Continental collision including a weak zone: the vise model and its application to the Newfoundland Appalachians

Continental collision generally occurs after a protracted history of rifting, sedimentation, and subduction. Inherited material heterogeneities and thermal effects from terrane accretion and amalgamation may control subsequent collisional deformation. Such strength contrasts may have played an important role in the Silurian evolution of the Newfoundland Appalachians. A cross-sectional plane-strain numerical model is used to investigate the effect of a weak zone embedded between stronger model crust (the "vise") on the deformation style of orogens. In contrast to collision of uniform strong crust in which deformation is directly related to the underlying subduction of mantle lithosphere, deformation in vise models can propagate out to the ends of the vise in a diffuse manner. This distributed tectonic style depends on the relative strength of the embedded weak zone, the degree of coupling of this zone to underlying layers, and the effect of gravity acting on thickened crust. For weak coupling at the base of the crust, results are insensitive to the behaviour of the underlying model mantle (e.g., subduction, subduction retreat, or pure-shear thickening). Vise model results are in first-order agreement with the following characteristics of Newfoundland Silurian tectonics: (i) a diffuse deformation style distributed over a weak core zone more than 100 km wide, (ii) lack of indirect evidence of mantle dynamics from crustal reflectivity fabric, and (iii) lack of clear evidence for significant crustal thickening during orogeny.