Superimposed features and deformation mechanism of Early Mesozoic folds in the Sangzhi-Shimen area, northern Hunan

Abstract

A series of Jurassic-type folds have continuously developed in the Mesozoic from the Sangzhi-Shimen area in northern Hunan. The analysis of the fold superimposition characteristics is of great significance to explore the deformation period and deformation mechanism of the Sangzhi-Shimen complex oblique folds, and also for the theoretical study of fold deformation controlled by faults. Based on the measurement of geological structure section and analysis of fold superimposition characteristics in Sangzhi-Shimen area, two fold structures in Indosinian and early Yanshanian were identified. The low-angle unconformity contact between the Jurassic and the Middle Triassic in the study area, and the high-angle unconformity contact between the Cretaceous and the Pre-Cretaceous reflect two deformation events of Indosinian and Early Yanshanian movement. It can be further inferred that the folds in the Sangzhi-Shimen area can be divided into two phases: the Indosinian movement and the early Yanshanian movement.The Indosinian fold axial trace mainly showed NEE to near EW; the early Yanshanian fold axial trace can be divided into two groups, one is NEE-near EW, the other is NNE. Two groups of early Yanshanian folds and pre Indosinian folds are superimposed to form two types of superposed folds, the enhanced type and the oblique limited type. The former is controlled by the inherited activities of the Indosinian NEE-near EW reverse fault; the latter is controlled by NNE trending reverse fault, which formed by NWW trending regional compressive stress in early Yanshanian, and it is limited by the pre NEE-near EW trending structure.This deformation mechanism well explains that the fold axis and thrust fault in the study area have no east or Southeast inclined polarity.

Keywords

• sangzhi-shimen complex syncline
• superposed fold
• structural analysis
• deformation period
• deformation mechanism