3. Spreading centres 2 Flashcards
What was Marie Tharp’s work, why is it important?
Marie Tharp created a series of physiographic diagrams of the configurations of transform valleys and median valleys in th eatlantic that were later explained by the seafloor spreading hypothesis.
Data was confidential sounding data collected by the US Navy as well as public data from the academic research fleets.
She had an important role in the development of plate tectonics
How did using widely spaced lines of soundings help Marie Tharp?
She was able to interpolate betwwen those lines because the valleys are so prominant
This may have not been possible if erosion was as effective as it is on the continents
A more dissected terrain typical of the continets may have resulted ina more chaotic map and thus features such as transform valleys would have been more difficult to recognise
Lynn Sykes (1967) located earthquakes along the ridge nd found that focal mechanisms in the transform valleys are typically strike-slip, with a sense expected from seafloor spreading. How do we locate earthquakes?
Because P-wave velocities are always greater than S-wave velocities for a given rock, the P-wave arrives first
The arrival time difference between P and S waves increases with range in a way that can be predicted form Earth velocity structure.
V=delta(X)/delta(t) X= distance from earthquake
We work out the distance from each seismometer to the source from the P-S time, then work out the most likely source location
At present this is accurate to within 5km for magnitudes >5.0 and ~50km for magnitudes < 4.0
Focal mechanisms are constructed by projecting the first motions onto a sphere centred on the source. The focal mechanism is found by fitting two nodal planes to those data so that all dilational and compressional first arrivals fall into different quadrants
Explain the Richter scale
The Richter scale uses the amplitude of ground movement caused by the S-waves. Diagrams or equations can be used to correct for distance to the source (waves get smaller because of spreading of the wave front and attenuation. The result is local magnitude
Whats the alterative to the Richter scale? and why is it better?
'’seismic moment’’ is a better measure of solid-earth movement
It is called ‘‘moment’‘because it is effectively a measure of the force (or stress) applied to the host rock when the rupture occurs, multiplied by the distance over which that force operates, (because of the use of the shear modulus)
How do we know a lot of plate spreading occurs without earthquakes?
Solomon et al. (1988) modelled the shape of the first seismic waves to work out earthquake depths and found that theu tended to be deeper at slower spreading rates
They also worked out how much seismic moment would occur if all the plate motion were to occur during earthquakes, using the known spreading rate adn geometry of faults. Their results are much larger than the observed rate of seismic moment release even if we correct the earthquake occurences for the many small earthquakes that were not recorded.
What did Cowie (1998)&(1993) propose about plate movement?
Cowie proposed the majority of plate movement occurs magmatically, i.e. by intrusion of dykes, and the remainder is seismic coupling (earthquakes)
Coupling appears high, maybe 100%, at Mid-Atlantic Ridge whereas there is almost no coupling at the East Pacific Rise (very few earthquakes on maps of seismicity)
Why?
The thickness of the brittle part of the lithosphere is affected above by fluid pressure, hence it is limited by temperature Tu. At depth is it limited by the ductile-brittle transition Tl
the gap between the two is significant at slow spreading ridges such as the MAR, but may be non existant at fast-spreading ridges such as the EPR
Why do the Mid-Atlantic Ridge and other slow-spreading ridges have deep axial valleys surrounded by elevated crestal mountains to either side?
- As the mantle rises slowly, it is cooled by conduction of heat and transfer of melt
- Tapponier (1978) suggested:
- the rifting produces a ‘steady state necking’ of the lithosphere, producing a rift valley
- The hole created in the lithosphere is like an elastic plate that uplift occurs of a wide area, hence uplifting the crestal mountains
- Sleep (1969) suggested:
- the axial valley is produced by the loss of the pressure in the mantle as it rises beneath the axis
- the rise could be occuring in a narrow conduit, but there is no evidence
Describe the faults at spreading ridges
Earthquake focal mechanisms and interprettion of bathymetry from Deeptow sonar -> normal faults appeared to be steeply dipping with an average 45 degree dip
How are steeply dipping normal faults at mid-ocean ridges distributed? Why?
Eponentially
* The elongate geometry of weak brittle lithosphere fosters elongation of faults adn fault linkage
* Limited time in the extensional zone before locking
* Small thickness of the brittle layer
Explain the great complexity found at ‘‘tectonic windows’’
- Corrugated slip surface
- Streaks of high and low backscatter in TOBI sidescan sonar images due to mud filling corrugations
- faults probably high angle in the crust, rotating as they emerge at the seabed - hence no low-angle earthquakes
- zones of gabbro and peridotite altered to produce weak minerals, e.g. serpentinite containing serpentine and talc, forming steeply dipping plane of weakness
- As these faults slip, the footwall rotates, leaving the corrugated surface with a low dip at the seabed
- This resolves the dispcrepency between the earthquake data, which suggests there are no low-angle faults, and their seabed outcrops, which are shallow dipping
What is notable about the way hydrate dmantle rocks deform?
Hydrated mantle rocks tend to compact when deformed (non-dilatant deformation) - increase in pore pressure - makes rock weaker - promoting further movement
More commonly, rocks tend to dilate, decreasing pore fluid pressure and making the rock stronger
What could explain breccias recovered in off-axis drill sites?
Bathymetry of 0.5-1m resolution of the Mid-Atlantic Ridge revealed a series of landslides
Large rockslide removing block of normal fault escarpment
