Faults & Fault Zones Faults and Fault Zones Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 1 Faults & Fault Zones Faulting and Rock Mass Characterization: General Considerations • The comprehension and recognition of the tectonic style and architecture of a region of geotechnical interest is of crucial importance for the development of a valid engineering geological model • Even isolated brittle shears are part of a fault system in close relationship with a regional stress regime (present or past) • Assessment of fault kinematics and sense of displacement reflects the tectonic regime with respect to stress axes orientations and relative sizes at the time of faulting • There are good reasons for the assumption that the kinematics of youngest brittle faults represent the present state of regional stress in a rock mass (see: fault plane solutions of earthquakes) • Present tectonic regime (extensional, wrench,...) and according local fault characteristics determine to a high degree some geotechnically important properties within a rock mass, as there are: permeability, stand-up time and stability performance, stress relief phenomena . Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 2 1 Faults & Fault Zones Characterization of Brittle Faults - Faults are elongated complex zones of deformation, ranging from decimeters to kilometers in magnitude - A significant internal structure of shear and extensional fractures has developed, reflecting the geometry of the strain field and, consequently, the orientation of the principal stresses - The brittle deformation, such as particle size reduction by crushing of grains and reorientation of grains by shearing, generates the characteristic fine-grained gouge - Low-temperature solution transfer contributes substantially to the alteration of fault rocks, in particular of gouge, through transformation and neo-formation of clay minerals Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 3 Faults & Fault Zones Terminology • (Minor) Shear: Single fracture w/wo thin layer of crushed material (fault rock) between sheared - offset surfaces • Fault: General geological structure with shearing displacement of adjacent rock masses • Shear/Fault Zone: Elongate zone of displacement of considerable width, consisting of interlacing shear fractures or fault sets with or without fault rocks • Fault Rock: Product of host rock structural and/or mineralogical alteration due to brittle and/or ductile deformation during shearing; may be cohesive (coherent), incohesive or secondary indurated Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 4 2 Faults & Fault Zones Significance of Fault Zones - The significant structural feature is a substantial heterogeneity, reflected by the occurrence of more or less undeformed competent blocks which are typically surrounded by a fine- grained matrix consisting of gouge and highly fractured rocks. The matrix appears to be flowing around the blocks in an anastomosing pattern - The mainly lozenge shaped blocks exhibit a fractal distribution of dimensions, ranging from the microscale to hundreds of meters in length. Fault structures are scale independent - A considerable heterogeneity of the stress field may exist. Variations in the stress field might be an important cause of segmentary fault zone formation - Groundwater conditions are also highly variable. Water occurrence, pressures and flow directions may change dramatically across fault zones. Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 5 Faults & Fault Zones Investigation of Faults • Mapping of Morphological Features • Detailed Outcrop Studies - Characterization of Intact Rock - Characterization of Discontinuities - Paleostress Analysis • Subsurface Investigation - Trenches, Trial Pits - Core Drilling - Geophysical Survey Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 6 3 Faults & Fault Zones Detecting Faults... Geological Field Survey Geological phenomena • slickensides, striations: fault kinematics • major joints • severe fracturing Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 7 Faults & Fault Zones Detecting Faults... Geological Field Survey Morphological phenomena • Linear depressions associated with - Seeps and / or - Swamps - Landslides - Creeping areas • Fault escarpment Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 8 4 Faults & Fault Zones Indications of Faulting: Fault-Bound Rock Cliff (right) and Slickensided bedding, Sheared Beds in Outcrop Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 9 Faults & Fault Zones Indications for faulting in covered regions: Trace of Fault Indicated by Surface Morphology, Mass Instability and Underground Water accumulation (Well) Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 10 5 Faults & Fault Zones Rock mass instabilities are often associated with faults and fault rock material can be found frequently in particular at the base of sliding masses, both in natural and artificial outcrops Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 11 Faults & Fault Zones Fault Scarp of Sub-Recent Activity (N- Anatolia) Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 12 6 Faults & Fault Zones Active Fault Escarpment, NE-Anatolian Fault Zone Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 13 Faults & Fault Zones Activity: Offset of Paleosoils due to Neotectonic Fault Activity Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 14 7 Faults & Fault Zones Detecting faulting: Large planar discontinuities or slickensided shear planes Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 15 Faults & Fault Zones Detecting Faults: Severe fracturing and development of curved shear planes (phacoids, below) as indication for faulting Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 16 8 Faults & Fault Zones An estimate on potential fault activity and age is of utmost geotechnical importance in certain geotectonic settings. Activity assessment by: Seismic records, historical records, activity of associated mass movements (e. g. rock falls) Alterations of slickensided surfaces : Lustre, weathering, overgrowth, aperture, overprinting, Morphological features: Freshness of scarps – edges – steps - troughs, offset relations (ditches, fences, gutters, watercourses, soil or weathering horizons) development/age of vegetation, Mineralogical hints: Development of clay mineral assemblages, disturbed fill (overprinting/re-juvenation), crack-seal phenomena Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 17 Faults & Fault Zones Age relationships established by overprinting relationships (displacement of features by shear along schistosity) Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 18 9 Faults & Fault Zones Basic Principles of Tectonic Rock Deformation • Brittle Shear Zone (Fault Zone): Contains fractures and cataclastic fault rocks, often accompanied by hydrothermal alterations • Ductile Shear Zone: Contains mylonites, i.e. structures that have a metamorphic aspect and that formed by ductile flow (crystal plasticity) • Brittle-Ductile Shear Zone: Show evidence of both brittle and ductile deformation, due to intermediate PT conditions or changing PT conditions during deformation. Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 19 Faults & Fault Zones Definitions of brittle deformation • Brittle deformation is defined as strongly pressure dependent deformation involving an increase in volume as a result of cracking and it includes fracture and frictional sliding (Suppe, 1985). • A macroscopic deformation process is defined as “brittle” if it is rate –independent and strain softening in the post-peak region (Mandl, 2000). Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 20 10 Faults & Fault Zones Earthquake activity is an expression of brittle deformation in the upper 10 km of the crust. CHI-CHI - Earthquake, TAIWAN, 21. Sept. 1999, Magnitude 7.6, 2415 deaths with 8000 people injured Legacy of Prof. Riedmüller Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 21 Faults & Fault Zones CHI-CHI - Earthquake, TAIWAN, 21. Sept. 1999 Klima, Schubert LegacyINSTITUTE of Prof. FOR Riedmüller ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 22 11 Faults & Fault Zones CHI-CHI - Earthquake, TAIWAN 21. Sept. 1999 Legacy of Prof. Riedmüller Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 23 Faults & Fault Zones The three basic fault types, their geometry and their relation to the driving stresses as described by Anderson (1942 & 1951): normal fault thrust fault strike-slip fault Mandl (2000) Klima, Schubert INSTITUTE FOR ROCK MECHANICS AND TUNNELING Short Course Singapore 2011 24 12 Faults & Fault Zones Basic fault types and kinematic axes thrust fault normal fault left lateral strike-slip faults right lateral (sinistral) http://www.data.scec.org/glossary.html (dextral) Klima, Schubert INSTITUTE