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Title: Gravity anomalies, flexure, and the long-term rigidity of the continental lithosphere
Author: Jordan, Tom A. R. M.
ISNI:       0000 0004 2671 989X
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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The cause and distribution of spatial variations in the mechanical properties of the continental lithosphere are fundamental questions for modern geology. In this study variations in long-term lithospheric rigidity have been investigated. These investigations used profile- and grid-based flexural models of the lithosphere’s response to geologically imposed topographic, or buried, loads. These models were constrained by topographic and gravity data allowing recovery of best fitting rigidity values. In Oman a Cretaceous ophiolite acts as a significant load on the continental crust. Flexural models along profiles orthogonal to the ophiolite strike show that the observed gravity data can be best modelled by an elastic beam with standard thickness (Te) of 30 km. Along strike there is shown to be significant variation in the foreland shape and the observed gravity signal. This, it is proposed, relates to the complex tectonic processes which occurred as the ophiolite was obducted. The Himalayan foreland has been the focus of controversy over the recovered long-term rigidity of the continents, with recovered Te values ranging from 40 to over 90 km. Both profile- and grid-based techniques show that Te is high (>70 km) in the foreland region. Across the India-Eurasia collisional system as a whole Te values are variable. Beneath the Tibetan plateau recovered values are generally low (<10 km), while the plateau margins are marked by regions of higher rigidity. Recovered Te values across the Arabia-Eurasia collisional system range from over 60 km in the foreland region to close to zero beneath the high Zagros mountains. In the eastern part of the foreland, flexural models match the gravity data; however, they disagree with sediment thickness data for the material infilling the foreland. This discrepancy is interpreted in terms of de-coupling of the flexural lithosphere from the shallower crustal levels, caused by the presence of significant salt deposits in this region. Application of grid-based techniques to South America, North America and Europe recover a broad range of Te values from ∼0 to over 90 km. The low Te values are explained in active orogenic belts in terms of current processes acting to weaken the lithosphere, and in the continental interiors as the relics of past orogenic events. High Te values in the continental interiors correlate with ancient cratonic cores which have undergone little deformation since their formation in the Archean. This study shows that Te variations have a critical influence on the development of large compressional orogenic belts. In the Himalayan and Andean orogens there is a correlation between the over-thrusting of the orogenic belt and high Te foreland regions. Where lower Te regions are seen, less over thrusting is apparent, and in the case of the India-Eurasia collisional system out-flow of lower crustal material may be occurring.
Supervisor: Watts, A. B. ; Searle, M. P. Sponsor: Natural Environmental Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Earth sciences ; Earthquakes and tectonics ; Marine geology and geophysics ; Te ; India ; gravity model ; inversion