(1) Web design: "Structural geology of southwest U.S. and northwest Mexico" preceptorship vs. paid? Contact: Stuart GlogoffManager, Distributed Learning Projects
[email protected] (520) 626-5347
(2) -Assist in paleomagnetic laboratory, Geosciences- Paid position, start ASAP- Contact Dr. Bob Butler ([email protected]); 621-2324- second-year student preferred
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Deformation, Metamorphism, and Time
A major goal of structural geologists: to decipher magnitude and timing of deformation- history!
How much and when were rocks buried to depth?When were rocks deformed?
When were rocks metamorphosed?When were rocks brought up from depth (exhumed)?
How fast?How did this all happen?
To get at displacement on BIG structures- need to know depths/temperatures from which rocks
were brought up- thermobarometry
To get at timing- need geochronology and thermochronology
Geothermal gradient: T increase with depth
Geothermal gradients in different tectonic regimes
Some rocks get subducted deep into the mantle- ultra-high pressure metamorphism and diamonds
An introduction to metamorphic facies
mineral assemblages in rocks vary as a function of pressure, temperature, composition, and fluid comp.
greenschist "low grade": chlorite, epidote, actinolite
amphibolite: hornblende, maybe
mod. to high T
granulite: two types of pyroxenes--- very high T
blueschist: glaucophane, jadeite,
eclogite: garnet + pyroxene
High T and P
Folds in eclogite:
green (pyroxene) and red (garnet)
Mineral assemblages can give range of P-T
conditions. But we want to do better!! HOW?
Thermobarometry: Quantitative determination of temperature (T) and pressure (P) using
Example: kyanite, andalusite, and sillimanite have same composition but different crystal
structure- function of T and P
One reaction yields one line. To determine a T and P point, at least one other reaction is needed
Fortunately, there are tons of
reactions that are useful for
constraining T and P
A real example- with real
The concept of a P-T path and
P-T paths for deeply buried, then exhumed
Linking Deformation with Metamorphism
So far, we known how to determine P and T and timing of metamorphism relative to deformation
What about precise timing??
Exactly when? How fast or slow?
Isotopes: Elements with different numbers of neutrons
Radioactive isotopes: are unstable- they decay with time to another isotope. This decay rate has been
constant throughout the history of the Universe.
isotopes can be removed from mineral grain(s) by many methods: dissolved out using acids, burned out in a
furnace, blasted out using a laser, or tickled out using an ion beam
Isotopic abundances (more often, ratios) are measured with a mass spectrometer
In a mass spectrometer isotopes of different massesare separated using a magnet and collected & counted
With modern technology, it is possible to determine agesfor little spots in a single grain. Way cool!!
Also cool, is that different minerals loose daughterproducts due to diffusion at different temperatures.
Some minerals like to keep the daughter products, even at high T.
Other minerals loose daughter products, even at lowT.
Closure temperature: Temperature below which a mineral will not loose daughter products. At higher T,daughter products will "run-away".
THERMOCHRONOLOGY: determining the time when a rock was at a certain temperature
Calculated cooling history for a granite in New Zealand
An attempt at putting it all together (structure, metamorphism, and time)- an example from Tibet
Regional Geologic Setting
Fault places low-grade limestones on top of a ledge of cataclasite (fault rock)
Structural studies suggest that the fault is a normal fault, where the hanging wall moved to
the east relative to the footwall
Footwall rocks include blueschists + greenschists and amphibolites.
But more precisely what P and T are the blueschists?
Yikes! Thermobarometry suggests ~500 C
at 14 kbar (50 km!!)
Did the normal fault exhume the blueschists from this great depth?
Mylonites in the footwall of the normal fault are amphibolite facies.
Here's what they look like under the microscope
The shear zone was active at ~11
kbar (~40 km)- probably cuts
the entire crust!
The fault cuts granites and the shear zone is
intruded by undeformed granite
When was the fault active?
before 204 Ma and after 220 Ma
Thermochronology suggests rocks were
exhumed from >35 km depth in <10 Ma!!!!!!!!!