Rheological information from geological structures

Peter J Hudleston, Labao Lan

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

The contrast in rheological properties between layers of different composition or texture, and between stiff inclusions and their matrix, gives rise to perturbations in flow that result in structures. Theory and modeling allow us to understand the conditions necessary for such structures to form and, conversely, we can use the form of the structures to infer possible rheological conditions for the rocks during natural deformation. We review here several structures and their use as indicators of rheological behavior, based on theory, numerical and experimental models, and observations on naturl structures. Theory predicts that a dominant wavelength/thickness (Ld/h) exists for both folding and boudinage that depends on the ratio of viscosities of layer to matrix, the homogeneous shortening undergone by the layer, and the exponent, n, in a flow law of power-law type. The measurement of average L/h, and of the shortening within the layers allows an estimate of the power-law exponent of the stiff layer to be made. Also, numerical modeling shows that fold hinges become sharper as n increases and the limbs become relatively longer and straighter. The dynamic growth of pinch and swell instabilities only overcomes the kinematic decay in nonlinear flow, thus the existence of pinch and swell is by itself evidence of nonlinear behavior. Strain rate and strain, increase more rapidly away from the neutral surface for a layer of power-law rheology (with n>1) than for a layer of Newtonian rheology. Thus, strain gradient across a fold hinge, at fixed amplitude or limb dip, increases with increasing n. Porphyroclasts in mylonites develop characteristic rims of recrystallized grains that are drawn out into trails of σ or ° shape by the perturbed flow of the, material around the clast. Experimental evidence suggests that σ shapes occur if flow is linear, whereas nonlinear flow may give rise to the δ shape. The inference of rheological behavior from structures is complementary to the determination of rheological properties of rocks in the laboratory. What data there is suggest that constitutive relations for rocks undergoing ductile deformation in which many structures develop are highly nonlinear. There is general qualitative agreement between flow laws inferred on the basis of experimental results and those inferred from observation of structural characteristics.

Original languageEnglish (US)
Pages (from-to)605-620
Number of pages16
JournalPure and Applied Geophysics PAGEOPH
Volume145
Issue number3-4
DOIs
StatePublished - Sep 1 1995

Keywords

  • Rheology
  • numerical models
  • strain
  • structures

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