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Undergraduate opportunities

(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

Announcements

Bad news: It's not Thanksgiving yet

Good news: NO CLASS ON WED.

TODAY

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

garnet

mod. to high T

granulite: two types of pyroxenes--- very high T

blueschist: glaucophane, jadeite,

kyanite, lawsonite

High-P, Low-T

eclogite: garnet + pyroxene

High T and P

Folds in eclogite:

green (pyroxene) and red (garnet)

layers

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

equilibrium reactions

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

uncertainties

The concept of a P-T path and

zoned minerals

P-T paths for deeply buried, then exhumed

rocks

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

Geographic Setting

Regional Geologic Setting

Geometry

Fault places low-grade limestones on top of a ledge of cataclasite (fault rock)

Kinematics

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

Timing

When was the fault active?

before 204 Ma and after 220 Ma

Thermochronology suggests rocks were

exhumed from >35 km depth in <10 Ma!!!!!!!!!

Tectonic significance