Graphic derivation of the local sense of shear strain components in stretched walls of lithotectonic boundaries

Abstract

Lithotectonic boundaries (LTBs) are deformed interfaces between rock masses such as sheared strata, midcrustal thrust slices or sutured microcontinents. Structural analysis of strained LTB walls have focussed attention on rock cuts through the foliation normal and mineral lineation direction, but this practice is commonly inappropriate for investigating the tangential shear strain ( γ). A simple graphic technique is introduced whereby one may derive the sense of large shear strain components in two sections across an LTB wall segment; one section parallel to the lineation direction, the other normal to the foliation. The orientation of the strain ellipse is used on each section to deduce the sense of its shear strain component by visual inspection. The technique is applicable to metamorphic rocks with L– S mineral fabrics, but without reliable gauges of large incremental or total longitudinal strain. If the factor of prolateness/oblateness of the strain ellipsoid is unknown, then the γ direction cannot be fixed within an angle of <90° on the LTB surface (cf. accompanying paper by R. J. Lisle, Journal of Structural Geology, 20, 969–973). However, the γ direction may be accurately determined if an LTB cross section fortuitously contains the normal to foliation as well as the mineral lineation direction. Similarly, the γ direction can be determined if the local strain ellipsoid corresponds to an oblate or prolate spheroid. Where the ellipsoid is nearly spheroidal, one can therefore discern which of the two shear strain components under consideration has the larger magnitude. This proves advantageous in assessing the sense of Alleghanian tangential shear below the Brevard fault zone (Grandfather Mountain area), southern Appalachians. Here γ has the sense of a ductile thrust, >1 km into the stretched footwall. Close to the fault zone, however, γ has a large sinistral component, unless the LTB dip is >40°.