Three-Dimensional Growth of Flexural Slip Fault-Bend and Fault-Propagation Folds and Their Geomorphic Expression

Abstract

Abstract:The three-dimensional growth of fault-related folds is known to be an important process during the development of compressive mountain belts. However, comparatively little is known concerning the manner in which fold growth is expressed in topographic relief and local drainage networks. Here we report results from a coupled kinematic and surface process model of fault-related folding. We consider flexural slip fault-bend and fault-propagation folds that grow in both the transport and strike directions, linked to a surface process model that includes bedrock channel development and hillslope diffusion. We investigate various modes of fold growth under identical surface process conditions and critically analyse their geomorphic expression. Fold growth results in the development of steep forelimbs and gentler, wider backlimbs resulting in asymmetric drainage basin development (smaller basins on forelimbs, larger basins on backlimbs). However, topographies developed above fault-propagation folds are more symmetric than those developed above fault-bend folds as a result of their different forelimb kinematics. In addition, the surface expression of fault-bend and fault-propagation folds depends both on the slip distribution along the fault and on the style of fold growth. When along-strike plunge is a result of slip events with gently decreasing slip towards the fault tips (with or without lateral propagation), large plunge-panel drainage networks are developed at the expense of backpanel (transport-opposing) and forepanel (transport-facing) drainage basins. In contrast, if the fold grows as a result of slip events with similar displacements along strike, plunge-panel drainage networks are poorly developed (or are transient features of early fold growth) and restricted to lateral fold terminations, particularly when the number of propagation events is small. The absence of large-scale plunge-panel drainage networks in natural examples suggests that the latter mode of fold growth may be more common. The advective component of deformation (implicit in kink-band migration models of fault-bend and fault-propagation folding) exerts a strong control on drainage basin development. In particular, as drainage lengthens with fold growth, more linear, parallel drainage networks are developed as compared to the dendritic patterns developed above simple uplifting structures. Over the 1 Ma of their development the folds modelled here only attain partial topographic equilibrium, as new material is continually being advected through active axial surfaces on both fold limbs and faults are propagating in both the transport and strike directions. We also find that the position of drainage divides at the Earth's surface has a complex relationship to the underlying fold axial surface locations.

Lateral Fault Growth in the Kashi Anticline (Chinese Tian Shan): Insights From Seismic Interpretation, Shortening Distribution, and Trishear Methods

“…Since the pioneering research of Shelton (1966), many field studies have used geomorphic criteria to evaluate the direction and rates of lateral propagation of active structures. In a general manner, the asymmetry of drainage networks, lateral deflection of rivers, and/or the existence of wind or water gaps have been considered as evidence of lateral fold propagation (e.g., Bernal et al, 2018;Bretis et al, 2011;Delcaillau et al, 1998;Jackson et al, 1996;Keller et al, 1999;Mueller & Talling, 1997;Ramsey et al, 2008). Similarly, several studies have observed alongstrike variations in the timing and amount of deformation of geomorphic surfaces and interpreted these differences as caused by lateral fault growth and fold propagation (e.g., Benedetti et al, 2000;Bennett et al, 2005;Jackson et al, 2002;Hetzel et al, 2004;Li et al, 2013;Keller et al, 1999;Morewood & Roberts, 1999.…”