Determining the timing of the initial continental collision is a fundamental step in accurately reconstructing the paleogeodynamic evolution of orogenic belts. This process necessitates a comprehensive integration of evidence gathered through various analytical techniques, both in the field and in the laboratory, to achieve a conclusive understanding. We here use comprehensive methods, including sandstone petrography, U-Pb dating, trace element and Hf isotopic compositions of the detrital zircon, in the Haji Abad (eastern Zagros Orogen) and the Shamil areas (west of the Minab-Zendan Fault) to constrain the timing of the initial collision between Arabia and Eurasia. Zircon U-Pb dating in the Haji Abad area reveals that detritus predominantly originates from the Arabia Pan-African basement (∼640 − 539 Ma, ƐHf (t): -10 to + 10) at the base of the Upper Oligocene-Lower Miocene Razak Formation. This is subsequently replaced up-section by detritus from Cenozoic and Mesozoic Eurasia magmatic-arc sources (~ 54 − 10 Ma, ƐHf (t): -2 to + 16; ~110 − 89 Ma, ƐHf (t): -2 to + 20, and ~ 175 − 163 Ma, ƐHf (t): -4 to + 10). In contrast, the Shamil area shows that detritus from the Pan-African Arabia basement remained dominant until the Early Miocene. The minimum age of continental collision is characterized by a significant change in provenance, transitioning from detritus sourced from the Arabia lower plate to that derived from the Eurasia upper plate. This transition is documented from the Late Oligocene in the Neyriz and Haji Abad areas along the Main Zagros Thrust, to the Middle Miocene (Langhian) in the Shamil area along the Minab-Zendan Fault.
The Nile is generally regarded as the longest river in the world. Knowledge of the timing of the Nile's initiation as a major river is important to a number of research questions. For example, the timing of the river's establishment as a catchment of continental proportions can be used to document surface uplift of its Ethiopian upland drainage, with implications for constraining rift tectonics. Furthermore, the time of major freshwater input to the Mediterranean is considered to be an important factor in the development of sapropels. Yet the river's initiation as a major drainage is currently constrained no more precisely than Eocene to Pleistocene. Within the modern Nile catchment, voluminous Cenozoic Continental Flood Basalts (CFBs) are unique to the Ethiopian Highlands; thus first detection of their presence in the Nile delta record indicates establishment of the river's drainage at continental proportions at that time. We present the first detailed multiproxy provenance study of Oligocene–Recent Nile delta cone sediments. We demonstrate the presence of Ethiopian CFB detritus in the Nile delta from the start of our studied record (c. 31 Ma) by (1) documenting the presence of zircons with U–Pb ages unique, within the Nile catchment, to the Ethiopian CFBs and (2) using Sr–Nd data to construct a mixing model which indicates a contribution from the CFBs. We thereby show that the Nile river was established as a river of continental proportions by Oligocene times. We use petrography and heavy mineral data to show that previous petrographic provenance studies which proposed a Pleistocene age for first arrival of Ethiopian CFBs in the Nile delta did not take into account the strong diagenetic influence on the samples. We use a range of techniques to show that sediments were derived from Phanerozoic sedimentary rocks that blanket North Africa, Arabian–Nubian Shield basement terranes, and Ethiopian CFB's. We see no significant input from Archaean cratons supplied directly via the White Nile in any of our samples. Whilst there are subtle differences between our Nile delta samples from the Oligocene and Pliocene compared to those from the Miocene and Pleistocene, the overall stability of our signal throughout the delta record, and its similarity to the modern Nile signature, indicates no major change in the Nile's drainage from Oligocene to present day.
The Zambezi River rises at the center of southern Africa, flows across the low-relief Kalahari Plateau, meets Karoo basalt, plunges into Victoria Falls, follows along Karoo rifts, and pierces through Precambrian basement to eventually deliver its load onto the Mozambican passive margin. Reflecting its polyphase evolution, the river is subdivided into segments with different geological and geomorphological character, a subdivision finally fixed by man's construction of large reservoirs and faithfully testified by sharp changes in sediment composition. Pure quartzose sand recycled from Kalahari desert dunes in the uppermost tract is next progressively enriched in basaltic rock fragments and clinopyroxene. Sediment load is renewed first downstream of Lake Kariba and next downstream of Lake Cahora Bassa, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzo-feldspathic in the lower tract, where most sediment is supplied by high-grade basements rejuvenated by the southward propagation of the East African rift. Feldspar abundance in Lower Zambezi sand has no equivalent among big rivers on Earth and far exceeds that in sediments of the northern delta, shelf, and slope, revealing that provenance signals from the upper reaches have ceased to be transmitted across the routing system after closure of the big dams. This high-resolution petrologic study of Zambezi sand allows us to critically reconsider several dogmas, such as the supposed increase of mineralogical "maturity" during long-distance fluvial transport, and forges a key to unlock the rich information stored in sedimentary archives, with the ultimate goal to accurately reconstruct the evolution of this mighty river flowing across changing African landscapes since the late Mesozoic.
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