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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.
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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.
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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
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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.
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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.
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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).
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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
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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.
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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.
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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.
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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).
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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.
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