Download Slip Patterns - Structural Geology - Lecture Notes and more Study notes Geology in PDF only on Docsity!
Brittle Failure: Faults IV: Slip Patterns and Segmented Fault Evolution
Ch. 8: p. 159-‐161; 166-‐171; 174-‐
1. Fault Damage Zone: The fault damage zone is a volume of rock alongside the fault core where the density of fractures exceeds the background level. It may contain shear fractures, opening fractures, deformation bands, and pressure solution surfaces.
The damage zone thickness is about 10-‐100% of the total slip.
[Fig. 8.11. Fault damage zone along the Moab fault, UT] [Fig. 8.12A. Fault damage zone thickness versus fault slip]
2. Fault Formation and Growth: The fault core and surrounding damage zone reflects the progression of fault growth through time. Faults are shear fractures, which are adept at creating fractures near their tiplines (e.g., tailcracks; pinnate fractures).
In fact, the fault tipline is surrounded by a damage zone within which a multitude of small cracks are created, called the process zone.
[Fig. 7.28. Horsetail fractures that formed in the process zone at the tip of a shear fracture] [Figure: Pinnate fractures at Valley of Fire State Park, NV]
3. Fault Formation and Growth: Faults grow by breaking through the process zone fractures, leaving a trail of fractures to either side of the fault plane (the fault damage zone) as a new process zone forms ahead of the advancing tip. Frictional wear along the fault then breaks the fractured rock down to form the fault core, while additional fractures may develop in the adjacent damage zone as the fault slips.
[Fig. 8.24. Propagation of a fault plane through the process zone fractures (here, deformation bands), leaving a damage zone alongside the fault. Ongoing shearing along the fault plane creates the fault core]
4. Fault Drag: As faults grow, adjacent layered rocks may get bent along the fault. This is fault drag. It is common where rocks are soft enough to be ductile at shallow depths (e.g., shale, siltstone, salt). It may only extend a few meters away from the fault but can extend much further, such that it is visible in seismic data.
[Fig. 8.28. Fault drag within soft sedimentary beds that behave in a ductile fashion in response to brittle deformation along the fault]
5. Fault Drag: Fault drag in seismic data may indicate a heterogeneous lithologic section with many soft layers. Folding alongside a fault may also be caused by changes in fault dip, forming features like rollovers.
[Fig. 8.26. Hanging wall rollover along a normal fault, Sinai, Egypt] [Fig. 8.29. Fault drag visible in seismic reflection data]
6. Fault Drag: Fault drag may also result from folding ahead of the propagating tip of the fault, called fault-‐ propagation folding.
One possibility is trishear, in which a triangular bead of ductile deformation occurs ahead of the tip, creating fault drag along the fault once the tip propagates through.
[Fig. 8.30. Fault drag caused by trishear fault-‐propagation folding ahead of an advancing fault tip]
7. Fault Drag and Clay Smear: Two fault segments that meet across a soft layer may cause it to deform and smear out between the faults. This is a form of fault drag and results in clay smear along the fault zone.
[Fig. 8.31. Clay smear or shale smear along a fault caused by a soft unit being smeared out between two segments of a fault] [Fig. 8.47. Clay smear between two fault segments near Moab, UT]
8. Fault Slip Distribution: Similar to opening fractures, faults grow away from their nucleation points in such a way that the tiplines define elliptical shapes.
The most slip occurs in the center of the fault, decreasing to zero at the tipline. The distribution of slip across the fault surface is also approximately elliptical (for an isolated fault).
[Fig. 8.16. Idealized elliptical fault tipline shape in 3D and the elliptical distribution of slip across the fault surface]
9. Fault Slip Distribution: The distribution of fault slip across the entire fault surface cannot be observed directly, but it can be inferred by interpolating between data points measured along seismic lines, in boreholes, or in mine tunnels.
[Fig. 8.15. Fault slip distribution interpolated from seismic data] [Figure: Normal fault slip distribution determined from seismic data in the English Channel (Kattenhorn & Pollard, 2001)]
10. Fault Slip Profiles: At the Earth’s surface, the distribution of slip or throw can be measured along the length of the fault trace to produce a slip profile. The slip will be zero at the fault tips and a maximum near the fault center. The shape of the profile will depend on where the Earth’s surface cuts through the fault in 3D.
[Fig. 8.13. Fault slip profile measured at the Earth’s surface along the length of the fault trace] [Fig. 8.14. Slip profile shapes may vary depending on the location of the fault trace on the fault surface]
11. Fault Slip Events: The shape of the slip or throw profile is commonly elliptical too. However, the total slip is the end result of numerous slip events. Most faults slip at rates of <10 mm/year, and earthquakes commonly produce ~1 m of slip per event (for M6.5-‐6.9). So hundreds of events may be needed to accrue the total slip we can measure along a fault.
[Fig. 8.13. Fault slip profile measured at the Earth’s surface along the length of the fault trace]
12. Fault Slip Events: During slip events, either: (a) the entire fault will rupture, causing individual, elliptical slip profiles to superimpose over time, creating cumulative elliptical profiles; or (b) the fault may slip in patches. The cumulative effect of the patches is also to create an overall elliptical slip profile shape.
[Fig. 8.36. Accumulation of fault slip through time to produce an overall elliptical slip profile shape]
18. Slip Distributions in Seismic Data: These low points in the slip can be seen in seismic data. Formerly unlinked segments create localized slip highs along the linked fault surface, with slip lows between them marking the linkage locations where relay zones were breached.
[Figure. Slip distribution on a fault surface in 3D. Local highs mark the locations of initial segments. From Kattenhorn and Pollard (2001)]
19. Slip Accommodation in Relay Zones: The relay ramp shape depends on the fault segment configuration. Not only does it permanently take up part of the fault throw, it also gets internally deformed and may develop joints, deformation bands, and subseismic faults.
[Fig. 8.41. Relay ramp at Arches National Park, UT]
20. Relay Zones in Faulted Oil Reservoirs: Locating relay zones in seismic reflection data is important to the oil industry because the relay zones may act as pathways for fluids to migrate across a fault zone without being blocked by the fault planes. They also affect fault slip distributions, which may affect the sealing capacity of the fault core.
[Box. 8.4. Oil migration through a relay ramp in a reservoir]
21. Geometry of Segmented Fault Systems: Given that evolved fault systems are comprised of numerous segments that were slightly out of plane from each other before linking together, fault traces tend to be irregular.
Small jogs or steps may occur, marking the locations of relay zones that were ultimately breached during fault growth.
[Fig. 8.42. Irregular fault traces interpreted from seismic data indicate fault linkage locations]
22. Geometry of Segmented Fault Systems: Given that evolved fault systems are comprised of numerous segments that were slightly out of plane from each other before linking together, fault traces tend to be irregular.
Small jogs or steps may occur, marking the locations of relay zones that were ultimately breached during fault growth.
[Figures. Segmented normal fault systems on Mars and in the Canyonlands National Park, Utah]
23. Geometry of Segmented Fault Systems: Linked segments may also give the fault trace the appearance of being comprised of multiple curved parts, particularly within normal fault systems.
[Fig. 8.43. Arcuate fault traces along normal faults in the North Sea, from seismic data] [Fig. 8.44. The segmented nature of the Wasatch fault in Utah has resulted in an irregular fault trace]
24. Geometry of Segmented Fault Systems: Linkage of segments across relay ramps is relevant to normal and thrust faults in map view (i.e., the segmentation is along strike). Segmentation is also possible in map views of strike-‐slip faults, or cross section views of normal and thrust faults, in the dip direction.
[Fig. 8.4. Complexity of normal faults occurs mostly in map view due to segmentation in the strike direction only] [Fig. 8.45. Segmentation in both the strike direction and the dip direction along a normal fault]
25. Segment Linkages in the Slip Direction: Segments separated along the dip direction may also attempt to link together, but it is sometimes more difficult because of the effect of the rock layers (mechanical stratigraphy). Instead, the layers may warp, rotate, thicken, or thin within the overlap zone, depending on the fault configuration and sense of motion.
This results in either restraining overlap zones or releasing overlap zones.
[Fig. 8.46. Restraining and releasing overlap zones where segments are offset along the dip direction]
26. Segment Linkages in the Slip Direction: Overlap zones in the dip direction usually develop because faults preferentially nucleate first in the more brittle units (e.g., sandstone, limestone) then attempt to link across the more ductile layers (e.g., shale, salt).
Identifying vertical overlap zones is also important in oil fields as they may affect oil migration, but they can be difficult to detect in the absence of good seismic reflectors.
[Fig. 8.47. Overlap zone controlled by mechanical stratigraphy. Attempted linkage is occurring across a shale bed, resulting in clay smear along the fault] [Figure. Vertical step along a normal fault in the Wytch Farm oilfield, English Channel. Kattenhorn and Pollard (2001)]
27. Mechanical Stratigraphy and Fault Shape: Segment linkages create faults that tend to be longer than they are tall. Mechanical stratigraphy has the same effect, tending to focus growth along the brittle layers. Such faults are vertically constrained, but may eventually break across the layering.
Vertically constrained faults are limited in how much throw they can accrue, so faults may appear to be very long relative to their maximum slip amounts. The slip may then “catch up” once the fault breaks across the layer boundaries.
[Fig. 8.49. Mechanical stratigraphy focuses fault growth along brittle units and affects the fault slip behavior]
28. Mechanical Stratigraphy and Fault Shape: These faults in the Wytch Farm oilfield, southern England, are longer in the horizontal direction than the down-‐dip direction because mechanical stratigraphy and segmentation along strike caused preferential fault growth along strike.
[Figure. Wytch Farm oilfield normal faults. Kattenhorn and Pollard (2001)]
29. Displacement-‐Length Scaling: Larger faults are older and more evolved, so tend to have more total slip than smaller faults. The relationship between fault length and slip is affected by fault segmentation and mechanical stratigraphy, as mentioned before, as well as fault shape, but overall, there is a fairly regular pattern to the relationship between fault length and total slip, governed by a power law: D (^) max = γ L n where D (^) max is the total slip, L is the fault trace length, and γ and n are power law constants. The value of n likely falls between 1 and 1.5.
[Fig. 8.50. Fault displacement (total slip) versus length on a log-‐log graph, indicating a power law relationship]
