Surface, neotectonic and compaction structures.

Lecture index (this material is a bit of a hodgepodge, a mixed bag of topics that are useful to know about):


Key words and concepts:

View of fault scarp created during 1983 Borah Peak earthquake (magnitude 7.3) in Idaho. Given the material that supports the fault scarp - how long will the scarp last? Photo source: http://quake.usgs.gov/prepare/factsheets/Wasatch/FaultScarp.gif .


Part 1 - Topographically driven/influenced structures

This is a hodgepodge of items. Many of these topics in this section are not neotectonic (e.g. glaciers and mass wasting). However, there is a gray zone between the tectonic and geomorphology worlds (e.g. gravitational collapse of mountains and continental shelves), and these are phenomena where we can learn some basic lessons about deformational processes through direct observation, and this is a convenient place to put them in this course.

Glaciers

Glaciers: A lot can be learned about deformation patterns from observing glacial form. Because of their lower rheidity this can be done in a human time frame. Additionally, glaciers show both shallow brittle features, and ductile deeper features.

Crevasse patterns:

USGS photo of crevasses on small alpine glacier. Note the person for scale, and how the length of the crevasse is perpendicular to the down slope direction. Photo source: http://www-atlas.usgs.gov/articles/government/IMAGES/usgs_shastina.gif .

Internal creep versus basal slip.

Glacial surges:


Slumps

Similar to glaciers, mass wasting features can be instructive as to deformational behavior in general.

Diagram from USGS site on mass wasting: http://wrgis.wr.usgs.gov/wgmt/elnino/deserten/processes.html .

Small scale slump. Photo source: http://wrgis.wr.usgs.gov/wgmt/elnino/enimages/slump.JPG .

Note the faulted white layer in the lower part of the cliff section (which shows about 150 m of relief). These are white sandstones of the Cretaceous Helvetiafjellet Formation on the eastern side of Spitsbergen. Clearly the faulting is broadly synsedimentary as the overlying white sandstones are unfaulted and truncate the faulted sequence. These are interpreted to be Cretaceous delta-front collapse structures. Since deltas have fairly good preservation possibilities, such delta-front collapse structures are not uncommon in the sedimentary record. The fact that you have a coarsening upwards trend with relatively weak muds at the base helps to set up the situation where delta-front collapse occurs.


Gravitational collapse/spreading

Related to mass wasting - these can be thought of as not so much as slope related, but as 'mound' related and internal pressure driven.

The influence of topography and surface processes on tectonics?


Tectonically driven surface features

Active fault scarps

For seismic risk assessment purposes, and for understanding deformation patterns from a uniformitarian perspective, it is useful to identify active faults.

How are active fault scarps identified?

Red lines are the traces of the Wasatch fault, with known times of movement. Note the segmentation of the fault. Image source: http://quake.usgs.gov/prepare/factsheets/Wasatch/ .

View of detailed cross section diagram obtained from trenching across the San Andreas fault showing how recent alluvial sediments are offset, along with image from Pallett Creek locality. Image to left source: http://earthquake.usgs.gov/research/hazmaps/haz101/faultpop.php .

Meers fault in Oklahoma as an example: two well dated Late Holocene events. Presently seismically quiescent.

Map of location of Meers fault from USGS site: http://earthquake.usgs.gov/regional/qfaults/eusa/sokla.php

USGS summary report on Meers fault.

 

Where is the fault scarp? This is an image of the slopes of Lihogda on the western side of Bockfjorden in northwest Spitsbergen. A close look shows an unusual, lighter colored linear feature that is subparallel to the shoreline and that cuts across streams. It is lighter colored because it is a topographic scarp (and not a road or path) where fresh slope colluvium and alluvium is exposed (whereas the surrounding material is darker because of vegetative cover and lichen). This very likely represents a somewhat degraded modern fault scarp due to a geologically recent earthquake. It follows an underlying old fault zone in the bedrock. This is presently an intraplate setting, and this fault can be considered as an active tectonic feature.


Active fold growth

Photo of the anticline and diagram of how terraces were cut into the flank as it grew. Image sources: http://quake.usgs.gov/research/deformation/modeling/papers/scientam/scientam.html .


Detecting surface deformation

GPS Geodesy: With base stations and the right set up you can get a position accuracy of mm per year, well within the ability to detect year to year deformation.

GPS detected motions in Southern California associated with the San Andreas fault. Image source: http://pasadena.wr.usgs.gov/office/hudnut/scec/97_SCEC_E_summary.html .

GPS geodesy data can also be used to monitor movement associated with an earthquake. Note here that most of country moved. Image source with more detailed information at: USGS site: Preliminary Geodetic Slip Model of the 2011 M9.0 Tohoku-chiho Taiheiyo-oki Earthquake - http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/031111_M9.0prelim_geodetic_slip.php

Link to site describing GPS geodesy in Sumatra area.

Earth article on use of GPS in understanding tectonics.


Satellite radar interferometry.

Link to USGS site describing the technique of interferometry.

"Interferogram of Mount Peulik volcano, Alaska: October 1996 and September 1998 (Lu and others, 2001)" Source: http://volcanoes.usgs.gov/About/What/Monitor/Deformation/Peulik.html

Diagram showing uplift by interferometry associated with magma movement 3 miles west of South Sister. Image source: http://volcanoes.usgs.gov/insar/ .

Here the image is of compaction related subsidence due to water withdrawal, where the pattern is partially controlled by a fault, that in turns influences the aquifer geometry. Image source: http://ca.water.usgs.gov/insar/ .

USGS article on using interferometry to track Yellowstone activity.


Structures formed by differential compaction

As sediments are buried and undergo diagenesis and lithification they compact. Sand may not compact significantly at all, but mud typically compacts 50%, and even more. Limestones can compact due to pressure solution and styolite seams are the result. Such compaction represents significant deformation on its own, and can fuel structures caused by differential compaction. Such structures occur on a small and larger scale, and include (but are not limited to):

Cross section image of siderite concretion in Triassic siltstones from Edgeoya, Svalbard. Lower center is the tip of a walking stick for scale. The dashed lines show an interpretation of the trace of the fissility which is bent because of the differential compaction. Assuming the concretion has suffered no compaction then the amount of compaction is roughly 50% here, a 2 to 1, uncompacted to compacted ratio. This is significant strain!

This is an additional siderite concretion from Edgeøya, but one where the earlier bedding lamination is much better preserved. Note how, as you trace a layer from the interior of the concretion outwards, the layer thickness is constant in the core, but then thins more gradually farther out. This suggests that the core formed pre-compaction, and that the outer and less developed concretion portions formed during compaction. In this case and in many other cases concretions form by localized cementation and replacement. Portion of walking stick to right for scale.

The above is a polished slab of sparry limestone from the wall of a hallway in the Joslyn Art Museum in Omaha. The field of view is roughly 20 cm. The dark seams with interfingering of the rock on either side are styolites, which are irregular solution seams. The dark coloration is insoluble residue left behind at the solution of the calcite progresses. Note the diffuse banding in the limestone which represents bedding and the diffuse coloration mottling. The sharp color difference across the lower styolite seam is because the intervening material has been dissolved. In 3-d the interfingering takes the form of interpenetrating cones, and the interpretation is that sigma one is in the direction of that cone axis. In this case, sigma one would be roughly perpendicular to bedding, an orientation consistent with being gravity/compaction driven. The relief of interfingering is argued to represent the minimum amount of material dissolved away. In this case it would represent a compaction or vertical shortening of several percent. Styolites are common in carbonates, but can also occur in quartzites in some cases. The irregular white band is a late calcite vein that occurred.

This is also part of a wall in the Joslyn Art Museum, and nicely shows a more complicated linkage of small calcite veins and styolite seams, indicating that the two features formed together. Given the orientation of the linking vein, it makes sense that if the two sides across a styolite seam are moving towards each other, there would be a component of opening across the vein direction.


Structural diagenesis

Structural diagenesis: an endeavor that considers the interplay between diagenetic and structural processes. Diagenesis includes the transformation of sediment to rock, a major mechanical transformation. Volume changes can be involved and this can drive deformation. In turn fracturing can change fluid flow, which in turn influences diagenesis.

Suite of structural features that can form during diagenesis:


References: