Studies of strain and rheology of conglomerates

• Published in: Journal of structural geology 2002-01, Vol.24 (10), p.1541-1567
• Main Authors: Treagus, Susan H, Treagus, Jack E
• Format: Article
• Language: English
• Subjects: Conglomerate; Rheology; Rock viscosity ratios; Strain
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/S0191-8141(01)00162-6

Conglomerates can show significant strain variations among their component rock types. Previous studies have exploited this feature as a method of quantifying the effective viscosity ratios among different rock types, using inclusion–matrix models that we review here. We further investigate the rheology of conglomerates via models of multiphase mixtures of spherical clasts with varying phase fractions. Using idealised models with two to four phases, we can quantify the influence of phase viscosities and volume fractions on the bulk viscosity, and reveal how this controls the strain partitioning among the different phases. We consider practical methods of analysing conglomerates, and choose R f- φ analysis for measuring clast strain, and Fry analysis for whole-rock strain. The methods are illustrated through case studies of two very different conglomerates, the Port Askaig Tillite, Scotland, and Cesson Conglomerate, Brittany. We define the important rock phases in each, and assess their strain variations. Although providing strain measurements of regional interest, our main purpose is to quantify the viscosity ratios among the component rocks. We find that among the most common rock types (semipelite, quartzite, psammite, volcanics, granite), the viscosity ratios span about one order of magnitude, agreeing with previous studies. These small numerical values for viscosity ratios among many rock types, in many conglomerates, lead us to conclude that these rocks each deformed at approximately constant viscosity: i.e. as linearly viscous, rather than according to a power law.