Controls on intrusion of near-trench magmas of the Sanak-Baranof Belt, Alaska, during Paleogene ridge subduction, and consequences for forearc evolution

Abstract

A belt of Paleogene near-trench plutons known as the Sanak-Baranof belt intruded the southern Alaska convergent margin. A compilation of isotopic ages of these plutons shows that they range in age from 61 Ma in the west to ca. 50 Ma in the east. This migrating pulse of magmatism along the continental margin is consistent with North Pacifi c plate reconstructions that suggests the plutons were generated by migration of a trench-ridge-trench triple junction along the margin. On the Kenai Peninsula the regional lower greenschist metamorphic grade of the turbiditic host rocks, texture of the plutons, contact-metamorphic assemblage, and isotopic and fl uid inclusion studies suggest that the plutons were emplaced at pressures of 1.5-3.0 kbars (5.2-10.5 km) into a part of the accretionary wedge with an ambient temperature of 210-300 °C. The presence of kyanite, garnet, and cordierite megacrysts in the plutons indicates that the melts were generated at a depth greater than 20 km and minimum temperature of 650 °C. These megacrysts are probably xenocrystic remnants of a restitic or contact metamorphic phase entrained by the melt during intrusion. However, it is also possible that they are primary magmatic phases crystallized from the peraluminous melt.Plutons of the Sanak-Baranof belt serve as time and strain markers separating kinematic regimes that predate and postdate ridge subduction. Pre-ridge subduction structures are interpreted to be related to the interaction between the leading oceanic plate and the Chugach terrane. These include regional thrust faults, NE-striking map-scale folds with associated axial planar foliation, type-1 mélanges, and an array

Petrogenesis of Early-Permian sanukitoids from West Junggar, Northwest China: Implications for Late Paleozoic crustal growth in Central Asia

“…For instance, in modern orogens like the south-central Alaska Range and the southwest Japan (Cole et al, 2006Hung et al, 2007;Kusky et al, 2003;Tsuchiya et al, 2005). Ridge subduction occurred during the Late Paleocene to the Early Eocene time along southern Alaska (Ayuso et al, 2009) and during the Early Cretaceous in southwest Japan (Kamei, 2004;Maruyama et al, 1997).…”

Accretionary orogens through Earth history

“…Flat-slab subduction and the resultant transitory plate coupling has been invoked as an important mechanism of orogenesis in the accretionary Lachlan orogen (Collins 2002a), in the North American Cordillera (Dickinson & Snyder 1978;Saleeby 2003), and in development of the Japanese accretionary orogen (Osozawa 1988;Underwood 1993;Isozaki 1996;Maeda & Kagami 1996;Brown 1998a). The mechanism for increased buoyancy with flat-slab subduction depends on the nature and rate of input of the thermal anomaly Kusky et al 2003). Ridge subduction will be associated with a progressive increase in buoyancy, whereas plateau or hotspot subduction will induce a rapid change in crustal thickness and, hence, buoyancy.…”

P–T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

“…Late sulfide-rich shear zones also truncate the tectonic foliation and the kink bands, and further north in the Chugach terrane we were able to date similar structures, and correlate them with a period of faulting and mineralization associated with Paleocene passage of a triple junction along the southern margin of the Chugach-Prince William terrane (Bradley et al, 2003;Kusky et aI., 1997aKusky et aI., , 2003.…”