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. 439 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 --7 / /pip 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