Journalof StructuralGeology, Vol. 10, No. 1, pp. 9 to 19, 1988 0191-8141/88 $03.00+0.00 Printed in Great Britain Pergamon Press plc Fault-block tilting: the Gebel Zeit example, Gulf of Suez I. MORETTIand B. COLLETrA Institut Francais du P6trole, B.P. 311, 1/4 av de Bois Pr6au, 92506 Rueil Malmaison C6dex, France (Received 5 September 1986; accepted in revised form 4 September 1987) Abstract--The Gebel Zeit consists of the eroded crest of a tilted block in the southern part of the Suez rift. The Zeit block displays a typical asymmetrical geometry: it is bordered to the east by a 35--45 ° E-dipping normal fault with kilometric throw and has a 30° SW-dipping homoclinal flank. Part of the pre-rift sedimentary sequence has been preserved on this flank and is unconformably covered by Upper Burdigalian (NN4) Globigerina marls. In the southern part of the block crest, the complete pre-rift series has been eroded and evaporites of Langhian age (NN5) rest directly on the Precambrian basement. Field evidence indicates alternations from erosion to sedimentation at the crest of the Zeit block. In an attempt to characterize its tilting, subsidence curves were computed along a cross-section with the backstripping method. Results indicate three stages in the evolution of the block: (1) a rapid subsidence between 22 and 16 My; (2) a pause between 16 and 10 My; and (3) a slow subsidence until present time. During the tectonic quiescence, the sedimentary loading effect alone produced an increase of 8° in the tilt angle. A simple kinematic model of tilting along a circular fault is proposed to quantify the Zeit rotation. Depth of the brittle--ductile transition is estimated at 10 km to explain the tilting. Strong driving of the tilting by the listric fault induces conjugate movements between the crest and trough of the block and explains the discrepancy between the Zeit and regional tectonic subsidence. INTRODUCTION THE Gulf of Suez (Fig. 1) is a 300 km long Cenozoic intracontinental rift trending NNW-SSE, which cuts through the Arabo-Nubian shield, where a 200-1500 m thick continental and marine series of Cambrian- Eocene age overlies a Precambrian metamorphic and granitic basement. Extensional movements began in late Oligocene-early Miocene times, as evidenced by K-Ar ages (between 32 and 18 My) of basaltic dykes around the Gulf province (Siedner 1973, Meneisy & Kreuzer 1974, Steen 1982). First marine deposits are Aquitanian to Burdigalian in age (NN3-NN4). The overall structure of the rift is controlled by normal faults and tilted blocks trending N140°-N150 °. On the south- western coast of the Gulf, the Esh el Mellaha Range and the Gebel Zeit consist of the eroded crests of two major tilted blocks dipping, respectively, 6° and 30° to the SW. The present work focuses on the Gebel Zeit block where tilting can be reconstructed using field evidence, subsid- ence analysis and simple kinematic modeling. GEOLOGICAL RECORD The Zeit block crest constitutes a low elongated N145 ° trending range (30 km long, 5 km wide). It consists of two prominent outcrops of basement rocks: Gebel Zeit itself is 14 km long with a maximum elevation of 465 m and Little Zeit is only 3 km long with a maximum elevation of 250 m. The Little Zeit is separated from the main range by a topographic saddle (maximum elevation 150 m) covered with Miocene evaporites. The strati- graphy of the Gulf of Suez has already been presented in previous works (i.e. Garfunkel & Bartov 1977, Scott & Govean 1984) and details concerning the Gebel Zeit area can be found in Perry (1983) and Colletta et al. (1986). Pre-rift sequence The basement consists mainly of metamorphic rocks intruded by pink (orthoclase and biotite) and grey (plagioclase) granites. These granites are cut by numer- ous dolerite dykes with a general N60 ° transverse trend. Basement rocks are overlain by a thick unit (--400 m) of Nubian sandstones. They are probably of continental origin, and could range from Cambrian to Lower Cre- taceous. They are conformably overlain by a much more diversified marine sequence; two main units can be attributed to the Cenomano-Turonian and to the Seno- nian. Two small isolated outcrops of cherty limestone have been attributed to the Eocene by facies analogy. These marine deposits are typical of a transgressive sequence on a stable continental platform, During Cenomanian, Turonian and Early Senonian times, abundant clastic sediments and gypsiferous marls indi- cate a nearshore environment. Pelagic conditions occur- red during late Senonian (Campanian Maestrichtian) and Eocene times. Syn-rift marine deposits A detailed analysis of sedimentary records and the main tectonic features are shown in Table 1. Miocene. The pre-rift marine units occur in the central and northern part of the West Zeit Range (Fig. 3). To the south, these units are progressively truncated by Miocene deposits and wedge out (Fig. 2). From north to south these Miocene deposits rest successively on Eocene, Upper Cretaceous, Nubian Sandstones and basement rocks (Fig. 2).