Convergent margin volcanism three topics
Download
1 / 50

Volcanism - PowerPoint PPT Presentation


  • 354 Views
  • Updated On :

Convergent Margin Volcanism Three topics MORs versus ARCs, a fruitful comparison What is the global population of arcs like? I add a wrinkle I have been trying to become comfortable enough with to publish. Volcano spacing decreases as plate convergence rate increases.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Volcanism' - omer


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Convergent margin volcanism three topics l.jpg
Convergent Margin VolcanismThree topics

  • MORs versus ARCs, a fruitful comparison

  • What is the global population of arcs like?

    • I add a wrinkle I have been trying to become comfortable enough with to publish. Volcano spacing decreases as plate convergence rate increases.

  • Central America is interesting

    • Vents b. Links c. Ba/La (Windows) d. Galapagos

      Read everything first (slides and notes) and then select specific slides (by number) to discuss


1 why can t the arcs be more like the ridges l.jpg
1. Why can’t the arcs* be more like the ridges?

  • Whenever I think of some possible new tectonic-volcanic/geochemical relationship for Central America, I check the RIDGE site and/or review the extensive literature on Mid-ocean ridges. The global set of convergent plate margins (CPMs) or arcs seems to be more complicated than the ridges, or do the arc groups just not talk to each other enough?

  • *arcs (sensu lato - because many convergent plate margins do not have an arc shape)


Spreading rates versus convergence rates narrower distribution for convergence rates l.jpg

MOR

CPM

Frequency

30

25

20

15

10

5

0

0

20

40

60

80

100

120

140

160

"Vc (Km/Ma)

10 Km/Ma =10 mm/yr = 1 cm/yr

Spreading rates versus convergence rates:Narrower distribution for convergence rates


Structures depending on rates l.jpg
Structures depending on rates

  • The MOR morphology, structure and gravity field has an interesting dependence on spreading rate. Slow spreading (mid Atlantic) has rugged topography and an axial graben. Fast spreading (EPR) has smooth topography and an axial high or crest.

  • At ARCs there is nothing like the MOR systematics with rate. There is some dependence of volcano spacing and convergence (see below). Oblique subduction may eventually define some global patterns.


Magma chemistry and crustal thickness l.jpg
Magma chemistry and crustal thickness

  • MOR depths/crustal thickness reflect magma chemistry. The thicker the crust, the higher the degree of melting and the lower the Na2O content (Klein and Langmuir and a whole host of papers)

  • ARC crust may affect magma chemistry in a similar way but the community does not seem impressed (Plank and Langmuir proposed this using Central America as an example that works pretty well, but the community resisted this idea.) I think it is a reasonable idea


Age history l.jpg
Age/history

  • MOR - what history? The axis is zero age. Plate geometry causes ridges to form and jump. Hotspots influence ridge locations and ridge geochemistry.

  • ARC - history is vital on both plates (e.g. Hotspot chains on subducting plate commonly indent CPMs and/or shut off volcanism for a period of time).


2 what is the global population of arcs like l.jpg
2. What is the global population of arcs like?

  • There are relatively few global compilations of arc properties. The recent G-Cubed paper by Syracuse and Abers is a good start. It refers to Jarrad (1986?) who made a global compilation of arc parameters. Another useful paper is d’Bremond d’Ars et al. 1995 in JGR. They looked globally at volcano spacing and found it random, not periodic.


Spacing of volcanic centers at arcs decreases as plate convergence rate increases l.jpg

70

Island arc: no active back-arc spreading

60

Continental arc

50

Poisson Spacing (Km)

Island arc: active back-arc spreading

40

30

20

10

0

50

100

150

Plate convergence rate normal to arc (mm/yr)

Spacing of volcanic centers at arcs decreases as plate convergence rate increases

80

Uyeda and Kanamori (1979) classification

Michael J. Carr IGC G10.07 August 22, 2004


Why examine this question because volcano spacings differ significantly l.jpg
Why examine this question?Because volcano spacings () differ significantly

Central America

N

Northern Sumatra

N

= 23 Km

= 65 Km

500 Km

500 Km


Aleutian volcanoes have spacings intermediate between central america and northern sumatra l.jpg

N

Aleutian volcanoes have spacings intermediate between Central America and northern Sumatra

Aleutians

= 40 Km

500 Km


Defining volcano spacings l.jpg

Use Central America as a guide

Ignore the back-arc

Focus on the volcanic front

Define Volcanic centers

Use Smithsonian’s GVP reference list

Defining volcano spacings



Slide13 l.jpg
Why ignore the little volcanoes? conesFlux derived melts at volcanic frontDecompression melts in back-arc


A simple composite cone is a center l.jpg
A simple composite cone is a conesCenter

Agua volcano in Guatemala


A cross arc alignment is a center l.jpg
A cross-arc alignment is a conesCenter

Atitlán-Toliman-Cerro de Oro in Guatemala


Make decisions defining discrete centers l.jpg

Central America cones

Volcanic center

N

Secondary cone in a center

Holocene activity doubtful

Back-arc cone

500 Km

Make decisions defining discrete centers

Data are from Smithsonian's Global Volcanism Program


Use poisson distribution to estimate spacing l.jpg
Use Poisson distribution to estimate spacing cones

  • Calculate nearest neighbor spacing

  • Create histogram using 10 Km or 20 Km bins

  • Vary  in Poisson equation to fit histogram

Poisson is a discrete probability function

xe- 

x = 0,1,2,3,…

f(x, ) =

x!


Volcano spacing in central america 23 km l.jpg

15 cones

Poisson distribution

n=36, bin=10

 =2.3 or 23 Km

10

Frequency

5

0

0

10

20

30

40

50

60

70

80

90

100 Km

Volcano Spacings in 10 Km bins

Volcano spacing in Central America= 23 Km


Volcano spacing in kuriles kamchatka 17 km l.jpg

20 cones

Poisson distribution

n=62 bin=10

 =1.7 or 17 Km

15

Frequency

10

5

0

0

20

40

60

80

100 Km

Volcano Spacings in 10 Km bins

Volcano spacing in Kuriles-Kamchatka= 17 Km

Suggestion of a second mode at 75 Km.


Volcano spacings determined here agree with those published by d bremond d ars et al 1995 l.jpg
Volcano spacings determined here agree with those published by d’Bremond d’Ars et al.1995

80

70

60

45º

50

d'Ars et al 1995 spacing (Km)

40

30

Cascades - an outlier because d’Ars used Guffanti and Weaver’s list not Smithsonian’s

20

10

0

0

10

20

30

40

50

60

70

80

Poisson Spacing (Km)


Negative correlation between plate convergence rate normal to arc and volcano spacing l.jpg
Negative correlation between plate convergence rate normal to arc and volcano spacing

80

70

n = 15

r = -0.82

60

Marianas

Ryukyus

Tonga

ignored in regression

50

Poisson Spacing (Km)

40

30

20

10

0

0

50

100

150 km

Plate convergence rate normal to arc (mm/yr)


Why a negative correlation l.jpg
Why a negative correlation? to arc and volcano spacing

1. Raleigh-Taylor gravitational instability and diapirs

μ2

If viscosity of lower layer, μ1 << μ2 then

h

wavelength, λ ~ h(μ2/μ1)1/3 - Whitehead and Luther (1975)

μ1

Higher convergence rate could increase the thickness of the buoyant layer (h) or lowers its viscosity, μ1

Unlikely: a. effect of μ1 has to be > than effect of h b. distributions of spacings are random

2. Multiple generations of cavity plumes – d’Bremond d’Ars et al. (1995)

Higher convergence rate increases the rate of cavity plume production, resulting in closer spacings


Central america is interesting a the volcano distribution l.jpg
Central America is interesting. to arc and volcano spacinga. The volcano distribution

  • Stoiber and Carr 1973, after Sapper (1897) and Dollfus and Montserrat (1868), showed that the large volcanoes define several right-stepping lines or volcanic segments.

  • What if you look at all the volcanoes? That is, ignore size and just plot vent locations?


Volcanic segments based on centers l.jpg
Volcanic segments based on “Centers” to arc and volcano spacing


Vents 600 ka in central america l.jpg
Vents <600 ka in Central America to arc and volcano spacing


Vents younger than 600 ka with arcs l.jpg
Vents younger than 600 ka with to arc and volcano spacingarcs


Slide27 l.jpg

Distance to arc and volcano spacing

3b. To link Volcanology and geochemistry

We study the entire volcanic chain. We often plot our volcanological and geochemical data against Distance


Regularities in the distribution and geochemistry of central american volcanoes l.jpg

150 to arc and volcano spacing

100

Ba/La

7

0

El Salvador

Nicaragua

Costa Rica

6

0

50

5

0

4

0

3

0

2

0

0

1

0

0

Guatemala El Salvador Nicaragua Costa Rica

400

300

Volcano volume Km3

200

100

0

0

1000 Km

Regularities in the Distribution and Geochemistry of Central American Volcanoes

=

Zr/Nb


Volcanic front consists of right stepping lines l.jpg
Volcanic front consists of right stepping lines to arc and volcano spacing

Stoiber and Carr (1973) suggested the subducting slab was segmented but the Zr/Nb result of Bolge (2006) requires a smooth slab (e.g. Syracuse and Abers, Protti, etc) thus volcanic segments are an upper plate phenomenon


Volume distribution along volcanic front l.jpg

Guatemala El Salvador Nicaragua Costa Rica to arc and volcano spacing

Guatemala El Salvador Nicaragua Costa Rica

400

400

Atitl

Atitl

á

á

n

n

Iraz

Iraz

ú

ú

Santa Ana

Santa Ana

300

300

Masaya

Masaya

Tecapa

Tecapa

Volcano volume Km3

Barva

200

200

San

San

Crist

Crist

ó

ó

bal

bal

Rinc

Rinc

ó

ó

n

n

100

100

Mv

Arenal

0

0

0

0

500

500

1000

1000

Distance Km

Volume distribution along volcanic front

Carr et al. (2007) modified from Stoiber and Carr (1973).

This mostly ignored pattern can now be linked to the volcanic segmentation and aspects of the geochemistry.

Volcanic segments


Slide31 l.jpg

7 to arc and volcano spacing

0

El Salvador

Nicaragua

Costa Rica

6

0

5

0

b

4

0

N

/

r

Z

3

0

2

0

1

0

0

3

0

0

5

0

0

7

0

0

9

0

0

1

1

0

0

Distance along the arc (km)

Zr/Nb or Nb depletion correlates with volcanic segmentation (Bolge, 2005)

Zr/Nb decreases along each segment then steps up at the beginning of the next segment (except for Central Costa Rica, where there is no step in the volcanic line)

Yojoa-back-arc, no slab signal

0

El Salvador

Nicaragua

Costa Rica

)

m

5

0

Zr/Nb is similar to the saw-tooth pattern of depths to slab beneath volcanoes (from Syracuse and Abers, 2006).

k

(

b

a

l

s

e

1

0

0

h

t

o

t

h

t

p

1

5

0

e

D

2

0

0

3

0

0

5

0

0

7

0

0

9

0

0

1

1

0

0

Distance along the arc (km)


Volcanic segments are oblique to gently curved axis that connects the large volcanoes l.jpg

QSC to arc and volcano spacing

Volcanic segments are oblique to gently curved axis that connects the large volcanoes

Axis of volcanic productivity, similar to contours of seismic zone; 150 km in Nicaragua, 90 km contour in Costa Rica


Within the same segment magma paths vary let zr nb slab signal l.jpg

Cocos Plate to arc and volcano spacing

Sed melt

Water

Within the same segment, magma paths vary, let Zr/Nb = slab signal

NW

SE

Upper plate stress field controls where the wedge is tapped

Variable reactive path lengths

Caribbean Plate

Lower output with short path, higher slab signal

Decompression

melt

Maximum output, taps everything

Zoned region of flux melt

Lower output with long path, lower slab signal


A plausible model of zr nb variation basalt reacts with mantle during ascent l.jpg
A plausible model of Zr/Nb variation: basalt reacts with mantle during ascent

80

AFC model

Part.Coefs. for cpx

R=1

Massimilant/Mmagma=2

Cosigüina - short path

60

to DM

Zr/Nb

40

to EM

DM

Mantle compositions

Momotombo-long path

20

EM

0

0

50

100

150

200

Ba/La


New insights on volcanic segmentation l.jpg
New insights on volcanic segmentation mantle during ascent

  • Zr/Nb saw-tooth requires the smooth slab imaged in modern seismicity studies

  • Volcanic segments are upper plate structures

  • A volcano’s size depends on its location relative to melt zone

  • Nb depletion is sensitive to depth to the slab

  • Need to know: What causes the segments?


3c what causes the regional variation in slab signal ba la l.jpg

Guatemala | El Salvador | Nicaragua mantle during ascent| Costa Rica

150

100

Ba/La

50

0

0

500

1000 Km

Distance

3c. What causes the regional variation in Slab signal (Ba/La)?

DSDP 495

DSDP 1039


Incoming sedimentary sections are similar but substantial unmeasured variation may exist l.jpg

e mantle during ascent

Incoming sedimentary sections are similar but substantial unmeasured variation may exist


Dsdp 495 sediment and morb l.jpg
DSDP 495 sediment and MORB mantle during ascent

Low variance

maximum in carbonate

maximum in hemipelagic

High variance

---------Local---------

--------Regional--------


See regional variation if sediments are similar see local variation if sediments differ l.jpg
See regional variation if sediments are similar mantle during ascentSee local variation if sediments differ

Note parallel arrays in local variation

TWO DIFFERENT WINDOWS!!


La carries the regional signal not ba l.jpg

Guatemala El Salvador Nicaragua Costa Rica mantle during ascent

1

5

0

S

i

O

<

55

w

t

.

%

2

1

0

0

0

B

a

1

0

0

B

a

/

L

a

S

i

O

<

55

w

t

.

%

2

5

0

0

0

.

4

5

0

1

/

L

a

0

0

0

.

2

0

5

0

0

1

0

0

0 Km

5

0

0

1

0

0

0 Km

D

i

s

t

a

n

c

e

D

i

s

t

a

n

c

e

0

.

0

La carries the regional signal, not Ba

Black crosses are estimated mantle contributions


Eiler et al 2005 strong evidence for a serpentine component in nicaragua from 18 o data l.jpg

serpentine mantle during ascent

Eiler et al. 2005, strong evidence for a serpentine component in Nicaragua from 18O data

carbonate sed


Slide42 l.jpg

Iraz mantle during ascentú-Turrialba volcanic center Costa Rica

Turrialba

569±6 ka

Irazú

136±5 ka

594±16 ka

855±6 ka pre Irazú


Interplay of geology and geochronology improved both age and volume estimates l.jpg
Interplay of geology and geochronology mantle during ascentimproved both age and volume estimates




Masaya volcano nicaragua mantle contribution 7 5 melt of dm l.jpg

Masaya contribution

100

10

7.5% melt of DM source

1

Cs

Rb

Ba

Th

U

Nb

Ta

K

La

Ce

Pb

Pr

Sr

P

Nd

Zr

Sm

Eu

Ti

Dy

Y

Yb

Lu

Masaya volcano, Nicaraguamantle contribution: 7.5% melt of DM

For subduction contribution

Ba estimate is robust! La is not!

Balava = 100 Bamantle = 4Basubducted=96%

Lalava = 14Lamantle = 8

Lasubducted= 43%


Constant flux for highly enriched elements cs ba k pb sr l.jpg
Constant flux for highly enriched elements (Cs, Ba, K, Pb, Sr)

Very weak model of mantle contribution

If a variable flux of subducted fluids occurs, then highly enriched elements, like Ba, should decrease from NW to SE. They do not.

La increases from NW to SE but has high error.



Slide49 l.jpg

Himu Pb, Sr)

High-μ


Slide50 l.jpg
END Pb, Sr)


ad