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Home , Geology of Ethiopia, Horst (geology)

DUBI LEVITTE Geological Survey of Israel, Jerusalem, Israel JOHN COLUMBA Geological Survey of Ethiopia, Addis Ababa, Ethiopia PAUL MÖHR Smithsonian Astrophysical Observatory, Cambridge, Massachusetts 02138

Reconnaissance of the Amaro Horst, Southern Ethiopian

ABSTRACT roughly equal to the 75-km width of the rift farther north (Baker and others, 1972, Fig. 14). The Amaro horst is a 90-km by 25-km b iock of sialic crust uplifted 1.5 km above the axis of the main Ethiopian rift, near its southern termination. The Amaro horst resembles the famous Ruwenzori horst of the Western rift, but additionally the former has had a Precambrian Rocks Neogene- volcanic history ths.t reveals episodic uplift of Precambrian rocks are exposed along a narrow, upfaulted crustal the horst from the axis of a bilaterally downwarping-downtilting rift sliver forming the summit ridge of the northern part of the Amaro floor. Vertical forces that produced normal faults uplifted the horst horst (Fig. 2). In the central and southern part, Precambrian rocks to stratigraphic elevations higher than the outside the rift. are exposed across the width of the horst, although the summit ridge and other high ground are capped by Tertiary volcanic rocks. INTRODUCTION Shallow-dipping mica schists form most of the Precambrian rocks (De Angelis d'Ossat and Millosevich, 1899; Kazmin, 1971). In the The main Ethiopian divides south of Abaya (or western part of the Amaro horst, muscovite schist overlies biotite Margherita), near lat 6.5° N. (Fig. 1). The north-trending Amaro schist, with having southwest to northwest dips between (Amaro horst) separate the Ganjuli to the west 10° and 30°. This thick, monotonous sequence is cut by abundant from the Galana graben to the east. The lorst and both die intrusions of quartz-feldspar pegmatite. In the northern part of the out near lat 5° N. on the southern fringe of the Ethiopian swell; the horst, biotite schist with aplitic gneiss and quartzite bands is injected rift is transposed en echelon west into the southward-continuing by coarse feldspathic pegmatite and quartz veins. Less common Stefanie graben (Baker and others, 1972). rock types within the schist sequence include serpentinite (possibly The only previous work on the geology of the Amaro region is that from altered sills) and chlorite schist. A prominent inselberg in the of Maurizio Sacchi, a member of the ill-fa ted second Bottego expedi- west-central part is formed of gneissose granodiorite, again injected tion. Sacchi's traverse observations, published by De Angelis with pegmatite (Fig. 2). d'Ossat and Millosevich (1899), showed Precambrian rocks at high The regional foliation of the Precambrian rocks was thus imposed elevations on the Amaro Mountains, higher indeed than on the after a localized intrusive episode. No unmetamorphosed granite of neighboring plateaus east and west of the rift valley. the type occurring southeast of Yavello (Rogers and others, 1965; Dainelli (1943) considered the Amaro Mountains a see Fig. 1) has yet been discovered in Amaro. remnant, left standing within a collapsed rift valley, and drew his The Precambrian sequence of the Amaro horst can be correlated provocative map of basement isohypsals accordingly. Reconnais- with the "Lower Group" of Chater and Gilboy (1970), who mapped sance surveys in the late 1950s (Mohr, 1960) showed, however, that in detail the Precambrian rocks of the Shakisso-Arero area —100 km the Amaro Mountains are formed by a horst uplifted from the rift east of the Amaro horst on the Bali plateau. At the southern end of floor. the Ganjuli graben floor is a similar sequence of Precambrian rocks (Mohr and Gouin, 1968) that consist of regionally extensive biotite PHYSIOGRAPHY schist and subordinate biotite gneiss and marble with foliation The Amaro horst trends N. 5° E. for a length of 90 km. The typical dipping south to southeast between 0° and 20° and with profuse width is —25 km but increases to the south. A continuous, intrusions of feldspathic pegmatite. Rogers and others (1965) have knife-edged summit ridge runs close to the eastern, steeper margin of dated two tectonothermal events affecting the Precambrian rocks of the horst at elevations of —2,600 m. The northernmost segment of southern Ethiopia: an event at least 600 m.y. ago presumably the horst is stepped down from the central sector and has the form of associated with the regional foliation and an event 500 m.y. ago a narrow, linear, bladed ridge, similar to the northern "nose" of the associated with the intrusion of posttectonic granite. The original Ruwenzori horst (Holmes, 1965). age of the sediment now occurring as schist remains unknown (see West of the Amaro horst, the Ganjuli graben contains the Kazmin, 1972). southern end of Lake Abaya (1,175-m elev), which overflows a narrow land bridge, the Tosa Sucha ( 'Bridge of God"), into Lake Mesozoic(P) Rocks Chamo (1,130-m elev). The Ganjuli graben is 25 to 30 km wide and Thin beds of gritstone and conglomerate occur in patches between continues the south-southwest trend of the main Ethiopian rift. West the Precambrian schist and the Amaro flood basalt (see below). The of the graben is an abrupt rise to the extensive plateau whose summit outcrops are too narrow to be indicated on Figure 2; however, they is the Gughe Mountains (—3,500-m elev). are well exposed on the upfaulted Golole ridge, bordering the East of the Amaro horst, the Galana graLen trends nearly due eastern shores of Lake Chamo south of the Sagan effluence, and also north. Its eastern side rises regularly to the edge of the Bali south of Kelyi. Variegated, subhorizontal fine gritstone, 2 to 5 m (Somalian) plateau, a major watershed whose elevation ranges thick, rests unconformably on muscovite schist in the Golole region between 2,000 and 2,500 m. and was more or less transformed into pisolitic lateritite before The divergence from the main Ethiopian rift is slight. The total extrusion of the Amaro flood basalt. Farther east, in the upper Sagan width of the two grabens plus that of the intervening horst remains valley, equivalent gritstone and angular conglomerate are <1 m

Geological Society of America Bulletin, v. 85, p. 417-422, 2 figs., March 1974

417

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thick; still farther east, on the summit ridge of the horst, the An:aro flood basalt rests on nonlateritized schist without any intervening sedimentary rocks. The age of the gritstone and conglomerate is uncertain, but the lateritization suggests a late Mesozoic age by analogy with the well-dated lateritites of central Ethiopia (Dainelli, 1943; Mchr, 1962). Mohr and Gouin (1968) concluded that the 20-m-thick sequence of the Gatto Grits, at the southern end of the Gan uli graben, was probably of Mesozoic rather than Tertiary age. The age of the lithologically similar Turkana gritstone of the Lake Rudolf region has recently been revised from to Cretaceous by Arambourg and Wolff (1969).

Upper Tertiary to Quaternary Rocks Amaro Flood Basalt. A thick sequence of flood basalt is developed over much of the Amaro horst. On the eastern and western fringes of the south-central part, the Amaro flood basalt does not exceed 100 m in thickness (10 to 15 flows), although in places this may represent a minimum owing to denudation. In the Baraka valley, thicker flood-basalt flows total 200 to 250 m; a similar thickness, veneered with silicic tuff, caps the summit ridge of the Amaro horst southwest of Kelyi (Amaro basalt on Fig. 2). South of Kelyi, ~200 m of flood basalt occurs on the tilted blocks of the faulted eastern escarpment of the horst; this sequence is absent from the summit ridge immediately to the west. The Amaro flood basalt thickens northward and exceeds 1,000 :n on the northern part of the horst, where the full sequence is last exposed. This northward thickening matches the general pattern on the plateaus west and east of the rift (Mohr and Gouin, 1968). Tbe Amaro flood basalt is commonly olivine-phyric, particularly the lower flows. Light-colored, amygdaloidal (agate and calcite) basalt, interspersed with feldspar-phyric flows, characterizes the upper part of the sequence. Numerous feeder dikes for the Amaro flood basalt are exposed on the western flank of the horst. In the Sagan valley (~10 km south of the southern limit of Fig. 2), a swarm of dikes cuts the Precambrian rocks of the graben axis. The swarm is 2 to 3 km wide, and the dikes trend between north and north-northeast and dip steeply west. Figure 1. Location map of the Amaro horst near the southern termination of the Typical lengths are 600 to 1,200 m, and right en echelon main Ethiopian rift (elevations higher than 2,000 m indicated by stippling). transpositions occur. On the southeastern part of the horst, north-trending dikes dip steeply east and are slightly but definitely part. The tuff shows a tendency to be draped on the existing oblique to the N. 10° E. strike of the dipping lava flows. In the topography of the horst, indicating an unconformity with the Baraka valley, thin (<1 m) vertical dikes trend north to underlying basalt. north-northeast. One of these dikes, sampled from the upper part ol The age of the tuff is uncertain but must be later than early the flood-basalt sequence, has yielded a radiometric age of 21 m.y. Miocene. A very extensive province of Miocene to Pliocene welded (Table 1); the dike rock is hawaiite (LeBas and Mohr, unpub. data; tuff covers much of southern and central Ethiopia (Mohr, 1968). On and is reversely magnetized. the Bali plateau, near Alghe, white welded crystal tuff is 50 to This early Miocene age, obtained for a specimen from the upper 150 m thick; a specimen from the base of the sequence has yielded part of the sequence of the Amaro flood basalt, fits well with other a radiometric age of 35 m.y. (Table 1); this apparent early Oligo- dated lava sequences from central Ethiopia (Megrue and others, cene age requires confirmation by dating of the underlying Trap 1972; Mohr, 1971). Nevertheless, the lithology of the Amaro flood Series basalt, but it hints that the Amaro ignimbrite was formed basalt is somewhat distinct from the Trap Series flood basalt of the from younger episodes of silicic volcanism. Unfortunately, the plateaus (Dainelli, 1943). The latter, both on the Gughe Mountains Amaro ignimbrite is too weathered to provide suitable specimens for and on the Bali plateau, is typically zeolitized basalt without olivine radiometric dating. phenocrysts, although occasionally having feldspar-phenocrysts. No centers for the Amaro ignimbrite have been identified, but The -only similar basalt found on the horst and its flanks forms a numerous large centers of late Tertiary to Quaternary age occur on small isolated exposure of purple, zeolitized basalt with plagioclase the rift floor and on the rift margins directly north of Lake Abaya. phenocrysts in the walls of the Gumide wadi, close to the Trachyte and Rhyolite E'omes. Trachyte domes, typically 1 to southeastern shores of Lake Chamo; relations with neighboring 1.5 km in diameter, are found sporadically along the 70-km length exposures are hidden by thick, wadi gravels. of the zone on the western boundary of the Amaro horst (Fig. 2, Amaro Ignimbrite. A veneer of tuff, rarely >100 m thick, lies on and regions to the south). Tie domes pierce the Amaro flood basalt, the basalt to form the highest summits of the central part of the but their relations, if any, with the Amaro ignimbrite are unknown. Amaro horst. Tuff is absent from the northern part; in the south, East of Gumide, a prominent was formed in successive pulses however, tuff overlaps directly onto Precambrian rock before with an intervening episode of boundary faulting. Rhyolite domes rapidly thinning out. occur on the northeastern part of the Amaro horst, as well as along The tuff beds are soft, light gray, and well bedded in their lower the eastern margin of the Galana graben where they were related to part. They grade up into badly weathered, welded tuff in the upper the final stages of ash-flow tuff eruption.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/3/417/3433393/i0016-7606-85-3-417.pdf by guest on 26 September 2021 QUATERNARY LACUSTRINE a FLUVIATILE SEDIMENTS

r^ ALLUVIAL FANS (LARGELY COVERING AMARO BASALT)

rvi TRACHYTE-RHYOLITE DOMES ^ (PLIOCENE - PLEISTOCENE )

Fxl QUATERNARY IGNIMBRITE

IV771 AMARO IGNIMBRITE ^ (? PLIOCENE)

I 5 "'JGOCENE-MIOCENE >—ä IGNIMBRITE

mrm NECH SAR BASALT U™ (PLIOCENE-QUATERNARY)

B AMARO BASALT (MIOCENE)

wmI TRAP SERIES BASALT 11223 (-)

¡23 PRECAMBRIAN GRANODIORITE

W PRECAMBRIAN SCHIST

northern Annaro hörst Somalian Galalactu plateau

Figure 2. Geologic map of northern and central parts of the Amaro horst and adjacent regions. Faults are along a latitude through Galalactu and thus crosses the northern part of the Amaro horst and the Tosa Sucha. In the indicated by thick traces; dipping strata, by arrows (numbers are dips, in degrees). ERTS-1 imagery reveals that absence of any reliable maps, topography is estimated. The faults are drawn with true dips, disregarding the latitude and longitude values previously assigned to this region must be in appreciable error, and they are omitted horizontal-vertical scale difference. from this map. (Note: Distortion from aerial photographic basemap has not been corrected.) The cross section is

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Nech Sar Basalt. Quaternary Nech Sar basalt occurs on the floor TABLE 1. K-Ar ANALYSES OF UPPER TERTIARY VOLCANIC ROCKS FROM THE AMARO REGION of the Ganjuli graben, west of the horst (no comparable basalt is Sample K20 Atmospheric Radiogenic Age known from the Galana graben). This young flood basalt forms (X) contamination ""Ar/a sample (m.y.) 3 the Nech Sar, a distinctive planar topographic feature on the (%) (ran ) 2 3 Basalt* 1.501 82.9 2.292 x 10" 21.3 ± 1.1 northeastern side of Lake Chamo, ~90 km in area and ~100 to 2 150 m above lake level. The basalt can be traced westward into trie Tufft 4.421 37.7 1.122 xlO" 35.2+1.4 Note: Analyzed by Ccochron Laborazories (replicate analyses) . Xg = 4.72 X 10 10 Tosa Sucha, between Lake Chamo and Lake Abaya, where me 1 per yr,- \e = 0.585 X 10 per yr. basalt has been cut by intense faulting. The occurrence of spatter * AM 101 (whole rock); Basa.'.t dike, southern side of middle Baraka valley, western cones along some fault scarps indicates that the faulting was part of the Amaro horst. + EMD57 (whole rock): Whiti? welded tuff exposed at second escarpment fault, 3 km contemporaneous with the termination of volcanic activity. north of Mount Jabasire on eastern rim of Galara graben. The Nech Sar basalt is fresh, coarse grained, olivine-phyric, and magnetite-rich. It is commonly exceedingly scoriaceous. Some of the spatter,cones ejected bombs rich in calcite. The Amaro horst is asymmetric; it is step-faulted abruptly on its Defining limits to these younger flood-basalt flows has proved eastern side but more complexly faulted over a wider belt on its difficult in the field. In the west, near Arba Minch (Fig. 2), the western side (except in the southern part; see below). The eastern contact with the Trap Series basalt of the Gughe Mountains is escarpment marks a total displacement between 1,000 and 1,500 m faulted and covered with alluvium, but it probably takes the form of upward from the Galana graben floor; this is accomplished in as ponding of the Nech Sar basalt flows against rift-margin fault scarps many as four steps, one of which usually predominates at any given that were subsequently rejuvenated. On the opposite side of the latitude, except in the sou thern part of the horst, where the steps are Ganjuli graben, the Nech Sar basalt can be traced eastward across of similar magnitude. The step faults of the eastern escarpment the important limiting fault of the Nech Sar plain into the delineate blocks tilted down to the west; the outer blocks are more wedge-shaped, dissected terrain west of the Sarmayli valley (Fig. 2). steeply tilted (20° to 30°). At the rare off sets of the escarpment faults, This terrain exposes steeply east-dipping Amaro flood basalt, but, ramp structures are more common than cross faults. Two large, near the southern apex of the wedge, the Nech Sar basalt extends annular drainage features occur on the eastern escarpment. The subhorizontally over the older, inclined basalt, thereby providing a more northerly feature, centered 5 km south of Kelyi (Fig. 2), has unique opportunity to study the relation between the two sequences. been formed at least in part by subsidence; there is no cogent We have so far been unable to discover an unconformity between evidence for any rotations.l movements (see Bond and others, 1972). the two sequences; we propose a progressive down warping against Along the western edge of the Amaro horst, the boundary faults of the Amaro horst causing dip to increase with depth (and age) in the the Ganjuli graben effect a total upthrow of at least several hundred pile (see discussion on , below). meters. More faulting of a complex pattern intervenes between the Flood basalt, 230 m thick and lying directly on Precambrian rocks boundary faults and the summit ridge. This intervening zone is at 1,200-m elev, occurs in the Aday valley west of Yavello at the occupied by the Sagan River, which flows south along a 5-km-wide southern end of the Amaro horst (Mohr and Gouin, 1968). This graben of lower elevation than that of the Ganjuli graben west of the basalt could be equivalent to the Nech Sar basalt, but the degree of boundary fault. Therefore, the upfaulting of the Amaro horst is denudation is greater. associated with localized severe downfaulting. Quaternary Ignimbrite of the Ganjuli Graben Floor. Ths The Sagan graben is separated from the adjacent summit ridge of denuded remnants of an ignimbrite sheet, originally at least 3 00 km 2 the horst by a single fault. This remarkably linear fault in area, occur on the northwestern part of the horst and the adjacert counterthrows the four la rge step faults on the eastern side of the islands of Lake Abaya. Farther south, in the upper Sagan valley, ridge and has an estimated total displacement of 800 to 1,000 m. It small patches of white ash-flow crystal tuff occur, described bv abruptly divides the flat graben floor from the faceted spurs of a Weber (1906) as olivine comendite; similar rocks lie on the Nech widely dissected zone forming the western face of the summit ridge. Sar basalt of the Tosa Sucha and Gumide regions. The ages of the The top of the ridge is here formed by a narrow flat plateau on which various ash-flow tuff units of the Ganjuli graben floor and any cor- is superimposed a very nar row, small, but continuous topmost ridge. relations between the units remain uncertain. Despite the straightness o:: the eastern boundary fault of the Sagan Sediment. Pliocene(P) through Quaternary lake sediment occurs graben, we can detect no evidence for any transcurrent extensively in the Lake Abaya basin and extends up the length of the displacements. Normal faulting appear;; to have been progressively Galana graben. The sediment occurs upfaulted as much as 300 m active into recent time. above the present lake level of Lake Abaya, well above the Tosa The floor of the Sagan graben exposes Precambrian rocks cut by Sucha threshold level, suggesting tectonic uplift and tilting of th; an axial dike swarm. The graben dies out northward into the central basin margins in late Quaternary time. Well-developed alluvial fans part of the Amaro horst; the faults are transposed westward and and beach deposits have built up along the foot of the eastern decrease in magnitude. The western boundary faults of the graben escarpment of the Amaro horst; they are cut by the most recent curve between north and northwest; northwest-trending faults are a faults. significant, though subsidiary, feature of the tectonic pattern of the west-central part of the Amaro horst. TECTONIC INTERPRETATION The faults on the western boundary oi: the Amaro horst are again Major faults and regions of tilted strata are indicated in Figure 2. associated with inward-tilted (dip, 10° to 20° E.) fault blocks, which for the northern and central parts of the Amaro horst. Its structure is are upthrown on their western side. Such tilting also occurs within defined by inward tilting of antithetically faulted blocks to either the horst, ir. the upper Sags n graben (Fig. 2). The thick Amaro flood side of a horst that is upfaulted normally from the axis of the rift basalt succession exposed in the northwestern part of the Amaro valley. horst reveals steepening easterly dip with increasing depth in the The fault trend is N. 5° to 10° E., parallel to the main Ethiopian rift pile. Thus, the preservation of the top of the basalt sequence adjacent structures. Important faults that are more east trending and to the granodiorite inselberg on the west-central part of the horst, upthrown to the southeast occur in the western part of the horst, between the Sarmayli and 3araka valleys, exposes flows dipping 5° however. They delineate the northern from central parts and also to 10° to the ivest, whereas north and south along the strike, but at broaden the horst to the south (Fig. 2). A notably similar feature lower elevations, the flows dip steeply eastward. causes southward broadening of the Ruwenzori horst (McConnell, The Amaro flood basalt of the western part of the horst shows a 1972, Fig. 6). regional, very gentle southerly decline in elevation, closely matching

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the gradient of the Sagan graben. This suggests a greater degree of evidence, that major uplift of the Amaro horst followed eruption of uplift of the horst to the north, away from a southern hinge line. the Amaro flood basalt later in Miocene time and that this basalt was The age range of the Amaro faulting, and whether the faulting has deeply eroded on the horst and its flanks. It was at this time that a been episodic or essentially continuous, is still poorly known. Some sharp summit ridge, still preserved as a topographic feature on the major faults in the upper Baraka valley have no topographic narrow plateau of the present high horst, was carved along the expression as scarps; because these faults displace the Amaro flood length of the proto-horst. basalt, they could be as old as Miocene. Rejuvenation of fault Rhyolitic, pyroclastic, ash-flow tuff (Amaro ignimbrite) was scarps is commonly seen, both on the eastern and western thinly draped over the horst between late Miocene and Pleistocene escarpments of the horst (Mohr, 1960), including the Sagan graben, time. The source of this tuff is not known, but the preserved where faceted spurs are typical. Quaternary fault movements are exposures are thickest at the highest elevations, unlike the indicated by displacements cutting volcanic cones on the Nech Sar underlying basalt. Recessional erosion of an originally uniformly plain and by the manifest freshness of some scarps. thick tuff may account for this phenomenon. No reverse faults, nor any fault planes dipping <45°, have been Major uplift of the Amaro horst occurred after deposition of some observed in the Amaro region. Furthermore, we have sought both scattered silicic tuffs on the northwestern part of the horst. This field and photogeologic evidence of transcurrent faulting. Except for phase of uplift can be tentatively correlated with the Pleistocene small dextral offsets of some stream valleys along the eastern faulting of the main Ethiopian rift margins (Baker and others, 1972); escarpment faults south of Kelyi, we observed no evidence for the Sagan graben and drainage system was probably initiated at this transcurrent faulting. time. Trachyte domes were extruded in association with movement An understanding of the structure of the Galana and Ganjuli along the western boundary faults on which they are situated. grabens is essential to a study of the Amaro horst. Briefly, the Galana Tilting of the Ganjuli graben floor toward the Amaro horst graben is an asymmetric structure, with the abruptly faulted east- continued into Quaternary time, after such movements had ern escarpment of the Amaro horst facing a broad zone of west- terminated in the Galana graben; the tilting was accompanied by dipping, antithetically faulted Trap Scries basalt and overlying ongoing flood-basalt eruptions west of the north-central part of the Oligocene to Miocene ignimbrite. This warped zone leads up via Amaro horst (Nech Sar and Tosa Sucha areas). the narrow Galalactu marginal graben to the synthetically faulted Renewed tectonism occurred during late Quaternary time, rim of the Bali plateau. notably along the boundary faults of the Amaro horst and the Sagan The Ganjuli graben is more complexly and recently deformed. graben. Also, the axis of the Ganjuli graben was finely fractured by South of Lake Chamo, the Precambrian surface slopes downward, the Wonji fault belt. This belt marks a line of crustal thinning along toward the Amaro horst (Mohr and Gouin, 1968). The Tosa Sucha the whole length of the main Ethiopian rift. The Galana graben was separating Lake Chamo and Lake Abaya is cut by numerous normal not affected by this tectonism. faults. Their 45° W. dip, allied with the tilting and upwarping (on the upthrown side) of the faulted blocks, gives the Tosa Sucha a ratchet CONCLUSIONS profile. Thus, the Tosa Sucha and the main rift floor immediately Massive crustal uplift of the Amaro horst from the downwarped north of Lake Abaya preserve evidence of a Quaternary episode of axis of the Ethiopian rift occurred, probably episodically, during the crustal extension (the Wonji fault belt of Mohr, 1960) in the Ganjuli late Tertiary and Quaternary. Both the Galana and the Ganjuli graben. No comparable evidence was found in the Galana graben. grabens, east and west of the Amaro horst, respectively, are faulted The Ganjuli graben also differs from the Galana graben in not more cleanly on their western sides: the Amaro horst is thus an manifesting a broadly warped zone along the margin facing the asymmetric structure. This asymmetry is complicated by important Amaro horst, although in the Ganjuli graben subsidence and northeast-trending faults on the less-cleanly faulted, western side of flooding by Lake Chamo inhibit the evidence. Massive, east-dipping the horst, where the bladelike prow of the northern part broadens or vertical faults form the western margin of the Ganjuli graben into the central part. In the northern part, the total amount of uplift where it adjoins the gently west-dipping flows of the Gughe above the rift floor has exceeded 1,500 m; the uplift decreases to Mountains. 1,000 m in the central part. Comparison with the Gughe Mountains west of the Ganjuli graben introduces an unexplained phenomenon: EVOLUTION OF THE the elevation of the Precambrian surface is higher on the floor of the AMARO HORST graben than it is under the elevated Gughe Mountains volcanic pile When the formation of the Amaro horst began is not known. (Mohr and Gouin, 1968). Schottenloher (1938) indicated that the Certainly, the Mesozoic(P) sedimentary rocks of the horst are Gughe Mountains are the most easterly of three extensive, thinner than those on the Bali plateau or in the Ganjuli graben northeast-trending horsts, with intervening grabens, which occur (Mohr and Gouin, 1968), suggesting a higher topography on the between the main rift and the lower Omo valley, but he did not horst even then. These sediments, lat<;ritized at a later date, were discuss the relation of the Precambrian surface to the Neogene removed from the axial region of the Amaro horst before the tectonism. eruption of the Amaro flood basalt in Oligocene(P) to early Miocene The existence of the Amaro sialic block, uplifted from the rift floor time. to high elevations, invites comparison with the renowned Some middle Tertiary Amaro flood basalt was erupted from dikes Ruwenzori horst of the Western rift (Holmes, 1965). The along the already partially uplifted axis of the Amaro horst, but Ruwenzori horst (5,120-m elev) is composed entirely of more of this basalt originated from presumed fissures near the Precambrian rocks (McConnell, 1972) and easily forms the highest margins of the Ganjuli graben. Dike dips indicate that the flanks of nonvolcanic in Africa. Holmes (1965, p. 1065-1067) the Amaro horst were marked on both sides by downwarping that considered that the 3,000-m excess uplift of the Ruwenzori horst acted toward the horst progressively during formation of the relative to the plateau bordering the Western rift could not be sat- basaltic pile. Downwarping was more severe to the north, where the isfactorily explained by "isostatic or eustatic effects," that is, by horst was most uplifted. The occurrence of a thin basalt succession vertical adjustments among sectors of a homogeneous crustal plate on the central sector of the summit ridge suggests that, after an initial split by inclined fault planes (Heiskanen and Vening-Meinesz, flooding, the horst rose above the eruptive surface; however, 1958, p. 390). Indeed, the relative rarity of high horsts in de- posteruptive denudation probably also influenced the present mands a further explanation. thinness of the basalt. Modeling of grabens and horsts by subjecting density-stratified We currently consider, particularly from the geoniorphologic clay cakes to horizontal acting at their base (Cloos, 1968;

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Stewart, 1971) has thus far not succeeded in elevating a "horst" Baker, B. H., Mohr, P. A., and Williams, L.A.J., 1972, Geology of the eastern above the original surface. Holmes (1965) was impelled to propese a rift system of Africa: Geol. Soc. America, Spec. Paper 136, 67 p. "drastic [decrease] of density in the underlying depths" to explain Beater, B. E., and Maud, R. R., 1960, The occurrence of an extensive fault the uplift of the Ruwenzori horst. But gravity surveys of the African system in S. E. Zululand and its possible relationship to the evolution of rift valleys have revealed the existence of dense intrusions at high a part of the coastline of southern Af rica: Geol. Soc. South Africa Trans, and Proc., v. 63, p. 51-61. levels within the axial zone, appropriately associated with Bond, J. G., Price, S. A., and Breeser, P. J., 1972, Deformational history of graben-in-graben subsidence (Baker and others, 1972) and not with the Columbia River basalts of western Idaho as interpreted from drain- sialic horsts. age development analysis: Geol. Soc. America, Abs. with Programs The normal faulting of the Amaro horst shows that the forces (Cordilleran Sec.), v. 4, no. 3, p. 129-130. pushing up the horst have been essentially vertical ones, associated Chater, A. M., and Gilboy. C. F., 1970, Stratigraphic and structural relations with a component of crustal extension; the forces have acted at the in the Shakisso-Arero region of southern Ethiopia: Univ. Leeds, 14th axis of the rift, which is unique in Ethiopia by being downwarpec or Ann. Rept. Research Inst. Afr. Geology, p. 8—11. tilted from both margins. This uniqueness we relate to the Cloos, E., 1968, Experimental analysis of Gulf Coast patterns: Am. occurrence of the Amaro horst in a narrow-angled structural Assoc. Petroleum Geologists Bull., v. 52, p. 420-444. bifurcation of the main Ethiopian rift. It is of interest that the Dainelli, G., 1943, Geologia dell'Africa Orientale: Roma, Reale Accad. Ital., 3 vols, text, 1 vol. maps. Ruwenzori horst likewise occurs in a narrow-angled bifurcation of De Angelis d'Ossat, G., and Millosevich, F., 1899, Cenni intorno alle rac- the Western rift and that downwarping has played a role in the colte geologiche, in Vannutelli, L., and Citerni, C., eds., L'Omo (Sec- development of the Western rift also (Hepworth, 1964). onda Spedizione Bottego): Milano, Soc. Geog. Ital., p. 575—594 (and Inward-tilting of faulted blocks toward a horst has also been map). described from coastal Natal (Beater and Maud, I960). There, :he Heiskanen, W. A., and Vening-Meinesz, F. A., 1958, The earth and its grav- now-denuded Ngoye horst, a 60-km by 5-km Precambrian block ity field: New York, McGraw-Hill Book Co., 470 p. upfaulted during the rifting and splitting of Gondwanaland in Late Hepworth, J. V., 1964, Explanation of the geology of sheets 19, 20, 28, 29 time, is bordered by Karroo sedimentary rocks that dip in (southern West Nile): Uganda Geol. Survey, Rept. 10, 123 p. from both west and east toward the horst; this is a marked anomaly Holmes, A., 1965, Principles of physical geology (2d ed.): London, Nelson, in the general coastward dip of the region. Maud (1961) has at- 1288 p. Kazmin, V., 1971, Structure of the Precambrian in Ethiopia: Nature, v. 230, tempted to explain the uplift of the Ngoye horst in terms of cruscai p. 176-177. tension acting across arcuate-faulted terrain, a hypothesis that de- 1972, Granulites in Ethiopian basement: Nature Phys. Sei., v. 240, p. serves further study but does not appear applicable to the Amaro or 90-92. Ruwenzori horsts. Maud, R. R., 1961, A preliminary review of the structure of coastal Natal: We currently speculate that, whatever forces were responsible for Geol. Soc. South Africa Trans, and Proc., v. 64, p. 247-256. the downwarping and subsidence of the rift floor relative to the McConnell, R. B., 1972, Geological development of the rift system of east- plateaus, the forces imposed their crustal effects against mantle ern Africa: Geol. Soc. America Bull., v. 83, p. 2549-2572. hydrostatic pressure. This pressure has typically been released in Megrue, G. H., Norton, E., i.nd Strangway, D. W., 1972, Tectonic history of the Ethiopian rift as deduced by K-Ar ages and paleomagnetic Ethiopia by periodic ascent of magma up fissures along zones cf measurements of basaltic dikes: Jour. Geophys. Research, v. 77, p. warping and crustal attenuation. At the Amaro horst, the occurrence 5744-5754. at a rift bifurcation of a bilateral downwarping toward the rift axis, Möhr, P. A., 1960, Report on a geological excursion through southern with a presumed correspondingly intensified reaction against mantle Ethiopia: Geophys. Obs. Addis Ababa Bull., no. 3, p. 9—19. hydrostatic pressure, forced the escape and pressure uplift of a 1962, The geology of Ethiopia: Addis Ababa, Univ. Coll. Addis Ababa crustal wedge along the axial zone of weakness. Gravity data should Press, 268 p. help distinguish between this model and the "density" model cf 1968, The Cainozoic volcanic succession in Ethiopia: Bull. Volcanol., v. Holmes: The former, assuming increasing density with depth in the 32, p. 5-14. crust, will show "highs" at the horst margin on the final Bouguer 1971, Ethiopian Tertiary dike swarms: Smithsonian Astrophys. Obs. Spec. Rept. 339, 51 p. anomaly profile, whereas the latter will reveal a "low" over the Möhr, ?. A., and Gouin, P., 1968, Gravity traverses in Ethiopia (4th interim horst, with wavelength dependent on the depth to the anomalous rept.): Geophys. Obs. Addis Ababa Bull., no. 12, p. 27-56. mass. Observations from extensive gravity surveys in the southern Rogers, A. S., Miller, J. A., a nd Möhr, P. A., 1965, Age determinations on Ethiopian rift region are currently being reduced and evaluated by some Ethiopian basement rocks: Nature, v. 206, p. 1021—1023. G. R. Marsden (Univ. Newcastle, Newcastle, England). Schottenloher, R., 1938, Bericht über eine forschungreise in Südäthiopien: Bayerische Akad. Wiss. Sitzungsber., Math.-Naturw. Kl., p. 205-210. ACKNOWLEDGMENTS Stewart, J. H., 1971, Basin and Range structure: A system of horsts and grabens produced by deep-seated extension: Geol. Soc. America Bull., A review of the manuscript by M. L. Hill led to substantial v. 82, p. 1019-1044. improvement in its presentation. Our research program was Weber, M., 1906, Die Petrographische Ausbeute der Expeditionen O. expedited by an Educational Expeditions International (EEI) group Neumann-v. Erlanger nach Östafrika und Abessynien, 1900-1901: organized by R. A. Citron in 1971. Frances Dakin of Haile Selassie I Geog. Gesell., München Mitt., v. 1, p. 637—660. University assisted with field work on that occasion. This research was supported in part by U.S. National Aeronautics and Space Administration Grant NGR 09-015-002.

REFERENCES CITED Arambourg, C., and Wolff, R. G., 1969, Nouvelle données paleontologique sur l'âge des "grès du Lubur" ["Turkana grits"] à l'ouest du lac MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 19, 1973 Rodolphe: Soc. Géol. France Compte Rendu, v. 6, p. 190—192. REVISED MANUSCRIPT RECEIVED SEPTEMBER 17, 1973

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