Tectonic Wedge Development and Syn-collisional Delamination in Convergent Orogens

Tectonic wedges are observed in the crust from scales of cm to tens of km. Triangle zones and smaller wedge structures are dependent on the pattern of mechanical layering. Some workers have suggested that wedge development and syn-collisional delamination of the lithosphere is likewise controlled by strength contrasts in the lithosphere. Based on a comparison of characteristics from >35 convergent orogens explored by deep geophysical techniques, we find no correlation between crustal type or composition and wedge development. Delamination of subducting passive margins is more common than over-riding active margins. These observations indicate that conditions in the lithospheric mantle may control delamination and wedge development. Estimates of the density with depth and mechanical layering of representative active and passive margin lithosphere show that the lithospheric mantle of the passive margin may contain net negatively buoyant mass (-M_B). In contrast, the active margin does not. The thermal structure of passive and active margins is based on conductive geotherms (surface heat flows of 50-60 mWm^-2) and an empirically derived geotherm for volcanic provinces, respectively. Significant -M_B indicates a propensity for the initiation of delamination during a subsequent collisional event. The top of the lower lithospheric layer with the greatest -M_B is aligned with a strength minimum near the Moho. Thus, passive margins may have the potential for delamination near the Moho whereas active margins do not. Transformation of the mafic lower crust to mafic eclogite may also generate a lower lithospheric layer containing -M_B. The top of this layer may be aligned with a strength minimum near the top of the metamorphosed lower crust. For delamination above the Moho to initiate, the lower crust must first be subducted or depressed to eclogite grade conditions. The initiation and propagation of delamination may depend on factors such as the subduction angle and amount of passive margin subducted before collision initiates. The process of crustal/lithospheric scale tectonic wedge development differs mechanically from smaller scale wedge structures in that the density structure, in addition to the mechanical layering, controls delamination.