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Dr Mark Hoggard

Dr Mark  Hoggard

Post-doctoral Research Fellow

Bullard Laboratories, Madingley Road, Cambridge, CB3 0EZ. UK.
Email:
Office Phone: +44 (0) 1223 337179

Research Interests

Dynamic Topography

Since its formation 4.5 billion years ago, the Earth has been steadily cooling, solidifying and segregating into a dense iron core and silicate mantle. Heat is continuously transported upwards, and convection within the mantle generates some fraction of surface topography that varies as a function of space and time.

Pioneering work in the 1980s predicted the pattern of this dynamic topography from inferred mantle density anomalies by assuming a radial viscosity model. This approach has been substantially developed over the last 30 years, resulting in a plethora of predictive models that suggest dynamic topography varies on wavelengths of ~10,000 km with amplitudes of ±2 km.

The principal focus of my PhD has been generating an observational database of dynamic topography using measurements of residual depth in the oceanic realm. A database of 2,120 observations has been constructed from over 1100 seismic reflection profiles, 200 modern wide-angle and 350 vintage slope-intercept refraction experiments. The bulk of observations are on old ocean floor adjacent to the continental margins. Care is taken to remove other causes of topography, including sedimentary loading and the effects of compaction, variable oceanic crustal thickness, and lithospheric thickening with age away from mid-ocean ridges.

Residual depth anomalies have amplitudes of ±1 km and wavelengths of ∼1,000 km. These features are too large to be flexural and therefore represent the pattern of dynamic topography. One of the most striking results is from the west coast of Africa where two full ∼2, 000 km wavelengths have been captured that positively correlate with long-wavelength gravity anomalies. Positive anomalies are also associated with the Icelandic, Afar, Hawaiian, Galapagos, Azorean and Cape Verde plume swells. Prominent depressions are located along the east coast of North America, Gulf of Mexico, Argentine Abyssal Plain and in the Australian-Antarctic Discordance.

Stratigraphic observations from continental margins show that these signals rapidly evolve with time. Sedimentary architecture and subsidence histories constrain the rates of uplift and drawdown, such as on the Northwest Shelf of Australia where ~600 m of subsidence has ocurred since 10 Ma.

The continental geological record also contains evidence for past vertical motions. Thermochronology, marine incursions, palaeo-shorelines, magmatism, longitudinal river profiles and sedimentary flux estimates can be integrated to produce continental uplift histories. Locations studied so far include Arabia, Mexico, and India where multiple independent constraints indicate significant Neogene growth of dynamic topography.

Key Publications