Shortening Histories in Active Detachment Folds Based on Area-of-relief Methods

Abstract

The thickness variations and shapes of strata deposited over actively growing folds provide a convolved record of the interaction of deformation and sedimentation. Here we show how key elements of the history of shortening can be extracted using area-of-relief measurements on many stratigraphic horizons in well-imaged structures. In pregrowth strata, the shortening is equal to the vertical gradient in the area of structural relief, S = dA/dz. In growth strata, this relationship must be modified because the observed gradient in the area of relief now includes the effects of both structural thickening and stratigraphic thinning caused by deposition over the growing structure, S = (dA/dz)obs (dA/dz)strat. This stratigraphic thinning cannot be measured directly from thickness variations because of later structural thickening but can be determined through consideration of the ratio of the mean shortening rate to the sedimentation rate, , which is a key parameter. We apply these concepts to a set of actively growing detachment folds: Nankai Trough, Japan; Cascadia, offshore Oregon; Yaken anticline, western China; and Agbami anticline, Niger Delta. These examples show a considerable diversity and complexity, including the effects of excess area in weak basal detachment layers, multiple detachment levels, and large differences in . All show not only constant shortening as a function of height in pregrowth strata and approximately constant shortening rates, , over substantial stratigraphic thicknesses, but also abrupt and large increases in shortening rate by factors of 5–10 and significant hiatuses in fold growth. The typical shortening rates in these frontal structures are in the range of 0.1–3 mm/yr (0.003–0.12 in./yr) and represent only 1–10% of the regional plate tectonic rates of their larger tectonic settings (2–6 cm/yr [0.8–2.4 in./yr]), indicating that shortening is not currently concentrated in these frontal zones of these mountain belts and accretionary wedges.