Lab X: Fault Rocks

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 cataclasistypically 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, orultramylonitedepending on the matrix to clast ratio; Table 1). Mylonitestypically 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.

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 tonalitepluton in British Columbia. How can you tell that this is a brittle fault rock? (sample consists of randomly-oriented, angular to subroundedclasts 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)

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 ispseudotachylite 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 well-defined 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)

SamplesM.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).

PART 2: DUCTILE FAULT ROCKS

SamplesBH, 1-51, and 2-11(hand samples): Sample BH is an undeformedgranodioritefrom 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 granodioriteprotolith. 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)

Samples4-636 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 rotationrecrystallization. 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 recrystallizationdominates, 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 10-12s-1), these recrystallization mechanisms correlate reasonably well with temperature. In lower greenschistfacies conditions (~300-400°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.

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. (thesesamples are clear examples of bulging recrystallization (3-143); subgrain rotation recrystallization (LR-1), 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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