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 riftin g, sedimentation, and subduction. Inherited material heterogeneities and th ermal effects from terrane accretion and amalgamation may control subsequen t collisional deformation. Such strength contrasts may have played an impor tant role in the Silurian evolution of the Newfoundland Appalachians. A cro ss-sectional plane-strain numerical model is used to investigate the effect of a weak zone embedded between stronger model crust (the "vise") on the d eformation style of orogens. In contrast to collision of uniform strong cru st in which deformation is directly related to the underlying subduction of mantle lithosphere, deformation in vise models can propagate out to the en ds 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 thi ckened crust. For weak coupling at the base of the crust, results are insen sitive to the behaviour of the underlying model mantle (e.g., subduction, s ubduction retreat, or pure-shear thickening). Vise model results are in fir st-order agreement with the following characteristics of Newfoundland Silur ian tectonics: (i) a diffuse deformation style distributed over a weak core zone more than 100 km wide, (ii) lack of indirect evidence of mantle dynam ics from crustal reflectivity fabric, and (iii) lack of clear evidence for significant crustal thickening during orogeny.