OROCLINE TRIGGERED LITHOSPHERIC DELAMINATION

Late- to post-orogenic oroclinal bending in conjunction with thinning of the lithospheric mantle is potentially an important component of the waning stages of plate convergence in collisional orogenies. We address the possible and hitherto unexplored cause-effect relationships between oroclinal bending of an originally linear belt and lithospheric thinning and delamination with an example from the Western European Variscan Belt (WEVB). The WEVB is a continent-scale oroclinal bend, which makes it an outstanding case study for investigating the genetic links between oroclinal bending, lithospheric thinning and delamination. We will endeavor to show that most of the apparently unrelated geological features of the Iberian portion of the WEVB, commonly described as "late Variscan", can be more clearly understood and interconnected if geodynamic links between oroclinal bending and lithospheric delamination are considered. We suggest that late- to post-orogenic bending of the lithosphere around a vertical axis resulted in a longitudinal tangential deformation mechanism that shortened and thickened the inner arc while simultaneously stretched the outer arc. The effect of stretching and thinning the lithospheric mantle underneath the outer arc was: i) upwelling of asthenospheric mantle and concomitant melting in the lower crust (reflected in post-tectonic granitoid intrusions); and ii) rapid topographic uplift that caused gravity driven deformation and massive continental sediment discharge into the foreland. The mass imbalance of lithospheric mantle that resulted (i.e. thinning below the outer arc and swelling beneath the inner arc) gave rise to a gravitational instability that resulted in delamination of the lithospheric root. Evidence for this final phase is found in lower crustal melting, widespread magmatism, low pressure - high temperature metamorphism, and fluid flow and coeval mineralization occurring in a time span of ca. 10 Ma. This model is consistent with the chronology of sedimentary, tectonic, magmatic and hydrothermal events recorded in the WEVB. Moreover, this hypothesis could explain the preservation of the lithospheric root in other linear orogens, like the Urals, which lack any significant modification following the initial phase of "orthogonal" lithospheric thickening.