Contraction along a previously extended plate boundary; analogue modelling of the Iberian - Eurasian suture zone
Abstract
The Iberian - Eurasian plate boundary can be roughly subdivided into a continent-continent and a continent-ocean collision zone in the east and west, respectively. This is due to the extensional phase that predates the contraction that formed the present day mountains in the area. A narrow ocean seaway separated the Iberian and Eurasian plates, whereas a wider ocean opened up towards west, where the present day Bay of Biscay lies. The deep seismic structures under the eastern segment show a subduction of the Iberian plate under the Eurasian plate, whereas the western segment is less well constrained and leave room for discussion regarding deep geometries and the nature of the collision zone. An analogue experiment was designed to represent the tectonic setting at the boundary at the culmination of the extensional phase in the early Cretaceous and then contracted to explore how surface topography and deep structures are affected by changes in upper mantle strength and contraction rate. The model is composed of layers of silicone putty and sand, tailored to simulate the assumed lithospheric geometries and strength-viscosity profiles along the plate boundary zone, and comprises two 'continental' plates separated by a thinner 'oceanic' plate that represents the narrow seaway that separated the eastern areas, and opens up to a 30° angle in the west, representing the Bay of Biscay. The experiment floats on a substrate of sodium polytungstate, representing mantle. The experiment was run 24 times, varying the thickness (and thus strength) of the upper mantle lithosphere, and the contraction rate. Keeping all other parameters identical for each experiment, the models were shortened by a computer-controlled jackscrew while time-lapse images were recorded. After completion, the models were saturated with water and frozen, allowing for sectioning and profile inspection. Of the 19 successful iterations of the experiment, three shortening rates were tested, each value representing an order of magnitude higher than the previous, and three thickness values (and thus strength) were applied to the sand layer representing upper mantle lithosphere. The results show how upper mantle strength appears to be the most important factor in determining whether an inversion of subduction direction occurs. A weak upper mantle layer (weaker than the oceanic crust) leads to obduction of the oceanic crust in the western segment of the model, forcing a significantly different scenario compared to observations in NW Spain today. In model iterations where the upper mantle is stronger than the oceanic crust, most model outcomes show a reversal in subduction polarity from northerly in the east, to southerly in the west. The transition zone is located where the narrow, parallel 'seaway' opens up towards west, near the centre of the model. Surface inspection of the models also show consistent patterns of faults breaking the surface, and are comparable to the present day major fault patterns mapped along the Pyrenean and Cantabrian mountains. It is concluded that both deep and surface structures are at least in part a result of the inherent zone of weakness that developed along the Iberian - Eurasian plate boundary.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T22B..05M
- Keywords:
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- 8155 TECTONOPHYSICS / Plate motions: general;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution