Dynamics of Brittle-Viscous Accretionary Wedges as Revealed by Geophysical and Drilling Data and Analog Modeling of the Barbados Prism

Mega-accretionary prisms do not exhibit the characteristics of the classical Coulomb wedge theory. In the example of the Barbados accretionary prism, a system of elongated extensional trenches ("basin-&-range" type) developed in the thickest zone of the tectonic wedge and this system has been active since at least Miocene times. Synchronously with this extension, compression occurred at the front of the accretionary prism (frontal accretion zone), and compression was also active at the inner edge of the prism (characterized by back-thrusting). The interpretation proposed in this study invokes a spreading of the superficial part of the accretionary prism due to a viscous response associated to a tectonic thickening of the deepest parts of the prism. This interpretation is consistent with the fact that extension is limited to the axial part of the prism and coincides with the zone where the prism is the thickest. The deep zone of viscous behavior is interpreted as shale-rich sedimentary domains intensively deformed by penetrative deformation. Analog sand-silicone models monitored by X-ray CT scanner have been used to simulate the distribution of the deformation in the Barbados accretionary prism as observed on available seismic reflection data. A bilayer model with an upper brittle zone and a lower ductile part is best suited to account for observed deformation. Compressive structures at the front and at the rear of the prism correlate with the zones of high concentration of maximum shear strain.