Post-collisional formation of the Alpine foreland rifts
Abstract
Pokolizyjna geneza ryftów przedpola Alp
A series of Cenozoic rift zones with bimodal volcanic rocks form a discontinuous arc parallel to the Alpine mountain chain in the foreland region of Europe from France to Czechoslovakia. The characteristics of these continental rifts include: crustal thinning to 70-90% of the regional thickness, in cases with corresponding lithospheric thinning; alkali basalt or bimodal igneous suites; normal block faulting; high heat flow and hydrothermal activity; regional uplift; and immature continental to marine sedimentary rocks in hydrologically closed basins. Preceding the rifting was the complex Alpine continental collision orogeny which is characterized by: crustal shortening; thrusting and folding; limited calc-alkaline igneous activity; high pressure metamorphism; and marine flysch and continental molasse deposits in the foreland region. Evidence for the direction of subduction in the central area is inconclusive, although northerly subduction likely occurred in the eastern and western Tethys.
The rift events distinctly post-date the thrusthing and shortening periods of the orogeny, making “impactogen” models of formation untenable. However, the succession of tectonic and igneous events, the geophysical characteristics, and the timing and location of these rifts are very similar to those of the Late Cenozoic Basin and Range province in the western USA and the Early Permian Rotliegendes troughs in Central Europe. This analogy suggests a similar origin by tensile stresses caused by viscous drag of mantle material which is mechanically and thermally coupled to the subducting plate. This model is broadly compatible with published estimates of lithospheric yield strengths and of tensile shear stresses produced by descending Slabs, especially if the foreland was fractured by prior collision. It is hypothesized that compression at the trench before and during the orogeny counteracts this tensile shear stress until subduction ceases, whereupon the descending plate breaks off and continues to sink, causing extension in the overlying foreland crust and lithosphere and creating the foreland continental rift zones.
A series of Cenozoic rift zones with bimodal volcanic rocks form a discontinuous arc parallel to the Alpine mountain chain in the foreland region of Europe from France to Czechoslovakia. The characteristics of these continental rifts include: crustal thinning to 70-90% of the regional thickness, in cases with corresponding lithospheric thinning; alkali basalt or bimodal igneous suites; normal block faulting; high heat flow and hydrothermal activity; regional uplift; and immature continental to marine sedimentary rocks in hydrologically closed basins. Preceding the rifting was the complex Alpine continental collision orogeny which is characterized by: crustal shortening; thrusting and folding; limited calc-alkaline igneous activity; high pressure metamorphism; and marine flysch and continental molasse deposits in the foreland region. Evidence for the direction of subduction in the central area is inconclusive, although northerly subduction likely occurred in the eastern and western Tethys.
The rift events distinctly post-date the thrusthing and shortening periods of the orogeny, making “impactogen” models of formation untenable. However, the succession of tectonic and igneous events, the geophysical characteristics, and the timing and location of these rifts are very similar to those of the Late Cenozoic Basin and Range province in the western USA and the Early Permian Rotliegendes troughs in Central Europe. This analogy suggests a similar origin by tensile stresses caused by viscous drag of mantle material which is mechanically and thermally coupled to the subducting plate. This model is broadly compatible with published estimates of lithospheric yield strengths and of tensile shear stresses produced by descending Slabs, especially if the foreland was fractured by prior collision. It is hypothesized that compression at the trench before and during the orogeny counteracts this tensile shear stress until subduction ceases, whereupon the descending plate breaks off and continues to sink, causing extension in the overlying foreland crust and lithosphere and creating the foreland continental rift zones.