LAB X: FAULT ROCKS
Fault rocks can provide deformed very useful geologic data, including information on
deformation conditions, movement directions of faults/shear zones, strain, and mineralization
history. In shallow portions of fault zones (<10-15 km), deformation is primarily accommodated
by brittle fracturing (Figure 1). Cataclasis or cataclastic flow is a process in which brittle
fracturing is accompanied by the sliding and rotation of broken grains. Rocks deformed by
cataclasis typically consist of a wide range of grain sizes and broken fragments with angular or
sharp outlines. Brittle fault rocks usually lack well-defined foliation. At shallow levels (e.g. <5
km), fault rocks may be incohesive. Incohesive fault rocks are classified as fault breccia if
visible fragments (to the naked eye) comprise >30% of the rock or gouge if <30% of the rock
consists of visible fragments (Table 1). Cohesive brittle fault rocks are classified as
protocataclasite, cataclasite, or ultracataclasite depending on the percentage of visible clasts
to matrix (Table 1).
In deeper portions of fault zones (>~300°C), temperatures may be high enough to
accommodate plastic flow and dynamic recrystallization of minerals (Figure 1). Rocks that have
undergone ductile shearing and dynamic recrystallization are classified as mylonites
(protomylonite, mylonite, or ultramylonite depending on the matrix to clast ratio; Table 1).
Mylonites typically have well-developed foliations and stretching lineations (L-S tectonites).
Although mylonites and cataclasites form by different processes, they are both characterized by
grain size reduction. At deep crustal levels (e.g. in middle to upper amphibolite facies),
temperatures may be high enough that recrystallization does not reduce grain sizes. Rocks
formed in shear zones at these high temperatures are generally characterized by gneissic foliation
and relatively coarse, equigranular grain size.
Figure 1. Idealized cross section of a major fault zone showing how fault rock varies with depth.
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Table 1. Simplified fault rock classification (after Sibson, 1977)
PART 1: BRITTLE FAULT ROCKS
Sample RQ (hand sample): This sample is from a brittle fault zone within a tonalite pluton in
British Columbia. How can you tell that this is a brittle fault rock? (sample consists of
randomly-oriented, angular to subrounded clasts surrounded by fine-grained matrix that lacks a
foliation). Classify this fault rock using the brittle fault rock classification (note: not all of this
rock has the same classification). (sample is clearly cohesive – cannot be a breccia or a gouge;
it is mostly a cataclasite with ~60% matrix, but there is a zone of ultracataclasite with ~100%
matrix). Fluid flow and mineralization are common along brittle fault zones. What type of
mineralization was taking place during development of this fault zone? (rock is dominated by
pistachio-green epidote)
Sample Tsp (hand sample): This is a sample of a fault plane from western Arizona. Note the
beautiful slickenlines on the polished, hematite-coated surface. Classify the fault rock below the
fault plane. (good example of protocataclasite with ~20% matrix)
Sample CF (hand sample): Careful with this sample – it’s very friable. This is gouge from the
Calico fault zone in the Mojave Desert (southern California). What mineral do you think is
dominant in this gouge zone? (clay)
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Sample 4-291 (hand sample): This is a sample of ultracataclasite from the Buckskin detachment
fault in western Arizona. The rock was once a granite, but it is now 100% ground up matrix.
Hematite (red-brown) and chrysocolla (hydrated Cu-silicate – turquoise-blue) were forming
during faulting. The polished surface on top is the main fault plane. There are two sets of
slickenlines on the main fault plane (you will need a hand lense to see one set). This sample is
oriented, with the strike and dip on top striking due N and dipping 10°E. What are the
orientations of these slickenlines? (measure rake of slickenline sets (39 & 140°) and determine T
& P with a stereonet: 39,6 & 140, 7)
Sample 3-259 (hand sample and thin section): Another sample from the Buckskin detachment
fault. Based on the hand sample, how would you classify this rock? (looks like a cataclasite with
~30-40% clasts). Now look at the thin section. What are the “clasts” in this rock? (clasts of
older cataclasite).
How would you classify this rock based on the thin section?
(ultracataclasite: ~100% matrix)
Sample “Pseudotachylite” (small hand sample): The dark vein is pseudotachylite from an
Oligocene strike-slip fault in British Columbia. What is pseudotachylite and where does it
usually form? (pseudotachylite is glassy material formed by frictional melting during an
earthquake slip event. This rare type of fault rock is generally restricted to dry fault zones at
depths <15 km)
Sample “E.Mitchel Range” (hand sample): This is ultracataclasite from the Waterman Hills
detachment fault in the Mojave Desert. Not all brittle fault rocks lack foliation. Foliation may
be present in clay-rich gouges and ultracataclasites. Note that this sample has a moderately welldefined foliation. In this case, a thin section may be needed to identify whether the rock is an
ultracataclasite or an ultramylonite. What would you look for in order to make this distinction?
(ultracataclasite: brittle fracturing dominates; ultramylonite: dynamic recrystallization
dominates)
Samples M.MX and E.B.: (hand samples): More examples of fault plane slickenlines. Sample
M.MX are calcite slickenfibers from a fault zone in carbonate rocks, whereas E.B. are
slickenlines in chalcedony which formed along a fault in granite (no questions).
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PART 2: DUCTILE FAULT ROCKS
Samples BH, 1-51, and 2-11 (hand samples): Sample BH is an undeformed granodiorite from
western Arizona that has locally undergone mylonitization in a ductile shear zone. Samples 1-51
(protomylonite) and 2-11 (mylonite) are derived from this same granodiorite protolith. Describe
the textural changes that have taken place during mylonitization of 1-51 and 2-11. (feldspar
grains have undergone grain size reduction; quartz grains have been deformed into lenses and
ribbons, biotite grains have become aligned parallel to a foliation)
Samples 4-636 and 1-121 (hand samples): More mylonites from western Arizona. Do these
samples have a foliation? A lineation? If so, what defines them? Would you classify this
mylonite as an L-, LS-, or S-tectonite? (sample 4-636: LS-tectonite with foliation defined by
aligned biotite and layers of recrystallized quartz and feldspar; the lineation is defined primarily
by quartz ribbons and streaks of biotite; sample 1-121: L-tectonite lacking a clear foliation; the
lineation is defined by quartz ribbons and streaks of chlorite)
Sample W.C.A: (hand sample): This sample is from an amphibolite-facies shear zone in British
Columbia. Classify this rock and discuss how it is different texturally than sample 2-11 or 4-636
(Amphibolite gneiss with gneissic, compositional layering. Compared to samples 2-11 or 4-636,
sample W.C.A. is coarse-grained and equigranular.)
Dynamic recrystallization mechanisms
Grain size reduction in mylonites is accomplished primarily by dynamic recrystallization
(recrystallization during deformation). Dynamic recrystallization results in the formation of new
grains that have fewer dislocations than older, strained grains. There are three main types of
dynamic recrystallization: bulging recrystallization, subgrain rotation recrystallization, and grain
boundary migration recrystallization (Figure 2). These recrystallization mechanisms are
primarily dependent on temperature, strain rate, and water content. At relatively low
temperatures and/or high strain rates, bulging recrystallization is dominant, resulting in the
formation of small (e.g. ~5-30 μm), blurry grains. At moderate temperatures/strain rates,
subgrains begin to form, and recrystallization gives way to subgrain rotation recrystallization.
Grains formed by subgrain rotation recrystallization typically have uniform sizes and polygonal
shapes similar to adjacent subgrains. At higher temperature/lower strain rate conditions, grain
boundary migration recrystallization dominates, typically resulting in the formation of
recrystallized grains with variable sizes (typically >100 μm) and irregular, lobate boundaries. At
strain rates typical of most shear zones (e.g. 10-14 to 10-12 s-1), these recrystallization mechanisms
correlate reasonably well with temperature. In lower greenschist facies conditions (~300400°C), quartz typically undergoes bulging recrystallization, whereas feldspar deforms primarily
by brittle fracturing. At upper greenschist-facies conditions (~400-500°C), subgrain rotation
recrystallization is dominant in quartz, whereas feldspar may undergo bulging recrystallization.
At amphibolite facies conditions (>500°C), grain boundary migration recrystallization is
dominant in quartz, and feldspar may undergo subgrain rotation recrystallization.
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Figure 2. Sketches illustrating the characteristic features of the three dynamic recrystallization mechanisms.
Samples 3-143, LR-1, and LB-69 (hand samples and thin sections): These are quartzite
mylonites in which quartz recrystallization is dominated by either bulging recrystallization,
subgrain rotation recrystallization, or grain boundary migration recrystallization. In each sample
draw a sketch of the recrystallized quartz grains illustrating their main textural features. In each
sketch, label the sample and recrystallization mechanism, and include a scale. (these samples
are clear examples of bulging recrystallization (3-143); subgrain rotation recrystallization (LR1), and high-temperature grain boundary migration recrystallization (LB-69))
Samples 5-1, 2-22, LB-126, and H-1 (hand samples and thin sections): Each of the following
samples are fault rocks derived from granitic rocks. Give each sample a fault rock classification
name and identify the dominant deformation/recrystallization mechanisms for quartz and
feldspar in each sample (fracturing, bulging recrystallization, subgrain rotation recrystallization,
or grain boundary migration recrystallization). Also, rank the samples in terms of their relative
deformation temperature based on their textural features and deformation mechanisms.
(5-1: mylonite, quartz: subgrain rotation recrystallization, feldspar: bulging recrystallization;
2-22: ultracataclasite – fracturing; LB-126: mylonite/ultramylonite (~90% matrix), quartz:
grain-boundary migration, feldspar: subgrain rotation recrystallization; H-1: mylonite, quartz:
subgrain rotation recrystallization, feldspar: fracturing; relative deformation temperature from
lowest to highest: 2-22, H-1, 5-1, LB-126)
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