This faulting tectonics analysis concerns the southernmost segment of the Dahomeyide Orogen and the West-African craton eastern margin in southeast Ghana. The analysis of strike-slip faults in the frontal units of the Dahomeyide Belt indicates that four distinct compressive events (NE-SW, ENE-WSW to E-W, ESE-WNW to SE-NW and SE-NW to SSE-NNW) originated the juxtaposition of the Pan-African Mobile Zone and the West-African craton. These paleostress systems define a clockwise rotation of the compressional axis during the structuring of the Dahomeyide Orogen (650–550 Ma). The SE-NW and SSE-NNW to N-S compressional axes in the cratonic domain and its cover (Volta Basin) suggest that the reactivation of the eastern edge of the West African craton is coeval with the last stages of the Pan-African tectogenesis in southeast Ghana. An extensional episode expressed as late normal faulting is also recorded in this study. This E-W to SE-NW extension, which is particular to the southernmost part of the Dahomeyide Belt, appears to be post-Pan-African. This extension probably contributed to the formation of a major Jurassic rifting zone that originated the Central Atlantic and the Benue Trough.
Southeast Ghana is a geologically complex area (Figure
Schematic geological map with the “Stable Zone” (comprising the southeastern margin of the West african craton and its cover represented by the Volta Basin) and the Pan-African “Mobile Zone” (i.e., the frontal structural units of the Pan-African Dahomeyide belt): 1 = Eburnean basement complex (WAC); 2 = Neoproterozoic to Paleozoic cover (Volta Basin); 3 = internal and external gneiss-migmatite units; 4 = kyanite bearing micaceous quartzites; 5 = basic and ultrabasic massifs of the suture zone; 6 = Atacora or Akwapim structural unit; 7 = Buem structural unit; 8 = Meso-Cenozoic cover of the Gulf of Guinea Basin; 9 = thrust contact; 10 = Kandi fault mylonitic zone; A-B = schematic section line (Figure
The sedimentary sequence in the Volta Basin rests unconformably on the Eburnean basement complex. This basement and its cover constitute the western “Stable Zone” of our study area (with rock assemblages that did not undergo the Pan-African thermotectonic events); the eastern “Mobile Zone” is represented by the frontal part of the Pan-African Dahomeyide Orogenic Belt (Figures
Schematic cross section (A-B, Figure
At the scale of satellite images or aerial photographs and outcrops, this Pan-African tectogenesis is strongly expressed as a dense network of polyphase lineaments and fractures. The analytical study of these fractures will lead to the following: (1) characterization of the Pan-African paleostresses in the southernmost Dahomeyide segment, and (2) identification of the Pan-African markers on the southeastern margin of the Eburnean basement complex and its cover represented by the Volta Basin (i.e., on the Stable Zone). Following the work done on the Pan-African fracturing in southwest Niger [
In the geological framework of this tectonic study, it is important to distinguish two different lithostructural domains as follows: the West-African Craton and its cover (Volta Basin) as the “Stable Zone” and the Pan-African domain made up of the frontal part of the Pan-African Dahomeyide Orogenic Belt as the “Mobile Zone” (Figures
The Stable Zone corresponds to the southeastern margin of the Guinea [
In Ghana, the West-African Craton is roughly equivalent of a gneiss-migmatite basement complex that includes multiple generations of granitic bodies. This complex contains supracrustal belts represented by volcanic, metavolcano-sedimentary or mainly metasedimentary formations defined as the Birimian and Tarkwaian units or groups [
The supracrustal belts include a lower assemblage, the Birimian unit, and an upper assemblage, the Tarkwaian unit [
As regards the Tarkwaian formation, it is described essentially as a detrital assemblage unconformably on the Birimian unit [
The sedimentary sequence of the Volta Basin lies unconformably on the southeastern margin of the West-African Craton (Figure
Structurally, the lower and middle megasequences show significant eastward thickening in the Volta Basin where they are folded and partially incorporated into the frontal units of the Pan-African Dahomeyide Orogenic Belt [
The Pan-African Dahomeyide Belt is located on the southeastern margin of the West-African Craton (Figures
The Buem structural unit corresponds to the outermost structural unit of the Dahomeyide Belt and has been directly overthrusted onto the Volta Basin or the Eburnean basement complex (Figures
The internal structural units are located to the East of the suture zone massifs, exposed as the western margin of the Benino-Nigerian metacraton [
Such a lithostructural assemblages in the Pan-African Dahomeyide Belt indicates a collision belt and defines an eastward increasing metamorphic gradient [
The satellite image of the study area (Figure
Satellite cover (MNT image) of southeast Ghana highlighting the juxtaposition of two highly contrasting morphostructural domains corresponding to the Pan-African Mobile Zone to the East and the Stable Zone (WAC and Volta Basin) to the West.
Lithostructural sketch highlighting the network of kilometric lineaments (a) and their distribution in the Stable Zone (b) and Mobile Zone (c): 1 = trace of Eburnean or Pan-African (Sn+1) foliation, 2 = major thrust contact, 3 = lineament.
The relative complexity of the lineament network in the Stable Zone is interpreted to be due to the coexistence of Eburnean and Pan-African fractures. Such an hypothesis leads to an inference that, in response to the Pan-African tectogenesis, the Eburnean cratonic basement and its cover records Pan-African fractures and that the Eburnean fractures were reactivated.
The rectilinear character of all the lineaments observed in the study area indicates the presence of subvertical fracture planes. They are thus distinguishable from the thrust contacts that separate the nappes in the frontal part of the Dahomeyide Orogeny. Finally, the offset of quartzitic bands leads to consider some lineaments as true sinistral or dextral strike-slip faults (Figure
About 800 orientation measurements of striated planes were collected at fourteen faulted sites developing in different geological units in southeast Ghana (Figure
Location of the fourteen faulted sites (1 = Oterkpolu, 2 = Twum, 3 = Somanya, 4 = Peduase-N, 5 = Peduase-S, 6 = Bani-Kpeve, 7 = Akosombo, 8 = Adukrom, 9 = Ablikuma, 10 = Akuse, 11 = Ahunda, 12 = Aboabo, 13 = Nsawam, 14 = Nkurakan) in different geological units in southeast Ghana (1 = Eburnean basement, 2 = Bombouaka Supergroup, 3 = Oti Supergroup, 4 = external and internal polycyclic nappes, 5 = granulitic nappes of the suture zone, 6 = Atacora or Akwapim structural unit, 7 = Buem structural unit, 8 = thrust contact.
A manual stereographic examination has allowed completing the characterization of the striations. Computer processing of the data was done by using Angelier’s [
Synthesis of the results of the processing of fracture planes studied at all faulted sites in southeast Ghana. The table summarizes, for each site, the geographic coordinates, the rock type (Rt) involved (DC = dolomitic carbonate, S-Q = sandstone or quartzite, MQ = micaceous quartzite, QS = quartzite and schist, G = granulite, OG = orthogneiss, MG = migmatitic gneiss, M-G = migmatite and granite), the number of planes (Np), plane type (Pt) analyzed (SSF = strike-slip faults, RF = reverse faults, and NF = normal faults), and trend and plunge of main paleostress axes (
Site | Coordinates | Rt | Np | Pt |
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Latitude | Longitude | ||||||||
Pan-African Mobile Zone | |||||||||
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1 | Oterkpolu | N06°11′37.7′′ | W00°04′42.9′′ | DC | 24 | SSF | 302-01 | 37-73 | 212-17 |
2 | Twum | N06°10′29.9′′ | W00°03′36.9′′ | S-Q | 12 | SSF | 109-25 | 282-65 | 18-03 |
3 | Somanya | N06°05′54.5′′ | W00°01′45.6′′ | Q | 33 | SSF | 37-13 | 166-69 | 303-16 |
22 | SSF | 269-07 | 143-78 | 00-09 | |||||
25 | SSF | 310-01 | 44-80 | 220-10 | |||||
9 | NF | 287-83 | 177-02 | 87-07 | |||||
4 | Peduase-N | N05°47′56.1′′ | W00°11′03.4′′ | QS | 36 | RF | 303-02 | 213-02 | 70-87 |
23 | SSF | 20-06 | 146-79 | 289-08 | |||||
23 | SSF | 260-03 | 161-73 | 350-17 | |||||
5 | Peduase-S | N05°47′28.8′′ | W00°11′33.1′′ | QS | 31 | SSF | 141-02 | 31-84 | 231-06 |
47 | NF | 259-72 | 52-16 | 144-08 | |||||
6 | Bani-Kpeve | N06°40′78.6′′ | E00°21′14.0′′ | Q | 21 | SSF | 199-09 | 74-74 | 291-13 |
28 | SSF | 158-04 | 53-75 | 249-15 | |||||
27 | NF | 262-81 | 13-03 | 104-08 | |||||
7 | Akosombo | N06°18′16.0′′ | E00°03′18.3′′ | QS | 22 | SSF | 210-06 | 95-76 | 301-13 |
15 | NF | 94-82 | 01-00 | 271-08 | |||||
8 | Adukrom | N06°00′40.6′′ | W00°03′59.4′′ | QS | 32 | SSF | 149-03 | 48-76 | 239-14 |
9 | Ablikuma | N05°37′03.9′′ | W00°17′39.3′′ | Q | 13 | SSF | 202-01 | 301-83 | 111-06 |
11 | SSF | 64-03 | 307-84 | 154-06 | |||||
10 | Akuse | N06°05′14.9′′ | W00°04′04.7′′ | G | 9 | SSF | 186-21 | 356-69 | 94-03 |
18 | SSF | 142-02 | 49-60 | 233-30 | |||||
11 | Ahunda | N06°18′29.9′′ | E00°32′39.7′′ | OG | 14 | SSF | 174-02 | 288-86 | 84-04 |
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Stable Zone | |||||||||
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12 | Aboabo | N06°07′30.1′′ | W00°13′56.9′′ | MG | 10 | SSF | 120-09 | 274-80 | 29-04 |
18 | SSF | 173-02 | 290-86 | 83-04 | |||||
13 | Nsawam | N05°48′58.9′′ | W00°22′24.4′′ | M-G | 30 | SSF | 340-07 | 234-66 | 73-23 |
14 | Nkurakan | N06°08′06.0′′ | W00°12′10.4′′ | S-Q | 24 | SSF | 02-14 | 137-70 | 269-13 |
The Mobile Zone sites were the first to be analyzed in order to reconstruct Pan-African paleostresses. The data collected in the Stable Zone (the Eburnean basement complex and the lower sandy cover, Figure
Most of the sites in the Pan-African domain display a polyphase fracture network. Some characteristics of such a fracture network are presented below.
The fracture in the Buem unit is well exposed in the Oterkpolu dolomitic carbonate quarry. The Bedded or banded bluish grey dolomitic carbonates show stratification planes that were strongly involved in decimetric (i.e., tens of centimeters) to metric (i.e., more than one meter) isoclinal folds (Figure
Structures of ductile and brittle deformations observed in southeast Ghana: (a) metric folds overturned to West and with submeridian axes in dolomitic carbonate of the Oterkpolu quarry; (b) N20-18E reverse fault plane in dolomitic carbonate of the Oterkpolu quarry (S0, N05-65E); (c) major N60-80NW sinistral strike-slip fault, with channel-like striation plunging at 15°NE (quartzite in a road section at Akosombo); (d) two generations of extension joints trending N35 and N120, inscribed on a Dn+2 decimetric fold (N60–20NE axis) and materializing the NE-SW and SE-NW
Stereograms obtained by processing striated fracture planes recorded in the Mobile Zone in southeast Ghana: (a), sets of strike-slip faults in dolomitic carbonates of the Oterkpolu quarry; (b), reverse faults or thrust planes in nappe pile of Peduase; (c), sets of primary strike-slip faults in quartzites at the Somanya quarry; (d) and (e), sets of secondary strike-slip faults in quartzites and schists (Somanya quarry and Peduase-N road section); (f) and (g), normal faults in quartzites and schists (Peduase-S and Bani-Kpeve road sections); (h) sets of strike-slip faults in augen gneiss at the Ahunda quarry. The stars represent poles of the
Seven sites such as road cuts and artisanal quarries are chosen for paleostress analysis (Figure
Along the road section between Peduase and Ayimensa, many thrust planes are particularly well exposed. The identification of these planes as reverse faults allows the constitution of two northwest verging nappe piles. The analysis of these E-SE dipping reverse faults suggests that the nappe piles of the Akwapim Structural Unit (in Peduase sector) were emplaced during the maximum N302-02
The thrust contacts in the Peduase sector are cut by numerous strike-slip faults related to two or three conjugated sets in the Akwapim quartzites. The relatively old system includes sets of major planes with mineral recrystallization (quartz, sericite). These planes, with submeridian and NE-SW (N40 to N75) trends, are dextral and sinistral respectively (Figure
Two families of dry fractures corresponding to conjugated planes are superimposed on the first strike-slip faults identified above. They are (1) NE-SW trending (N40 to N65) dextral and ESE-WSW trending (N105 to N125) sinistral, and (2) ENE-WSW to ESE-WNW (N80 to N125) dextral and N140 to N165 or N350 to N10 sinistral (Figures
Finally, the Akosombo, Bani-Kpeve, Somanya, and Peduase sites display tectoglyphe bearing planes of characteristic normal faults (Figure
The fracture data from the eastern sector (suture zone and internal structural units) were collected at an industrial granulite quarry in the Shai hills (Akuse site) and in the Hô augen gneiss (Ahunda site, Figure
At the Ahunda quarry, the SE-NW dextral and N20 to N40 sinistral conjugated strike-slip faults were identified with a primary N170
From the analysis of the striated planes in the Mobile Zone in southeast Ghana, three or four compressional axes and one late extensional axis can be identified (Figure
The paleostresses deduced from the analysis of the fracture planes in the Buem dolomitic carbonates (Oterkpolu site) are comparable to those from the last episodes in the Akwapim structural unit. Similarly, the paleostress systems represented by the strike-slip faults identified in the eastern part (suture zone and internal structural units) are superposable on the paleostresses recorded in the Buem. The development of late strike-slip faults thus appears to be common in the Pan-African Mobile Zone.
The imprint of the E-W to SE-NW extensional episode suggests normal faulting. This post-Pan-African or very late tectonic episode is responsible for the reactivation of former fracture planes, leading to the superposition of high pitch striations that is incompatible with the primary movements.
Limited outcrops are discovered in the Stable Zone. Consequently only three faulted sites (Aboabo, Nsawam, and Nkurakan) were studied in the Eburnean basement complex and in the basal part of the lower megasequence of the Volta Basin (Figure
The Aboabo industrial quarry reveals migmatitic paragneisses with quartzo-feldspathic leucozones and dark centrimetric boudins. These facies of the Eburnean basement complex are also cut by two generations of aplitic or pegmatitic veins. More recent NE-SW centrimetric veins cut through the sinistral submeridian strike-slip faults. In the quarry, two fracture generations were identified. The older planes are generally covered with biotite or amphibole and quartz lineations. The latter ones are often associated with recrystallized fibres parallel to the subhorizontal lineation. The tectoglyphes indicate homogeneous strike-slip movements which appear to reflect late reactivations of older planes. The conjugated strike-slip faults are SE-NW trending (N120 to N155) dextral and NE-SW trending (N30 to N50) sinistral, indicating a compression with
Stereograms obtained by processing striated planes recorded in the Eburnean basement complex and the basal part of the Volta Basin in southeast Ghana: (a), sets of strike-slip faults in migmatitic paragneisses of the Aboabo quarry; (b), sets of strike-slip faults in the Eburnean basement complex of the Nsawam quarry; (c), sets of strike-slip faults in quartzites at the Nkurakan quarry. The stars represent poles of the
In the Nsawam quarry, dark gneisso-migmatitic facies of the Eburnean basement complex display numerous amphibolitic inclusions. They are associated with a complex of metric veins of leucogranites, aplites, and pegmatites. The assemblage is cut by the first generation type fractures as identified at the Aboabo site (Figure
Outcrops at the Nkurakan site are characterized by massive bands of quartzites or quartzitic sandstones with limited black shale intercalations. These facies belong to the Mpraeso Formation (lower part of Bombouaka or Kwahu Supergroup, [
The Eburnean basement complex and the lower part of its cover display the imprints of two fracturing episodes due to compressions. The main episode is attributable to a system of paleostresses with a principal N-S to NNW-SSE trending
In summary, the set of fractures observed in the frontal part of the Dahomeyide Belt in southeast Ghana are resulted from four Pan-African compressive phases followed by a very late or post-Pan-African extension probably of Devonian age [
The compressive episodes are materialized by west verging thrust planes and conjugated strike-slip faults. The latter allow determination of the main paleostress
Correlation between the main Pan-African compressional axes in northern Togo and southeast Ghana.
Pan-African deformation phases | Compressional axis ( |
Compressional axis ( |
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Mobile Zone | Mobile Zone | Stable Zone | |
Dn+1 | NE-SW to ENE-WSW (N40 to N80) | NNE-SSW to NE-SW (N20 to N40) | |
Dn+2 | ESE-WNW (N100 to N115) | ENE-WSW to E-W (N80 to N90) | |
Dn+3 | SE-NW (N120 to N130) | SE-NW (N120 to N130) | SE-NW (N120) |
Dn+4 | SE-NW to SSE-NNW (N140 to N150) | SE-NW to SSE-NNW (N140 to N175) | SSE-NNW to S-N (N340 to N02) |
In the cratonic domain and its cover (Stable Zone), only two compressional axes might be defined. They are comparable to those related to the Dn+3 and Dn+4 Pan-African episodes (Table
In short, the partial reactivation of the cratonic margin in southeast Ghana occurred during the major rotation peak of the Pan-African compressional axis. Two successive events materialize this reactivation as follows: (1) the development of ENE-WSW and SSE-NNW fractures indicating a main N120-02
In southeast Ghana, the late extensional movement is materialized by normal faults that are particularly well developed in the quartzites of the Akwapim structural unit. It is also responsible for normal faulting reactivations of strike-slip fault planes resulting from Pan-African compressional episodes. Popoff [
The analysis of the dense fracture network in southeast Ghana has led the reconstitution of the superposition of Pan-African to late- or post-Pan-African paleostresses recorded in the frontal margin of the Pan-African Dahomeyide Belt and the southeastern margin of the West-African Craton (WAC). Above a clear morphostructural contrast, the distribution of lineaments indicates a juxtaposition of two very different domains and suggests the coexistence of Eburnean and Pan-African fractures in the stable domain constituted by a part of the WAC (of Archean to Paleoproterozoic age) and of its Neoproterozoic cover. In the lithostructural units of the Pan-African orogeny, the recorded fractures define four successive compressional axes oriented NE-SW, ENE-WSW, SE-NW, and SSE-NNW to S-N. These axes are comparable to those which are well known in the northern and central segments of the Dahomeyide Belt and correlated with four Pan-African tectogenesis phases (Dn+1 to Dn+4). As in northern Togo, the superposition of the paleostresses deduced from these deformation phases defines a major clockwise rotation (of more than 140°) of the compressional axis during the erection of the southern segment of the belt, while a rotation of 90 to 110° was expressed in the central segment of this orogen.
Despite the complexity of the lineament network characterizing the cratonic margin and its cover in southeast Ghana, the studied sites in this Stable Zone only present a few families of fractures and reworked strike-slip faults. An analysis of these structural elements has led to determine major paleostresses
In southeast Ghana, the peculiarity of brittle deformation is the recording of a post-Pan-African extensional phase. This late normal faulting corresponds to an E-W to SE-NW extension. It may be attributed to early (Cambrian to Devonian) episodes of the major Mesozoic fragmentation of the Western Gondwana. This fragmentation has resulted in the birth of the Central Atlantic and the Benue Trough in Jurassic.