OVERVIEW

Our proposal called for evaluating the suitability of SIR-C/X-SAR imagery for mapping Precambrian basement structures in hyperarid regions with subdued relief where Visible and Near Infrared (VNIR) imagery revealed only sand cover. Ancient rocks (550 to 870 million years old) of the 'Arabian-Nubian shield', exposed in the eastern Sahara Desert on the flanks of the Red Sea, were the general target. We originally planned to use SIR-C/X-SAR imagery to define the southern extension of the ca. 600 million-year old Hamisana Shear Zone in NE Sudan (Fig. 1). Before SIR-C/X-SAR data became available, we were able to advance our understanding of structural framework of basement rocks in the vicinity of the study area. These studies included the first detailed geological studies of the previously but poorly known Nakasib and Keraf sutures and led to the discovery of the Atmur-Delgo suture. These studies also revealed that the Hamisana Shear Zone terminates just south of its southernmost oucrop. This disappointment was short-lived because we simultaneously realized that a similar N-S structure called the Keraf Suture lay just to the west of Hamisana, and that the Keraf Suture was both more important and poorly exposed than the primary target (Fig. 1). Our effort was redirected towards resolving the extent and structural evolution of the Keraf Suture, including using SIR-A images to study the partially exposed northern part of the suture. This prepared us to use SIR-C/X-SAR data to document for the first time the evolution of the southern part of the Keraf Suture, and relate this important structure to the collision which formed the supercontinent Gondwanaland about 600 million years ago.

Investigations of the Keraf Suture using SIR-C/X-SAR imagery and field studies led us to discover that a portion of the Nile (along what is known as the 'fifth cataract stretch') closely follows the much older, N-trending Precambrian structures. We studied SIR-C/X-SAR imagery over a segment of the Nile farther downstream (known as the 'third cataract stretch') and discovered E-W trending faults of much younger age (<90 million years old) than those revealed along the fifth cataract stretch. The Nile follows these younger structures when it makes sharp, 90_ turns to the east or west. The success of SIR-C/X-SAR for revealing structural controls of the northward-flowing portions of the Nile encouraged us to use this imagery to understand the origin of the 'Great Bend', where the river flows southwest for 300km. We failed to identify any structures oriented in this direction but discovered a paleochannel of the Nile and a ~1000 km wide zone of E- and N-trending faults that indicate the Nile has been recently diverted by uplift along the east-west oriented 'Nubian Swell'. Some of the E-W trending faults associated with the Nubian Swell appear to be very young and perhaps seismically active, and so could be hazardous to structures such as the Aswan Dam. We continue to use SIR-C/X-SAR imagery along with seismic data to understand this potentially important application to water resources management.

RESULTS

Results prior to SRL-1 and 2 flights

Results using Landsat TM imagery

Evaluating the suitability of SIR-C/X-SAR imagery for resolving hidden geologic structures in the Sahara Desert required that we first outline the structures exposed in the Red Sea Hills of Sudan. Landsat TM imagery was powerful for outlining exposed structures because of an absence of soil or vegetation cover, and because many of these structural belts are nearly vertical and are decorated by rock units that are spectrally distinctive on Landsat TM imagery. We employed techniques developed to map ophiolitic rocks using Landsat TM imagery in NE Africa (Sultan et al., 1986; 1987) to reveal the complex structural evolution of the Hamisana Shear Zone and the older sutures that it offest (Stern et al. 1990). This approach was also used in an effort to understand the structural and tectonic evolution of the poorly known Nakasib Suture (Abdelsalam and Stern, 1993a and b), where it was less effective because the rock units and structures have gentle dips. We used Landsat TM images to discover the Atmur Suture (Schandelmeier et al., 1994). These discoveries contributed significantly in understanding the tectonic and structural evolution of the region which allowed us to develop regional syntheses (Stern, 1994; Abdelsalam and Stern,1996)

Results using SIR-A imagery

Prior to receiving the SIR-C/X-SAR data, SIR-A data were used to study the structural evolution of the northern part of the Keraf Suture (Abdelsalam et al., 1995). The northern part of this N-S structure is almost completely buried under thin (1-6 m) of dry sand. This study demonstrated the advantage of radar imagery over Visible and Near Infrared (VNIR) imagery in structural mapping in arid regions with subdued relief.

Results using SIR-C/X-SAR imagery

Discovery of the southern continuation of the Keraf suture

SIR-C/X-SAR data were used to define the southern continuation of the Keraf Suture (Abdelsalam and Stern, 1996). The southern Keraf Suture lies east of the Nile along its fifth cataract stretch before it starts its great bend (Fig. 2A). Exposures of rocks deformed along the Keraf suture are very scarce (Fig. 2B) because the landscape is very low-lying and partially covered by windblown sand. VNIR imagery from the region revealed extensive cover of sand (Fig. 2C). However, SIR-C/X-SAR imagery revealed details of structures associated with the suture (Figs. 2D). Revealing structures in southern Keraf suture using SIR-C/X-SAR imagery (Fig. 3) represents an important step forward not only for understanding the geology of the Sudan. but also for how and when continental collision 600 million years ago resulted in the formation of the supercontinent Gondwanaland. We used L- (wavelength = 24) and C-bands (wavelength = 6) with like- and cross-polarization, and X-band (wavelength = 3) to evaluate radar penetration as a function of wavelength and polarization. We concluded that, in hyperarid regions, radar bands with longer wavelength (L-band) penetrate loose dry sand better than shorter wavelengths (C- and X-band). Moreover, cross-polarization enhances penetration.

SIR-C/X-SAR imagery from the Keraf Suture were also used to outline major structures as well as planning ground checking. These investigations revealed that the structures in the southern Keraf Suture resulted from sinistral transpression which associated with oblique collision between east and west Gondwana about 600 million years ago (Abdelsalam et al., in press)

Nile Paleodrainage

SIR-C/X-SAR images were used to understand what controlled the course of the Nile in Nubia, and led directly to the discovery of a previously unknown, 25 km long, 1 km wide, paleochannel (Fig. 4). The location of the paleodrainage relative to the present river indicates that the Nile has recently changed its course to the south near the great bend (Stern and Abdelsalam, 1996). SIR-C/X-SAR images also revealed that the course of the Nile in Nubia is either controlled by N-trending Precambrian (ca 600 million year old) structures or E-trending faults that are younger than 90 million years old (Fig. 5). SIR-C/X-SAR allowed us to discover that the younger faults define a ~1000 km wide zone which extends from the Bayuda Desert in the Sudan in the south to the central part of the Western Desert in Egypt. These faults are interpreted to reflect recent uplift of the Nubian Swell (Stern and Abdelsalam, 1996).

Future plans for using SIR-C/X-SAR data

Neotectonics in Nubia and its impact on water resources

As outlined above, SIR-C/X-SAR imagery allowed us to identify and map basement structures that control parts of the course of the Nile. The N-S structures are ancient and the river follows these because certain N-S layers are more easily eroded or because younger uplift has exploited these older structures, for example as the Red Sea Hills have been uplifted relative to the interior). However the E-W faults are much younger and may still be active, particularly if uplift of the Nubian Swell continues, and such faults may exert a more active control on the river's course. Recent seismic activity reported from around the Aswan area (Toppozada et al., 1984) suggests that some of the E-W faults are still active, posing a potential risk to the Aswan Dams (Fig 6). We plan to use SIR-C/X-SAR data to understand neotectonics in Nubia and its impact on water resources in the Middle East. A preliminary effort will include co-registration of SIR-C/X-SAR images, Landsat TM images, lithological and structural data, gravity data, and seismicity data to analyze them in a GIS environment in an attempt to understand the relationship between seismic activity and faults distribution and geometry.

References

Abdelsalam, M.G., and Stern, R.J., 1993a. Tectonic evolution of the Nakasib suture, Red Sea Hills, Sudan: Evidence for a Late Precambrian Wilson Cycle. J. Geol. Soc. London, 150, 393-404.

Abdelsalam, M.G., and Stern, R.J., 1993b. Structure of the late Proterozoic Nakasib suture, Sudan. J. Geol. Soc. London, 150, 1065-1074.

Abdelsalam, M. G., Stern, R. J., Schandelmeier, H., and Sultan, M., 1995, Deformational history of the Keraf zone in NE Sudan revealed by Shuttle Imaging Radar, Journal of Geology, 103, 475-491.

Abdelsalam, M.G., Stern, R.J., Copeland, P., Elfaki, E.M., Elhur, B., and Ibrahim, F.M., in press. The Neoproterozoic Keraf Suture in NE Sudan: Sinistral Transpression along the Eastern Margin of West Gondwanaland. Journal of Geology.

Abdelsalam, M.G., and Stern, R.J., 1996. Mapping Precambrian Structures in the Sahara Desert with SIR-C/X-SAR Radar: The Neoproterozoic Keraf Suture, NE Sudan. Journal of Geophysical Research, 101(E10), 23,063-23,076

Abdelsalam, M.G., and Stern, R.J., 1996. Sutures and Shear Zones in the Arabian- Nubian Shield. J. African Earth Sciences, 23, 289-310.

Schandelmeier, H., Wipfler, E., Küster, 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 Northeast Africa. Geology 22, 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 Science 22, 319-351.

Stern, R.J., and Abdelsalam, M.G., 1996. The Origin of the Great Bend of the Nile from SIR-C/X-SAR Imagery. Science, 274, 1696-1698

Stern, R.J., Nielsen, K.C., Best, E., Sultan, M., Arvidson, R.E., and Kröner, A., 1990. Orientation of late Precambrian sutures in the Arabian-Nubian Shield. Geology 18, 1103-1106.

Sultan, M., Arvidson, R.E., and Sturchio, N.C. 1986. Mapping of serpentinites in the E Desert of Egypt by using Landast Thematic Mapper data. Geology 14, 995-999.

Sultan, M., Arvidson, R.E., Sturchio, N.C., and Guiness, E.A. 1987. Lithologic mapping in arid regions with Landsat thematic data: Meatiq dome, Egypt. Geol. Soc. Amer. Bull. 99, 748-762.

Toppozada, T. R., Boulos, F. K., Hennin, S. F., El-Sherif, A. A., El-Sayed, A. A., Basta, N. Z., Shatiya, F. A., Melek, Y. S., Cramer, C. H., and Park, D. L. 1984. Seismicity near Aswan High Dam, Egypt, following the November 1981 earthquake. Ann. Geol. Surv. Egypt 14, 107-126.Figure Caption

Figure 1: (A) Advanced Very High Resolution Radiometer (AVHRR) mosaic of NE Africa. (B) Major structures in the Nubian Desert.

Figure 2: (A) Space Shuttle Hand-Held Photograph (HHP) of the Nubian Desert showing the location of the Keraf Suture. (B) Ground photograph of landscape around the Keraf Suture. (C) HHP of part of the Keraf Suture. (D) SIR-C image covering the same area covered by the HHP in figure 2B. In this image Lhh is red, Chh is green and Lhv is blue.

Figure 3: (A) AVHRR mosaic of NE Africa showing the location of the Keraf Suture. (B) SIR-C image covering part of the Keraf Suture. In this image Lhh is red, Chh is green and Lhv is blue. The image revealed for the first time the complex structures in the Keraf suture.

Figure 4: (A) AVHRR mosaic of NE Africa showing the location of the paleochannel which was discovered using SIR-C/X-SAR imagery. (B) HHP of the area of the paleochannel. Note that the paleochannel cannot be identified in the HHP. (C) SIR-C image covering the same area covered by the HHP in figure 4B. In this image 1/Chh is red, 1/Lhh is green, and 1/Lhv is blue.

Figure 5: (A) AVHRR mosaic of NE Africa showing the location of a Cretaceous, E-trending fault which cuts in half a 90 million year old ring complex. (B) Landsat TM image of the area occuppied by the faulted ring complex. In this image band 5 is red, band 4 is green, and band 3 is blue. The image revealed dominantly sand cover. (C) SIR-C image covering the same area and revealing much more structural information. In this image 1/Chh is red, 1/Lhh is green, and 1/Lhv is blue.

Figure 6: (A) AVHRR mosaic of NE Africa showing the location of the Aswan Dams. (B) SIR-C image of the Aswan Dams and part of lake Nasser. In this image Chh is red, Lhh is green, and Lhh/Chh is blue. The image reveals E- and N-trending high-angle faults close to the dam.