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Simple extensional models that involve stretching by listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various sedimentary basins, continental shelves, and the Central graben in the North Sea. The case where extension thins the crust by a different amount from the subcrustal lithosphere has been considered by several authors, but their treatment of two-layer extension is overly complicated and partly incomplete. In this paper, we present a simplified analysis of the two-layer extensional model for the elementary case in which extension is instantaneous, the crust is thinned by a different amount from the subcrustal lithosphere, the effects of radioactivity and dike intrusion are i nored, and local isostatic compensation is assumed at all times. We show how the thinning parameters can be obtained from the subsidence data through the use of a simple and powerful method of data analysis. We show that conservation of mass during a process of non-uniform extension implies that much greater thicknesses of sediment can be deposited in a young basin than in the case of uniform extension of both crust and subcrustal lithosphere. Further, we show that such an extensional process produces significant uplift of the flanks of a graben and that, as a result of erosion of the uplifted areas, the effective area of the basin can be increased as much as 25 to 30%, depending on the rate of erosion, compared to the area that
would have been created by a process of uniform extension. Finally, we consider the forces of uplift on the flanks in the situation where the crust is treated as a thin elastic plate floating on a fluid upper mantle, the graben is bounded by two major normal faults, and there is subcrustal thinning under the flanks. We show that such normal faults produce uplift of the flanks and that this uplift can be significantly increased by the subcrustal thinning. Both kinds of forces give rise to uplift which can, in certain places, be misinterpreted as pre-rift doming. However, the uplift and erosion on the flanks of the graben associated with non-vertical faulting does not introduce significant error into the calculation of extensional parameters from subsidence analysis of a cross section o a basin.
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