Fault & Basin Growth/GSA Abstract

Rift basin evolution and the growth of normal faults

Abstract--The three-dimensional displacement geometry associated with micro-normal faults in sedimentary strata closely resembles the pattern of subsidence and uplift following earthquakes on parts of normal fault systems. Both are similar to the large-scale geometry of half-graben sedimentary basins and their uplifted footwall blocks. These observations suggest that (1) half-graben evolve as a consequence of repeated slip events on basin-bounding normal faults (and associated sediment-infilling, erosion, and isostatic adjustments) and (2) normal fault systems grow in length through time as displacement accumulates. Furthermore, half-graben should grow in depth, width, and length through time, and details of the evolution of basins and their border fault systems (BFS) should be recorded in the basin fill. Indeed, Triassic synrift strata progressively onlap "basement" along the margins of many eastern North American rift basins, implying that these basins grew in length and width. A progressive decrease in accumulation rates through the Triassic section is also consistent with basin growth. Forward modeling involving basin growth, however, indicates that both features need not always occur, depending on the relationship between the size of the structural basin (largely tectonically controlled) and that of the depositional basin (controlled by the interplay between basin capacity and sediment supply).

On the largest scale, a typical Mesozoic rift basin approximates a large syncline (as represented by the map-view trace of the "basement"-sediment contact) plunging toward its associated BFS. This geometry suggests that cumulative fault displacement and associated basin subsidence were highest near the center of the map trace of the BFS. On a smaller scale, transverse growth folds in the hanging wall of the BFS are associated with a segmentation of the BFS: synclines formed where fault segment displacement was highest; anticlines formed where displacement was lower, generally at the boundaries of fault segments. Thus, along-strike variations in fault displacement occur on a variety of scales. In addition, the map trace of the "basement"-sediment contact is apparently unaffected by the transverse folds and the segmentation of the BFS, implying some form of regional--and possible physical--linkage among the segments of the BFS and perhaps other faults within the basin.

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