Structure of fault zones in cohesive volcanic sequences
Creators: name: Holland, Marc
Publisher: name: RWTH Aachen University
Year: 2011
Subjects: subject: 550
subject: Vulkanboden
subject: Vulkan
subject: Hawaii
subject: Geologie
subject: Strukturgeologie
subject: Störung
subject: Geowissenschaften
subject: analoge Modelle
subject: Island of Hawai'i
subject: analog models
subject: geology
subject: volcano

description: Aachen, Techn. Hochsch., Diplomarbeit, 2004; Aachen : Publikationsserver der RWTH Aachen University 54 S: (2011). = Aachen, Techn. Hochsch., Diplomarbeit, 2004
description: Normal fault systems are basic features in commercially important geological structures like e.g. sedimentary basins. While the interpretation of seismic data sets reveals the structure of the strata and its offset by larger faults, the properties of the fault planes itself remain undetermined. The information on e.g. permeability of laterally confined fault systems is derived from outcrops or theoretical models. The scope of the faulting research focuses on softer unconsolidated materials as they occur in sedimentary domains. Harder cohesive rock sequences were so far largely neglected. The Koa´e fault zone on Big Island/Hawai'i is characterized to act as an example for a normal fault system in cohesive materials. The Koa'e system is located in a sequence of subaerial emplaced basalt flows. The main features of the fault zone are subvertical scarps up to 25 m height, as well as large opening magnitudes beyond 5 m width, showing the discrepancy to the soft sedimentary systems. The governing properties of the basalt are the high strength in combination with a dense pattern of potential weakness planes like cooling joints or delamination planes. The material is therefore less strong than laboratory measurements suggest. Initial deformation of the rigid material is evident by a rotation and shifting of the predefined fragments. The system is able to compensate some deformation by the displacement of the fragments until the offset exceeds the fragment size or until the stresses are high enough to crush the fragments. The deformation is spread and not localized until this stage. The Koa'e fault system shows open fissures and faults documenting an initial mode-I movement preceding a further mode-II or III movement. The variety of observed cross-sectional morphologies of the fault system are reproducible by a scaled analogue model using gypsum powder. Documented by digital cameras time-lapse movies and their quantitative analysis by a PIV software are derived. Gypsum powder shows as fine-grained cohesive material similar properties as the basaltic rock. The powder forms irregular indenting clusters. These react in a shallow regime by the same rotation and shifting until prolonged deformation breaks down the clusters into smaller fragments. Localization sets in and the relative strength of the fault zone is reduced. The material properties of the powder were measured at DelftChemTech, the Pharmazeutisches Insitut Würzburg and at the Endogene Dynamik Aachen. A tensile strength of 33 Pa together the measurements of the shear experiments document a curved yield locus for the gypsum powder in the Mohr space. The curvature of the Mohr circles and the yield locus of the powder are similar in the shallow regime. Since the two touch each other at multiple points the system obtains a high sensitivity, governed by local heterogeneities and not by the local stress field. This results in a non-linear system with different fault locations and dip angles. The corresponding surface morphologies are similar but show a variety. The ramp and scarp type end members are observed in the field as well. Since the first deformation structures at the surface govern the development of the morphology, the system has a low reproducibility. The morphologies in the Koa'e zone may therefore be a result of the freedoms in the system and not a result of different initial boundary conditions. The material characterization of the powder reveales the importance of the yield envelope shape to the evolving structures. This applies not only for the Koa'e fault zone model but also to analogue modelling in general. The standard material sand lacks a tensile strength and true cohesion since it is not able to support vertical structures. Interpolated shear strengths of sand indicate a cohesion if using a linear Mohr-Coulomb relationship. This way of material handling is only justified by the fact, that the material properties of the rock material are described in the same manner. In deeper regimes the linear part of the yield envelope may be similar, but the material behaviour in shallow regimes deviate. Vertical rock walls as seen in the Koa'e fault zone or in any cliff document the tensile strength and cohesion of rock material and indicate that sand is not an adequate model material for strong rocks. Powder however is capable to mimic these conditions if the material characterization is done properly. With a tensile strength of 33 Pa, a cohesion of 62 Pa and a linear yield locus in deeper parts, the gypsum powder represents a material that has obvious advantages in respect to sand. Deformation structures like open fissures, progressive fragmentation, block rotation and thin nondilatant deformation zones display the large potential of this material.
description: Published by Publikationsserver der RWTH Aachen University, Aachen