From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora, Mexico - PowerPoint PPT Presentation

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

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Passive Rifting/Slab Roll Back Model. Modified from Lawton & McMillan, 2000. Phase 1 ... Age/temp wrong for slab roll back. Arc migrates wrong direction ... – PowerPoint PPT presentation

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


1
From Subduction to Extension/Transtension A
Case Study in Transitional Geochemistry from
Sonora, Mexico
  • Christy B. Till
  • Phillip B. Gans
  • Frank J. Spera
  • University of CA, Santa Barbara

2
  • Research Questions
  • 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?

3
Why Sonora?
RTJ
RTJ
(Atwater, 2004)
4
Why Sonora?
5
Why Sonora?
Increasing Distance from Gulf of California
West
East
Volcanic Sections From Regions Sampled
6
Subduction-related volcanism
(modified from Francis, 1993)
(modified from Wilson, 1989)
7
Rift-related volcanism
(Wilson, 1989)
(modified from Francis, 1993)
(Atwater, 2004)
(modified from Best Christiansen, 2001)
8
Major Element Geochemistry
9
Major Element Geochemistry
10
The major element geochemistry reveals little
or no change after subduction shuts off.
What does the trace element geochemistry tell
us?
11
Trace Element Geochemistry
All andesites basaltic andesites from Sonora
12
Trace Element Geochemistry
enriched in LILS
NVZ Andes
depleted in HFS
All andesites basaltic andesites from Sonora
13
Trace Element Geochemistry
All andesites basaltic andesites from Sonora
14
Trace Element Geochemistry
All andesites basaltic andesites from Sonora
15
Trace Element Geochemistry
Less enriched in LILS
Less depleted in HFS
All andesites basaltic andesites from Sonora
16
Trace Element Geochemistry
greater arc signature
La/Nb through time
17
Trace Element Geochemistry
SSU
greater arc signature
coast
SL
eastern
SSU
SL
coast
La/Nb through time
18
Trace Element Geochemistry
19
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?
20
Passive Rifting/Slab Roll Back Model
Phase 1
Modified from Lawton McMillan, 2000
21
Passive Rifting/Slab Roll Back Model
Phase 2
Modified from Lawton McMillan, 2000
22
Passive Rifting/Slab Roll Back Model
Thick, cold, old slab required
Phase 2
Modified from Lawton McMillan, 2000
23
Passive Rifting/Slab Roll Back Model
Phase 3
Modified from Lawton McMillan, 2000
24
Passive Rifting/Slab Roll Back Model
never get Phase 3 volcanism
Phase 3
Modified from Lawton McMillan, 2000
25
Passive Rifting/Slab Roll Back Model
Future Gulf rift
Phase 3
Modified from Lawton McMillan, 2000
26
Active Rifting/Slab Assimilation Model
Phase 1
Concept from Severinghaus Atwater, 1990
Atwater, 1989
27
Active Rifting/Slab Assimilation Model
Phase 2
Concept from Severinghaus Atwater, 1990
Atwater, 1989
28
Active Rifting/Slab Assimilation Model
arc migrates west
Phase 2
Concept from Severinghaus Atwater, 1990
Atwater, 1989
29
Active Rifting/Slab Assimilation Model
Phase 3
Concept from Severinghaus Atwater, 1990
Atwater, 1989
30
Active Rifting/Slab Assimilation Model
Phase 4
Concept from Severinghaus Atwater, 1990
Atwater, 1989
31
Active Rifting/Slab Assimilation Model
Future Gulf rift
Phase 4
Concept from Severinghaus Atwater, 1990
Atwater, 1989
32
Conclusions
  • 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
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