Oceanic crustal fabric and fracture zones: correlation with plate tectonics and sedimentary unconformities and deformation of adjacent continents

This project will A PhD project involving mapping oceanic crustal features from gravity data and identifying their correlation with plate tectonic events and sedimentary unconformities. 

 

Summary

This PhD project aims to build on existing oceanic crustal texture and fracture zone observations in order to investigate the changes in direction of oceanic fracture zones and deformation within adjacent continents and timing of sedimentary unconformities.  The project will research further into the uniformity and variability of these observations and thereby assess the constraints that can be placed on plate tectonic models, the relative and absolute movements of continents as well as deformation within continents.

 

Background

The generation of satellite derived gravity data sets has vastly improved the imaging of crustal textures within the world’s oceans.  The early gravity maps of Haxby (1985) were a revelation in terms of imaging of fracture zones. These data sets have continued to improve (Green et al., 2019) such that the detail along the fracture zones and the intervening texture of the oceanic crust can now be observed in full resolution.  The geometry of these fracture zones relates to the relative movement of lithospheric plates, but the shapes of fracture zones are not simple, because movements of plates change over geological time due primarily to plate collisions (e.g., Europe into Africa) and deformation within continents.

Fairhead et al., (2013) mapped changes in direction of some Atlantic fracture zones (Figure 1) and correlated the ages of these “kinks” with unconformities within the Mesozoic sedimentary basins of the West and Central African Rift System – which can be considered as a buffer zone between the north and southern African sub-plates.  Fairhead and Wilson (2005) have further shown that there are distinct areas of textural change within the oceanic crust some of which are highlighted in Figure 2. The aim of this project is to further investigate and understand these textural changes and correlations using the highest resolution satellite data and assessing their constraints on plate tectonic models.

Figure 1.  Sandwell and Smith (1997) satellite gravity over parts of Central (A) and Equatorial (B) Atlantic showing the varying orientation of fracture zones (after Fairhead et al., 2013). 

Figure 2.  Sandwell and Smith (1997) satellite gravity over parts of South Atlantic (C) showing tracks of ‘V’ shaped volcanic lineaments (highlighted in red) at latitude of St Helena (D) showing inverted ‘V’ shaped volcanic lineaments (highlighted in yellow) south of the Equatorial Fracture Zone. Diagrams taken from Fairhead and Wilson 2005. The white dashed line marks the axis of the Mid-Atlantic Ridge.

 

Aims and Objectives

The aim of this project is to assess the constraints that the shapes of oceanic fracture zones and intervening crustal textures can impose of plate tectonic models and to develop a process for taking value from the geometry of these fracture zones.

Specific objectives would be:

• to map a wide range of fracture zones
• to assess the consistency in geometry locally (e.g., within the Central Atlantic) and between regions (e.g., between the Central and South Atlantic) and to produce a model of the process involved
• to compare the observations with plate models, identify causative events and assess differences between plate models
• to compare with available sedimentation records and analyse these together with information about basin shapes, orientations and fault patterns
• to develop a process for constraining plate models based on fracture zone geometry

 

Project outline

We envisage the project to proceed along the following lines, but different directions for the research are likely to become apparent:

• Look at a large number of FZs in different parts of the Atlantic
• Formulate more automated (and consistent) approach to track the position of FZs from satellite gravity and calculate their direction (horizontal azimuthal derivative)
• Identify correlations and discrepancies in local areas
• Identify correlations and discrepancies between regions
• Attempt to correlate “kinks” with unconformities in basins (to the extent available) and along the continental margins.
• Compare “kinks” to “events” in plate models.  Look at different plate models to assess which ones fit
• Assess which types of “events” are seen in some local areas and not in others and interpret results
• Possibly look at different areas – Indian Ocean/north Atlantic Ocean
• Possibly look at features that are not perpendicular to the ridge – e.g., various areas in mid-Atlantic, some with numerous features.  Track how common they are and model how they might form.  This would be a bit of a fringe project

 

Impact

This project will develop the understanding of how shapes of oceanic fracture zones relate to Plate Tectonics.  High-resolution satellite derived gravity data will allow the student to develop understanding of more subtle features. The results of this study will improve our understanding of asthenosphere volcanic processes with respect to absolute motions of plates. It will identify the timing (via magnetic isochron data of the oceanic crust) of plate motion changes based on deformation within both the oceanic and continental parts of plates.

Student profile

Applicants should have a good BSc (or equivalent) degree in geophysics, geology, earth sciences or a related discipline.  The project would suit students with an aptitude for geometric problems, who have good mathematical skills and programming ability.  The student can develop particular skills by sitting in on modules from the Masters programmes in the School of Earth and Environment – especially Structural Geology with Geophysics and Exploration Geophysics.

 

References

FAIRHEAD, J. D., GREEN, C. M., MASTERTON, S. M. & GUIRAUD, R. 2013. The role that plate tectonics, inferred stress changes and stratigraphic unconformities have on the evolution of the West and Central African Rift System and the Atlantic continental margins. Tectonophysics, 594, 118-127.

FAIRHEAD, J. D. & WILSON, M. 2005. Sea-floor spreading and deformation processes in the South Atlantic Ocean: Are hot spots needed? www.MantlePlumes.org

GREEN, C. M., FLETCHER, K. M. U., CHEYNEY, S., DAWSON, G. J. & CAMPBELL, S. J. 2019. Satellite gravity – enhancements from new satellites and new altimeter technology. Geophysical Prospecting, 67, 1611-1619.

HAXBY, W. F. 1985. Gravity field of the world’s oceans. US Navy, Naval Office of Research.

SANDWELL, D.T. and SMITH W.H.F. 1997 Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. Journal of Geophysical Research 102:10,039-10,054.