Thrust systems and contractional tectonics

Collision mountain belts are sites of crustal thickening and tectonic contraction. On their outer margins this deformation is commonly accommodated by thrust faulting, where strata are carried up over each other, in places carried for many tens of kilometres. This chapter examines the structures of thrust systems and the approaches that are used to unravel their complexities. These discussions build from historical perspectives that review the development of structural interpretation, starting in the Moine Thrust Belt in NW Scotland and moving through the foothills of the Canadian Cordillera. Further examples presented here come from the Western Alps, Appalachians, Pyrenees, Himalayas, Zagros mountains and Sichuan basin. Thrust systems also form at the toes of gravitationally-collapsing sedimentary prisms accumulating on continental margins, sites far-removed from mountain ranges but which yield spectacular seismic images. Contractional tectonics involves more than just faulting - rocks can fold and simply thicken up by distributed strain. These variations, together with the role of crustal-scale structures, commonly reflect heterogeneities in the inherited structure of sedimentary basins that become incorporated in collision mountain belts.