Using Apatite Fission Track Thermochronology to date Cenozoic Range Growth:

Examples from NW Argentina

Edward Sobel

OUTLINE

•  Introduction to fission track dating •  Orographic Barriers •  Plateau-margin Topography •  Marginal Sedimentary Basins

~1 Ma offset Central Naryn Fault along the Kadjerty river, Tien Shan

after Thompson et al., 2002

Neogene ss

Neogene ss

QIII(2) terrace

gravel

gravel

26 ± 2 Ma

112 ± 8 Ma

Maidan fault, Tian Shan

Fission track methodology

~10 µm

c axis

Fission tracks on an internal surface of an apatite crystal

.

c axis

Redrawn after Brown, Summerfield and Gleadow, 1994

accumulation of spontaneous fission tracksin apatite crystal

SurfaceConfined

polished section through crystal

spontaneous tracks etched

mica external detector attached

thermal neutron irradiation

induced fission tracks register in detector

induced fission tracks etched only in detector

From Gallagher, Brown and Johnson, 1998

Track Production (1)

From Gallagher, Brown and Johnson, 1998

Plan view ofseveral crystals

Mirrorimage

Grain mountshowing spontaneous tracks

in the individual apatite grains

External detector mountshowing induced tracksdefining grain outlines

A

B C

A'

B'C'

Track Production (2)

Mount (apatite) Print (mica)

200

100

60

40

20

-2

300

150

20

2 6 10 15

50 25 10 715 8 6%

Precision Index (X)

% Error on Age

200

100

60

40

300

150

20

2 6 10 15

50 25 10 715 8 6%

(b) Tightly ClusteredSingle Grain Ages

200

100

60

40

300

150

20

2 6 10 15

50 25 10 715 8 6%

(c) Broad Spread in SingleGrain Ages, with Young Cluster

(a) Reading Single GrainAges and Their Uncertainties

Single GrainFT Age,

RadialScale(Ma)

Read theAge Here

±2σ confidenceinterval

lower preci-sion grain

symbol for deposi-tional age range

all single grain ageshave ±2σ error bars ofthis length, which arenormally not shown

Almost all grains clusterwithin ±2σ swath(statistically concordent,passes chi square test)

Youngest grainscluster within ±2σswath

±2σ swath of oldestpopulation of grainsin sample

higher precisiongrain

±2σ confidenceinterval

Read theAge Here

Dumitru, 2000

Radial Plots

Precision

2 6 10 15

50 25 10 715 8 6%

300200150100604020

Confined lengthsReflected light Transmitted light

10

20

30

40

5 10 15 20

10.92 ± 0.16 µm1.64 S.D.n = 100

Track Length (µm)

Dumitru, 2000

Otway Basin Fission Track Data

. .

45°C

Outcrop(10°C)

86°CClustered

80-90°C

0 10 20

>≈110°CToo fewtracks tomeasure

Track Length(µm)

102°-110°C

90-101°C

60-70°C

40-50°C

0

20

40

0

20

40

0

20

40

Outcrop(10°C)

0

20

40

0

20

40

0

20

40N

umbe

r of T

rack

s (N

orm

aliz

ed to

100

)

0

100

50

Sample DownholeTemperature (°C)

0 50 100 150Sample Fission Track Age (Ma)

0 5 10 15Mean Track Length (µm)

Track-LengthHistograms

Histograms of Single-Grain Ages, with

Age Spectra Curves

92°CSpread

73°C12

109°C

2468

0

2000 100

2468

0

(Extrapolated)

>125°CClustered

20

Single Grain Age (Ma)

150

200

Observed SampleFission-Track Ages

- 202

>125°C10080

125150

175

65504030

2010

410

0 20 40

- 202

109°C10080

125150

175

65504030

2010

410

- 202

92°C10080

125150

175

65504030

2010

410

- 202

86°C10080

125150

175

65504030

20

- 202

73°C10080

125150

175

65504030

20

- 202

45°C10080

125150

175

65504030

20

- 202

11°C10080

125150

175

65504030

20

Precision Index

Radial Plotsof Single-

Grain Ages

2468

0

2468

0

(Extrapolated)

0

20

40

0

20

40

0

20

40

0

20

40

Clustered

Well Clus-tered Single-Grain Ages

Clustered

Num

ber o

f Gra

ins

(Nor

mal

ized

to 2

0)

Single-Grain Age

Broadly SpreadSingle-Grain Ages

Error bars ±2σ

Observed SampleMean Fission-Track Lengths

FT P

artia

l Ret

entio

n Zo

ne (a

lso

know

n as

Parti

al A

nnea

ling

Zone

)

Very BroadSpread in Single-

Grain FT Ages[P(χ2)<0.1%]

WellClusteredSingle-Grain FTAges[P(χ2)>5%]

All Grains Have Zero Age(Tracks Totally Erased)(Extrapolated)

No Tracks at All(Extrapolated)

F i s s i o n T r a c k D a t a

Tc Ta

Schematic exhumation

Base of PAZ

T1young old

Base ofexhumedPAZ

apparent exhumation rate

Age Elevation

60˚C

110˚C

conceptual basis for 'exhumed'partial annealing zone (PAZ)

100˚C

'exhumed'PAZ

blockingtemperature

PAZ modernPAZ

Age (Ma)

50

0

40

20

0

50

40

0

2010

break inslope

time1 time2(=time1 + 20 My)

break inslope

Conceptual basis for exhumed partial annealing zone (PAZ)

Denali

64

161310

119 86

65

565

6

Mt McKinley (Denali) Apatite FT ages (Ma)

Slide courtesy of P. Fitzgerald

Simple bedrock exhumation

Apatite age profile collected from the west flank of Denali. The * marks the onset of rapid cooling brought about by denudation. Fitzgerald

et al., 1995

~2000 m/Ma or

~2 mm/yr

Mean length (µm) Standard deviation

Influence of Orographic Barriers

Digital elevation model of the NW Argentine Andes with superposed precipitation patterns

Strecker et al., 2007

Effect of an orographic barrier on exhumation: The Sierras Pampeanas

Modified from Trauth et al., 2000 Modified after Sobel and Strecker 2003

Precipitation (mm/yr)

3 Landscape Views

N S

N S

NS

Santa Maria basin

Santa Maria basin

Campo Arenal

Sierra Aconquija

Cumbres Calchaquies transect

transect

A

B

C

Sobel and Strecker, 2003

Modified from Kleinert and Strecker, 2001

Santa Maria Basin Strata

Summary of AFT results

.

2 3 4 5 6 7 80 12

3

4

5

Age (Ma)

length (µm)

Age (Ma)9080706050

151413121110 321Dpar (µm)

Elev

atio

n (k

m)

curve fit excludes the highest sample

Cumbres CalchaquíesAconquija

2

3

4

5

Elev

atio

n (k

m)

Cumbres CalchaquíesAconquija

Sobel and Strecker, 2003

~1.5 km/Ma

CCA1

Loebens et al., 2013

Newer Acon results

pseudosection: orange line: ZHe data offset by 3.5 km to match AFT data using 20°C/km geotherm. Blue line – minimum offfset 35°C/km geotherm

Loebens et al., 2013

Newer Acon results

pseudosection: orange line: ZHe data offset by 3.5 km to match AFT data using 20°C/km geotherm. Blue line – minimum offfset 35°C/km geotherm

Cumbres Calchaquies

AFTSolve model

Tmax = 84°C

Sobel and Strecker, 2003

.

0

25

50

75

100

125

150

175

200

020406080100120

oldesttrack

modelage

TA

CCA1

Age (my)

Wt% Cl = 0.005

T (°

C)

key

TA

temperatureconstraint

best fit

good fit(length and age ≥ 0.5

acceptable fit(length and age ≥ 0.05

Total Annealingtemperature

Schematic evolution of Cumbres Calchaquíes

Present

3 Ma

~5 Ma

~13 to ~6 Ma

rainshadow created

1st basement-derived conglomerates

Terrestrial deposits

4800 m asl

~2000 - 2500 m asl

>1600 m exhumation

150 to 1000 m asl

Sobel and Strecker, 2003