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Fault Kinematics: Understanding Fault Geometry, Evidence, and Terminology, Study notes of Geology

West Bengal State UniversityGeology

This lecture from the 'brittle failure: faults ii' series covers the fundamentals of fault kinematics, including fault geometry, evidence, and terminology. Topics include normal and reverse faults, fault slip components, and fault kinematic terminologies such as slip trend, rake, and dip separation.

Typology: Study notes

2012/2013

Uploaded on 07/22/2013

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bg1
% % Lecture%8%
!
1!
Brittle%Failure:%Faults%II:%Terminology%and%Kinematics%
"
Ch."8:"p."154+157;"162+163"
"
!
1.!Fault!Geometry:"Normal"faults"that"dip"towards"each"other"form"a"down+dropped"block"called"a"graben."Normal"
faults"dipping"away"from"each"other"form"an"uplifted"block"called"a"horst."
"
A"major"fault"may"have"smaller"faults"nearby"that"either"dip"in"the"same"direction"(synthetic"faults)"or"in"the"
opposite"direction"(antithetic"faults)."
"
[Fig.&8.5.&Fault&geometries&in&normal&faulting&environments]&
"
"
2.!Fault!Evidence:"Fault"slip"can"be"resolved"into"a"number"of"components"that"tell"us"about"the"relative"motions"of"
the"two"sides"of"the"fault."We"rely"on"markers"(e.g.,"beds,"contacts,"intrusions)"that"have"been"relatively"displaced."
"
[Figure:&Relatively&displaced&beds&in&the&Permian&Cutler&Formation,&Arches&National&Park,&UT]&
"
"
3.!Fault!Kinematics:"Become"familiar"with"the"following"terminologies"regarding"fault"kinematics:"
"
[Fig.&8.6.&Components&of&motion&along&a&normal&fault&having&a&rightGlateral&component&of&slip]&
"
"
4.!Fault!Kinematics:"Slip:"also"called"the"net"slip,"true"slip,"or"the"displacement"discontinuity."It"is"the"sum"of"the"
two"displacement"vectors"on"each"side"of"the"fault"(which"are"mutually"opposed)."Slip"represents"the"cumulative"
result"of"many"slip"events."
"
[Fig.&8.6.&Slip&vectors&connect&two&points&across&the&fault&plane&that&used&to&be&together.&The&slip&vectors&of&different&
slip&events&may&be&different.&Also,&the&slip&vectors&can&vary&across&the&surface&of&the&fault]&
"
"
5.!Fault!Kinematics:"Slip"trend"and"plunge:"the"horizontal"projection"of"the"slip"vector"points"in"the"compass"
direction"of"slip"and"is"called"the"slip"trend"or"azimuth."The"angle"it"makes"with"the"fault"plane"is"called"the"plunge."
"
[Fig.&8.6.&Slip&trend&and&plunge&are&not&shown&on&this&figure]&[Figure:&Slip&trend&and&plunge]&
&
"
6.!Fault!Kinematics:"Rake:"the"angle"between"the"strike"of"the"fault"and"the"slip"vector,"measured"in"the"plane"of"
the"fault."Also"called"the"pitch."The"slip"vector"is"sometimes"discernable"from"scratches"imbedded"in"the"fault"
plane,"called"slickenlines"or"grooves."The"polished"fault"surface"is"a"slickenside."
"
[Fig.&8.6.&The&rake&is&typically&measured&as&an&acute&angle,&but&here&angle&f&is&obtuse.&The&benefit&of&always&using&
the&angle&measured&away&from&the&true&strike&(obeying&the&rightGhand&rule)&is&that&the&exact&direction&of&the&slip&
vector&in&space&is&obtained]&[Figure:&Slickenlines]&
"
"
7.!Fault!Kinematics:"Dip"separation:"any"cross"section"through"the"fault"produces"a"component"of"motion"parallel"
to"the"fault"trace,"also"called"the"apparent"slip"or"offset."Only"in"a"cross"section"oriented"parallel"to"the"slip"vector"
does"this"become"the"true"slip."In"nature,"we"typically"observe"apparent"slip,"not"true"slip."
"
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Download Fault Kinematics: Understanding Fault Geometry, Evidence, and Terminology and more Study notes Geology in PDF only on Docsity!

Brittle Failure: Faults II: Terminology and Kinematics

Ch. 8: p. 154-­‐157; 162-­‐

  1. Fault Geometry: Normal faults that dip towards each other form a down-­‐dropped block called a graben. Normal faults dipping away from each other form an uplifted block called a horst.

A major fault may have smaller faults nearby that either dip in the same direction (synthetic faults) or in the opposite direction (antithetic faults).

[Fig. 8.5. Fault geometries in normal faulting environments]

  1. Fault Evidence: Fault slip can be resolved into a number of components that tell us about the relative motions of the two sides of the fault. We rely on markers (e.g., beds, contacts, intrusions) that have been relatively displaced.

[Figure: Relatively displaced beds in the Permian Cutler Formation, Arches National Park, UT]

  1. Fault Kinematics: Become familiar with the following terminologies regarding fault kinematics:

[Fig. 8.6. Components of motion along a normal fault having a right-­‐lateral component of slip]

  1. Fault Kinematics: Slip: also called the net slip, true slip, or the displacement discontinuity. It is the sum of the two displacement vectors on each side of the fault (which are mutually opposed). Slip represents the cumulative result of many slip events.

[Fig. 8.6. Slip vectors connect two points across the fault plane that used to be together. The slip vectors of different slip events may be different. Also, the slip vectors can vary across the surface of the fault]

  1. Fault Kinematics: Slip trend and plunge: the horizontal projection of the slip vector points in the compass direction of slip and is called the slip trend or azimuth. The angle it makes with the fault plane is called the plunge.

[Fig. 8.6. Slip trend and plunge are not shown on this figure] [Figure: Slip trend and plunge]

  1. Fault Kinematics: Rake: the angle between the strike of the fault and the slip vector, measured in the plane of the fault. Also called the pitch. The slip vector is sometimes discernable from scratches imbedded in the fault plane, called slickenlines or grooves. The polished fault surface is a slickenside.

[Fig. 8.6. The rake is typically measured as an acute angle, but here angle f is obtuse. The benefit of always using the angle measured away from the true strike (obeying the right-­‐hand rule) is that the exact direction of the slip vector in space is obtained] [Figure: Slickenlines]

  1. Fault Kinematics: Dip separation: any cross section through the fault produces a component of motion parallel to the fault trace, also called the apparent slip or offset. Only in a cross section oriented parallel to the slip vector does this become the true slip. In nature, we typically observe apparent slip, not true slip.

[Fig. 8.6. Any cross section through the fault that shows a component of motion along the fault trace is showing offset, apparent slip, or dip separation. If the cross section is parallel to the slip vector, the offset is equal to the true slip]

  1. Fault Kinematics: Strike separation: this is the offset measured along the fault trace in map view. The offset sense can be counter-­‐intuitive depending on the relative dips of the beds and the fault and the true strike-­‐slip component of motion.

[Fig. 8.6. Strike separation is observed in map view. Here, an apparent left-­‐lateral offset is actually produced by right-­‐lateral motion! Also, strike separation can occur when only dip-­‐slip motions happen because of the dips of the beds]

  1. Fault Kinematics: Throw: this is the vertical component of motion and is the same regardless of the orientation of the cross section relative to the slip vector.

[Fig. 8.6. Throw is caused by dip-­‐slip motion components and is simply the height of the scarp or the vertical component of slip]

  1. Fault Kinematics: Heave: this is the horizontal component of the dip-­‐slip vector and so is only accurately measured in a cross section oriented perpendicular to fault strike. It is used to estimate the fault-­‐perpendicular strain across a region.

[Fig. 8.6. Heave is the horizontal component of motion measured parallel to the dip direction. For any other cross section orientation, the horizontal component is greater than the heave]

  1. Footwall and Hanging wall Cutoffs: The heave component of motion results in the top of fault scarp and the bottom of the fault scarp being separated in map view. The fault trace thus has a width to it defined by the footwall cutoff and the hanging wall cutoff.

[Fig. 8.8. Footwall and hanging wall cutoffs define a certain width to fault traces in map view. This is how faults are most accurately represented on a map]

  1. Fault Kinematics: Separation: the relative offset of a marker from one side of the fault to the other, measured perpendicular to the strike of the marker. In cross section, if the beds are horizontal, this is also called the stratigraphic separation and is equal to the throw.

[Fig. 8.6. Separation is different to dip separation and strike separation. However, if beds strike perpendicular to the fault, the strike separation and the separation are identical]

  1. Missing or Repeated Section: Subsurface data from wellbores may show missing section where the well intersected a normal fault. Repeated section is possible where drilling intersects a thrust fault or where a normal fault dips more steeply than the well plunge. It is important to remember that these effects can also occur in response to lateral facies changes and unconformities.
  1. Geomorphic Evidence of Faulting: In mountainous regions, scarps may be covered by alluvial fans that obscure the scarp until a slip event occurs.

[Figure: Normal fault obscured by alluvial fans at Death Valley, CA]

  1. Geomorphic Evidence of Faulting: Lateral offsets of small alluvial fans may provide evidence of fault motions, as well as doglegs along river drainages.

[Figure: Offset river drainage (dogleg) along the San Andreas fault, CA]

  1. Geomorphic Evidence of Faulting: Fault traces may also be delineated by lines of vegetation or aligned geothermal springs.

[Figure: Hot springs delineate a fault trace in the Alvord Basin, Oregon]