KaliGandaki

83o E

29oN

0 10 20 305Km

MCT

STD

Granites

Tethyan Series

Formation II/III Lesser Himalaya

Greater Himalaya

85o E

84o E

84o E

29oN29oN

28oN28oN

28oN

85o E

83o E

Kathmandu

Pokhara

Barahtpur

X10-3

1 2 3 40

0. 4

0. 8

1. 2

83∞E

29∞N

X10-3

1 2 3 40

2

4

6

8

X10-3

1 2 3 40

2

4

6

0. 2

0. 6

1

X10-3

1 2 3 40

X10-3

1 2 3 40

0. 4

0. 8

1. 2

1. 6Manang

Beni

Sample ID River Namen=# of ages

Pro

babili

ty

Age (Ga)

Key

F2

F4

F1

Marsy-andi

Seti Trisuli

Narayani

n=174

Kali Gandaki

n=101

Seti K.n=99

Marsyandin=217

K

Trisulin=96

F3

0.20.40.60.8

11.21.41.6

x 10 -3

1 2 3 400

0. 2

0. 6

1

1. 4

x 10 -3

1 2 3 40

0. 2

0. 6

1

1 2 3 40

x 10 -3

0

1

2

3

x 10 -3

1 2 3 40

1

2

1 2 3 40

x 10� -3

1 2 3 40

0. 5

1

1. 5

2

2. 5 x 10 -3

1 2 3 40

x 10 -3

0. 511. 522. 533. 5

0

1

2

1 2 3 40

x 10 -3

1 2 3 40

x 10 -3

1

2

3

4

5

1 2 3 40

x 10 -3

0

1

2

3

1 2 3 40

x 10 -3

1

2

Pro

babi

lity

Age (Ga)

Key

A

B

F

D

C

K

I

BG

E

J

E

Legend

STD (Colchen)

MCTSTD (Searle)

Formation II/IIIFormation ILesser Himalaya

Tethyan SeriesManaslu Granite

LesserHimalaya

Formation I

MCT

STDFm.II, III

TethyanSeries

Marsyandi@ FI-FIIn=105

Thoran K.Tethyan S.

n=98

Marsyandi@ MCTn=105

Chudi K.Lesser H.

n=96Lowest Marsy-

andin=217

MarsyandiTethyan S.

n=101Ghatte K.

Formation IIIn=102

Danaque K.Formation II

n=95

Syange K.Formation I

n=131

Darondi FI+ L.H. n=96

River NameLithology

n=# of ages

0 6 12 18 Km

Paudi K.Lesser H.

n=94

UCSB Construction of Foreland Mineral Populations: Insights From Mixing of U-Pb Zircon AgesWilliam H. Amidon, Douglas W. Burbank, George E. Gehrels^ * Department of Geological Sciences, University of California Santa Barbara

^Department of Geosciences, University of Arizona**Many detrital mineral studies assume that each contributing source region has equal potential to contribute heavy minerals such that the proportion of detrital minerals from a specific source is proportional to its exposure area. To test this assumption, this study evaluates the importance of three fundamental variables on construction of the foreland mineral population: exposure area of a given source lithology; concentration of heavy minerals in source rock; and variable erosion rates within the source area. We use U-Pb zircon ages from 15 samples of modern river sediment in the Marsyandi drainage of central Nepal to trace zircons from specific source lithologies within the Himalaya through the fluvial system and into the foreland. The U-Pb zircon-age signature of each lithology is assessed through samples of tributary catchments dominated by single lithologies, whereas trunk-river samples reflect the relative importance of contributions from each lithology. The relative contributions of zircon from each lithology are calculated by iterative and inverse statistical techniques developed by Amidon et al. (in review). A simple model is then constructed in which the observed zircon mixing proportions are compared with the mixing proportions predicted based on the relative exposure area and estimated zircon concentration of each lithology. Relative erosion rates are then modeled which account for the discrepancies between observed and predicted zircon mixing ratios. These predicted erosion rates are compared with known erosional proxies to evaluate the validity of the model. One important result is that although the Manaslu Granite comprises up to 12% of the drainage area upstream of some samples, no zircons reflecting its Miocene age are recovered in detrital samples because of its low zircon concentration. Results also show that in the Marsyandi drainage, higher erosion rates offset low zircon concentrations in the Lesser Himalaya, while lower erosion rates offset higher zircon concentrations in the Greater and Tethyan Himalaya to create a consistent flux of zircons from all regions of the drainage. We also compare U-Pb zircon age distributions from four adjacent drainages in central Nepal spanning ~250 km along the Himalayan front, and a fifth sample downstream of their confluence. Age distributions for each sample were compared using the Kolmogorov-Smirnov test, and zircon mixing proportions to evaluate the spatial homogeneity of ages contributed from different regions of the mountain range. Results reveal relative consistency in age distributions across the mountain front, but show that the Tethyan series is under-represented in several samples due to low erosion rates on the southern edge of the Tibetan Plateau.

Overview

[P(A)-P(B)]φ

[(P

(C)-

P(B

)]

***

*

*

*

*

*

*

*

*

*

**

**

***

*

* predictedrelationship

actual relationshipat each age (i)

Observed PDF A

+ =φ* (1-φ)*P(A) P(B) P(C)art

Artificial PDF C Observed PDF B

Age (Ma) Age (Ma)Age (Ma)

P PP

Area of Mismatch

Artificial P(C)

Observed P(C)

Age (Ma)

P

1. Calculating the proportion of zircon derived from each lithology

First, age probability density functions (PDFs) are defined from 100+ U-Pb zircon ages for each sample. If: (1) the PDFs are accurately defined; and (2) the downstream sample is a mixture of zircons derived only from upstream lithologies, then the mixing relationship is defined by:

φP(A) + (1-φ)P(B) = P(C)where PDFs A and B define upstream lithologies, C is a downstream mix, and φ is a mixing proportion such that 0<φ<1. The mixing proportions can be estimated using either an iterative or inverse approach as described below.

Iterative Approach: Combines the component PDFs in every possible combination until the mixing proportions are found that minimize the % area mismatch between the observed mixed PDF and the artificial mixed PDF.

Inverse Approach: The above equality is rewritten in the form Ax=B:

[P(A)-P(B)]φ=[P(C)-P(B)]and PDFs are expressed as vectors in which each 1 Ma age increment has an associated probability value. A least-squares inversion determines the best estimate of φ by minimizing the sum of the squared difference between the two sides of the equality for all age increments.

2. Estimating Zircon Concentration 1. Separate all zircons from a known mass of rock or sediment

2. Weigh the resulting heavy mineral concentrate and 2-3 aliquots

3. Digitally photograph, measure and count all zircons in aliquot

4. Determine the proportion of zircon >60 µm in width for each sample

5. Estimate total number of >60 µm zircons in original samples and

the number of datable zircon grains per gram of rock or sediment

6. Verify counting results by independently estimating concentration

Conc.=6

(Aa*Ca)/[(Ab*Cb)+(Aa*Ca)]=.5(Ab*Cb)/[(Ab*Cb)+(Aa*Ca)]=.5

Aa*Ca=.1875Ab*Cb=.1875

Combined Mixing Prediction:1a:1b

Age (Ma)

Pro

babi

lity a

Age (Ma)

Pro

babi

lity b Age (Ma)

1a:1bc

30 km2

10 km2 Aa=.75Ab=.25

a

b

c

1. Make an Area Prediction:

2. Make a Concentration Prediction:

3. Combine the Predictions:

4. Compare predicted mixing proportions with Observed Mixing of U-Pb age distributions:

2

1

3

Aa=.25Ab=.75

a

b

c

Conc.=2

3. Predicting the proportion of zircon derived from each

lithology

0 1000 2000 3000 4000

0. 10. 20. 30. 40. 50. 60. 70. 80. 91

Marsyandi (K)

Kali Gandaki (F3)Narayani (F4)

Seti (F2)

Trisuli (F1)

Age (Ma)

Cum

ulat

ive

Pro

babi

lity

4. Observed U-Pb zircon ages in the Narayani watershed

5. Observed U-Pb zircon ages in the Marsyandi drainage

ALithologic area

prediction

Contributingareas

Sample Site

Attributes

BArea plus zirconconcentration

prediction

CObserved frommixing of U-Pb

ages

DRelative

erosion rates

FRelative

erosion rates(FII = Ord. TSS)

EArea plus zirconconc. prediction(FII = Ord. TSS)

Sample E(FI-FII contact)

Sample G (MCT)

Sample K(Lowest Mars.)

Sample G (MCT)

Sample K(Lowest Mars.)

Sample K(Lowest Mars.)

Tethyan Series

Lesser Himalaya

Formation I

Manaslu Granite

Formation II/III

Mixed Upstream Lithologies

Sample J (Darondi)

(.76)

(.11)

(.25)

(.19)

(.75)

(.81)

(.17)(.10)(.10)

(.63)

(.25)

(.00)

(.74)

(.02)

(.00)

(.32)

(.68)

(.00)(.02)

(.12)

(.19)

(.46)(.35)

(.19)

(.44)(.56)

(.76)

(.24)(.32)

(.24)

(.44)(.32)

(.14)

(.54)

(.30)(.22)

(.06)(.05)

(.36)

(.32) (.35) (.31) (.34) (.35)

(.65)

(.45)

(.01) (.00)

(.35)

(.19)

(.61)

(.20)

(.20)

(.68)

(.32) (.32)

(.12)

(.00)

(.23)(.33)

(.36)

(.20)

(.60)

(.19) (.21)

(.41) (.45)

(.13)

(.05)

(.78)

(.00) (.05)(.12)

(.28) (.20)

(.52)

(.22) (.26)

(.52)

(.40)

(.60)

(.56)

(.12)

(.15)

(.40)(.28) (.20)

(.52)

(.21) (.16)

(.63)

(.28)

(.62)

(.18)

(.00)

(.43)(.57)

(.74)

(.26)

(.41)

(.59)

(.24) (.22)

(.00)

(.31)

(.13)

(.00)

(.05)(.00)

(.29)

(.00) (.00)

(.53)

(.21) (.26)

(.07)

(.86)

(.00)(.07)

(.00)

(.42)

(.30)

(.00)

(.46)(.54)

.98

.95

(.71)

E JK3K2K1G2G1

To test the assumption that the proportion of detrital zircons derived from a given lithology is proportional to the upstream exposure area of that lithology, we compare: (A) area based mixing predictions; and (B) combined predictions based on area and zircon concentration; with (C) observed zircon mixing proportions for four Marsyandi samples (E,G,K, and J). Row D shows the lithology-specific relative erosion rates that would create a match between the predicted and observed mixing proportions. Three important results are that: (1) Formation II/III zircons are over-represented; (2) Manaslu Granite zircons are not represented despite accounting for up to 12% of upstream drainage area; and (3) modeled erosion rates are consistently ~2-fold higher in the Lesser Himalaya than in the Greater and Tethyan Himalaya.

7. Comparison of predicted and observed zircon mixing proportions and modeled erosion rates

F1(Trisuli)

F4(Narayani)

F3(Kali Gandaki)

F2 (Seti)

K(Marsyandi)

? ?

(.17)

(.08)(.32)(.24)

(.06)

(.38) (.31)(.35)(.34)

(.20)

(.56)

(.24)(.16)

(.04)

(.42)(.39)

(.18)

(.01)

(.33)

(.20)(.12)

(.35)

(.54)

(.23)

(.57)

(.21)(.22)

(.62)

(.07)(.01)

(.30)

(.76)

(.12)(.04)

(.37)(.32)

(.11)

Key

Teth

yan

Ser

ies

Less

er H

imal

aya

For

mat

ion

I

For

mat

ion

II/III

(.xx)(.xx)(.xx)

(.xx)

ProportionsALithologic area

prediction

BObserved frommixing of U-Pb

ages

AttributesSample Site

(.06)

Contributingareas

eastwest

8. Predicted and observed zircon mixing proportions for four adjacent drainages in the Narayani watershed

Comparison of zircon mixing predictions based on lithologic exposure area with observed zircon mixing proportions reveals that: (1) the relative importance of Formation II/III zircons decreases from west to east reflecting the diminishing exposure area of this unit along strike of the range; and (2) Tethyan Series zircons are under-represented in the Trisuli and Kali Gandaki drainages because the Tethyan Units are exposed in the upper reaches of the drainages along the southern margin of the slowly eroding Tibetan Plateau. This demonstrates how differential erosion rates can bias the detrital mineral population.

9. Our model supports the correlation of Formation II with the Cambro-

Ordovician units of Tethyan Series

0 6 12 18 Km

A

257 km2

B

Legend

STD (Colchen)

MCTSTD (Searle)

Formation II/IIIFormation ILesser Himalaya

Tethyan SeriesManaslu Granite

813 km2

Our results show that Formation II/III zircons are over-represented in the detrital population (rows B and C, panel 7). One explanation for this is that Formation II is correlative with the Cambro-Ordovician units of the lower Tethyan Series. As shown below, this would expand the source area for the unique "Formation II/III" age distribution from 257 km2 to 813 km2. However, a larger source area alone does not obtain agreement between observed and predicted mixing ratios, a higher concentration for the source is neccessary (rows E and F, panel 7).

10. Comparison of CDFs and K-S statistics for four adjacent drainages

To evaluate the spatial consistency of detrital U-Pb zircon ages on the orogen scale, we compared PDFs, CDFs and K-S statistics for ages from four adjacent drainages spanning ~250 km along the range front, and a fifth sample from below their confluence in the foreland. Results (panel 4 and below) show that all drainage pairs are statistically similar, but slight differences exist between the eastern and western drainages. This suggests slight differences in the U-Pb age distributions derived from different regions of the orogen.

Kolmogorov-Smirnovtest statistics

Trisuli (F1)

Marsyandi (K)

Seti (F2)

Kali Gandaki (F3)

n=96 n=217 n=99 n=101Narayani (F4) n=174 0.0917 0.1208 0.0543 0.067Kali Gandaki (F3) n=101 0.1335 0.1225 0.0823Seti (F2) n=99 0.1075 0.0867Marsyandi (K) n=217 0.115

ConclusionsThe purpose of this study is to: (1) evaluate the importance of bedrock zircon concentration and differential erosion rates on construction of the foreland zircon population; and (2) to assess the spatial homogeneity of the U-Pb zircon age distribution on the orogen scale. We conclude that:

1. Our method accurately measures zircon concentration in rocks and river sediments, but lithologic concentrations are best defined in river sediments.2. Zircon concentration can strongly bias the detrital population as shown by the lack of detrital Manaslu Granite ages.3. Erosion rates exert a strong control on detrital populations as shown by: (A) the over-representation of Lesser Himalayan zircons despite low bedrock zircon concentrations; and (B) the under-representation of Tethyan Series drainages in the Trisuli and Kali Gandaki.4. We believe the apparent over-representation of Formation II/III ages suggests a correlation between FII and the lower Tethyan Series. 5. The U-Pb zircon age distribution derived from four drainages spanning ~250 km along the Himalayan front are statistically similar, suggesting the foreland population is not biased towards one region of the orogen.

Relative concentration of zircon in mono-lithologic river sediments

Formation II/III 1.0

Lesser Himalaya 3.2 1

Tethyan Series 6.5

Formation I 8.1 2.5 - 4.0

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