Abstract
Along mid-ocean ridges the extending crust is segmented1 on length scales of 10–1,000 km. Where rift segments are offset from one another, motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis. Where segments overlap, non-transform offsets with a variety of geometries2 accommodate shear motions. Here we use micro-seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north Iceland. Between the rift segments, we identify a series of faults that are aligned sub-parallel to the orientation of the main rift. These faults slip through left-lateral strike-slip motion. Yet, movement between the overlapping rift segments is through right-lateral motion. Together, these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf-faulting mechanism, named after its resemblance to a tilting row of books on a shelf3. The faults probably reactivated existing crustal weaknesses, such as dyke intrusions, that were originally oriented parallel to the main rift and have since rotated about 15° clockwise. Reactivation of pre-existing, rift-parallel weaknesses contrasts with typical mid-ocean ridge transform faults and is an important illustration of a non-transform offset accommodating shear motion between overlapping rift segments.
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Acknowledgements
Seismometers were borrowed from the Natural Environment Research Council SEIS-UK (loans 914 and 968), and the work was financially supported by a research grant and studentships from the NERC and Shell. We thank S. Steinþórsson, H. Soosalu and all those who have assisted in fieldwork in Iceland since 2009. We are grateful to Á. R. Hjartardóttir and P. Einarsson for providing raw data on fracture orientations and for helpful comments on the manuscript, and A. Copley for advice. The Icelandic Meteorological Office kindly provided additional data from their seismometers in northeast Iceland. Dept. Earth Sciences, Cambridge contribution number ESC3000.
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All authors participated in data collection and the interpretation of results. Data processing of the microseismic swarms was carried out by R.G.G., with the addition of picks for the easternmost swarm by R.S.W. The manuscript was written by R.G.G and R.S.W.
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Green, R., White, R. & Greenfield, T. Motion in the north Iceland volcanic rift zone accommodated by bookshelf faulting. Nature Geosci 7, 29–33 (2014). https://doi.org/10.1038/ngeo2012
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DOI: https://doi.org/10.1038/ngeo2012
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