INCLINATION SHALLOWING AND PREFERRED TRANSITIONAL VGP PATHS

Abstract

Transitional VGP paths recorded in sediments cluster into two antipodal preferred longitude bands that tend to lie 90° away from their site longitudes, the latter also being clustered. VGP paths obtained from lava flow sequences, though much fewer, appear not to show these biases, suggesting a rock-magnetic influence on VGP paths recorded in sediments. Inclination shallowing of detrital magnetic remanence, enhanced under low transitional field strengths, is the most likely candidate.We illustrate the effects of inclination shallowing by applying a simple shallowing model (tan IR = f tan IA, where IA is the inclination of the magnetic remanence and IA is the inclination of the ambient field) with field variation to hypothetical data sets. Shallowing-induced clustering increases as f decreases and becomes extreme as f approaches 0.1.We have used the model to ‘de-shallow’ the available set of transitional VGP sediment records for various values of f. The probability that the observations arise from inclination shallowing of a uniform random distribution of paths increases as f decreases. When f drops to 0.13 there is a 50% chance of getting at least as much grouping as observed. To decide if inclination shallowing is a dominant factor in the clustering, we need to know whether such extreme shallowing is widespread in sedimentary records under transitional field conditions. Field and laboratory redeposition data are not yet adequate to resolve this question.