GY403 Structural Geology Lab - University of South …...Oblique: Normal dip‐slip and rt.‐lat....
Transcript of GY403 Structural Geology Lab - University of South …...Oblique: Normal dip‐slip and rt.‐lat....
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
=dip angle=50°
A
B
C
D
G
E
F
H
dip trend=030
Vertical plane
030 azimuthStrike & Dip: 300, 50NERight‐Hand: 300, 50Dip line trend and plunge: 030, 50
Horizontal plane
Dip line
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
β=plunge angle=22°
A
B
C
G
F
D
β
Horizontal plane
Mineral lineation
Eγ
γ=rake angle=30°
Trend & Plunge: 320, 22Rake angle: 30NW
N
Planar Attitude Examples0
90
180
270
0
90
180
270
0
90
180
270
0
90
180
270
0
90
180
270
0
90
180
270
0
90
180
270
0
90
180
270
0
90
180
270
(A) (B) (C)
(D) (E) (F)
(G) (H) (I)
60
330, 60SW
25
060, 25SE
05
290, 05NE
090, 42N –or‐ 270, 42N
42
horizontal 000, 75W
75
045, 90 060, 35NW OT
35
Rt. hand rule: 120, 39
39
Linear Attitude Examples
90
180
270 90
180
270 90
180
270
0
90
180
270
0
90
180
270
0
90
180
270
(D) (E) (F)
15
000, 15 060, 60
05
210, 05
240, 00 ‐or‐ 060, 00 Vertical ‐or‐ plunge=90 75, 330
60
75
Note: all above linear attitudes are in “azimuth, plunge” format,except (F) that is in “plunge, azimuth” format.
Geologic TimePeriod Symbol
Quaternary Q
Tertiary T
Cretaceous K
Jurassic J
Triassic Tr
Permian P
Pennsylvanian |P
Period Symbol
Mississippian M
Devonian D
Silurian S
Ordovician O
Cambrian ‐C
Precambrian p‐C
Young
Old
Young
Old
Rule of “V’s” for Geologic Contacts Crossing Stream Valleys
5020 90
• “V” in dip direction is less pronounced with larger dip angle• A vertical bed shows no “V”
Bedding Strike & Dip Symbols
‐Cs Oc Sr Dc
35 35 35 35
‐Cs Oc Sr
Dc
DcSr
Oc
p‐Ca
35
• 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
Bedding Strike & Dip Symbols
‐Cs Oc Sr Dc
75 75 75 75
‐Cs OcSr Dc
Dc
p‐Ca
75
• “V” in dip direction is less pronounced with larger dip angle
Bedding Strike & Dip Symbols
‐Cs Oc Sr Dc
‐Cs Oc Sr Dc
Dc
90
• Vertical beds have no “V” across a stream valley• Note the special symbol for a vertical planar attitude
Bedding Strike & Dip Symbols
Sr Oc ‐Cs p‐Ca
35 35 35 35
SrOc ‐Cs
p‐Ca
Dc
35
• Note the overturned symbol and that the dip direction is toward older beds
p‐Ca
‐CsOc
Apparent Dips
Oc Sr
DcMt
Mt
Dc
Sr
Oc
Dc
SrOc
Mt
>35
35
>35
>35
• 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.
‐Cs Oc Mf |Pp
35 35 35 35
‐Cs Oc Mf
|Pp
|Pp
Mf
Oc
p‐Ca
35
• 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
Unconformable Contacts
Sr Dc OcDc Sr Oc Sr Dc
Folding and 3D Blocks
Dc
Sr
Oc
60
60 40 50
60
60
4040 40
40
50 50 50
Dc
Sr
Oc
‐Ca ?p‐Ca
Mf ‐Ca
40
• 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.
Dc Mf |Pp SrMf Dc Oc Sr Dc Mf
Non‐plunging Anticlines & Synclines
|PpMf
Dc
Sr
Oc
?‐CrOc
Sr
Dc
DcMf
Sr
• 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.
Axial traces of folds
Anticlinesymbol
Syncline symbol
Geologic Domes and Basins
MtDc
Sr
Sr
Sr
Oc
Oc
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
‐Cs
Oc
Sr
Dc Mf
Oc
Dc
p‐Ca
p‐Ca‐Cs Oc
Sr
SrOc
‐Cs
p‐Ca
p‐Ca
‐CsOc
Sr
Oc
Dc
• 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.
?
Plunging anticlineAxial trace
Plunging synclineaxial trace
Plunge direction offold hinge
Mf Dc Sr Dc Mf |Pp Mf Dc Sr
?O
SrDcMfMfDcSr
?O
Dc
Mf
|Pp
Overturned Folds
• 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
Overturned anticlinesymbol
Overturned synclinesymbol
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.
‐Ca‐Cp
Ox
Sj
DoOx
‐Cp
‐Cp
Fault Classification:____________________________Left‐lateral strike slip
‐Ca
35
35
35
35
35
35
Do
Sj
Ox
‐Cp‐Ca
70
70
Strike‐slip Fault
HW FW
• Note that “HW” always on the dip direction tic mark side of fault contact.• Note the arrows indicating left‐lateral (sinistral) strike slip.
Note: slicken‐side striations were found to be horizontal in the fault zone.
Sj
+ ‐
‐Ca‐Cp
Ox
Sj
DoOx
‐Cp
‐Cp
Fault Classification:____________________________Reverse dip slip
‐Ca
35
35
35
35
35
35
Do
Sj
Ox
‐Cp‐Ca
70
70
Dip‐slip Fault
HW FW
• 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.
Note: slicken‐side striations were found to be down‐dip in the fault zone. U D
Sj
Fault Classification:______________________________________
Mf
Dc
Sr
Oc
Qal
Tr Kt
Jl
Trd
Oblique: Normal dip‐slip and rt.‐lat. Strike‐slip
Oblique‐slip Fault
UD
HWFW
MfDc
SrOc
KtJl
Trd
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
+‐