The generally accepted model of rifting (the McKenzie model) has certain geometric and spatial constraints that seem to preclude its operation in the earliest stage of rifting. It may be a more advanced stage of the rifting process, if it is correctly described.

An aborted rift system can be studied in the subsurface of the Permian basin. The Delaware, Val Verde, and Marfa basins formed a rift-rift-rift triple junction in mid-Pennsylvanian time, but it never progressed far enough to cause permanent extension. It apparently rose thermally, and then settled back down in place during the cooling cycle. The details of earliest rifting are preserved.

Several geometric factors need to be considered in the rift model. The first is that the earth is a sphere. On a sphere, uplift causes extension, and downwarping causes compression. The dominant fracture system in the brittle crust tends to be vertical, and on a sphere, vertical planes converge at the center.

The rheology of the basement and the overlying sedimentary rocks is different. The basement can be extended areally by dilating the fracture system during uplift and extension, but the sedimentary rocks will be stretched plastically. During the cooling cycles, vertical fractures can close, but there will be sediment to spare. The rocks will be buckled, crinkled, and overturned during the cooling cycle as they are lowered from

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the arc toward the chord. Many interpret this as evidence of external compression.

A Permian basin model can explain many diverse and, to many geologists, unrelated structural and stratigraphic phenomena. If this model is correct, it may mean that many other aborted rift systems are not being properly described. The North Sea is a case in point.

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