sedimentary basins Rift basins - Structural aspects -
In regions of extensional tectonics, a knowledge of the geometry and kinematics of large-scale faults is of crucial importance to understand the internal structure of sedimentary basins and the amount of extension involved. The relationship between the geometry of main faults and the internal structure of sedimentary basins is best illustrated through sandbox experiment. Since the upper crust on top of which sediments accumulate deform via brittle failure, simple physical experiments using sand as analogue for the brittle crust can be used to study the evolution of extensional systems. Over the past two decades, the team of McClay (Univ of London, UK) and that of Brun (Univ of Rennes, FR) have produced a very large number of physical experiments to illustrate the evolution of extensional systems.
Extension above 2 rigid plates moving apart. Basal discontinuity in the velocity field leads to symmetric extension through the activation of planar faults dipping in opposite direction. Estimating extension from faults- Extension of brittle crust involves the rotation of planar fault blocks (dominos). The horizontal stretch related to individual block rotation is: (F-F')/F'
Extension above a rubber mat pulled from its left end. This basal simple shearing leads to asymmetric extension through the activation of parallel planar faults rotating toward slower dips.
Extension above a rigid wedge, extension is driven by pulling on a basal rigid plate. This leads to a complicated set of first order and second order planar faults some rotating to become sub-horizontal.
Sandbox models from Ken McClay & Tim Dooley, Fault Dynamics Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK.
Jackson and White (1989) show that large seismogenic normal faults on continents appear to be restricted to a dip range of 30º-60º, and that fault dips of <20º have not been observed in fault plane solutions of large earthquakes.