Vent distribution and crustal thickness in stretched continental crust: The case of the Afar Depression (Ethiopia) Francesco Mazzarini* Istituto Nazionale di Geofi sica e Vulcanologia, Sezione di Pisa, Via della Faggiola 32, 56100 Pisa, Italy ABSTRACT volcanic activity (Rubin, 1993; Petford et al., layering of the crust (Mazzarini, 2004). Vents 2000; Canon-Tapia and Walker, 2004). It has tend to cluster according to a power-law distri- The spatial clustering of vents in basaltic been proposed that fractures fi lled by magma bution defi ned over a range of lengths approxi- volcanic fi elds within a stretched continen- (i.e., dikes) tend to coalesce during their ascent mating the thickness of the fractured medium tal crust is here used as a proxy for crustal to the surface, thereby controlling the fi nal level (crust). This correlation has been studied in thickness. Basaltic monogenetic vents show of magma emplacement and the distribution of volcanic fi elds within extensional continental self-similar clustering with a power-law dis- volcanic vents at the surface (Takada, 1994a, settings in backarcs, such as in southernmost tribution defi ned by the correlation expo- 1994b; Ito and Martel, 2002). Evidence for Patagonia (Mazzarini and D’Orazio, 2003), and nent (D) computed in a range with lower hydrofracturing has also been observed in man- in continental rifts, such as the Ethiopian Rift and upper cutoffs. The upper cutoff for the tle rocks from mid-ocean ridges (Cannat, 1996) system (Mazzarini, 2004). fractal clustering of vents yields the thick- and in ophiolites (Nicolas et al., 1994). The hypothesized link between vent cluster- ness of the crust. The spatial distribution of The link between volcanism and tectonics has ing and crustal thickness is investigated in the vents is analyzed in the Afar Depression (the long been recognized (e.g., Nakamura, 1977; Afar Depression, the most stretched portion of northern termination of the East African Rift Takada, 1994a; van Vyk de Vries and Merle, the East African Rift system connecting the Red system in Africa), where the continental crust 1996), and essentially depends on two main Sea and Gulf of Aden oceanic spreading axes has thinned considerably. More than 1700 parameters: (1) brittle deformation of the crust with the Main Ethiopian Rift (e.g., Bonini et al., vents were identifi ed and mapped in the Afar (fracture network formation), and (2) magma 2005). The East African Rift system is a classic region through the use of Landsat ETM+ availability (magma supply rate and eruptive seismically and volcanically active continental (enhanced thematic mapper) satellite image style). The link between fractures and volcanic rift extending several thousands of kilometers in mosaics. Vents cluster in seven main groups vents has been established, especially for mono- a N-S direction (e.g., Rosendahl, 1987; Braile et corresponding to the principal structural genetic volcanoes (Tibaldi, 1995). Monogenetic al., 1995; Chorowicz, 2005); it accommodates features of the Afar Depression. The mapped volcanoes are volcanic vents formed during extension between the Nubian (Africa) and vents are generally younger than 2 Ma, and single episodes of volcanic activity, and occur in Somalian plates (e.g., Chu and Gordon, 1999). most are Holocene age. The Afar vents show volcanic fi elds composed of tens to hundreds of The bulging and extension of the crust and the self-similar clustering (D = 1.39 ± 0.02) in the monogenetic vents (Connor and Conway, 2000). consequent widespread volcanism have been ~2–23 km range. The upper cutoff of ~23 km Volcanic fi elds are common in continental rifts ascribed to the impinging of one or two plumes matches well the thickness of the crust in the and in backarc extensional areas; they often on the base of the East Africa crust (Ebinger Afar region as derived from seismic and grav- comprise both monogenetic and polygenetic and Sleep, 1998; Rogers et al., 2000) or, more ity data (~25 km). The distribution of vents in volcanoes that are essentially basaltic in com- recently, to a broader mantle upwelling (the Afri- the Afar Depression is compared with that of position (Connor and Conway, 2000; Mazzarini can superplume); the Afar hotspot is a surface vents in the northern Main Ethiopian Rift. and D’Orazio, 2003; Mazzarini, 2004; Mazza- manifestation of this (Benoit et al., 2006). Sev- rini et al., 2004). The basaltic composition of eral volcanic fi elds occur in the Afar Depression, Keywords: volcanic fi elds, cone distribution, cones in volcanic fi elds testifi es to the presence where volcanic rocks are widespread; this region crustal thickness, rift, Africa, Afar. of deep crustal or subcrustal magma reservoirs is thus a natural laboratory to test the hypoth- requiring a connected fracture network through- esized correlation between crustal thickness and INTRODUCTION out the crust to feed cones. the upper limit (upper cutoff) of the power law The correlation between vent distribution and describing the distribution of vents. This correla- Volcanic eruption requires hydraulically open fracture network properties is such that the spa- tion has been described for the northern part of pathways that allow magmas to move upward tial distribution of vents may be studied in terms the Main Ethiopian Rift (Mazzarini, 2004) and is from crustal or subcrustal reservoirs to the sur- of self-similar (fractal) clustering (Pelletier, here investigated for the Afar Depression, where face. The bulk permeability of the crust may be 1999; Mazzarini, 2004), as in the case of frac- the locations of several monogenetic vents have enhanced through fracturing; rock-fracturing ture networks (Bour and Davy, 1999; Bonnet been identifi ed and their spatial clustering has processes allow the ascent of magma at rates et al., 2001). Findings based on this approach been analyzed in terms of self-similar cluster- that are akin to the time scale characterizing suggest that, for basaltic volcanic fi elds in a ing. Results will be compared with crustal thick- stretched continental crust, the distribution of nesses derived from existing geophysical data *E-mail: [email protected]. monogenetic vents is linked to the mechanical on the selected study sites. Geosphere; June 2007; v. 3; no. 3; p. 152–162; doi: 10.1130/GES00070.1; 8 fi gures; 4 tables. 152 For permission to copy, contact [email protected] © 2007 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/3/152/865205/i1553-040X-3-3-152.pdf by guest on 02 October 2021 Vent in Afar GEOLOGICAL SETTING OF THE AFAR The Danakil depression, or North Afar, is is ~10 km long and 3 km wide. Recent volca- DEPRESSION between the Danakil-Aysha’a blocks and the nism in the Manda Inakir–Asal-Ghoubbet rifts western rift plateau, and represents an area of has produced basaltic lavas, shield volcanoes, The Afar Valley is located at the confl uence highly thinned crust (e.g., Redfi eld et al., 2003) and numerous spatter cones; these volcanic of the Main Ethiopian Rift, the western Gulf of affected by active volcanism and crustal exten- centers have Pleistocene and Holocene ages Aden, and the southern Red Sea (Fig. 1). The sion (Wright et al., 2006). This area is character- (Manighetti et al., 1998; Lahitte et al., 2003a, low-lying part of the Afar triple junction covers ized by the occurrence of NNW-SSE–trending 2003b), and formed above volcanic surfaces an area of ~200,000 km2 called the Afar Depres- axial volcanoes that mark the inland propaga- dated as 3 Ma (Lahitte et al., 2003a, 2003b). sion. It is fl anked to the west and the southeast tion of the Red Sea ridge (Manighetti et al., The South Afar is dominated by N- to NE- by the Ethiopian and Somali Plateaus and to the 1998, and references therein). Basaltic volca- trending horst and graben structures; it connects east by the Danakil and Aysha’a blocks (Fig. 1). nism is Quaternary to Holocene (Lahitte et al., the NE-SW– and NNE-SSW–trending fault sys- Elevations in the adjacent plateaus reach 3000 m, 2993a, 2003b, and references therein) and has tems of the northern Main Ethiopian Rift with and in the Danakil Alps exceed 2100 m. These produced several scoria cones and eruptive fi s- the NW-SE– and E-W–trending rifts of Central elevations are in marked contrast with those of sures (Fig. 3). Faults and fractures strike NW- Afar (Tesfaye et al., 2003; Fig. 2). the depression, which range from about +800 m SE and NNW-SSE, following the rift trends. The Afar region is characterized by strong to −100 m. This low-lying region is dotted with In Central Afar, the Aden Ridge spurred the crustal attenuation: Bouguer gravity data indi- a number of topographically high shield volca- development of minor rift systems (Manighetti cate crustal thinning (Makris and Ginzburg, noes (CNR–CNRS-Afar team, 1973; Barberi et al., 1998, and references therein). The area 1987; Woldetinsae and Gotze, 2005) with an and Varet, 1977; Mohr, 1983). is characterized by the intersection of the N- average thickness of ~25 km. Inverse model- The geology of the Afar Depression has S faults marking the western escarpment, the ing of gravity data shows a crustal thickness been investigated since the early 1970s (Gass, NE-SW faults of the Main Ethiopian Rift, and of 23 km in the Afar Depression and 24 km in 1970; CNR–CNRS-Afar team, 1973; Barberi the NW-SE and E-W faults of the propagat- South Afar (Tiberi et al., 2005). Differences in and Varet, 1977; Zanettin et al., 1978; Merla et ing rifts (Manda Hararo–Goba’ad and Manda the elastic thickness of the lithosphere derived al., 1979; Zanettin, 1993; Tefera et al., 1996; Inakir–Asal-Ghoubbet rifts).