Lithospheric stretching, detached normal faulting and footwall uplift

Abstract

Summary Lithospheric stretching can successfully account for the overall evolution of many sedimentary basins, and detached normal faulting the detailed geometry of the upper crust. In an instantaneously stretched, isostatically compensated basin, the equations which describe these two processes can be combined to define a ‘notional depth to decollement’. Only at this level can the sole to the normal fault system maintain a constant depth below sea-level during extension. The notional depth to decollement depends primarily on the mean density of the basin fill and for a constant-density basin fill (e.g. sea water) coincides with the level of no vertical motion during stretching. In a sediment-filled basin, the notional depth to decollement will increase with the stretching factor β as early-deposited sediments are compacted and the mean density of the sediment column increases. In general, a physical sole fault will not lie at the notional depth to decollement, and must move vertically to maintain isostatic equilibrium: such movement precludes the use of balanced cross-section techniques to determine the physical depth to decollement. In typical crustal situations, uplift of the sole fault will be more common than subsidence and will in turn cause uplift of any residual, unfaulted basement blocks which rest upon it. Footwall uplift can also be modelled using area-balance constraints, referred to the notional depth to decollement rather than to the physical sole fault. The amount of uplift depends primarily on the initial fault spacing. Three fields can be distinguished: one in which footwalls subside at an increasing rate as β increases, one in which they are uplifted above sea-level then subside below sea-level, and one in which they are uplifted then subside, but always remain above sea-level. Similar relationships exist in an uncompensated basin, where the depth to the physical sole fault replaces the notional depth to decollement. Curves showing uplift and subsidence versus β have been constructed in dimensionless form (referred to depth to decollement) and for a model basin with an exponential sediment compaction relationship. They agree closely with uplift/subsidence histories inferred from seismic and well data for the North Sea, and with published descriptions of other areas (the Armorican margin, the Aegean Sea).