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Title: Dynamic topography of Madagascar and its surroundings
Author: Stephenson, Simon
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Some portion of long-wavelength topography and bathymetry in the southwestern Indian Ocean is supported by convective processes within the Earth's mantle. Predictive models suggest that both southern Africa and the southwestern Indian Ocean are anomalously elevated by 0.5--1.5~km. In contrast, observed residual depth anomalies of the oceanic plate vary over wavelengths of $10^{3}$~km with amplitudes of $\pm 1$~km around the African margin. Positive long-wavelength free-air gravity anomalies, sub-plate seismic velocity anomalies, and geochemical observations along the Southwest Indian Ridge suggest that these changes in ridge depth are controlled by thermochemical anomalies within a 150~km channel located immediately beneath the plate. Onshore, lithospheric and crustal thickness estimates can be combined with free-air gravity anomalies to gauge dynamic support. Madagascar and southern Africa have an admittance of $40\pm10$~mGal~km$^{-1}$ at wavelengths of 500--3000~km indicative of some component of dynamic support. Differences between gravity and topographic signals at longer wavelengths suggest that deeper mantle anomalies may partly support the Southern African Plateau. Both Madagascar and Southern Africa are located in distal equatorial regions which are routinely exploited for the study of Quaternary eustatic sea level. A significant problem is that sea level markers fringing Madagascar may have experienced vertical motions arising from the spatio-temporal evolution of dynamic topography. A suite of coral reef terraces were surveyed, mapped and sampled across northern Madagascar. The intermediate terrace is dated to the last interglacial period using U-series techniques ($130.7\pm13.2$~ka). A tilt in terrace elevations from 9.3--2.8~m cannot be explained by glacial-isostatic adjustment. Instead, growth of a dynamic topographic swell may have tilted the land surface during Quaternary times. A suite of apatite fission track and apatite helium analyses from northern and central Madagascar are presented and combined with published measurements to provide constraints on landscape evolution. Inverse modelling of these observations is used to determine thermal histories, which are a proxy for burial and exhumation. Cooling of between 10--80~$^{\circ}$C during Neogene times suggests that a pulse of uplift of 100--500~m occurred at $30\pm10$~Ma. Samples at the base of escarpments have cooled more significantly than those located higher up, suggesting that erosion is diachronous. The uplift history of Madagascar is also determined by jointly inverting a suite of longitudinal river profiles. Erosional parameters are calibrated by minimising the misfit between observed and calculated river profiles for a range of parameter values and by comparing to independent geological observations such as uplifted marine limestones. Mafic magmatism started between 10 and 30~Ma on the northern and central plateaux. A major element thermobarometer and inverse modelling of rare earth element compositions are used to estimate asthenospheric temperatures. Recovered values are consistent with estimates made by converting upper mantle shear velocity anomalies to temperature. Potential temperatures are $1330\pm30$~$^{\circ}$C beneath northern Madagascar and $1315\pm30~$^{\circ}$C beneath central Madagascar. In conclusion, a combination of both sub-plate convection and lithospheric thinning are required to generate uplift.
Supervisor: White, Nicky Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Dynamic Topography ; Convective Support ; Sea level ; Landscape Evolution ; Madagascar ; Indian Ocean