Wrangellia oceanic plateau

Introduction

The accreted Wrangellia oceanic plateau in the Pacific Northwest of North America is perhaps the most extensive accreted remnant of an oceanic plateau in the world where parts of the entire volcanic stratigraphy are exposed.

Wrangellia flood basalts formed as an oceanic variety of a large igneous province (LIP) in the Middle to Late Triassic, with accretion to western North America occurring in the Late Jurassic or Early Cretaceous (e.g. Richards et al., 1991).

Wrangellia flood basalts form the core of the Wrangellia terrane, or Wrangellia, one of the largest outboard terranes accreted to western North America (Jones et al., 1977).

Current exposures of the flood basalts are preserved in a narrow belt over 2300 km in length extending from southern British Columbia (BC) through Yukon and into Alaska (Fig. 1).

An ongoing research project investigates the stratigraphic and geochemical architecture of the erupted lava sequences of the Wrangellia oceanic plateau (Greene et al., 2008a; 2008b; 2008c; 2008d).

There are approximately 10 oceanic plateaus that have formed in the last 130 Myr, ranging in size from 100,000 to 2 million km², and collectively they cover an area of about 10 million km² (~3%) of the ocean basins, mostly in the Pacific and Indian oceans (Fig. 2; e.g. Eldholm & Coffin, 2000).

Studying oceanic plateaus in the ocean basins is very difficult because of the technological challenges of drilling into more than the uppermost few hundred meters of the submerged flood basalt sequences, which can be six or more kilometers thick (e.g. Kerguelen, Ontong Java, etc.).

The volcanic stratigraphy of oceanic plateaus can also be studied on land where the deeper levels of accreted volcanic stratigraphy can be sampled [e.g. Caribbean (Kerr, 2003), Ontong Java (Mahoney et al., 1993; Tejada et al., 1996)].

These studies can be used to constrain aspects of mantle plume-related magmatism that occur beneath the oceans in a similar way that continental flood basalts (CFBs) have been used to understand the sequential development of mantle plumes and rifting beneath continental lithosphere (e.g. Peate, 1997; Storey et al., 1997).

The Wrangellia oceanic plateau formed mostly within the Late Ladinian and Carnian stages of the Triassic Period (ca. 231-225 Ma; e.g. Carlisle & Suzuki, 1974; Parrish & McNicoll, 1992), as the continents were gathered into a great landmass.

The Wrangellia flood basalts erupted onto different-aged Paleozoic arc volcanic and marine sedimentary sequences in both shallow and deep marine settings.

Paleontological and paleomagnetic studies indicate that the Wrangellia flood basalts probably erupted in the eastern Panthalassic Ocean in equatorial latitudes
(Fig. 3; Jones et al., 1977; Katvala & Henderson, 2002).

Paleomagnetic measurements of the flood basalts have not yet revealed any magnetic reversals and, together with the absence of intervening sediments between the flows, this suggests a short duration for the eruptions (Hillhouse, 1977; Yole & Irving, 1980; Hillhouse & Coe, 1994).

The eruption of Wrangellia flood basalts broadly coincides with major biotic and environmental changes worldwide that occurred at the end of Carnian time (Furin et al., 2006). The accretion of the Wrangellia oceanic plateau to western North America was a major tectonic event and represents a significant addition of oceanic mantle-derived material to western North America (Condie, 2001).

Much of the original stratigraphic thickness of the Wrangellia plateau is intact and is defined as the Karmutsen Formation on Vancouver and Queen Charlotte Islands (Haida Gwaii), and as the Nikolai Formation in southwest Yukon and south-central Alaska (Fig. 4).

In Alaska and Yukon, the volcanic stratigraphy (~3.5 km) is predominantly massive subaerial flows with a small proportion of submarine flows along the base. Smaller elements in southeast Alaska may be correlative with the Wrangellia flood basalts. Throughout areas of Wrangellia the flood basalt stratigraphy is bounded by Middle to Late Triassic marine sediments.

The ongoing study of the Wrangellia oceanic plateau integrates stratigraphic, geochemical, and geochronological results to provide constraints on the nature of the mantle source, conditions of melting and magmatic history of the basalts, and the duration of volcanism in the context of the construction of this major oceanic plateau (Greene et al., 2008a; 2008b; 2008c; 2008d).

In the 1970s, Jones and co-workers (1977) defined a terrane of fault-bound blocks of crust that contain diagnostic Triassic flood basalts in BC, Yukon, and Alaska as Wrangellia, named after the type section in the Wrangell Mountains of Alaska .

A back-arc setting was initially proposed for the formation of Karmutsen basalts on Vancouver and Queen Charlotte Islands based on major- and trace-element geochemistry of 12 samples (Barker et al., 1989).

Richards and co-workers (1991) proposed a plume initiation model (Fig. 5 and Fig. 6) for the Wrangellia flood basalts based on evidence of rapid uplift prior to volcanism, lack of evidence of rifting associated with volcanism (few dikes and abundant sills), and the short duration and high eruption rate of volcanism.

The accreted Wrangellia oceanic plateau is an exceptional natural laboratory on land to study and sample the volcanic stratigraphy of an oceanic plateau. These exposures hold answers to some of the important questions about the development of oceanic plateaus that remain mostly hidden beneath the surface of the ocean.

Wrangellia flood basalts formed in submarine and subaerial environments in the Middle to Late Triassic during a single, short-lived eruptive phase that probably lasted less than 5 Myr.

Volcanic stratigraphy in Alaska and Yukon (Nikolai Formation) was initially emplaced in a shallow marine environment, but the majority of the volcanic stratigraphy formed as compound pahoehoe flow fields in a subaerial environment, in a similar way to subaerial flows in CFBs (Fig. 7 and Fig. 8).

In contrast, the Wrangellia flood basalts on Vancouver Island (Karmutsen Formation) grew from the ocean floor and accumulated >3000 m of pillowed and massive submarine flows before emplacement of 400-1500 m of hyaloclastite and pillow breccia as the plateau formed in shallow water and reached sea level (Fig. 9 and Fig. 10).

The lowest pillowed flows were emplaced on unconsolidated and lithified fine-grained siliciclastic and carbonaceous sediments containing Daonella beds (ca. 235-232 Ma) that are intruded by abundant Karmutsen mafic sills.

Minor volumes of picritic pillow lavas occur in the upper part of the pillowed flow sequence in areas of the plateau exposed on northern Vancouver Island (Nixon et al., 2008).

On Vancouver Island, approximately 1500 m of subaerial flows were emplaced on top of the submarine flows and intervolcanic sedimentary lenses in the upper parts of the subaerial stratigraphy formed in shallow water as eruptions waned and the plateau was subsiding.

After volcanism ceased, for areas of accreted stratigraphy in Alaska, Yukon, and British Columbia, the plateau continued to subside for more than 25 Myr and was overlain by hundreds to >1000 m of limestone and siliciclastic deposits.