Jean Klerkx

Short-lived Eburnian orogeny in southern Mali. Geology, tectonics, U-Pb and Rb-Sr geochronology

Abstract This work is based on a comprehensive field survey, including mapping of the Massigui degree sheet in southern Mali. The investigated area belongs to the northern part of the Man shield which is mainly composed of Lower Proterozoic volcano-sedimentary sequences (Birimian) and of large areas of granitoids, the whole being affected by the Eburnian orogeny. Structural and geochronological (U-Pb and Rb-Sr) results from this study point to a short period of time (∼ 30 Ma) for the Eburnian orogenesis between 2100 and 2070 Ma, the events recorded later (1984 ± 30 Ma) being due to tectonic reactivations. On the other hand, no trace of events older than 2100 Ma have been found. Coupled with geochemical and Rb-Sr isotopic systematics, this leads to invoke a fundamental link between Birimian volcanics and Eburnian granitoids, both being high-K calc-alkaline in composition and coming from a single, evolving mantle source in a palaeo-environment which has some of the characteristics of modern subduction (including back-arc) or docking zones. The main regional structure of the area strikes roughly NNE-SSW with an important strike-slip component, escorted by a greenschist metamorphism. The deformation is the most intense in the N40° to N10° 5 km large Banifin shear-zone where metamorphism reached the lower amphibolite facies. Here one observes subvertical strong foliation, non-cylindrical folding (locally sheath folds) and subhorizontal pencil lineations, the deformation being regionally recorded by light subisoclinal folds with subvertical axes compatible with a sinistral movement. This regional shear tectonics (D2,2074+9/-8 Ma) is preceded by an isoclinal deformation (D1, bracketed between 2098 ± 5 Ma and D2) only known as relics, from which it likely derived. A later phase (D3) occurred along the Banifin shear belt, at a more superficial level, even if still ductile, generated by a dextral movement and having induced retrograde metamorphism. This last tectonic phase, which is not accompanied by plutonism nor volcanism, has been dated at 1982±30 Ma being responsible for the complete resetting of the Rb-Sr isotopic system in the plutons present in the Banifin corridor.

The Palaeoproterozoic Ubendian shear belt in Tanzania: geochronology and structure

The Ubendian belt is a linear, NW-SE trending orogenic belt in western Tanzania. It is part of a larger Palaeoproterozoic orogen, developed around the west and south-western margin of the Archaean Tanzanian craton. The Ubendian Belt has experienced several periods of reactivation since the Palaeoproterozoic, acting as a zone of displacement during successive orogenic and rift-forming events. The Ubendian Belt is characterized by an early deformation and granulite-facies metamorphism, isotopically dated at 2100–2025 Ma, and marked by an E-W to ESE-WNW trending foliation. This phase also affected the adjacent Usagaran (and Bangweulu?) Belt and is interpreted as a product of collisional orogeny along the SW margin of the Tanzanian, and possibly Congo, cratons. A second phase of deformation, apparently restricted to the Ubendian Belt, is characterized by large, NW-SE trending, dextral shear zones. This phase is responsible for the creation of the eight crustal blocks developed throughout the belt and overprints much of the earlier deformation fabric. This second deformation phase is terminated by late- to post-kinematic calc-alkaline granitic batholiths (ca 1860 Ma). A phase of tectonic reactivation occurred locally at ca 1725 Ma. Notable by its absence, is any evidence of Kibaran isotopic ages in the Ubendian Belt. Major Kibaran orogenic belts occur to the north (Burundi) and south (Zambia) of the Ubendian Belt, although they are apparently not isotopically recorded within it. A third phase of deformation is characterized by Neoproterozoic (ca 750 Ma) reactivations of the Ubendian shear zones. This deformation is marked by the development of sinistral, brittle-ductile shear zones, displaying retrograde metamorphic mineral assemblages, and intruded by alkaline plutons. These late shear zones were the preferential locus for the brittle rift faults of the western branch of the East African Rift.

Persistent fault controlled basin formation since the Proterozoic along the Western Branch of the East African Rift

Abstract The Western Branch of the East African Rift System is outlined by elongate sedimentary basins, frequently occupied by Cenozoic rift lakes. The role of the inheritance of the leading rift faults from pre-existing basement structures has often been invoked. Recent studies in western Tanzania confirm the extent of the northwest orientated Palaeoproterozoic Ubende Belt contribution to the Phanerozoic Rift. Attention is drawn here on the occurrence of different Meso- and Neoproterozoic sedimentary basins that developed along the ductile shear belt as a result of repeated sinistral wrench fault reactivation. These basins partly overlap each other and typically bear shallow and weakly evolved sediments. North of the Ubende Belt, the Mesoproterozoic Kibara Belt is inferred to have originated as a basin controlled by the complex termination of the Ubende wrench fault. Phanerozoic rift basins also develop along the northwest orientated Ubende Belt structure. They display the same elongate shape as the Proterozoic basins. In Late Palaeozoic-Early Mesozoic the Karoo rift basins formed from a dextral lateral shear reactivation of the inherited Proterozoic shear faults. During the first phase of development the Lake Tanganyika Basin is belived to bear the same characteristics as all previous basins along the Ubende Shear Belt, mainly controlled by strike-slip movements along pre-existing shear faults. The present Lake Tanganyika Basin is subdivided in two sub-basins, separated by the transverse Mahali Shoal, which is an active structure located on the Ubende Shear. The deep lake basin mainly developed outside the Ubende Belt. The northern sub-basin appears to be structurally controlled by the reactivation of the Mesoproterozoic sinistral wrench fault termination of the Ubende shear faults. Structural control of the Palaeoproterozoic basement is however unclear for the southern sub-basin of Lake Tanganyika: this part of the rift segment is flanked by Palaeoproterozoic basement which has not been affected by the Ubende Shear.

1,000-year environmental history of Lake Issyk-Kul.

Lake Issyk-Kul constitutes one of the most important economic resources in the Republic of Kyrgyzstan, with more than 100 recreational centers along its shore. Some 370,000 holidaymakers visit the lake annually, and this number is expected to increase in the near future given the growing interest in natural environments (Romanovsky, 1990; Savvaitova and Petr, 1992). Thus, a fuller understanding of the past and present evolution of this ecosystem is essential for promoting and sustaining this natural habitat.

Structural evolution of the Teletsk graben (Russian Altai)

Abstract Lake Teletskoye in the northeastern part of the Altai mountain range has attracted the attention of geo-scientists for a long time, because it fills an impressive tectonic depression. The lake is 77 km long and 4 km wide, and it has a maximum water depth of 325 m. The vertical offset of the basement surface is up to 3000 m. A multidisciplinary study of the Teletsk graben was carried out during the last few years, including satellite image and air photo analysis, bathymetric-, structural- and geomorphological mapping, high-resolution seismic profiling and seismic refraction. The structural study revealed that reactivation of preexisting weak basement zones is important in controlling the basin formation. These zones separate different tectonic terranes at the contact of which the Teletsk graben developed. This study identifies the significance of the basin in the regional neotectonic context. It shows that the major vertical movements are restricted to the basin itself, but do not characterize the whole region. Outside of the basin, recent tectonic structures have the same pattern as adjacent areas of Northeast Altai and West-Sayan. Quaternary glaciations have had no major influence on the basin formation. Two stages of faulting are identified. First, transpressive movements restricted to discrete (reactivated) fault zones controlled the opening of the basin. In the second stage, normal faulting is dominant and is responsible for the modern basin outline. An echo-sounding survey led to the recognition of several morphological characteristics of the lake bottom. In the southern part, the uppermost sediments seem slightly disturbed, whereas further north, transverse ridges and slope breaks are increasingly common. The deepest part of the lake is located in a highly disturbed zone of normal fault-bounded blocks. The structural difference between the southern and northern subbasins is supported by the interpretation of a deep seismic refraction profile which indicates a substantial increase of basement isochores in the area where the reactivated Teletsk (Paleozoic) shear zone crosses the lake. Correlation of high-resolution seismic profiles suggests that the Teletsk graben started to evolve during the Pleistocene, and that its present shape was formed in two stages. The first stage was responsible for the opening of the southern basin. It probably started in the Middle Pleistocene. A second kinematic stage induced by a sinistral reactivation of the NE striking West-Sayan fault initiated the opening of the different segments of the northern subbasin due to opposite movements between the reactivated Teletsk and West-Sayan faults. This second stage was active after the end of Late Pleistocene glaciations and during the Holocene. The recent lateral extension and the related N–S-trending normal faults result from a change in tectonic regime, with related extensional movements along the main reactivated fault zones. These recent movements result in the lateral escape of the lake borders and the collapse of the area between them.