Igor B. Morozov, Scott B. Smithson Lincoln S. Hollister, and Nikolas I. Christensen
Abstract:
The ACCRETE project is among the first detailed multidisciplinary studies
of a continental arc where new crust has been formed through accretion
of exotic terranes and generation of new crust. Employing marine
wide-angle seismic techniques in a fjord in southeastern Alaska and western
British Columbia, we took advantage of dense subsurface coverage, and high-quality
shear-wave data combined with excellent geologic control and laboratory
measurements of rock samples to constrain the interpretation of the seismic
results. A crustal model resulting in Vp/Vs is compared with lab velocity
measurements. The crust across ACCRETE profile consists of three units;
the Wrangellia-Alexander (WA) far-traveled terrane, the Coast Mountains
batholith (CMB), and the Stikinia
fold and thrust belt. The crust under the WA is 23 km thick with
high velocity lower crust and Vp/Vs appropriate for oceanic crust.
The CMB has high average velocity crust and lower crust with Vp/Vs of 1.84,
compatible with interlayered garnet pyroxene granulite and quartzofeldspathic
restite. The crust under Stikinia has a lower average velocity and
lower Vp/Vs. The crust under the CMB is relatively thin, with the total
Moho relief within the ACCRETE seismic section going from 23 to 32 km.
Moho is sharp (~200 m), highly reflective and continuous under all terranes
with horizontal correlation length on the order of 5-15 km. Low Pn velocity
(7.9 km/s) indicates anomalously hot mantle and high temperatures of 800+/-
100(C in the lower crust. Accordingly, the lower quarter of
the crust under the CMB is presently in the granulite facies of metamorphism,
and garnet should be stable in pyroxene granulite. Geologic and seismic
evidence for crustal-scale extension of the CMB and massive intrusion of
tonalite and diorite plutons suggests that the lower crust is made of a
mafic garnet granulite, the metamorphic product of the mantle-derived basalt
that provided the ultimate heat source for driving the extension and the
crustal melting. To the SW, the contrast between the crust of the Coast
Mountains Batholith and of the mid-Cretaceous thrust belt, across the CSZ
is apparent in the Vp/Vs domain and
is associated with the difference in their tectonic origin as a crustal
suture. Our results suggest that generation of new crust within a former
continental arc results in average crustal velocities of about 6.55-6.6
km/s in the lower 3/5 of the crust. This would lead to an average
crustal velocity of ~6.4 km/s with 10-15 km of the upper crust restored.
The crustal section under the Coast Mountains Batholith (CMB) represents
the lower two thirds of normal crust inflated by intrusions of tonalite
and gabbro. The above structure can account for the diversity of
continental crust.
