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