The Tectonic and Climatic Evolution of High Plateaux

High topography significantly affects climate and atmospheric circulation, often separating areas of intense precipitation from relatively arid rainshadows inland. Temporal variations in climate on high plateaux have been inferred from both rocks and ice and may be related to changes in global climate, local atmospheric circulation, and/or changes in surface elevation. Constraints on how and when surface topography was generated not only provide insight into the relationship between high plateaux and climate, but help us distinguish between different geodynamic mechanisms responsible for their formation. The following research employs multiple techniques across the Andean Plateau, the Pamir, and Tibetan Plateau, to better understand both the tectonic evolution of high plateaux and how they affect climate and atmospheric circulation, particularly in continental settings.;The Andean Plateau in South America is the second highest and most extensive topographic feature on Earth. Paleoelevation constraints from fossil leaf physiognomy and stable isotopes of sedimentary carbonate suggest that significant surface uplift of the northern Andean plateau, on the order of 2.5 +/- 1 km, occurred between ∼10.3 and 6.4 million years ago (Ma). South American teeth from modem and extinct mammal taxa spanning from the Oligocene (∼29 Ma) to present were collected as they preserve a record of surface water isotopes and the type of plants that animals ingested. Previous studies have shown that the isotopic composition of oxygen (delta18O) in modern precipitation and surface waters decreases systematically with increasing elevations across the central Andes. Results from high elevation sites show substantially more positive delta18O values for late Oligocene tooth samples compared to <10 Ma tooth delta18O values. Late Oligocene teeth collected from low elevation sites in southeast Brazil show delta18O values within 2 per mil (‰) of contemporaneous teeth collected at high elevation in the Eastern Cordillera. This suggests that the Andean plateau was at a very low elevation during the late Oligocene. Late Oligocene teeth from the northern Eastern Cordillera also yield consistent carbon isotopic compositions (delta13C) of about -9‰, indicating the environment was semi-arid at that time. Latitudinal gradients in delta18O values of late Miocene to Pliocene fossil teeth are similar to modern values for large mammals, suggesting that by ∼8 Ma in the northern Altiplano and by ∼3.6 Ma in the southern Altiplano, both regions had reached high elevation and established a latitudinal rainfall gradient similar to modern.;The Pamir Plateau is the western expression of topographic growth related to Indo-Eurasian convergence. New stratigraphic, provenance, and oxygen stable isotope data (delta18O) from Jurassic to Miocene strata along the Pamir's northeastern margin provide better constraints on the region's tectonic and climatic history. Prominent ∼40 Ma U-Pb ages in Oligocene to early Miocene detrital zircon grains record the exhumation of an Eocene belt of shoshonitic rocks in the central to southeastern Pamir. This is roughly coincident with an ∼4‰ shift in carbonate delta18O values during the Eocene and/or Oligocene (from an average of -8.7‰ to -12.6‰), suggesting the Pamir became a significant topographic barrier to westerly atmospheric flow during that time. A change from Eocene to Jurassic aged detrital zircon grains in the early to middle Miocene indicates provenance shifted from source rocks in the central Pamir toward the Tarim Basin to the hanging wall of the Main Pamir Thrust (MPT), consistent with prograding facies at that time. Our results corroborate north-northeastward advancement of Himalayan deformation, affecting all margins of the Tarim Basin by the middle Miocene.;Paleoaltimetry on the central and northern Tibetan Plateau is challenged by the lack of a clear relationship between stable isotopes of meteoric water and elevation north of the Himalaya. New delta18O and deuterium excess (d excess) results from modem surface water along a roughly south-north transect on the eastern margin of the Tibetan Plateau corroborate a positive trend in meteoric water delta18O that is linear (∼1.5‰ per degree latitude) and robust (R2 = 0.94). A positive trend northward is also observed in d excess of surface water from large rivers. These observations can be explained by a simple model of Rayleigh distillation modified by surface water recycling. Isotopic trends are consistent with roughly parallel transects to the west, suggesting that moisture recycling exerts control over isotope evolution across the entire plateau, regardless of possible changes in oceanic source. Assuming the northern Tibetan Plateau was equally far from an oceanic source during late Eocene- Miocene time, paleoelevations of the Hoh Xil Basin are recalculated to account for the modern latitudinal gradient in delta18O, increasing estimates by 1.1-2.7 km.