Half-graben Filling Models/GSA Abstract

Stratigraphic constraints on models of half-graben evolution

Abstract--Three end-member models of half-graben development evolve differently through time and produce different patterns of stratal geometry. In the fault growth model in which the basin-bounding fault grows longer through time and consequently the basin grows wider and longer, the rate of increase in basin volume is positive. Fluvial strata (deposited when the available volume of sediment exceeds the capacity of the basin) progressively onlap "basement" rocks of the hanging wall block, whereas lacustrine strata (deposited when basin capacity exceeds the available volume of sediment) pinch out against older fluvial strata at the center of the basin but onlap "basement" rocks along the lateral edges. In the linked fault system model in which a basin-bounding fault passes into a subhorizontal detachment, the rate of increase in basin volume during uniform extension is zero, and younger strata consistently pinch out against older synrift strata rather than "basement" rocks. In the domino fault block model in which both the basin-bounding fault and the intervening fault blocks rotate during extension, the rate of increase of basin volume is negative during uniform extension. Basin fill commonly forms a fanning geometry (all stratal wedges converge at a single line) during fluvial sedimentation, whereas lacustrine strata pinch out against older synrift strata.

The fault growth model is best at predicting the following features observed in numerous continental half-graben of all ages: (1) the transition from initial fluvial sedimentation to lacustrine deposition; (2) progressive onlap of younger strata onto "basement" rocks on the hanging wall block; and (3) the hanging wall block as the major source of sediment to the basin. The domino model is also capable of predicting feature (3).

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