Clutch tectonics and the partial attachment of lithospheric layers

Abstract

Clutch zones, in analogy to the clutch in an au- tomobile, explain the mechanical communication between the necessarily different displacement fields of rheologically- distinct lithospheric layers. In contrast to detachment zones, these sub-horizontal shear zones act as partial attachment zones between lithospheric layers. Two cases are possible: 1) Top-driven clutch systems, such as in extensional core complexes, in which kinematics are controlled by gravita- tional forces; or 2) Bottom-driven clutch systems in which kinematics are imposed from below. We focus on three dif- ferent clutches that are associated with the lithosphere: the crustal brittle/ductile transition, the crust/mantle transition (lower crust), and the lithosphere/asthenosphere transition. The clutch model predicts coupled crustal (lithospheric?) deformation resulting from bulk flow of the lithospheric and asthenospheric mantle. An indication that orogenic systems are generally bottom-driven, rather than side-driven, is the pervasive sub-horizontal shearing which occurs within the lowermost crust. This model questions several aspects of classical plate tectonics by suggesting that: 1) Vertical dis- placement gradients are as significant as horizontal displace- ment gradients; 2) Areas of coherent upper crustal move- ment (plates?) do not encompass the entire lithosphere; and 3) Orogenic deformation results from bottom, rather than side, boundary conditions.