Basin Research - Volume 2, Issue 1, 1989

Abstract The Amadeus Basin, a broad intracratonic depression (800 times 300 km) in central Australia, contains a complex Late Proterozoic to mid‐Palaeozoic depositional succession which locally reaches 14 km in thickness. The application of sequence stratigraphy to this succession has provided an effective framework in which to evaluate its evolution. Analysis of major depositional sequences shows that the Amadeus Basin evolved in three stages. Stage 1 began at about 900 Myr with extensional thinning of the crust and formation of half‐grabens. Thermal recovery following extension was well advanced when a second less intense crustal extension (stage 2) occurred towards the end of the Late Proterozoic. Stage 2 thermal recovery was followed by a major compressional event (stage 3) in which major southward‐directed thrust sheets caused progressive downward flexing of the northern margin of the basin, and sediment was shed from the thrust sheets into the downwarps forming a foreland basin. This event shortened the basin by 50–100 km and effectively concluded sedimentation.

The two stages of crustal extension and thermal recovery produced large‐scale apparent sea‐level effects upon which eustatic sea‐level cycles are superimposed. Since the style of sedimentation and major sequence boundaries were controlled to a large degree by basin dynamics, depositional patterns within the Amadeus and associated basin are, to a large degree, predictable. This suggests that an understanding of major variables associated with basin dynamics and their relationship to depositional sequences may allow the development of generalized depositional models on a basinal scale.

The Amadeus Basin is only one of a number of broad, shallow, intracratonic depressions that appeared on the Australian craton during the Late Proterozoic. The development of these basins almost certainly relates to the breakup of a Proterozoic supercontinent and in large part, basin dynamics appears to be tied to this global tectonic event. Onlap and apparent sea‐level curves derived from the sequence analysis appear to be composite curves resulting from both basin dynamics and eustatic sea‐level effects. It thus appears likely that sequence stratigraphy could be used as a basis for inter‐regional correlation; a possibility that has considerable significance in Archaean and Proterozoic basins.

Abstract SEASAT altimetric measurements are used to determine the gravity anomalies across two passive continental margins: the western margin of the Rockall Plateau, UK, and the Exmouth Plateau off north‐west Australia. The small gravity anomalies observed over the starved western margin of the Rockall Plateau require the existence of a major density contrast within the crust, as well as the Moho, and show that the elastic thickness is less than 5 km at the time of rifting. The gravity anomaly over the Exmouth Plateau is compared with the gravity anomaly calculated from the sediment loading of a thin elastic plate, taking account of the variation in crustal thickness. This comparison shows that the Exmouth Plateau also has a small effective elastic thickness of 5 km, even for loads emplaced between 60 and 120 Myr after rifting. Elastic thicknesses of about 5 km have also been reported for other sedimentary basins, and are to be expected if the rheological properties of the crust and mantle depend on the ratio of the present temperature to the melting temperature. Flexural effects are therefore likely to be of minor importance in sedimentary basins.

Abstract A formal stratigraphical scheme is proposed for the Palaeozoic succession encountered in the subsurface of the Western Desert of Egypt, and is compared to other published schemes in North Africa. Two groups and seven formations span the interval from the Mid Cambrian to the Early Permian. The resolution between these superficially similar clastic‐dominated units is enhanced by the use of the palyno‐stratigraphical zonation devised by Gueinn & Rasul. It is apparent from an analysis of this rock sequence that two depositional regimes can be recognized. In the first of these, the Ghazalat Basin lay over the central Western Desert, and was bounded by high axes to the west and south‐east. An inversion event of Early Tournaisian age preceded a new depositional regime, centred along the Libyan border: the Tehenu Basin. A poorly understood sequence of strike‐slip movements on the Trans‐African Lineament is shown to be the most likely cause of this complex basin history. One consequence of this hypothesis is, however, to throw into question the validity of correlations made with the Gulf of Suez Palaeozoic succession, across the Trans‐African Lineament.

Abstract Kinematics of faults in the Northern Aegean show three extensional tectonic regimes the tensional directions of which trend (1) WNW‐ESE, (2) NE‐SW and (3) N‐S. These were active during the Upper Miocene, Pliocene‐Lower Pleistocene and Mid Pleistocene‐Present day, respectively. The main characteristics of the stress patterns (1) and (2) on the overall Aegean is tentatively explained by variations of the horizontal lithospheric stress value σzz due to the slab push and of the vertical lithospheric stress value σzz due to mass heterogeneities. During the Mid Pleistocene‐Present, due to the slab push, tectonics were compressional along the arc boundary: σzz was σ1. In the Aegean basins, tectonics were extensional, c2Z was σ1 as a consequence of the thickness of the continental crust and, possibly of an updoming asthenosphere; thus σzz became σ2, allowing tension σ3 to be orthogonal to the compression along the arc, i.e. to be roughly parallel to the arc trend. During the Pliocene‐Lower Pleistocene, the extensional regime was distinctly different. The tensional directions were roughly radial to the arc. It is suggested that σzz was weakly compressional, or eventually tensional, due a seaward migration of the slab so that σzz became σ3. In the Northern Aegean, the stress pattern has been also controlled by the westward push of the Anatolian landmass. During the Mid Pleistocene‐Present day, this was typically extensional (al was vertical) and the right lateral strike‐slip motion on the North Anatolian Fault transformed into a N‐S‐stretching, E‐W‐shortening of the Northern Aegean. Dextral strike‐slip motions along the North Aegean Trough fault zone were possible on NE‐SW‐striking faults. During the Pliocene‐Lower Pleistocene, normal fault components were higher; however, because the angle between the NE‐SW trend of the tensional axis and the strike of the fault zone was acute, dextral strike‐slip components were possible on all the faults striking NE‐SW to E‐W. A clockwise 15^o rotation of Limnos with respect to Samothraki, Thraki and Thassos, suggested by structural data, was probably associated with these dextral motions. The WNW‐ESE trending tension during the Upper Miocene indicates that the dextral North Anatolian Fault had not yet merged into the North Aegean Trough fault zone at that time. We propose that the formation of Aegean basins during the Cenozoic was related to the activity of two major Hellenic arcs. The ‘Pelagonian‐Pindic Arc’ resulted in the formation of the subsident Aegean basins of Middle Eocene‐Lower Miocene age and of the older Northern Aegean orogenic volcanism. The ‘Aegean Arc’ resulted in the formation of the subsident Aegean basins of Middle Miocene to Present day age and of the Southern Aegean orogenic volcanism. Were these arcs associated with a unique subduction zone or with two such zones ? In the first case, the slab is no more than 16 Myr old, in the second it may be as old as 45–50 Myr. The answer depends on the accuracy of the seismic tomography profiles.