9 Structural Geology I: Faulting and Folding: Timing, Geometry, and Processes

Monday, 5 November 2001
8:00 AM - 12:00 PM

FOLDING BY CATACLASTIC FLOW: EVOLUTION OF CONTROLLING FACTORS DURING DEFORMATION

Paper number: 9-6
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Initial folding at upper crustal levels, occurs by bending of beds and flexural slip between beds. As the fold’s interlimb angle decreases, changes in bed thickness and limb rotation are accommodated by various mechanisms, depending on deformation conditions. In the elastico-frictional (EF) regime, cataclastic flow is the dominant mechanism for fold tightening. In detail, the degree of cataclastic deformation varies significantly across the fold due to minor variations in initial bedding thickness, grain size, matrix composition, etc. The Canyon Range (CR) syncline, located in the Sevier belt of central Utah, involves four thick quartzite units, with slight lithological variations between them. Fold tightening took place in the EF regime (<2 km overburden) by cataclastic flow, involving collective movement on a distributed network of fractures and deformation zones (DZs) from the micro- to the outcrop-scale (Ismat & Mitra 2001). A cooperative relationship exists across different scales, and the fracture networks result in a continuous deformation that is homogeneous at the largest scale.

The initial outcrop scale fracture/DZ network geometry is a product of the growth and linking of micro-scale cataclasite zones, which in turn is controlled by primary lithological variations. The material behavior of the fractured rock is unlike that of the original rock, with sliding of fracture bound blocks accomplishing large scale cataclastic flow. Thus, initial lithological variations at the microscale largely control the final deformation behavior at the largest scale. As folding continues, additional factors begin to regulate cataclastic flow, such as DZ reactivation or healing during folding. Although initial lithological variations in different units may produce unique network geometries, each unit’s behavior may also depend upon the behavior of adjacent units. In the CR syncline, during the initial stages of cataclastic flow, the inherent nature of each quartzite unit results in unit-specific network geometries. As deformation progresses, unit-specific networks begin to interact with those in surrounding units, resulting in feedback mechanisms regulating the later stages of network development. Thus, the nature of cataclastic flow changes dramatically from the initial to the final stages of folding.