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.
Readings:
Terms:
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 '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 orgogenic welts as a tectonic process.
Ductile shear zones can also be major zones of intraplate brittle fault reactivation (e.g. Billefjorden fault zone in Spitsbergen has Devonian reactivation of a Caledonian mylonite zone). This may be because of their mineralogy, 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. Cataclastites are where brittle processes dominate at a hand specimen scale. Breccias and gouges are types of cataclastites. Below that one gets into the realm of ductile fault rocks.
mylonite: Initially there was fruitful debate on what characterizes a mylonite. The following is my summary of the outcomes of that debate.
Two types of mylonites are ultramylonies 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 lense 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.
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.
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 srength and localizes strain). Because of the ability to date intrusives, especially granites, they also provide constraints on the timing of deformaiton. 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
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.
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.
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.
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 srtength profile - source of diagram USGS site on geothermal energy.
However, consider the crust as it is, extremely heterogenous, 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 oriention. 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.
Copyright Harmon D. Maher Jr., This may be used for non-profit educational purposes as long as proper attribution is given. Otherwise, please contact me. Thank you.
