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Application of paleostress analysis for the identification of potential instability precursors within the Benue Trough Nigeria

Application of paleostress analysis for the identification of potential instability precursors... Background: Structures such as faults, joints and fractures of diverse patterns have acted as precursors of several slope instability cases within the Benue Trough Nigeria. In some cases, the structures by their nature weakened and also created avenues that streams took advantage to further destabilize the rock slopes. In other cases, structure orientation played significant roles in the mobility and eventual runout distance of debris flow and avalanches in the region. Detailed field-based structural, fracture and paleostress analyses were therefore carried out to determine the fractural patterns that correlate to reported instability and landslide cases in the region; and to produce models that reveal areas with heightened risk. Results: Three fracture sets were isolated from analysis of fracture orientations and field relationships: Pre-folding (JT), Syn-Folding (JS) and Post Folding (JC) fracture systems. Paleostress analysis carried out on these fracture systems using the TENSOR™ software tool yielded three paleostress tensors corresponding to transtensional stress tensor with ENE-WSW direction of maximum extension (S ), oblique compressive (transpressional) tensor with HMIN NW-SE direction of maximum shortening (S ), and transtensional tensor with WNW-ESE direction of maximum HMAX extension (S ). HMIN Conclusion: These tensors are related to the prevailing plate tectonic stress regimes affecting the entire Benue trough and the West and Central African Rift System (WCARS). Our pre- and post-tectonic models have revealed the reasons for instability and the likely places where future failures may be located. This is the first such analyses in the region and it is hoped that the results can broaden the use and applicability of paleostresses in failure-prone terrains for future risk and disaster reduction/assessment within the Trough and in other areas prone to structure- controlled landslides disaster. Keywords: Paleostress, Instability, Landslides disaster, WCARS, Transtension, Transpression Background In 2010, a rock-debris avalanche, unprecedented in Geologic structures have been reported as precursors scale and form, occurred on the hillslopes bounding and control of several medium to large-scale rainfall- Nigeria and Cameroon. Igwe et al (2015) reported the induced landslides within the Benue Trough Nigeria avalanche (Fig. 2) initiated as distinct slides on two (Fig. 1) and the Cameroon Line (Igwe 2015a,b; Igwe et al slopes weakened by ubiquitous fractures. The observed 2015; Igwe et al 2016). These and other slope move- surface displacements and predicted mechanisms of ments cause considerable loss of resources in a country movement indicated that the slides started from differ- where poverty and sundry socio-cultural circumstances ent blocks at speed between 8 and 20 km/h. Soon after rarely permit the implementation of disaster/risk reduc- however, a structurally-controlled coalescing of the two tion strategies. slides, the subsequent movement of the coalesced mass slope along an expanded fracture surface, and the flow of water along the same fracture system aided a quick * Correspondence: Ogbonnaya.igwe@unn.edu.ng; igwejames@hotmail.com transformation to a highly mobile mass movement that Department of Geology, Faculty of Physical Sciences, University of Nigeria, attained speed between 55 and 80 km/h. Half way down Nsukka, Nigeria © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 2 of 15 Fig. 1 Location and geological attributes of the Benue Trough the slope, the moving masses were transferred to a sur- the basal lithologic units comprising the slopes. The oc- face with numerous foliations that were perpendicular to currence of fractures and their study are therefore import- the direction of movement which enhanced mobility ant not only in slope stability risk assessment but also in (>110 km/h) until reaching a distance of over 2.5 km disaster reduction and management. The understanding where gradual deposition commenced. Several lives, of the stress orientation will improve the knowledge of de- acres of land, farms, trees and economic treasures were formation mechanisms, which is crucial for the imple- lost during the episode. mentation of a viable monitoring system. Similarly, Igwe (2015b) described a fractured slope in Irfan (1999), Revellino et al (2010), Grelle et al (2011), which streams took advantage of discontinuities to trigger Aucelli et al. (2013), Prakash et al (2015) have reported a landslide in an area without any previous history of fail- structurally-controlled landslides. Investigation of the land- ure. It became obvious afterwards that this particular case slides revealed that the geo-structural settings predisposed was clearly a case of a disaster waiting to happen because the slopes to factors triggering mass movements, which is researchers had not observed the myriad of fractures in consistent with Fookes and Wilson (1966), Zaruba and Mencl (1969), and Varnes (1978). A structurally-controlled landslide is also documented in Luzon et al (2016) where it was reported that the 2006 rockslide-debris avalanche in Southern Leyte, one of the largest known landslides in the Philippines in recent history, occurred on a weakened slope at an area where there was continuous movement along the Philippine Fault. The characteristics and mechanisms of the Leyte landslides reported in Sassa et al (2004) and Catane et al (2008) are similar to those of the Nigeria- Cameroon border avalanche. Brittle fractures are the consequences of the action of stresses on a macroscopic scale. A rock body subject to a known stress regime (that produces fractures) has an unambiguous relationship among the fracture planes and the orientations of the stresses. This concept can then be used to reconstruct the orientation of forces that created Fig. 2 The 2010 rock-debris avalanche showing the source and a the fractures that were active in the past based on present part of the landslide toe where the researcher is standing day orientations. To fully understand the applicability of Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 3 of 15 paleostress technique in risk assessment, it is neces- The study area falls within the southern part of the sary to analyze ancient stress regimes in the context Benue trough (Fig. 1) a 1000 km long northeast trending of their role as potential precursory agents. Kayen et intracontinental structure stretching from the beneath al. (2011) noted that stress analysis is a useful and the Niger Delta and Anambra Basins to the south to popular tool for structural and seismological ele- the Chad Basin in the North (Benkhelil 1989; Ofoegbu ments. Kaymakci (2006) reported that the state of and Onuoha 1990; Ojoh 1992; Nwajide 2013). It forms stress in rocks is generally anisotropic and is defined a part of the eastern flank of the Abakaliki synclinor- by stress ellipsoid axes, which characterize the magni- ium which forms the major structural unit in the tudes of the principal stresses. The paper determined Southern Benue trough. The synclinal structure is Paleostress orientations and relative paleostress mag- formed by Albian to Coniacian sediments folded in the nitudes (stress ratios) using the reduced stress con- Santonian (Fig. 3). The folds are generally open to gen- cept for the purpose of improving the understanding tle and asymmetrical with asouthwest plunge. In the of the kinematic characteristics of a Basin. core of the synclinorium to the south are deposited At the moment, there is no previous paleostress Campanian and younger clastic sediments belonging to study of the study area, which has in part hindered the Anambra basin formed after the Santonian folding knowledge of the potential instability precursors in episode (Nwajide and Reijers 1996; Obi and Okogbue the zones of frequent slope failures. Even though a 2004). century of geological study has enabled an extensive The stratigraphy of the Afikpo synclinorium is simi- understanding the geology of the Benue Trough, it lar to the southern Benue trough as a whole (Fig. 4). was only in the later part of the 20th century that a With predominantly clastic shallow marine deposition picture of the structural framework, within which the with cycles of transgressions and regressions. Detailed trough evolved, began to emerge. The controversies descriptions of the stratigraphy of the Benue Trough surrounding the tectonic evolution of the Benue have been addressed by Ojoh (1992) and Nwajide trough have been largely resolved; with the over- (2013). whelming evidence leaning towards the interpretation of the Benue trough as a collection of wrench related Methods pull apart basins related to transcurrent movement Fracture analysis along deep-seated oceanic transform faults (Benkhelil Fracture data were obtained from 844 fractures from 1982, 1989; Guiraud et al 1989; M. Guiraud 1993). ten locations (Table 1). The area sampled was kept The evolution of the basin has also been incorporated small enough (1000–2500 m2) in order to guarantee into a framework of genetically related basins in west homogeneity of results (Delvaux and Sperner 2003). and central Africa: The West and Central African Rift Data type obtained include attitude (Strike, dip and System (WCARS) (Binks and Fairhead 1992; Genik dip direction) of the fracture plane as well as nature 1992; Guiraud et al 1992; Guiraud and Maurin 1992; of fracture surface, cross-cutting relations, mean sep- Fairhead et al 2013;). There is only limited field based aration between the fractures, bedding orientation structural studies especially in the central and south- and relationship and others which may be of use in ern parts due to the nature of the units which do not determining the relative age relationships between allow for preservation, and the tropic climate which the fractures and dividing them into fracture sets makes for a difficult terrain to carry out detailed and systems. Three sets of fractures were observed a structural studies necessary to obtain information steeply dipping NNW-SSE pre-Fold system of frac- which could be used to deduce structural regimes tures (JT), a NE-SW syn-folding fracture set with that can be correlated to the precursor factors re- lower dips (JS), and a WNW-ESE set of post folding ported in several slope failures within the trough, and fractures predominant in the Post-folding sediments data required for the various methods of stress inver- (JC). These fracture systems were used to define the sion (Benkhelil 1986). initial subsets used in paleostress inversion of the fractures to obtain the reduced stress tensor. Geologic and stratigraphic setting The Afikpo synclinorium, forming a part of the Southern Paleostress inversion Benue Trough (Fig. 1), offers a unique opportunity to The most common and extensively used method of study and understand the deformational processes and stress inversion typically involves use of faults with to determine the tectonic stresses active in the southern slickenlines that record the direction of slip relative to Benue trough as the highly indurated nature of the sedi- the fault plane (Hancock 1985; Angelier 1994; Ramsay ments allow for a relative abundance of outcrops where and Lisle 2000). Their use is based on the Wallace-Bott structural data useful for inversion could be collected. hypothesis which states that the slip on a planar Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 4 of 15 Fig. 3 Cross sections taken in different directions across the study area showing rock history and arrangement Fig. 4 Stratigraphic synopsis of the Southern Benue Trough and Anambra basin (After Ojoh 1992 and Nwajide 2013) Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 5 of 15 Table 1 A summary of discontinuities’ characterization in the study area Site ID Locality Rock type Fracture set No. of fractures Strike Dip Average fracture spacing (m) Fracture infilling 002 Itigidi Sandstone JC-1 30 300–310 80–90 1.5 Ferruginized 006 Aboine River Akpoha Shale JT-1 38 340–355 80–90 0.1 None JT-2 8 310–320 85–90 0.3 JT-4 3 260–270 80–90 2.0 008 Amaseri Ridge Sandstone JT-1 5 330–360 80–90 0.7 None JS-1 9 045–060 45–60 1.5 012 Ohaozara Sandstone JT-1 40 330–350 85–90 0.15 None 013 Asu River Ohaozara Shale JT-1 156 330–360 70–90 0.1 None JT-2 50 300–330 55–80 0.1 JT-3 3 290–300 85–90 0.1 JS-1 11 040–060 25–30 0.6 45–60 70–75 JS-2 10 060–080 60–65 0.55 80–85 014 Asu River Akpoha Shale JT-2 100 310–330 70–80 0.05 None JS-1 13 40–50 30–35 2.0 55–60 JS-4 29 280–300 40–45 1.6 55–60 017 Aboine River Isinkoro Shale JT-1 18 355–050 90 0.2 None JT-2 51 310–340 70–75 0.2 JT-3 1 290–300 75–80 0.5 021 Aba-Omega Ugep Road Sandstone JT-1 12 350–360 70–75 0.2 None JT-2 9 320–330 80–85 0.2 026 Asu River Shale JT-2 28 330–340 70–80 0.2 None JT-2 57 320–340 75–80 0.3 JS-6 55 340–350 45–50 0.3 JS-5 59 310–320 30–35 0.2 JT-1 35 330–340 60–65 0.2 027 Afikpo Road Sandstone JS-3 5 020–030 40–45 1.0 Ferruginized JC-1 8 280–290 55–60 1.0 structure is assumed to occur parallel to the greatest re- deformational event, that the rocks themselves are fairly solved shear stress (Bott 1959). Similar assumptions can homogenous, the fractures do not significantly perturb be made for extension fractures (e.g. Joints) and contrac- the stress field in their vicinity and also that the struc- tional (e.g. Stylolites) fractures -that they form perpen- tures have not rotated significantly since their initiation dicular or at a high angle to the minimum (σ ) and (Ramsay and Lisle 2000). maximum (σ ) principal stress direction respectively- or The aim of paleostress inversion is to characterize for conjugate shear fractures where the maximum what is known as the reduced stress tensor. The re- principal stress (σ ) bisects the acute angle between the duced stress tensor has four parameters of the six conjugate planes while the minimum principal (σ ) stress needed to define the full stress tensor: the principal bisects the obtuse angle between the fracture planes. stress axes σ (maximum), σ (intermediate) and σ 1 2 3 The structures can be used separately or collectively to (minimum) and the ratio of principal stress differ- constrain the stress field that led to their formation. The ences, R =(σ − σ )/(σ − σ ). The parameter R defines 2 3 1 1 assumption being that the fractures formed in the same the shape of the stress ellipsoid. Only the directions homogenous stress field i.e. related to the same of the principal stresses (known as Euler angles) can Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 6 of 15 be determined for the stress tensor from inversion. The index R' defines the stress regime completely and Their relative magnitudes are represented by the is convenient for computing the mean regional stress fourth parameter R. The two additional parameters of regime from a series of individual stress tensors in a the full stress tensor arc the ratio of extreme princi- given area (Benkhelil et al 1989). On structural maps, pal stress magnitudes (σ /σ )and theisotropic com- the stress tensors are displayed with the orientation of 1 3 ponent of the stress tensor (the Mean stress), but both horizontal principal stress (SHmax) and horizontal these cannot be determined from fracture data only. minimum stress axes (SHmin) as recommended by The methods of paleostress inversion are numerical and Guiraud et al (1989) (Fig. 5). currently involve the use of computer programmes to statistically analyse fracture data in order to characterize Results the stress field responsible for them (Etchecopar et al The stress fields were determined for the three joint 1981; Angelier 1994; Ramsay and Lisle 2000; Delvaux and systems based on field-based age relationship criteria. Sperner 2003; Célérier et al. 2012). This study makes use The tensors were determined after applying the Right of TENSOR™ program (Delvaux 1993; Delvaux et al. 1997; Dihedron and Rotational Optimization described Delvaux and Sperner 2003). This program is a tool above. for controlled interactive separation of fault slip or focal At a location named Aboine River, Akpoha, two mechanism data and progressive stress tensor optimization tensors were determined. The first tensor was calcu- using successively the Right Dihedron method and the lated from 46 Joints belonging to the JT fracture Rotational Optimization method. Detailed explanation of system, giving a Strike-slip extensional regime with how these methods are utilized in TENSOR can be found parameters σ =12/150, σ = 12/150 and σ = 00/060 1 2 3 in Delvaux et al. (1997) and Delvaux and Sperner (2003). with an NNW-SSE direction of maximum shortening The stress regime is determined by the nature of the and a stress regime value of 1.00 (Fig. 6a). A single vertical stress axes: extensional when σ is vertical, strike- conjugate shear fracture yielded a pure compression slip when σ is vertical and compressional when σ is tensor with parameters σ = 00/337, σ = 20/067 and 2 3 1 2 vertical. The stress regimes also vary, within these three σ = 70/247 with a stress regime index of 2.50 and a main types, as a function of the stress ratio R : Radial NNW-SSE direction of maximum shortening. Being extension (σ vertical, 0 < R <0.25), Pure extension (σ verti- the only shear fracture data it is considered unreliable 1 1 cal, 0.25 < R <0.75), Transtension (σ vertical, 0.75 < R <1 andrankedE(Fig. 9a). or σ vertical, 1 > R >0.75), Pure strike-slip (σ vertical, At a location named Amaseri Ridge, analysis was 2 2 0.75 > R >0.25),Transpression (σ vertical, 0.25 > R > 0 or σ carried out on 14 joints (extension fractures) with two 2 3 vertical, 0 < R < 0.25), Pure compression (σ , vertical, 0.25 tensors determined. A strike-slip extensional tensor <R <0.75) and Radial compression (σ vertical, 0.75 < R < characterized by σ = 76/008, σ = 13/163 and σ = 07/ 3 1 2 3 I) (Delvaux et al. 1997; Delvaux and Sperner 2003). The 254 and a stress regime index of 1.00 and a NNW-SSE type of stress regime can be expressed numerically using direction of maximum shortening, characterized the JT an index R', ranging from 0.0 to 3.0 and defined as fracture system (Fig. 6b). An oblique radial compressive follows: tensor with σ = 47/348, σ = 19/237 and σ =37/131 and a 1 2 3 R' = R when (σ is vertical; extensional stress regime) stress regime index of 3.00 and a NE-SW direction of R' = 2 - R when (σ is vertical; strike-slip stress regime) maximum shortening, characterized the gently dipping JS R' =2+R when (σ is vertical; compressional stress fracture system (Fig. 9b). Similarly at another location called regime). Amichi, analysis was carried out on 40 joints (extension Fig. 5 Stress tensor representation for different stress regimes. After Guiraud et al (1989) Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 7 of 15 Fig. 6 Cenomanian to Turonian tensors for calculated for three locations in the study area (a) Aboine River Akpoha (b) Amaseri Ridge (c) Amichi fractures) with a single strike-slip extensional tensor adjacent location (Asu-River Akpoha), two tensors were characterized by σ = 88/072, σ = 00/341 and σ = determined. The first tensor was calculated from 100 Joints 1 2 3 02/251 and a stress regime index of 1.00 and a NNW-SSE giving a Strike-slip extensional regime with parameters direction of maximum shortening, determined for the σ =73/023, σ = 07/136 and σ = 16/228 with a NW-SE 1 2 3 JT fracture system (Fig. 6c). direction of maximum shortening and a stress regime value Within the Asu River, Okposi Road, a single tensor was of 1.00 (Fig. 7b). The second tensor was calculated from all determined. The first tensor was calculated from two hun- the JS system joints combined together (43 planes). The dred and nine (209) Joints giving a Strike-slip extensional tensor parameters are σ =42/343, σ = 21/094 and 1 2 regime with parameters σ =78/295, σ = 09/152 and σ = σ = 41/204 with a WNW-ESE direction of maximum 1 2 3 3 07/061 with an SSE-NNW direction of maximum shortening and a stress regime value of 1.91 (Oblique radial shortening and a stress regime value of 0.99 (Fig. 7a). At an compressive) (Fig. 9c). Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 8 of 15 Fig. 7 Cenomanian to Turonian tensors for calculated for three locations in the area (a) Asu River Okposi RD (b) Asu River Akpoha (c) Aboine River Isinkoro Analysis was carried out at Aboine River, Isinkoro on 70 were determined from a total of 234 joints. The first joints with a single tensor determined with the parameters tensor, determined from 120 joints gave the following σ = 68/261, σ = 09/146 and σ = 19/053 and a stress parameters: σ = 64/280, σ = 15/155 and σ =20/060 1 2 3 1 2 3 regime index of 0.84 and a NW-SE direction of maximum and a stress regime index of 1.00 and a NNW-SSE shortening representing a Strike-slip extensional regime direction of maximum shortening representing a (Fig. 7c). At Abaomege-Ugep road, analysis was carried out Strike-slip extensional regime (Fig. 8b). on 22 joints with a single tensor determined with the The second tensor belonged to an oblique radial parameters σ = 69/305, σ = 18/160 and σ =11/066 and a compressive regime with parameters σ = 42/225, σ 1 2 3 1 2 stress regime index of 1.00 and a NNW-SSE direction of =07/321and σ =47/058 with a stress regime index maximum shortening representing a Strike-slip exten- of 2.67 and a NNW-SSE direction of maximum short- sional regime (Fig. 8a). At Asu-River, two tensors ening (Fig. 9a). Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 9 of 15 Fig. 8 Cenomanian to Turonian tensors for calculated for two locations in the area (a) Ugep Road (b) Asu River Within Afikpo Road area, two tensors were deter- settings despite some existing limitations. This work is mined from a total of 13 joints. The first tensor gave all the more useful because there is very little informa- the following parameters: σ = 33/072, σ = 21/176 tion about the link between instability and discontinu- 1 2 and σ =21/176 and a stress regime index of 2.30 and ities in the unstable regions of the country. a NNE-SSW direction of maximum shortening repre- From the results three major tensors can be charac- senting an oblique radial compressive regime (Fig. 9b). terized for the study area. They are directly related to The second tensor belonged to an oblique strike-slip the three fracture systems earlier described, with extensional regime with parameters σ = 42/225, σ = indications that these directions are a manifestation 1 2 07/321 and σ =47/058 with a stress regime index of of stress permutations in the region which are 1.37 and a WNW-ENE direction of maximum short- contemporaneous. Shearing along these fractures leads ening (Fig. 10b). Finally, at Itigidi, analysis was carried to deformational pathways that may be similar to out on 29 joints (extension fractures) only one tensor the mechanisms espoused in Scheidegger (1998) and was determined for the JC fracture system found in Grelle and Guadagno (2010). Interestingly, the domin- the area. This tensor is characterized by σ = 66/310, ant fracture orientations can be correlated to the σ = 22/107 and σ =08/200 with a stress regime general trends of the fractures that have created in- 2 3 index of 0.96 and an ESE-WNW direction of max- stability and predisposed the unstable slopes in the imum shortening (Fig. 10a). The tensor type is Strike- region to several failures. The knowledge gained from slip extensional or Transtensional. the unambiguous relationships among the fracture planes and the orientations of the stresses can be Discussion applied for the analysis of risks at specific areas of For a long time now, paleostress inversion techniques high instability such as the Iva valley in Enugu and have been successfully applied to various tectonic the Nigerian-Cameroon mountain range. Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 10 of 15 Fig. 9 Santonian tensors for calculated for three locations in the area (a) Aboine River (b) Amaseri Ridge (c) Asu River Akpoha Cenomanian-Turonian transtension Tethys through the Benue trough and the Termit A most prominent strike-slip extensional or transten- basin (Petters 1980). This dominant transtensional sional tensor is found in the pre-folding fractures stress regime is also likely related to Lead-Zinc and (JT) which are dated Cenomanian- Turonian (Fig. 11), Barite mineralization characteristic of the Benue with a general NE-SW maximum extension (SH ). trough. The mineralization has been established to MIN The period was a general period of rifting in the have occurred from the Albian to the Turonian, pos- Benue trough and the other basins of the WCARS sibly related to magmatic activity in that period. (Genik 1992; Guiraud and Maurin 1992; Fairhead et Most significant though is that the structural trend al 2013). The Turonian was also a time of maximum of these vein mineralization is strikingly similar the basin subsidence rates (Ojoh 1992) and eustatic sea fractured trend related to this stress regime (NNW- levels leading to a connection between the equatorial SSE and N-S) (Ezepue 1984; Etim et al 1988; atlantic (Petters 1980; Benkhelil et al 1989) and the Benkhelil 1989). Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 11 of 15 Fig. 10 Campanian to Maastrichtian tensors for two locations in the study area (a) MGIDI (b) Afikpo Road Fig. 11 Geologic map with Cenomanian to Turonian tensors Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 12 of 15 Fig. 12 Geologic map of the study area with Santonian tensors shown Santonian transpression are also seen to be related to deformation bands and The second tensor is an oblique radial compressive probably to nearby faulting. These normal faults have regime with a slight strike-slip (transpressive) compo- been already established to post-date the folding nent (Fig. 12). This tensor is related to the folding episode. The post santonian was one of a return to (SH directions are perpendicular to the fold axes) tension stress regimes (Benkhelil 1986; Guiraud et al MAX and probably to the weakly develop sub-vertical axial 1992; Guiraud and Bosworth 1997). This could be fracture cleavage seen in some of the shales. The attributed to the flexuring of the south-eastern and Santonian phase is one of compression and structural western flanks of the folded and uplifted southern inversion In other parts of the southern Benue trough Benue trough into the Afikpo and Anambra Synclinoria the Santonian phase is marked by intense folding and also stress release which followed the santonian and low grade regional metamorphism with the devel- inversion stage. The tension fractures therefore show opment of sub-vertical axial cleavage which is poorly transtensional tensors with a NNW-SSE direction of developed in the study area as NE-SW fracture cleav- maximum extension. The tensional regime is also marked age (Benkhelil 1989; Guiraud 1993). by a peak in magmatic activity and intrusion of Campa- nian to Maastrichtian sills and minor intrusion in the Afikpo to Ugep region. A set of Normal faults also cut Campanian-Maastrichtian transtension into the Turonian to Campanian Sandstones and shales The third tensor is also transtensional but with a with evidence of synsedimentary deformation. The pre- change in direction from the typical NE-SW (Fig. 13) and post-santonian tectonic models of the area created to NNE-SSW maximum extension (SHMIN). This from the study indicate the zones of potential instability tensor was calculated from fractures that are seen to (Figs. 14 and 15). It is understood that areas of in- post-date the folding episode and are the youngest creased instability are the areas that have probably system of fractures (JC). This fracturing is related to experienced faulting, folding, and has several discon- the major lineament directions of the Anambra basin tinuities. Using this knowledge therefore, it will be and significantly the trend of the major dolerite sill possible to predict that the areas of frequent landslide intruding the Eze-aku shale which is Campanian- activity within the Trough. Maastrichtian in age (Benkhelil 1986). The fractures Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 13 of 15 Fig. 13 Campanian to Maastrichtian tensors on the geologic map of the study area Conclusions analysis using the software TENSOR™ have enabled three This research undertook the structural characterization stress regime phases to be characterized for the study of small and large‐scale discontinuities to properly area from the Cenomanian to the Maastrichtian. A ceno- understand their roles as potential precursors of instabil- manian to Coniacian transtensional phase, a Santonian ity. Detailed field-based structural and paleostress transpressional phase and a Campanian to Maastrichtian Fig. 14 Pre-Santonian schematic model for the study area Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 14 of 15 Fig. 15 Post tectonic schematic model for the study area transtensional phase. These stress regimes are related to not only weakened the rocks but are also now being regional plate scale tectonics affecting the Benue Trough exploited by factors aggravating failure tendencies. as a whole. Interestingly, the dominant stress orienta- Finally, this work will enable the prediction of the tions correspond to the reported orientations of the likely fractural trends in any area within the Trough, and fractures predisposing slopes to catastrophic failures in may aid the development of sustainable disaster manage- the region; which are indications that the fractures origin ment and risk reduction strategies. is related to the regional paleostress history of the Benue Trough. Acknowledgements The authors wish to acknowledge the support of the staff and post-graduate A most prominent strike-slip extensional or transten- students of the Department of Geology, University of Nigeria, Nsukka during sional tensor, with a general NE-SW maximum extension and after data collection. We would like to thank Dr A. W. Mode who is the (SH ) is strikingly similar the fractural trend related to coordinator of the petroleum trust fund activities in the University for his MIN Support. major episodes of landslide activity in the region. Additionally, The third tensor which is transtensional Authors’ contribution but with a change in direction from the typical NE-SW to OI conceived, designed, modified and approved the research project. OI also NNE-SSW maximum extension (SHMIN) are fracture examined the data, validated the results of analysis, interpreted the data, and drafted the manuscript. IAO was my MSc, and Ph.D student. This manuscript orientations related to deformation bands and probably to was part of IAO MSc and Ph.D work under the supervision of OI. IAO collected nearby faulting, which are all signs of instability. the field data, made substantial contribution in data analysis and interpretation, Furthermore, the paleostress analysis has aided the created the maps and Figures, was involved in the design and scoping of the project, edited the draft manuscript. Both authors read and approved the final production of accurate pre- and post- tectonic models manuscript. of the area which can be used as a reference in future stress analysis and interpretation. It is understood Competing interests that areas of increased instability are the areas that The authors declare that they have no competing interests. have probably experienced faulting, folding, intru- Received: 15 July 2016 Accepted: 15 September 2016 sions, and are criss-crossed by several discontinuities. Using this knowledge therefore, it will be possible to predict the areas of frequent landslide activity within the Trough References are the areas. Before the tectonic activities, the Trough Angelier, J. 1994. Fault slip analysis and paleostress reconstruction. Continental Deformation, 53–100. Oxford: Pergamon Press. seemed generally stable. This stability appeared to have Aucelli, PPC, E Casciello, M Cesarano, SP Zampelli, and CM Rosskopf. 2013. been lost following the Santonian tectonic activity A deep, stratigraphically and structurally controlled landslide: the case of which created instability pathways. These pathways Mount La Civita (Molise, Italy). Landslides 10(5): 645–656. Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 15 of 15 Benkhelil, J. 1982. Benue trough and Benue chain. Geological Magazine Igwe, O. 2015b. Predisposing factors and the mechanisms of rainfall-induced 119(02): 155–168. slope movements in Ugwueme South-East Nigeria. Bulletin of Engineering Benkhelil, J. 1986. Structure and geodynamic evolution of the intracontinental Geology and the Environment. doi:10.1007/s10064-015-0767-0. 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Landslides and their control, 205. Prague: on landslides: a methodological approach for computer-aided mapping Elsevier-Academia. analysis. Natural Hazards and Earth System Sciences 11: 1395–1409. Guiraud, M. 1993. Late Jurassic rifting-early Cretaceous rifting and late Cretaceous transpressional inversion in the upper Benue basin (NE Nigeria). Bulletin Des Centres de Recherches Exploration-Production Elf Aquitaine 17(2): 371–383. Guiraud, M, O Laborde, and H Philip. 1989. Characterization of various types of deformation and their corresponding deviatoric stress tensors using microfault analysis. Tectonophysics 170(3–4): 289–316. Submit your manuscript to a Guiraud, R, RM Binks, JD Fairhead, and M Wilson. 1992. Chronology and journal and benefi t from: geodynamic setting of Cretaceous-Cenozoic rifting in West and Central Africa. Tectonophysics 213(1): 227–234. 7 Convenient online submission Guiraud, R, and W Bosworth. 1997. Senonian basin inversion and rejuvenation of 7 Rigorous peer review rifting in Africa and Arabia: synthesis and implications to plate-scale tectonics. Tectonophysics 282(1–4): 39–82. 7 Immediate publication on acceptance Guiraud, R, and JC Maurin. 1992. Early cretaceous rifts of Western and Central 7 Open access: articles freely available online Africa: an overview. Tectonophysics 213(1–2): 153–168. 7 High visibility within the fi eld Hancock, PL. 1985. Brittle microtectonics: principles and practice. Journal of 7 Retaining the copyright to your article Structural Geology 7(3-4): 437–457. Igwe, O. 2015a. The geotechnical characteristics of landslides on the sedimentary and metamorphic terrains of South-East Nigeria, West Africa. Submit your next manuscript at 7 springeropen.com Geoenvironmental Disasters. doi:10.1186/s40677-014-0008-z. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

Application of paleostress analysis for the identification of potential instability precursors within the Benue Trough Nigeria

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Environment; Environment, general; Earth Sciences, general; Geography, general; Geoecology/Natural Processes; Natural Hazards; Environmental Science and Engineering
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Abstract

Background: Structures such as faults, joints and fractures of diverse patterns have acted as precursors of several slope instability cases within the Benue Trough Nigeria. In some cases, the structures by their nature weakened and also created avenues that streams took advantage to further destabilize the rock slopes. In other cases, structure orientation played significant roles in the mobility and eventual runout distance of debris flow and avalanches in the region. Detailed field-based structural, fracture and paleostress analyses were therefore carried out to determine the fractural patterns that correlate to reported instability and landslide cases in the region; and to produce models that reveal areas with heightened risk. Results: Three fracture sets were isolated from analysis of fracture orientations and field relationships: Pre-folding (JT), Syn-Folding (JS) and Post Folding (JC) fracture systems. Paleostress analysis carried out on these fracture systems using the TENSOR™ software tool yielded three paleostress tensors corresponding to transtensional stress tensor with ENE-WSW direction of maximum extension (S ), oblique compressive (transpressional) tensor with HMIN NW-SE direction of maximum shortening (S ), and transtensional tensor with WNW-ESE direction of maximum HMAX extension (S ). HMIN Conclusion: These tensors are related to the prevailing plate tectonic stress regimes affecting the entire Benue trough and the West and Central African Rift System (WCARS). Our pre- and post-tectonic models have revealed the reasons for instability and the likely places where future failures may be located. This is the first such analyses in the region and it is hoped that the results can broaden the use and applicability of paleostresses in failure-prone terrains for future risk and disaster reduction/assessment within the Trough and in other areas prone to structure- controlled landslides disaster. Keywords: Paleostress, Instability, Landslides disaster, WCARS, Transtension, Transpression Background In 2010, a rock-debris avalanche, unprecedented in Geologic structures have been reported as precursors scale and form, occurred on the hillslopes bounding and control of several medium to large-scale rainfall- Nigeria and Cameroon. Igwe et al (2015) reported the induced landslides within the Benue Trough Nigeria avalanche (Fig. 2) initiated as distinct slides on two (Fig. 1) and the Cameroon Line (Igwe 2015a,b; Igwe et al slopes weakened by ubiquitous fractures. The observed 2015; Igwe et al 2016). These and other slope move- surface displacements and predicted mechanisms of ments cause considerable loss of resources in a country movement indicated that the slides started from differ- where poverty and sundry socio-cultural circumstances ent blocks at speed between 8 and 20 km/h. Soon after rarely permit the implementation of disaster/risk reduc- however, a structurally-controlled coalescing of the two tion strategies. slides, the subsequent movement of the coalesced mass slope along an expanded fracture surface, and the flow of water along the same fracture system aided a quick * Correspondence: Ogbonnaya.igwe@unn.edu.ng; igwejames@hotmail.com transformation to a highly mobile mass movement that Department of Geology, Faculty of Physical Sciences, University of Nigeria, attained speed between 55 and 80 km/h. Half way down Nsukka, Nigeria © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 2 of 15 Fig. 1 Location and geological attributes of the Benue Trough the slope, the moving masses were transferred to a sur- the basal lithologic units comprising the slopes. The oc- face with numerous foliations that were perpendicular to currence of fractures and their study are therefore import- the direction of movement which enhanced mobility ant not only in slope stability risk assessment but also in (>110 km/h) until reaching a distance of over 2.5 km disaster reduction and management. The understanding where gradual deposition commenced. Several lives, of the stress orientation will improve the knowledge of de- acres of land, farms, trees and economic treasures were formation mechanisms, which is crucial for the imple- lost during the episode. mentation of a viable monitoring system. Similarly, Igwe (2015b) described a fractured slope in Irfan (1999), Revellino et al (2010), Grelle et al (2011), which streams took advantage of discontinuities to trigger Aucelli et al. (2013), Prakash et al (2015) have reported a landslide in an area without any previous history of fail- structurally-controlled landslides. Investigation of the land- ure. It became obvious afterwards that this particular case slides revealed that the geo-structural settings predisposed was clearly a case of a disaster waiting to happen because the slopes to factors triggering mass movements, which is researchers had not observed the myriad of fractures in consistent with Fookes and Wilson (1966), Zaruba and Mencl (1969), and Varnes (1978). A structurally-controlled landslide is also documented in Luzon et al (2016) where it was reported that the 2006 rockslide-debris avalanche in Southern Leyte, one of the largest known landslides in the Philippines in recent history, occurred on a weakened slope at an area where there was continuous movement along the Philippine Fault. The characteristics and mechanisms of the Leyte landslides reported in Sassa et al (2004) and Catane et al (2008) are similar to those of the Nigeria- Cameroon border avalanche. Brittle fractures are the consequences of the action of stresses on a macroscopic scale. A rock body subject to a known stress regime (that produces fractures) has an unambiguous relationship among the fracture planes and the orientations of the stresses. This concept can then be used to reconstruct the orientation of forces that created Fig. 2 The 2010 rock-debris avalanche showing the source and a the fractures that were active in the past based on present part of the landslide toe where the researcher is standing day orientations. To fully understand the applicability of Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 3 of 15 paleostress technique in risk assessment, it is neces- The study area falls within the southern part of the sary to analyze ancient stress regimes in the context Benue trough (Fig. 1) a 1000 km long northeast trending of their role as potential precursory agents. Kayen et intracontinental structure stretching from the beneath al. (2011) noted that stress analysis is a useful and the Niger Delta and Anambra Basins to the south to popular tool for structural and seismological ele- the Chad Basin in the North (Benkhelil 1989; Ofoegbu ments. Kaymakci (2006) reported that the state of and Onuoha 1990; Ojoh 1992; Nwajide 2013). It forms stress in rocks is generally anisotropic and is defined a part of the eastern flank of the Abakaliki synclinor- by stress ellipsoid axes, which characterize the magni- ium which forms the major structural unit in the tudes of the principal stresses. The paper determined Southern Benue trough. The synclinal structure is Paleostress orientations and relative paleostress mag- formed by Albian to Coniacian sediments folded in the nitudes (stress ratios) using the reduced stress con- Santonian (Fig. 3). The folds are generally open to gen- cept for the purpose of improving the understanding tle and asymmetrical with asouthwest plunge. In the of the kinematic characteristics of a Basin. core of the synclinorium to the south are deposited At the moment, there is no previous paleostress Campanian and younger clastic sediments belonging to study of the study area, which has in part hindered the Anambra basin formed after the Santonian folding knowledge of the potential instability precursors in episode (Nwajide and Reijers 1996; Obi and Okogbue the zones of frequent slope failures. Even though a 2004). century of geological study has enabled an extensive The stratigraphy of the Afikpo synclinorium is simi- understanding the geology of the Benue Trough, it lar to the southern Benue trough as a whole (Fig. 4). was only in the later part of the 20th century that a With predominantly clastic shallow marine deposition picture of the structural framework, within which the with cycles of transgressions and regressions. Detailed trough evolved, began to emerge. The controversies descriptions of the stratigraphy of the Benue Trough surrounding the tectonic evolution of the Benue have been addressed by Ojoh (1992) and Nwajide trough have been largely resolved; with the over- (2013). whelming evidence leaning towards the interpretation of the Benue trough as a collection of wrench related Methods pull apart basins related to transcurrent movement Fracture analysis along deep-seated oceanic transform faults (Benkhelil Fracture data were obtained from 844 fractures from 1982, 1989; Guiraud et al 1989; M. Guiraud 1993). ten locations (Table 1). The area sampled was kept The evolution of the basin has also been incorporated small enough (1000–2500 m2) in order to guarantee into a framework of genetically related basins in west homogeneity of results (Delvaux and Sperner 2003). and central Africa: The West and Central African Rift Data type obtained include attitude (Strike, dip and System (WCARS) (Binks and Fairhead 1992; Genik dip direction) of the fracture plane as well as nature 1992; Guiraud et al 1992; Guiraud and Maurin 1992; of fracture surface, cross-cutting relations, mean sep- Fairhead et al 2013;). There is only limited field based aration between the fractures, bedding orientation structural studies especially in the central and south- and relationship and others which may be of use in ern parts due to the nature of the units which do not determining the relative age relationships between allow for preservation, and the tropic climate which the fractures and dividing them into fracture sets makes for a difficult terrain to carry out detailed and systems. Three sets of fractures were observed a structural studies necessary to obtain information steeply dipping NNW-SSE pre-Fold system of frac- which could be used to deduce structural regimes tures (JT), a NE-SW syn-folding fracture set with that can be correlated to the precursor factors re- lower dips (JS), and a WNW-ESE set of post folding ported in several slope failures within the trough, and fractures predominant in the Post-folding sediments data required for the various methods of stress inver- (JC). These fracture systems were used to define the sion (Benkhelil 1986). initial subsets used in paleostress inversion of the fractures to obtain the reduced stress tensor. Geologic and stratigraphic setting The Afikpo synclinorium, forming a part of the Southern Paleostress inversion Benue Trough (Fig. 1), offers a unique opportunity to The most common and extensively used method of study and understand the deformational processes and stress inversion typically involves use of faults with to determine the tectonic stresses active in the southern slickenlines that record the direction of slip relative to Benue trough as the highly indurated nature of the sedi- the fault plane (Hancock 1985; Angelier 1994; Ramsay ments allow for a relative abundance of outcrops where and Lisle 2000). Their use is based on the Wallace-Bott structural data useful for inversion could be collected. hypothesis which states that the slip on a planar Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 4 of 15 Fig. 3 Cross sections taken in different directions across the study area showing rock history and arrangement Fig. 4 Stratigraphic synopsis of the Southern Benue Trough and Anambra basin (After Ojoh 1992 and Nwajide 2013) Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 5 of 15 Table 1 A summary of discontinuities’ characterization in the study area Site ID Locality Rock type Fracture set No. of fractures Strike Dip Average fracture spacing (m) Fracture infilling 002 Itigidi Sandstone JC-1 30 300–310 80–90 1.5 Ferruginized 006 Aboine River Akpoha Shale JT-1 38 340–355 80–90 0.1 None JT-2 8 310–320 85–90 0.3 JT-4 3 260–270 80–90 2.0 008 Amaseri Ridge Sandstone JT-1 5 330–360 80–90 0.7 None JS-1 9 045–060 45–60 1.5 012 Ohaozara Sandstone JT-1 40 330–350 85–90 0.15 None 013 Asu River Ohaozara Shale JT-1 156 330–360 70–90 0.1 None JT-2 50 300–330 55–80 0.1 JT-3 3 290–300 85–90 0.1 JS-1 11 040–060 25–30 0.6 45–60 70–75 JS-2 10 060–080 60–65 0.55 80–85 014 Asu River Akpoha Shale JT-2 100 310–330 70–80 0.05 None JS-1 13 40–50 30–35 2.0 55–60 JS-4 29 280–300 40–45 1.6 55–60 017 Aboine River Isinkoro Shale JT-1 18 355–050 90 0.2 None JT-2 51 310–340 70–75 0.2 JT-3 1 290–300 75–80 0.5 021 Aba-Omega Ugep Road Sandstone JT-1 12 350–360 70–75 0.2 None JT-2 9 320–330 80–85 0.2 026 Asu River Shale JT-2 28 330–340 70–80 0.2 None JT-2 57 320–340 75–80 0.3 JS-6 55 340–350 45–50 0.3 JS-5 59 310–320 30–35 0.2 JT-1 35 330–340 60–65 0.2 027 Afikpo Road Sandstone JS-3 5 020–030 40–45 1.0 Ferruginized JC-1 8 280–290 55–60 1.0 structure is assumed to occur parallel to the greatest re- deformational event, that the rocks themselves are fairly solved shear stress (Bott 1959). Similar assumptions can homogenous, the fractures do not significantly perturb be made for extension fractures (e.g. Joints) and contrac- the stress field in their vicinity and also that the struc- tional (e.g. Stylolites) fractures -that they form perpen- tures have not rotated significantly since their initiation dicular or at a high angle to the minimum (σ ) and (Ramsay and Lisle 2000). maximum (σ ) principal stress direction respectively- or The aim of paleostress inversion is to characterize for conjugate shear fractures where the maximum what is known as the reduced stress tensor. The re- principal stress (σ ) bisects the acute angle between the duced stress tensor has four parameters of the six conjugate planes while the minimum principal (σ ) stress needed to define the full stress tensor: the principal bisects the obtuse angle between the fracture planes. stress axes σ (maximum), σ (intermediate) and σ 1 2 3 The structures can be used separately or collectively to (minimum) and the ratio of principal stress differ- constrain the stress field that led to their formation. The ences, R =(σ − σ )/(σ − σ ). The parameter R defines 2 3 1 1 assumption being that the fractures formed in the same the shape of the stress ellipsoid. Only the directions homogenous stress field i.e. related to the same of the principal stresses (known as Euler angles) can Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 6 of 15 be determined for the stress tensor from inversion. The index R' defines the stress regime completely and Their relative magnitudes are represented by the is convenient for computing the mean regional stress fourth parameter R. The two additional parameters of regime from a series of individual stress tensors in a the full stress tensor arc the ratio of extreme princi- given area (Benkhelil et al 1989). On structural maps, pal stress magnitudes (σ /σ )and theisotropic com- the stress tensors are displayed with the orientation of 1 3 ponent of the stress tensor (the Mean stress), but both horizontal principal stress (SHmax) and horizontal these cannot be determined from fracture data only. minimum stress axes (SHmin) as recommended by The methods of paleostress inversion are numerical and Guiraud et al (1989) (Fig. 5). currently involve the use of computer programmes to statistically analyse fracture data in order to characterize Results the stress field responsible for them (Etchecopar et al The stress fields were determined for the three joint 1981; Angelier 1994; Ramsay and Lisle 2000; Delvaux and systems based on field-based age relationship criteria. Sperner 2003; Célérier et al. 2012). This study makes use The tensors were determined after applying the Right of TENSOR™ program (Delvaux 1993; Delvaux et al. 1997; Dihedron and Rotational Optimization described Delvaux and Sperner 2003). This program is a tool above. for controlled interactive separation of fault slip or focal At a location named Aboine River, Akpoha, two mechanism data and progressive stress tensor optimization tensors were determined. The first tensor was calcu- using successively the Right Dihedron method and the lated from 46 Joints belonging to the JT fracture Rotational Optimization method. Detailed explanation of system, giving a Strike-slip extensional regime with how these methods are utilized in TENSOR can be found parameters σ =12/150, σ = 12/150 and σ = 00/060 1 2 3 in Delvaux et al. (1997) and Delvaux and Sperner (2003). with an NNW-SSE direction of maximum shortening The stress regime is determined by the nature of the and a stress regime value of 1.00 (Fig. 6a). A single vertical stress axes: extensional when σ is vertical, strike- conjugate shear fracture yielded a pure compression slip when σ is vertical and compressional when σ is tensor with parameters σ = 00/337, σ = 20/067 and 2 3 1 2 vertical. The stress regimes also vary, within these three σ = 70/247 with a stress regime index of 2.50 and a main types, as a function of the stress ratio R : Radial NNW-SSE direction of maximum shortening. Being extension (σ vertical, 0 < R <0.25), Pure extension (σ verti- the only shear fracture data it is considered unreliable 1 1 cal, 0.25 < R <0.75), Transtension (σ vertical, 0.75 < R <1 andrankedE(Fig. 9a). or σ vertical, 1 > R >0.75), Pure strike-slip (σ vertical, At a location named Amaseri Ridge, analysis was 2 2 0.75 > R >0.25),Transpression (σ vertical, 0.25 > R > 0 or σ carried out on 14 joints (extension fractures) with two 2 3 vertical, 0 < R < 0.25), Pure compression (σ , vertical, 0.25 tensors determined. A strike-slip extensional tensor <R <0.75) and Radial compression (σ vertical, 0.75 < R < characterized by σ = 76/008, σ = 13/163 and σ = 07/ 3 1 2 3 I) (Delvaux et al. 1997; Delvaux and Sperner 2003). The 254 and a stress regime index of 1.00 and a NNW-SSE type of stress regime can be expressed numerically using direction of maximum shortening, characterized the JT an index R', ranging from 0.0 to 3.0 and defined as fracture system (Fig. 6b). An oblique radial compressive follows: tensor with σ = 47/348, σ = 19/237 and σ =37/131 and a 1 2 3 R' = R when (σ is vertical; extensional stress regime) stress regime index of 3.00 and a NE-SW direction of R' = 2 - R when (σ is vertical; strike-slip stress regime) maximum shortening, characterized the gently dipping JS R' =2+R when (σ is vertical; compressional stress fracture system (Fig. 9b). Similarly at another location called regime). Amichi, analysis was carried out on 40 joints (extension Fig. 5 Stress tensor representation for different stress regimes. After Guiraud et al (1989) Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 7 of 15 Fig. 6 Cenomanian to Turonian tensors for calculated for three locations in the study area (a) Aboine River Akpoha (b) Amaseri Ridge (c) Amichi fractures) with a single strike-slip extensional tensor adjacent location (Asu-River Akpoha), two tensors were characterized by σ = 88/072, σ = 00/341 and σ = determined. The first tensor was calculated from 100 Joints 1 2 3 02/251 and a stress regime index of 1.00 and a NNW-SSE giving a Strike-slip extensional regime with parameters direction of maximum shortening, determined for the σ =73/023, σ = 07/136 and σ = 16/228 with a NW-SE 1 2 3 JT fracture system (Fig. 6c). direction of maximum shortening and a stress regime value Within the Asu River, Okposi Road, a single tensor was of 1.00 (Fig. 7b). The second tensor was calculated from all determined. The first tensor was calculated from two hun- the JS system joints combined together (43 planes). The dred and nine (209) Joints giving a Strike-slip extensional tensor parameters are σ =42/343, σ = 21/094 and 1 2 regime with parameters σ =78/295, σ = 09/152 and σ = σ = 41/204 with a WNW-ESE direction of maximum 1 2 3 3 07/061 with an SSE-NNW direction of maximum shortening and a stress regime value of 1.91 (Oblique radial shortening and a stress regime value of 0.99 (Fig. 7a). At an compressive) (Fig. 9c). Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 8 of 15 Fig. 7 Cenomanian to Turonian tensors for calculated for three locations in the area (a) Asu River Okposi RD (b) Asu River Akpoha (c) Aboine River Isinkoro Analysis was carried out at Aboine River, Isinkoro on 70 were determined from a total of 234 joints. The first joints with a single tensor determined with the parameters tensor, determined from 120 joints gave the following σ = 68/261, σ = 09/146 and σ = 19/053 and a stress parameters: σ = 64/280, σ = 15/155 and σ =20/060 1 2 3 1 2 3 regime index of 0.84 and a NW-SE direction of maximum and a stress regime index of 1.00 and a NNW-SSE shortening representing a Strike-slip extensional regime direction of maximum shortening representing a (Fig. 7c). At Abaomege-Ugep road, analysis was carried out Strike-slip extensional regime (Fig. 8b). on 22 joints with a single tensor determined with the The second tensor belonged to an oblique radial parameters σ = 69/305, σ = 18/160 and σ =11/066 and a compressive regime with parameters σ = 42/225, σ 1 2 3 1 2 stress regime index of 1.00 and a NNW-SSE direction of =07/321and σ =47/058 with a stress regime index maximum shortening representing a Strike-slip exten- of 2.67 and a NNW-SSE direction of maximum short- sional regime (Fig. 8a). At Asu-River, two tensors ening (Fig. 9a). Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 9 of 15 Fig. 8 Cenomanian to Turonian tensors for calculated for two locations in the area (a) Ugep Road (b) Asu River Within Afikpo Road area, two tensors were deter- settings despite some existing limitations. This work is mined from a total of 13 joints. The first tensor gave all the more useful because there is very little informa- the following parameters: σ = 33/072, σ = 21/176 tion about the link between instability and discontinu- 1 2 and σ =21/176 and a stress regime index of 2.30 and ities in the unstable regions of the country. a NNE-SSW direction of maximum shortening repre- From the results three major tensors can be charac- senting an oblique radial compressive regime (Fig. 9b). terized for the study area. They are directly related to The second tensor belonged to an oblique strike-slip the three fracture systems earlier described, with extensional regime with parameters σ = 42/225, σ = indications that these directions are a manifestation 1 2 07/321 and σ =47/058 with a stress regime index of of stress permutations in the region which are 1.37 and a WNW-ENE direction of maximum short- contemporaneous. Shearing along these fractures leads ening (Fig. 10b). Finally, at Itigidi, analysis was carried to deformational pathways that may be similar to out on 29 joints (extension fractures) only one tensor the mechanisms espoused in Scheidegger (1998) and was determined for the JC fracture system found in Grelle and Guadagno (2010). Interestingly, the domin- the area. This tensor is characterized by σ = 66/310, ant fracture orientations can be correlated to the σ = 22/107 and σ =08/200 with a stress regime general trends of the fractures that have created in- 2 3 index of 0.96 and an ESE-WNW direction of max- stability and predisposed the unstable slopes in the imum shortening (Fig. 10a). The tensor type is Strike- region to several failures. The knowledge gained from slip extensional or Transtensional. the unambiguous relationships among the fracture planes and the orientations of the stresses can be Discussion applied for the analysis of risks at specific areas of For a long time now, paleostress inversion techniques high instability such as the Iva valley in Enugu and have been successfully applied to various tectonic the Nigerian-Cameroon mountain range. Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 10 of 15 Fig. 9 Santonian tensors for calculated for three locations in the area (a) Aboine River (b) Amaseri Ridge (c) Asu River Akpoha Cenomanian-Turonian transtension Tethys through the Benue trough and the Termit A most prominent strike-slip extensional or transten- basin (Petters 1980). This dominant transtensional sional tensor is found in the pre-folding fractures stress regime is also likely related to Lead-Zinc and (JT) which are dated Cenomanian- Turonian (Fig. 11), Barite mineralization characteristic of the Benue with a general NE-SW maximum extension (SH ). trough. The mineralization has been established to MIN The period was a general period of rifting in the have occurred from the Albian to the Turonian, pos- Benue trough and the other basins of the WCARS sibly related to magmatic activity in that period. (Genik 1992; Guiraud and Maurin 1992; Fairhead et Most significant though is that the structural trend al 2013). The Turonian was also a time of maximum of these vein mineralization is strikingly similar the basin subsidence rates (Ojoh 1992) and eustatic sea fractured trend related to this stress regime (NNW- levels leading to a connection between the equatorial SSE and N-S) (Ezepue 1984; Etim et al 1988; atlantic (Petters 1980; Benkhelil et al 1989) and the Benkhelil 1989). Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 11 of 15 Fig. 10 Campanian to Maastrichtian tensors for two locations in the study area (a) MGIDI (b) Afikpo Road Fig. 11 Geologic map with Cenomanian to Turonian tensors Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 12 of 15 Fig. 12 Geologic map of the study area with Santonian tensors shown Santonian transpression are also seen to be related to deformation bands and The second tensor is an oblique radial compressive probably to nearby faulting. These normal faults have regime with a slight strike-slip (transpressive) compo- been already established to post-date the folding nent (Fig. 12). This tensor is related to the folding episode. The post santonian was one of a return to (SH directions are perpendicular to the fold axes) tension stress regimes (Benkhelil 1986; Guiraud et al MAX and probably to the weakly develop sub-vertical axial 1992; Guiraud and Bosworth 1997). This could be fracture cleavage seen in some of the shales. The attributed to the flexuring of the south-eastern and Santonian phase is one of compression and structural western flanks of the folded and uplifted southern inversion In other parts of the southern Benue trough Benue trough into the Afikpo and Anambra Synclinoria the Santonian phase is marked by intense folding and also stress release which followed the santonian and low grade regional metamorphism with the devel- inversion stage. The tension fractures therefore show opment of sub-vertical axial cleavage which is poorly transtensional tensors with a NNW-SSE direction of developed in the study area as NE-SW fracture cleav- maximum extension. The tensional regime is also marked age (Benkhelil 1989; Guiraud 1993). by a peak in magmatic activity and intrusion of Campa- nian to Maastrichtian sills and minor intrusion in the Afikpo to Ugep region. A set of Normal faults also cut Campanian-Maastrichtian transtension into the Turonian to Campanian Sandstones and shales The third tensor is also transtensional but with a with evidence of synsedimentary deformation. The pre- change in direction from the typical NE-SW (Fig. 13) and post-santonian tectonic models of the area created to NNE-SSW maximum extension (SHMIN). This from the study indicate the zones of potential instability tensor was calculated from fractures that are seen to (Figs. 14 and 15). It is understood that areas of in- post-date the folding episode and are the youngest creased instability are the areas that have probably system of fractures (JC). This fracturing is related to experienced faulting, folding, and has several discon- the major lineament directions of the Anambra basin tinuities. Using this knowledge therefore, it will be and significantly the trend of the major dolerite sill possible to predict that the areas of frequent landslide intruding the Eze-aku shale which is Campanian- activity within the Trough. Maastrichtian in age (Benkhelil 1986). The fractures Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 13 of 15 Fig. 13 Campanian to Maastrichtian tensors on the geologic map of the study area Conclusions analysis using the software TENSOR™ have enabled three This research undertook the structural characterization stress regime phases to be characterized for the study of small and large‐scale discontinuities to properly area from the Cenomanian to the Maastrichtian. A ceno- understand their roles as potential precursors of instabil- manian to Coniacian transtensional phase, a Santonian ity. Detailed field-based structural and paleostress transpressional phase and a Campanian to Maastrichtian Fig. 14 Pre-Santonian schematic model for the study area Igwe and Okonkwo Geoenvironmental Disasters (2016) 3:17 Page 14 of 15 Fig. 15 Post tectonic schematic model for the study area transtensional phase. These stress regimes are related to not only weakened the rocks but are also now being regional plate scale tectonics affecting the Benue Trough exploited by factors aggravating failure tendencies. as a whole. Interestingly, the dominant stress orienta- Finally, this work will enable the prediction of the tions correspond to the reported orientations of the likely fractural trends in any area within the Trough, and fractures predisposing slopes to catastrophic failures in may aid the development of sustainable disaster manage- the region; which are indications that the fractures origin ment and risk reduction strategies. is related to the regional paleostress history of the Benue Trough. Acknowledgements The authors wish to acknowledge the support of the staff and post-graduate A most prominent strike-slip extensional or transten- students of the Department of Geology, University of Nigeria, Nsukka during sional tensor, with a general NE-SW maximum extension and after data collection. We would like to thank Dr A. W. Mode who is the (SH ) is strikingly similar the fractural trend related to coordinator of the petroleum trust fund activities in the University for his MIN Support. major episodes of landslide activity in the region. Additionally, The third tensor which is transtensional Authors’ contribution but with a change in direction from the typical NE-SW to OI conceived, designed, modified and approved the research project. OI also NNE-SSW maximum extension (SHMIN) are fracture examined the data, validated the results of analysis, interpreted the data, and drafted the manuscript. IAO was my MSc, and Ph.D student. This manuscript orientations related to deformation bands and probably to was part of IAO MSc and Ph.D work under the supervision of OI. IAO collected nearby faulting, which are all signs of instability. the field data, made substantial contribution in data analysis and interpretation, Furthermore, the paleostress analysis has aided the created the maps and Figures, was involved in the design and scoping of the project, edited the draft manuscript. Both authors read and approved the final production of accurate pre- and post- tectonic models manuscript. of the area which can be used as a reference in future stress analysis and interpretation. It is understood Competing interests that areas of increased instability are the areas that The authors declare that they have no competing interests. have probably experienced faulting, folding, intru- Received: 15 July 2016 Accepted: 15 September 2016 sions, and are criss-crossed by several discontinuities. Using this knowledge therefore, it will be possible to predict the areas of frequent landslide activity within the Trough References are the areas. 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