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Mark Hoggard

Mark  Hoggard

Post-doctoral Research Fellow

Bullard Laboratories, Madingley Road, Cambridge, CB3 0EZ. UK.
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 to the surface by convection within the mantle. This flow generates some fraction of topography that varies as a function of space and time.

The principal focus of my research has been generating an observational database of this dynamic topography. The primary method for constraining the present-day pattern is using measurements of residual depth in the oceanic realm. A database of over 2,000 observations has been constructed from thousands of seismic reflection profiles, modern wide-angle and vintage slope-intercept refraction experiments from a range of industry and academic sources. The bulk of observations are on old ocean floor adjacent to continental margins. Additional causes of topography are removed, including sedimentary loading, 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.

The temporal evolution of dynamic topography can also be carefully constrained. Stratigraphic observations from continental margins show that these vertical motions rapidly evolve with time. For example, a large angular unconformity on the Angolan shelf cuts down into Pliocene deltaic foresets. Careful compaction analysis using seismic stacking velocities indicates that the margin has uplifted by ~500 metres over the last 10 Ma. This uplift coincides with raised Pleistocene marine terraces and large knickzones in the river profiles that drain the 2 km tall mountains of the Bié Dome. A contrasting picture is observed on Australia's Northwest Shelf, where a Miocene carbonate reef rapidly switches from prograding to aggrading at 9 Ma, before eventually drowning. This behaviour is consistent with the sudden onset of dynamic drawdown and continuing subsidence, reaching up to 600 m at the present-day.

The continental geological record contains a lot of evidence for transient vertical motions. Thermochronology, marine incursions, palaeo-shorelines, magmatism, longitudinal river profiles and sedimentary flux estimates can be integrated to produce continental-scale uplift histories. Locations that we have studied so far include Arabia, Mexico and western USA, South America, India, Borneo and Australia. In all of these locations, multiple independent constraints indicate significant growth of dynamic topography during the Neogene period.

Key Publications

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