Scientific Challenges: Continental rifting and other margins Don Forsyth MARGINS Workshop, February 15, 2010

1.What is the relationship of mantle upwelling and melt production to rift initiation and segmentation? Or, What the heck are those “red” blobs in the vicinity of rifts in seismic tomography images of the upper mantle?

Beutel, et al Afar triangle overlies major hotspot Typical magmatic and rift segment scale km

Bastow et al.,2005 Velocity anomalies at 75 depth are displaced from the shallow magmatic segments

Benoit et al., 2006 Major low velocity centers spaced km apart

% Average Shear Velocity Anomaly km Shear velocity anomaly at centered at a depth of ~ 60 km Velocities slow along plate boundary (red) with 3 (4?) centers of particularly low velocity km apart. Low velocity anomalies are displaced somewhat from spreading or rifting centers.

Profile of anomalous shear velocities along Gulf of California. We interpret low velocity centers as indicating higher melt concentrations, which may induce convective upwelling in underlying asthenosphere. Low absolute shear velocities extend to km.

Low velocity anomalies near Marianas backarc spreading centers spaced about 250 km apart with maximum anomalies at depth of ~ km. Pyle et al., JGR submitted, 2009

Melt Porosity Dynamic upwelling due to melt depletion buoyancy, melt retention and temperature anomalies predicted at km wavelengths Magde and Sparks, 1997

Does the km spacing of shallow-upper-mantle, low-seismic-velocity anomalies represent dynamic upwelling of the asthenosphere on this length scale? If so, how does the magma produced in the upwelling zones migrate to the shallow rift segments that are on a significantly smaller length scale? Or does the displacement of the anomalies from the rift centers simply indicate that melt is produced in a relatively broad region, but extracted more efficiently in the vicinity of the rifts?

2.How does the subduction of a spreading center affect the tectonic and volcanic evolution of the plate boundary? How does the cessation of spreading just before ridge subduction and the persistence of slab fragments affect the plate boundary? Where does the subducting slab detach from the unsubducted plate fragment?

Atwater and Stock, 1998

Nicholson, et al. 1994

Current view. Farallon subduction left unsubducted fragment(s) behind - the Monterrey microplate offshore California. Nicholson et al., 1994

Magnetic anomalies from Wilson, et al.,2005 after Lonsdale, 1991 and Severinghaus & Atwater, 1989 Monterey microplate Spreading ceased ~ 19 Ma Guadalupe microplate Magdalena microplate Spreading ceased ~ 12.5 Ma

Burkett &Billen, 2009

Post-subduction volcanism From Negrete-Aranda and Canon-Tapia (2008)

Post-subduction volcanism in Baja California Negrete-Aranda & Canon-Tapia, 2008 Infer presence of slab, melting of oceanic crust and release of fluids from fossil slab, with resurgence of volcanism in last 4 Ma. Slab extends to 150+ km.

High-Mg Andesites Horizontal slice at 100 km shows high shear velocities corresponding to slab fragment still attached to unsubducted microplate remanents Spreading ceased at ~ 12.5 Ma High-Mg andesites are post-subduction volcanism thought by many to involve partial melting of subducted oceanic crust Wang, et al. V41B-2190

Isabella or Central Valley anomaly Colorado Plateau Shear velocity at 100 km depth

At 70 to 90 km, low velocity region coincides with region of Quaternary volcanism (black dots). Dashed line indicates extent of high-K volcanism at ~ 3.5 Ma. Solid line indicates total extent of volcanism at that time (after Manley et al., Geology, 2000). Note more circular shape of the Great Valley anomaly.

Drip may be associated with delamination of the lower crust and lithospheric mantle from beneath the eastern Sierra Nevada

Alternative Suggestion: Instead of oceanic lithosphere detaching, opening a slab window, followed by lithospheric instability and delamination, the Isabella anomaly is simply a piece of stalled oceanic lithosphere still attached to the unsubducted Monterey microplate.

Schmandt and Humphreys, 2010

% P wave anomaly Latest P-wave tomography model across Isabella anomaly, Schmandt and Humphreys, AGU 2009 W36°NE

Wilson et al., 2005 Gap in post 20 Ma volcanic activity due to absence of slab window? Requires coast ranges to translate along with Monterey microplate and Pacific plate.

Brocher, et al., 1999 Oceanic crust has been traced close to the San Andreas fault with seismic reflection and refraction. What happens at the San Andreas and further inland is unknown. Could the San Andreas be the location where the Pacific/Farallon lithosphere begins to plunge downward, decoupling the Pacific plate from North America?

Berkeley Quake Map

A major question: Could the subducted slab fragment translate 100s of km along with the Pacific plate/Monterey microplate and still remain intact and attached?

Benz, et al., JGR 1992

Furlong and Schwartz, 2004

Conclusions There are remnants of the subducting Farallon slab beneath Baja California extending to depths of ~150 km that are still attached to the unsubducted Guadalupe and Magdalena microplates The Isabella or Great Valley or Central Valley anomaly in southern California is probably a slab remnant still attached to the Monterrey unsubducted microplate rather than sinking lithosphere related to delamination of the lithosphere beneath the Sierra Nevada These attached fragments of subducted oceanic lithosphere translate laterally for hundreds of km along with the microplates that are now part of the Pacific plate