important to emphasize that basaltic of vents and the thickness of the crust in the fracture toughness, crustal mechanical disconti- monogenetic vents testify to the presence of volcanic fi elds is thus proposed in the light of nuities, and magma availability, dykes may stop deep crustal or subcrustal magma reservoirs the percolation theory. at some levels in the crust or, eventually, they directly connected via fracture network to may also construct magmatic chambers or reach the surface, involving a hydraulic connection INTRODUCTION directly the surface generating eruptions (e.g., through the whole crust or a large portion of Ida, 1999; Dahm, 2000a; Gudmundsson, 2002; it between source and surface. Moreover, the Fluids commonly migrate through the Taisne and Tait, 2009). correlation between vent distribution and frac- Earth’s crust in hydro-fractures such as min- In particular, eruptions of basaltic magmas ture network properties is such that the spatial eral veins or dykes, stopping at various depths imply the transfer of magmas from deep res- distribution of vents may be studied in terms in the crust (e.g., Watanabe et al., 1999; Dahm, ervoirs up to intermediate magma chambers of self-similar (fractal) clustering (Mazzarini 2000a, 2000b; Gudmundsson and Brenner, in middle upper crust or directly to the Earth’s and D’Orazio, 2003; Mazzarini, 2004, 2007; surface. Polygenetic volcanoes repeatedly erupt Mazzarini et al., 2008), as in the case of frac- from the same general vent (summit or crater) ture networks (Bonnet et al., 2001). Findings *E-mail: [email protected] Geosphere; October 2010; v. 6; no. 5; p. 567–582; doi: 10.1130/GES00569.1; 7 fi gures; 5 tables. For permission to copy, contact [email protected] 567 © 2010 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/5/567/3341583/567.pdf by guest on 02 October 2021 Mazzarini and Isola based on this approach suggest that, for basal- and in the Springerville Volcanic Field in Ari- The connectivity of fractures defi nes the por- tic volcanic fi elds in a deformed continental zona (Connor et al., 1992; Condit and Connor, tion of the existing fracture network that hydrau- crust, the distribution of monogenetic vents is 1996). Vent alignment has often been used to lically connects the system boundaries, allowing linked to the mechanical layering of the crust. infer the direction of the minimum horizontal fl uids to fl ow (Margolin et al., 1998; Darcel et Vents tend to cluster according to a power-law principal stress (Nakamura, 1977; Lutz, 1986; al., 2003). Connectivity mainly depends on distribution defi ned over a range of lengths Wadge and Cross, 1988), and vent distribution fracture size (length), density, orientation, and bounded between a lower limit (Lower cutoff, has been used as evidence for structural con- on the spatial correlation among fractures (e.g., Lco) and an upper limit (Upper cutoff, Uco); the trol on vent location (Connor, 1990; Connor Renshaw, 1999; Berkowitz et al., 2000; Darcel upper cutoff approximates the thickness of the et al., 1992) and to outline the importance of et al., 2003). fractured medium (crust). This correlation has strain rate in the style of volcanism (Takada, Fracture lengths in nature often display a been studied in volcanic fi elds within exten- 1994a; Alaniz-Alvarez et al., 1998; Mazzarini power-law distribution in the form sional continental settings in back-arcs, such et al., 2004). as in southernmost Patagonia (Mazzarini and Volcanoes tend to form clusters, showing sta- N(l) ∝ l – a, (1) D’Orazio, 2003) and in continental rifts such as tistical self-similarity in both space and time as the Main Ethiopian Rift (Mazzarini, 2004) and documented in hot spots on oceanic crust (Shaw where N(l) is the number of fractures whose Afar (Mazzarini, 2007), as well as in contrac- and Chouet, 1991) and in continental magmatic length is longer than l and a is the fractal expo- tional continental settings as the Andean fore- arcs (Pelletier, 1999). Vent clustering in basaltic nent (e.g., Bonnet et al., 2001, and references land (Mazzarini et al., 2008). volcanic fi elds has been also described in the therein). In this work we present a qualitative model to Newer Volcanic Province, southeast Australia A robust way to defi ne how fractures fi ll explain the observed relationships between the (Lesti et al., 2008) and in Afar, east Africa (Maz- space is to analyze their self-similar cluster- upper limit of the fractal distributions of basal- zarini, 2007). ing (Bonnet et al., 2001). This is accomplished tic monogenetic vents and the crust’s thickness As previously discussed, the occurrence by computation of the correlation sum that on the base of the percolation theory (Stauffer of basaltic monogenetic volcanism implies accounts for all the points at a distance of less and Aharony, 1992). In this model the mono- the existence of a portion of the fracture net- than a given length l (Bonnet et al., 2001, and genetic vents represent the intersection of the work allowing a direct connection between references therein) following the equation connected part of the actual fracture network, the magma reservoir at depth and the surface which effectively ties the source of magma to (vents). Clearly this feature points to the geo- C(l) ∝ l – D, (2) the surface (i.e., the backbone in the percolation metric and hydraulic characteristics of the theory, see below), with the surface and their actual fracture network.