SEDIMENTARY BASINS AND CRUSTAL THICKENING

We consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust. Well-known examples are foreland basins; others are ramp basins and buckle basins. We have reproduced various styles of compressional basins in experiments, properly scaled for gravity. A multilayered model lithosphere, with brittle and ductile layers, floats on a model asthenosphere. A computer-driven piston provides shortening and thickening, synchronous with erosion and sedimentation. After a first stage of lithospheric buckling, thrust faults appear, mainly at inflection points. Slip on an isolated reverse fault is accompanied by flexure. Footwall flexure results in a foreland basin and becomes accentuated by sedimentation. Hangingwall flexure is less marked, but may become accentuated by erosion. Motion on a fault leads to hangingwall collapse at the surface. Either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault. Slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin. Simultaneous slip produces a symmetric ramp basin, whereas alternating slip results in a butterfly-shaped basin, with superposed foredeeps. Some well-developed ramp basins become pushed down, until bounding faults meet at the surface and the basin disappears from view. At this stage, the basin depth is equivalent to 15 km or more. Slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline, between two distinct foredeeps. In Central Asia and in Western Europe, Cenozoic crustal thickening is due to continental collision. For Central Asia (Western China, Kyrgyzstan, Uzbekistan, Tajikistan), we have compiled a regional structure-contour map on the base of the Tertiary, as well as 4 regional sections. Foreland basins and ramp basins are numerous and associated with Cenozoic thrusts. Large basins (Tarim, Junggar, Fergana, Tajik) occur around and between mountain ranges, but smaller basins (Issyk-Kul, Naryn) occur within them. In Western Europe, the Alps and Pyrenees are surrounded by foreland basins, ramp basins or intermediate styles. In the Andes and its foreland, Neogene thrusts and compressional basins are due to subduction of oceanic lithosphere. In Colombia, they account for much of the Cordillera Oriental; in NW Argentina, for the Altiplano; in West-Central Argentina, for the Sierras Pampeanas. Compressional basins are also common in other areas of older crustal thickening.