From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora,...

From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora,

Mexico

Christy B. TillChristy B. TillPhillip B. GansFrank J. Spera

University of CA, Santa Barbara

Research Questions 1.How does the character of

volcanism change as the tectonic setting changes?

• Distinct changes in major element chemistry? Trace element?

• Timescale for geochemical change?

• Is change sharp or progressive?

2.Do extant petrotectonic models explain the changes we observe?

Why Sonora?

(Atwater, 2004)

RTJ

RTJ

Why Sonora?

Why Sonora?

Volcanic Sections From Regions Sampled

Increasing Distance from Gulf of California EastWest

Subduction-related volcanism

(modified from Francis, 1993)

(modified from Wilson, 1989)

Rift-related volcanism

(modified from Francis, 1993)

(Atwater, 2004)

(Wilson, 1989)

(modified from Best &Christiansen, 2001)

Picro-basalt

BasaltBasalticandesite

AndesiteDacite

Rhyolite

Trachyte

TrachydaciteTrachy-andesite

Basaltictrachy-andesiteTrachy-

basalt

TephriteBasanite

Phono-Tephrite

Tephri-phonolite

Phonolite

Foidite

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

Na2O+K2O

SiO2

Major Element Geochemistry

Alk MgO

FeO*

Calc-Alkaline

Tholeiitic

Alk MgO

FeO*

Calc-Alkaline

Tholeiitic

Major Element Geochemistry

The major element geochemistry reveals little or no change after subduction shuts off.

What does the trace element geochemistry tell us?

1

10

100

1000

Rb Ba Th U Nb Ta K La Ce Sr Nd Sm Zr Ti Gd Y

Rock/Chondrites Sun 1980

Trace Element Geochemistry

All andesites & basaltic andesites from Sonora

1

10

100

1000




enriched in LILS

depleted in HFS

NVZ Andes




1

10

100

1000





1

10

100

1000


Rock/Chondrites Sun 1980 Less enriched in LILS

Less depleted in HFS


gre

ate

r a

rc s

ign

atu

re

La/Nb through time


gre

ate

r a

rc s

ign

atu

re

La/Nb through time

eastern

SSU

SL

SLcoast

coast

SSU


0 5 10 15 20 25 300

5

10

15

20

Rb/Nb

age

0 5 10 15 20 25 300

100

200

300

400

Ba/Nb

age

0 10 20 301

3

5

7

9

La/Nb

age

0 5 10 15 20 25 300

10

20

30

40

50

La/Yb

age

Geochemical Chronology

Until 15 Ma: subduction & arc volcanism migrates west

15 - 12.5 Ma: subduction of very young hot slab, no volcanism

12.5 - 8 Ma: progressive change from subduction-related toward rift-related volcanism

How does this compare to models?

Passive Rifting/Slab Roll Back Model

Modified from Lawton & McMillan, 2000

Phase 1



Phase 2



Phase 2

Thick, cold, old slab required



Phase 3



Phase 3

never get Phase 3 volcanism



Phase 3

Futu

re G

ulf

rift

Active Rifting/Slab Assimilation Model

Phase 1

Concept from Severinghaus & Atwater, 1990 & Atwater, 1989


Phase 2



Phase 2


arc migrates west


Phase 3



Phase 4



Phase 4


Futu

re G

ulf

rift

• A geochemical change in Sonora is only evident on the trace element scale

• Geochemical change in Sonora is progressive and spans

several million years (time scale to flush mantle wedge)

• Sonora does not resemble petrotectonic models:

•Age/temp wrong for slab roll back•Arc migrates wrong direction

• Need to be cautious when interpreting historical geochemistry

Conclusions

From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora,...

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