CORDILLERAN THRUST BELTS AND FORELAND DEFORMATION: UNSOLVED MYSTERIES

The foreland thrust belts and external basement uplifts of the North and South American Cordilleras seem the perfect complement: The latter is still active with geophysical data from earthquakes and GPS surveys, and a well defined plate geometry and convergence history. The Jurassic to Eocene North American foreland has had more extensive geologic mapping and importantly, the hinterland has been dismembered and denuded by subsequent extension thus exposing deeper crustal levels. The similarities noted between the two Cordilleras more than 30 years ago still stand: Thin- and thick-skinned deformation are/were active simultaneously. A relatively thin pre-orogenic stratigraphic cover over basement appears to be a necessary but not sufficient condition for foreland basement uplifts, whereas flat subduction is a necessary condition. Based on deep earthquakes and deep seismic reflection profiles, thick-skinned deformation involved brittle frictional deformation of nearly the entire continental crust. Likewise, in both Cordilleras, synorogenic strata have been crucial for defining the thrust history showing foreland propagation of the thrust front and, especially in South America, demonstrating simultaneous and out-of-sequence motion of the major plates.

The still unexplained differences are more interesting: The Subandean belt and the Canadian and Idaho-Wyoming thrust belts differ in width by less than a factor of two, yet the duration of deformation is nearly an order of magnitude longer in North America. In South America, removal of lithospheric material occurs via two mechanisms: subduction erosion has removed as much as 200 km since the Jurassic such that the west end of the Precordillera thrust belt is just 300 km from the trench making crustal balancing a challenge. Likewise in both flat and steep segments of the Nazca subduction zone the lithospheric mantle lid must have been largely removed. In contrast, North America was an accreting margin, including terrane accretion, during much of the Mesozoic. The role of delamination in the Mesozoic of North America is less clear. Finally, Southern Tibet style detachments are completely missing from the Central Andes and the hinterland precursors to the metamorphic core complexes, if present, remain completely buried.