Three-dimensional (3-D) finite strain at the central Andean orocline and implications for grain-scale shortening in orogens

Abstract

Three-dimensional (3-D) finite strain analyses from across the central Andes are used to document the contribution of grain-scale strain in quartzites and sandstones to the total shortening budget. The results are compared to thermal, stratigraphic, and strain data from other fold-and-thrust belts to determine the influence of lithologic strength and deformation temperature on strain accommodation during orogenic evolution. In the central Andes, 3-D best-fit ellipsoids are inconsistently oriented relative to structural trends, have short axes at high angles to bedding ( Z , mean plunge = 78° ± 21°), and have bedding-parallel long axes ( X , mean plunge = 6° ± 24°). Ellipsoid shapes are dominantly oblate ( X = Y > Z ), indicate low natural octahedral shear strains (e S = 0.03–0.19), and have axial ratios that range from 1.02:1:0.81 (e S = 0.19) to 1.02:1:0.97 (e S = 0.03). Highly variable R f -ϕ data ( R f = 1.0–5.0, ϕ fluctuations exceeding 100°) indicate detrital grain shapes may overwhelm any measurable tectonic strain fabric recorded by grain geometry. The best-fit ellipsoids may reflect either weak compaction strain, or they may be related to a depositional fabric. At a minimum, granular strain was insufficient to reset the detrital grain fabric, and therefore grain-scale strain in quartzites and sandstones is not a significant factor in deformation. We suggest that the nonstrained nature of these stiffer lithologies indicates a lack of regional, penetrative strain in the central Andes like that quantified in similar lithologies in other orogens. The regional lack of strain may be due to deformation temperatures