Paleomagnetism of ocean layers 2 and 3: evidence from the Metchosin Complex, Vancouver Island

Abstract

Abstract The early Eocene Metchosin Complex is an ophiolitic sequence and is the northernmost part of the Crescent Terrane. It was underthrust beneath Wrangellia later in the Eocene. Directions of magnetization in sheeted diabase (layer 2) and gabbroic intrusions (layer 3) from the complex are internally consistent, and both normal and reversed polarities are present. Mean directions, irrespective of sign, differ not only from the expected Eocene palaeofield but also from one another. These divergences have not, therefore, been caused by tectonic rotation of the Metchosin Complex as a whole, but by variable tilting of bodies within the complex. It is argued that these tilts probably were imposed during the emplacement of the Metchosin Complex adjacent to and beneath Wrangellia. The systematic clockwise rotations found further south in Eocene rocks of the Crescent Terrane of Washington and Oregon do not occur. The remanent magnetizations of the gabbros of layer 3 are commonly strong and stable. These are the only rocks in the Metchosin Complex that, had they been part of the present oceanic crust, would be capable of contributing to marine magnetic anomalies. The stable magnetization of diabase of layer 2 is too weak, the dominant magnetization being a soft viscous remanence imposed by the present geomagnetic field. The remanence of basalt lavas of layer 2, as previous work on the Metchosin Complex has shown, is weak and scattered in direction.