Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 Structural Analysis of Recent Seismic Fissures and Neotectonics In Yemen Mustafa R. Salih Al-Ubaidi Dep. of Geology, College of Sciences, University of Baghdad, Iraq [email protected] Abstract Recent seismic fissures in various trends are distinguished in the alluvium covering and the bedrocks of many basins in Yemen such as Amran Basin. Obviously, these fissures are systematic and signal continuous propagation in the area. These fissures in the Jahran basin as an example, have been analyzed to know the new mechanism style and paleostress systems that have prevalent in the Yemen area. These fractures have be caused by shear and extensional stresses with faults propagated in this basin. These groups of fissures serve as the geometrical classification applied with the ancient groups that are spread in the Tertiary and Quaternary Volcanic Zone, indicating that the fault activities are being reactivated, and this activity is the result of the recent tectonic and volcanic processes in the southern parts of the Red Sea. Keywords: Jahran Basin, recent fissures, paleostress, new activities, slip الخالصة حدوث تشققات زلزالية حديثة في اتجاهات مختلفة قطعت الترسبات والصخور في العديد من االحواض اليمنية مثل حوض تم تحليل هذه التشققات في حوض. ومن الواضح أن هذه التشققات كانت عبارة عن شقوق نظامية مستمرة االنتشار في المنطقة.عمران أن هذه.عمران كمثال للتعرف على نمط اآللية الميكانيكية للضغوط الحديثة وانظمة االجهادات القديمة التي تعرضت لها االرض اليمنية هذه المجموعات من الشقوق تم تصنيفها هندسيا.الكسور ناجمة عن قوى قصية وتمددية مصاحبة للصدوع المنتشرة في هذا الحوض حيث تشير إلى أن الصدوع االولية قد،ومطابقتها مع مجموعة الكسور القديمة التي تنتشر في كال من االنطقة البركانية الثالثية والرباعية . وهذا النشاط هو نتيجة للعمليات التكتونية والبركانية التي حدثت مؤخ ار في االجزاء الجنوبية من البحر األحمر، اعادة نشاطها . االزاحة, النشاط التكتوني الحديث, االجهادات القديمة, التشققات الحديثة, حوض جهران: الكلمات المفتاحية 1. Introduction The recent seismic and tectonics are showing today in different areas in Yemen (e.g. the recent volcanic activities in Jabal Alteer in the Red Sea (30/9/2007) and volcanic activities in Zabed Island in the Red Sea in 2011, new fissures in many basins in the last 10 years and many land-sliding incidences reported in various areas). According to Plate tectonic theory, Yemen is located in the south west of Arabian Plate. So, the movement of the Arabian plate away from Africa in an NE direction led to the opening of two young oceanic basins; the Red Sea between Africa (Nubia) and Arabia, and the Gulf of Aden, between Somalia and Arabia. Jahran Basin as an example is located within the Central Yemeni Highland geomorphological province, which is located in the western mountainous range (fig1). This basin is located about 70 kilometer south of Sana`a city, at an elevation of 2200m (Davison, 1994). The main problem at this location appears to be that a lot of fissures have been forming at the basin; this may be related to the sub-surface, which will be the objective of our field investigations. The distribution of physical and mechanical characteristics of the surface and sub-surface soil and rock layers will be a study in the investigated area, in order to define the competence of materials. 1624 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 Recent fissures Fig. 1 The geological map of Jahran Basin (Robertson Group) In 1990, fissures in Jahran Basin extended as a liner forms. The fissures locally showed 50 cms of vertical displacement. The main fissure was the longest (approaching 10 km long) and remained to be the most affected by erosion and infiltration of flood water. Fissures have been discovered astride a large section in Jahran and many other basins in Yemen (Al-Ubaidi, 2008), (Al-Ubaidi, 2009) (fig 2 and 3). These fissures are characterized by N30W, N40E and N50W orientations, and they have different width ranges between 1cm–3m, depths between 1cm-10m and length ranges between some cm to 300m. These fractures are in zigzag shape and en-echelon arrangement with elongated shape of holes on the surface. Our field observations of these fissures outcrops have also shown that a main rupture is developed along the Jahran fault over a length of ~ 5 kms, and a depth of 5-10 m of a secondary rupture over a length ~1 kms formed along the southern segment of the Jahran fault. Furthermore, two minor surface breaks occur between the two principal ruptures mentioned above: The faults are equally characterized by NW-SE, N-S faulting together with right-lateral slip. From east to west, the NW trending rupture dips from near-vertical to 50-60 degrees to the North West, and the amount of net slips (i.e. displacement) are also noted to have decreased gradually. 1625 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 Fig. 2 Photographs of (a) facing to the south west along the fissures. It shows the west side up displacement on the main crack and an adjacent tilted block on the East. (b) Zigzag shape with elongated shape of holes on the surface. (c) Fissures in the rocks and the fissures had been vertical offset as high as 50 cm. (d) the main rupture developed along the Jahran Basin. a b Fig.3 Stereographic projection for fracture sets and systems in Jahran basin. Pole: normal to fracture planes (a) recent crack planes (b) Faults and fractures. These fissures have become the focus of attention at this time in these basins, because of their importance in controlling engineering structures and their impact on the recharge of ground water. This feature is a problem because of the large area affected. Before this study began with the aerial photographs, an image interpretation indicated some small active faults or fissures already formed in the area. Since then, the growths of these fissures have been increasing, starting from South to the North West crossing the Jahran Basin. The aim is to determine the orientations and the magnitudes of the principal stress axes at the time of faulting in the study area, using the striation analysis. This uplift with displacement is receiving close attention because similar deformation has occurred, prior to some earthquakes in the same area (e.g. Dhamar earthquake in 1982) and elsewhere. The deformation apparently began in 1990, and since then, it has grown northwestward to include area of about 150 square 1626 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 kilometers. The uplift with the displacement discovery has resulted from analyses or repeated measurements taken over a period of years (2006-2010) by various stations. However, the study will correlate with paleo-stress and the corresponding analysis, to show if there is any relation with the modern phenomena that have been observed in Yemen. Jahran Basin is a part of the Dhamar-Rada`a Volcanic Field, and it is surrounded by a number of volcanic cones (Al-Ubaidi. 2001). These volcanic rocks are related to the tectonic movements leading to the opening of the Red Sea and Gulf of Aden. Tectonically, the features of Yemen contain plate boundaries, local faults, dike swarms, volcanic activities and thermal mineral springs. The expanding Red Sea due to the Arabian plate rotation during the Tertiary and Quaternary are the major controlling factors. The extensional tectonic regime in Yemen has resulted in the extrusion of large volumes of effusive rocks (2000 m thick) during the late Oligocene-early Miocene eras. The area which is located at the north of Jahran is affected by major tectonic structures striking in NW-SE, E-W and NNE-SSW directions. These trends correspond to the regional trends of faults that control the tectonic configuration of the western part of Yemen. According to available geological maps ( Fig.1), this basin is dominated by Tertiary felsic and granitoid rocks to the west, northwest and north east and Late Miocene and Quaternary mafic volcanic to the south east. The area is surrounded by many planar extensional faults bordering blocks; some of which are horsts. These faults are sub vertical and located on the margin of these horsts. The eastern and western margins are bounded by faults, which far exceeds 3000 m. Another important set of faults is trend in an N40E direction and they comprise the continuation of the Oceanic transforms present in the Red Sea trough. 2. Assumptions of this study The first assumption of this study depends on the theory of Jaeger (Jaeger. 1962). The assumptions of this theory are given in the following. 1- Within a rock body containing a multitude of fracture discontinuities, the superimposition of a stress may result in the movement on several sets of faults. 2- Slip is most likely occurring on planes of high shear and low normal stress. The second assumption: According to Bott (Bott. 1959), faulting may frequently arise from the existence of old planes of fracture within the rocks. From a stress of a given orientation, it is found that the slip may occur in any possible direction within the plane, with the direction depending on the relative of the three principal stresses. The theory suggests that when a preexisting fault is subjected to the re-orientation stress the system of the movement after fracture is usually oblique. Third assumption: Fissure's structure in this area is believed to be associated with the same trends of many earthquakes in the area, like the 1982 Dhamar earthquake fault trend extended farther north into Jahran Basin and out into the other area (Shoalan, Mohammed , Al Thour 1986). The epicenter was 10 kms south of Jahran Basin. This earthquake was noted to have started on the dipping fault (350/80) on the west, known as the Jahran fault, and propagated unilaterally to the northwest over a distance of ~ 50 kms. 1627 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 This area is located along several sub parallel faults. The steep topography of the Jahran Basin area is related to the big escarpment of the tectonic structures, which indicate that much of the recent fracture rise has occurred along this border fault, while other faults in the area are apparently less active. 3. Estimation of the orientation of the principal stresses According to the previous studies (Justin, Gaulier . 1994),(Huchon, Kanbari,2003),(AlUbaidi, and Al-Kotbah . 2003), the area affected early on by NE-SW compression produced a series of folds and first fracture sets and systems, and eventually contributed to the development of major thrusts and later, the E-W extension produced major and minor normal fault systems. Stress and strain analyses provide useful constrains for understanding the geodynamic evolution of the deformed tectonics. This study concerns the deformational history of active faults or fractures within this area. Different methods were used (P and T, Right dihedral, brittle shear zones, Direct Inversion methods) with the aim of comparing their results on the basin data (fig 4). In all groups, the principal stress axes show a similar pattern of orientation, whereby two of them are usually horizontal in the NE-SW or NW-SE direction, and one is vertical. The estimated orientations of the principal stress axes are consistent with observations on the conjugated faults within the basin. The regional stress field, characterized by the NE-SW compression and NW-SE extension is related to the northeastward motion of the Arabian plate. The sub-vertical and sub horizontal orientations of the σ2 and σ3 axes are respectively shown by the direct inversion method at the station basin, which are due to the lateral movements. The Jahran Basin kinematic model of several closely-spaced basement faults, which could be the re-activations of older structures, share a detachment which transfers displacement onto a cover strand localized above the most prominent basement scarp. The results from all the methods reveal that brittle shear zones developed in the dip-slip regime with sub vertical σ1 and σ3 axes are directed towards NNE and EES respectively. It is evident that the brittle mode of deformation was more dominant during this event as a result of which, the echelon fractures were developed. Our results of an earlier strike-slip, and later, normal fault event, as well as the orientations of principal axes during these two events are similar to those derived by others. 1628 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 a b Fig. 4 Shows (a) Comparison between results from new stress (fissures) and paleostress (fractures in rocks) (b) results of the stress analysis of the Brittle shear zones using different methods. 4. The magnitude of the paleostresses and new stresses Several authors (Sassi and Carey . 1987),(Angelier .1989),(Al-Ubaidi, 1990) have applied the reactivated fault model to estimate the magnitudes of the stresses. Mohr`s circle representation of stresses in three dimensions forms the basis of the method used to examine the notion that the fault's analyses had behaved as reactivated faults. The mean of estimates of the maximum thickness of overburden in this area is between 100-400m of loss deposit sediment (soil) in the Basin. This soil is consolidated in depth. However, if we agree with an assumption that the basement is now reactivating, in this case, the estimated thickness is about 1000m of different rock types. Otherwise, if we agree with the assumption that the basin sediment is only reactivating, in this case the estimated thickness is about 100m. The vertical stress σ v is given by; σ v = pgz (Price, 1966)where p is the crustal density, g is the acceleration to gravity and z the depth estimated above. The confining pressure for the maximum vertical stress σv at the rock unit containing these measurements of faults would not be much more than 450 bars. The limits of frictional angles were found to lie between 25 degrees for soil and 45 degree for the rocks. Cohesion values of both 0 and 100 bars are shown in (fig.5). 1629 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 Fig. 5 Mohr's diagram shows the stress magnitudes of faults in the Jahran Basin. Table (1) the calculated magnitudes (in bars) of the principal stresses for faults. Fault Types Strike – Slip Strike – Slip Strike – Slip Normal Normal Normal σ1 σ2 σ3 σ1 σ2 σ3 Soil 100 Bars 75 Bars 50 Bars 2 Bars - 4 Bars - 10 Bars Rock σ1 830 Bars σ2 450 Bars σ3 80 Bars σ1 450 Bars σ2 170 Bars σ 3 - 100 Bars The above table (1) shows that the differential stress measurements formed at an estimated depth of 1000m, show that stress magnitudes increase with depth from 100 bars in the upper few hundred meters to below 1000m depth. Throughout the site, the stress was clearly high enough to produce faulting because of the regional stress. In the group of strike-slip, the horizontal stress (σh) is greater than the vertical stress (σv) but in the normal fault σ1 is vertical and corresponds to lithostatic pressure σv. The Mohr diagram (fig.5) shows that the stress magnitudes were sufficient to induce the reactivation of faults in the area. The magnitude of stresses changes gradually with the constant in their orientations. 5. Strain determination In the general case, when a rock deforms, each particle within it changes its position with respect to each other particle. From the conglomerate beds which are adjacent to these fissures, many oriented samples were measured. The distribution can be used to determine the state of 1630 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 strain when the distance between central point, and the neighbors are constant in the nondeformed state, but in the deformed state, the maximum distances between points occur parallel to the principal stretch direction S1, whereas the minimum distances occur parallel to S2. Therefore, this data can be used to calculate the finite strain (fig.6). Fig (6) The biaxial indicatrix showing the three principal axes of strain. Under this assumption, the Fry method is one of the best methods for the analysis of deformed ellipsoids. The theoretical limitation of method with the discussion of edge effects and other complications are given by Fry (1979) (Fry. 1979). This method is based on the assumption that an initially uniform anti-clustered distribution of points will change after deformation into a non-uniform distribution. The resulting plot on a sheet has an ellipticity (whereby the shape of the hole around the central point is elliptical) of the same ratio and orientation as the strain ellipse. The long and short axial and orientations of the principal axes are measured directly. These plots are well within the prolate field of the strain ellipse. 1631 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 6. Discussion The above results mostly correspond with other studies (Hunchon, Contagrel . Gaulier , AlKahirbash and Gafanen 1991) and (Manetti , Capaldi , Chisa , Civetta ., Conticelli , Gasparon, La Vope , Osri , 1991). They have suggested a continuity of sea floor spreading in the Red Sea within the period from 10 Ma up to recent times, and the evolution of the Arabian plate boundary can be explained largely by the process of the micro plate, and the normal faulting is generally produced due to the tensile stress acting at right angle to the ridge's axes of the Red Sea. In this study, many assumptions are ble to elaborate on the development of these fissures in these areas. 1) The Dikes of Tertiary and Quaternary age cut through a thick overburden of rocks in this area. These dikes are part of the northwest-southeast orientated volcanic cones, which have led to the upcoming overburden and active extensional faulting. According to Mastin and Pollard (Mastin and Pollard, 1988), the fissures and faults along the area might have formed above a shallow intrusion of magma. 2) Most fissures in the sedimentary section result from the reactivation of basement shear zones due to fracturing at +_ 30° to maximum compressive stress, as hypothesized by the strain theory (Andersonian theory of stress). Compression results from the reactivation of preexisting basement faults not at the right angles to maximum compressive stress. In fact, as this angle changes from perpendicular to more oblique direction, the reactivated basement fault would exhibit only the strike-slip movement. These faults occur when faults are formed in areas where a set of parallel basement faults is not at right angles to the maximum compressive stress. 3) The calculated three-dimensional orientations of the axes of strain shape show a definite relationship with faulting. The directions of the maximum strain are perpendicular in orientation to the strike of the faults or fissures. The intermediate and minimum strains are approximately parallel to the fault plans. The results of the strain analysis show that the regional finite strain (prolate) is heterogeneous and generally increases southeastwards of the study area. According to these data, we can discuss possible mechanical reasons for the observed paleostresses and present a working hypothesis that relates these fissures to the different propagation ages of the deformations and changes in the transport direction, or it is related to the rejuvenation of faults in the basins at this time. 4) They are weakly and strongly dependent on the lithology. Clay, gravel and sand in the basin are the least competent and the most strained, while igneous rocks at the margins of the basin are the most competent and the least strained rock types. These rocks are faulted, fractured and their behavior is related to the competence contrasts between layers of different lithology. In the Jahran area, there is a range of competence from basaltic rocks to loose and sedimentary rocks. As a result of these data, the high strain is localized within areas dominated by low competence rocks (soil and loss materials located in the basin). These zones usually originate from discrete horizons separating domains of more competent rocks, which show little evidence of deformation. The displacements are zonal, accommodated by low competence rocks. The zones of intense deformation are localized within beds of clay and sand. However, this area is affected by a gradual change in a stress regime from the shear stress phase to the extensional phase of faulting. These phases are staying in the same direction (NESW). The changes cause the reactivation of faults from the existence of the old phase faults (strike-slip) within the Jurassic rock units to normal faults. 1632 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 According to these data, the maximum compressive stresses (N10-20E) were resolved into low normal stress acting in perpendicular to a fault surface and high shear stress acting parallel to the transform fault surface (N40E), or parallel to the fissure trend (N50W). With an increasing displacement on strike-slip faults, the magnitude of maximum principal stress axis decreases, and its axis rotates towards the vertical, resulting in normal faults (fig. 7). In the Precambrian-Mesozoic age, the stress pattern changed to NE-SW and the transport direction shifted to NE and at this time, the Red Sea was beginning to open. This event mostly shows as strike-slip or faults formed in the N40E trend and normal faults in N50W. In the Cenozoic era, the stress pattern changed due to the decreasing horizontal stress and had become lower than the overburdening pressure. The maximum principal stress, in effect, became equal to the vertical stress. Therefore, stresses of the gravitational origin are responsible for producing the last formation of normal faults. The E-W extension corresponds to the dip-slip of conjugated faults trending towards N-S and NW-SE. This extensional event tends to occur when the maximum principal stress axis is equal to the vertical stress. Fig. 7 Tectonic model of strike-slip fault and zig-zag fractures in the area. 7. Conclusions In the present time, the slip that occurred on the pre-existing discontinuities in the north western regions of Yemen is due to the re-seismic and volcanism activities at the south of the Red Sea along the transform faults towards N40E. The study area has been affected by the deformation, controlled by simultaneous but heterogeneous, slip on all the faults in the area, with greater rate of deformation near the recent fissure's disturbances. The following generalized sequence applies to the principal surfaces at many locations in the area under study (fig 8): a- Strike-slip faults along the Transform faults at the early stages of deformation. b- -Normal faults are a result of gravitational forces at the end-stage of deformation. 1633 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 The model (fig.9) shows why the study area was affected by continuous strike-slip/normal fault's sequence of tectonic deformation. However, the following model can be proposed: In the present study, stress field, failure elements, fault patterns and strain determination are in agreement with the active faulting of the region. The horizontal stress in the basement is high enough to produce a reactivated faulting in the basin because of the fact that it is greater (about 850 bars) than the vertical stresses (about 80 bars). The calculated three-dimensional fabric of finite strain, which is (prolate) characterises some possible results from the super-imposition of the tectonic strain associated with faulting. The movement of the Arabian plate away from Africa in a NE direction (Davison et al. 1994) has led to the opening of two young oceanic basins: the Red Sea, and the Gulf of Aden and it allows for the development of the series of faults and fissures with different orientations. Fig. 8 Pattern of the strike – slip faults oblique to the Transport direction ( T.D. ) , U ( uplift ), D (subduction) σs ( shear stress ) σn ( normal stress ). Fig. 9 Sketch showing the different stages of development of the fissures and the relationship between a) strike-slip and b) normal faults (Talbotand Alavi , 1996). 1634 Journal of Babylon University/Pure and Applied Sciences/ No.(4)/ Vol.(23): 2015 Acknowledgments I would like to thank the Geological Survey and Mineral Resources Board (YGSMRB) (YEMEN). I also wish to extend my gratitude to Dr. Ismail Naser AL-Ganad, the Director of the Geological Survey. References Al-Ubaidi, M.R, 1990. The structural analysis of the Trimsaran, NW part of South Wales Coalfield, Ph.D. Thesis Wales University U.K Al-Ubaidi, M. R. 2001. Geomorphologic and Structure Analysis of Wadi Dhahr NW of Sana’a. J of geographical Soc Yemen, vol.1 266-288 Al-Ubaidi, M and Al-Kotbah A. 2003. The Magnitude of the principal Stress of Yemen Faults. Faculty of Science (Univ. of Sana’a) Vol. 15, Oct Al-Ubaidi, M. R. 2008. Structural study of the Sana`a International Airport and the northern parts of Sana`a Basin (unpublished report) project submitted to the Yemen geological Survey and Mineral Resources Board Al-Ubaidi, M. R. 2009. General interpretation of Amran Basin (structural geology). A project submitted to the Yemen Geological Survey and Mineral Resources Board in behalf of BGR Integrated Water Resources Management Angelier, J .1989. From orientation to magnitudes in paleo stress determinations using fault slip data. J. Struct. Geol. 11:37-50 Bott, M.H. 1959. The mechanics of oblique slip faulting. Geol. Mag. 96, 106-117 Davison, I. 1994. Geological evolution of the southeastern Red Sea Rift margin, Republic of Yemen. Bull. Geol. Soc. Am., 106, 1474-1493 Fry, N. 1979. Random point distributions and strain measurements in the rock. Tectonophysics 60 89-105 Hunchon, P., Contagrel J.M. Gaulier J.M., Al-Kahirbash S., and Gafanen A. 1991. Extensional deformations in Yemen since Oligocene and the Africa-Arabian-Somalia triple junction. Annales Tectonic, vol. v, N2 141-163 Huchon. P., Kanbari K. 2003. Rotation of the syn-rift stress field of the northern Gulf of Aden margin, Yemen. Tectonophysics 364, 147-166 Jaeger, J.C. 1962. Elasticity Fracture and flow. Methuen, London Justin, F. Huchon P, Gaulier J. M. 1994. The Somalia plate and the east African Rift System: present kinematics. Geophys J. Int. 116, 637-654 Manetti P., Capaldi G., Chisa S., Civetta L., Conticelli S., Gasparon M., La Vope L., Osri G. 1991. Magmatism of the Eastern Red Sea Margin in the Northern Part of Yemen from Oligocene to Present: Tectonophysics, v. 198, PP. 181-202.125. Mastin, L G, and Pollard D D, 1988. Surface deformation and shallow dike intrusion processes at Inyo Craters, Long Valley, California. Journal of geophysical Research, v. 93, no. B11, 221-235 Price, N J. 1966. Fault and joint development in brittle and semi brittle rock, Pergamon Press, Oxford Sassi W and Carey E. 1987. Interpretation mecanique du glissent sur les failles: introduction dun critere de frottement , Anales Tectonically, 139-154 Shoalan, J.M., Mohammed O., Al Thour K. 1986. December 13th, 1982 Earthquake- Seismological Features Ground Cracks, Water Table Level, Springs Variations and Rock Falls. Report, PP 22. Talbot, C.J. and Alavi M. 1996. The past a future syntaxis across the Zagros. Salt Tectonics, Geological Society No.100 89-90. 1635