Focused melt supply at the Cobb Hot Spot / Juan de Fuca Ridge

Focused melt supply at the Cobb Hot Spot / Juan de Fuca Ridge

Focused melt supply at the Cobb Hot Spot /
Juan de Fuca Ridge intersection
Michael West*, William Menke**, Maya Tolstoy**
* New Mexico State University
** Lamont−Doherty Earth Observatory
Co
ax
ial
Co
b
b
Juan de Fuca Ridge / Cobb−Eickelberg Chain
Blanco F.Z.
rift
Axial Volcano
so
uth
Brown
Bear
Va
n
se c e
g.
Brown
Bear
no
rt
hr
ift
Cobb−Eickleberg chain
Axial
Caldera
Principle question
Hot spot
Ridge
small (compared to Iceland,etc.)
long−lived (>9 Mya)
intermediate rate (5−6 cm/yr)
How do they behave in close proximity?
Airgun survey of Axial Volcano & region
5000+ shots
1225 km of shot lines
120 sec. shot interval
~250 meter spacing
Magma reservoir under caldera
Mid−crustal structure from Pg phases
Low velocities (40%) under caldera
Velocities indicative of partial melt
5−21 km3 of melt
>> 0.1−0.2 km3 emplaced during
1998 eruption (Fox et al. 2001)
Vp in km/s
EWING
water
Pg
crust
% melt
mantle
Sample PmP record
(150 traces shown out of ~30,000)
Strong PmP throughout
1500+ PmP traveltimes
Traced through 3D crust
EWING
PmP
water
PmP
Pg
Pn
crust
PmP
mantle
Crustal thickness
Crustal thickness from PmP
PmP "bounce points" show
regional thickness
(depth error < 1 km)
EWING
water
crust
mantle
"bounce
point"
Crustal thickness II
11 km under caldera
Radially symmetric
Steep Moho topography
Consistent with gravity
What does it mean?
Very thick crust
... Large flux of magma (relative to ridge)
Thickens rapidly beneath Axial
...magma supply is focused beneath volcano
High flux, point source of magma
...Cobb hot spot dominates supply to Axial system
Passive upwelling vs. hotspot flow − I
Upwelling beneath JdF ridge is passive
Cobb hot spot upwelling is active (9 My seamount chain)
Hot spot has turned off upwelling
yr
/
m
6c
surface eruption
JdF ridge
dike intrusion
flow from
hotspot source
transition
passive
upwelling
transition
underplating or
dike accretion?
passive
upwelling
Passive upwelling vs. hotspot flow − II
/yr
6 cm
surface eruption
JdF ridge
passive
upwelling
ift
so
uth
r
flow from
hotspot source
transition
passive
upwelling
transition
Brown
Bear
Va
se nce
g.
underplating or
dike accretion?
no
rth
rift
dike intrusion
Co
axi
al
Co
bb
Hot spot feeds 50−100 km of rift via diking and eruption
Upwelling where central supply is weak
Both processes exist along rift tips
Transition is evident at rift / segment overlap
Axial
Caldera