Lower lithosphere flow and continental strike-slip faults

A geophysical fault is a break in rock structure that occurs when pressures in the Earth's crust are strong enough to cause fracture and displacement, and earthquakes are common at suchbreak points. A strike-slip fault is a movement parallel to the fault plane, and the San Andreas fault of California is of thistype. The term "lithosphere" refers to the outer layer of the Earth, comprising the crust and upper mantle, and extending to adepth of 50 to 70 kilometers. The traditional view of tectonics (changes in the structure of the Earth's crust) is that the lithosphere consists of a strong brittle layer overlying a weak ductile layer, the system producing two forms of deformation, namely, brittle fracture in the upper layer (accompanied by earthquakes), and a seismic (without earthquakes) ductile flow in the lower layer. The current consensus is that this view is generally correct but imprecise, since the accumulated evidence is now interpreted to indicate that frictional events along faultlines, rather than new fractures, are the causes of earthquakes. The essential idea is that fault lines, which are the interfaces between the crustal plates, build up stresses resulting from the movements of the plates, and at intervals these stresses are suddenly relieved by interface slippages the surface manifestations of which are earthquakes. One important question is how movements of the lithosphere relate to the faults and to earthquakes, with one view holding that lithosphere deformations are secondary, and the other view holding that lithosphere deformations are of great significance for fault behavior. The Marl-borough fault zone is found in the South Island of New Zealand.Geodetic measurements, in this context, are measurements of contemporary terrestrial structure, as opposed to determinations based on geological (historical) analysis. ... ... Bourne et al (University of Oxford, UK) report an investigation of the consequences of the lithosphere deformation model. They tested predictions of the model in two plate boundary regions, the Marlborough fault zone and the Southern California fault zone, and they found that slip rates on the faults predicted from geodetic measurements are in good agreement with geologically determined slip rates. The authors suggest their results support the view that flow in the ductile portion of the lithosphere drives the accumulation of strain in the brittle upper layer where faulting occurs, and must play a role in the post-seismic relaxation of strain resulting from slip during earthquakes, and that the upper-crustal blocks must follow the flow of the lower layer.

QY:S.J. Bourne S.J.Bourne@siep.shell.com
Nature 12 Feb 98

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