Abstract High topography in the realm of the rifted East African Plateau is commonly explained by two different mechanisms: (1) rift-flank uplift resulting from mechanical and/or isostatic relaxation and (2) lithospheric uplift due to the impingement of a mantle plume. High topography in East Africa has far-reaching effects on atmospheric circulation systems and the amount and distribution of rainfall in this region. While the climatic and palaeoenvironmental influences of high topography in East Africa are widely accepted, the timing, the magnitude and this spatiotemporal characteristic of changes in topography have remained unclear. This dilemma stems from the lack of datable, geomorphically meaningful reference horizons that could unambiguously record surface uplift. Here, we report on the formation of high topography in East Africa prior to Cenozoic rifting. We infer topographic uplift of the East African Plateau based on the emplacement characteristics of the c. 300 km long and 13.5 Ma Yatta phonolitic lava flow along a former river valley that drained high topography, centred at the present-day rift. The lava flow followed an old riverbed that once routed runoff away from the eastern flank of the plateau. Using a compositional and temperature-dependent viscosity model with subsequent cooling and adjusting for the Yatta lava-flow dimensions and the covered palaeotopography (slope angle), we use the flow as a ‘palaeo-tiltmeter’. Based on these observations and our modelling results, we determine a palaeoslope of the Kenya dome of at least 0.2° prior to rifting and deduce a minimum plateau elevation of 1400 m. We propose that this high topography was caused by thermal expansion of the lithosphere interacting with a heat source generated by a mantle plume. Interestingly, the inferred Mid-Miocene uplift coincides with fundamental palaeoecological changes including the two-step expansion of grasslands in East Africa as well as important radiation and speciation events in tropical Africa.

Abstract Since the discovery of ophiolite sequences, the Bou-Azzer inlier has been considered a key area for understanding the evolution of the northern margin of the West African craton during the Pan-African orogeny. For about 20 years, it had been commonly accepted that the Bou-Azzer inlier represents an accretionary mélange accreted onto the West African craton under blueschist metamorphic conditions, similar to the Franciscan Complex and the Sanbagawa facies series. This would imply that a low geothermal gradient was prevalent during the subduction of the Pan-African oceanic plate, and that the ocean was subducted with a high convergence rate. A reinvestigation of the metamorphic conditions by a thermodynamic approach shows that the ophiolite sequence of Bou-Azzer underwent HT greenschist metamorphic conditions instead of blueschist metamorphic conditions. We propose that the ophiolites of Bou-Azzer are not similar to the Sanbagawa facies series or to the Franciscan Complex, but bear similarities to the Albanian or Cyprus ophiolites, which represent dismembered ophiolite sequences overprinted by greenschist conditions.

Abstract We summarize ages of the high-pressure/low-temperature (HP/LT) metamorphic evolution of the central and the western Alps. The individual isotopic mineral ages are interpreted to represent either: (1) early growth of metamorphic minerals on the prograde path; (2) timing close to peak metamorphism; or (3) retrograde resetting of the chronometers at still-elevated pressures. Therefore, each individual age cannot easily be transferred to a geodynamic setting at a certain time. These different data indicate a subduction-related metamorphism between 62 and 35 Ma in different units (e.g. Voltri Massif, Schistes Lustrés of the western Alps, Tauern Window). Oceanic and continental basement units show isotope ages related to eclogitic or blueschist facies metamorphism between 75 and 40 Ma. Most of these ages may record equilibration along the retrograde path, except of some Lu/Hf garnet ages and some zircon SHRIMP ages, which provide information on the prograde path. These different isotope ages are interpreted as different steps along pressure–time paths and so may provide some information on the geodynamic evolution. The data record a continuous subduction, which is ongoing for several tens of millions years. In a large-scale picture, we have to assume fragmentation of the downgoing plate in order to explain the available P–T and t data. This interpretation questions the ongoing driving force for subduction during the disappearance of the Alpine Tethys.

Abstract Major discoveries in metamorphic petrology, as well as other geological disciplines, have been made in the Alps. The regional distribution of Late Cretaceous–Tertiary metamorphic conditions, documented in post-Hercynian metasediments across the entire Alpine belt from Corsica–Tuscany in the west to Vienna in the east, is presented in this paper. In view of the uneven distribution of information, we concentrate on type and grade of metamorphism; and we elected to distinguish between metamorphic paths where either pressure and temperature peaked simultaneously, or where the maximum temperature was reached at lower pressures, after a significant temperature increase on the decompression path. The results show which types of process caused the main metamorphic imprint: a subduction process in the western Alps, a collision process in the central Alps, and complex metamorphic structures in the eastern Alps, owing to a complex geodynamic and metamorphic history involving the succession of the two types of process. The western Alps clearly show a relatively simple picture, with an internal (high-pressure dominated) part thrust over an external greenschist to low-grade domain, although both metamorphic domains are structurally very complex. Such a metamorphic pattern is generally produced by subduction followed by exhumation along a cool decompression path. In contrast, the central Alps document conditions typical of subduction (and partial accretion), followed by an intensely evolved collision process, often resulting in a heating event during the decompression path of the early-subducted units. Subduction-related relics and (collisional/decompressional) heating phenomena in different tectonic edifices characterize the Tertiary evolution of the Eastern Alps. The Tuscan and Corsica terrains show two different kinds of evolution, with Corsica resembling the western Alps, whereas the metamorphic history in the Tuscan domain is complex owing to the late evolution of the Apennines. This study confirms that careful analysis of the metamorphic evolution of metasediments at the scale of an entire orogen may change the geodynamic interpretation of mountain belts.