A tale of two kinds of normal fault: the importance of strain weakening in fault development
W. Roger Buck, Luc L. Lavier, 2001. "A tale of two kinds of normal fault: the importance of strain weakening in fault development", Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea, R. C. L. Wilson, R. B. Whitmarsh, B. Taylor, N. Froitzheim
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We search for a description of fault formation consistent with the main features of two very different types of extensional faults: (1) large-offset, low-angle normal faults; (2) small-offset, high-angle normal faults. We use an advanced numerical model to predict how the pattern of faulting varies as a function of the imposed magnitude and rate of weakening of an extending Mohr-Coulomb layer. We assume that fault weakening is due to reduction of cohesion with fault offset. Faults initiate and slip at high dip angles. When the fault offset is large (i.e. comparable with layer thickness) then the inactive footwall fault surface can be rotated to a flat orientation. We find two requirements for development of a large-offset fault. The magnitude of cohesion loss must be greater than c. 20% of the initial total extensional yield strength. Also, the rate of weakening with fault offset has to be moderate: the fault offset to lose cohesion has to be less than c. 2 km and more than c.100m, with the lower bound being harder to define. Using the same cohesion and rate of offset weakening, extension of a thick layer can lead to development of multiple, small-offset, high-angle faults rather than a single ‘low-angle’ fault. For cohesion reduction of 20 MPa a brittle lithosphere thicker than 20 km leads to multiple faults. Finally, we show that inclusion of thermal advection weakening can shift the transition to thinner layers for the same magnitude and rate of cohesion weakening.
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Non-volcanic continental margins may form up to 30% all present-day passive margins, and remnants of them are preserved in mountain belts. The papers in this volume demonstrate the benefits of integrating offshore and onshore studies, and illustrate the range of information obtained at different scales when comparing evidence from land and sea. Data sets collected across a range of spatial scales are evaluated: thin sections, cores, outcrops, seismic reflection profiles, and other geophysical data. The outcrop scale is crucial because it enables the spatial gulf to be bridged between DSDP and ODP cores and marine seismic data. There is also the problem that basins on land and beneath the sea inevitably have had different post-rift histories resulting in their contrasting present-day elevation. In mountain belts, portions of continental margins and oceanic crust are superbly exposed, but dismembered by subsequent compressional tectonics. Off present-day passive margins, extensional features have only been slightly deformed, if at all, by compressional movements, but are buried beneath significant thicknesses of post-rift sediments and so can only be sampled by ocean drilling at a small number of points.
The first paper reviews the synergies that have occurred between investigations of the eastern North Atlantic non-volcanic margins and remnants of similar Mesozoic margins preserved in the Alps, and some later papers return to this theme. However, papers describing margins from other parts of the world show that it may be premature to use models based on the Atlantic and the Alps as the paradigm for all non-volcanic margins. The following 25 papers in the book are grouped under the following headings: (1) Margin overviews; (2) Exhumed crust and mantle; (3) Tectonics and stratigraphy; (4)Numerical models of extension and magmatism.