Rock Structure and Fault Activity chapter 9
Dec 16, 2015
What is structural geology
The study of the forms of the Earth’s crust and the processes which have shaped it
• analysis of displacement and changes in shape of rock bodies (strain)
• reconstruct stress that produced strain
Structural DeformationRocks deform when
stresses placed upon them exceed the rock strength
• Brittle deformation (e.g. fractures)
• ductile deformation (e.g. folds)
Driving Forces• Plate tectonics – plate convergence and ridge
spreading• Deep burial of sediments• Forceful intrusion of magma into the crust• Meteorite impacts
Evidence of Crustal Deformation
• Folding of strata• Faulting of strata• Tilting of strata• Joints and fractures
Evidence of Crustal Deformation
• Folding of strata• Faulting of strata• Tilting of strata• Joints and fractures
Evidence of Crustal Deformation
• Folding of strata• Faulting of strata• Tilting of strata• Joints and fractures
Evidence of Crustal Deformation
• Folding of strata• Faulting of strata• Tilting of strata• Joints and fractures
Applications of structural geology
• subsurface exploration for oil and gas
• mining exploration• geotechnical investigations• groundwater and environmental
site assessment
Geological structures
• Geologic bed contacts• Primary sedimentary structures• Primary igneous structures• Secondary structures
Fundamental Structures
Three fundamental types of geologic structures:
• bed contacts• primary structures - produced during
depositionor emplacement of rock body• secondary structures - produced by
deformationand other process after rock is emplaced
Bed Contacts
Boundaries which separate one rock unit from another
• two types:1. Normal conformable contacts2. Unconformable contacts
(‘unconformities’)
Conformable Bed Contacts
Horizontal contact between rock units with no break in deposition or erosional gaps
• no significant gaps in geologic time
Book Cliffs, central Utah
Unconformable Contacts
Erosion surfaces representing a significant break in deposition (and geologic time)
• angular unconformity• disconformity• non-conformity
Angular UnconformityBedding contact which discordantly cuts
across older strata• discordance means strata are at an angle to
each other• commonly contact is erosion surface
DisconformityErosional gap between rock units
without angular discordance• example: fluvial channel cutting
into underlying sequence of horizontally bedded deposits
Nonconformity
Sedimentary strata overlying igneous or metamorphic rocks across a sharp contact
• example: Precambrian-Paleozoic contact in Ontario represents a erosional hiatus of about 500 ma
Grand Canyon, USA
Structural Relations
The structural relations between bed contacts are important in determining:1. presence of tectonic
deformation/uplift and;2. relative ages of rock units
• principle of original horizontality• principle of cross-cutting• principle of inclusion
Principle of Original Horizontality
Sedimentary rocks are deposited as essentially horizontal layers• exception is cross-bedding (e.g. delta foresets)• dipping sedimentary strata implies tectonic uplift and
tilting or folding of strata
Principle of Cross-cutting
Igneous intrusions and faults are younger than the rocks that they cross-cut
Mafic dike cutting across older sandstones
Cross-cutting Relations
Often several cross-cutting relationships are present
• how many events in this outcrop?
Principle of Inclusion
Fragments of a rock included within a host rock are always older than the host
Fundamental Structures
Three fundamental types of structures:
• bed contacts• primary structures• secondary structures
Primary Sedimentary Structures
Structures acquired during deposition of sedimentary rock unit
Stratification - horizontal bedding is most common structure in sedimentary rocks
Primary Sedimentary Structures
Cross-bedding - inclined stratification recording migration of sand ripples or dunes
Primary Sedimentary Structures
Ripples - undulating bedforms produced by
unidirectional or oscillating (wave) currents
Primary Sedimentary Structures
Graded bedding - progressive decrease in grain size upward in bed• indicator of upwards direction in deposit• common feature of turbidites
Primary Sedimentary Structures
Mud cracks - cracks produced by dessication of clays/silts during subaerial exposure
Primary Sedimentary Structures
Sole marks - erosional grooves and marks formed by scouring of bed by unidirectional flows
• good indicators of current flow direction
Primary Sedimentary Structures
Fossils – preserved remains of organisms, casts or moulds
• good strain indicators• determine strain from change in shape of fossil• relative change in length of lines/angle
between lines
Primary Igneous Structures
Flow stratification • layering in volcanic rocks produced
by emplacement of successive lava sheets
• stratification of ash (tephra) layers
Primary Igneous Structures
Flow stratification • layering in volcanic rocks produced
by emplacement of successive lava sheets
• stratification of ash (tephra) layers
Importance of Primary Structures
1. Paleocurrents - determine paleoflow directions
2. Origin – mode of deposition, environments3. Way-up - useful indicators of the direction of
younger beds in stratigraphic sequence4. Dating - allow relative ages of rocks to bedetermined based on position, cross-cuttingrelations and inclusions5. Strain indicators - deformation of primarystructures allows estimates of rock strain
Secondary Structures
Secondary structures - deformation structures
produced by tectonic forces and other stresses in crust
Principle types:• fractures/joints• faults/shear zones• folds• cleavage/foliation/lineation
Fractures and Joints
Fractures – surfaces along which rocks have broken and lost cohesion
Joints - fractures with little or no displacement parallel to failure surface
• indicate brittle deformation of rock
FaultsFaults - fracture surfaces with appreciable displacement of strata
• single fault plane
• fault zone - set of associated shear fractures
• shear zone - zone of ductile shearing
Shear Zones
Shear zone - zone of deformed rocks that are more highly strained than surrounding rocks
• common in mid- to lower levels of crust
• shear deformation can be brittle or ductile
Fault Terminology
Hanging wall block- fault block toward which the fault dips
Footwall block - fault block on underside of fault
Fault plane – fault surface
Fault Slip
Slip is the fault displacement described by:• direction of slip• sense of slip• magnitude of slip
Fault Types
Strike-slip – fault slip is horizontal, parallel with strike of the fault plane• right-handed (dextral)• left-handed (sinistral)
Fault Types
Oblique slip – Combination of dip- and strike-slip motion• dextral-normal• dextral-reverse• sinistral-normal• sinistral-reverse
Folds
Folds – warping of strata produced by compressive deformation• range in scale from microscopic
features to regional-scale domes and basins
• indicators of compression and shortening
Fold Terminology
Hinge (Axial) plane - imaginary plane bisecting fold limbs
Hinge line - trace of axial plane on fold crestPlunge - angle of dip of hinge line
Fold Terminology
Anticline - convex in direction of youngest beds
Syncline - convex in direction of oldest beds
Antiform - convex upward fold (stratigraphy unknown)
Synform - concave upward fold
Fold Terminology
Synformal Anticline - overturned anticline
Antiformal Syncline - overturned syncline
Foliation and CleavageFoliation - parallel alignment of planar fabric elements
within a rockCleavage - tendency of rock to break along planar surface
cleavage is a type of foliation• resemble fractures but are not physical discontinuities
Foliation and CleavageFoliation - parallel alignment of planar fabric elements
within a rockCleavage - tendency of rock to break along planar surface
cleavage is a type of foliation• resemble fractures but are not physical discontinuities
Lineations
Lineation - sub-parallel to parallel alignment of elongate linear fabric elements in a rock body
• e.g. slickenlines and grooves on fault plane surface
Structural analysis
Involves three steps1. Descriptive or geometric analysis2. Kinematic analysis3. Dynamic analysis
Geometrical analysis
Measurement of the 3-dimentional orientation and geometry of geological structures
simplified into:
• lines• planes
lines or linear geological structures
• liniation – any linear feature observed in a rock or
on a rock surface – any imaginary line – such as a fold axis
Orientation of linear structures
LINESTrend – azimuth direction measured clockwise from north 360°Plunge – angle of inclination of line measured from the horizontal (0 - 90°)
Examples of linear structures
• Primary – flute casts, grooves, glacial striae• Secondary – slickenlines, mineral lineations
Glacial striations on bedrock sole marks
Examples of linear structures
• Primary – flute casts, grooves, glacial striae• Secondary – slickenlines, mineral lineations
Grooves on fault plane Slickenlines on fault surface
Orientation of Linear Structures
linear structures on an other planar surface:
• pitch angle– angle from horizontal measured
within the planePitch angleStriations
on a fault plane
Planar Geological Structures
• bedding planes and contacts• foliation• joint surfaces• fault planes• fold limbs• fold axial planes (imaginary
surface)
Examples of Planar Structures
Bedding planes – most common • primary depositional surface• erosional surface
inclined bedding plane
Examples of Planar Structures
Foliation – cleavage planes produced by metamorphism
• common in slates and phyllites
foliated phyllite
Orientation of Planar Structures
The attitude of a plane can be established from any two lines contained in the plane, provided they are not parallel
Orientation of Planar Structures
Strike – azimuth direction of a horizontal line in a plane
Dip – angle of inclination of line measured from the horizontal (0 - 90°)
Orientation of Planar Structures
Appearent dip– dip measured
along a line other than 90 to strike
– apparent dip will always be less than the true dip angle
Measurement of orientation
Strike (plane) Trend (line)azimuth orientation
measured with a compass
Dip (plane)Plunge (line)inclination measured
using an inclinometer
Measurement of Strike Direction
Right hand rule???When your thumb (on your right
hand) is pointing in the direction of strike your fingers are pointing in the direction of dip!!
Structural Data
Symbols represent different structural data
Symbols are placed on the map:– in the exact field orientation– where the data is measured
Exercises
• geological maps• structure contour and structure
maps• three-point problems• cross sections• sterionets
Geological Maps
• distribution of rock types and contacts– symbols on map represent structures
(strike and dip, fold axes, faults etc.)– map and structure symbols allow you
to infer subsurface structures
Outcrop patterns
Outcrop patterns controlled by attitude (strike and dip) of beds and topographic relief
Block models
Relations between outcrop patterns and subsurface structures
map view on bottom – cross sections in blocks on top
Structural Contour Maps
Map showing the relief of a subsurface geological surface– top or bottom of bedding planes,
faults or folded surface– constructed from borehole data
Structure Contour Maps
Structure contour lines are lines of equal elevation• show elevation relative to horizontal datum• values are often negative since subsurface
elevations are commonly below sea level
Constructing Structural Contours
Points of equal elevation along a bed contact• intersection of contact with topo contour• draw structure contours through points of
equal elevation
Rules of Contouring
1) contours cannot cross or bi-furcate2) contours cannot end in the middle of
the map, except at a fault or other discontinuity
3) same contour interval must be used across the map and elevations must be labelled
4) elevation is specified relative to datum (e.g. m above sea level)
Determining Dip
Dip direction and angle can be determined from structure conour maps
• measure horizontal separation X and find difference in Z• tan = Z/X, = tan –1 (Z/X)• e.g. = tan –1 (10m/100m), = 6°
Three-point problem
A minimum of three points are required to uniquely define the orientation of a plane
Three-point problem
• Find min and max values
• Draw line between these and divide distance into intervals
• Connect points of equal elevation
• Two points in a plane at the same elevation lie in the line of strike
Three-point problem
• Find min and max values
• Draw line between these and divide distance into intervals
• Connect points of equal elevation
• Two points in a plane at the same elevation lie in the line of strike
Isochore Map
Drill hole logs giving the thicknesses in the drilled (often vertical) direction
Apparent thickness – true thickness = perpendicular to bedding
Cross-sections
Cross-section is a 2-D slice through stratigraphy
• construct perpendicular to dip = true dip
• constructed at any other direction = apparent dip
Engineering properties of faulted or folded rock
• shear strength– loose materials– compressive materials– permeable materials
hydrology of fault zones
• water in fault zones common due to fractured rock– fault zone may be either an aquifer or an
aquiclude• crushed to gravel• crushed to clay
hydrology of fault zones
• water in fault zones common due to fractured rock– fault zone may be either an aquifer or an
aquiclude• crushed to gravel• crushed to clay
Problems due to water in fault zones
• leakage of waste water under a landfill• leakage of water under a dam• sudden collapse and inflow of water into
a tunnel• hydrothermal alteration of rocks to clay
minerals along faults – variable physical, mechanical and hydrological properties
• soluble rocks - cavities
Activity of faults?
• Risk for further movement– active fault – has moved in the last 100 000
to 35 000 years– dormant fault – no recorded movement in
recent history
Indicators of fault movement
• fault scarps• stream displacement • sag ponds lineaments• vegetation displacement
Risk potential depends upon:
1. duration of the quake2. intensity of the quake3. recurrence of the quake
Potential trigger’s
stess > stength• water in a reservoir – added weight and
lubrication• storage of fluids in old mines• blasting• surface excavation• ground water mined from aquifers• extraction of oil and gas from aquifers
Case studies
• Auburn Dam – wide slender arch dam on the American River, upstream of Sacramento, California
• Fig. 9.31• pre investigations
– detailed mapping– 8 km trenches– 2 km exploratory tunnels– 30 km borings
Auburn Dam
• geology– metamorphic competent amphybolite– metasediments– included vertical weak zones and lenses of chlorite
schist, talc schist and talcose serpentinites up to 30 m wide, aligned with foliation
– series of sub parallel mineralized reverse faults with strike transverse to the dam axis dipping 40 to 55 degrees into the abutment
– two of the longest faults are tangential to the dam, close or under the dm on the left abutment
– no active faults in the area– the area was supposed to be low seismicity
Auburn Dam
• foundation construction– earthquake occurred 5.7– regional fault study– reassessment
• 32 km trenches• more borings• surface excavations
aim to establish the time relationship of the faults
Auburn Dam
Concluded that the faults wee formed in another tectonic setting than the present (compressional rather that extensional stress field)
A review of the dam – will it withstand vibrations from a 6.5 magnitude quake on a fault < 8 km from the dam??
“Off set” design recommended to withstand 25 mm to 900 mm
NO DAM built due to discussions on safety!
Baldwin Hill reservoir – failed 1963
• 1 principle embankment, 47 m high, and 5 smaller embankments
• excavated hollow in between at the top of a mountain range
Baldwin Hill reservoir – failed 1963
• geology– friable deposits of the Pliocene Pico Formation, massive
beds of clayey, sandy siltstone– Pleistocene Ingewood Formation. interbedded layers of
sand, silt, and clay, with some thin linestone beds; some of the sand and silt beds are unconsolidated and erodable
– Both formations contain calcareous and limonitic concretions
– bedding dips slightly 5 to 7 degrees, striking roughly parallel to the Inglewood fault
– major active fault, Inglewood, passes just 150 m west of the reservoir
– the fault is a right lateral strike slip with a vertical component
– fault acts as a subsurface dam for a major oil field in the hills
Baldwin
• Excavation phase– 7 minor faults wee mapped– mostly normal faults– 3 to 100 mm silty gouge– largest fault had a total displacement
of more than 8 m
Baldwin
• Design– rock foundation lined with
• asphalt and • gravel drain layer• covered with compacted clay• covered with asphalt
Baldwin
• Construction phase 1947-51– fault 1 caused problems– slide initiated revealing that the fault
passed beneath the inlet/outlet tower– the tower was relocated 48 ft
Baldwin
• after completion– liner cracked along the trace of the fault– emptied in 1957– cracks repaired– cracks were also observed in the surrounding
area of the reservoir– the cracks dipped steeply– trend NS parallel to the faults– some exhibited small sinkholes – indicative of
extensional strain– offset dip slip
Baldwin
• nearby oil fields – oil was being extracted– resulted in subsidence due to collapse of the aquifer– subsidence of 2.7 m between 1917 and 1962
Baldwin
• Failure 1971– emptied completely in 4 hours– seepage along the fault had enlarged
to a pipe– then to a tunnel and – then the collapse of the roof– a canyon eroded completely through
the all of the reservoir