The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures

Close to surface, cohesive rocks fail in extension, which results in open fractures that can be several tens of meters wide, so-called massively dilatant faults. These open fractures make fault slip analysis in rifts challenging, as kinematic markers are absent. Faults in rifts often have oblique sl...

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Main Authors: Christoph von Hagke, Michael Kettermann, Nicolai Bitsch, Daniel Bücken, Christopher Weismüller, Janos L. Urai
Format: Article
Language:English
Published: Frontiers Media S.A. 2019-02-01
Series:Frontiers in Earth Science
Subjects:
Online Access:https://www.frontiersin.org/article/10.3389/feart.2019.00018/full
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spelling doaj-2d422b7e47e0420b8a9a7ab759f9257e2020-11-24T21:17:13ZengFrontiers Media S.A.Frontiers in Earth Science2296-64632019-02-01710.3389/feart.2019.00018415415The Effect of Obliquity of Slip in Normal Faults on Distribution of Open FracturesChristoph von Hagke0Michael Kettermann1Nicolai Bitsch2Daniel Bücken3Christopher Weismüller4Janos L. Urai5Institute of Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Aachen, GermanyInstitute of Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Aachen, GermanyInstitute of Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Aachen, GermanyInstitute of Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Aachen, GermanyInstitute of Neotectonics and Natural Hazards, RWTH Aachen University, Aachen, GermanyInstitute of Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Aachen, GermanyClose to surface, cohesive rocks fail in extension, which results in open fractures that can be several tens of meters wide, so-called massively dilatant faults. These open fractures make fault slip analysis in rifts challenging, as kinematic markers are absent. Faults in rifts often have oblique slip kinematics; however, how the amount of obliquity is expressed in the surface structure of massively dilatant faults remains enigmatic. Furthermore, the structures of oblique dilatant faults at depth is largely unconstrained. To understand the subsurface structures we need to understand how different obliquities of slip influence the surface structures and the corresponding structures at depth. We present analog models of oblique massively dilatant faults using different cohesive materials in a sandbox with adjustable basement fault slip obliquity from 0° to 90°. Experiments with different mean stress and material cohesion were run. Using photogrammetric 3D models, we document the final stage of the experiments and investigate selected faults by excavation. We show that fault geometry and dilatancy changes systematically with angle of obliquity. Connected open fractures occur along the entire fault to a depth of 6–8 cm, and as isolated patches down to the base of the experiments. Using the scaling relationship of our models implies that transition from mode-1 to shear fracturing occurs at depths of 250–450 m in nature. Our experiments show the failure mode transition is a complex zone and open voids may still exist at depths of at least 1 km. We apply our results to the dilatant faults in Iceland. We show that the relationship between angle of obliquity and average graben width determined on faults on Iceland matches experimental results. Similarly, fracture orientation with respect to fault obliquity as observed on Iceland and in our experiments is quantitatively comparable. Our results allow evaluation of the structure of massively dilatant faults at depth, where these are not accessible for direct study. Our finding of a complex failure mode transition zone has consequences for our understanding of fracture formation, but also influences our interpretation of fluid flow in rift systems such as magma ascent or flux of hydrothermal waters.https://www.frontiersin.org/article/10.3389/feart.2019.00018/fullfaultmechanicsgeometrydilatantanalog modelingIceland
collection DOAJ
language English
format Article
sources DOAJ
author Christoph von Hagke
Michael Kettermann
Nicolai Bitsch
Daniel Bücken
Christopher Weismüller
Janos L. Urai
spellingShingle Christoph von Hagke
Michael Kettermann
Nicolai Bitsch
Daniel Bücken
Christopher Weismüller
Janos L. Urai
The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures
Frontiers in Earth Science
fault
mechanics
geometry
dilatant
analog modeling
Iceland
author_facet Christoph von Hagke
Michael Kettermann
Nicolai Bitsch
Daniel Bücken
Christopher Weismüller
Janos L. Urai
author_sort Christoph von Hagke
title The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures
title_short The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures
title_full The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures
title_fullStr The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures
title_full_unstemmed The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures
title_sort effect of obliquity of slip in normal faults on distribution of open fractures
publisher Frontiers Media S.A.
series Frontiers in Earth Science
issn 2296-6463
publishDate 2019-02-01
description Close to surface, cohesive rocks fail in extension, which results in open fractures that can be several tens of meters wide, so-called massively dilatant faults. These open fractures make fault slip analysis in rifts challenging, as kinematic markers are absent. Faults in rifts often have oblique slip kinematics; however, how the amount of obliquity is expressed in the surface structure of massively dilatant faults remains enigmatic. Furthermore, the structures of oblique dilatant faults at depth is largely unconstrained. To understand the subsurface structures we need to understand how different obliquities of slip influence the surface structures and the corresponding structures at depth. We present analog models of oblique massively dilatant faults using different cohesive materials in a sandbox with adjustable basement fault slip obliquity from 0° to 90°. Experiments with different mean stress and material cohesion were run. Using photogrammetric 3D models, we document the final stage of the experiments and investigate selected faults by excavation. We show that fault geometry and dilatancy changes systematically with angle of obliquity. Connected open fractures occur along the entire fault to a depth of 6–8 cm, and as isolated patches down to the base of the experiments. Using the scaling relationship of our models implies that transition from mode-1 to shear fracturing occurs at depths of 250–450 m in nature. Our experiments show the failure mode transition is a complex zone and open voids may still exist at depths of at least 1 km. We apply our results to the dilatant faults in Iceland. We show that the relationship between angle of obliquity and average graben width determined on faults on Iceland matches experimental results. Similarly, fracture orientation with respect to fault obliquity as observed on Iceland and in our experiments is quantitatively comparable. Our results allow evaluation of the structure of massively dilatant faults at depth, where these are not accessible for direct study. Our finding of a complex failure mode transition zone has consequences for our understanding of fracture formation, but also influences our interpretation of fluid flow in rift systems such as magma ascent or flux of hydrothermal waters.
topic fault
mechanics
geometry
dilatant
analog modeling
Iceland
url https://www.frontiersin.org/article/10.3389/feart.2019.00018/full
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