Lateral extrusion in the eastern Alps, Part 1: Boundary conditions and experiments scaled for gravity

Lateral extrusion encompasses extensional collapse (gravitational spreading away from a topographic high in an orogenic belt) and tectonic escape (plane strain horizontal motion of wedges driven by forces applied to their boundaries). In the Eastern Alps it resulted from (1) an overall northerly compression (Apulia against Eurasia), (2) a strong foreland (Bohemian massif), (3) lack of constraint along a lateral boundary (Carpathian region), and (4) a previously thickened, gravitationally unstable, thermally weakened crust (Eastern Alpine orogenic belt). Six indentation experiments reproduce lateral extrusion at lithospheric scale. The models have two to four lithospheric layers, with a Mohr/Coulomb rheology for the upper and a viscous rheology for the lower crust. The lithosphere rests upon a low-viscosity asthenosphere. A broad indenter, a narrow deformable area, and a weakly constrained eastern margin fullfill as closely as possible conditions in the Eastern Alps. Indentation produces both thickening in front of the indenter and escape of triangular wedges. Lateral variations in crustal thickness become attenuated by gravitational spreading. The overall fault pattern includes domains of reverse, strike-slip, oblique normal, and pure normal faults. Strike-slip faults in conjugate sets develop serially. The narrow width of the deformable area and the strength of the foreland determine the angles between the sets. Gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin. Opposite the unconstrained margin, the indenter front shows thrusts and folds intersecting with the conjugate strike-slip sets. A triangular indenter favors spreading. High velocity of indentation favors escape. High confinement limits lateral motion, inhibits spreading, and favors thickening. Lateral extrusion in the Eastern Alps is best modeled by (1) a weak lateral confinement, (2) a broad and straight indenter, (3) a narrow width of the deformable area, and (4) a rigid foreland. Crustal thickening, lateral escape, and gravitational spreading all contribute to the overall deformation.

[1]  N. Kusznir,et al.  Intraplate lithosphere deformation and the strength of the lithosphere , 1984 .

[2]  F. Bergerat.  From pull-apart to the rifting process: the formation of the Pannonian Basin , 1989 .

[3]  A. Bosellini,et al.  Eoalpine and mesoalpine tectonics in the Southern Alps , 1987 .

[4]  Jean-Claude Sibuet,et al.  Geological evolution of the tethys belt from the atlantic to the pamirs since the LIAS , 1986 .

[5]  P. R. Cobbold,et al.  Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine , 1982 .

[6]  R. Madariaga,et al.  Numerical modeling of intraplate deformation: Simple mechanical models of continental collision , 1982 .

[7]  M. Tsenn,et al.  Flow properties of continental lithosphere , 1987 .

[8]  J. Dewey.  Extensional collapse of orogens , 1988 .

[9]  G. Peltzer,et al.  On the mechanics of the collision between India and Asia , 1986, Geological Society, London, Special Publications.

[10]  B. Burchfiel,et al.  Carpathian Foreland Fold and Thrust Belt and Its Relation to Pannonian and Other Basins , 1982 .

[11]  F. Bergerat.  Stress fields in the European platform at the time of Africa‐Eurasia collision , 1987 .

[12]  G. Peltzer,et al.  Formation and evolution of strike‐slip faults, rifts, and basins during the India‐Asia Collision: An experimental approach , 1988 .

[13]  L. Ratschbacher,et al.  Extension in compressional orogenic belts: The eastern Alps , 1989 .

[14]  D. Roeder.  South-Alpine thrusting and trans-Alpine convergence , 1989, Geological Society, London, Special Publications.

[15]  W. Frisch.  Tectonic progradation and plate tectonic evolution of the Alps , 1979 .

[16]  J. Dewey,et al.  Kinematics of the western Mediterranean , 1989, Geological Society, London, Special Publications.

[17]  S. Schmid,et al.  The role of the Periadriatic Line in the tectonic evolution of the Alps , 1989, Geological Society, London, Special Publications.

[18]  M. Parish,et al.  Kinematics of the Alpine arc and the motion history of Adria , 1989, Nature.

[19]  P. England,et al.  A thin viscous sheet model for continental deformation , 1982 .

[20]  Richard C. Morgan,et al.  Intraplate deformation due to continental collisions: A numerical study of deformation in a thin viscous sheet , 1985 .

[21]  P. Davy,et al.  Indentation tectonics in nature and experiment. I: Experiments scaled for gravity , 1988 .

[22]  P. Tapponnier.  Evolution tectonique du systeme alpin en Mediterranee; poinconnement et ecrasement rigide-plastique , 1977 .

[23]  O. Merle.  Strain models within spreading nappes , 1989 .

[24]  J. Byerlee.  Brittle-ductile transition in rocks , 1968 .

[25]  Peter Molnar,et al.  Slip-line field theory and large-scale continental tectonics , 1976, Nature.

[26]  J. Selverstone,et al.  Petrologic constraints on imbrication, metamorphism, and uplift in the SW Tauern Window, eastern Alps , 1985 .

[27]  A. Glazner,et al.  Evolution of lithospheric strength after thrusting , 1985 .

[28]  H. Laubscher.  The problem of the deep structure of the Southern Alps: 3-D material balance considerations and regional consequences , 1990 .

[29]  P. England,et al.  The mechanics of the Tibetan Plateau , 1988, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[30]  Lothar Ratschbacher,et al.  Lateral extrusion in the eastern Alps, PArt 2: Structural analysis , 1991 .