Lecture index: Larger context and significance of ductile shear zones. / Ductile fault rocks. / Composite fabrics in originally homogeneous rock. / Composite fabrics in originally layered/ foliated rocks. / Porphyroclasts and other related sense of shear indicators. / Crustal strength profiles and thin-skinned tectonics.
The San Andreas plate boundary must extend through lithosphere, circa 100 km. The seismogenic (earthquake producing) zone is the upper 20 km or so. Therefore below 15 km ductile processes must be accommodating fault movement. The same is true of other plate boundary related faults. It is this realm of 'ductile faulting' that we are focusing on in this lecture.
The image to the right is a block diagram of the portion of the San Andreas fault system near Los Angeles, and depicts the brittle to ductile transition. Note by the way the presence of thrust faults as part of the deformation system. We will discuss this later. Image source: http://pubs.usgs.gov/fs/1999/fs110-99/ .
Understanding ductile fault zone kinematics has allowed us to understand the large scale behavior of mountain belts better. One can argue that it has been responsible for the recognition of gravitational collapse of orogenic welts as a tectonic process.
Ductile shear zones can also be major zones of intraplate brittle fault reactivation later in their history (e.g. Billefjorden fault zone in Spitsbergen has Devonian, Carboniferous and Tertiary brittle reactivation of a Caledonian mylonite zone). This may be because of the presence of a well developed fabric, and their continuity.
A ductile shear zone rock classification is reviewed in your readings. If you check out various text books or publications there is some dispute as to the classification and exact definitions of different ductile fault rock types. It is typically depicted that the fault zone width increases with depth as one passes through the transition from brittle to ductile. This has implications for fault rock classification. One common scheme identifies likely fault rock types on the basis of depth in the crust. Cataclasites are where brittle processes dominate at a hand specimen scale. Breccias and gouges are types of cataclasites. Below that one gets into the realm of ductile fault rocks.
Factors that influence ductile fault rock character?
mylonite: initially there was fruitful debate on what characterizes a mylonite. The following is one summary of the outcomes from that debate.
Two end member types of mylonites are ultramylonites and blastomylonites.
ultramylonite - these are characterized by extreme grain size reduction so that they are very fine grained. They can appear flinty. There is a debate as to relative roles of cataclastic milling and dynamic recrystallization, and so they are often characterized by mixed brittle-ductile processes. Common to have inclusions (porphyroclasts). Such mylonites can result either from very high strain rates, or form at the brittle-ductile portion of the crust were thermal recovery processes are not efficient.
This is slide of part of the Goat Rock Fault zone in the Appalachian hinterland in Georgia. The fine grain size (camera lens cap for scale) indicates this is an ultramylonite. Originally it was coarse-grained migmatitic gneiss, and so it has suffered a severe grain size reduction of probably two magnitudes. The small pinkish grains that look like phenocrysts are feldspar porphyroclasts. Such rocks have been mistaken for phenocrystic volcanics! Note the well developed foliation. The sense of movement on this fault zone is dextral, and it moved during the Alleghanian orogenic episode.
This is a fine-grained mylonite (tending toward an ultramylonite) in basement rocks of NW Spitsbergen. It comes from a several hundred meter thick ductile detachment that is thought to be part of a Late Silurian to Devonian metamorphic core complex. Note the well developed lineation on the mylonitic foliation that can be seen on the overhang surface in shadow. The white lenses are deformed and boudined quartz veins. A close look will show some of the quartz lenses to have an asymmetric geometry, consistent with a foliation parallel simple shear component of deformation. In thin section biotite and garnet suggest these rocks attained lower amphibolite metamorphic grade. The grain size is much finer than typical for rocks at this metamorphic grade because of the grain reduction by ductile shear.
This another photo from the same several hundred meter thick mylonite zone as above. The cross section view is basically parallel to the lineation and transport direction. Here the asymmetric geometry and sense of shear associated with the deformed quartz veins is clearly developed (sinistral). Note the asymmetric bondinage of the quartz vein that continues to the left of the tip of the mechanical pencil.
This is another mylonite with a slightly more mica rich composition from the same mylonite zone. One question is why all the quartz veins?
This is a marble from within the same ductile fault zone as above. Since marble is typically a weaker rock it has likely suffered even greater strain than the enclosing dark mylonites shown above. Calcite recovers and recrystallizes so easily it often doesn't show the same textures as typical mylonites, but in this one more competent inclusions, the well developed foliation and the intrafolial folds all help document the very high levels of strain. A term that can be used for this rock is marble tectonite.
Here one is looking at the foliation surface of the same highly strained marble unit as depicted above. A well developed lineation can be seen, which results from the elongation of the individual calcite crystals, and likely reflects elongation in the direction of shear in the mylonite zone.
blastomylonite - These are often coarser grained, recrystallized into a sugary appearance, often without a strong fabric evident to the eye. Thermal recovery, static recrystallization produced a late stage or subsequent increase in grain size, and so these are considered more typical of rocks deforming at deeper levels in the ductile portion of the crust. Context or other structures indicate it is a mylonite.
Both images are mylonites from a large complex shear zone in the Caledonian basement rocks of NW Spitsbergen. In both cases the images are 3.3 mm across in horizontal view, and under cross nichols. The left image is a blastomylonite, where thermally aided dynamic recrystallization (and possible post deformation static recrystallization) allowed the grain size to remain relatively coarse. Note the quartz grains are relatively free of undulose extinctions, indicating the dislocations have easily migrated to form grain boundaries. One can see some evidence that feldspar is recrystallizing. The right image is thought to be the same initial rock, but comes from the top of the shear zone under lower-T conditions, where dynamic grain size reduction through recrystallization has formed an ultramylonite with distributed and discrete dextral slip surfaces. Note the offset of the recrytallized quartz-feldspar aggregate. Micas dominate the deformation behavior of this ultramylonite, and the fine-grained sericite (white mica) likely formed from the breakdown and retrograde metamorphism of feldspar. Since sericite deforms much more easily than feldspar, this process greatly weakens the rock, concentrating the deformation into these rocks. This phenomena is common in large scale crustal shear zones associated with crustal extension.
foliated cataclasites - these are fault rocks truly in between brittle and ductile, and consist of angular fragments in a finer grained foliated matrix. Brittle communition of grains creates the fine-grained matrix. Grain boundary sliding, grain rolling, and pressure solution then allow for very distributed deformation in the matrix that gives it the ductile-like foliated character. Foliated cataclasites often occur in associated with ultramylonites.
Black and white thin section image (plain light, 3.3 mm horizontal field) of a foliated cataclasite found in association with ultramylonite from Spitsbergen depicted above. The hanging-wall rocks for the cataclasite are Devonian sediments and so the cataclasite formation must involve Devonian fault movement. Many of the clasts in the matrix are mylonites and ultramylonites, indicating a local progression from ductile to brittle deformation in the shear zone.
L- tectonite - highly deformed rock with a strong linear fabric.
S- tectonite - highly deformed rock with a strong planar fabric.
mylonites can be L or S tectonites, or a L-S tectonite.
phyllonites: these are rocks dominated mechanically by micas.
Intrusives often intrude along ductile shear zones, and then become deformed by continued movement. This has some interesting mechanical implications, including melt lubrication (where the weak magmatic material decreases the fault strength and localizes strain). Because of the ability to date intrusives, especially granites, they also provide constraints on the timing of deformation. In such deformed granites, 2 surfaces, one more penetrative and the other more spaced, are often seen spatially associated with each other. The old interpretation was that of a later crenulation being imposed on an earlier flow fabric; i.e. two different deformation episodes. It is now realized they form synchronously as a result of ductile shear.
S - surface
C - surface
Schematic diagram of S and C surface development in a dextral ductile shear zone, along with accompanying asymmetric porphyroclasts. In reality, the C surfaces tend to anastomose some (split and join).
Sense of motion given by S - C geometry. C-S intersection assumed to be perpendicular to movement.
S-surface tracking across the mylonite zone and net movement (Naruk , in metamorphic core complex):
Critical factor - original orientation of DSF - dominant slip foliation, usually penetrative. This is an pre-existing surface that is utilized as a slip surface. Can develop in a separate episode of deformation, or early in a protracted episode.
NSC - normal slip crenulations, these are spaced and are not parallel to the shear zone walls.
Operation of DSF as a slip plane will cause shear zone thickening and they will rotate out of a favorable slip position. NSC is compensating structure; allows slip planes to keep operating. Important point is that there are two slip systems operating.
RSC - reverse slip crenulations are usually spaced, or isolated fold structures. In this case DSF is oriented clockwise from slip plane in dextral slip and the operation of DSF will cause shear zone thinning. RSC are then the compensating thickening structure. These are much more uncommon than NSC.
Naturally can get evolution of shear zone as S surfaces get formed, rotated, and utilized as slip planes.
This view is of a phyllonite in the Irmo Shear Zone, exposed on the Clarks Hill Reservoir on the South Carolina side that shows well developed composite surfaces.. This is part of a system of Late Alleghanian dextral shear zones that pervade the Piedmont region. Some of the NSC are ornamented with yellow dots, while examples of DSF are ornamented with green dots. The shear zone walls are well defined here and approximately bisect the acute angle between NSC and DSF.
This is a thin section of an ultramylonite from Caledonian basement rocks in NW Spitsbergen. It is under plain light and the horizontal field of view is 3.3 mm, and in black and white (there wasn't much color in the color version). In this view the DSF is running diagonally and the NSC is running more horizontal, and the sense of shear is dextral. Note the very fine grain size. The dark coloration is due to a combination of very fine-grained biotite and biotite, and the protolith of this sheared rock was an amphibolites grade, biotite gneiss/schist.
From petrology you are familiar with the term porphyroblast - a mineral of greater grain size because it grew larger. A porphyroclast is a grain that is greater in grain size because deformation reduced the grain size of the surrounding material. Porphyroclasts are usually formed by competent minerals, that strain less. The most common type of porphyroclast is feldspar, and the classic feldspar 'augen' (eye in german) in gneisses are a common example. Garnets can also form good porphyroclasts.
Left image: This is thin section image (crossed nichols w gypsum plate, 3.3 mm horizontal dimension) from an ultramylonite in Caledonian basement rocks in NW Svalbard. The large grain at extinction is a garnet porphyroclast. Note how the garnet is behaving brittlely, both extending with some quartz infill, and with a sinistral offset along a surface at a high angle to the mylonite foliation. Overall the sense of shear is dextral, and the porphyroclast offset can be thought of as due to antithetic slip. The age of deformation is unclear, but could be Silurian or Devonian. Right image: This is a thin section image of another specimen from nearby the one described above, and is a good example of an asymmetric porphyroclast, in this case with a sinistral sense of shear in this view direction. Note how the irregular margins of the garnet. retrograde metamorphism is changing the garnet into sericite, and that mica material is being deformed into porphyroclast tails (also known as pressure shadows. It almost like a wake area that is created as the surrounding material flows by with the sinistral sense of shear.
This is a thin section from a sheared amphibolite from the Piedmont of Georgia, showing a feldspar porphyroclast with an asymmetric shape (outlined in blue) and an adjacent NSC surface that has possibly separated the two feldspar grains. The photo is under plain light, and the brown flaky material is biotite, while the green material is hornblende.
Utilization of this new understanding has led to new understanding of large scale mechanics of mountain belts by informing us about their internal dynamics.
Remember from the general positive slope of the Mohr failure envelope that the amount of deviatoric stress needed for brittle failure increases with depth, and so in the brittle realm of the crust, the rocks get stronger. Indeed, the larger earthquakes nucleate from the brittle-ductile transition, and in many diagrams this is where the crust has its maximum strength. Below that crustal rocks are weaker because they are hotter, and distributed ductile strain dominates.
Link to diagram that shows simplified upper crustal strength profile - source of diagram USGS site on geothermal energy.
However, consider the crust as it is, extremely heterogeneous, and includes material and surfaces of greatly varying strength. What features may cause significant departures from a simple strength profile?
Many of these have an initially sub-horizontal orientation The crust can then be considered to be mechanically laminated, and it is then no surprise that it delaminates along detachments. This produces a phenomena known as thin-skinned tectonics.
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