No abstract

The Cenozoic East African Rift System (EARS) extends from the Red Sea to Mozambique. Here we use seismic reflection and bathymetric data to investigate the tectonic evolution of the offshore branch of the EARS. The data indicate multiple and time transgressive neotectonic deformations along~800 km of the continental margin of northern Mozambique. We observe a transition from a mature rift basin in the north to a juvenile fault zone in the south. The respective timing of deformation is derived from detailed seismic stratigraphy. In the north, a~30 km wide and more than 150 km long, N-S striking symmetric graben initiated as half-graben in the late Miocene. Extension accelerated in the Pliocene, causing a continuous conjugate border fault and symmetric rift graben. Coevally, the rift started to propagate southward, which resulted in a present-day~30 km wide half-graben, approximately 200 km farther south. Since the Pleistocene, the rift has continued to propagate another~300 km, where the incipient rift is reflected by subrecent small-scale normal faulting. Estimates of the overall brittle extension of the matured rift range between 5 and 12 km, with an along-strike southward decrease of the extension rate. The offshore portion of the EARS evolves magma poor, similar to the onshore western branch. The structural evolution of the offshore EARS is suggested to be related to and controlled by differing inherited lithospheric fabrics. Preexisting fabrics may not only guide and focus extension but also control rift architecture.

Upper Carboniferous Coal Measures strata have been interpreted traditionally in terms of cyclothems bounded by marine¯ooding surfaces (marine bands) and coal seams. Correlation of such cyclothems in an extensive grid of closely spaced coal exploration boreholes provides a robust stratigraphic framework in which to study the Lower Coal Measures (Namurian C±Westphalian A) of the Ruhr district, north-west Germany. Three distinct types of cyclothem are recognized, based on their bounding surfaces and internal facies architecture.(1) Type 1 cyclothems are bounded by marine bands. Each cyclothem comprises a thick (30±80 m), regionally extensive, coarsening-upward delta front succession of interbedded shales, siltstones and sandstones, which may be deeply incised by a major¯uvial sandstone complex. The delta front succession is capped by a thin (<1 m), regionally extensive coal seam and an overlying marine band de®ning the top of the cyclothem. (2) Type 2 cyclothems are bounded by thick (»1 m), regionally extensive coal seams with few splits. The basal part of a typical cyclothem comprises a thick (15±50 m), widespread, coarsening-upward delta front or lake in®ll succession consisting of interbedded shales, siltstones and sandstones. Networks of major (>5 km wide, 20±40 m thick), steep-sided, multistorey¯uvial sandstone complexes erode deeply into and, in some cases, through these successions and are overlain by the coal seam de®ning the cyclothem top. (3) Type 3 cyclothems are bounded by regionally extensive coal seam groups, characterized by numerous seam splits on a local (0á1±10 km) scale. Intervening strata vary in thickness (15±60 m) and are characterized by strong local facies variability. Rootpenetrated, aggradational¯oodplain heteroliths pass laterally into single-storeȳ uvial channel-®ll sandstones and coarsening-upward, shallow lake in®ll successions of interbedded shales, siltstones and sandstones over distances of several hundred metres to a few kilometres. Narrow (<2 km) but thick (20± 50 m) multistorey¯uvial sandstone complexes are rare, but occur in a few type 3 cyclothems. Several cyclothems are observed to change character from type 1 to type 2 and from type 2 to type 3 up the regional palaeoslope. Consequently, we envisage a model in which each cyclothem type represents a different palaeogeographic belt within the same, idealized delta system, subject to the same allogenic and autogenic controls on facies architecture. Type 1 cyclothems are dominated by deltaic shorelines deposited during a falling stage and lowstand of sea level. Type 2 cyclothems represent the coeval lower delta

No abstract

No abstract