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)