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Explaining the presence of normal faults in overall compressive settings is a challenging problem in understanding the tectonics of active mountain belts. The Himalayan-Tibetan orogenic system is an excellent setting to approach this problem because it preserves one of the most dramatic records of long-term, contemporaneous shortening and extension. Over the past decades, several studies have described extensional features, not only in the Tibetan Plateau, but also in the Himalaya. For a long time, the favored model explained the function of the Southern Tibetan detachment system, a major fault zone in the Himalaya, as a decoupling horizon between the regime of crustal shortening forming the Himalayan wedge to the south and the extensional regime of the Tibetan Plateau to the north. However, in recent years, increasing evidence has shown that N-S-trending normal faults in the Central Himalaya crosscut not only the Southern Tibetan detachment system, but also the Main Central thrust. Here, we present new structural data and geologic evidence collected within the NW Indian Himalaya and combine them with previously published seismicity data sets in order to document pervasive E-W extension accommodated along N-S-trending faults extending as far south as the footwall of the Main Central thrust. We conducted a kinematic analysis of fault striations on brittle faults, documented and mapped fault scarps in Quaternary sedimentary deposits using satellite imagery, and made field observations in the Greater Sutlej region (Spiti, Lahul, Kinnaur) and the Garhwal Himalaya. Studies of extensional features within the regionally NW- SE-trending NW Indian Himalaya provide the advantage that arc-parallel and E-W extension can be separated, in contrast to the Central Himalaya. Therefore, our observations of E-W extension in the Indian NW Himalaya are well suited to test the applicability of current tectonic models for the whole Himalaya. We favor the interpretation of E-W extension in the NW Indian Himalaya as a propagation of extension driven by collapse of the Tibetan Plateau.
The development of rise Cenozoic East African Rift System (EARS) profoundly re-shaped the landscape and significantly increased the amplitude of short-term environmental response to climate variation. In particular, the development of amplifier lakes in rift basins after three million years ago significantly contributed to this exceptional sensitivity of East Africa to climate change compared to elsewhere on the African continent. Amplifier lakes are characterized by tectonically-formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area. Such amplifier lakes respond rapidly to moderate, precessional-forced climate shifts, and as they do so apply dramatic environmental pressure to the biosphere. Rift basins, when either extremely dry or lake-filled, form important barriers for migration, mixing and competition of different populations of animals and hominins. Amplifier lakes link long-term, high-amplitude tectonic processes and short-term environmental fluctuations. East Africa may have become the place where early humans evolved as a consequence of this strong link between different time scales. (C) 2010 Elsevier Ltd. All rights reserved.
Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the similar to 300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2 degrees, suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa.