The tectonic expression slab pull at continental convergent boundaries

Abstract

Examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style, topographic elevation, denudation, metamorphism, postcollisional convergence, and foredeep basin geometry and facies. The distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary. At subduction boundaries where the rate of overall plate convergence is less than the rate of subduction (termed here retreating subduction boundaries) the transmission of horizontal compressive stress across the plate boundary is small, and regional deformation of the overriding plate is by horizontal extension. The tectonic expression of these retreating subduction boundaries includes topographically low mountains, little erosion or denudation, low‐grade to no metamorphism, little to no involvement of crystalline basement in shortening, little to no postcollisional convergence, anomalously deep foredeep basins, and a protracted history of flysch deposition within the adjacent foredeep basin. Analysis of deflection and gravity data across three retreating subduction boundaries (Apennine, Carpathian and Hellenic systems) shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km (Carpathians), 50 and 150 km (Apennines) and 50 and 250 km (Hellenides). The total mass anomalies represented by the slabs are approximately 3×10 12, 6×10 12 and 12×10 12 N/m, respectively. The slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland, and partially supported by dynamic (viscous) stresses in the asthenosphere. At subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction (termed here advancing subduction boundaries) the transmission of horizontal compressive stress across the plate boundary is large, and regional deformation of the overriding plate is by horizontal shortening. The tectonic expression of these advancing subduction boundaries includes topographically high mountains, antithetic thrust belts, large amounts of erosion and denudation, exposure of high‐grade metamorphic rocks at the surface, extensive deformation of crystalline basement to midcrustal depths, protracted postcollisional convergence (tens of millions of years), and a protracted history of molasse deposition within the adjacent foredeep basins. Analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere (Western to Eastern and Southern Alps and Himalayas) shows that the thrust sheets have been translated for great distances over the foreland lithosphere (relative to the point at which the subduction forces are applied), thus obscuring any flexural and gravity signals from the subducted slab. However, it appears that far‐field stresses, presumably related to global plate motions, drive most of the convergent motion across these subduction boundaries. The concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved. Appendix B is available with entire article on microfiche.Order from the American Geophysical Union, 2000 FloridaAvenue, N.W., Washington, D.C. 20009. Document T92‐004; $2.50. Payment must accompany order.