Dr. Robert J. Stern
Center for Lithospheric Studies
University of Texas at Dallas
Box 688
Richardson, TX 75083
Co-Investigators:
Timothy H. Dixon, JPL/Caltech
Kent C. Nielson, Univ. of Texas at Dallas
Mohammed Sultan, Washington Univ., St. Louis
SIR-C
Studies of the Precambrian Hamisana and Nakasib Structures, NE Sudan, in Arid
Regions of Low Relief and in the Subsurface
OBJECTIVES
a) Develop techniques for optimizing structural analysis of basement trends in arid
regions with extremely subdued topography and/or thin aeolian cover.
b) Apply results of (a) to map the southern extension of the Hamisana Shear Zone and
the western extension of Nakasib Suture.
c) Apply results of (b) to constrain the roles of terrane accretion and strike-slip
re-organization for late Precambrian crustal evolution in NE Africa.
PROGRESS
Our proposal called for evaluating the suitability of
SIR-C/X-SAR
imagery for resolving
basement structures in hyperarid regions where visible and near-infrared imagery
revealed only sand covering. Specifically, we proposed using the radar data to define
the southern extension of the Hamisana Shear Zone in NE Sudan. This Precambrian basement
structure lies in the Sahara Desert, earth's most hyperarid environment. We had
previously exhausted efforts to use conventional imagery to resolve the structure.
The study area that we originally proposed is shown as 'Original Proposed Study Area'
in Fig. 1. In preparing for the
SIR-C/X-SAR
experiments, field studies led us to
recognize that a parallel but more westerly structure was also important. This is
the poorly exposed and consequently very poorly known Keraf Shear Zone. Following preliminary
field studies which indicated that the Hamisana may terminate just south of its present
outcrop limit, and that the Keraf Shear Zone may be more important than previously thought, we redirected our efforts towards studying the Keraf. This is a slight
change in direction that has been justified by subsequent discoveries resulting from
our interpretation of the
SIR-C/X-SAR
imagery.
Prior to receiving the
SIR-C/X-SAR
data, we carried out a field program based on structural
interpretation of
SIR-A
data over the northern part of the Keraf Shear Zone. This
was done in order to better prepare us to interpret the
SIR-C/X-SAR
images and to define the fundamental structural characteristics of the Keraf that were likely to
be so identified. The location of this 'SIR-A pilot study' is shown in Fig. 1, and
the results of this investigation have been published (Schandelmeier et al., 1994;
Stern et al., 1993) or are now in press (Abdelsalam et al., in press). We also took advantage
of the preparation phase to prepare an overview summarizing the sequence of events
and global significance of the tectonic evolution of the Arabian-Nubian Shield (Stern 1994).
Progress in the Past 1.5 years
Our experiment continues to focus on using the multi-parameter characteristics of
the
SIR-C/X-SAR
imagery to find, define, and interpret geologic features in hyperarid
regions, and to understand the extent to which the radar is resolving structure by
penetration through sand or back-scattering from subtle topographic expressions of structure.
Once the
SIR-C/X-SAR
data were acquired, we compiled
SRL-1
and -2 survey data over
all of NE Sudan. This involved obtaining reconnaissance L-band data takes (Alden prints) and compiling these with reference to known geologic structures. This comprehensive
data set was then studied to identify where further processing efforts and field
studies (ground-truth efforts) should be concentrated. This work was completed
at the end of Spring 1995, and because the quality of the L-band Alden prints is very
similar to that of
SIR-A,
we were well prepared to interpret these images on the
basis of our pilot study in the northern Keraf Zone. This phase of our study indicated
that the original focus - tracing the southern continuation of the Hamisana Shear Zone
- was not likely to be fruitful, and we switched our primary focus to the heretofore
unknown southern part of the Keraf Shear Zone, where we are confident that our original objectives can be attained. This area is labeled 'New Area of Primary Concentration'
in Fig. 1. The first and most exciting result to date was our discovery on the radar
imagery and subsequent ground verification of two previously unrecognized shear zones (which we have named 'Abu Hamed Shear Zone' and 'Abu Dis Shear Zone'; Fig. 2)
along the Nile north of Atbara. This is the southern part of the Keraf zone, and
while its existence has been long suspected to lie in this region on the basis of
isotopic considerations, it was not until we examined the
SIR-C/X-SAR
imagery that the location,
trend, and even the existence of this important lithospheric structure in this region
was demonstrated.
This discovery was the result of the
SIR-C/X-SAR
imaging capabilities, and we are
now concentrating on understanding and exploiting this result. We are convinced
that this discovery dramatically demonstrates the importance of radar to geological
studies in hyperarid, poorly known regions, among which the Sahara Desert is the type example.
After careful study, we conclude that almost none of the structures visible on the
radar images (Fig. 3) can be seen on images produced from a wide range of visible
and near infrared sensors. We have studied Landsat
TM
images, Shuttle hand-held photography
(acquired during the April 1994 flight), and 1:80,000 air photos in efforts to discern
structures that are vividly revealed in the radar images. We conclude that there is no way that the visible and near-infrared images could have been processed or
interpreted that could have resolved these structures.
It is presently unclear how much of the radar's sensitivity to structure is the result
of penetration of sand drapes and how much is due to subtle variations in topography
and low, nearly-hidden outcrops. Answering this question is an important part of
the work that remains to be done in the next two years.
Just how spectacular the
SIR-C
images of the area are is shown in Fig. 3, a very small
part of the region shown in Fig. 2. This scene was processed at
UTD.
The radar images
show remarkable detail over the shear zone, revealing a complex, intensely folded
region to the east of the Abu Dis shear zone and a less intensely deformed region
to the west of the shear zone. We carried out three weeks of field studies in the
region last April. This concentrated on refining our structural interpretations
of selected portions of the region shown in Fig. 3, and on understanding how much of the structure
is buried beneath dry, wind-blown sand (and how thick is this cover). Our preliminary
estimate is that a variable thickness of sand covers a basement peneplain. Our preliminary estimate is that about 10-20% of the desert surface is low basement outcrop
and the other 80-90% is 2-200 cm of windblown sand. The situation in the field is
shown in Fig. 4, as is how the region appears with visible and near-infrared imagery.
SIGNIFICANT RESULTS
Our work is continuing, but we have prepared two abstracts on our results (Abdelsalam
et al., submitted; Stern et al., submitted) that will be presented at the 1995 national
meeting of the Geological Society of America meeting in November. We are also preparing a manuscript (Stern et al., in prep.) that will be submitted to the special
volume of JGR-Green on geologic results from
SRL-1
and 2. Another manuscript nearing
completion reports the results of our structural studies of southern Keraf Shear
Zone, based on interpretation of the
SIR-C/X-SAR
data and field investigations (Abdelsalam
et al., in prep.). Several other publications are planned following completion of
our studies.
FUTURE PLANS
Our top priority is completing the analysis of data sets that cover the 'New Area
of Primary Concentration' outlined in Fig. 2, and publication of these results.
We have all of the radar data sets that we need, and we are proceeding with the following
work plan: 1) Assembly of L-band total power image (black and white) over the entire
area outlined in Fig. 2.; 2) Generation of individual false-color L-, C-, and L/C
images for subscenes covering all of Fig. 2; 3) Coregistration of L-, C-, and X-band
data sets for selected parts of the region outlined in Fig. 2; and 4) Field checking of
results following final image processing. Items 1 and 2 are well in hand, and these
tasks should be completed sometime in the next 2 months. We place a very high value
on item 3. One of the few disappointments of the
SIR-C/X-SAR
experiment has been the
scarcity of images that use all three wavelengths. This is unfortunate for two reasons.
First, the international nature of the experiment means that a very real part of
the collaboration is reflected by the products that use both data sets, and the relative
lack of these products can all too easily be interpreted as indicating a lack of
true co-operation. Second, the
SIR-C/X-SAR
platform is unique in acquiring simultaneous
multi-wavelength data, and this aspect should be considered for future orbital radar
platforms. Coregistration of all three wavelengths is a very high priority, and
this task is being carried out with S. Okonek and other scientists at
JPL.
An important
part of this effort will be comparing images produced from the three-wavelength coregistered
data with images produced using only L-, C-, and L/C.
Finally, we require another 2- to 3-week field expedition to Sudan. This is needed
in order to carry out GPS-controlled field checks of our structural interpretations,
collect a few samples for radiometric age determinations needed to constrain the
age of deformation, and to quantify the depth of sand cover and percentage of basement exposure.
This work will be the focus of our efforts during most of
FY
96.
In late
FY
96 and early
FY
97, following completion of the primary tasks outlined
above, we will turn our attention to application of
SIR-C/X-SAR
data to another poorly
exposed region along the Nile in Sudan, shown as 'New Area of Secondary Concentration'
in Fig. 2. The work plan for this area will be similar to that outlined above or
the Keraf area: 1) Assembly of L-band total power image; 2) Generation of detailed
false-color images, either coregistered X-, C-, and L-band data, or L-, C-, and L/C,
depending on the results of our study outlined above; and 3) Field work to verify and modify
interpretations and provide data regarding amounts and depths of sedimentary cover.
The data sets that we need to conduct this phase of the work are about half complete, and we expect to submit requests for another 10-15 precision products to both
JPL
and
DLR.
Finally, in late
FY
97, we hope to apply our refined techniques to a study of the
Allaqi-Heiani suture. This fundamental lithosphere structure in SE Egypt is poorly
known, but appears to be deflected by the northern extension of the Keraf Zone.
If time and funds allow, we would like to carry out the sorts of investigations outlined above
to studying this structure. If this work moves forward, we expect to require another
20 precision products from both
JPL
and
DLR.
PUBLICATIONS
Abdelsalam, M.G., Stern, R.J., Daniels, A.T., Elfaki, E.M., Elhur, B., and Ibrahim,
F., in press. Tectonic evolution of the Neoproterozoic Keraf Zone, Sudan. Geol.
Soc. America, Ann. meeting (abs.).
Abdelsalam, M.G., Stern, R.J., Schandelemeier, H., and Sultan, M., in press. Deformational
history of the Neoproterozoic Keraf Zone, NE Sudan revealed by Shuttle Imaging Radar.
Journal of Geology.
Schandelmeier, H., Wipfler, E., Kuster, D., Sultan, M., Becker, R., Stern, R.J., and
Abdelsalam, M.G., 1994. Atmur-Delgo suture: A Neoproterozoic oceanic basin extending
into the interior of NE Africa. Geology
v.22, p. 563-566.
Stern, R.J. 1994. Arc Assembly and Continental Collision in the Neoproterozoic East
African Orogen: Implications for the Consolidation of Gondwanaland. Annual Reviews of Earth and Planetary Sciences
v.22, p. 319-351.
Stern, R.J., Abdelsalam, M.G., and Daniels, A.T., in press. Shuttle Imaging Radar
reveals a major deformation belt in the Sahara Desert of Sudan. Geol. Soc. America,
Ann. meeting (abs.).
Abdelsalam, M.G., and Stern, R.J., submitted. Shuttle Imaging Radar imaging of buried
and poorly exposed basement structures in hyper-arid terrains. J. Geophys. Res.
- Planetology.
Stern, R.J., Abdelsalam, M.G., Schandelmeier, H., and Sultan, M., 1993. Carbonates
of the Bailateb Group, NE Sudan: a Neoproterozoic (ca. 750 Ma) passive margin on
the eastern flank of Gondwanaland, Geol. Soc. America, abs. with progr. v.27, p.
49.
Figure Captions
Figure 1 (401K GIF):
Locality map of Precambrian basement outcrops in NE Sudan and the locations
of various study areas proposed in the text. Note that the area labeled "New Area
of Primary Concentration" also shows the location of Figure 2.
Figure 2 (20K GIF):
Geologic sketch map of the southern Keraf Zone. The geology from west
of the Nile is generally well-exposed and is taken from various published reports
and our own field investigations. The geology east of the Nile is not exposed and
represents our interpretation of
SIR-C
L-band survey images supplemented by field checking.
Notice that the geology of this part of the Keraf Zone is complex, with ultramafic
rocks defining the folded surface of the suture between the pre-1.0 Ga crust of the
East Saharan Craton and the 0.85 - 0.65 Ga juvenile crust of the Arabian-Nubian Shield,
and younger left-lateral strike-slip faults (Abu Hamed shear zone and Abu Dis shear
zone) disrupting the older suture. The area of detailed work to date is shown in
the rectangle labeled 'Figure 3.'
Figure 3 (376K GIF):
Area of detailed work on the Abu Hamed and Abu Dis shear zones. 'A' shows
a
SIR-C
image
(SRL-1,
Data take 82.42) processed (HH polarization, red = L-band power,
green= C-band power, blue = L/C). This image reveals a variety of previously unknown structures, interpreted in 'B.' Our field work in April 1995 concentrated on
understanding the geometry of these structures. The location of the detailed image
shown in Figure 4 is outlined by rectangle in 'A.'
Figure 4 (531K GIF):
Comparison of images of the study area. 'A' shows terrain along the Abu
Dis shear zone, looking SSW from arrow 'a' in 4. The <1m high ridge is made up of
mylonitized granite emplaced in the shear zone during deformation. 'B' shows terrane
looking east from arrow 'b' in 4. Notice the low hills in the distance; these can be
recognized as two NE-SW elongated ridges about 6 km to the east. Notice very subdued
outcrop in foreground. 'C' shows the
SIR-C
scene itself, processed as described
for figure 3. This scene is about 20 km wide. 'D' shows the region in 4 outlined by a
rectangle as photographed during
SRL-1
(photo STS059-L19-OCF). Notice the absence
of any structural features. All visible and infrared images are similarly featureless.