SUMMARY OF WEEK 1 READINGS
T&M pp. ix -34; skip Box 2.1, 2.2, and 2.3
Preface: Twiss & Moores try to integrate structural geology and tectonics as a unified subject divided only by the scale that each look at. Need to integrate our understanding of deformations on all scales in order to understand the dynamics of the Earth. Sructural Geology deals with the submicroscopic to the regional, and tectonics scales are the regional to the global.
Chapter 1: Overview
- Structural geology is concerned with the reconstruction of the motions that have shaped the Earth's outer layers.
- There are two main types of motion:
- Rigid Body Motion- transport without damage or deformation.
- Deformation- breaks or changes in rock that leaves a permanent record or imprint.
- Scale can range from submicroscopic (ie. crystals) to regional (ie. entire plates)
- Recent developments that have enhanced the understanding of Structural Geology
- Continuum Mechanics- model material properties as homogeneous or smoothly variant in any direction
- able to approximate rocks as elastic materials for ease in modeling motions.
- Direct deformability lab tests on rock samples- correlate to field responses.
- Microscopic studies of crystal behaviors.
- Theory of Plate Tectonics.
- Growing importance for Geophysical data as a "window" into the subsurface.
- Use of Models- must create and study in order to extrapolate large scale motions
- Geometric- displays where structures are and how they are oriented with respect to each other.
- Kinematic- shows the motions that occur to produce stuctures.
- Mechanic- describes the forces involved and how applied to produce structures.
- Limitation!!- Can only predict how model will behave with 100% accuracy.
- Must be careful of how accurate assumptions and correlations are between the model and real life motions.
- The Interior of the Earth.
- Core- very dense material believed to be made of Iron-Nickel alloy
- inner=solid, outer=liquid
- Mantle- lower density magnesium-iron silicates
- Crust- outer layer of low density material
- Asthenosphere- liquid layer believed to support plate motions.
- Lithosphere- make up plates consisting of crust material and upper mantle (100km deep)
- Moho- bottom boundary of crust material and upper mantle.
- Temperature gradient= 30 degrees C/km
- Study of Structural Geology focuses on upper 20-30km of Earth- merely a thin rind of the entire Earth.
- The Earth's Crust
- Divided into Continental (Granitic) and Oceanic (Basaltic)
- Bimodally distributed- not much else in between continental and oceanic crust
- Plate Tectonics- characteristics of crust created from direct or indirect results of plate motions.
- 7 major plates and several minor ones- treat as rigid bodies
- Geologic stuctures formed dependent on the zone of formation.
- Types of Plate Boundaries:
- Divergent- basaltic upwelling usually at mid-ocean ridges.
- Normal faults account for crustal stretching.
- Convergent- sediments scraped and melted usually creating volcanic arcs
- Thrust faults deal with shortening and contraction.
- Conservative- strike-slip stuctures
- Variety of secondary/mixed stuctures not necesarily indicative of boundary type.
Chapter 2: Techniques of Structural Geology and Techtonics
- Basis of Structural Geology is describing the attitude (orientation) of lines and planes (stuctures).
- Strike and Dip- House roof example
- The peak of the roof represents the strike (horizontal direction of horizontal line in a given planar feature with respect to geographic north).
- The angle the roof face makes with the horizontal plane is the dip angle (angle between horizontal plane and planar feature)
- Direction of steepest slope on the roof is the dip direction (always perpendicular to strike)
- Trend and Plunge are analogous terms for a linear feature
- Trend- strike of vertical plane that contains linear feature.
- Plunge- angle with horizontal plane measured within vertical plane.
- Tools for describing structural components
- Geologic maps- 2-D representations of earth's surface with a variety of geologic data.
- Can show distribution of rock types, locations and attitudes of structures
- Contacts with differing degrees of certainty
- Sedimentary contacts- stratigraphy.
- tectonic- faults.
- intrusive- invades another stucture.
- Distortion must be taken into account.
- Highly interpretive representations of depth.
- Vertical exaggeration can distort dips.
- Ideally oriented perpendicular to strike of planar features- if attitude changes then is not an accurate depiction of feature.
- can also represent with "3-D" block diagrams.
- Stratigraphic sequence indicators- help to determine relative ages.
- Structures in Sedimentary Rocks
- Bottom Markings- turbidity currents, underside of sandstone beds
- Flute casts- wavy bottom
- Load casts- Sand filling in holes of clay sediments. In-filling can describe stratigraphic up.
- Graded bedding- changes in grain size within a sedimentary layer (ie. fine down to coarse)
- Metamorphism can change direction of grade due to fines crystal growth
- Cross bedding- results of depostions in ripples or dunes
- Channel or scour-fill- deposits fill-in stream or sut-out into seds.
- Primary Structures in Igneous Rocks- most telltale in extrusive but not as abundant as sedimantary.
- Flow-top Beccia- top of flow solidifies then breaks up
- Can have vesicles that fill up with precipitates- if half filled can tell direction of stratigraphic up
- Pillow Lava- extrusion of lavas underwater.
- Disconformities- time gaps within parallel layers.
- Angular unconformity- erosional surfaces that cut older beds that have deformed
- Nonconformity- contacts between sedimentary rocks and underlying metamorphic and igneous.
- Graphical representation of Orientation Data
- Data with Constant Strike
- Histograms- bar graph that describes frequency of occurance for given stike and dip range.
- Rose Diagram- radial histogram (more intuitive representation of different dip directions).
- Spherical Projections- all planes and lines pass throught center, attitudes defined by intersection with sphere.
- Stereographic/equal angle projection
- Lambert/equal area projection
- Geophysical techniques- used to gather information at depth
- Seismic Studies- track oscillations of elastic deformation propagating away from source.
- P-waves (primary) travel faster than
- S-waves (shear)- velocity of waves change with rock density and elasticity enabling the mapping of boundaries.
- Seismic Refraction- measures waves propagated through boundaries.
- low velocity layers cannot be detected.
- deep structures detected only at surface distances greater than depth.
- properties are averaged over large distances.
- non-horizontal/discontinuous layers difficult to resolve.
- Seismic Reflection- mearsures waves propagated off of boundaries.
- Stacking- reflection of same subsurface point from different angles gains greater resolution.
- Analysis of Gravity Anomalies- measure density differences
- Fit measurements to a model along a traverse- many corrections to data required:
- Free-Air- correction due to change in altitude (ie. change in distance to Earth's center)
- Bouguer- change in gravity anomaly due to gain/loss of crustal mass.
- Terrain- effects of local topography to gravity anomaly.
- Geomagnetic Studies- study differences in the expected measurement of the Earth's gravit field.
- Paleomagnetism- study of the orientation of the magnetic field recorded in rocks.
EPS116 - FAQ from previous classes - Week 1
- Why, following the onset of spreading (and thinning) beneath continental crust, is there not a constant, even slope from the continental edge to the new oceanic crust? i.e., why is there a "shelf break"?
Due to natural sedimentation patterns, a drop is formed between the continental crust and the oceanic crust. Uplift of the continents can also play a role.
The slope break corresponds, roughly, to the transition between less dense continental crust and more dense oceanic crust. While the nature of the suture zone between these two types of crust is generally unknown, it can be inferred that the shelf break is due, at least initially, to the density differences at the boundary between continental and oceanic crust. Over time, as sediment from the eroding continents is deposited in the oceans, the slope break is enhanced. This is due to the fact that most of the sediment will fall out of suspension and be deposited closer to the coast (due to things like water velocity and grain size). Thus a continental shelf builds up and the slope develops from this.
- I'd be interested to learn more details and history of the theory
you mentioned in class (about the vertically moving sections of crust)
that was prevalent prior to tectonics.
These vertical motion theories explained that there were sediments but not how the structures came to be positioned. Plate Tectonics explained the motions.
Early (1800s and early 1900s) theories regarding movements of the continents and ocean basins relied on various ideas. One of those, the theory of permanentism, described orogenies as due to the filling of very large sedimentary basins called geosynclines. The theory, though, did not suggest a mechanism by which geosynclinal sediments were uplifted; no vertical mechanism was suggested, just the observation that sedimentary rocks could be found in mountain ranges. For more on this, see our web page "Before Wegener
Geology in the 19th century" .
- Flute casts: if a turbidite-flow is rushing over an area, scouring the underlying mud, how is the sand going to settle out (be deposited) at such high velocity?
These structures can be modeled as underwater landslides usually near continental margin sedimentation zones.
Turbidites lithify when flow conditions are slow reflecting the flow history/pattern of an area.
Generally, sand size particles will not settle out of suspension until the velocity of the water has slowed enough to permit deposition. Features such as flute casts may form by molding of existing material by a turbidite current or by deposition of sand from the current and then molding. Either way, flute casts can still be used as up-down indicators in a sedimentary sequence.
Turbidites are episodic gravity currents, primarily derived from active tectonic margins. Thought to be triggered by earthquakes (e.g., Pacific NW).
- Channel deposits:
- Proximal deposits: (massive, poorly developed, weak grading, little clay or mud)
- Turbidites (classic): graded bedding, oriented erosion and fill markings (sole marks), interbedded pelagic clays.Bouma sequences. => Useful up-down and current orientation indicators.
- Distal turbidites: Distant from source, thin, fine grained, cross-laminated.
- Could you explain how the shape of a flute cast reflects the
direction of the water flow in the environment of formation? I've come
across this feature before but have never been clear as to why the
flute cast tapers at the end.
One theory states that vortexes (mini underwater tornadoes) are formed along the sandy bottom and "drill" out material dissipating energy as it moves downstream (Steep slope points upstream).
The shape of flute casts are strictly due to the interaction between cohesive forces in the sand and shaping forces of the water current. If the current begins scouring out a sand bed, it initially has a lot of energy . As a result, it will scour more. As it loses energy (i.e. as the mini-vortex dissapates), cohesive forces outdo scouring forces and the shape becomes more tapered. I'm sure there are much better fluid dynamic models that explain this but that is the gist of it.
- There wasn't an example of what a load cast looks like. Could you
show us a picture to give a visual idea of what it looks like?
Sand filling in holes within a clay matrix. The sand in-filling can sink and indicate stratigraphic up.
Picture this: a sand bed overlies a layer of clay. The clay is water-saturated (the platy clay particles are not aligned) so that the clay is less dense than the overlying sand. Because of this density instability, the sand begins seeping into the clay layer, forming balls of intruding material. Because the sand seeps down into the clay, these features are used as up-down indicators in a sedimentary sequence.
Other bedding markings that can be used to identify which way is up: Animal tracks, clast imbrication, mudcracks, raindrop impressions, ripple marks, cross bedding, worm burrows, load casts.
- The angular unconformity figure suggests that stratigraphy is bent
and then sediments form on top. Isn't this a much more gradual
process? Shouldn't the figure not show such an abrubt change in
The figure was just an extreme example
where motion of the turned beds had stopped and sedimentation began. Can get unconformities with
both turning and sedimentation occuring at the same time.
The issue here is time-scale. Remember, an angular unconformity will occur whenever deposition ceases for some amount of time (for example, if a submerged sequence becomes uplifted or sea level drops) and erosion is allowed to eat away at the tilted stratigraphy. If sedimentation then restarts, the resulting stratigraphy will be horizontal
in direct contrast to the tilted bedding below.
If deformation occurs together with sedimentation (penecontemporaneous) we find evidence for "growth". Evidence for synsedimentary deformation includes systematic thickness variations across structure and onlap features.
- I didn't quite grasp how "equal-area" projections preserve area, and "equal-angle" projections angular relations...
Equal-area: an object of given area on a sphere will project with the same area anywhere on the 2-D
Equal-angle: The area of a given object will not be preserved in the projection but the relative
position between objects will be maintained.
The answer here is a geometrical one. The projection of a three-dimensional feature onto a two-dimensional plane can be accomplished in a variety of ways. Two of these ways result in the stereographic projections mentioned above: equal-area and equal-angle. This is an argument best looked at visually so check out Marshak and Mitra for some further insight.
- How can one distinguish between migration of a continent and slight movement of the magnetic pole itself? Do the magnetic poles precess (?) about the axis?
There are measurable variations in the Earth's magnetic field but they are difficult to measure without an outside point of reference.
It appears that the magnetic poles do precess about an axis, and that accounts for some of the variation seen in nature in the magnetic field. To distinguish between the movement of a continent and changes in the magnetic pole, you need to compare the magnetic pole given by a rock in one continent with that of another continent. If the two polar wander paths given from these two samples show parallel tracks, then there was no relative continental migration. However, if the paths diverge at some point, then it can be inferred that there was some relative movement between these two points. This movement can then be tied into paleoplate movements.
- What are the specific properties of rock that reveal the magnetic
reversals of the earth? Does it have to do with the type or mineral?
Certain minerals have the ability to line up with a magnetic field (paramagnetism). Some minerals line up in a certain orientation and are resistant to change (ferromagnetism).
Some minerals have no preferance of orientation when exposed to a magnetic field (diamagnetism). This describes the magnetic susceptibility of a mineral.
Some sediments are able to line up. Metamorphism can change the magnetic alignment of minerals.
There are minerals in rocks of basaltic composition that align themselves with magnetic fields. An example of these are iron-containing minerals such as magnetite (Fe3O4). These ferromagentic minerals crystallize from magma at mid-ocean ridges and "freeze" a specific magnetic orientation into the rock. Then, when ships go along dragging a magnetometer, they can measure this orientation and see reversals in the rocks.
Depends on magnetic susceptability of a material which is strongly related to content of small amounts of magnetite and haematite and describes the strength of the induced magnetic field strength when an external field is applied. Above the Curie temperature (580°C - magnetite, 680° haematite) the orientation gets erased. Also sedimentary and chemical remanent magnetism is possible.
- Why will compressional (P) waves always travel faster than shear (S) waves?
Compressional waves move with the wave propagation. Shear waves move perpendicular to wave propagation.
Following this observation, it may be easier to understand why p-waves move faster than s-waves in a given medium. P-waves rely on small translations in the rock to propagate through a given medium; by contrast, s-waves rely on larger translations to propagate. Thus, it takes longer for s-waves to travel (more "distance" to cover) than p-waves. It is easier to represent this visually
check out Twiss and Moores again or ask one of us to draw you a picture.
- How important is it to understand the geophysical methods
described? Is it good enough to simply know that these methods exist?
Understand what information can be gotten from the methods and how the data can be used.
- Could you explain migration? I didn't understand the books
See textbook by Fowler, C.M.R., The Solid Earth - An Introduction to Global Geophysics, p. 144-145 for a basic explanation of migration problems and how one goes about fixing them.
- I'm unclear as to why low-velocity layers cannot be deteced in
seismic refraction surveys.
If a low-velocity layer (seismic velocities decreases with depth) there will be no critically refracted head wave from the interface.