Vent distribution and crustal thickness in stretched continental crust: The case of the Afar Depression ()

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 (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 Takada, 1994a; van Vyk de Vries and Merle, the East African Rift system connecting the Red system in ), where the continental crust 1996), and essentially depends on two main Sea and 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 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 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 , 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 and Holocene ages Aden, and the southern (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). The Manda by gravity data inversion (Ebinger and Hay- Manighetti et al., 1998; Lahitte et al., 2003a, Hararo–Goba’ad system consists of NNW-SSE, ward, 1996; Hayward and Ebinger, 1996) indi- 2003b; Kidane et al., 2003; Bosworth et al., NW-SE, and E-W rifts as long as 80 km and cate that the Main Ethiopian Rift and Afar have 2005; Beyene and Abdelsalam, 2005). The several kilometers wide. Rifts are character- undergone different degrees of stretching. Seis- depression is mainly fl oored by Pliocene and ized by active deformation and axial volcanic mic refraction data imaged a crustal thickness of younger volcanic rocks. Based on the above ranges. Both silicic and basaltic magmas have 28–30 km in the northern Main Ethiopian Rift, studies, the following divisions can be made: erupted (Lahitte et al., 2003a, 2003b, and refer- 23–25 km in the southern Afar region, and only (1) a pre-rift sequence consisting of Neopro- ences therein); the oldest silicic lavas are ca. 1.3 15 km in the Danakil depression to the north terozoic basement rocks, sedimen- Ma, and 30 ka basalts are widespread (Lahitte et (Berckhemer et al., 1975; Prodehl and Mechie, tary rocks, and pre- volcanic and igne- al., 2003a). Deformation in the Manda Hararo 1991; Prodehl, et al., 1997). Analysis of receiver ous rocks; (2) lava fl ows of the Afar Stradoid rift segments becomes younger southeastward functions from broadband seismic data (Dugda series; (3) lava fl ows of the late Stradoid basal- (Manighetti et al., 1998), and basaltic volcanism et al., 2005) reveals that the crust is 27–30 km tic series; (4) deposits of Pliocene–Pleistocene (spatter cones and eruptive fi ssures) postdates thick in the Main Ethiopian Rift, and 25 km silicic centers; (5) basalts of the rift segments; the formation of large silicic volcanoes (Lahitte thick in the Afar Depression. Both gravity and and (6) Quaternary sediments (Fig. 2). et al., 2003a, 2003b). Three main fault systems seismic data reveal a crustal thickness of 35– Interaction between the southern Red Sea and interact in the Manda Hararo–Goba’ad area: the 40 km in the shoulders of the rifts. Aden oceanic ridges and the Afar stretched conti- NW-SE Red Sea, the E-W Aden Ridge, and the nental crust led to the formation of rifts and asso- NE-SW South Afar systems. On the basis of SPATIAL DISTRIBUTION OF ciated volcanism (Manighetti et al., 1998; Lahitte structural and geomorphic considerations, Tes- VOLCANOES et al., 2003a, 2003b, and references therein). faye et al. (2003) located the present position of Five main physiographic units thus defi ne the the Afar triple junction in this area. The Manda The spacing of volcanoes in relationship to Afar Depression: the rift plateaus (Ethiopia and Inakir–Asal-Ghoubbet system is composed of crustal thickness, fracture patterns, and litho- Somalia Plateaus), the Danakil-Aysha’a blocks, several rifts resulting from the encroachment on spheric thickness at convergent and divergent the Danakil depression (North Afar), the Central land in the Afar region by the Aden Ridge (e.g., plate boundaries has been debated since the early Afar, and the South Afar (Fig. 2). Manighetti et al., 1998). All these structures 1970s (e.g., Vogt, 1974); in extending oceanic The rift plateau unit comprises the highest mark the southeast to northwest propagation of and continental plates, it has been linked to the areas fl anking the Afar Depression. It consists of seismically (e.g., Hofstetter and Beyth, 2003) response of the lithosphere to the load of the vol- pre-Miocene and Miocene basaltic and rhyolitic and volcanically (e.g., Lahitte et al., 2003a) canic pile (e.g., ten Brink, 1991). In particular, the lava fl ows (Trap series); a series of fault escarp- active rifts. The southeastern segments (Asal- spacing of central volcanoes within continental ments mark the transition between this unit and Ghoubbet rifts), ~40 km long and 10 km wide, rift settings has been linked to the elastic thick- the rift depression (e.g., Lahitte et al., 2003a; strike NW-SE. The northwestern propagation of ness of the lithosphere (Mohr and Wood, 1976). Beyene and Abdelsalam, 2005). deformation is associated with volcanism lead- Vent alignment has often been used (1) to infer The Danakil and the Aysha’a blocks fl anking ing to the formation of spatter cones and erup- the direction of the minimum horizontal princi- the eastern border of the Afar Depression con- tive fi ssures. The northwestern Manda Inakir pal stress (Lutz, 1986; Wadge and Cross, 1988), sist of basement rocks, old Miocene volcanic rift is a NW-SE structure with a NNW-trend- (2) as evidence for structural control on vent rocks, recent lava fl ows, and volcanoes (Fig. 2; ing, right-stepping array of three main segments location (Connor, 1990; Connor et al., 1992), e.g., Beyene and Abdelsalam, 2005). (Manighetti et al., 1998). The Manda Inakir rift and (3) to outline the importance of strain rate in

Geosphere, June 2007 153

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 Mazzarini

25 30 35 40 45 50 55

Figure 1. Location map of the (digital elevation map [DEM] from GEOTOPO30 data set; http://edc.usgs.gov/products/ elevation/gtopo30/gtopo30.html). AD—Afar Depression; EP—Ethi- opian Plateau; SP—Somalian Plateau; MER—Main Ethiopian Rift. Axes represent degrees lati- tude and longitude. Inset: Physio- graphic units in the Afar Depres- sion (DEM from GEOTOPO30 data set; http://edc.usgs.gov/prod- ucts/elevation/gtopo30/gtopo30. html). DB—Danakil block; AB— Aysha’a block; NA—North Afar; CA—Central Afar; SA—South Afar; EP—Ethiopian and Soma- lian Plateaus; EA—Erta Ale, Ala- yuto, and Tat Ale axial volcanoes; MIG—Manda Inakir–Ghoubbet rifts; MHG—Manda Hararo– Goba’ad rifts.

154 Geosphere, June 2007

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

the style of volcanism (Takada, 1994a; Alaniz- Alvarez et al., 1998; Mazzarini et al., 2004). A strong correlation between fractures and vent alignments has been identifi ed in active Holo- cene volcanic fi elds (e.g., Iceland, Kamchatka); these geometric relationships have been ascribed N to the exploitation of favorably oriented preexist- Red Sea ing structures by the ascending magma (Connor

u and Conway, 2000, and references therein). The Danakil Depression feeder system of monogenetic apparatuses (see Connor and Conway, 2000) consists of dikes, Danakil Block and the fl ow of magma may be represented as a 14° channeled fl ow through a rock volume enhanced by secondary permeability (i.e., permeability

opian Platea opian due to fractures in the rock volume). Each feeder may be used only one time, as the cooled magma

Ethi seals the hydraulic pathway. This condition is met in cinder and spatter cones within volcanic Assab fi elds. Several types of eruptive vents may be assumed to be monogenetic apparatuses: cones, small vents along fractures, and eruptive frac- tures. I (Mazzarini, 2004) developed a simple model for visualizing the relationship between vents and the geometric properties of fractures. This model assumes that the aperture of a frac- Central Afar ture is greatest at its barycenter and that volcanic vents erupt at the point of maximum fracture aperture (Fig. 4); the resulting vent distribution is thus closely linked to the hydraulic properties Ays of both the crust and fractures.

haa Block

HYDRAULIC PROPERTIES OF FRACTURES AND VENT

DISTRIBUTION opian Plateau opian The connectivity of fractures defi nes the por- tion of the existing fracture network that hydrau-

Ethi South Afar lically connects the system boundaries, allowing fl uids to fl ow (Margolin et al., 1998; Darcel et 10° al., 2003). In a rock volume the connected net- 100 km work is a subset of the existing fracture network Somalian Plateau (e.g., Roberts et al., 1998, 1999), defi ned as the 40° NMER 43° backbone in percolation theory (Stauffer and Figure 2. Geological sketch map of the Afar Depression (after CNR– Aharony, 1992). CNRS-Afar team, 1973; Merla et al., 1979; Manighetti et al., 1998; Hydraulic features of fractures such as frac- Lahitte et al., 2003a, 2003b; Kidane et al., 2003; Bosworth et al., ture connectivity and aperture are scale invariant 2005; Beyene and Abdelsalam, 2005). White—Quaternary sediments (Bonnet et al., 2001). In particular, the spatial and volcaniclastic deposits. Red—Pleistocene–Holocene basaltic vol- clustering of a fracture network, represented as canism. Yellow—Pliocene–Pleistocene silicic centers. Orange—lava the fracture barycenter, has been directly linked fl ows of the late Afar Stratoid basaltic series (late Pliocene). Light to the hydraulic properties of the fracture net- gray—lava fl ows of the Afar Stratoid series (Miocene–Pliocene). Dark work (Renshaw, 1999; Bour and Davy, 1999; gray—pre-rift sequence consisting of Neoproterozoic basement rocks, Darcel et al., 2003). Assuming a direct genetic Mesozoic sedimentary rocks, and pre-Miocene volcanic and igne- and spatial link between fracture and vent (Con- ous rocks. Thick dashed lines—main tectonic features; thin black nor and Conway, 2000; Mazzarini, 2004), scale lines—faults (after Manighetti et al., 1998; Lahitte et al., 2003a, 2003b; invariance in vent distribution refl ects the frac- Kidane et al., 2003; Bosworth et al., 2005; Beyene and Abdelsalam, tal properties of the connected part of a fracture 2005). NMER—northern Main Ethiopian Rift. network (i.e., the backbone). The main geometric features of a fracture net- work are generally measured and mapped (frac- ture attitude, aperture, spacing, intersections,

Geosphere, June 2007 155

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 Mazzarini

EA N N NA

a EP MIG Fig. 3B a MHG CA 8 km B Figure 3. RGB (red, green, blue) false color composite of Landsat ETM (enhanced thematic mapper) image mosaic (courtesy of the University of Maryland University Global Land Cover Facility; http://glfc.umiacs.umd.edu/index.shtml), band ETM+ band 7 in the red channel, ETM+ band 4 in the green channel, and ETM+ band SA 2 in the blue channel. (A) General overview of the Afar Depression. NA—North Afar; CA—Central Afar; SA—South Afar; EP—Ethio- A pian Plateau; EA—Erta Ale volcano Range; MIG—Manda Inakir– Ghoubbet rifts; MHG—Manda Hararo–Goba’ad rifts. (B) Close-up view of the MIG area clearly showing recent and older scoria cones. 200 km Red and orange hues are old cones, and dark blue hues are young cones (a in the fi gure).

length, and density) at different scales of obser- length, density), and that these are functions of vation, from satellite images to fi eld mapping, mechanical layers and rock properties. Experi- showing scale invariance spanning several mental studies on normal fault populations sug- orders of magnitude (Bonnet et al., 2001; Mar- gest the presence of upper and lower cutoffs in rett et al., 1999). The way in which fractures fi ll the power law describing the distribution of the space (i.e., the spatial distribution of fractures) geometric properties of fractures (Ackermann et depends strictly on the spacing of fractures. The al., 2001). Moreover, the thickness of both sedi- H latter geometric feature is correlated with the mentary beds and the crust controls the scaling thickness of the fractured medium calculated on law of fractures and earthquakes (Pacheco et al., the basis of the stress saturation model (Wu and 1992; Davy, 1993; Ouillon et al., 1996). Crust Pollard, 1995; Gross et al., 1995; Ackermann The dependence of the fracture network spa- and Schlische, 1997). tial distribution on the rheologic layering of the Wet fracture (dike) A robust way to defi ne how fractures fi ll space medium (i.e., the crust) can also be inferred is to analyze their self-similar clustering (Bonnet from the connected part of the network. The et al., 2001). The defi nition of self-similar clus- connected fracture network allows basaltic Dry fracture tering for the analyzed spatial correlation of frac- magma to rise to the surface from deep crustal tures (i.e., computation of the fractal exponent; or subcrustal reservoirs, passing through most Vent see Bonnet et al., 2001, and references therein) is or the whole of the crust. Analysis of the fractal performed for a range of lengths (the size range) character of the spatial distribution of vents can Sill between a lower and an upper cutoff. thus reveal the mechanical layering of the crust. The upper cutoff is here considered to be Figure 4. Conceptual model of relationships directly linked to the mechanical layering of the DATA ACQUISITION among the fracture network, dikes, and medium. Mandelbrot (1982) suggested that there vents in a volcanic fi eld. Only the connected are upper and lower cutoffs for the scale-invari- Landsat 7 ETM+ (enhanced thematic map- portion of the fracture network feeds the ant characteristics of fractures (e.g., spacing, per) images were used to identify and map vents. H is the crust thickness.

156 Geosphere, June 2007

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

volcanic vents (Goward et al., 2001; http://land- sathandbook.gsfc.nasa.gov/handbook.html). A mosaic of 29 ETM images was used to map a 64.40 large portion of East African Rift system (cour- tesy of the Maryland University Global Land Cover Facility; http://glfc.umiacs.umd.edu/ index.shtml). The images are georeferenced to 76.27 the Universal Transverse Mercator projection (zone 37 N –WGS84) and displayed as RGB (red, green, blue) false color composites (with 88.13 ETM+ band 7 in the red channel, ETM+ band Similarity (%) 4 in the green channel, and ETM+ band 2 in the blue channel). The original spatial resolution of Landsat ETM+ images is 30 × 30 m. The 15 100.00 × 15 m spatial resolution of the Landsat ETM+ observations mosaic was obtained through a color transform Figure 5. Dendrogram representing the amalgamation of observations into one cluster. The using the 15 × 15 m geometric resolution of similarity from an initial value of 100% (1725 clusters) decreases to a value of ~89% (pas- the Landsat ETM+ panchromatic band (Janza sage from red to black trees). At this step there are eight clusters. et al., 1975; Vrabel, 1996). One Landsat 7 ETM+ scene was also processed to investigate the spectral behavior of basaltic scoria cones (path 167 row 051, acquisition date 28 Novem- ber 2000, courtesy of the Maryland University were identifi ed for the analyzed vents (Fig. 5). lavas of the Erta Ale area (data set NA) and Cen- Global Land Cover Facility; http://glfc.umiacs. They are characterized by the number of vents, tral Afar (data sets CA-1 and CA) and the rela- umd.edu/index.shtml). the location of the cluster center (centroid), and tively older basaltic volcanism. Young basaltic Images clearly show volcanic features as well by the average distance of all the vents within cones generally have dark blue hues, well-pre- as fractures and faults (Fig. 3), and vent posi- the cluster from the centroid of their respective served fl anks, and a defi ned crater rim (Fig. 3A), tions were identifi ed in a geographic informa- cluster. Three clusters are in the Northern Afar with an average basal diameter of 370 m. Old tion system environment (ArcView 3.3). The unit, two are in the Danakil block, two are in the cones have red hues, show disrupted cones and locations of vents were identifi ed on the satellite Central Afar unit, and one is in the South Afar gullies on their fl anks, and erosion often high- images with an accuracy of one pixel (i.e., 15 × unit (Fig. 6; Table 1). lights cone breaching; their average basal diam- 15 m). This error in vent location is lower than The identifi ed clusters were subsequently eter is 439 m (Fig. 3B). Old vents share similar that arising through the use of 1:50,000 scale grouped into data sets according to the follow- morphologic features both in areas with basaltic topographic maps (Mazzarini, 2004). Moreover, ing criteria: (1) each data set had to include more rocks and in those with silicic rocks. They are all relationships between vents and fractures are than 100 vents to ensure statistical signifi cance assumed to be basaltic. For example, the vents well imaged by the synoptic view of satellite (see Mazzarini, 2004); and (2) the data set had of data set CA and those of data set SA are all images. As many as 1725 vents were identifi ed, to spatially relate to major structural features of assumed to be basaltic, although the cones are and their coordinates were stored in a fi le. The the Afar region. located in areas where silicic volcanic centers vents are located in structurally controlled areas, On this basis, six new data sets were gener- also occur (Fig. 2). Using the pixel values in where they are unevenly distributed. ated. They are localized in the following struc- satellite images, a rough spectral comparison tures of the Afar Depression: two data sets are between the vents of data set CA (in an area Afar Vents in the Danakil block: data set DB-na (north of with silicic volcanic rocks) and those of data Assab) and data set DB-a (Assab area); one set DB-na (in an area with recent basaltic rocks) In order to identify the occurrence of vent data set is in the North Afar depression: data was performed to validate the assumption that clusters in the Afar Depression, the spatial set NA (zone with Erta Ale, Alayuto, and Tat all the detected vents have a basaltic composi- distribution of vents was investigated through Ale volcanoes); two sets are in the Central Afar tion. The sampled pixels belonging to the CA multivariate analysis by applying a clustering depression: data set CA-1 (Manda Hararo and and DB-na vents show similar pixel values that approach based on an agglomerative hierarchical Goba’ad rifts) and data set CA (Manda Inakir, clearly differ from those of pixels sampled on method (using the MINITAB statistical software Asal, Ghoubbet rifts and the western border of silicic volcanic centers (Fig. 7). package). This approach is used when clusters the Danakil block); and the last data set is in the are initially unknown. The optimal number of South Afar depression: data set SA (junction DATA ANALYSIS clusters is derived by analyzing the dendrogram between Afar and the Main Ethiopian Rift). (Fig. 5) that depicts the amalgamation of obser- A data set (Afar) of all the detected vents in Methods vations into one cluster. The similarity at any the Afar Depression was also created (Fig. 6). step is defi ned as the percent of the minimum The identifi ed vents (mainly scoria cones) The Afar vents were analyzed in terms of distance at that step relative to the maximum are generally of Pleistocene and Holocene age self-similar clustering and mean vent separation interobservation distance (i.e., the maximum (younger than 1.8 Ma) and are mainly basaltic (Mazzarini, 2004); cone density was also inves- distance between the vents in this case). The in composition (Lahitte, et al., 2003a, 2003b; tigated. The spatial distribution of vents was step where values change abruptly may mark the Kidane et al., 2003). The detected vents (1725) analyzed by calculating the correlation exponent point for cutting the dendrogram. Eight clusters may be roughly divided into the recent basaltic D (Bonnet et al., 2001). A two-point correlation

Geosphere, June 2007 157

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 Mazzarini

Danakil Block

Figure 6. Distribution of identifi ed clusters (red North Afar squares) and of the six data sets in the Afar. Pink dots—NA data set; white dots—DB-na data set; green dots—DB-a data set; dark blue dots—CA data set; light blue dots—CA-1 data set; orange dots—SA data set (see text). Dashed Central Afar white lines divide the Afar Depression into North, Cen- tral, and South Afar; dot- ted white circle marks the present position of the Afar triple junction according to Tesfaye et al. (2003). Black thin lines are main fault trends. The shaded eleva- South Afar tion model (illumination from the north) is derived from SRTM data (Shuttle Radar Topography Mission; http://srtm.usgs.gov/).

TABLE 1. IDENTIFIED CLUSTERS IN AFAR VENTS DISTRIBUTION Cl Vents cc-easting cc-northing adist-cc Location Data set (m) (m) (m) 1 294 68,7192 1,453,772 30,807 North Afar NA 2 10 65,6653 1,539,766 8927 North Afar NA 3 45 65,3600 1,404,244 8067 North Afar NA 4 594 81,5092 1,404,542 52,198 Central Afar CA 5 230 81,3794 1,509,105 20,437 Danakil block DB-a 6 215 87,0634 1,441,364 18,338 Danakil block DB-na 7 203 76,4145 1,247,056 30,131 Central Afar CA-1 8 134 70,6441 1,121,904 21,541 South Afar SA Note: cl—cluster number; cc—cluster centroid; adist-cc—average distance from cluster centroid.

158 Geosphere, June 2007

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

vent spacing (1.14 km). The vent spacing dis- tribution can be described by the coeffi cient of variation, CV. Data sets DB-na (CV = 0.88) and DB-a (CV = 0.77) have an anticlustered (homo- geneous) distribution of vent spacing, whereas the remaining data sets show CV values >1, indicating a clustered distribution of vent spac- ing (Table 2). The higher the CV value, the more clustered the distribution. Self-similar clustering of vents is described by the correlation exponent D (the higher the correlation exponent D the more homogeneous the distribution i.e., anti-clustered; e.g., Bonnet et al., 2001). Data sets show different degrees of cluster- ing (Table 3), with D = 1.42 ± 0.02 for the Afar vents as a whole. Data sets NA (D = 1.34 ± 0.02) and SA (D = 1.19 ± 0.04) show the highest self- similar clustering of cones. Data sets DB-a (D = 1.69 ± 0.02), CA-1 (D = 1.52 ± 0.02), and DB-na (D = 1.50 ± 0.01) show low self-similar clustering. The D exponent for the CA data set is Figure 7. Pixel values (DN) of basaltic vents (DB-1 and DB-2) and silicic volcanics (see text) very similar to that for the Afar vents as a whole sampled in a Landsat ETM+ (enhanced thematic mapper) satellite image. (1.43 ± 0.02). Overall, the vents in the Afar Depression show a well-established fractal behavior over more than one order of magnitude (1.2–23 km); the function method was used to measure the fractal separation. Vent separation (or spacing) is ana- computed upper cutoff is 23.4 ± 2.0 km. Three dimension of the vent population. For a popula- lyzed by computing the average minimum dis- data sets (CA, DB-na, and SA) have an upper tion of N points (vent centers), the correlation tance between vents. The coeffi cient of varia- cutoff of ~23 km (Table 3). Data sets DB-a and integral C(r) is defi ned as the correlation sum tion CV (Gillespie et al., 1999, and references CA-1 have upper cutoff values of 12.5 ± 1.6 km that accounts for all the points at a distance of therein) for the distribution of vent separation and 11.5 ± 0.8, respectively. The NA data set has less than a given length r (Bonnet et al., 2001, describes the degree of vent clustering. CV > 1 an upper cutoff of 14.2 ± 2.3 km. and references therein). In this approach, the indicates clustering of vents, CV = 1 indicates The vents closeness (VC) density was com- term C(r) is computed as: a random or Poisson distribution of vents, and puted using 5 km, 2.5 km, and 1.4 km search CV < 1 indicates anticlustering (a homoge- radii. The smallest radius (1.4 km) was used 2 × Nr() , (1) neous distribution) of vents. CV is defi ned as: because it is close to the maximum computed Cr()= ×− spacing of vents (1.30 km) (Fig. 6; Table 2). As NN()1 where N(r) is the number of pairs of points S , (2) a whole, VC ranges from 0.1 to 0.3 vents km–2 CV = whose distance is

Geosphere, June 2007 159

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 Mazzarini

TABLE 2. STATISTICS OF VENT SEPARATION FOR THE ANALYZED DATA SETS DB-na DB-a NA CA-1 CA SA Afar Mean (km) 1.14 1.30 1.27 1.24 1.23 1.17 1.24 Standard deviation (km) 1.01 1.00 1.46 1.32 1.37 1.41 1.30 Maximum (km) 5.22 6.28 14.03 8.79 18.40 10.20 18.40 Minimum (km) 0.13 0.19 0.12 0.01 0.09 0.14 0.01 CV 0.88 0.77 1.16 1.07 1.10 1.20 1.05 Note: DB-na—Danakil block north of Assab; DB-a—Assab area; NA—North Afar; CA-1—Central Afar (Manda Hararo and Goba’ad rifts); CA— Manda Inakir, Asal, Ghoubbet rifts and the western border of the Danakil block); SA—South Afar (junction between Afar and the Main Ethiopian Rift); CV—coefficient of variation.

TABLE 3. VENT SELF-SIMILAR CLUSTERING IN AFAR DB-na DB-a NA CA-1 CA SA Afar Vent number 221 227 349 203 591 134 1725 D 1.50 ± 0.01 1.69 ± 0.02 1.34 ± 0.02 1.52 ± 0.02 1.43 ± 0.02 1.19 ± 0.04 1.42 ± 0.02 c 3 x 10–7 7 x 10–8 8 x 10–7 3 x 10–7 2 x 10–7 5 x 10–6 7 x 10–7 Lower cutoff (km) 1.2 ± 0.2 1.2 ± 0.2 1.4 ± 0.4 1.2 ± 0.2 1.7 ± 0.3 1.7 ± 0.8 1.2 ± 0.5 Upper cutoff (km) 23.1 ± 1.4 12.5 ± 1.6 14.2 ± 2.3 11.5 ± 0.8 23.5 ± 2.5 22.8 ± 1.5 23.4 ± 2.0 R2 0.99 0.99 0.99 0.99 0.99 0.99 0.99 Note: DB-na—Danakil block north of Assab; DB-a—Assab area; NA—North Afar; CA-1—Central Afar (Manda Hararo and Goba’ad rifts); CA— Manda Inakir, Asal, Ghoubbet rifts and the western border of the Danakil block); SA—South Afar (junction between Afar and the Main Ethiopian Rift). D— correlation exponent; c—normalization coefficient; R2—correlation coefficient.

Rift may thus refl ect different degrees of crustal different degrees of stretching in an extending TABLE 4. MEASUREMENTS stretching (e.g., Hayward and Ebinger, 1996). continental crust; extension is mainly localized OF VENT DENSITY Moreover, the observed Uco value for the Afar in areas where the propagation of the spreading Data sets Vent closeness Depression fi ts well with the crustal thickness ridge (the Red Sea for the NA data set and the (vent/km2) of ~25 km derived from geophysical data from Gulf of Aden for the CA-1 data set) (Manighetti r = 5* r = 2.5 r = 1.4 the region. et al., 1998) leads to strain localization. Afar 0.2 0.2 0.3 Most of the analyzed data sets show similar Global plate reconstruction based on geo- DB-na 0.1 0.2 0.2 Uco values (DB-na, CA, and SA); the average detic, geophysical, and geological data (Chu DB-a 0.2 0.2 0.2 NA 0.1 0.2 0.3 of 23.2 ± 0.5 km clearly matches the Uco value and Gordon, 1999; Sella et al., 2002; Kreemer CA-1 0.2 0.2 0.2 computed for the Afar vents (23.4 ± 2.0 km). et al., 2003; Fernandes et al., 2004) yields an CA 0.2 0.2 0.3 The NA data set, from northern Afar where average spreading rate of 1.6–7 mm yr–1 for the SA 0.2 0.2 0.2 large volcanoes occur, has a Uco value of 14.2 East African Rift system. Strain rates in the East *r = search radius in km. ± 2.3 km that closely matches the crustal thick- African Rift system vary from south to north, ness of the area based on seismic refraction data and an average strain rate of 5–7 mm yr–1 can be (e.g., Prodehl et al., 1997). Data set DB-a also assumed in the Afar. Using these values as proxy has a low Uco value (12.5 ± 1.6 km) along an E- for the strain rate in the Afar Depression, and that crustal extension in the Afar region is ongo- W volcanic range close to Assab in the Danakil assuming pure shear deformation, a crustal neck- ing and not uniform. The local-scale distribution block (Fig. 6). The CA-1 data set in the Manda ing of ~5 km is accomplished in ~1–3 m.y. The of vents can thus provide insight into localized Hararao–Goba’ad rifts shows the lowest Uco basaltic vents used to determine the thickness of crustal extension. value (11.5 ± 0.8 km). The low Uco value for the the crust in the Afar region are younger than 2 The exposure and tectonic setting of the Manda Hararo–Goba’ad rifts (CA-1) is consis- Ma, and most are Holocene age (Manighetti et Afar Depression make it an ideal location for tent with data (seismic, synthetic aperture radar al., 1998; Lahitte et al., 2003a, 2003b; Kidane investigating the link between vent distribution interferograms, and fi eld surveys) collected in et al., 2003); the vents thus formed just after or (self-similar clustering) and crustal thickness. In September–October 2005 in the northwestern during the late stages of crustal extension. The general, several factors (e.g., changes in strain termination of the Manda Hararo rift during the basaltic magma thus rose from deep crustal or rate) may change the style and chemistry of vol- formation of eruptive fi ssures and vents, and subcrustal reservoirs through a thinned crust to canism. In the Afar region, for example, basaltic during the emplacement of a dike extending the surface, giving rise to monogenetic vents activity marks periods of high extensional strain from 2 to ~9 km in depth (Wright et al., 2006), and forming volcanic fi elds. In the Afar Depres- (e.g., Lahitte et al., 2003a). The occurrence suggesting a brittle crust thickness of ~10 km. sion, the spatial distribution of vents imaged the of monogenetic basaltic volcanism requires a The inferred Uco values for the Afar Depres- heterogeneous thickness of the crust (see the channeling of magma from deep reservoirs to sion can be considered proxy measures of analyzed data sets, Table 3), thereby confi rming the surface without formation of intracrustal

160 Geosphere, June 2007

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

systems in geological media: Reviews of Geophysics, v. 39, p. 347–383, doi: 10.1029/1999RG000074. Bosworth, W., Huchon, P., and McClay, K., 2005, The Red Sea and Gulf of Aden Basins: Journal of African Earth Sciences, v. 43, p. 334–378, doi: 10.1016/j.jafrearsci.20 05.07.020. Bour, O., and Davy, P., 1999, Clustering and size distribu- tion of fault patterns: Theory and measurements: Geo- physical Research Letters, v. 26, p. 2001–2004, doi: 10.1029/1999GL900419. Bour, O., Philippe Davy, P., Darcel, C., and Odling, N., 2002, A statistical scaling model for fracture network geometry, with validation on a multiscale mapping of a joint network (Hornelen Basin, Norway): Jour- nal of Geophysical Research, v. 107, p. 2113, doi: 10.1029/2001JB000176. Braile, L.W., Keller, G.R., Wendlandt, R.F., Morgan, P., and Khan, M.A., 1995, The East African Rift system, in Olsen, K.H., ed., Continental rifts: Evolution, struc- ture, tectonics: New York, Elsevier, Developments in Geotectonics, v. 25, p. 213–231. Cannat, M., 1996, How thick is the magmatic crust at slow spread- ing oceanic ridges?: Journal of Geophysical Research, v. 101, p. 2847–2857, doi: 10.1029/95JB03116. Canon-Tapia, E., and Walker, G.P.L., 2004, Global aspects of volcanism: the perspectives of “plate tectonics” and “volcanic systems”: Earth-Science Reviews, v. 66, p. 163–182, doi: 10.1016/j.earscirev.2003.11.001. Chorowicz, J., 2005, The East African Rift System: Jour- nal of African Earth Sciences, v. 43, p. 379–410, doi: 10.1016/j.jafrearsci.2005.07.019. Chu, D., and Gordon, R.G., 1999, Evidence for motion Figure 8. Plot of log(r) versus log(C[r]) for the Afar (black diamonds) and Main Ethiopian Rift between and Somalia along the Southwest Indian (MER) vents (red dots). Black crosses are the local slope for the Afar vent. Vertical dashed Ridge: Nature, v. 398, p. 64–67, doi: 10.1038/18014. black lines mark the upper and lower cutoffs for the Afar vents. Vertical dashed red lines mark CNR-CNRS-Afar team, 1973, Geology of northern Afar (Ethiopia): Revue de Geographie Physique et de Geol- the upper and lower cutoffs for the MER vents. MER data are from Mazzarini (2004). The ogie Dynamique, v. 2, p. 343–390. slope of the curves is the correlation exponent (D). The Afar Depression and the MER have Connor, C.B., 1990, Cinder cone clustering: Journal of Geo- physical Research, v. 95, p. 19,395–19,405. nearly equal slopes. The MER upper cutoff is higher than the Afar upper cutoff (see text). Connor, C.B., and Conway, F.M., 2000, Basaltic volcanic fi elds, in Sigurdsson, H., ed., Encyclopedia of volca- noes: New York, Academic Press, p. 331–343. Connor, C.B., Condit, C.D., Crumpler, L.S., and Aubele, J.C., Geology, v. 25, p. 1127–1130, doi: 10.1130/0091– magma chambers, regardless of the brittle layer 1992, Evidence of regional structural control on vent 7613(1997)025<1127:AOSNFA>2.3.CO;2. distribution: Springerville volcanic fi eld, Arizona: Jour- thickness. The style of volcanism may change Ackermann, R.V., Schlinsche, R.W., and Withack, M.O., nal of Geophysical Research, v. 97, p. 12,349–12,359. through time, generating more evolved magma 2001, The geometric and statistical evolution of normal Darcel, C., Bour, O., Davy, P., and de Dreuzy, J.R., 2003, Con- and fi ssure magmatism and determining the for- fault systems: An experimental study of the effects of nectivity properties of two-dimensional fracture networks mechanical layer thickness on scaling laws: Journal of mation of central volcanoes (e.g., Lahitte et al., with stochastic fractal correlation: Water Resources Structural Geology, v. 23, p. 1803–1819, doi: 10.1016/ Research, v. 39, p. 1272, doi: 10.1029/2002WR001628. 2003a). In this case the distribution of volcanic S0191–8141(01)00028–1. Davy, P., 1993, On the frequency-length distribution of the vents may be controlled by strain rate and brittle Alaniz-Alvarez, S.A., Nieto-Samaniego, A.F., and Fer- San Andreas fault system: Journal of Geophysical rari, L., 1998, Effect of strain rate in the distribu- and/or ductile layering of the crust. Research, v. 98, p. 12,141–12,151. tion of monogenetic and polygenetic volcanism in Dugda, M.T., Nyblade, A.A., Julia, J., Langston, C.A., More data sets on the distribution of vents the Transmexican volcanic belt: Geology, v. 26, Ammon, C.J., and Simiyu, S., 2005, Crustal structure in basaltic volcanic fi elds within extensional or p. 591–594, doi: 10.1130/0091–7613(1998)026<0591: in Ethiopia and Kenya from receiver function analy- EOSRIT>2.3.CO;2. contractional continental tectonic settings are sis: Implications for rift development in eastern Africa: Barberi, F., and Varet, J., 1977, Volcanism of Afar: Small-scale Journal of Geophysical Research, v. 110, p. B01303, required to better defi ne the link between crustal plate tectonic implications: Geological Society of Amer- doi: 10.1029/2004JB003065. thickness and the upper vent distribution cutoff ica Bulletin, v. 88, p. 1251–1266, doi: 10.1130/0016– Ebinger, C.J., and Hayward, N.J., 1996, Soft plates and hot 7606(1977)88<1251:VOASPT>2.0.CO;2. spots: Views from Afar: Journal of Geophysical Research, value. Once this hypothesis is confi rmed by Benoit, M.H., Nyblade, A.A., and VanDecar, J.C., 2006, v. 101, p. 21,859–21,876, doi: 10.1029/96JB02118. robust statistics, the distribution of basaltic vents Upper mantle P-wave speed variations beneath Ethio- Ebinger, C.J., and Sleep, N.H., 1998, Cenozoic magma- can be used as a proxy measure of crustal and/or pia and the origin of the Afar hotspot: Geology, v. 34, tism in central and east Africa resulting from impact p. 329–332, doi: 10.1130/G22281.1. of one large plume: Nature, v. 395, p. 788–791, doi: lithospheric thickness in large and remote areas Berckhemer, H., Baier, B., Bartlesen, H., Behle, A., Burkhardt, 10.1038/27417. of the Earth or on planets with evidence of vol- H., Gebrande, H., Markris, J., Menzel, H., Miller, H., Fernandes, R.M.S., Ambrosius, B.A.C., Noomen, R., Bastos, canic activity. and Vees, R., 1975, Deep seismic soundings in the Afar L., Combrinck, L., Miranda, J.M., and Spakman, W., region and on the highland of Ethiopia, in Pilger, A., and 2004, Angular velocities of Nubia and Somalia from Rosler, A., eds., Afar Depression of Ethiopia: Stuttgart, continuous GPS data: Implications on present-day rel- ACKNOWLEDGMENTS Germany, E. Schweizerbart, p. 89–107. ative kinematics: Earth and Planetary Science Letters, Beyene, A., and Abdelsalam, M.G., 2005, Tectonics of the v. 222, p. 197–208, doi: 10.1016/j.epsl.2004.02.008. Afar Depression: A review and synthesis: Journal of Gass, I.G., 1970, The evolution of volcanism in the junction I thank G. Corti and two anonymous review- African Earth Sciences, v. 41, p. 41–59, doi: 10.1016/ area of the Red Sea, Gulf of Aden and Ethiopian rifts: ers for comments and suggestions that improved j.jafrearsci.2005.03.003. Royal Society of London Philosophical Transactions, Bonini, M., Corti, G., Innocenti, F., Manetti, P., Mazzarini, ser. A, v. 267, p. 369–382, doi: 10.1098/rsta.1970.0042. the manuscript. F., Abebe, T., and Pecskay, Z., 2005, Evolution of the Gillespie, P.A., Johnston, J.D., Loriga, M.A., McCaffrey, Main Ethiopian Rift in the frame of Afar and Kenya K.J.W., Walsh, J.J., and Watterson, J., 1999, Infl u- REFERENCES CITED rifts propagation: Tectonics, v. 24, p. TC1007, doi: ence of layering on vein systematics in line samples, 10.1029/2004TC001680. in McCaffrey, K.J.W., et al., eds., Fractures, fl uid fl ow Ackermann, R.V., and Schlische, R.W., 1997, Anticlus- Bonnet, E., Bour, O., Odling, N.E., Davy, P., Main, I., and mineralization: Geological Society [London] Spe- tering of small normal faults around larger faults: Cowie, P., and Berkowitz, B., 2001, Scaling of fracture cial Publication 155, p. 35–56.

Geosphere, June 2007 161

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 Mazzarini

Goward, S.N., Masek, J.G., Darrel, L., Williams, D.L., Irons, Mazzarini, F., Corti, G., Manetti, P., and Innocenti, F., 2004, Journal of Geophysical Research, v. 107, doi: J.R., and Thompson, R.J., 2001, The Landsat 7 mis- Strain rate bimodal volcanism in the continental rift: 10.1029/2000JB000033. sion: Terrestrial research and applications for the 21st Debre Zeyt volcanic fi eld, northern MER, Ethiopia: Stauffer, D., and Aharony, A., 1992, Introduction in percola- century: Remote Sensing of the Environment, v. 78, Journal of African Earth Sciences, v. 39, p. 415–420, tion theory (second edition): Philadelphia, Pennsylva- p. 3–12, doi: 10.1016/S0034–4257(01)00262–0. doi: 10.1016/j.jafrearsci.2004.07.025. nia, Taylor and Francis, p. 190. Gross, M.R., Fischer, M.P., Engelder, T., and Greenfi eld, R.J., Merla, G., Abbate, E., Azzaroli, A., Bruni, P., Canuti, P., Takada, A., 1994a, The infl uence of regional stress and mag- 1995, Factors controlling joint spacing in interbedded Mazzuoli, M., Sagri, M., and Sacconi, P., 1979, A matic input on styles of monogenetic and polygenetic sedimentary rocks: Integrating numerical models with geological map of Ethiopia and Somalia (1973) at volcanism: Journal of Geophysical Research, v. 99, fi eld observations from the Monterey Formation, USA, 1:2.000.000 scale: Florence, Italy, Consiglio Nazionale p. 13,563–13,573, doi: 10.1029/94JB00494. in Ameen, M.S., ed., Fractography: Fracture topogra- delle Ricerche, p. 1–95. Takada, A., 1994b, Development of subvolcanic structure by phy as a tool in fracture mechanics and stress analysis: Mohr, P., 1983, The Morton-Black hypothesis for the thin- the interaction of liquid-fi lled cracks: Journal of Vol- Geological Society [London] Special Publication 92, ning of continental crust—Revisited in western Afar: canology and Geothermal Research, v. 61, p. 207–224, p. 215–233. Tectonophysics, v. 94, p. 509–528, doi: 10.1016/0040– doi: 10.1016/0377–0273(94)90004–3. Hayward, N.J., and Ebinger, C.J., 1996, Variations in the 1951(83)90032-X. Tefera, M., Chernet, T., and Haro, W., 1996, Explanation of along-axis segmentation of the Afar Rift system: Tec- Mohr, P.A., and Wood, C.A., 1976, Volcano spacings and the geological map of Ethiopia: Addis Ababa, Ethio- tonics, v. 15, p. 244–257, doi: 10.1029/95TC02292. lithospheric attenuation in the Eastern Rift of Africa: pian Institute of Geological Surveys, p. 3–79. Hofstetter, R., and Beyth, M., 2003, The Afar Depression Earth and Planetary Science Letters, v. 33, p. 126–144, ten Brink, U., 1991, Volcano spacing and plate rigidity: interpretation of the 1960–2000 earthquakes: Geo- doi: 10.1016/0012–821X(76)90166–7. Geology, v. 19, p. 397–400, doi: 10.1130/0091– physical Journal International, v. 155, p. 715–732, doi: Nakamura, K., 1977, Volcanoes as possible indicators of tectonic 7613(1991)019<0397:VSAPR>2.3.CO;2. 10.1046/j.1365–246X.2003.02080.x. stress: Journal of Volcanology and Geothermal Research, Tesfaye, S., Harding, J.D., and Kusky, T.M., 2003, Early Ito, G., and Martel, J., 2002, Focusing of magma in the upper v. 2, p. 1–16, doi: 10.1016/0377–0273(77)90012–9. continental breakup boundary and migration of the mantle through dike interaction: Journal of Geophysical Nicolas, A., Boudier, F., and Ildefonse, B., 1994, Dike pat- Afar triple junction, Ethiopia: Geological Society of Research, v. 107, p. 2223, doi: 10.1029/2001JB000251. terns in diapirs beneath oceanic ridges: Constraints America Bulletin, v. 115, p. 1053–1067. Janza, F.J., Blue, H.M., and Johnston, J.E., eds., 1975, from volcanological and geochemical investigations, Tibaldi, A., 1995, Morphology of pyroclastic cones and Manual of remote sensing: Theory, instruments and in Ryan, M.P., ed., Magmatic system: San Diego, Cali- tectonics: Journal of Geophysical Research, v. 100, techniques: Volume I: Bethesda, Maryland, American fornia, Academic Press, p. 77–95. p. 24,521–24,535, doi: 10.1029/95JB02250. Society of Photogrammetry, 867 p. Ouillon, G., Castaing, C., and Sornette, D., 1996, Hierarchical Tiberi, C., Ebinger, C., Ballu, V., Stuart, G., and Oluma, Kidane, T., Courtillot, V., Manighetti, I., Audin, L., Lahitte, geometry of faulting: Journal of Geophysical Research, B., 2005, Inverse models of gravity data from the Red P., Quidelleur, X., Gillot, P.-Y., Gallet, Y., Carlut, J., v. 101, p. 5477–5487, doi: 10.1029/95JB02242. Sea–Aden–East African rifts triple junction zone: Geo- and Haile, T., 2003, New paleomagnetic and geochro- Pacheco, J.F., Scholz, C.H., and Sikes, L.R., 1992, Change in physical Journal International, v. 163, p. 775–787, doi: nologic results from Ethiopian Afar: Block rotations the frequency-size relationship from small to large earth- 10.1111/j.1365–246X.2005.02736.x. linked to rift overlap and propagation and determi- quakes: Nature, v. 355, p. 71–73, doi: 10.1038/355071a0. van Vyk de Vries, B., and Merle, O., 1996, The effect of vol- nation of a ~2 Ma reference pole for stable Africa: Pelletier, J.D., 1999, Statistical self-similarity of magmatism canic constructs on rift fault patterns: Geology, v. 24, Journal of Geophysical Research, v. 108, p. 2102, doi: and volcanism: Journal of Geophysical Research, v. 104, p. 643–646. 10.1029/2001JB000645. p. 15,425–15,438, doi: 10.1029/1999JB900109. Vogt, P.R., 1974, Volcano spacing, fractures, and thickness Kreemer, C., Holt, W.E., and Haines, J., 2003, An inte- Petford, N., Cruden, A.R., McCaffrey, K.J.W., and Vigner- of the lithosphere: Earth and Planetary Science Letters, grated global model of present day plate motions and esse, J.L., 2000, Granite magma formation, transport v. 21, p. 235–252, doi: 10.1016/0012–821X(74)90159–9. plate boundary deformations: Geophysical Journal and emplacement in the Earth’s crust: Nature, v. 408, Vrabel, J., 1996, Multispectral imagery band sharpening International, v. 154, p. 8–34, doi: 10.1046/j.1365– p. 669–673, doi: 10.1038/35047000. study: Photogrammetric Engineering and Remote 246X.2003.01917.x. Prodehl, C., and Mechie, J., 1991, Crustal thinning in rela- Sensing, v. 62, p. 1075–1083. Lahitte, P., Gillot, P.-Y., Kidane, T., Courtillot, V., and tionships to the evolution of the Afro-Arabian rift Wadge, G., and Cross, A., 1988, Quantitative methods for Abebe, B., 2003a, New age constraints on the timing system—A review of seismic refraction data: Tecto- detecting aligned points: An application to the volca- of volcanism in central Afar, in the presence of propa- nophysics, v. 198, p. 311–327, doi: 10.1016/0040– nic vents of the Michoacan-Guanajutato volcanic fi eld, gating rifts: Journal of Geophysical Research, v. 108, 1951(91)90158-O. Mexico: Geology, v. 16, p. 815–818, doi: 10.1130/0091– p. 2123, doi: 10.1029/2001JB001689. Prodehl, C., Fuchs, K., and Mechie, J., 1997, Seismic-refrac- 7613(1988)016<0815:QMFDAP>2.3.CO;2. Lahitte, P., Gillot, P.-Y., and Courtillot, V., 2003b, Silicic tion studies of the Afro–Arabian rift system––A brief Walsh, J.J., and Watterson, J., 1993, Fractal analysis of frac- central volcanoes as precursors to rift propagation: The review: Tectonophysics, v. 278, p. 1–13, doi: 10.1016/ ture pattern using the standard box-counting technique: Afar case: Earth and Planetary Science Letters, v. 207, S0040–1951(97)00091–7. Valid and invalid methodologies: Journal of Structural p. 103–116, doi: 10.1016/S0012–821X(02)01130–5. Redfi eld, T.F., Wheeler, W.H., and Often, M., 2003, A kine- Geology, v. 15, p. 1509–1512, doi: 10.1016/0191– Lutz, T.M., 1986, An analysis of the orientation of large matic model for the development of the Afar Depres- 8141(93)90010–8. scale crustal structures: A statistical approach based sion and its paleogeographic implications: Earth and Woldetinsae, G., and Gotze, H.-J., 2005, Gravity fi eld and on areal distribution of pointlike features: Journal of Planetary Science Letters, v. 216, p. 383–398, doi: isostatic state of Ethiopia and its adjacent areas: Jour- Geophysical Research, v. 91, p. 421–434. 10.1016/S0012–821X(03)00488–6. nal of African Earth Sciences, v. 41, p. 103–117, doi: Makris, J., and Ginzburg, A., 1987, The Afar Depression: Renshaw, C.E., 1999, Connectivity of joints networks with 10.1016/j.jafrearsci.2005.02.004. Transition between continental rifting and sea fl oor power-law length distributions: Water Resources Research, Wright, T.J., Ebinger, C., Biggs, J., Ayele, A., Gesahegn, spreading: Tectonophysics, v. 141, p. 199–214, doi: v. 35, p. 2661–2670, doi: 10.1029/1999WR900170. Y., Keir, D., and Stork, A., 2006, Magma-maintained 10.1016/0040–1951(87)90186–7. Roberts, S., Sanderson, D.J., and Gumiel, P., 1998, Fractal rift segmentation at continental rupture in the 2005 Mandelbrot, B.B., 1982, The fractal geometry of nature: analysis of Sn-W mineralization from Central Iberia: Afar dyking episode: Nature, v. 442, p. 291–294, doi: New York, W.H. Freeman, p. 468. Insights into the role of fracture connectivity in the for- 10.1038/nature04978, doi: 10.1038/nature04978. Manighetti, I., Tapponnier, P., Gillot, P.Y., Jacques, E., Cour- mation of an ore deposit: Economic Geology and the Wu, H., and Pollard, D.D., 1995, An experimental study of tillot, V., Armijo, R., Ruegg, J.C., and King, G., 1998, Bulletin of the Society of Economic Geologists, v. 93, the relationships between joint spacing and layer thick- Propagation of rifting along the Arabia-Somalia plate p. 360–365. ness: Journal of Structural Geology, v. 17, p. 887–905, boundary Into Afar: Journal of Geophysical Research, Roberts, S., Sanderson, D.J., and Gumiel, P., 1999, Fractal doi: 10.1016/0191–8141(94)00099-L. v. 103, p. 4947–4974, doi: 10.1029/97JB02758. analysis and percolation properties of veins, in McCaf- Zanettin, B., 1993, On the evolution of the Ethiopian volca- Margolin, G., Berkowitz, B., and Scher, H., 1998, Structure, frey, K.J.W., et al., eds., Fractures, fl uid fl ow and min- nic province, in Abbate, E., et al., eds., Geology and fl ow, and generalized conductivity scaling in fracture eralization: Geological Society [London] Special Pub- mineral resources of Somalia and surrounding : networks: Water Resources Research, v. 34, p. 2103– lication 155, p. 7–16. Istituto Agronomico per L’Oltremare, Firenze, Relazi- 2121, doi: 10.1029/98WR01648. Rogers, N., Macdonald, R., Fitton, J.G., George, R., Smith, oni e Monografi e Agrarie Subtropicali e Tropicali, Marrett, R., Ortega, O.J., and Kelsey, C.M., 1999, Extent M., and Barreiro, B., 2000, Two mantle plumes Nuova Serie, v. 113, p. 279–310. of power-law scaling for natural fractures in rock: beneath the East African Rift system: Sr, Nd and Pb Zanettin, B., Justin-Visentin, E., and Piccirillo, E.M., 1978, Geology, v. 27, p. 799–802, doi: 10.1130/0091– isotope evidence from Kenya Rift Basalts: Earth and Volcanic succession, tectonics and magmatology in 7613(1999)027<0799:EOPLSF>2.3.CO;2. Planetary Science Letters, v. 176, p. 387–400, doi: central Ethiopia: Atti Memorie Accademia Patavina Mazzarini, F., 2004, Volcanic vent self-similar clustering 10.1016/S0012–821X(00)00012–1. Scienze Lettere Arti, v. 90, p. 5–19. and crustal thickness in the northern Main Ethiopian Rosendahl, B.L., 1987, Architecture of continental rifts with Rift: Geophysical Research Letters, v. 31, p. L04604, special reference to East Africa: Annual Review of doi: 10.1029/2003GL018574. Earth and Planetary Sciences, v. 15, p. 445–503, doi: Mazzarini, F., and D’Orazio, M., 2003, Spatial distribution 10.1146/annurev.ea.15.050187.002305. of cones and satellite-detected lineaments in the Pali Rubin, A.M., 1993, Dikes vs. diapirs in viscoelastic rock: Aike Volcanic Field (southernmost Patagonia): Insights Earth and Planetary Science Letters, v. 119, p. 641– into the tectonic setting of a Neogene rift system: Jour- 659, doi: 10.1016/0012–821X(93)90069-L. MANUSCRIPT RECEIVED 28 SEPTEMBER 2006 nal of Volcanology and Geothermal Research, v. 125, Sella, G., Dixon, T.H., and Mao, A., 2002, REVEL: A REVISED MANUSCRIPT RECEIVED 21 FEBRUARY 2007 p. 291–305, doi: 10.1016/S0377–0273(03)00120–3. model for recent plate velocities from space geodesy: MANUSCRIPT ACCEPTED 7 MARCH 2007

162 Geosphere, June 2007

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