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Proterozoic metasediments of the Itremo Group in central Madagascar probably represent a passive margin sequence predating Gondwana assembly. The quartzites are well-sorted quartz arenites that contain flat lamination, wave ripples, current ripple cross-lamination, and dune cross bedding. The carbonate rocks preserve abundant stromatolites and algal laminates. A continental source is indicated by mudrock major and trace element chemistry. The combination of lithologic association, sediment architecture and mudrock chemistry indicates that the sequence was deposited on a continental shelf or platform. SHRIMP data from detrital zircons indicate that the source area included early Proterozoic and late Archean rocks with ages between 1.85 and 2.69 Ga, and that the depositional age of the Itremo Group must be less than 1855 ± 11 Ma. The sequence has been deformed into a series of large-scale folds separated by ductile shear zones. SHRIMP data indicate both massive lead loss from detrital zircons and new zircon growth in the metasediments at 833 ± 112 Ma, which we interpret as the age of metamorphism of the sequence. Comparison of detrital grain ages with basement ages in East Africa and in India indicates that the source area for the Itremo Group probably lay on the present African mainland.

A B S T R A C TIreland's Aran Islands are an excellent place to test whether coastal boulder deposits-including individual rocks weighing several tens of tonnes near sea level and clasts weighing several tonnes transported at tens of meters above sea level-require a tsunami for emplacement or whether storm waves can do this work. Elongate deposits of cobbles, boulders, and megagravel are strung along the Atlantic coasts of the Aran Islands. No tsunamis have affected this region in recent centuries, so if these deposits are forming or migrating at the present time, they must be storm activated. We find a diverse range of evidence for recent ridge activity. First, shells of Hiatella arctica (subtidal rockboring bivalves preserved in life position within ridge boulders) yield radiocarbon ages from ≈200 AD to modern (post-1950 AD). Second, recent motion is attested to by eyewitness accounts that pin the movement of several individual 40-80-t blocks to a specific 1991 storm and by repeat photography over the last few field seasons (2006)(2007)(2008)(2009)(2010)(2011) that captures the movement of boulders (masses up to ≈10.5 t) even in years without exceptionally large storms. Finally, geographic information system comparison of nineteenth-century Ordnance Survey maps with twenty-firstcentury orthophotos shows that in several areas the boulder ridges have advanced tens of meters inland since the mid-nineteenth century, overrunning old field walls. These advancing ridges contain boulders with masses up to 78 t at 11 m above high-water mark, so wave energies sufficient to transport those blocks must have occurred since the 1839 survey. Thus, there is abundant evidence for ridge activity since the 1839 mapping, and as there have been no tsunamis in the northeastern Atlantic during that time period, we conclude that the Aran Islands boulder ridges are built and moved by storm waves.Online enhancement: appendix.

Erosion via lavaka formation is widespread in Madagascar, but controls on why and where lavakas occur are not understood. Geographic information system analysis reveals a spatial correlation between lavaka abundance and the frequency of seismic events: most lavakas occur in or near areas where recorded earthquakes (magnitude 0.5-5.6) are most frequent. This correlation explains the unevenness of lavaka distribution in the Malagasy highlands, and highlights the importance of natural factors in lavaka formation. Seismic activity appears to precondition the landscape to lavaka formation, although the mechanism by which this happens is not yet known. Recognizing the connection, however, allows us to pinpoint areas prone to future lavaka development in zones of active deforestation. Areas with the greatest frequency of seismic events are most at risk for high-density lavaka development. This idea came originally from a conversation with Mike Zavada. Neil Wells kindly shared his fi eld data. Sharron Macklin and David Backus helped with the geographic information systems analysis. Land-Sat images came from the Global Landcover Facility, University of Maryland. Didier Bertil and the Institut et Observatoire de Géophysique d'Antananarivo (IOGA) provided seismic data.