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More people die on dusty days(Thorax, 2013)
How do we date landscapesand measure erosion rates?
slope profile evolution by diffusion
erosion
convex upwards curvature
concave upwards curvature
deposition
Applied to dated fault scarps
Talus wedge
VE = 2
Scarp diffusion methods:1) midpoint method
Erf:
Scarp diffusion methods:1) full-fit method (Pelletier et al., 2006)
Which scarp is younger? What are the approximate ages of the scarps? Do they represent one earthquake or several?
Please read: Pearthree and Calvo (1987)
Radio-isotope chronometers
“Exposure” chronometers
CosmoIsotopeproductionversus depth
Gosseand Phillips,2001
The case of glacial erosion
TCN Accumulation
Stable TCN – linear increase Radioactive TCN – initial increase to steady state
Con
cent
rati
on (
atom
s/g)
Time
3 He, 21 Ne
N P
(1 e t )
t ln 1 N
P
.
N=concentrationP=production rate=decay constantT=time
Exposure dating requires:
(1) TCN Production increases with latitude.
(2) TCN production increases/decreases with changes in geomagnetic field.
(3) TCN Production increases with elevation.
Sea Level
50,000 m
Shielding of cosmic rays by surrounding topography
Production Rates
- Production rates vary with TCN and mineral type.
- Target atoms have to be bigger than spallation product.
TCN Prod. Rate
(at/g/yr) target3He 115 olivine21Ne 21, 38 Qtz, Ol10Be 6 quartz14C 15 quartz26Al 37 quartz36Cl 67, 137 Ca, K
Production (and accumulation) of TCN also affected by:(1) self-shielding(2) Topographic shielding(3) Erosion(4) Burial
Uncertainties in TCN dating:(5) Calibration/measurement of production rates.
(1) Changes in geomagnetic field over time, particularly Holocene.
(6) Previous exposure.
With constant exposureratio of isotope production eventually decreases
Upon burial or shieldingratio decreases below the constant exposure line
Sampling Strategies :- surface stability (i.e., desert pavements, desert varnish).- Highest, flattest surface on deposit.- Largest, flattest boulder on deposit.
Sample Preparation- crush rocks - Physical and chemical mineral-separation processes.- 3He, 21Ne: melt mineral at 1400 C under vacuum, measure gas on
mass spectrometer.- Radioactive TCN: chemical processes to extract element of
interest. Isotopic ratios measured on AMS.
Applications of TCN: dating surfaces, estimate rates of geomorphic processes.
(1) Estimating Fault Displacement Rates.
0
5
10
15
20
25
30
050100150200250300
Fault Scarp Profile of 75 ka Alluvial Surfacenear the Bar Ten flow
(Whitmore 98529-3)
Distance (m)
3.5
21.75
4.2
vertical displacement = 7 m
Displacement Rates on the Toroweap and Hurricane faults
Thermoluminescence /Optically stimulated luminescence
Background
TL/OSL measurement
TL ‘saturation’
Uranium-series dating I
U-238
Po-210Pb-206 Pb-210
U-234
Rn-222
Th-230 Ra-226
(stable)
4.5 x 109
years years
days
years
years days
2.5 x 105 7.5 x 104
22 3.8138
1.6 x 103
years
U = uranium; Th = thorium; Ra = radium; Rn = radon; Pb = lead; Po = polonium
Uranium-series dating II
U = uranium; Pa = protactinium; Th = thorium; Ra = radium; Pb = lead;
U-235 Pa-231
Pb-207
Th-227
Ra-223
(stable)
7.1 x 108
years years
3.2 x 104
19days
days
11
Blisniuk and Sharp (2003)