Transcript Gulf of California • Rifting is an interesting problem Mantle-related questions

Gulf of California
Some results and problems in need of mantle imaging
• Rifting is an interesting problem
- dominated by crustal studies, deformation modeling
• Mantle-related questions
- Where is the Farallon slab, are there slab windows?
(What is/has been it’s role in rifting and spreading?)
- What is the thinning profile of the lithospheric mantle?
(Can this be related to numerical models of lithospheric rupture)
- What is the state of the mantle, does it vary along axis?
(How has this affected magmatism and rifting?)
- How is rift obliquity accommodated in the mantle?
(How does this affect magmatism and rifting?)
Gulf of California
• Oblique Rift
• Subduction stalled
• Plate boundary
stepped east
• ~13 my of extension
Fletcher et al., 2006
~35 - 13 Ma
Basin and Range
style extension
13 Ma
“reconstruction”
~13 - 0 Ma
Gulf Extensional
Province
• Subduction stalled
• Baja coupled to PP
• Strain-rate increased
• Extension localized
• Details of this …
Style of Extension
(shape and forcing functions)
The style (mode and pattern) of extension
• What is the style?
- Imaging
(wide, narrow,
symmetric,
asymmetric,…)
• What controls it?
- Infer from modeling
(rheology profile,
geotherm,
strain rate,
magmatism,
fault mechanics, … )
(Huismans and Beaumont, JGR, 2003)
Many Models, Few Observations
(Huismans and Beaumont, JGR, 2003)
Crustal Seismic Results
• Crustal structure imaged
along three conjugate-margin
transects:
- Guaymas Basin
- Alarcon Basin
- Cabo/Puerta Vallarta
Lizarralde, Axen, Kent, Fletcher,
Gonzalez, Harding, Holbrook,
Umhoefer
Extension
• Approx. same amount of
extension, but
• Style varies non-uniformly
along axis
In a location where major forcing functions (strain rate, gross crustal
composition, mantle state, rift duration) are constant,
• The mode and pattern of extension vary substantially
• “Lithospheric strength” varies
on small spatial scales
• Style of extension is more
sensitive than expected
• Things vary in a way we don’t
understand
• Or maybe it’s the slab
Controls on Style
~250 km
Oskin et al. [2001]
• Slab geometry
• Slab window
Controls on Style
• Mesozoic arc
• Mesozoic suture
• Magmatism
Sierra Madre Occidental
Thick ignimbrite
System Elements
M. Roy
Slab rollback
Basin and range extension
Slab breakoff
Miocene bajaites
Convection in slab window
Rifting
J. Fletcher
Slab Detachment
Explains Trans-Mexican
Volcanic province
(Ferrari, Geology, 2004)
We need a hi-res image of the slab
Composite of VS – upper-mantle model from Van der Lee & Nolet (1997)
lower mantle from Grand (1994)
(Schmid, Goes, Van der Lee, and Giardini, EPSL, 2002)
Lithospheric structure of the Rio Grande rift
David Wilson1, Richard Aster1, Michael West2, James Ni2, Steve
Grand3, Wei Gao3, W. Scott Baldridge4, Steve Semken5 & Paresh
Patel3
NATURE|VOL 433|24 FEBRUARY 2005|www.nature.com/nature
MT – Seeing the slab?
(Romo, Gomez-Trevino, Esparza, Garcia-Abdesiem, and Flores-Luna, 2003)
Mantle State
(magmatism, arc and rift)
Crustal Thickness
• Crust ~2 km thicker in
Guaymas
• Sediments may enable
more efficient melt
extraction
The upper mantle beneath western
N. America and the Gulf of California
• Asthenosphere is shallow
• Velocities are slow (hot mantle?)
• Anomalies need to be localized
(Ritzwoller et al., JGR, 2002)
Mantle Flow Patterns
(obliquity, slab and cont. lith. control)
“Ridge” jumps
• Evidence for jumps of
similar distance
• At similar times, ~3.5 Ma
The Final Jump
• Synchronous (~3.5 Ma)
• Westward
• Toward upwelling?
Mantle flow?
- Obliquity
- Slab
The NARS-Baja array
(Network of Autonomously
Recording Stations)
NARS-Baja/RESNOM/RESBAN: SKS Splits
• Mainland
- basin and range deformation
• Baja
- slab or slab detachment
• Despite
- shallow asthenosphere
(Obrebski, Castro, Valenzuela, Benthem, and Rebollar, GRL, 2006)
Mantle-scale work in the GEP
The NARS-Baja array: 3D Regional-scale velocity model
DiLuccio, Clayton and Persaud
Cross sections…
… look like asthenospheric mantle bound by continental lithosphere
DiLuccio, Clayton and Persaud
Ongoing and
(to be) proposed work
• Gaherty/Collins
- OBS deployment
• Blackman, Vernon, et al.
- massive deployment
Figure: Blackman
Owens Valley is similar to the early
stages of the Gulf extensional
province
• Extension localized at the western edge
of the basin and range along and arc
batholith
• Transtensional deformation, with strikeslip to the west
• More work here would provide useful
analogues for GEP evolution
… an example is delamination
invoked for the southern Sierras.
Don Forsyth, using NARS-Baja data, sees
evidence for delamination at the latitude of
the Salton Trough.
Forsyth et al.
Gulf of California
Problems in need of mantle imaging
• Mantle-related questions
- Where is the Farallon slab, are there slab windows?
(What is/has been it’s role in rifting and spreading?)
- What is the thinning profile of the lithospheric mantle?
(Can this be related to numerical models of lithospheric rupture)
- What is the state of the mantle, does it vary along axis?
(How has this affected magmatism and rifting?)
- How is rift obliquity accommodated in the mantle?
(How does this affect magmatism and rifting?)
• GoC rifting involves problems analogous to western U.S.
Seismicity is poorly located with the current network
• Too sparse to confidently locate
events smaller than 5 mb
• Compare this with the Salton Sea
area …
Rebollar
1981 and 2005 Salton Trough Earthquake Swarms
The temporal pattern of a
swarm of small magnitude
events in advance of a large
event is recorded, in two
instances.
For the 2005 event, InSAR
data show that the precursory
swarm was driven by
aseismic slip.
McGuire
Lohmann