The Surface Expression of Mantle Convection in North America:
Uplift, Erosion and Sedimentary Flux During the Last 100 Million Years
Supervisors
Dr. Gareth Roberts, Dr. Alex Whittaker ([email protected])
Background
The aim of this project is to determine the history of dynamic (i.e sub-crustal) support of
the North American continent during the last 100 million years. In most places on Earth
high topography is generated by thickening the crust. In western and eastern North America
crustal thickening alone cannot be responsible for generating high topography. For example,
the Colorado Plateau, which has an elevation of 1–2 km, has the same crustal thickness (35–45
km) as the Great Plains, which has an elevation of ∼ 0.5 km (see Figure).
Columbia
river
Great
Plains
Colorado
river
Atlanitc Ocean
Mississippi
river
Rio
Grande Gulf of Mexico
Pacific Ocean
Topography, crustal thickness (circles) and drainage (blue lines) of North America. Dashed
outline = Colorado Plateau (CP). Note CP and Great Plains have similar crustal thicknesses.
In western North America, the high coherence between long wavelength (> 1000 km) free-air
gravity anomalies and topography, and slow mantle seismic velocities imaged using tomographic
methods indicate regional dynamic support. More broadly, residuals from the ocean-age depth
curve indicate that the oldest ocean floor surrounding the continent is dynamically supported
both positively and negatively. Clearly mantle convective processes exert an important influence
on the topography of North America. The age and elevation of uplifted marine and lacustrine
sedimentary rocks, the delivery of sediment to the Gulf of Mexico, and local rock cooling ages
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indicate that North America had a polyphase Cenozoic uplift and erosion history1,2,3 . These
spot-measurements of uplift and erosion place important constraint on the evolution of dynamic
support, however they are limited by their spatial resolution. Recently, new techniques have
been developed which allow drainage patterns to be inverted for a continental-scale history
of uplift4 . A preliminary study indicates that this approach allows us to quantitatively link
independent incision and uplift estimates with fluvial erosion and rock cooling of the Colorado
Plateau with sediment delivery to the Gulf of Mexico. This project will constrain the
uplift and erosional history of the continent by combining new drainage inversion
techniques with local incision, denudation and uplift constraints.
Methods
1. The student will use flow-routing algorithms to extract continental-scale drainage patterns
from high resolution digital elevation data (e.g. 30 m NED; http://ned.usgs.gov). The
data will be benchmarked against satellite imagery and spot measurements of elevation.
2. Existing and new numerical and analytical techniques will be used to invert for a continental uplift rate and exhumation history4 .
3. A body of published stratigraphic, thermochronological, incision rate and uplift history
information exists for parts of North America1,3 . These data will be used to calibrate
erosional models and to benchmark calculated uplift histories. In some places where
those data are sparse or do not exist (e.g. upper reaches of Colorado, Green, Little
Colorado rivers) we will have to collect and analyse samples. The student will be part of
a sampling campaign, which will collect data to constrain the rock cooling and incision
history along those rivers. This fieldwork will involve rafting parts of the Colorado river
system including the Grand Canyon. These data will be analysed in collaboration with
the London Geochronology Centre.
4. These results will be combined with new high resolution geophysical datasets (e.g. USArray; http://www.usarray.org) to constrain the evolution of dynamic support through
time. Results will be used to test geodynamic simulations of mantle convection5 .
Outcomes
This research programme will combine a range of thermochronological, digital elevation and
stratigraphic data with new analytical techniques to determine the uplift history of western
North America. This uplift history will constrain the mechanism of topographic support including mantle convection. It is expected that field and modelling results will be published in
high-impact journals and that the student will present their results at UK and international
conferences.
Training
This PhD is ideally suited to a geologist/geomorphologist or geophysicist/physicist with a range
of interests including tectonics, mantle convection and geomorphology. It will combine fieldwork,
laboratory experience and development and use of new software. Training will include numerical
and analytical techniques, ArcGIS, Python, Fortran and Linux. This project would provide an
excellent starting point for a student aiming for a future career in academia or industry.
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References
1. Flowers, R. M., B. P. Wernicke, & K. A. Farley (2008), Unroofing, incision, and uplift history
of the southwestern Colorado Plateau from apatite (U-Th)/He thermochronometry, Geol. Soc.
Am. Bull., 120(56), 571587.
2. Galloway, W. E., T. L. Whiteaker, & P. Ganey-Curry (2011), History of Cenozoic North
American drainage basin evolution, sediment yield, and accumulation in the Gulf of Mexico
basin, Geosphere, 7(4), 938973.
3. Huntington, K. W., B. P. Wernicke, & J. M. Eiler (2010), The influence of climate change and
uplift on Colorado Plateau paleotemperatures from carbonate clumped-isotope thermometry,
Tectonics, doi:10.1029/ 2009TC002449.
4. Roberts, G. G., N. J. White, G. L. Martin-Brandis, & A. G. Crosby (2012), An Uplift
History of the Colorado Plateau and its Surroundings from Inverse Modeling of Longitudinal
River Profiles. Tectonics, doi:10.1029/2012TC003107.
5. Spasojevic, A. & Gurnis, M. (2012), Sea level and vertical motion of continents from dynamic
earth models since the Late Cretaceous, AAPG Bulletin, 96(11), 2037–2064.
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