Geology of British Columbia

The geology of British Columbia is a function of its location on the leading edge of the North American continent. The mountainous physiography and the diversity of rock types and ages hint at the complex geology which is still undergoing revision despite a century of exploration and mapping.

The mountain ranges which are the most prominent expression of the geology are a part of the North American Cordillera which stretches from Southern Mexico to Alaska.

Terrane Theory

Terrane theory was first proposed by Jim Monger of the GSC and Charlie Rouse in 1971 as explanation for a set of fusilinid fossils found in central British Columbia. Rather than use facies changes or seaways (which were common explanations at the time) they proposed that the fossils in question had been part of an assemblage of rocks which had migrated across the Pacific Ocean to their present location. This theory was then developed by Porter Irwin and Davy Jones of the US Geological Survey to its common definition of "fault bounded regional geologic entities, each characterized by a different geologic history than its neighbours"Fact|date=October 2008.

Terranes are most commonly associated with different tectonic elements such as island arcs, volcanic plateaus, subduction zones, continental margins, mid-oceanic ridges and continental fragments. These terranes are gradually joined together by elements such as overlap assemblages and stitching plutons and are then accreted to the continent. In some cases a terrane can contain multiple tectonic elements. The Cache Creek terrane is composed of a massive carbonate component, an oceanic floor component and a subduction melange component.

Architecture and Composition

There are five morpho-geological belts which define the geology of British Columbia from east to west, The Foreland, The Omenica, The Intermontain, The Coast and The Insular Belt. Each has a separate geology, metamorphic, physiograhpic, metallogenic and tectonic history.

The Foreland Belt is composed of weakly metamorphosed sedimentary rocks which are 1.4 billion to 33 million years in age, and represents a rift sequence followed by a passive margin, which was turned into a retro arc fold and thrust belt with synsorogenic sedimentation. The region is very rugged except in the northeast of the province where it flattens out to become a wide plain.

The Omenica belt is composed of highly metamorphosed, pericratonic (near craton) terranes and fragments of north America from 2 billion to 180 million year in age. Terranes in the belt include the Slide Mountain oceanic terrane, the Yukon -Tannana Terraine and the Cassiar Terrane. This belt goes from low hills to high mountains across it's length, with the majority of the region being extremely rugged.

The Intermontain belt is a flatter more rounded region composed of three terranes, Stikinia, Quenellia and the Cache Creek terrane. It is of a lower metamorphic grade than the Omenica belt and ranges from 400 million to recent with volcanic activity occuring in the past 10,000 years.

The Coast belt is the single largest outpouring of Granite and granodiorite in the phanerozoic, it contains heavily metamorphosed fragments of both the terranes of the insular belt and the intermontain belt. In the south east there are a series of small terranes of both oceanic (Bridge River and Chilliwack) and continental affinity (JackAss Mountain, Ladner). The hard weathering granite is unrelievedly rugged the length of the belt.

The Insular belt is composed of the outboard terrain with no connection to North America before accretion. There are two main terranes Wrangellia and the Alexander and few smaller ones such as the Leech River and Crescent terranes. Since it is the most tectonically active of the belts it has the greatest relief differences from the depths of Queen Charlotte sound to the heights of the Wrangell- St Elias mountains. The ages is from 600 million to recent with metamorphic grade varying depending on the age and host of the rock type.