McKenzie Model Research Articles

Interaction of deep and shallow processes in the evolution of the Kenya rift

The start of volcanism before rifting in the northern Kenya rift suggests that an asthenospheric thermal anomaly was responsible, not decompression melting due to lithosphere stretching. This volcanism may be partly related to the Ethiopian rift, or even the Anza graben, not the Kenya rift. In the northern Kenya rift the first stage of deformation was the formation of isolated sediment-filled half-graben basins during the Late Oligocene-Early Miocene, perhaps superimposed on lower Tertiary basins. During the Miocene, the location of basins shifted eastwards. This shift is interpreted as being due to strain hardening of the lithosphere during extension caused by a relatively slow strain rate. Relocation of the zone of extension progressively eastwards was possibly caused by migration of the asthenospheric thermal anomaly to the east (which lowered the strength of the crust above the thermal anomaly). The simple McKenzie model of uniform lithosphere stretching does not apparently fit the Kenya rift. Uniform extension may have affected the entire lithosphere but uniform stretching can only be demonstrated for the continental crust. The shape of the geophysically defined base lithosphere under the rift shows much more thinning of the mantle lithosphere than the crust. Consequently, thermal thinning of the mantle lithosphere has to be invoked to explain the discrepancy. Where the asthenosphere lies almost at the base of the crust the surface rift above displays swarms of minor faults and a linear array of Pliocene recent volcanoes. Thus the deep thermal history and the shallow brittle structures of the Kenya rift appear to be closely linked and each has influenced the evolution of the other. Extension estimates for the upper crust and the lower crust are similar, indicating that addition of magma to the crust has not caused an underestimate of lower crust extension. This suggests that either the ratios of magma emplaced within the crust to surface volcanism are much smaller (1:1 to 2:1) than previously thought (5:1 to 10:1), or that entry of magma into the crust is offset by an equal loss of crustal material into the asthenosphere.

Carboniferous

Abstract Carboniferous geology has developed very rapidly over the past two decades since ideas on sedimentology and new radiometric scales, together with new biostratigraphical schemes based on miospores, conodonts and foraminifera have been synthesized with ideas on plate tectonic processes, climatic changes and oscillations of sea level. The application of the McKenzie model of basin development to the Carboniferous, and the relating of eustatic sea-level changes, which had a profound effect on Carboniferous palaeo-geography, to fluctuations in the size of the Gondwanan ice sheets, have been major factors in the understanding of Carboniferous processes. The Carboniferous evolution of Britain took place to the (present) north of a plate suturing occurring along a line extending from Galicia, through Armorica and the Massif Central to the Vosges, during mid Devonian to early Carboniferous times. In front of this collision zone was the Rheno-Hercynian zone which opened probably during mid Devonian times and which was partly floored by oceanic crust; its closure can be traced through to late Carboniferous times as a series of northwardly directed thrust complexes. To the (present) north of this zone was a foreland on which basins developed on either side of a persistent Wales-London-Brabant High. Studies by Dewey (1982) and Leeder (1982, 1987, 1988) have shown that late Devonian to early Carboniferous extension, involving β factors of up to 2, produced a series of grabens and half-grabens in the Caledonian basement. These developing structures were the dominant control on early Carboniferous sedimentation and produced a sea-bed topography which was

Initial Process of Rifting: ABSTRACT

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 End_Page 252------------------------------ 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. End_of_Article - Last_Page 253------------

Thermal Model of Ocean Ridges

MCKENZIE'S model of crustal creation at the ocean ridges1,2 and its derivatives3,4 predicts such features as the topography and high heat flow of the ridges. In spite of this success there are some unsatisfactory aspects of the model; for example, the arbitrary temperature distribution in the intrusive zone gives rise to infinite heat generation and the lithospheric thickness is a free parameter not determined by the physics. We offer here a simple refinement of McKenzie's model that overcomes these difficulties. The essential difference stems from the inclusion of terms in the boundary conditions to account for the evolution of latent heat in places where the plate is growing. We first describe the physical basis of the model.