Mid-ocean ridge tectonics

Fast-spreading mid-ocean ridges have an axial high (the "rise crest") whereas slow-spreading ridges have deep axial rift valleys. This difference in morphology is reflected in the ruggedness of the flanks of ridges. Fast-spread ridge flanks are smooth whereas slow-spread ridge flanks are very rugged, and consequently vast areas of the ocean floors are covered with rough or smooth abyssal hills depending on the type of ridge crest they were produced at. The difference is mostly due to different styles and intensity of faulting at ridges so understanding the faulting is important for understanding the structure of substantial parts of the ocean floors. The following are contributions towards this effort.

Tectonic strain of the Mid-Atlantic Ridge, 29 degrees N

At mid-ocean ridges, dykes and other forms of volcanism fill the space left by the separating tectonic plates. A significant component of plate separation, however, also occurs by slip on extensional faults. This project was conceived to estimate the amount of tectonic extensional strain for a section of the Mid-Atlantic Ridge using sidescan sonar data collected with the TOBI deeply towed system. The image to the right shows multibeam sonar data from the survey area collected during the research cruise of RRS Charles Darwin (CD99).

Collaborators: Patience Cowie, Javier Escartin and Simon Allerton (Edinburgh), Chris MacLeod (Cardiff), Roger Searle and Peter Slootweg (Durham) and Team TOBI (Southampton).

Neotectonics on Iceland

Although we know that faulting at mid-ocean ridges occurs near the ridge axes, we know very few further details on the timing of fault growth, which is important for understanding their mechanical behaviour. Unfortunately there are few ways of dating fault motion over the bare volcanic rock of ridges, as traditional methods based on stratigraphy, which are useful for understanding sedimentary basin evolution, cannot usually be applied. We collected high-resolution CHIRP sediment profiler records from Thingvallavatn lake in Iceland, which crosses Iceland's main western rift zone, in order to attempt to infer fault evolution from the lake sediments. The records and sidescan sonar images collected during the same survey also show details of fracturing in the lake floor and changes in sediment deposition probably related to changes in water circulation in the lake.

Collaborators: Jon Bull, Justin Dix, Angus Best and Tim Minshull (Southampton) and Kjartan Thors (Reykjavik).

Triple junctions

Triple junctions occur where three tectonic plates and their boundaries meet at a point.

The Bouvet triple junction, South Atlantic

This project, funded by the NERC, was concerned with the junction between South America, Africa and Antarctica, which occurs near Bouvet Island. We mapped its structure using sidescan sonar images and bathymetry collected using the University of Hawaii HMR1 system aboard the British Antarctic Survey ship James Clark Ross in 1995. Because the velocity-space diagram for the relative plate motions is almost an isosceles triangle near Bouvet, some authors have speculated that the triple junction may be a ridge-fault-fault type (with the Mid-Atlantic Ridge terminating at two transform faults).
The upper figure is an image of the gravity field around the triple junction from satellite altimetry data (Sandwell and Smith, 1997) with plate boundaries shown. The lower figure is bathymetry over the triple junction and Spiess Ridge, a giant volcanic structure (abstract and full article (PDF)*).

The growth of Spiess Ridge adjacent to the triple junction has led to peculiar deformation around it which does not obviously fit either the RRR or RFF models (abstract). Our surveying just north of the Bouvet and Conrad transform valleys, however, showed that the triple junction did probably evolve as a simple RFF for a few millions of years. Interestingly the Mid-Atlantic Ridge fabrics created during that RFF evolution are now over-printed by a massive oblique extensional faults, possibly representing the largest zone of transtensional deformation found so far in the oceans (abstract and full article (PDF)*).

Collaborators: Roy Livermore (British Antarctic Survey), Enrico Bonatti, Marco Ligi, Paola Fabretti and Gabriela Carrara (ISMAR, CNR, Bologna, Italy)


The Rodriguez or Indian Ocean triple junction

The Rodriguez junction between Africa, Antarctica and Australia is arguably the simplest of the oceanic triple junctions, with three spreading ridges almost meeting at a discrete point. The triple junction consists of two medium spreading ridges and one slow spreading ridge (the Southwest Indian Ridge). GLORIA sidescan sonar and French Sea Beam bathymetry data suggest that the two medium spreading ridges almost form one continuous ridge. Seafloor created by them is slowly rifted by the slow extension of the Southwest Indian Ridge, producing a progressively broadening and deepening rift. The image to the right shows Sea Beam bathymetry data from the triple junction, courtesy of R. Schlich and M. Munchy, IPG Strasbourg, and illustrates the broadening SWIR rift valley (on the left) away from the roughly N-S Central Indian and Southeast Indian Ridge rift valleys. Abstract

A view of the triple junction from the east, showing the uplifted rift flanks of the Southwest Indian Ridge.


This work was carried out when I was a graduate student with thesis advisors John Dewey (Oxford) and Lindsay Parson (SOC).



Also see work on the Azores and on submersible dive observations

Relevant publications

Funding for the above work was provided by Research Fellowships from the Royal Society and the NERC, and by a studentship from the NERC. The Indian Ocean work was carried out while I was a student funded by the NERC and continued in New York with funding from the Roy. Comm. for the Exhibition of 1851. The field work was supported by a number of agencies including the NERC (cruise CD99 and the Indian Ocean and Bouvet triple junctions) and the Royal Society (Thingvallavatn, Iceland).

*The American Geophysical Union owns the copyright to these documents. Further reproduction or electronic distribution of them is not permitted.


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