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 (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.

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