The strain history of a dalradian slide: Using pebbles with low fluctuations in axis orientation

Abstract

The strain associated with the Horn Head Slide, a major tectonic break in the Dalradian rocks of NW Ireland, is recorded by pebbles in an adjacent quartzite horizon. Mean X/Y ratios of the deformed pebble shapes in excess of 8.0 are seen closest to the slide and the field of three-dimensional shapes lies along the K = 1 line. The usual methods of separating initial shape ratio ( R i ) and tectonic strain ratio ( R s ) from the deformed shape ratio ( R f ) of ellipsoidal markers are based on measurements of variation in fluctuation (e.g., the ( R f /φ technique). However, due to the high X/Y strains in this situation and since the pebbles initially lay parallel to bedding and to a principal plane of the finite strain ellipsoid, fluctuation is generally very low. Thus, except for the least deformed X/Y data, the R f/φ technique is inapplicable and other methods are used. For X/Y data with mean ( R f > 4.0 : R s is calculated as the harmonic mean of R f ; maximum R i values only are obtained from the range of R f data. For all Y/Z and X/Z data: R i is calculated from the logarithmic range ( ω log ) of R f ; R s is simply obtained from the geometric mean of R f modified by R i . It is concluded from this that a varying prolate tectonic strain ( K ∼- 1.5) reaching X/Y values in excess of 8.00 was coaxially superimposed on an initial oblate shape fabric to produce the present field of deformed pebbles in the quartzite near the slide.