GY403 Structural Geology Lab
Geological Attitudes and 3D Block Diagram Interpretation
Terminology & Definitions
•
Azimuth: compass direction of a line measured relative to north=0, east=090, south=180, west=270 (increases clockwise from north). •
Bearing: compass direction of a line in quadrant format (ex. N30W, S20E)
•
Trend: azimuth or bearing direction of a line (ex. 330 or N30W). A vertical line has no definable trend.
•
Strike: trend of the horizontal line contained in a geological plane (ex. bedding). By convention strike lines are recorded to a north quadrant (0‐90; 270‐360)
•
Inclination: angle measured from a line of plane to the horizontal in a vertical plane. The maximum possible angle is 90. Planar structures have inclinations that are termed “true dip”. Linear structures have an inclination that is termed “plunge”.
•
Dip: inclination angle measured on a geological plane. If the angle is measured perpendicular to strike it is a “true” dip, otherwise the angle is an “apparent” dip. Apparent dips are always less that the true dip.
•
Plunge: inclination of a line measured from the line up to the horizontal in a vertical plane.
•
Trace: intersection of a plane with the horizontal map surface. For example the axial trace is the intersection of the axial plane with the map surface. If the map surface is horizontal the trace is also the strike line of the plane. •
Rake (Pitch): angle between a line contained in a plane and the strike line of the plane. A rake angle needs to be followed by the quadrant end of the strike line from where the rake was measured (ex. 33SW in a plane that is oriented N50E, 60SE). The maximum possible angle is 180.
Attitude (Orientation) of Geological Structures
• Geological structures that can be measured are either geometric “planes” or “lines”. • Planar: geological structures such as bedding, faults, joints, axial planes are planar geometries.
• Linear: geological structures such as fold hinges, elongated minerals, cleavage/bedding intersection are linear geometries
Planar Attitudes
•
A plane’s attitude in 3D space is determined by measuring 2 non‐parallel lines that lie within the plane.
•
Strike and Dip: strike is the trend of the horizontal line that lies in the geological plane. By convention this trend is always recorded to a north quadrant. The trend can be either azimuth or bearing format.
•
The dip is the maximum angle of inclination of the plane. Because the dip trend is always perpendicular to the strike only the quadrant direction (NE, SE, NW, SW) is needed for the dip.
•
Azimuth Example: 050, 68SE. The north end of the strike line is trending at azimuth 050. The true dip inclination angle is 68 degrees trending at 140 azimuth (southeast quadrant 90 degrees from strike). The two non‐parallel lines that define the plane are the strike line (azimuth=050, plunge=0), and the dip line (azimuth=140, plunge=68).
•
Quadrant Bearing example: N30W, 55SW. The north end of the strike line is trending 330 azimuth (=N30W), and the dip line is trending 240 azimuth (SW quadrant 90 degrees from strike). The plunge of the dip line is 55. Other Planar Attitude Conventions
• Right‐Hand rule: rather than reading the strike to a north quadrant (NE: 0‐90; NW: 270‐360) the strike trend is recorded in the azimuth direction such that the true dip (incline) of the plane is to the observers right. This removes the need for a quadrant direction for the dip.
• Example: A strike and dip of 310, 55SW would be measured as 130, 55.
• Note that a right‐hand rule measurement appears no different than a bearing and plunge for a line so the note‐
taker must be careful to distinguish planar form linear data. Other Planar Attitude Measurement Conventions
• Dip Trend and Angle: Because the strike of a plane always has a trend 90 degrees from the dip trend, one may simply record the dip trend and dip angle of a plane. The strike can always be calculated as the line trending perpendicular to the dip trend. • As with the right‐hand rule, the note‐taker must be careful to distinguish planar from linear data.
Planar Geometry
Strike & Dip: 300, 50NE
Right‐Hand: 300, 50
Dip line trend and plunge: 030, 50 030 azimuth
B
Vertical plane
F
A
G
E
dip trend=030

=dip angle=50°
Horizontal plane
Dip line
H
D
C
Linear Attitude
• Linear attitudes are specified by a bearing (trend) and plunge measurement. Note that no quadrant direction is needed therefore 2 numbers completely specify the attitude.
• Example: 220, 15 (line is trending at azimuth 220, and the plunge incline is 15 degrees).
• Note that a line may trend in any azimuth direction (0‐360). • Note that a line plunging 90 degrees (vertical) has no definable trend. Linear Geometry
N
Trend & Plunge: 320, 22
Rake angle: 30NW
B
G
A
E
γ
β
F
C
Horizontal plane
Mineral lineation
β=plunge angle=22°
γ=rake angle=30°
D
Planar Attitude Examples
(A)
0
(C)
0
(B)
0
05
270
90
60
90
270
25
180
180
180
290, 05NE
060, 25SE
330, 60SW
(D)
90
270
0
(E)
0
(F)
0
42
90
270
090, 42N –or‐ 270, 42N
90
270
000, 75W
0
35
90
75
180
horizontal
(H)
0
270
270
180
180
(G)
90
270
0
(I)
90
90
270
39
180
045, 90
180
060, 35NW OT
180
Rt. hand rule: 120, 39
Linear Attitude Examples
15
90
270
60
270
90
90
270
05
(D)
180
180
000, 15
060, 60
(E)
0
180
210, 05
0
(F)
0
75
90
270
180
240, 00 ‐or‐ 060, 00
90
270
180
Vertical ‐or‐ plunge=90
90
270
180
75, 330
Note: all above linear attitudes are in “azimuth, plunge” format,
except (F) that is in “plunge, azimuth” format.
Geologic Time
Young
Old
Period
Symbol
Q
Mississippian
M
Tertiary
T
Devonian
D
Cretaceous
K
Silurian
S
Jurassic
J
Ordovician
O
Triassic
Tr
Cambrian
‐C
Permian
P
Precambrian
p‐C
Pennsylvanian
|P
Period
Symbol
Quaternary
Young
Old
Rule of “V’s” for Geologic Contacts Crossing Stream Valleys
20
50
90
• “V” in dip direction is less pronounced with larger dip angle
• A vertical bed shows no “V”
Bedding Strike & Dip Symbols ‐Cs
35
Oc
35
Sr
35
35
35
‐Cs
Dc
Dc
Sr
Dc
Oc
Sr
p‐Ca
•
•
•
•
Unless strata is overturned dip should be toward younger beds
Strike is parallel to contact
Dip angle is measured in plane perpendicular to strike Stream valley produces a contact “V” that points in dip direction Oc
Bedding Strike & Dip Symbols ‐Cs
75
Oc
75
Sr
75
Dc
75
Dc
75
‐Cs
Sr
Dc
Oc
p‐Ca
• “V” in dip direction is less pronounced with larger dip angle
Bedding Strike & Dip Symbols ‐Cs
Oc
Sr
Dc
Dc
90
‐Cs
Oc
Sr
Dc
• Vertical beds have no “V” across a stream valley
• Note the special symbol for a vertical planar attitude
Bedding Strike & Dip Symbols Sr
35
Oc
35
‐Cs
35
35
Sr
p‐Ca
35
p‐Ca
‐Cs
p‐Ca
Oc
‐Cs
Dc
• Note the overturned symbol and that the dip direction is toward older beds
Oc
Apparent Dips
Oc
Sr
Sr
Oc
>35
Dc
>35
>35
35
Dc
Mt
Mt
Mt
Dc
Oc
•
Sr
Note that the 35 degree angle on the front face must be an apparent dip because the contacts on the map surface are clearly not perpendicular to the front face. The true dip must therefore be >35 degrees.
Unconformable Contacts
‐Cs
35
Oc
35
Mf
35
35
35
‐Cs
|Pp
|Pp
Mf
Oc
|Pp
Oc
Mf
p‐Ca
• Unconformities are marked by hachures on the map surface on the younger side of the contact; in a cross‐section view by a irregular contact line depicting erosional relief.
• Note the missing Silurian & Devonian formations producing the disconformity
Folding and 3D Blocks 60
60
40
40
40
40
50
60
50
Mf
Dc
Dc
Sr
Oc
‐Ca
40
60
Dc
Sr
40
Sr
50
Oc
Sr
Dc
Oc
50
Dc
Sr
Oc
‐Ca
•
?p‐Ca
Typical fold problem presentation‐ add strike & dip symbols, complete contacts on sides of block, add anticline and syncline symbols. Problem solution is in blue color. Use dashed line and “?” for speculative contacts and age labels.
Non‐plunging Anticlines & Synclines
Axial traces of folds
Anticline
symbol
Syncline symbol
Mf
Dc
Mf
|Pp
Mf
Dc
Sr
Oc
Sr
Dc
Dc
Sr
Mf
|Pp
Mf
Dc
Dc
Sr
Sr
?‐Cr
Oc
Oc
•
•
•
•
Note that the anticline has the oldest strata in the core of the structure, and the syncline has the youngest, and note the axial trace symbols for anticline and syncline.
Because there are no overturned beds the strata always dip in the younging direction.
Non‐plunging folds have straight contact lines in the map view (horizontal) surface.
Vertical sides of the block diagram perpendicular to the axial traces of folds may contain curved contact lines.
Geologic Domes and Basins
Oc
Sr
Dc
Oc
Mt
Sr
Sr
Oc
Sr
?‐C
?‐C
•
•
•
Oc
This example is a basin because the youngest strata are in the core (center).
Note that dip direction is toward the core of the basin, which is also toward younger beds. Strike direction is “tangent” to the curved contacts in domes & basins.
Plunging Folds
Dc
Plunge direction of
fold hinge
Mf
Dc
Sr
Plunging anticline
Axial trace
Sr
Sr
Dc
Oc
Sr
Oc
Oc
‐Cs
p‐Ca
Plunging syncline
axial trace
Oc
‐Cs
p‐Ca
‐Cs
‐Cs
Oc
p‐Ca
?
•
•
Oc
p‐Ca
Note that anticlines still have oldest strata in the core of the structure, vice versa for synclines. Dip direction is away from anticline core, toward syncline core.
Plunge direction always points with the “V” direction of the contact in the anticline, opposite the “V” in contacts for the syncline.
Overturned Folds
Overturned anticline
symbol
?O
Mf
Dc
Dc
Sr
Mf
|Pp
Mf
Dc
Sr
Mf
Dc
|Pp
Sr
Dc
Mf
Mf
Dc
Overturned syncline
symbol
Sr
?O
•
•
•
Overturned folds have one limb containing overturned strata.
The above anticline & syncline pair share an overturned limb.
Note the overturned symbols for the anticline & syncline
Fault Classification
• Dip‐Slip Faults
– Hanging wall down = Normal dip slip
– Hanging wall up = Reverse dip slip
• Normal faulting accommodates lateral stretching
• Reverse faulting accommodates lateral compression
• Strike‐slip faults
– Right‐handed offset across fault contact = Right‐lateral (dextral)
– Left‐handed offset across fault contact = Left‐lateral (sinistral)
• Fault slip that combines both dip‐slip and strike‐
slip components is oblique‐slip.
Strike‐slip Fault
Left‐lateral strike slip
Fault Classification:____________________________
‐Ca
Note: slicken‐
side striations were found to be horizontal in the fault zone.
HW
‐Ca
35
35
‐Cp ‐Ca
Sj
35
35
Ox
35
70
Ox
35
Ox
Do
Sj
Do
+
•
•
‐Cp
70
‐Cp
‐Cp
FW
‐
Sj
Note that “HW” always on the dip direction tic mark side of fault contact.
Note the arrows indicating left‐lateral (sinistral) strike slip.
Dip‐slip Fault
Reverse dip slip
Fault Classification:____________________________
‐Ca
Note: slicken‐
side striations were found to be down‐dip in the fault zone.
HW
‐Ca
•
•
•
35
Ox
35
35
U
D
35
‐Cp ‐Ca
Sj
35
Ox
35
Do
‐Cp
‐Cp
70
‐Cp
Ox
FW
70
Sj
Do
Sj
Note that “HW” always on the dip direction tic mark side of fault contact.
Note the arrows indicating reverse dip slip (HW up relative motion).
Note that the “U” up‐thrown symbol on fault block where strata is displaced in dip direction. Oblique‐slip Fault
Oblique: Normal dip‐slip and rt.‐lat. Strike‐slip
Fault Classification:______________________________________
Trd
Oc
Sr
D
Tr
U
Kt
Jl
Qal
Dc
Trd
HW
FW
Mf
Mf
Dc
Sr
•
•
•
Jl
‐
Oc
Kt
+
Jl
Note that a strike‐slip motion would not “match” strata in opposite blocks.
Offset on axial trace determines the right‐lateral slip.
The fact that “Mf” is in the synclinal core in the west block as compared to “Qal” in the east block proves that the west block has been uplifted