Geology, geochronology, and rift basin development in the central sector of the Main Ethiopia Rift
GIDAY WOLDEGABRIEL* ) n „ „, JAMES L ARONSON J DePartment °J Geological Sciences, Case Western Reserve University, Cleveland, Ohio 44106 ROBERT C. WALTER Institute of Human Origins, 2453 Ridge Road, Berkeley, California 94709
ABSTRACT 37°30'-40°. A satellite thematic mapper image of this classic rift region (Fig. 1) shows all but the southwestern part of the study area. The MER is Based on stratigraphic relationships and K/Ar dating of volcanic divided geographically into three sectors: northern, central, and southern rocks from both of the escarpments, flanking plateaus, and from the (inset map, Fig. 2), and most of the central sector of the rift proper is in this rift floor of the central sector of the Main Ethiopian Rift, six major image. The MER divides the 1,000-km-wide uplifted Ethiopian volcanic volcanic episodes are recognized in the rift's development over a time province asymmetrically into the northwest and southeast plateaus (inset span from the late Oligocene to the Quaternary. Using the K/Ar data, map, Fig. 2). Volcanic sequences that cover an area several hundred correlation of volcanic units from the six periods of activity through- kilometers across are more voluminous and widespread on the northwest out the study area forms the basis for establishing six time- plateau than on the opposite side. Contrary to previous suggestions that stratigraphic chronozones for the central sector that are related to volcanism migrated from the northwest plateau toward the Ethiopian Rifts volcanism in the Ethiopian Cenozoic volcanic province. The oldest (MER and Afar) with time (Pilger and Rosier, 1975; Zanettin and basalt and rhyolite flows exposed along the rift margins of the central Justin-Visentin, 1975; Morton and others, 1979; Zanettin and others, sector are time correlative to, or older than, those in river canyons (for 1980), the margins of the Ethiopian rifts locally expose pre-Tertiary sedi- example, Blue Nile) on the adjacent northwest plateau. A thinned mentary and basement rocks unconformably overlain by Paleogene vol- Mesozoic stratigraphic sequence along the Guraghe western rift mar- canic flows that are just as old as the flows covering the northwest plateau, gin suggests that doming may have preceded volcanism and rifting of as demonstrated in this paper and elsewhere. the Cenozoic. Most of the geologic sections exposed along the rift margins are By late Miocene time, at least by 8.3 Ma and 9.7 Ma, the eastern dominated by Tertiary volcanic rocks except for a few locations where and western faulted margins, respectively, of the rift had formed at crystalline basement is unconformably overlain by Mesozoic sedimentary Guraghe and at Agere Selam as indicated by containment of flows of and/or Tertiary volcanic rocks. Such pre-Tertiary rocks covered by Ter- that age within the rift wall during eruption. A paroxysm of calc- tiary basalt are present along the eastern, western, and southern Afar alkaline ignimbrite activity produced voluminous flows nearly fully margins (Hutchinson and Engels, 1970; Mohr, 1970; Zanettin and Justin- contained within the rift during the Pliocene epoch. The Munesa Crys- Visentin, 1974; Black and others, 1975; Chessex and others, 1975), the tal Tuff (3.5 Ma), a prominent marker tuff exposed on both rift mar- western rift margin (Guraghe Mountain) of the central sector of the MER gins, is present at depth in a geothermal well beneath the rift floor and (WoldeGabriel and Aronson, 1986), and in the Amaro Horst of the south- indicates a minimun of 2 km of downthrow in the central sector since ern sector of the MER (Levitte and others, 1974; Zanettin and others, its eruption. 1978). In the broad rift zone of southwest Ethiopia, crystalline basement is Structural and stratigraphic relationships in the central sector unconformably overlain by various Tertiary (12.7-49.4 Ma) and Quater- indicate a two-stage rift development. This is characterized by an early nary volcanic rocks, including the oldest known (Eocene) flood basalts in phase (late Oligocene or early Miocene) of a series of alternating Ethiopia (Davidson and Rex, 1980; Davidson, 1983). opposed half-grabens along the rift with alternating polarity, such as Several K/Ar data have been previously published on volcanic rocks that in the present Gregory and Western Rifts of East Africa and from the northern and southern sectors of the MER and the rift flanks, but symmetrical rifts that evolved from these grabens in late Miocene or outside of the rift floor, the central sector has been very little studied. The early Pliocene time. Thus, evolution from alternating half-graben to a volcanic rocks along the boundary faults of the northern sector of the full symmetrical graben with a medially located neovolcanic zone that MER range in age from Oligocene to Pliocene (28.0-5.0 Ma) (Rex and is bifurcated to marginal grabens in the northern part of the study area others, 1971; Justin-Visentin and others, 1974; Kunz and others, 1975; may be a fundamental part of the rifting process. The study indicates Morbidelli and others, 1975; Jones, 1976; Morton and others, 1979; Kaz- that there are major petrologic and tectonic differences between the min and others, 1980). A single date of 27.8 Ma and Plio-Pleistocene dates Main Ethiopian Rift and the Gregory (Kenyan) Rift. (5.3-1.2 Ma) were reported from the rift margins and shoulders of the central sector of the MER (Mohr and Potter, 1976; Merla and others, INTRODUCTION AND BACKGROUND 1979; Kazmin and others, 1980; Mohr and others, 1980; Zanettin and others, 1980). Like the northern sector, the boundary faults of the southern The Main Ethiopian Rift (MER) is a symmetrical graben with up- sector of the MER expose Oligocene to Miocene (31.7-11.9 Ma) volcanic lifted flanks and steep border faults; it lies between lat. 5°- 9°N and long. rocks (Levitte and others, 1974; Zanettin and others, 1978). The central sector of the MER itself is more than 175 km long and 75 *Present address: Los Alamos National Laboratory, ESS-1/D462, Los Ala- km wide, widening toward the northeast (Afar) and narrowing southward mos, New Mexico 87545. (Mohr, 1967). During two field seasons, in 1983 and 1984, most of the
Geological Society of America Bulletin, v. 102, p. 439-458, 12 figs., 2 tables, April 1990.
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Figure 2. The central sector of the MER and adjacent areas. Thick line segments 26-33; Area 4:34-43; Area 5:44-52; Area 6:53-58; Area 7:59-61; Area 8:62-73; Area 9: represent rift margin faults with ticks on the downthrown side. Pointed stars represent 74-77. Lakes are dotted, fine lines are rivers, and medium lines are major roads. The rift-shoulder central volcanoes, and asterisks are Quaternary peralkaline rhyolite centers of inset map displays the area (dotted) of the thematic mapper image shown in Figure 1. the rift axis. The dual marginal Quaternary rift axes on the rift floor of the northern part of MER rift sectors: I, Northern; II, Central; and III, Southern; and the major Ethiopian the central sector of the MER are expressed by tightly defined lines. Numbers represent volcanic regions: N.W., northwestern; C.E., central eastern; S.E., southeastern; and S.W., sample locations at each of the nine major sections: Area 1:1-22; Area 2: 23-25; Area 3: southwestern.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/4/439/3380803/i0016-7606-102-4-439.pdf by guest on 28 September 2021 TABLE 1 . K/Ar AGE DATA OF VOLCANIC ROCKS FROM THE CENTRAL SECTOR OF THE MAIN ETHIOPIAN RIFT
40 Sample Locations Rocks K2O Ar* «Ar- Aget numbers dated (wt %) (IO"11 mole/g) IS) (m.y.)
Guraghe-Kella western rift margin 1 ET3 8°15'N38°28'E Basalt 1.444 6.785 92 32.30 ± 1.6 2 ET4 8°20'30"N38°30"E Basalt 1.463 6.883 89 32.40 ± 1.6 3 BT10 8°18'N38°30"E Plagioclase 0.179 0.266 16 4.00 ± 0.5 4 BT15 8°l 5'31 "N38°°29'50"E Basalt 1.240 0.663 55 3.70 ± 0.2 5 BT17 8°16'30*N38o29'10"E Tuff 4.273 3.236 71 3.63 ± 0.2 6 ET25 8°16'N38°29'E Tufrf 7.098 3.636 64 3.60 ± 0.2 7 KELI 8°16'N38°29'E Tuff 4.257 2.499 85 4.07 ± 0.2 8 W8340E 8°17'N38°28'30'E Tuff 4.188 2.486 80 4.10 ± 0.2 9 BTI6 Tuff 4.572 2.576 76 3.90 ± 0.2 10 BT51 Plî-NM^'E Tu(# 5.983 2.301 61 2.67 ± 0.1 11 ET7A 8°1'30"N38,'18'E TuffS 5.640 2.107 70 2.59 ± 0.1 12 BT57 8°12'58"N38°17'30*E Basalt 1.111 1.460 54 9.10 ± 0.5 13 BT68 8°12'58"N38°17'30"E Basalt 1.313 1.637 76 8.60 ± 0.5 14 BT69 8° 12'58"N38° 17'30"E Basalt 1.255 1.546 12 8.50 ± 0.8 15 BT72 8° 12'58"N38°17'30"E Basalt 1.443 1.937 53 9.30 ± 0.5 16 BT73 8°12'58-N38°17'30"E Basalt 1.373 3.098 92 10.60 ± 0.5 17 BT58 8°l2'58-N38°l7-30-E Tuff 4.370 2.428 85 3.85 ± 0.2 18 BT120 8°22TÖ8°24'E Rhyolite 5.035 7.263 96 9.98 ± 0.5 19 BT122 8021'35"N38°24'E Basalt 0.578 0.786 67 9.50 ± 0.5 20 BTI19 8°21'N38°24'E Tuff 4.610 2.326 87 3.50 ± 0.2 21 BT26 8°H'N38°20'E Tuff 3.889 4.694 85 8.37 ± 0.4 22 BT54 8°I4"N38°21'E Tuff 4.780 6.827 84 8.30 ± 0.4
Ambo fault scarp (lineament) 23 ET 111 9°4'N37°55'E Basanite 1.832 8.437 93 31.70 ± 1.5 24 ET 109 9°10'N37°55'E Basalt 1.525 5.651 65 25.60 ± 1.4 25 ET 112 9°1N37°55'E Basalt 1.237 1.215 19 0.68 ± 0.05
Guraghe-Munesa rift floor 26 BT24 S'lSTOSWE Basalt 0.858 0.017 8 0.13 ± 0.02 27 BTI08 8°9'30*N38°20'E Obsidian 4.027 9.181 6 1.58 ± 0.2 28 BT06 7°56'N38°39'30"E Rhyolite 4.273 0.817 58 1.30 ± 0.1 29 BT82 7°49'N38°45'30"E Obsidian 4.420 0.027 8 0.04 ± 0.01 30 BT79A 7°44'30"N38°47'30*E Rhyolite 4.697 0.054 3 0.08 ± 0.02 31 LA3 7°47'30*N38°48'E Tuff$ 9.311 1.864 26 1.39 ±0.1 32 BT95 7°40'N38°53'E Trachyte 3.679 0.442 23 0.83 ± 0.1 33 BT92 7°52'N38°54'30"E Hawaiite 1.436 0.059 6 0.29 ±0.1
Munesa-Asela eastern rift margin 34 BT-87A 7°35'N38°54'E Tuff 4.460 2.267 72 3.53 ± 0.2 35 BT87A 7°35'N38°54'E Feldspar 5.875 2.969 55 3.51 ± 0.2 36 BT87B 7°35'N38°54'E Obsidian 4.839 2.059 28 2.95 ± 0.2 37 BT87B 7°35"N38054'E Feldspar 5.447 2.220 31 2.83 ± 0.2 38 BT86 7°35'N38°54'E Basalt 0.807 2.948 47 2.54 ± 0.1 39 BT88A 7°35'30"N38°51'30"E Tuff 4.860 2.440 56 3.48 ± 0.2 40 BT89 7°3i'N38°5ri0'E Tuff 4.840 2.230 75 3.19 ± 0.2 41 W8339 7°57'30"N39°5'E Basalt 1.083 0.307 26 1.97 ± 0.2 42 BTI13 7°57'30"N39°5'E Basalt 2.041 0.518 22 1.76 ± 0.1 43 BTI 14 7°57'30"N39°5'E Tuff 4.473 1.069 61 1.66 ± 0.1
Central volcanoes of the eastern rift shoulder 44 BT131A 8°2'N39°H'E Trachytic tuff 3.042 1.113 86 2.54 ± 0.1 45 BT135 7055'N39°1'E Trachybasalt 1.811 0.455 48 1.74 ±0.1 46 BT 143 7°31'30"N39°18'E Basalt 0.806 0.219 47 1.88 ± 0.1 47 BT140 7°24'N39°29'E Mugearite 2.068 0.764 67 2.56 ±0.1 48 BT 142 7°24'N39°13'E Trachyte 4.188 1.649 88 2.70 ±0.1 49 ET19A 7°31'N39°rE Phonolite 4.911 9.032 82 12.10 ± 0.6 50 ET19B 6°55'N37°37°30'E Phonolite 2.954 4.953 83 11.60 ±0.6 51 BT148 7°44'N39°5'E Tuff 4.497 0.199 50 0.31 ± 0.0 52 BT150 7°44'N39°5'E Tuff 4.560 0.025 8 0.38 ± 0.0
Wabi Shebele River canyon 53 ET81 7°14'40"N39°28'E Mugearite 3.748 9.181 69 16.90 ± 0.8 54 ET83-2 7°14'40"N39°28'E Feldspar 5.337 6.692 97 16.60 ± 0.8 55 ET83-4 7°14"40"N39°28'E Feldspar 6.043 12.214 95 14.00 ± 0.6 5« ET84 7°14'40"N39°28'E Trachyte 5.332 12.385 97 16.10 ± 0.7 57 ET86 7°14'40"N39°28'E Trachybasalt 3.799 1.567 43 2.86 ± 0.2 58 ET87 7°14'40"N39°28'E Basalt 1.282 0.523 50 2.82 ± 0.2
Wagebeta-Sodo western rift margin 59 ET75 7°22"N37°46'30°E Feldspar 4.666 2.842 83 4.20 ± 0.2 60 ET74 7°18'N37°46'E Rhyolite 4.182 2.191 79 3.63 ± 0.2 61 ET72 7°56'30"N37°44'30"E Trachyte 5.563 2.357 65 2.94 ± 0.1
Awasa eastern rift margin and the adjacent rift floor 62 ET43 7°16'N38°4'E Feldspar 6.617 0.198 28 0.21 ± 0.01 63 ET39 6°55'N38°22'30"E Rhyolite 4.912 1.761 90 2.49 ± 0.1 64 ET45 7°3'N38°20'E Benmorite 2.607 0.477 56 1.27 ± 0.1 65 ET28 7°6'N38°36'E Obsidian 4.160 0.579 45 0.96 ± 0.1 66 ET77 7°5'30"N38°35'30"E Rhyolite 4.533 0.306 52 0.47 ± 0.02 67 ET33 7°10'45"N38°29'E Obsidian 4.464 0.015 2 0.02 ± 0.01 68 ET78 7°5'N38°38'30"E Mugearite 2.119 2.975 86 9.70 ± 0.5 69 ET30 7°5'N38°38'30"E Feldspar 5.281 11.967 78 9.69 ± 0.7 70 ET79 7°5'N38°38'30*E Feldspar 6.245 3.320 87 3.69 ± 0.2 71 ET29E 7°5'N38°40'E Feldspar 0.567 0.151 24 1.85 ± 0.1 72 ET29B 7°5'N38°40'E Feldspar 5.852 0.929 51 1.10 ±0.1 73 HE8416 6°57'30'N38°33'E Basalt 1.189 0.282 34 1.60 ± 0.1
Agere Selam eastern rift margin 74 ET53 6°35'N38°27'E Basalt 1.023 4.293 60 28.90 ± 1.6 75 ET48 6°35'N38°27'E Rhyolite 4.327 19.515 47 31.10 ± 1.6 76 ET49 6°29"N38°32'E Mugearite 2.929 12.668 96 29.70 ± 1.3 77 ET50 6°29'N38032'E Feldspar 6.400 2.133 82 2.30 ± 0.1
•Radiogenic. tDetermined from decay constants and isotopic abundance of ^K according to Steiger and Jäger (1977). ^Feldspars dated.
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major geologic sections along the rift margins, floor, and adjacent river stratigraphic units (chronozones) for the central sector as a whole. This canyons were surveyed (Fig. 1). Sampling employed the utmost effort to stratigraphic framework for the central sector allows us to relate the vol- obtain fresh and unaltered rocks. Major- and trace-element analyses define canic history of the central sector to that previously established elsewhere the geochemical and petrological characteristics of these rocks (Wolde- in the Ethiopian Cenozoic volcanic province. Thirdly, the stratigraphic Gabriel, 1987; Hart and others, 1989) and their petrologic classification framework is used to integrate the data from each stratigraphic section into based on Le Bas and others (1986). Using field relations, petrology, and three generalized cross-rift sections, one each across the northern, middle, K/Ar dating, we develop a systematic stratigraphic framework for the and southern parts of the study area from the western to the eastern rift previously little-known central sector of the MER. On the basis of this new shoulder. By combining data from the cross-rift stratigraphic sections to- information, the volcanotectonic history of the central sector is interpreted. gether with structural information, the history of rifting and volcanism is deduced. Finally part four summarizes the evolutionary relationships be- SAMPLES AND ANALYTICAL PROCEDURES tween rifting and volcanic petrology for the central sector and compares them with the regional tectonomagmatic development of the rest of the All samples analyzed were chosen from petrographic indications of MER and the East African rift system. freshness and were thoroughly cleaned, crushed, and sieved. Mesh frac- tions of 14-40 were used for whole rock and 40-60 or 60-80 for feldspar GEOLOGIC AND GEOCHRONOLOGIC RESULTS K/Ar dating, following the methods outlined in Hart (1982). Whole-rock and feldspar separates from the same samples were dated to check for The nine principal areas with prominent sections that form the focus contamination from older rock fragments or xenocrysts in the welded tuffs. of this study are shown in Figure 2. First, the six areas from the northern K/Ar data for all samples appear in Table 1, numbered in the order part of the study area are discussed west to east, followed by the three discussed in the text. sections from the southern part of the central sector. Argon was analyzed with an MS-10 spectrometer equipped with an online extraction system and a bulb-pipetted 38Ar tracer calibrated by the 1. Western Rift Margin at Guraghe LP-6 interlaboratory standard (19.3 x 10~10 mole/g radiogenic argon) that yielded 19.55 x 10 10 ± 0.12 mole/g (average and standard deviation of The arcuate western rift escarpment at Guraghe is a key area for 23 runs). The K2O analyses were made on duplicate samples with a flame recording many details about the structural and petrologic history of the photometer on acid solutions of sample beads fused in lithium metaborate. MER. The composite escarpment exposes a sliver of crystalline basement A sanidine from the Fish Canyon Tuff with the best current date of 27.79 with its cover of Mesozoic sedimentary strata, and a 1.5-km-thick section Ma (Kunk and Sutter, 1985) gave 27.89 Ma. Repeat runs of several of basalt-dominated volcanic rocks (Di Paola, 1972) along several step samples gave reasonable evidence of reproducibility (Table 2). faults (Figs. 3A and 3B). Until now only a small amount of information has been available about the pre-Tertiary rocks, discovered in 1978 (Arno NATURE OF THE STUDY and others, 1981). Because of its thick character and by analogy to the plateau basalts, the basaltic succession at Guraghe has been regarded as In any rift setting as large as the central sector of the MER, the lateral Eocene or Oligocene in age (Mohr, 1970; Di Paola, 1972). A single age of distribution of volcanic units is often local with complex interfingering 4.2 m.y. was reported from a welded tuff along the northwestern slopes of relations, and units are disrupted by faulting or buried by rift fill. Thus Guraghe (Zanettin and others, 1980). K/Ar dating of volcanic units is essential to correlate between sections and The crystalline basement is represented by an altered biotite gneiss to establish the spatial and temporal relationships necessary to understand intruded by swarms of northwest-trending quartzofeldspathic pegmatites, the volcanotectonic history. neither of which were dated here, but radiometric dates on crystalline There are four parts to our presentation. First we describe the founda- basement elsewhere in Ethiopia range from 976-370 Ma (Page and others, tion of the study. Namely this is the stratigraphic relations and geochrono- 1972). The Mesozoic sequence (Fig. 4A), which is widely present across logic data obtained from the nine high-relief areas that contain the most Ethiopia, unconformably overlies the crystalline basement at Guraghe. complete and best exposed composite sections of the volcanic sequences Compared to a typical total thickness elsewhere of more than 1,000 m (for of the rift shoulder, margin, and floor in the study area. Secondly, we example, Blue Nile or Abbay River gorge, inset Fig. 2), the Mesozoic present a summary of the correlations among sections indicated by the sequence is much thinner: 150 m of Early Jurassic Adigrat Sandstone, 20 K/Ar data and show that there are six logically separated time- m of varigated shale, and 30 m of Jurassic Antalo Limestone (Arno and others, 1981). The Cretaceous strata present elsewhere in the platform Mesozoic sequence of Ethiopia is absent at Guraghe, due to either non- TABLE 2. K/Ar DATA OF SAMPLES RUN REPEATEDLY TO CHECK FOR REPRODUCIBILITY deposition or erosion. Fine-grained Oligocene basaltic flows (32 Ma; sam- AND CONTAMINATION FROM OLDER ROCK FRAGMENTS OR XENOCRYSTS IN WELDED TUFF ples nos. 1 and 2, Table 1) overlain by fluvial strata are exposed along both
40 Sample K20 Ar> mole/g «""At* Aget flanks of the narrow fault blocks containing the pre-Tertiary rocks (Fig. number (wt%) (10"") (m.y.) 4A). These 40- to 60-m thick lavas are exposed along fault-controlled
ET49 2.959 12.581 97 29.30 ± 1.4 stream cuts transverse to the rift margin. The fluvial strata (2-10 m thick) ET49 2.929 12.668 96 29.70 ± 1.3 suggest that subsidence at least locally accompanied volcanism in this area. ET49 2.914 12.671 97 29.90 ±1.4 ET795 6.256 3.365 80 3.70 ± 0.2 The pre-Tertiary rocks, the Oligocene basalt, and the fluvial strata are ET79§ 6.245 3.320 87 3.69 ± 0.2 ET79§ 6.256 3.305 66 3.67 ± 0.2 unconformably overlain by a thick sequence of Pliocene basalt flows BT87A" 4.460 2.267 72 3.53 ± 0.2 (4.2-2.5 Ma, nos. 3 and 4) and particularly crystal-rich welded tuff (nos. BT87A§ 5.875 2.970 55 3.51 ± 0.2 W8340C« 4.520 2.476 89 3.80 ± 0.2 5-9), that together exceed a total thickness of 400 m (Fig. 4A). A single W8340C§ 1.812 2.333 20 3.60 ± 0.2 crystal-rich tuff (15-40 volume percent crystals, 3.5 Ma, nos. 5 and 6)
'Radiogenic. dominates the section (200-250 m thick). A number of thin (2-10 m) t Determined from decay constants and isotopic abundance of 40K according to Steiger and Jäger (1977). ET49 - Mugearite lava. ash-fall tuffs that cover the individual step-faulted blocks in front of the ^Sanidine. main Guraghe scarp are late Pliocene, having ages between 2.72 and 2.59 "Welded-tuff whole rock. m.y. (nos. 10 and 11).
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South along the strike of the pre-Tertiary rocks, the rift escarpment is flows are blanketed by 30- to 40-m-thick, mid-Pliocene, crystal-rich tuff offset sharply to the west at the arcuate Guraghe escarpment (Figs. 3A and (3.5 Ma; no. 20). 3B). There at least 14 thick, generally aphyric, basalt flows and intercal- Along the foothills of the main Guraghe scarp, a geologically signifi- ated vitric ash are exposed (Fig. 4B); these yielded ages of 10.6 and 9.1 cant glassy and perlitic welded tuff abuts and thickens against the rift wall m.y. (nos. 12-16) for the basal and top flows of the section, respectively. (Fig. 3B). Two samples of this tuff (nos. 21 and 22) collected 10 km apart The base of the section is not exposed. Some lavas are separated by along the base of the escarpment gave consistent dates of 8.3 Ma and scoriaceous horizons with no apparent erosional surface between the lavas, provide a minimum age for the formation of the boundary fault. implying a rapid extrusion. The top of the section at Guraghe is capped by 20-m-thick Pliocene welded tuff (3.85 Ma; no. 17). The Gash Megal 2. Ambo Lineament topographic high perched along the rift shoulder, just north of the Guraghe escarpment (Figs. 2 and 3), is a late Miocene rhyolite overlying time- In the Ambo area, 100 km northwest of the Guraghe western rift correlative basaltic flows (10 Ma; nos. 18 and 19). The basalt and rhyolite margin, more than 500 m of volcanic rocks are exposed along a normal fault of the east-west-trending Ambo lineament (Le Bas and Mohr, 1968; Moore and Davidson, 1978; Merla and others, 1979) (Figs. 2 and 4C).
km EASTERN SHOULDER Approx Sca,e RIFT FLOOR EASTERN MARGIN WESTERN MARGIN
•j Gash Megal
AIN \ v \ W \ URAGHE' Guraghe SECTION / / A
tpy Late Miocene B —/-^y Ignlmbrite
PRE-TERTIARY ROCKS 0 I- luaternary km Basalts Scale (Horizontal)
Figure 3. Schematic block diagrams of (A) the northern half of the central sector of the MER and (B) the western rift margin at Guraghe. WFB, Wonji fault belt; SDZFZ, Silti-Debre Zeit fault zone.
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Here the Cretaceous Upper Sandstone (Beauchamp, 1977; Merla and consists of reworked pyroclastic rocks separated by a thin (30-cm) spheru- others, 1979) is unconformably overlain by an Oligocene basanite flow litic obsidian flow that yielded an age of 1.58 m.y. (no. 27). The top of the (32 Ma; no. 23, Table 1). Contacts between the lavas above the basal flow section is blanketed by bedded ash falls erupted from Quaternary silicic are obscured by talus. The top of the section is represented by a 26-m.y.- centers of the rift axis (Aluto and Bora, Figs. 2 and 3). South of the Meki old basalt (no. 24), which forms a 30- to 40-m-high cliff capped by lateritic River section, the Gademota caldera forms an arcuate structure of rhyolite soil. The downthrown block is covered by a 3-m-thick, 0.68-m.y.-old ridges and domes (Fig. 3A). This arcuate feature is an eroded western rim basalt (no. 25) that crops out on top of unconsolidated ash-fall from the of a large (30-km diameter?) caldera with an eastern rim buried under Wonchi caldera 25 km to the southwest (Fig. 2). The basalt is the youngest younger rocks (Laury and Albritton, 1975; Lloyd, 1977, unpub. Ethiopian flow observed outside the rift floor of the central sector. Its presence on Geol. Survey Report). Our 1.3-m.y. (no. 28) age of the rhyolite from the strike with the Quaternary Tepi Basalt fields of southwest Ethiopia (Da- eastern wall compares well to previous 1.27- and 1.28-m.y. sanidine ages vidson, 1983) suggests that the Ambo lineament was reactivated in this (EIGS-GLE, 1985) but is older than the 1.05 m.y. reported by Laury and area during the Quaternary period. Albritton (1975). The Wonji fault belt (WFB), which is the main rift axis in the MER 3. Basins of the Rift Floor: Lakes Ziway-Langano-Abiata (Mohr, 1967), is displaced from the median of the rift floor to become the eastern marginal graben of the central sector. Unlike the Silti-Debre Zeit The rift floor in the study area consists of three caldera-related basins fault zone, the WFB has peralkaline silicic centers along it. Southeast of occupied by Lakes Ziway-Langano-Abiata, Shala, and Awasa, and the the Gademota caldera, the Aluto volcanic center rises more than 600 m Bilate River drainage basin (Fig. 2). These basins are connected by the from the surrounding floor (Fig. 3A). Two samples from the bases of the volcanotectonically active Wonji fault belt (Mohr, 1967). In the northern northwestern and southeastern slopes are 0.042 m.y. and 0.078 m.y. old, central sector (Ziway-Langano-Abiata basin), the rift axis consists of two respectively (nos. 28 and 29), within the range of 0.27-0.021 m.y. pre- en echelon Quaternary volcanotectonic zones, as narrow grabens along viously reported on Aluto rocks (EIGS-GLE, 1985). each magin of the rift floor. Elsewhere in the MER, the rift axis forms a Geothermal well LA3 drilled from the Aluto caldera floor bottomed single median zone (Fig. 1). at 2,143-m depth in a thick (>400 m), hydrothermally altered, crystal- On the Ziway-Langano-Abiata rift floor adjacent to the Guraghe rich, welded tuff. Sample no. 31 from the basal section of this tuff at escarpment, the active western marginal graben, here called the "Silti- 1,779-m-depth is an altered crystal-rich tuff. It is petrographically and Debre Zeit fault zone" (modified after Di Paola, 1972), contains lacustrine geochemically very similar to the main crystal-rich tuff exposed along the sediment and welded tuff on which are several interspersed coalescing western margin at Guraghe (nos. 5 and 6) (WoldeGabriel, 1987). The nested scoria cones aligned parallel to the Guraghe escarpment (Figs. 2, measured age of 1.39 m.y. on a feldspar separate is clearly a minimum age 3A). Basalt flows erupted along the axis of this graben for more than 60 due to thermal alteration of the feldspars (30%-40% volume) that are km along strike. An age of 0.13 m.y. (no. 26, Table 1) was obtained on completely replaced by authigenic potassium feldspar, sericite, and kaolin- one of the aphyric flows, which is younger than the 0.24 m.y. reported ite. A high whole-rock K2O content of 9% indicates that major potassium from this general area (Mohr and others, 1980). Faulting and stream erosion along the Meki River near Dugda expose a stratigraphic section (Tuff) more than 100 m thick (Figs. 2 and 3A). This steep scarp forms the eastern 3.8 wall of the western marginal graben. The bulk of this section (60 m) 9.2 100 50 LOCATION MAP 9.04 •L 0 (Vitric TUff) Meters
3.6 8.6 BLUE NILE
RIVER MUGHER 4.0 4.2 8.5 26.5 (Vitric Tuff) BASALT 3.7 Fluvial 32.0
9.3 ' MESOZOIC 10.6
4B 31.0 BIOTETE GNEISS Cretaceous WABI SHEBELE Sandstone JéJ/\ RIVER 4A 4C / / i \ ML.?} y Figure 4. Stratigraphic succession along the western rift margin !MAS (Guraghe area) and the adjacent plateau: A, Kella section; B, main / / i* WAS% «HASHEMENr^ / / (* ^ /SHASHEMENE Guraghe section; and C, Ambo area. Map shows the location of the
J; A>ILAT#'// O 20 KM SODÒ, three sections. Roads indicated by broken lines; line segments are |RIVER / ' I I rift-margin faults with ticks on the downthrown side, rivers are repre- *\AQERE SELAM sented by solid lines, and lakes are shaded.
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metasomatism has affected this unit. Bottom-hole temperatures during beds and agglomerate. The agglomerate grades to a 1.76-m.y.-old scori- drilling reached as high as 300 °C or more (EIGS-GLE, 1985). aceous basalt (no. 42). A 1,66-m.y.-old vitric tuff (no. 43) crops out on top A trachyte (no. 32) from one of the horsts east of Aluto and a of the basalt, and the top of the section is blanketed by a crystal-rich hawaiite (no. 33, Table 1) from east of Lake Ziway yielded 0.83 and 0.29 perlitic tuff. m.y., respectively. This suggests that lava production along the WFB On the basis of similar stratigraphy, petrology, and geochronology, started about 0.83 m.y. ago. Volcanism may have shifted at about this time the crystal-rich tuff (>300 m thick) at Munesa is correlated with the from the median toward the marginal zone as indicated by a pattern of altered tuff in the lower half of geothermal well LA3 (-400 m thick), and decreasing age of volcanic rocks toward the margin of the rift floor. to the crystal tuff (-250 m thick) across the rift at the Guraghe section. The Shala caldera basin of the rift floor to the south of Aluto is Because of the stratigraphic and tectonic significance of this major tuff blanketed by pyroclastic rocks erupted from Quaternary calderas along the which is correctable across the central sector of the MER, we refer to it as rift axis (for example, Aluto, Shala, and Corbetti) (Di Paola, 1972; Mohr the "Munesa Crystal Tuff." The distribution, penological, and geochemi- and others, 1980; Lloyd, 1977). Mohr and others (1980) indicated that the cal relationships (WoldeGabriel, 1987) imply that the source of this vo- 300-m-thick pyroclastic sequence exposed in the caldera wall belongs to luminous ash-flow tuff was on the rift floor between Guraghe and Munesa. the Shala caldera. Previous ages of 0.28 m.y. and 0.24 m.y. for a pre- The absence of caldera remnants, from which such an immense unit might caldera rhyolite and a welded tuff related to the caldera collapse, respec- have erupted in the Pliocene epoch, is attributed to burial under subse- tively (Mohr and others, 1980), suggest that Aluto and Shala formed quent eruptions and tectonic subsidence. Assumption of a radial-areal contemporaneously. distribution of the Munesa crystal tuff with a diameter equal to the width of the rift and an average thickness of 300 m yields a conservative estimate 3 4. Munesa-Asela Eastern Escarpment of the volume for this major eruption of 1,100 km . We consider a volume of more than 2,000 km3 obtained by using a diameter equal to the The WFB east of Aluto and Lake Ziway is bounded by steep (>60°) diagonal distance between the Munesa and Kella exposures as a better border faults of the Munesa-Asela eastern rift margin. The stratigraphic estimate of its volume. section exposed by the boundary fault decreases from more than 400 m high at Munesa to 20-30 m north of Munesa and increases again to 100 m 5. Eastern Rift Flank Volcanic Centers at Asela. The bulk of the Munesa section (Fig. 5A) is represented by a single 3.51- to 3.53-m.y.-old (whole-rock and sanidine ages), thick (>300- This region is of volcanologic and petrologic interest because six m), crystal-rich, welded tuff (nos. 34 and 35, Table 1) overlain by 2.83- to major shield volcanoes (Chilalo, Badda, Hunkuolo, Kaka, Chike, and 2.95-m.y.-old (nos. 36 and 37) vitrophyric tuff (1-2 m thick). Locally the Kubsa), ranging in elevation from 3,700-4,200 m above sea level, and the Pliocene tuff units are capped by a 2-m-thick basalt dated at 2.54 m.y. major Galama linear volcanic range (also called the Sagatu Range) are in (no. 38). Other tuffs overlying the crystal-rich unit along north- proximity to each other (Figs. 1,2, 3A). Previous geochronology led to the northeast-trending horsts in front of, and parallel to, the rift margin at notion that all centers formed during the Pliocene epoch (Kunz and others, Munesa are 3.48 and 3.17 m.y. old (nos. 39 and 40). At the Asela section, 1975; Mohr and Potter, 1976). Reconnaissance sampling of the central 40-50 km north of Munesa, a volcanic sequence is exposed that is both volcanoes and the Galama Range was carried out here to document and younger and lithologically more diverse than that exposed at Munesa (Fig. compare their ages. 5B). The base of the section is represented by a 5-m-thick, late Pliocene, K/Ar determinations on eight samples from these shield volcanoes aphanitic basalt (1.97 m.y., no. 41), overlain by horizontal sedimentary confirm that most are of mid-Pliocene age. Two samples from the lower and upper northern slope of Mount Chilalo are 2.31 and 1.74 m.y. old LOCATION MAP (nos. 44 and 45, Table 1). A basalt from the southern end of the Galama range south of Chilalo is 1.88 m.y. old (no. 46). South of the range, there are two large volcanoes, Hunkuolo and Kaka, located approximately 5 km
2.54 2.95 2.82 3.51 2.86 16.1 13.98 1.66 16.6 1.76 16.9
Sediments
WABI SHEBELE Sediments p RIVER Agglomerates 1.97 OMO RIVER/' 5A 5B 5C Figure 5. Stratigraphic succession along the Munesa-Asela east- iNWAS^ jSH ASHEMEHT^ ern rift margin and the adjacent Wabi Shebele River: A, Munesa ILAT^'7/ 0 20 kin section with the 3.5-m.y.-old Munesa Crystal Tuff; B, Asela section; RIVER'./' and C, Wabi Shebele River. Map shows the location of the three /'•, AGERE SELAM sections.
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southeast and 10 km southwest, respectively (Figs. 2 and 3A). The north- their absence farther downstream, and the upstream increase in thickness ern foothills of Mount Hunkuolo (3,850 m) are interbedded thick basal- of the 16 Ma trachytic lava indicates a close-by source, compatible with tic breccias (agglomerate) and lavas that yielded an age of 2.56 m.y. (no. their having been erupted eastward from Mount Chike. The 2.56 Ma 47). A greenish-gray trachyte dated at 2.70 Ma (no. 48) was collected K/Ar date obtained from Mount Hunkuolo only 10 km north of the from the middle section of the northeastern slope of Mount Kaka (4,245 canyon wall (Fig. 3A) suggests that it was the source for the 2.8 Ma basalt m). Mount Kubsa (late Pliocene; Lloyd, 1977) and Mount Chike are capping the Wabi Shebele section. northwest of Mount Kaka; Mount Chike is closest of all the centers to the edge of the rift. Our results show that the Mount Chike phonolite center 7. Wagebeta-Sodo Western Rift Margin and the Omo River Canyon has had a distinct history from all of the rest of the undersaturated centers of the eastern rift shoulder. Lavas from the middle and top part of its Between Guraghe and the Wagebeta caldera complex on the western northern slope are 11.6 and 12 m.y. old, respectively (nos. 49 and 50). In rift margin, the topography is subdued in part due to blanketing by pyro- line with its much older age, the summit caldera of Mount Chike is clastic flows from these calderas and from silicic centers and domes of the dissected and breached along the northern side. A 60-m-deep gorge cut by rift floor and margin. The Wagebeta calderas and associated domes are the Katar River in the flat region between Chilalo and Chike (Fig. 3A) arranged linearly transverse to the rift margin along the cross-rift Bonga exposes a stratigraphic succession of welded tuff and unconsolidated ash lineament (Merla and others, 1979) (Fig. 2). A feldspar separate from a flows. Two lithic-free welded tuffs from the basal and middle parts of the welded tuff along the eastern wall of the Wagebeta caldera yielded a date section yielded Quaternary ages of 0.31 and 0.38 m.y. (nos. 51 and 52, of 4.2 Ma (no. 59, Table 1), whereas a rhyolite lava 10 km to the southeast Table 1), respectively. of the caldera is 3.6 m.y. old (no. 60, Table 1). More than 35 km south of Similar K/Ar dates from the shield volcanoes and the Munesa-Asela the Wagebeta caldera, there is a major trachytic center, Mount Damot, eastern escarpment suggest that the lavas flowed riftward from these cen- which perhaps occupies a niche on the western shoulder analogous to the ters. For example, the 1.97 and 1.76 Ma basalt flows from the base of the more abundant centers on the eastern shoulder. A minimum age of 2.94 rift scarp at Asela (Fig. 5B) are similar to the Chilalo (1.74 Ma) and the m.y. obtained on a somewhat altered trachyte lava from a quarry at the Galama Range flows (1.88 Ma). Moreover, the basalt (dated at 2.54 Ma) northern base of Mount Damot (no. 61, Table 1), suggests that Damot's at the top of the Munesa section and the mugearite lava (2.56 Ma) from activity correlates with that of the trachytic centers on the eastern rift Mount Hunkuolo erupted contemporaneously. shoulder. Our ages from the Pliocene centers are similar to, or younger than, The Omo River canyon west of Damot (Figs. 2 and 6) exposes more the previously published dates from Chilalo of 1.9-1.2 Ma, from Badda of than 1.2 km of volcanic rocks and sedimentary strata in two dissected 3.6-2.1 Ma, and from Kaka of 3.4 Ma (Mohr and Potter, 1976; Mohr and fault scarps. The volcanic rocks along the eastern canyon wall are dominat- others, 1980). Older dates of 5.3 and 3.5 Ma were reported from the ed by mid-Miocene (16.9-10.2 Ma) rhyolite, trachyte, basalt, and pumi- Chilalo area (Kunz and others, 1975; Kazmin and others, 1980). ceous pyroclastic flows that are unconformably overlain by Pliocene (>4.00-1.90 Ma) vitric tuff, basalt, and welded tuff in ascending order 6. Wabi Shebele River Canyon (WoldeGabriel and Aronson, 1987). The chemistry of the vitric tuff in the inner canyon (WoldeGabriel, 1987), beneath the 3.94-m.y.-old basalt, One of the major rivers of Ethiopia, the Wabi Shebele, flows south- matches that of the Moiti Tuff at East Turkana, Kenya (Cerling and east from the mountains of the eastern rift shoulder across the Gadeb Plain Brown, 1982), with a well-established age of 4.10 ± 0.07 m.y. (McDou- into a deep canyon 50-60 km east of the Munesa-Asela eastern rift margin gall, 1985). The thickness and age make it possible that one of the (Figs. 2 and 3A). The canyon wall investigated here is downstream from Wagebeta calderas erupted the Moiti Tuff. Proceeding both south- the section investigated by Williams and others (1979) and 10 km south- southwest and north of the Omo Canyon, Eocene to Oligocene ages east of the trachytic center of Mount Hunkuolo. Here, the canyon is 350 m (42.7-25.0 Ma) have been previously reported on flood basalt and rhyolite deep, and all of the samples were collected from the northern wall. flows (Grasty and others, 1963; Merla and others, 1979; Davidson and The canyon section (Fig. 5C) in ascending order consists of more Rex, 1980). The lack of exposure of these widespread Paleogene flood than 150 m of agglomerate and bedded volcaniclastic strata overlain by basalts at Omo Canyon despite an elevation lower than the floor of the 25-m-thick, mid-Miocene, basic lava flows (16.9 m.y.; no. 53, Table 1). MER is likely due to burial and is consistent with our hypothesis that Omo The lavas are capped by at least four mid-Miocene pyroclastic units (>50 Canyon is an ancestral, now failed, rift valley (WoldeGabriel and Aron- m) represented at the base by a scoriaceous welded tuff which is 16.6 m.y. son, 1987). The 3.94-m.y.-old basalt in the canyon correlates in age with old (no. 54). Two bedded pumice-fall deposits are covered by lithic-rich the Mursi Basalt, which has widespread correlatives north and south of the welded tuff (no. 55) which yielded a stratigraphically inconsistent age of Omo Delta along the Ethio-Kenyan border (Frank Brown, University of 14 m.y. The pyroclastic rocks are covered by a 50-m-thick trachyte (no. Utah, 1987, written commun.; Davidson and Rex, 1980; Davidson, 1983). 56) dated at 16.1 Ma that correlates with a trachyte dated at 16.5 Ma from an upstream section by Williams and others (1979). The top of the section 8. Bilate River Basin-Awasa Caldera Rift Floor consists of three basaltic units dated between 2.82 and 2.86 Ma (nos. 57 and 58). The K/Ar data suggest that an unconformity separates the basic The rift floor east of the Wagebeta-Damot western rift margin consists lavas at the top of the section from the 16.1 Ma trachyte lava. Fifteen of two basins. The western half is represented by the rift-controlled Bilate kilometers downstream, the canyon deepens and exposes agglomerate and River basin, and the eastern half is the closed basin of the nested Awasa- sedimentary strata capped by several basic flows. The major sequence of Corbetti caldera complex (Figs. 1, 2). The nested caldera forms a giant mid-Miocene lavas and pyroclastic rocks of the upstream section are ab- elliptical depression 30-40 km wide on the rift floor where the Bonga sent here. lineament intersects the MER axis (WoldeGabriel, 1986). The rift floor The age of the Wabi Shebele mid-Miocene lavas and pyroclastic along the Bilate River drainage system is blanketed by welded tuff, lacus- rocks is similar to, but somewhat older than, the phonolitic lavas we trine strata, and fissural basalt and maars (Di Paola, 1972; Zanettin and investigated from Mount Chike. The coarse nature of the pyroclastic rocks, others, 1978).
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DAMOT Figure 6. A geologic section 2.94 , across the eastern canyon wall 1.!9 2000 10.2 of the Omo River west of Sodo V) 10.6 (Fig. 2). The section consists of OC UJ 7 basalt, trachyte, and rhyolite. h- 1500 Omo River 16.9 Ma (rhyolite), 15 Ma (tra- LU 16.7/ s 4 chyte), 3.94 Ma (basalt), 2 Ma 3 z ' (welded tuff), 16.7 Ma (tra- 1000 I- 2 0.25 km chyte), 10.6 Ma (basalt), 10.2 Ma (mugearite), 1.9 Ma (welded | 2.5 km tuff), and 2.94 Ma (trachyte). V.E. 10X
Sanidine from a glassy tuff exposed by a roadcut along the eastern two adjacent sections (2 km apart) consists of basic lavas and welded tuff bank of the Bilate River (Fig. 2) is 0.21 Ma (no. 62, Table 1). Ages of 0.54 (Fig. 7A). Late Miocene (9.8-9.7 Ma) aphanitic mugearite and a welded m.y. and 1.66 m.y. were previously reported from a rhyolite lava from this vitric tuff (nos. 68 and 69, Table 1) are exposed in the basal part of the area and a trachytic rhyolite 20 km northeast of Sodo, respectively (Zanet- western section unconformably covered by a 20- to 30-m-thick, 3.69-m.y.- tin and others, 1978). Rocks in and around the Awasa-Corbetti calderas old welded tuff (no. 70). The eastern section exposes several welded-tuff are similar in lithology and age to those observed in the Bilate River and ash flows bracketed between 1.85 and 1.1 m.y. in age (nos. 71 and drainage basin except for a 2.5-m.y.-old rhyolite (no. 63) from a horst 5 72). About 15 km farther south along strike, the rift margin is dominated km south of the Awasa caldera margin. All flows from the northwestern by basaltic flows that are capped by the same 1.1-m.y.-old welded tuff. part of the Awasa caldera are young, including a benmorite atop the The basal basaltic flow is 1.6 m.y. old (no. 73). western wall (1.27 m.y., no. 64) and two rhyolites on the northeastern wall The eastern rift margin increases in height from 500 m at Awasa to (0.96 m.y., no. 65; 0.47 m.y., no. 66). Corbetti is even younger, as docu- more than 700 m, 60 km to the south in the Agere Selam area. The section mented by a 0.023-m.y.-old obsidian flow (no. 67) from the eastern flanks at Agere Selam consists of basalt, trachybasalt, rhyolite, and intercalated of the Chabbi dome in this caldera. Basalt samples from the western and lacustrine strata (Mohr, 1970; Merla and others, 1979). The basal porphy- northern Corbetti walls are too young to date by the K/Ar method, in ritic olivine-basalt yielded an Oligocene age of 28.9 m.y. (no. 74) (Fig. 7B) contrast to 0.52-m.y.-old basalt on the floor of the Awasa caldera (Mohr which is anomalously young, considering that the middle of the section and others, 1980). contains a rhyolite dated at 31.0 Ma (no. 75), capped by a 29.7-m.y.-old mugearite lava (no. 76). These Oligocene dates agree with a single trachy- 9. Awasa and Agere Selam Eastern Escarpment basalt age (27.8 m.y.) previously obtained from the eastern rift margin 25 km northeast of Agere Selam by Merla and others (1979). The Oligocene The eastern wall of the Awasa caldera asymmetrically overlaps the rocks around Agere Selam are unconformably overlain by a late Pliocene eastern rift margin (Fig. 2). There, a 500-m-thick composite column from (2.3 Ma) crystal-rich welded tuff (no. 77), indicating a long hiatus in this area.
CORRELATION OF VOLCANIC ROCKS LOCATION MAP AND STRATIGRAPHIC NOMENCLATURE
The K/Ar data and field relationships presented above are now used to correlate the diverse volcanic rocks of the nine major sections with each
2.3 29.7 1.1
1.85 Figure 7. Stratigraphic suc- cessions along the eastern rift margin of the central sector of 31.8 3.70 the MER in the southern part of <%/' ÌLUTQ/ \ WABI SHEBELE 9.69 RIVER < the study area: A, eastern topo- 9.80 100 graphic wall of the Awasa cal- 50 7A dera; and B, the Agere Selam 1 lir-%'1 Z° r 0 margin. Map shows the location 28.9 iSHASHEMEN Meters \/l of the two sections. J 0 20 km ,AGERE SELAM 7B
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other (Fig. 8) and to establish the history of volcanism in the central sector. In the stratigrahic nomenclature we adopt for the volcanic rocks of Volcanism was episodic, based on these ages and lithologie correlations, the central sector of the MER, each chronozone is selected after a locality and we propose to group the Cenozoic rocks from the central sector into that contains a complete section for that particular unit and after the six chronostratigraphic units, from Oligocene to Quaternary. The North dominant rock type in the age group. This central sector chronostratigra- American Stratigraphie Code (1983) is not clear on how to subdivide and phy is shown in column 3 of Figure 9. Figure 9 is an attempt to correlate develop a correlation framework for a volcanic province. Because volcanic the Tertiary volcanic stratigraphic units previously employed throughout and interbedded sedimentary rocks are likely to be diverse and locally the Ethiopian volcanic terrane by various workers on the basis of K/Ar distinct in a rift setting, lithostratigraphy is not an appropriate stratigraphie data. Below we briefly present each of the central sector chronostratigraph- framework for such a large region as the central sector. Because we are ic units within a regional stratigraphic framework, beginning with the able to recognize region-wide distinct periods of volcanism and inter- oldest unit. spersed hiatuses from the K/Ar dating of the above described sections, we here refer to the rocks erupted in each period defined by the K/Ar geochro- 1. Kella Basalt (26-32 Ma) nology as an individual chronozone (North American Stratigraphie Code, 1983). Furthermore, within the central sector, each of these chronozones The oldest volcanic rocks in the study area are at Agere Selam, corresponds at least in a limited way to a distinguishable volcanic petrol- Ambo, and Kella (Figs. 2 and 8). These Oligocene rocks are dominated by ogy, and so the chronozones are partly specified petrologically. Thus, for basalt with localized rhyolite and sedimentary strata. Except at Ambo, the example, the Butajira Ignimbrite chronozone is dominated by calc-alkaline ignimbrite (WoldeGabriel, 1987), interbedded with sedimentary rocks and basalt flows largely confined to the rift floor and margins, and erupted from 4.2 to 3 Ma. The chronozones at present apply only to the central sector of the MER, but as shown in Figure 9, the chronozones in part are chronologically correlatable with previously established chronostratigraph- ic volcanic units documented elsewhere in the Ethiopian volcanic province.
Figure 8. Stratigraphic successions of various sections along the rift margins and the adjacent rift shoulders as presented sequentially in the text: la, Kella western margin; lb, Guraghe western margin; 2, Ambo fault scarp; 3, Geothermal well LA3 of the rift floor; 4a, Munesa eastern escarpment; 4b, Asela eastern escarpment; 6, Wabi Shebele River gorge; 7, Omo River gorge; 9a, Awasa eastern escarpment; and 9b, Agere Selam eastern escarpment. Correlative chronostratigraphic units of the central sector of the MER are represented by numbers from the oldest to the youngest: 1, Kella Basalt; 2, Shebele Trachyte; 3, Guraghe Basalt; 4, Butajira Ignimbrite; 5, Chilalo Trachyte; and 6, Wonji Group. Crystalline basement (+); dotted and shaded areas represent Mesozoic rocks and Cenozoic sedimentary units, re- spectively. Map displays the locations of the various sections. LOCATION MAP
|300 • 4 « 200 T ..4 . ! 150 5 5 -3- io » 5 100 I * oi x - — — - 2
--5 4 _6_
;5; 1
5 « 100 T + •+ 4 I 50 + + •f •* + + * ol h-3 H + -3- -5- n 1a 1b 4a 4b 9a 9b
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/4/439/3380803/i0016-7606-102-4-439.pdf by guest on 28 September 2021 Figure 9. Correlation chart of Cenozoic volcanic rocks in Ethiopia compiled from (1) Davidson and Rex, 1980; Davidson, 1983; (2) Levitte and others, 1974; Zanettin and others, 1978; (3) this study; (4) Juch, 1975; Kunz and others, 1975; Meyer and others, 1975; Morbidelli and others, 1975; (5) Barberi and others, 1975; Chessex and others, 1975; (6) Zanettin and Justin-Visentin, 1974; and (7) Berhe and others, 1987. Numbers in circles indicate the locations (map) of the various volcanic sections of the Ethiopian volcanic terrain. Fm., Formation; Ignimb., ignimbrite.
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bases of the Kella Basalts are not exposed, and older volcanic units may confined to the Wonji fault belt along the entire MER (Kazmin, 1979). exist below them (Fig. 8). Time-correlative lavas are widespread in the Included in the chronozone are the young flood basalts from the Blue Nile (Abbay) and Mugher Rivers of the northwest Ethiopian Plateau Silti-Debre Zeit fault zone and the 0.68-m.y.-old flood basalt in the Ambo north of Ambo, the broadly rifted zone of southwest Ethiopia, the Bale area. Outside the study area, the Tepi Basalts of southwest Ethiopia (Da- Mountains southeast of the central sector, and the margins of the southern vidson and Rex, 1980; Davidson, 1983) and the Batu Trachyte of the Bale sector of the MER (Grasty and others, 1963; Megrue and others, 1972; Mountains (Berhe and others, 1987) erupted contemporaneously with the Jones and Rex, 1974; McDougall and others, 1975; Morbidelli and others, Wonji Group, as well as comparable units of the rift floor elsewhere along 1975; Jones, 1976; Zanettin and others, 1974, 1978, 1980; Merla and the MER and the Afar (Fig. 9). others, 1979; Davidson and Rex, 1980; Berhe and others, 1987). CROSS-RIFT STRUCTURAL RELATIONSHIPS 2. Shebele Trachyte (12-17 Ma) AND AGE OF RIFTING This group of mid-Miocene age is dominated by undersaturated in- termediate and acidic volcanic rocks and consists of basalt, trachyte, The central sector of the MER is a symmetrical rift, mostly character- phonolite, rhyolite, and intercalated volcaniclastic strata. They are exposed ized by well-defined synthetic rift margins having variable throws along in deep river canyons of the Omo (west) and the Wabi Shebele (east) that the strike of the boundary faults. These margins are marked by high-angle cut into the shoulders on either side of the rift. Mount Chike is the only one normal faults with large throws that comprise several step-faulted blocks. of the numerous rift shoulder volcanoes that is made up of this group. The faulted margins show right-angled spurs and re-entrants similar to Time-correlative basalt, trachyte, phonolite, and rhyolite have been re- those documented in the broad rift zone of southwest Ethiopia (Moore and ported from the rift margins of the northern and southern sectors of the Davidson, 1978; Davidson, 1983). At places, the rift escarpments adjacent MER, the broad rift zone of southwest Ethiopia, and the Afar region (Fig. to the Quaternary silicic centers located along the rift axis (for example, 9) (Zanettin and others, 1974, 1978; Kunz and others, 1975; Morbidelli Aluto, Shala, and Corbetti calderas) are generally subdued in part due to and others, 1975; Davidson and Rex, 1980). pyroclastic accumulation along the margins (Fig. 2). Throws along the normal faults have been estimated where possible from displaced units; 3. Guraghe Basalt (8.3-10.6 Ma) more often, only minimum estimates are available from the height of the Late Miocene basalt and subordinate silicic flows are present at fault scarps. As displayed in Figure 2, the central sector is segmented into Awasa, Guraghe, and the Omo River canyon (Figs. 2 and 8). The 10.6- three zones: from north to south, they are (1) the Guraghe-Wabi Shebele m.y.-old basalt in Omo Canyon is underlain by several undated basalt River zone, (2) the Wagebeta-Awasa zone, and (3) the Omo River-Agere flows and mid-Miocene (15.0-16.9 Ma) Shebele group rhyolite and tra- Selam zone. The rift structures and their temporal and spatial relationships chyte flows. The base of the correlative units at Awasa and Guraghe are with the various volcanic rocks in each of these three zones are described not exposed. Time-correlative late Miocene rocks are widespread along below, and characteristic cross sections (A-A', B-B', and C-C') across the both rift margins and shoulders of the rift system (Fig. 9). rift are presented in Figure 10; these highlight the structure and physio- graphic features of the rift flanks, margins, and floor. 4. Butajira Ignimbrite (3-4.2 Ma) Guraghe-Wabi Shebele River Rift Zone (A-A') The early-middle Pliocene epoch (4.2-3 Ma) was characterized by the eruption of voluminous silicic pyroclastic material (from the Awasa At the Guraghe section, more than 1.5-km-thick flood basalt is ex- caldera, the Wagebeta caldera complex, and a major caldera probably posed by several major step-faults (Figs. 3B, 4A, 4B, and 10A) that buried beneath the Ziway-Langano-Abiata basin) and by subordinate ba- strike north-northeast and, in turn, are cut by northwest- to north- salt flows (Figs. 2 and 8). Chemical and petrologic data (WoldeGabriel, northwest-striking transverse faults. The truncation of most of the rift- 1987) distinguish rhyolitic rocks of the Butajira Ignimbrite from those of oriented fault blocks topographically down from, and east of, the main the Quaternary Wonji Group as being calc-alkaline or mildly peralkaline Guraghe escarpment by these transverse faults has resulted in the blocks rather than peralkaline. Although the proximal, welded pyroclastic rocks having a pattern of serrated edges and re-entrants (Fig. 3). Overall, the are confined to the MER and its immediate shoulders, primary and re- main rift margin at Guraghe (Fig. 3) seems to have been displaced west- worked distal air-fall units may be in northern Kenya, the Afar, and the ward as a net effect of this transverse faulting. Cumulative throw along Gulf of Aden (Brown, 1982; Sarna-Wojcicki and others, 1985; Wolde- step faults is at least more than 1.5 km, considering that the adjacent Gabriel and others, unpub. data). Voluminous time-correlative units are marginal graben of the rift floor is filled by an unknown thickness of present in the northern sector (Fig. 9). pyroclastic and volcaniclastic rocks that is capped by Quaternary basalt. Because of the complex structural relationships, the evolution of the 5. Chilalo Trachyte (1.6-3.5 Ma) Guraghe-Kella scarp undoubtedly has been complex and has involved a history of tectonic inversion in which originally downdropped blocks have This middle to upper Pliocene stratigraphic unit is a modification of been subsequently elevated. The schematic drawing of its evolution shown the Chilalo-Badda Trachyte (Kasmin and Berhe, 1978) so as to include the in Figure 11 is consistent with the field observations and geochronology products of the Pliocene centers of the eastern and western shoulder of the presented above. The pre-Tertiary rocks at Guraghe are cut into four tilted rift and compositionally correlative units from the Awasa caldera. This individual blocks by west-northwest-trending normal cross-faults that are unit includes trachyte, silicic rocks, and basalt that overlie units of either downthrown to the northeast. Now structurally low (D and E, Fig. 11), the Shebele Trachyte or the Butajira Ignimbrite. Outside the central sector, the block of pre-Tertiary rocks was probably subjected to a long period of time-correlative units are confined totally to the rift floor (Fig. 9). uplift and erosion, because the more than 200-m-thick Cretaceous Upper Sandstone (Ambo-Blue Nile or Abbay River gorge) (Mohr, 1970; Merla 6. Wonji Group (<1.6 Ma) and others, 1979), which unconformably underlies the Kella Basalt at Ambo 100 km to the north, is absent in the Guraghe area. Moreover, This previously named group consists of diverse Quaternary lava, Oligocene and Miocene lavas of the Kella and Guraghe Basalts were not pyroclastic rocks, and volcaniclastic strata (<1.6 Ma) that are generally observed on top of these older rocks. Instead, the Kella Basalt and overly-
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ing organic-rich fluvial strata devoid of clasts from either the Oligocene or al eruption of these lavas 11-9 m.y. ago was confined to a graben or Miocene basalts appear to lap against both flanks of the pre-Tertiary downfaulted block west of the uplifted horst (C, Fig. 11). This thick (>800 structural blocks (A-C, Fig. 11). The absence of any remnants of the late m), horizontally layered, basalt sequence was uplifted to form the main rift Miocene basalt flows (Guraghe Basalt) on top of the older rocks (that is, margin before 8.3 Ma (D, Fig. 11), as indicated by the containment of the pre-Tertiary and mid-Oligocene rocks) suggests that the voluminous fissur- westward-tilted 8.3-m.y.-old, glassy welded tuff that thickens against the main Guraghe escarpment (Figs. 3B and 11). After the formation of the main Guraghe escarpment, the previously uplifted fault blocks containing the pre-Tertiary rocks and the Oligocene Kella Basalt dropped (D, Fig. 10). As a result, fault terraces along edges of the block of pre-Tertiary rocks were partially covered by early Pliocene basalt which crops out alongside the pre-Tertiary rocks north of Kella (Fig. 2). All of the marginal fault blocks in the Kella area were completely buried in the Pliocene epoch by voluminous welded tuff of the Butajira Ignimbrite. South of Kella, most of these pyroclastic rocks were topograph- ically confined to the base of, and in front of, the main Guraghe scarp covering the 8.3-m.y.-old, glassy welded tuff (Fig. 11). The total thickness of Pliocene welded tuff exposed along fault blocks in front of the main Guraghe rift margin far exceeds the <40 m deposited on the rift shoulder. During the late Pliocene epoch, north-northeast-striking faults exposed the pre-Tertiary rocks again (Fig. 11E). These rift-oriented faults are cut by Quaternary cross-faults (transfer faults?) that displace 0.1 Ma basalt flows. Our interpretation that block faulting controlled the distribution of the Guraghe basalt flows west of the pre-Tertiary block sets 11 Ma as a minimum date for the formation of the rift margin. If the flow distribution
Figure 10. Simplified geologic sections across the northern (A-A'), central (B-B'), and southern (C-C) parts of the central sector of the MER and the adjacent plateaus; vertical exaggerations, 12.5x. Elevations in meters above sea level. Map LOCATION MAP shows locations of cross sections.
A
WAGEBETA
| Rift Axis Basalts [fiy,| Late Miocene Basalts Epp] Quaternary Pyroclastic & Mid-Miocene Basalts, Trachytes, Phonolites & Rhyolites ~~ Volcanlclastlc .Sediments l"-:'-] Oligocene Basalts I*«t1 Pliocene Pyroclastlcs (K^ Pliocene Trachytes H Pre-Tertiary Rocks §111 Pliocene Basalts
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of the western marginal graben is more than 100 km long and 2-5 km wide and converges at its southern end with the medially located Wonji fault belt (Fig. 2). Faults along the eastward-tilted medial block of the rift floor are obscured by thick pyroclastic flows from silicic centers of the Wonji fault belt and young volcaniclastic strata. The Wonji fault belt is the single throughgoing medial volcanotectonic axis of the MER except in the northern part of the central sector, where it forms the 5- to 10-km-wide eastern marginal graben (Mohr, 1967; Mohr and Wood, 1976). In the central sector, the fault belt is right laterally offset into four en echelon rift-axis segments: Gadamsa-East Ziway, Ziway-Shala, Shala-Awasa, and the Duguna-Abaya zones (Fig. 2). Faulting and fissural basalt flows along this belt are more intense than along the Silti-Debre Zeit fault zone. A major cross-fault south of the Aluto center crosscuts north- northeast-striking normal faults in the rift floor and extends to, and offsets, the eastern rift margin, forming a right-angled spur along the margin south of Munesa (Figs. 1, 2). Our interpretation of the presence of the Munesa Crystal Tuff in the base of geothermal well LA3 inside the Aluto caldera indicates that more than 2 km of vertical displacement between the top parts of the crystal tuff at Munesa (2,400 m elevation) and in the drill hole (322 m elevation) has occurred since mid-Pliocene time (Fig. 10A). Un- like elsewhere along the MER, faulting on the floor of the northern part of the central sector is progressively younger toward the margins. Continuing eastward from the rift floor on the cross section of Figure 10A, the eastern rift shoulder is a broad uplifted plateau of Plio-Pleistocene volcanic rocks, consisting of several trachyte centers and the 70-km-long Galama volcanic range, which is related to a structural control by a major line of Precambrian crustal weakness (Mohr and Potter, 1976). The north- northeast-striking boundary fault of the eastern margin between Munesa and Asela is cut by short oblique faults (north-northwest striking), E resulting in serrated edges and open fissures (Figs. 1, 2). North- northeast-striking open fissures more than 50 m deep and 5-30 m wide offset 0.3-m.y.-old welded tuff north of Munesa. Like the western escarp- ment, the eastern escarpment had fully formed along the general direction of the present-day boundary by Pliocene time, resulting in a symmetrical rift as confirmed by the total containment of the more than 300-m-thick, Figure 11. Schematic volcanotectonic development of the west- 3.5-m.y.-old, Munesa Crystal Tuff to the rift floor and margin blocks in the ern rift margin at Guraghe, central sector of the MER. A, block northern half of the central sector. Its total absence in the Wabi Shebele faulting and tilting of pre-Tertiary rocks and mid-OIigocene basaltic River canyon, 50 km east of the rift escarpment, is attributed to the eruption; B, sedimentation and faulting; C, eruption of thick, late Mio- presence of a high rift wall already existent prior to its eruption. This cene, flood basalts; D, late Miocene faulting, eruption of 8.3 Ma ig- suggestion is further supported by the banking of Butajira Ignimbrite units nimbrite (no. 6), and Pliocene flood-basalt and ignimbrite (Munesa (3.51-2.54 m.y. old) against the mid-Miocene Chike phonolite center, and crystal tuff, no. 8) eruptions; and E, Plio-Pleistocene faulting. 1, crys- the containment of Chilalo Trachyte units (1.66-1.97 m.y. old) against talline basement; 2, Mesozoic sedimentary rocks; 3, Oligocene basalts; older rocks (2.31 m.y. old) of the same chronozone at the northern base of 4, fluvial sediments; 5, late Miocene basalt; 6, late Miocene welded Mount Chilalo. As the rift wall at some places is fully composed of the tuff; 7, Pliocene basalt; 8, Pliocene welded tuff; and 9, rift-floor Qua- Munesa Crystal Tuff, the pre-existing eastern rift margin in the northern ternary rocks. half of the central sector was buried by this major eruption. Intense down- faulting along the buried fault after the eruption of the Munesa tuff must have subsequently re-established the escarpment in the late Pliocene of the Oligocene basalt on the flanks of the pre-Tertiary rocks is an original slightly inward from the pre-existing escarpment, thus exposing the full feature and not due to erosion, then uplift and faulting possibly occurred as thickness of the Munesa Crystal Tuff. far back as Oligocene time. As mentioned, the anomalously thin Mesozoic sequence of Guraghe possibly reflects a doming stage in the Mesozoic that Wagebeta-Awasa Calderas Rift Zone (B-B') preceded Tertiary volcanism and rifting. In the northern sector of the MER (Addis Ababa area), evidence for The subdued and eroded rift margin of poorly welded tuff between major rift faulting in the late Miocene epoch has been documented com- the Wagebeta caldera complex and the Guraghe escarpment (Fig. 10B) parable to that in the central sector. Upper Miocene basalt and Pliocene shows a right-angle spur where intersected by the transversely oriented rhyolite of the rift floor banked against lower Miocene silicic and mafic Bonga lineament (marked by its three aligned Wagebeta calderas). East of flows of the western rift margin (Justin-Visentin and others, 1974; Mohr, these calderas, the boundary faults cut, and are therefore younger than, the 1975; Morton and others, 1979). 4.2 to 3.6-m.y.-old welded tuff and rhyolite domes. The exclusion of The rift floor between Guraghe and Munesa is characterized by two significant amounts of younger volcanic rocks in the scarp suggests that the north-northeast-trending marginal grabens that are marked by swarms of present-day margin is late Pliocene in age. closely spaced normal faults and associated volcanic flows of the Quater- The cross section of Figure 10B shows two separate basins of the rift nary Wonji Group (Figs. 2, 3A, and 10). The Silti-Debre Zeit fault zone floor. Because the Silti-Debre Zeit fault zone terminates just south of
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Guraghe, few sparsely distributed faults are recognizable along the western Davidson, 1983) and the Gregory Rift of Kenya, characterized by a series half of the rift floor. The eastern half is occupied by the Shala-Corbetti rift of half-grabens, defined by a major border fault on one side and a series of axis segment of the throughgoing Wonji fault belt, which from here south smaller normal faults and monoclines on the opposing side, alternating in has a more medial location on the rift floor. This axis is marked by polarity along the rift (Bosworth, 1985; Rosendahl, 1987; Ebinger, 1989a, Pleistocene cinder cones and flood basalts, horsts, and graben associated 1989b). Considering the absence of early Tertiary flood basalts in the with closely spaced faults and fissures. Gregory and Western Rifts outside the Turkana region of northern Kenya, The boundary fault of the eastern rift margin is well defined where it is possible that these rifts are considerably younger than the MER. The the topographic rim of the asymmetric Awasa caldera overlaps it; how- early existence of such high-angle (>60°) border faults along the MER ever, the fault scarp decreases northward and is barely recognizable in the (Fig. 12) may explain the peculiar distribution pattern of mid- and late Shashemene area (Fig. 2). The rift margin south of the topographic rim of Miocene flows by having confined the volcanic piles to half-grabens and to the Awasa caldera in the Agere Selam area is a modestly steep, broad, the area separating the border faults of opposite polarity (accommodation dissected surface dropping more than 700 m to the rift floor. Boundary zones; Bosworth, 1985). For example, in the eastern margin of the north- faults are not sharply defined because of weathering and talus. A late ern sector of the MER (A, Fig. 12), Miocene and Pliocene volcanic rocks Miocene basalt (9.7 m.y.) of the Guraghe chronozone along the base of the and volcaniclastic strata are confined by Oligocene basalt of the Arba eastern topographic rim of the Awasa caldera is probably banked against an old (>9.7 m.y.) border fault scarp. No rocks of this age have been found in the escarpment, which consists entirely of Oligocene flows.
Omo River-Agere Selam Rift Zone (C-C)
This cross section (Fig. 10C) shows that south of the Wagebeta calderas, a narrow uplifted block forms the western shoulder between the ancestral Omo Rift and the MER. This block, with the 2.9-m.y.-old tra- chyte center (Mount Damot) and the 3.6- to 4.2-m.y.-old Wagebeta silicic domes and calderas 50 km to the north, represents a tectonic setting similar to the uplifted Munesa-Wabi Shebele block of the eastern rift margin (Fig. 10A), which also consists of Pliocene trachytic centers. Faults in the gently sloping margin east of Mount Damot are obscured by pyroclastic rocks from the adjacent Quaternary Duguna silicic center of the rift axis (Fig. 2). The medially located active Duguna-Lake Abaya singular rift axis of the rift floor, right laterally displaced from the Shala-Corbetti segment (Fig. 2), is marked by maars, basaltic cones, and flood basalts aligned along north-northeast-striking faults and fissures both north and south of the Duguna rhyolite center (Figs. 1, 2). The eastern half of the rift floor south of the Awasa caldera is broadly uplifted and dominated by pyroclas- tic rocks and volcaniclastic strata. The eastern escarpment in the Agere Selam area is part of the old dissected high-relief margin described in the Awasa area to the north (relief >700 m high). The complete absence of late Miocene basalt of the Gu- raghe chronozone in the Agere Selam escarpment but its presence against the escarpment at Awasa indicate that the Agere Selam margin had formed by 9.7 Ma, coincident with, or earlier than, the present Guraghe escarpment, which formed after the eruption of the Guraghe basalt but prior to 8.3 Ma. Cross section C-C' shows a minimum structural relief of 1.9 km from the highest Oligocene basalt on Agere Selam to the mid- Miocene rocks of the Omo Canyon floor. Figure 12. Fault traces in the MER and adjacent areas (modified after Mohr, 1980). Major border faults indicated by letters (A, B, C, TRANSITION FROM ASYMMETRICAL TO and D) are where steep escarpments expose thick sequences of Oligo- SYMMETRICAL RIFTING IN THE MER cene basalt against which Miocene and Pliocene volcanic rocks are banked. Proceeding south, these alternating opposed scarps are A, the Based on the contrasting nature of Oligocene lava sections and the eastern margin in the Arba Gugu Mountain region of the northern distribution patterns of subsequent volcanic flows, four main high-angle sector; B, the Guraghe western rift margin; C, the Agere Selam eastern border faults (>60°) bounding alternating and opposing half-grabens are margin; and D, the Chencha western escarpment. The condition of here inferred to have existed in an early tectonic phase (pre-Pliocene) of alternating opposed half-grabens was an early stage in the formation the central sector and the adjacent northern and southern sectors of the of the MER supplanted by the present mostly symmetrical rifting MER. This early phase of the MER is similar to the present configuration stage. Inferred accommodation zones are marked by cross-rift broken of the broad rift zones of southwest Ethiopia (Moore and Davidson, 1978; lines. Shaded areas are lakes of the rift floor.
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Gugu Mountain region that forms part of the eastern rift escarpment of the asthenospheric upwelling that induces melting within the underlying sub- northern sector of the MER (Juch, 1975; Kunz and others, 1975; Morbi- continental mantle. Chemical and isotopic data not presented here suggest delli and others, 1975; Kazmin and others, 1980; Berhe, 1986) similar to that the mantle below the MER was heterogeneous and that melting that observed at Guraghe. A change in asymmetry from the eastern margin involved enriched and depleted mantle reservoirs and ocean-island basalt half-graben of the northern sector (Arba Gugu Mountain) (A, Fig. 12) to (OIB)-type mantle sources (WoldeGabriel, 1987; Hart and others, 1989). the opposing faced Guraghe escarpment half-graben (B, Fig. 12) is Geographic distribution and compositional diversity among volcanic units achieved by transfer faults that strike obliquely to the high-angle border of the Ethiopian volcanic province indicate that there has been a relation- faults. Any accommodation zone linking the alternating opposed high- ship between magma composition and rifting processes (Barberi and oth- angle border faults of Arba Gugu with those of Guraghe is largely buried ers, 1980; Betton and Civetta, 1984; Hart and others, 1989). Volcanic and overprinted by the rift-oriented Quaternary volcanotectonic zones of rocks from the MER, Afar, and the plateaus on either side of the rifts the rift floor, but in crossing the rift, the Awash River follows this accom- generally straddle the tholeiitic-alkalic boundary (transitional) in an modation zone (Fig. 12). The early opposed scarps are still very high, but alkalies-silica diagram (Zanettin and others, 1978; Piccirillo and others, in contrast to the Gregory rift of Kenya, increased volcanotectonic activity 1979; Brotzu and others, 1980a, 1980b; Barberi and others, 1982; Wolde- in the MER has ultimately resulted in the formation of a symmetrical rift. Gabriel, 1987; Hart and others, 1989). The compositional (major- and The Awasa-Agere Selam eastern rift margin forms the opposite half- trace-element) similarity between Tertiary transitional basalt from the pla- graben border fault across from and south of Guraghe (C, Fig. 12). The teaus (thick crust) and the rifts (attenuated crust) has been attributed to temporal and spatial relationships of faulting and volcanism at Agere intensity of tensional tectonic movements during volcanism where basalts Selam are very similar to the Guraghe border fault but in the opposite of more tholeiitic affinity were associated with intense volcanotectonic direction. The existence of an accommodation zone between the border activities (Zanettin and Justin-Visentin, 1975; Zanettin and others, 1974, faults at Guraghe and Agere Selam is indicated by fault-controlled and 1978; Piccirillo and others, 1979). During the initial stages of volcanism aligned Pleistocene volcanic centers visible in the Landsat imagery (Fig. 1), and rifting, lithospheric melting predominated, and crustal contamination somewhat obscured by an overprint by the Wonji fault belt volcanotec- was significant and led to transitional basalt with more alkalic character. tonic axis. The arcuate border fault at Guraghe is truncated by transverse With time, depleted asthenospheric mantle became an important compo- faults at its southern end, reducing the 800-m-high escarpment to a broad nent, initially as an end member in mantle mixing, and eventually as the subdued margin southward. Similarly, the northern end of the Agere dominant mantle reservoir in more attenuated areas (for example, axial Selam border fault is truncated by the Bonga lineament. Farther south, the zone of MER, Afar) and led to a more tholeiitic transitional basalt (Hart Chencha western escarpment (1,000-2,000 m) of the southern sector of and others, 1989). the MER represents the alternating opposed major border fault 60 km From chemical data (WoldeGabriel, 1987; Hart and others, 1989), across from, and 50 km south of, Agere Selam (Fig. 2 and D, Fig. 12), and the six major volcanic cycles of the central sector are dominated by basalts it has a matching stratigraphy of upper Oligocene flood basalt and rhyolite. of transitional composition. The six cycles were related to the above two The Chencha scarp disappears northward truncated by transverse faults distinct tectonic stages, each with its own rifting process. The earliest and buried by young volcanic flows. transitional basalts of Oligocene age (Kella Basalt), probably associated Our dating and observed structural relationships of dated units with with an early stage of rifting, have an alkaline affinity presumed to have faults allow us to deduce that the transition from opposing and alternating been generated by lithospheric mantle melting with significant crustal high-angle border faults of the early phase of rifting in the MER to the contamination. By late Miocene time (11 Ma) when definite block faulting more symmetrical rift valley of today probably took place in late Miocene ensued, elemental and isotopic signatures of Guraghe Basalt show the or early Pliocene time. Between Guraghe and Munesa, all of the basalt and addition of a depleted asthenosphere mantle source to a lithosphere mantle most of the welded tuff of the Pliocene Butajira Ignimbrite chronozone source (Hart and others, 1989). was contained within both margins of the border faults. Our data indicate In the adjacent northern sector of the MER, the most definitive that the Munesa Crystal Tuff actually thickens, toward the eastern margin, evidence as to when rifting began is provided by peralkaline silicic rocks of which by early to middle Pliocene was no longer subdued. If this interpre- the Nazret Group (<9 m.y. old) and volcaniclastic deposits (Miocene tation of a transition from half-graben to full-graben basinal geometries is Chorora Formation) that are present on the rift floor banked against the valid, then it distinguishes the MER from the Gregory and Western Rifts eastern border fault (Kazmin and others, 1980). This demonstrates that the of the East African Rift System, which, by comparison, appears to be in rift margin had formed prior to their eruption and deposition, respectively, the earlier stage of evolution (WoldeGabriel and Aronson, 1987). The approximately synchronous with the formation of the present western rift two-stage history of the MER leading to the present symmetrical rift stage margin of the central sector at Guraghe and the eastern margin at Agere is consistent with the information from regional stratigraphy, geochronol- Selam. Continuous attenuation of the lithospheric crust in the northern ogy, and tectonic pattern, which clearly indicates that volcanotectonic sector is demonstrated by the change of transitional basalts to a more processes within the MER increased from the Miocene onward. This is tholeiitic character with time (Barberi and Varet, 1977; Brotzu and others, further supported by geochemical and isotopic data (WoldeGabriel, 1987; 1980b). The similarity in magmatic composition and timing of rifting Hart and others, 1989). demonstrate that the central and northern sectors of the MER evolved synchronously. Flow distributions of the Kella and Guraghe chronozone REGIONAL TECTONOMAGMATIC CONSIDERATIONS basalts at Guraghe with respect to the pre-Tertiary rocks suggest that the development of the present rift margin was preceded by even earlier As shown in the correlation diagram of Figure 9, the Ethiopian faulting there. volcanic terrane was constructed by several episodes of eruptions in the In the Afar, the timing of continental rifting is debated (Barberi and Cenozoic representing diverse petrologic compositions. Volcanism and others, 1975, 1982; Kazmin and others, 1980; Berhe, 1986); however, it rifting in major continental rifts generally are either attributed to, or cause, appears that rifting commenced in late Oligocene-early Miocene time
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(Barberi and others, 1975, 1982) consistent with the evidence of earliest middle and late Miocene flood phonolite (20-7 Ma). According to Karson rifting or faulting in the southern sector (Zanettin and others, 1978) and at and Curtis (1989), the magmatic system of the Gregory rift was induced Guraghe. Chemical and isotopic data from the Afar rocks also suggest a by broad asthenospheric upwelling. Consequent uplift and stretching of the close relationship between magmatism and rifting processes during litho- Kenyan dome initiated local lithospheric thinning and rifting on the crest spheric attenuation and continental breakup. Hybrid mantle sources were of the swell. The most important tectonic features in northern Kenya tapped before, and depleted mantle sources after, the formation of the Afar developed 7 Ma and 2-0.5 Ma (King, 1978; Williams and Truckle, proto-oceanic rift (Barberi and others, 1980, 1982; Betton and Civetta, 1980). During the first phase (7 Ma), the Gregory Rift was characterized 1984; Hart and others, 1989). Young rift basalt of the Wonji Group from by major faulting and contemporaneous eruptions of flood trachyte, basalt, the rift axis of the central sector of the MER is transitional in composition and particularly phonolite along much of its length in Kenya, whereas the and retains a hybrid geochemical signature of a mixture of enriched and younger phase (2-0.5 Ma) was largely responsible for the present rift depleted mantle reservoirs similar to the lavas erupted prior to continental morphology by rejuvenation of older structures. The major flood phono- breakup in the Afar (Hart and others, 1989). lite, so characteristic of the Kenya volcanic terrane, is virtually absent from Likewise proceeding south of the central sector of the MER, one can the MER, except for a single mid-Miocene center (Chike) along the east- see a correlation in magmatic evolution with time of rifting in (1) the ern rift shoulder of the central sector and sequences of phonolite and alkali southern sector of the MER, (2) the broad rift zone of southwest Ethiopia, trachyte recognized at two locations in the broad rift zone of southwest and (3) the adjacent Turkana region of northern Kenya (Moore and Ethiopia (Davidson, 1983; this study). Davidson, 1978; Zanettin and others, 1978; Bellieni and others, 1981; The Gregory rift, with its series of half-grabens which alternate along Davidson, 1983). Zanettin and others (1978) argued that the eruption of the rift with opposite polarity (Bosworth, 1985; Rosendahl, 1987; Ebinger, the Oligocene basalt flows predated rifting and that a distinct rift margin 1989a, 1989b), contrasts with the presently evolved structure of the MER. had formed by 12-13 Ma, when rhyolite flows of that age were confined In the MER, the present rift stage has progressed to a symmetrical graben, to what was then the rift floor. A single K/Ar date of 21 Ma for a dike of a stage yet to occur in the Kenyan Rift. As discussed above, however, the the Amaro Basalt, known only in the region of the Amaro Horst of the rift present pattern of a series of alternating half-grabens along the Gregory floor (Levitte and others, 1974) allows for a possibility of older age of rift Rift is similar to that deduced here for the ancestral stage (late development (Zanettin and others, 1978). According to Zanettin and oth- Oligocene-early Miocene) of the MER. By contrast the northernmost ers (1978), the five major volcanic cycles of the southern sector of the projection of the Gregory Rift into the broadly rifted zone of southwest MER (Fig. 9) are more alkaline in composition than elsewhere, and this Ethiopia at and north of Chew Bahir is symmetrical (A. Davidson, Geol. was attributed to less intense extension during volcanism compared to the Survey of Canada, 1989, written commun.). other rift sectors. The formation of a definite rift margin there by 12-13 The central sector of the MER fills an important gap in appraising the Ma is consistent with, or perhaps a little earlier than, the evidence of over-all volcanotectonic evolution of the East African Rift System. From a definite rift wall formation in the central and northern sectors. Within the general point of view, it appears that in each region along the rift system, limitations of the preserved geologic relationships and the geochronology volcanotectonic activity was episodic. The earliest flood basalt of Eocene data, well-defined border faults had developed along all of the sectors of age in southwest Ethiopia (Davidson and Rex, 1980) is the clearest indica- the MER by late Miocene time. tor of when definite volcanotectonism had commenced. Major flood- Uplift preceded volcanism in the broad rift zone of southwest Ethio- basalt volcanism was widespread across Ethiopia and probably was associ- pia (Moore and Davidson, 1978; Davidson and Rex, 1980). The basaltic ated with an early stage of normal faulting. By middle to late Miocene volcanism characterized by alkaline composition in the earliest flows (Eo- time, the MER was characterized by asymmetric half-grabens. Today cene and Oligocene) evolved to transitional with tholeiitic affinity in the proceeding south, one can see a progression of rift and concomitant tec- younger succession (Davidson, 1983), unlike basalt flows of the southern tonomagmatic evolution from the proto-oceanic structure of the Afar to sector and more like that of the central sector of the MER. Rifting in the mostly symmetrical rift of the MER, to the series of alternating half- southwest Ethiopia developed between late Miocene and middle Pliocene grabens of the Gregory Rift. time (Moore and Davidson, 1978), probably later than when definite faulted rift margins had formed in the three sectors of the MER in late CONCLUSIONS Miocene time. Volcanic stratigraphy and rift structures of the MER, Afar, and the New geological and geochronological information from the central Ethiopian plateaus provide a striking tectonomagmatic contrast to that of sector of the MER has been used to establish stratigraphic units and the the Gregory rift of Kenya. It is only in the Turkana region of northern temporal and spatial relationships of volcanism to rifting as summarized Kenya between the Ethiopian and Kenyan domes that the oldest volcanic below. rocks known in Kenya are present, represented by Oligocene transitional 1. Volcanism in the central sector of the MER and the adjacent (tholeiitic) basalt accompanied by alkaline basalt flows (Bellieni and oth- plateaus started as early as mid-Oligocene time or earlier, considering that ers, 1981). Five major cycles of volcanic eruptions (30 Ma to present) the bases of some of these basalts are not exposed. Six volcanic episodes have been documented in the Kenya volcanic terrane characterized by a spanning a time period from the Oligocene epoch to the present are general trend of increasing undersaturation away from the rift axis and recognized in the central sector of the MER, each separated by a hiatus. decreasing undersaturation with time leading to the eruption of young This led to a stratigraphic framework of six chronozones with stratigraphic transitional basalt along the axis of the rift (Lippard and Truckle, 1978; correlatives elsewhere in the Ethiopian volcanic province. We found no Williams, 1978; Williams and Truckle, 1980). Unlike the Ethiopian vol- evidence that volcanism has migrated from the plateaus toward the central canic terrane, which is dominated by transitional flood basalt beginning as sector of the MER. far back as Oligocene time, the Kenya volcanic field is dominated by 2. The relatively thin section of the Mesozoic strata exposed at Gu-
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raghe above the basement gneiss compared to elsewhere in Ethiopia is ger of the Geodynamics branch of the NASA Goddard Flight Center, evidence that the region now occupied by the MER was already being Greenbelt, Maryland is greatly appreciated. We appreciate reviews and arched in the late Mesozoic. Thickness variations and distributions of constructive comments by Jeffrey Karson, John Moore, and Anthony Oligocene flows suggest that initial rifting began in late Oligocene-early Davidson. We are especially indebted for the many thorough, scholarly Miocene time. written discussions on various versions of the manuscript by Dr. A. 3. Evidence for pre-middle Oligocene uplift and faulting in the cen- Davidson. tral sector is furnished by the structural relationship of the distribution of mid-Oligocene, Miocene, and Pliocene volcanic rocks with the pre- Tertiary rocks at Guraghe. These relationships imply that pre-middle Oligocene block faulting occurred in this region. 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Principal financial support of this research was provided by National Davidson, A., and Rex, D., 1980, Age of volcanism and rifting in southwestern Ethiopia: Nature, v. 283, p. 657 658. Di Paola, G. M, 1972, Ethiopian Rift Valley (between 7° and 8° 40' lat. North): Bulletin Volcanologique, v. 36, Science Foundation (NSF) Grant No. EAR83-06386. We appreciate the p. 517-560. encouragement and help of John Lance of NSF who flexibly allowed us to Ebinger, C. J., 1989a, Tectonic development of the western branch of the East African rift system: Geological Society of America Bulletin, v. 101, p. 885-903. shift more of our research emphasis to the MER. A fellowship to Wolde- 1989b, Geometric and kinematic development of border faults and accommodation zones, Kivu-Rusizi Rift, Africa: Tectonics, v. 8, p. 117-133. Gabriel from The L.S.B. Leakey Foundation is gratefully acknowledged. EIGS (Ethiopian Institute of Geological Surveys)-GLE, 1985, Geotherma! 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N.), in Pilger, A., and Rosier, A., eds., REVISED MANUSCRIPT RECEIVED AUGUST 24,1989 Afar Depression of Ethiopia: Stuttgart, West Germany, Schweizerbart, p. 362-369. MANUSCRIPT ACCEPTED SEPTEMBER 18,1989
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