OROCLINAL OROGENS: MANTLE WIND BLOWN POND SCUM?

Bent orogens, referred to as oroclines, are enigmatic tectonic features. The scale of oroclines can be enormous. Much of the Alaskan and Iberian peninsulas are attributable to coupled oroclines of the Cordilleran and Variscan orogens respectively. Paleomagnetic, structural and stratigraphic data demonstrate that these orogens were linear features that were subsequently folded about vertical fold axes. Attempts at analogue modelling of oroclines as buckled beams have failed, calling into question interpretations and models of oroclinal orogens as products of buckling of lithospheric-scale beams. In essence, neither the crust nor the lithospheric mantle have the strength necessary to buckle at the required scale.

An alternative interpretation is that oroclines are akin to wind blown pond scum that deforms against a buttress. In the case of oroclines, deformation is driven by a mantle wind, the scum is represented by a thin, crustal layer detached from the underlying crust and mantle, and the buttress is likely provided by a convergent margin. An appropriately oriented mantle wind appears to have been present during formation of the Cordilleran and Variscan oroclines. In the Cordillera, a consistent north-blowing mantle wind explains northward translation of terranes and continental ribbons since the Jurassic, a process that is ongoing as is indicated by Global Navigation Satellite System (GNSS) velocity vectors and the broad array of active dextral strike-slip structures. Similarly, a north-blowing mantle wind explains the consistent south to north transfer of Gondwana-derived terranes that were swept across the intervening Iapetan-Rheic oceanic realm before accreting to the Laurentian-Baltic-Asian upper plate in advance of the terminal Pangea-forming collisional event. In both cases, detachment and vertical axis folding of a thin, crustal lid accompanied interaction with a continental buttress (the Siberian craton in the Cordilleran case, and Laurentia-Baltic in the Variscan). The implication of the pond scum model is that oroclines are allochthonous with respect to the crust and lithospheric mantle that currently underpins them.