Measuring fold growth and crustal shortening: a case study from northern Tien Shan
Abstract
Active folding can generally be documented from structural geology, geomorphology or subsurface imaging techniques. From the age and elevation of a folded terrace, one can primarily infer incremental uplift or incremental tilting since terrace abandonment, a period of time generally covering several seismic cycles. Structural geology and subsurface geometry are more appropriate to estimate deformation over the longer term, typically a few 100 kyr at least. The identification and dating of growth strata is then key to estimate the chronology of the deformation. The pattern of uplift, and its time evolution, is the first accessible information one can retrieve. For most applications, horizontal shortening needs to be estimated as well. This step requires some structural models of the fold development. For example, in the case of mature fault-bend fold with no growth strata, converting the uplift pattern derived from a folded terrace into horizontal shortening is straightforward: uplift at any point along the section is equal to the product of the sinus of local dip angle and of the shortening across the fold. In many cases this simple approach cannot be applied. Dip angles may not parallel the fault at depths either because the beds may postdate fold initiation (growth strata) or because the fold might not be mature enough, or because of non conservation of bed thickness during deformation . We will show how the methodology can be adapted and expanded in partitucal to identify and overcome these difficulties. We will focus on a case study of an active fold along the northern front of the Tien Shan. The time evolution of shortening across that particular fold can be estimated accurately from combining geomorphic measurements, structural measurements and a magnetostratigraphic section.