Panafrican and older evolution in Tanzania

The existence of the supercontinent Gondwana on the southern hemispere during Paleozoic time (from c. 550 ma to c. 250 ma) is a fact in our knowledge of the history of earth. During the Pan-African orogeny (700-550 Ma) this supercontinent must have been amalgamated by collision of several continental fragments of western and eastern Gondwana. The aim of our project is to study the kinematics, geological time span, and petrological development of this collision in central and southern Tanzania. Our former work in southern Kenya suggested the existance of an eastern terrane of continental margin affinity, a central oceanic terrane with MORB affinity which contains a sinistral north-south trending wrench fault, and a western terrane which exhibits properties of an accretionary wedge. As the oceanic domain between an eastern passive continental margin and a western terrane was closed, it incorporated island arc and accretionary wedge material. Oblique convergence is suggested by wrench tectonics. Continuation of these terranes could be traced into central Tanzania. In southern Kenya, Sm-Nd garnet-whole rock ages on migmatic orthogneisses show 3 age domains: (1) Ages around 585 Ma in the western terrane. (2) An age of about 550 Ma in the central and eastern terrane. (3) An age of 530 Ma in the shear zone. Since large fault systems could explain these inconsistencies in ages, we need additional information about the role faults played in juxtaposing older and younger domains. Our present knowledge of the Pan-African major structures in East Africa is the following: NE-SW nappe stacking acted along high-temperature thrusts within a deep crustal level in the north, whereas in central Tanzania there was relatively cold W-vergent thrusting of nappes rooting in the eastern part. Strike-slip shear zones juxtapose areas with different age domains. The main open questions are: What is the distribution of juvenile rocks within the belt? Which terranes were incorporated into the Pan-African Belt? What is the date of the initial rifting (e.g. the break-up of the older supercontinent Rodinia)? Do Pan-African structures include - extensional structures (Rhodinia opening)? - west-east shortening that exhibits a regional trend with decreasing temperature conditions towards Archean forelannd units? - large wrench tectonics (major strike slip faults)? What was the major mechanism of the juxtaposition of different age domains? Which geodynamic model will explain both, Usagaran, and Pan-African Tectonics?

This research project was located in the East African Orogen in Tanzania, a mountain range that is north-south striking along the whole East African Coast. This mountain range was formed at circa 550 - 700 Ma ago in the eon of the `Neoproterozoic` leading to the formation of the supercontinent `Gondwana` on the southern hemisphere. This supercontinent was accreted from continental fragments of West- and East-Gondwana during the so-called `Pan- African Orogeny`. Major suture zones of this supercontinent are located in Tanzania. Those sutures were formed by complex tectonic processes over long time spans hundreds of million years ago and are therefore hard to discover and reconstruct. A distinction of these Pan-African structures from pre-existing and subsequent tectonic events as well as the understanding of the spatial and temporal history of this orogen was a major task of this project. In order to understand the Pan-African Orogen we had to work on different scales. Satellite images and digital elevation models were used to discover and characterize major shear zones and crustal blocks in between on orogenic scale. The compilation of available geological maps from the study area was an extremely useful approach to provide a tectonic overview map of central Tanzania in order to define tectonic key areas with distinct structural elements. Five field campaigns during the three project years from 8 earth scientists of the University of Graz were successfully carried out in cooperation with Tanzanian colleagues (University of Dar es Salaam). Circa 1500 rock samples were taken as well as numerous rock orientation data in the field. Microscopic studies on thin sections of the samples allowed a detailed reconstruction of the deformational, pressure and temperature history that experienced the various crustal blocks in the deep earth`s crust. A variety of samples was chosen for chemical and isotopic analyses in order to constrain rock chemistry and formation age. The combination of different scientific techniques led to a large set of data which allowed reconstruction of a model for the formation of the East African Orogen. The project results were presented at international conferences and can be read in international journals. The Insitute for Earth Sciences (University of Graz) gained unique Know-how by these studies and ensured the appreciation of international colleagues and an important position in this part of basic research.