Final Report Summary - DEFORM (Detachment faults in ophiolites)
New seafloor and underlying oceanic lithosphere are produced at constructive plate boundaries by a complex set of magmatic and tectonic processes known collectively as seafloor spreading. At ridges where plates spread slowly (< 4 cm / a), plate divergence is commonly taken up by displacement on large tectonic faults that cut through the entire crust, rather than by magmatic emplacement of new rocks. These large-offset 'oceanic detachment faults' have only recently been recognised, but are so widespread in slow- and ultraslow-spreading ridges (underlying some 50 % of the length of the Mid-Atlantic Ridge) that a new 'detachment-mode' of seafloor spreading has recently been defined that is fundamentally different to classical magmatic accretion. Slip on oceanic detachment faults leads to tectonic exposure of lower crustal and upper mantle rocks on the seafloor and to the development of large, uplifted massifs known as 'oceanic core complexes'. This uplift is achieved by rotation of the rocks beneath the fault surface (in the fault footwall), and this rotation has been shown to be a characteristic of oceanic core complexes in the Atlantic Ocean.
In the oceans detachment-mode spreading may be recognised by locating exposed fault surfaces and / or uplifted oceanic core complexes on the seafloor via detailed bathymetric mapping. However, the deposition of deep-water sediments soon blankets and obscures this bathymetric evidence, and so detachment-mode spreading has only been documented in relatively young (< 15 Ma) lithosphere. To look for evidence of this fundamental mode of spreading in the deeper geological past, we need to rely upon preservation of detachment faults and / or oceanic core complexes in slices of oceanic lithosphere preserved as ophiolites in ancient mountain chains. Unfortunately, the geological field evidence for these structures in ophiolites is difficult to decipher as seafloor morphologies are obliterated. In this project, we have used for the first time the characteristic rotation of oceanic core complex footwalls (as seen in geologically young examples in the modern oceans) to determine whether detachment-mode spreading also operated in the deep past. To do this, we conducted a detailed paleomagnetic analysis of the Mirdita ophiolite of Albania, where field relationships have previously been used to suggest presence of a fossil oceanic core complex, and used the magnetisation of the oceanic rocks to quantify the style and amount of potential footwall rotation. This is the only way that such rotations can be detected, and hence paleomagnetic analyses can uniquely provide insights into this problem.
The project proceeded via four work packages: literature review; field sampling; laboratory analyses; and consolidation and dissemination. After an initial literature review we carried out two field-sampling campaigns in June 2010 and July 2011 and collected more than 700 paleomagnetic samples from 72 sites located in the footwall of the detachment fault, within the peridotites and gabbroic intrusions of the Krabbi and Puka massifs, and from the overlying extrusive rocks of the hanging wall. Subsequent laboratory analyses allowed the determination of mean remanence vectors for each suite of sites, enabling assessment of the degree of relative rotation across the main detachment zone. Rock samples were demagnetised according to standard paleomagnetic techniques, and exhibited strong and stable magnetisation components acquired at the time of the formation of the oceanic crust (Jurassic). Remanence directions were first compared to an appropriate reference direction determined from the African apparent polar wander path. However, the most effective demonstration of rotation of the footwall relative to the hanging wall was achieved using a net tectonic rotation approach, yielding both the angle and axis of rotation, with associated confidence limits. This analysis demonstrated that significantly different magnetisations between the footwall and hanging wall blocks either side of the detachment can be explained by relative rotation during seafloor spreading. Monte Carlo modelling of permissible rotation solutions indicates c. 45-degree rotation around a nearly horizontal, ridge-parallel axis during detachment faulting. This style and amount of footwall rotation is directly comparable to that seen in well-constrained examples of oceanic core complexes in the Atlantic. The striking similarity between rotation parameters in the ancient Mirdita detachment fault system and young examples from the Atlantic now provides compelling evidence that the detachment mode of spreading also operated in the deep geological past and has been a characteristic of slow-spreading systems since at least the Jurassic.
This result is highly significant for the scientific community interested in modern ocean ridge dynamics as it extends the record of detachment-mode spreading back to 160 Ma, but also provides a proven methodology for identification of oceanic core complexes in ancient ophiolites, which will help to further unravel the kinematics of these major extensional structures.
In the oceans detachment-mode spreading may be recognised by locating exposed fault surfaces and / or uplifted oceanic core complexes on the seafloor via detailed bathymetric mapping. However, the deposition of deep-water sediments soon blankets and obscures this bathymetric evidence, and so detachment-mode spreading has only been documented in relatively young (< 15 Ma) lithosphere. To look for evidence of this fundamental mode of spreading in the deeper geological past, we need to rely upon preservation of detachment faults and / or oceanic core complexes in slices of oceanic lithosphere preserved as ophiolites in ancient mountain chains. Unfortunately, the geological field evidence for these structures in ophiolites is difficult to decipher as seafloor morphologies are obliterated. In this project, we have used for the first time the characteristic rotation of oceanic core complex footwalls (as seen in geologically young examples in the modern oceans) to determine whether detachment-mode spreading also operated in the deep past. To do this, we conducted a detailed paleomagnetic analysis of the Mirdita ophiolite of Albania, where field relationships have previously been used to suggest presence of a fossil oceanic core complex, and used the magnetisation of the oceanic rocks to quantify the style and amount of potential footwall rotation. This is the only way that such rotations can be detected, and hence paleomagnetic analyses can uniquely provide insights into this problem.
The project proceeded via four work packages: literature review; field sampling; laboratory analyses; and consolidation and dissemination. After an initial literature review we carried out two field-sampling campaigns in June 2010 and July 2011 and collected more than 700 paleomagnetic samples from 72 sites located in the footwall of the detachment fault, within the peridotites and gabbroic intrusions of the Krabbi and Puka massifs, and from the overlying extrusive rocks of the hanging wall. Subsequent laboratory analyses allowed the determination of mean remanence vectors for each suite of sites, enabling assessment of the degree of relative rotation across the main detachment zone. Rock samples were demagnetised according to standard paleomagnetic techniques, and exhibited strong and stable magnetisation components acquired at the time of the formation of the oceanic crust (Jurassic). Remanence directions were first compared to an appropriate reference direction determined from the African apparent polar wander path. However, the most effective demonstration of rotation of the footwall relative to the hanging wall was achieved using a net tectonic rotation approach, yielding both the angle and axis of rotation, with associated confidence limits. This analysis demonstrated that significantly different magnetisations between the footwall and hanging wall blocks either side of the detachment can be explained by relative rotation during seafloor spreading. Monte Carlo modelling of permissible rotation solutions indicates c. 45-degree rotation around a nearly horizontal, ridge-parallel axis during detachment faulting. This style and amount of footwall rotation is directly comparable to that seen in well-constrained examples of oceanic core complexes in the Atlantic. The striking similarity between rotation parameters in the ancient Mirdita detachment fault system and young examples from the Atlantic now provides compelling evidence that the detachment mode of spreading also operated in the deep geological past and has been a characteristic of slow-spreading systems since at least the Jurassic.
This result is highly significant for the scientific community interested in modern ocean ridge dynamics as it extends the record of detachment-mode spreading back to 160 Ma, but also provides a proven methodology for identification of oceanic core complexes in ancient ophiolites, which will help to further unravel the kinematics of these major extensional structures.