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doi:10.1144/0016-76492007-093 2008; v. 165; p. 535-547 Journal of the Geological Society
S. Tesfaye, M.G. Rowan, K. Mueller, B.D. Trudgill and D.J. Harding
Ethiopia and DjiboutiRelay and accommodation zones in the Dobe and Hanle grabens, central Afar,
Journal of the Geological Society
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© 2008 Geological Society of London
Journal of the Geological Society, London, Vol. 165, 2008, pp. 535–547. Printed in Great Britain.
535
Relay and accommodation zones in the Dobe and Hanle grabens, central Afar,
Ethiopia and Djibouti
S. TESFAYE 1, M. G. ROWAN 2, K. MUELLER 3, B. D. TRUDGILL 4 & D. J. HARDING 5
1Cooperative Research Programs, Lincoln University, 104 Foster Hall, 820 Chestnut Street, Jefferson City, MO 65102,
USA (e-mail: [email protected])2Rowan Consulting Inc., 850 8th Street, Boulder, CO 80302, USA
3Department of Geological Sciences, University of Colorado, Campus Box 399, Boulder, CO 80309, USA4Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden,
CO 80401, USA5NASA, Goddard Space Flight Center, Planetary Geodynamics Laboratory, Greenbelt, MD 20771, USA
Abstract: The right-stepping Dobe and Hanle grabens display a variety of structures that serve to transfer
extensional displacement. These structures range from relay zones between overlapping fault segments to
accommodation zones between interacting rift segments. The study reveals the presence of three examples of
displacement transfer structures: a breached relay zone, a large-scale accommodation zone that is partially
breached, and a composite zone that combines elements of both. All three examples exhibit common structural
elements. First, dipping ramps develop between horizontal horst blocks and graben floors. Second, these ramps
are cut by numerous faults, most of which are antithetic to the ramps and the graben boundary faults. The
antithetic faults bound elongate blocks that are rotated into the grabens. Third, crosscutting faults partially or
completely link the en echelon or overlapping graben boundary faults. The identification of precursory
structures (mode I fractures) at the leading edge of the Dobe–Hanle accommodation zone breaching faults
suggest that the breaching process may be continuing. The spatial alignment from north to south of the
crosscutting faults, open fractures and lineaments indicates that the breaching process is progressing from the
Dobe graben towards the Hanle graben.
Segmentation of extensional fault systems has been identified at
different scales. First, the segmentation of grabens and graben-
bounding faults has been documented in continental rift systems
from both surface mapping and subsurface geophysical investiga-
tions (Rosendahl et al. 1986; Ebinger 1989a, b; Morley et al.
1990; Nelson et al. 1992; Hayward & Ebinger 1996). The areas
between adjacent or overlapping rift segments have been termed
transfer zones (Morley et al. 1990; Nelson et al. 1992) or
accommodation zones (Bosworth 1985; Rosendahl et al. 1986;
Ebinger 1989a, b). In these regions of interaction between
boundary faults, complex arrays of smaller-scale faults conserve
extensional strain as displacement is transferred from one graben
to the other.
At a smaller scale, surface mapping and modern seismic data
show that faults are composed of linked segments (Peacock &
Sanderson 1991, 1994; Childs et al. 1995; Peacock 2002). These
may be hard-linked or soft-linked. Walsh & Watterson (1991)
defined hard-linked faults as those where the fault surfaces are
linked at the scale of the map or cross-section in use. Soft-linked
faults are isolated from one another at the scale of the map or
cross-section in use, with mechanical and geometric continuity
achieved by ductile strain of the intervening rock volume. The
zones of complex deformation between overlapping fault seg-
ments are referred to as relay ramps (Larsen 1988; Peacock &
Sanderson 1994; Trudgill & Cartwright 1994; Peacock et al.
2000), transfer zones (Morley et al. 1990) or relay zones (Childs
et al. 1995; Huggins et al. 1995; Densmore et al. 2003). These
structures accommodate the transfer of displacement between
interacting fault segments (Larsen 1988; Peacock & Sanderson
1991, 1994; Trudgill & Cartwright 1994; Childs et al. 1995;
Huggins et al. 1995; Peacock et al. 2000; Peacock 2002;
Densmore et al. 2003).
In this paper we use ‘displacement transfer’ to describe the
variable distribution of extensional strain that occurs between
interacting segments of faults or grabens. To describe the
geometries, we adopt ‘accommodation zone’ for the region
between adjacent or overlapping rift segments and ‘relay zone’
for the area between interacting en echelon or overlapping fault
segments.
We examine the right-stepping Dobe and Hanle grabens of
central Afar (Fig. 1) to determine the geometry and evolution of
displacement transfer in overlapping fault segments and interact-
ing rift segments. The study demonstrates that the processes that
accommodate displacement transfer are similar both in relay and
accommodation zones. Typical deformation includes ramp
development, normal faulting of the ramp, complex block
rotation on both synthetic and antithetic faults within the ramp,
and development of breaching faults that link the interacting
graben-bounding faults. We illustrate these geometries with three
examples: a breached relay zone; an accommodation zone that is
partially breached; and an intermediate structure that combines
elements of both relay and accommodation zones.
Our study in central Afar enriches the prevailing understand-
ing of relay and accommodation zone evolution. Relay and
accommodation zones are known to play various roles in
structural and tectonic studies. First, relay and accommodation
zones are important in the migration and trapping of hydro-
carbons (Larsen 1988; Morley et al. 1990; Nelson et al. 1992;
Peacock & Sanderson 1994; Peacock 2002). Second, relay
structures play a key role in understanding the growth and
scaling of faults (Dawers & Anders 1995; Cartwright et al. 1995;
Peacock 2002). Third, they are critical in determining fault
segment length, and thereby constrain maximum likely earth-
quake magnitudes on these structures (Jackson & Blenkinsop
1997; Ferrill et al. 1999).
The Dobe–Hanle graben area was chosen in this investigation
to seek an explanation for the anomalously high density of
normal fault occurrences adjacent to the least deformed fault
blocks in the area. The study is based on field investigations,
analysis of Advanced Spaceborne Thermal Emission Reflection
Radiometer (ASTER) imagery, aerial photographs, and digital
topographic data. The Dobe–Hanle graben area is ideal for using
remotely sensed images to investigate structures because of the
extremely good exposure of outcrops, except in the graben floors,
owing to the absence of vegetation and soil cover in the region.
Geological framework
The diffuse Afar triple junction, where the Red Sea, Gulf of
Aden and Ethiopian Rifts converge (Fig. 1, inset), is character-
ized by volcanic and seismic activity. Evidence for active rifting
is manifest as well-preserved fault scarps, voluminous volcanic
rocks, volcanic centres, and shallow earthquakes (CNR–CNRS
(Afar team) 1973; Manighetti et al. 1998; Beyene & Abdelsalam
2005). Magmatic activity commenced in the Afar region (south-
ern Red Sea rift) between 31 and 29 Ma (Hofmann et al. 1997;
Ukstins et al. 2002). Rifting in Afar, the result of divergence
between the African (Nubian) and Arabian plates that are
separated by the anticlockwise rotating Danakil microplate, has
been active since c. 25 Ma (Barberi et al. 1975; Ukstins et al.
2002; Beyene & Abdelsalam 2005).
Fig. 1. Generalized geological map of central Afar (modified from Abbate et al. 1995).
Fig. 2. Topographic image of the Dobe–Hanle graben area generated
from digital terrain elevation data. Elevation ranges from c. 100 m (dark)
to c. 1400 m (white). Diagonal lines are locations of topographic profiles
shown in Figure 4. Fault relief along major graben-bounding faults was
measured along X–X9 (Fig. 7b) and Y–Y9 (Fig. 14). The location of the
map is shown in Figure 1.
S . TESFAYE ET AL.536
Fig. 3. Georeferenced, Advanced
Spaceborne Thermal Emission and
Reflection Radiometer (ASTER) imagery of
the Dobe–Hanle graben area. Black areas
are water bodies, white represents evaporite
and lacustrine sediments and grey is Afar
stratoid volcanic rocks (alluvium is also
grey). The image is a black and white
reproduction of a false colour composite of
the visible and near IR bands of ASTER
data at 15 m spatial resolution. The location
of the image is shown in Figure 1.
Fig. 4. Topographic profiles extracted from
the digital terrain elevation data. The
profiles show a change in the geometry of
the grabens as a result of changes in the
graben-bounding fault geometries. Grabens
bounded by single escarpments show abrupt
elevation drops between graben shoulders
and floors, whereas gradual elevation
changes are observed in graben segments
bounded by relay or accommodation zones.
The location of the profiles is indicated in
Figure 2. Vertical exaggeration is
approximately 3:1 and is uniform in all
profiles.
RELAY AND ACCOMMODATION ZONES, DOBE – HANLE GRABENS 537
Central Afar, a graben- and horst-dominated topography, is
floored by volcanic and sedimentary sequences. Three stages of
volcanism are distinguished in this region. The first group
includes the Miocene to Early Pliocene alkali basalts and more
silicic rocks (Barberi et al. 1975) that are at present exposed
around the periphery of the region (Fig. 1, Lower Extrusive
Complex). The second group, Plio-Pleistocene flood basalts with
subordinate silicic interlayers and volcanic centres, are collec-
tively referred to as the Afar stratoid series (CNR–CNRS (Afar
team) 1973; Varet & Gasse 1978; Barberi & Santacroce 1980).
Available age data indicate that the stratoid series was emplaced
between 4.0 and 1.0 Ma and occupy the floor of central Afar
(Barberi et al. 1975; Barberi & Santacroce 1980; Lahitte et al.
2003). The third group encompasses the ,1 Ma axial volcanic
ranges (primarily basaltic) exposed along discrete zones that
trend NW–SE in the north and WNW–ESE farther south (Fig.
1, inset). The axial volcanic ranges are interpreted to represent
the subaerial expression of oceanic spreading ridges that link the
Gulf of Aden with the Red Sea rift (Barberi & Varet 1977;
Barberi & Santacroce 1980; Barberi et al. 1980; Manighetti et
Fig. 5. Aerial photography interpretation of
the Middle Dobe relay zone. A–A9 and
B–B9 are the field traverses illustrated in
Figure 6. The location of the map is shown
in Figure 2. The scale is approximate.
Fig. 6. Topographic profiles of the Middle
Dobe relay zone constructed from field
survey data. The relay ramp consists of
rotated fault blocks, with faults antithetic to
the graben-bounding faults being most
common. The relief of the relay ramp faults
is mostly less than 100 m whereas the
interacting graben-bounding faults show
relief in excess of 500 m. Vertical
exaggeration is 2.4:1 and is uniform in both
profiles. Profile locations are shown in
Figure 5.
S . TESFAYE ET AL.538
al. 2001). The tectonic basins are filled with detrital and
chemical sediments (Fig. 1) that range in age from Pleistocene to
Present (CNR–CNRS (Afar team) 1973; Barberi & Varet 1977).
The distinct graben and horst topography and rotation of
blocks in central Afar inspired the introduction of rigid micro-
blocks in the modelling of its tectonic evolution (Tapponnier et
al. 1990). The mode of deformation in central Afar, however, is
a contentious issue. Proposed models include rift propagation
induced ‘bookshelf’ style of fault rotations characterized by left
lateral displacements between rigid microblocks (Tapponnier et
al. 1990; Manighetti et al. 1998, 2001). Sigmundsson (1992)
modified the bookshelf model by introducing rift-normal exten-
sion in the grabens in addition to the fault-parallel, left lateral
displacements along the boundary of the rigid blocks. Acton et
al. (1991) favoured a rift-normal extension between the rigid
blocks. Souriot & Brun (1992) proposed a ‘crank-arm’ model
that suggests that external boundary conditions, primarily the 108
anticlockwise rotation of the Danakil horst, account for the
observed fault patterns and clockwise rotation of fault blocks in
central Afar.
Faults are ubiquitous in central Afar and affect rocks of all
ages (Abbate et al. 1995). The dominant fault trend varies from
NNW–SSE to WNW–ESE. North–south and east–west fault
trends are less common. Normal faulting is dominant, although
strike-slip motions along some faults (same strike as normal
faults) have been reported (Dakin et al. 1971; Souriot & Brun
1992; Abbate et al. 1995). Prevailing models of extensional
faults in the Afar region assume that they maintain a steeply
Fig. 7. (a) A perspective view of the
Middle Dobe relay zone generated from the
ASTER image and digital terrain elevation
data. (b) Plot of vertical relief (proxy for
throw) v. strike distance in the Middle Dobe
relay zone measured from digital terrain
elevation data between points X and X9
shown in Figures 2 and 7a. Only the
graben-bounding faults are shown.
RELAY AND ACCOMMODATION ZONES, DOBE – HANLE GRABENS 539
dipping, planar geometry through the entire seismogenic crust
(Morton & Black 1975; Vellutini 1990; Abbate et al. 1995;
Hayward & Ebinger 1996). The base of the seismogenic crust in
central Afar occurs between 3 and 10 km depth beneath axial
volcanic ranges (Gouin 1979; Lepine & Hirn 1992) and between
8 and 15 km elsewhere, including the Dobe–Hanle graben region
(Lepine & Hirn 1992).
Methods
A combination of remote sensing images and field surveys was used to
determine the geometry of the fault and graben systems. Fault patterns in
map view were obtained from digital terrain elevation data (DTED; Fig.
2), ASTER imagery (Fig. 3), and 1:60 000 aerial photographs. The
DTED, obtained from the National Aeronautics and Space Administration
(NASA), were instrumental in determining throw across faults, and
extracting topographic profiles (Fig. 4). The DTED are in the form of a
raster topographic image with 3 arc second (c. 90 m) x–y (horizontal)
resolution. Elevation values measured with respect to mean sea level are
recorded at 1 m intervals.
We use topographic profiles obtained from the digital elevation data as
a proxy for structural cross-sections because of the minimal amount of
erosion in this region. The ground surface generally represents the same
stratigraphic level of youngest lava flows, which cap the stratoid series.
Exceptions include the floors of the major grabens, where there is
alluvial, lacustrine and evaporite fill, and along fault scarps, where there
has been degradation of the original fault surfaces. We also gathered
high-resolution topographic data across fault scarps and grabens along
three field traverses. Surveying was carried out with a Wild Total Station
(electronic theodolite and distance measuring device). Continuous meas-
urements were taken along the width of fault blocks, at scarp top and
bottom, and intervening graben filling sediments. In addition, the
orientation of faults and fault block tilts were measured along the
profiles.
Geometry of displacement transfer
The NW–SE-striking Dobe–Hanle graben system, with a com-
bined length of c. 125 km and maximum width of c. 20 km
(scarp to scarp), is one of the prominent tectonic features of
central Afar (Fig. 2). The grabens are bounded by normal faults
with dips up to 808 that displace the Afar stratoid volcanic rocks
(Mohr 1971; Tesfaye 2005). There is significant elevation
difference between graben shoulders and graben floors, with the
lowest point (84 m above sea level) occurring in the middle of
Dobe graben and the highest point (1376 m above sea level)
found in Kadda Gamarri (Fig. 2), NW of the Hanle graben. The
floors of the grabens are filled with alluvial, lacustrine, and
evaporite deposits of unknown thickness (Fig. 3) (Varet & Gasse
1978). Recent volcanism (,1 Ma) is markedly absent in the
grabens (Varet & Gasse 1978; Abbate et al. 1995).
Anomalously high density of faulting marks the region be-
tween the right-stepping Dobe and Hanle grabens (Figs 2 and 3).
The area between the two grabens exhibits a geometry inter-
mediate between those of rift jumps and rift offsets (e.g. Nelson
et al. 1992). The symmetric nature of the grabens is evident in
the topographic profiles (Fig. 4). Along the central portions of
the grabens, the boundaries are defined by single escarpments
with sharp elevation drops between graben shoulders and floors
(Fig. 4a, f and g, both sides; Fig. 4b, NE side; Fig. 4c, SW side).
The graben shoulders (Adaghilu, Dida, and Kadda Gamarri
horsts) display one of the least deformed blocks in central Afar,
with nearly horizontal surfaces and insignificant faulting (Figs
2–4).
In contrast to the symmetric, unrotated nature of the major
horst blocks, the regions of displacement transfer are defined by
closely spaced, tilted and rotated fault blocks marked by a
gradual drop in elevation (Fig. 4b, d and e, SW sides). These
areas are the focus of this paper. We first examine a relay zone
along the western margin of middle Dobe graben and then the
accommodation zone between the two grabens. We then analyse
the composite relay–accommodation zone where the southern-
most boundary fault of the Dobe graben and the eastern
boundary fault of the Hanle graben interact. Measured topo-
graphic relief is used in all cases as a proxy for fault throw.
Middle Dobe relay zone
The Middle Dobe relay zone developed between two overlap-
ping, synthetic fault segments with a maximum separation of c.
4.5 km (Fig. 5). The NW–SE- to west–east-trending, right-
stepping, graben-bounding faults are linked by a connecting fault
(ramp-breaching fault) that strikes north–south. Whereas the
relief on the graben-bounding faults exceeds 500 m, maximum
relief on the connecting fault is c. 80 m. The ramp region
Fig. 8. (a) An aerial photograph of the Middle Dobe relay zone. The
location of the aerial photograph is shown in Figure 5. (b) Field
photograph of the Middle Dobe relay zone breaching fault. The figures
show that the NW–SE-striking faults (both ramp and graben bounding)
are offset by the north–south-striking relay zone breaching fault. The
arrow in both figures points to the same fault scarp along the breaching
fault.
S . TESFAYE ET AL.540
between the two interacting fault segments has an overall dip of
c. 98 towards the SE. Near the connecting fault, however, the
ramp dips 68 towards the south. The ramp is cut by a dozen or so
synthetic and antithetic normal faults with scarps generally less
than 100 m long (Fig. 6). Short fault segments and small fault
blocks resembling box faults (Griffiths 1980) are clustered
around the hinge zone near the connecting fault, whereas
elongate fault blocks parallel to the main graben-bounding faults
are more common elsewhere on the ramp. These blocks are
variably rotated about a horizontal axis, with dips of up to 308
towards the graben centre (Figs 5 and 6). The relief on the
overlapping graben-bounding faults is typical of that of interact-
ing faults in a relay zone (Peacock & Sanderson 1994; Huggins
et al. 1995); when the relief (throw) on one of the interacting
faults diminishes the relief on the other interacting fault in-
creases, with a deficit of throw occurring in the overlap zone
(Fig. 7a and b).
Based on field observations and the morphology of fault scarps
on the 1:60 000 scale aerial photographs, the NW–SE-striking
faults (both ramp and graben bounding) are offset by the north–
south-striking, connecting fault that links the right-stepping
escarpment of the Dobe graben (Fig. 8a and b). This observation,
the relay zone geometry, and the distribution of relief measured
along the strike of graben-bounding faults are all compatible
with published models of fault growth by segment linkage
(Peacock & Sanderson 1991, 1994; Trudgill & Cartwright 1994;
Cartwright et al. 1995; Childs et al. 1995). We infer that the
overlapping boundary faults were originally separated by a
dipping relay ramp. Only when net displacement increased was
the relay ramp breached by the connecting fault. The fact that
Fig. 9. Aerial photography interpretation of
part of the Dobe–Hanle accommodation
zone. The location of the map is shown in
Figure 2. The scale is approximate.
RELAY AND ACCOMMODATION ZONES, DOBE – HANLE GRABENS 541
Fig. 10. Shaded relief image of the Dobe–
Hanle graben area. NE-facing fault scarps
are in the shadow and SW-facing scarps
appear bright. The relief on the
accommodation zone breaching faults is
small compared with the major graben-
bounding faults, as indicated by the size of
the shadow cast. The dashed black line
(south–central part of figure) marks the
boundary between the rotated and unrotated
fault blocks in the accommodation zone
ramp. The white squares indicate the
epicentres of the 1989 Dobe earthquake
sequence (Sigmundsson 1992). The focal
plane solutions for some of the events are
obtained from the Harvard Centroid
Moment Tensor Catalog (http://
www.seismology.harvard.edu/
CMTsearch.html).
Fig. 11. ASTER image of a portion of the
Dobe–Hanle accommodation zone. It
shows the spatial relationship of the
interpreted structural elements, which
include accommodation zone breaching
faults, open fractures, lineaments, and
rotated and unrotated fault blocks. The
location of the image is shown in Figure 3.
S . TESFAYE ET AL.542
the connecting fault extends south of the intersection with the
outer Dobe fault (Figs 5 and 8a) suggests that linkage was not
accomplished by lateral propagation of one of the main faults,
but by development of a new, separate segment.
Dobe–Hanle accommodation zone
The accommodation zone between the right-stepping Dobe and
Hanle grabens is a c. 16 km wide region that is cut by closely
spaced faults. In this section, we focus on the part of the
accommodation zone where the southwestern footwall of the
Dobe graben, called the Adaghilu horst, plunges to the SE into
the centre of the Hanle graben (Figs 2 and 3). This densely
faulted zone displays variable topography that ranges from an
average elevation of c. 880 m above sea level in the NW to c.
100 m in the floor of Hanle graben in the SE.
The accommodation zone is cut by two distinct, north–south-
to NNW–SSE- and NW–SE- to WNW–ESE-trending, fault sets
(Fig. 9). The prominent, NW–SE- to WNW–ESE-trending,
normal faults can be further divided into two subsets. In the
northeastern quadrant of Figure 9, the faults trend NW–SE, dip
mostly to the SW, and bound blocks that are rotated toward the
NE. In the southern half of Figure 9, faults trend WNW–ESE
and dip both to the south and north, forming a series of narrow
(,1 km) horsts and grabens with no rotation discernible on aerial
photographs.
The dominant (NW–SE- to WNW–ESE-trending) fault set is
cut by swarm of NNW–SSE-trending, mainly east-dipping
normal faults, tens to hundreds of metres long (Fig. 9). They
have relatively small relief compared with the major graben-
bounding faults, as evidenced from the shadow cast by the scarps
on the shaded relief image of Figure 10. These faults are younger
Fig. 12. Aerial photography interpretation
of the South Dobe composite zone. A, B, C
and D are faults that make up the
southwestern boundary of the Dobe graben.
K–K9 is the topographic profile illustrated
in Figure 16. The location of the map is
shown in Figure 2. The scale is
approximate.
RELAY AND ACCOMMODATION ZONES, DOBE – HANLE GRABENS 543
than the dominant set (NW–SE to WNW–ESE) and terminate to
the south near the boundary between the rotated and unrotated
NW–SE-trending fault blocks (Figs 9 and 10). Two open
fractures with a north–south trend occur south of the terminus of
the crosscutting faults (Figs 9 and 11). These fractures are
narrow, up to tens of metres wide, and hundreds of metres long,
and no vertical separation can be discerned on the 1:60 000 aerial
photographs or on the DTED. Further south, close to the west
Hanle graben-bounding fault, lineaments with similar crosscut-
ting trend that have no identifiable opening or displacements are
observed on the ASTER image (Fig. 11).
The NNW–SSE-trending, crosscutting faults are linked, at the
northern end, with the southern tip of the southernmost boundary
fault system of the Dobe graben (Fig. 10). This, combined with
the crosscutting nature of the north–south-trending faults, sug-
gests that the Dobe–Hanle accommodation zone is in the process
of being breached. The presence of open fractures at the leading
edge of the NNW–SSE-trending crosscutting faults signifies
(Fig. 11) the importance of precursory structures (mode I
fractures) in the initial stages of normal faulting (e.g. Crider &
Peacock 2004). Continued slip on the crosscutting faults and
development of the precursory structures to through-going faults
could result in the linking of the western boundaries of the Dobe
and Hanle grabens. The spatial alignment from north to south of
the crosscutting faults, open fractures and lineaments could
indicate that the breaching process is progressing from the Dobe
graben towards the Hanle graben.
South Dobe composite zone
The South Dobe composite zone is so named because it
combines elements of both accommodation and relay zones. It is
part of the Dobe–Hanle accommodation zone, occurring between
the terminations of the southernmost boundary fault of the Dobe
graben and the eastern boundary fault of the Hanle graben (Figs
12 and 13). It also acts in part as a relay zone where the south
Dobe graben boundary fault steps southward to an en echelon
segment of the southernmost Dobe graben boundary fault (fault
D, Fig. 12). This fault (fault D) has a maximum relief of over
400 m (Fig. 14) and is linked to the south Dobe graben boundary
fault by three fault segments that trend north–south to NNW–
SSE (faults A, B and C, Fig. 12) with maximum relief of less
than 200 m (Fig. 14). The southern termination of fault D
coincides with the beginning of the NNW–SSE-trending, cross-
cutting faults that are in the process of breaching the Dobe–
Hanle accommodation zone (Figs 10, 12 and 13).
The South Dobe composite zone is dominated by a relay ramp
that dips 8–108 to the SE and fault blocks that are variably
rotated to the NE (Figs 12 and 15a, b). The relay ramp is cut by
numerous faults that are antithetic to the graben-bounding faults
(Figs 12, 15a, b and 16). Most of the fault blocks that make up
the ramp strike WNW–ESE and are rotated toward the graben
centre, similar to the northeastern boundary fault of the Hanle
graben, which extends into the ramp (Figs 12 and 13). The
faulting pattern in the ramp becomes more chaotic close to faults
A and B (Figs 12 and 15a), where fault blocks are shorter, more
highly rotated, and strike north–south to NNW–SSE. Some of
the WNW–ESE-striking faults veer towards a NNW–SSE
orientation as they approach the graben boundary faults (faults
A, B and C), and others are cut by NNW–SSE-trending faults.
The presence of an old stream drainage network that runs across
four of the rotated fault blocks (Figs 12 and 15a) indicates that
initial basinward tilting of the relay ramp occurred prior to the
antithetic faulting. No lateral offset is observed in the truncated
stream drainage, indicating dip-slip movement.
The width and tilt of the antithetic fault blocks in the South
Dobe composite zone show a systematic variation. The width of
fault blocks along profile K–K9 (Fig. 12) ranges from 22 m to
1173 m (Table 1) and generally decreases from NE to SW as fault
D is approached (Figs 16 and 17a). Fault block dip ranges from 48
to 258 (Table 1) and increases to the SW. In general, we observe
an inverse relation between fault block widths and dip (Fig. 17b).
Fig. 13. A schematic illustration of the interacting Dobe and Hanle
grabens interpreted from the ASTER image. It shows the spatial
relationship of the structural elements that play a role in the evolution of
the relay and accommodation zones in the area, including relay and
accommodation zone breaching faults, open fractures, and lineaments
(possibly fractures).
Fig. 14. Plot of vertical relief v. strike distance of graben-bounding faults
(SW Dobe and east Hanle) in the South Dobe composite zone. Relief is
measured from the digital terrain elevation data between points Y and Y9
in Figure 2.
S . TESFAYE ET AL.544
The increase in fault block width away from fault D is interpreted
to be produced by the increasing strength of the hanging-wall
block as it increases in thickness in the down-dip direction of the
graben-bounding fault. Assuming a wedge-shaped hanging wall,
thinnest near the escarpment and thicker toward the graben centre,
and given the same material property (basalt in this case), fault
spacing will be wider where layer thickness is greater during
extensional deformation (e.g. Mandl 1987).
Discussion
Displacement transfer in the Dobe–Hanle graben area manifests
itself as relay zones between synthetic fault segments and as
accommodation zones between interacting rift segments. The
geometries within the study area provide clues to the evolution
of displacement transfer as regional extension gradually in-
creases. Initially, en echelon fault or rift segments are separated
by dipping but otherwise undeformed ramps, as shown by the
incision of old stream drainage patterns (Figs 12 and 15a). As
displacement increases, the ramps presumably steepen, but more
importantly, become broken up to form elongate fault blocks,
usually rotated along antithetic faults (Figs 5, 6, 12 and 15a, b).
Crosscutting faults develop near the hinge zones of the dipping
ramps as extension persists (Figs 5, 9 and 10). These faults
appear to originate, at least in some cases, as open (mode I)
fractures (Fig. 11). With continued extension, some of the
crosscutting faults link together, forming a continuous fault
joining originally separate fault or rift segments and breaching
the relay or accommodation zone (Figs 5, 9 and 11).
Faults in both the relay and accommodation zones are
dominantly antithetic. The associated fault blocks are rotated (up
to 308) toward the grabens. In contrast, the grabens are generally
symmetric, bounded on both sides by major faults and separated
by horizontal horsts. The underlying reason for the observed
predominance of antithetic faulting geometries in the study area
Fig. 15. (a) Perspective image of the South
Dobe composite zone generated from the
ASTER image and digital terrain elevation
data showing the rotated fault blocks within
the relay ramp and the undisturbed footwall
block in the background. A trace of an old
stream channel that runs across four fault
blocks is shown at the centre of the image.
(b) Field photograph of the South Dobe
composite zone showing the horizontal
basaltic layers in the footwall (horst) and
rotated fault blocks within the relay ramp.
The picture was taken from fault D looking
towards fault A (refer to (a) for
orientation).
Fig. 16. Topographic profile across the South Dobe composite zone
constructed from field survey data. The relay ramp is cut by antithetic
faults bounding rotated fault blocks (numbered). The fault blocks tend to
become narrower and steeper closer to fault D. Vertical exaggeration is
3:1. Location of the profile is shown in Figure 12.
RELAY AND ACCOMMODATION ZONES, DOBE – HANLE GRABENS 545
is not clear. In the Dobe–Hanle accommodation zone, however,
the presence of both rotated and unrotated fault blocks appears
to be influenced by the accommodation zone breaching faults. In
the Dobe–Hanle accommodation zone the southern extent of the
basinward rotated fault blocks coincides with the termination of
the accommodation zone breaching faults (Figs 9, 10, 11 and
13). South of this boundary, faults form a series of narrow,
elongated horst and graben structures that display ramp dip but
no rotation. A possible explanation for such geometry is that for
the fault blocks in the ramp (hanging wall) to rotate they need to
be physically separated from the footwall by the accommodation
zone-breaching faults. This implies that the process of accom-
modation zone breaching precedes the rotation of fault blocks in
the ramp. It cannot be ascertained, from the available data, if this
is the case elsewhere in the Middle Dobe relay zone or South
Dobe composite zone.
The occurrence of open fractures at the leading edge of
Dobe–Hanle accommodation zone breaching faults signifies the
importance of precursory structures in the initiation and develop-
ment of extensional faults (Crider & Peacock 2004). The
identification of accommodation zone breaching faults, open
fractures, and lineaments close to the west Hanle graben-bound-
ing fault (Figs 11 and 13) indicates the advanced stage of the
accommodation zone breaching process. In addition, the spatial
alignment of these structural elements suggests that the accom-
modation zone breaching is progressing from the Dobe graben
towards the Hanle graben. Moreover, the presence of accommo-
dation zone breaching faults, open fractures and lineaments
between the interacting Dobe–Hanle graben-bounding faults
suggests that the west Hanle graben-bounding fault and the
southernmost Dobe graben-bounding fault (fault D) could be
connected at depth (e.g. Peacock & Parfitt 2002). Whether the
breaching process will be completed or not depends on the
persistence of the extensional deformation that produced it in
the first place. The 1989, moderate magnitude earthquake
sequence (mb 5–6) in the middle of Dobe graben (Fig. 10)
indicates that extensional deformation in the area is still continu-
ing (Sigmundsson 1992).
Conclusions
Relay and accommodation zones in the central Afar depression
result from regional, crustal-scale extension. They serve to
transfer displacement between fault and rift segments, respec-
tively. We have analysed displacement transfer over a range of
scales from fault segments that are several kilometres apart to
rift segments that are tens of kilometres apart. Characteristic
structures in the Dobe–Hanle region include dipping relay ramps
between overlapping faults, elongate fault blocks bounded by
faults that dip antithetically to the graben-bounding faults, and
crosscutting faults that link the major boundary faults. Observed
geometries suggest that these structures develop in this order as
net extension increases through time. The presence of accommo-
dation zone breaching faults, open fractures and lineaments
between the interacting Dobe–Hanle graben-bounding faults
indicates the evolving nature of the accommodation zone breach-
ing process. The spatial alignment from north to south of the
crosscutting faults, open fractures and lineaments indicates that
the breaching process is progressing from the Dobe graben
towards the Hanle graben.
We thank R. Bilham, whose support and encouragement made this work
possible. C. Ebinger, S. Agar and C. Childs are thanked for commenting
on an earlier version of the manuscript. We are grateful for the
Table 1. Fault block parameters along profile K–K9 (Figs 12 and 16)
Block number Distance fromfault D (m)
Block dip(degrees)
Block width(m)
18 44 25 8817 117 15 5716 202 16 11415 341 24 16414 434 24 2213 494 23 9812 669 24 25211 1053 18 51510 1325 10 309 1544 17 4088 1909 15 3217 2241 16 3436 2610 16 3954 3016 11 4183 3517 12 5832 4294 4 9721 5367 4 1173
Block distance is measured from fault D to the centre of each block.
Fig. 17. Patterns of fault block width and dip along profile K–K9 shown
in Figure 12. (a) The plot shows the width of the fault blocks increases
as the distance from the graben-bounding fault D increases. (b) Fault
blocks with smaller width tend to rotate more than those with greater
width.
S . TESFAYE ET AL.546
constructive comments made by D. Peacock and an anonymous reviewer,
whose input improved the content of the manuscript. The research work
was supported by NASA grants NAG5-2584 and NAG5-3072. During the
course of the investigation S.T. also received support from a CIRES
Graduate Fellowship at the University of Colorado, Boulder. The
assistance of the Geology and Geophysical Department and Geophysical
Observatory of Addis Ababa University and the Ethiopian Institute of
Geological Surveys in facilitating fieldwork is greatly appreciated.
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Received 11 January 2007; revised typescript accepted 3 July 2007.
Scientific editing by Tim Needham
RELAY AND ACCOMMODATION ZONES, DOBE – HANLE GRABENS 547