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Abstract.The cause and geodynamic impact of fiat subduction are investigated. First, the 1500 km long Peru fiat slab segment is examined. Earthquake hypocenter data image two morphologic highs in the subducting Nazca Plate which correlate with the positions of subducted oceanic plateaus. Travel time tomographic images confirm the three-dimensional slab geometry and suggest a lithospheric tear may bound the NW edge of the fiat slab segment, with possible slab detachment occurring down dip as well. Other fiat slab regions worldwide are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. Flat subduction is shown to be a widespread phenomenon, occuring in 10% of modern convergent margins. in nearly all these cases, as a spatial and temporal correlation is observed between subducting oceanic plateaus and fiat subduction, we conclude that fiat subduction is caused primarily by (1) the buoyancy of thickened oceanic crust of moderate to young age and (2) a delay in the basalt to eclogite transition due to the cool thermal structure of two overlapping lithospheres. A statistical analysis of seismicity along the entire length of the Andes demonstrates that seismic energy release in the upper plate at a distance of 250-800 km from the trench is on average 3-5 times greater above fiat slab segments than for adjacent steep slab segments. We propose this is due to higher interplate coupling and the cold, strong rheology of the overriding lithosphere which thus enables stress and deformation to be transmitted hundreds of kilometers into the heart of the upper plate.

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Abstract.New regional seismological data acquired in Colombia during 1993 to 1996 and tectonic field data from the Eastern Cordillera (

Summary Recent advances in global imaging have lead to tomographic mantle models with regional scale details. To improve these models further, we have extended the usual linearized approach to traveltime tomography to non‐linear tomography. Here ‘non‐linear’ means that seismic ray bending due to inferred velocity heterogeneity is taken into account in an iterative method in which inversion steps are alternated with 3‐D ray tracing to update ray paths and traveltimes. As a starting point for our non‐linear inversion we have used the mantle model of Bijwaard et al.(1998) and ray tracing is performed following Bijwaard & Spakman (1999a). We have not attempted a full exploration of the non‐linear nature of the traveltime inverse problem. This would at the very least require tests with different starting models and the relocation of all events in each of these models. The main results are as follows. We observe no overall dramatic change in anomaly patterns, but subtle changes on the global mantle scale lead to a small increase in variance reduction and model amplitudes. These small changes together with very similar resolution estimates for the linear and non‐linear inversions do not allow us to investigate formally possible model improvements. However, expected non‐linear effects such as the focusing of structures and a baseline shift towards lower velocities indicate an improved solution, which is also more consistent with expected physics than a fully linearized inversion. Apart from that, some very strong changes occur in distinct upper mantle regions such as below Japan, Tibet, South America, Europe and Tonga–Fiji, where 3‐D ray bending effects are substantially larger than in the deeper mantle since model amplitudes fall off rapidly with depth. In the lower mantle, how‐ever, increased focusing effects can be observed that may prove important for detailed interpretations.