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 (εS = 0.03–0.19), and have axial ratios that range from 1.02:1:0.81 (εS = 0.19) to 1.02:1:0.97 (εS = 0.03). Highly variable Rf-ϕ data (Rf = 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 <180 °C and the presence of five ≥1-km-thick shale detachments. In the Sevier and Appalachian orogens, granular strain fabrics are best developed where temperatures exceeded ∼180 °C, but they are also found where temperatures were <180 °C. The lack of distributed detachments in Appalachian and Sevier stratigraphy may have favored minor layer-parallel shortening at temperatures <180 °C rather than formation of numerous, low-offset faults as in the central Andes. Minimal slip on individual central Andean thrusts would have limited footwall burial and maintained low deformation temperatures.

You do not currently have access to this article.