Some basins also evolve through the merger of originally isolated subbasins whose border faults grew toward one another. The oldest strata form restricted sequences in each subbasin. Strata deposited shortly after consolidation thin toward the "intrabasin high" that forms in the merger zone, a region of short-term displacement deficit. If the merging faults grow together in the same plane, the merger zone is located at the center of the combined fault system, and displacement must rapidly increase to conform to the typical displacement profile. Thus, the youngest strata thicken toward the former location of the intrabasin high. If the growing faults do not lie in the same plane but overlap in the extension direction, displacement is distributed on numerous splay faults. The intrabasin high then has long-term expression, even though the summed fault displacement within the high is equal to or slightly greater than that of the deeper, flanking subbasins. Oblique-slip accommodation zones form in the overlap zone of basins whose propagating faults dip in opposite directions. In general, faults growing in length obviate the need for transfer faults at the fault tips. Transfer faults may form if both fault tips cannot propagate. If only one tip is fixed, the other tip propagates away from the fixed tip, and the depocenter migrates in the same direction.

The fault and basin growth models described above provide a useful framework for interpreting the stratigraphic record of extensional basins and extracting their tectonic development, as we will demonstrate with examples from the Mesozoic rifts of eastern North America and the northern Basin and Range.

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