Deformational styles in a sequence of olistostromal mélanges, Pacific Rim Complex, western Vancouver Island, Canada

Abstract

The Pacific Rim Complex is exposed in a fault-bounded slice along the west coast of Vancouver Island. It is divided into two parts: (1) the Ucluth Formation, a basement unit composed of lower Mesozoic arc-volcanic rocks, and (2) a suprajacent sedimentary sequence, more than 2 km thick, composed of Lower Cretaceous olistostromal melanges. An unconformity is present, at least locally, at the Ucluth-melange contact and indicates that the Ucluth Formation is stratigraphic basement to the melanges. Melange-style deformation is restricted to the sedimentary sequence and has affected virtually all parts of that sequence. The melanges formed without the development of a cleavage or pervasive cataclasis, indicating that deformation occurred prior to consolidation of the sediments. Three types of melanges have been recognized. The first consists of a matrix of inter-bedded mudstone, chert, sandstone, and green tuff, which encloses numerous exotic blocks of igneous rocks and limestone. Beds within the matrix show variable amounts of layer-parallel fragmentation, which is attributed to in situ liquefaction and lateral flowage within individual beds. The blocks in this melange, most of which were derived from the underlying Ucluth Formation, are considered to be submarine slide blocks, produced by rock falls from nearby basement scarps. The matrix sediments near the blocks commonly contain wispy lenses of poorly sorted plutonic and volcanic detritus (green tuff), interpreted as scree from the rock slides. The second and third types of melange are distinguished by the predominance of either mudstone or sandstone but are otherwise quite similar. These melanges lack exotic blocks and consist of a contorted assemblage of mudstone, turbidite sandstone, conglomerate, pebbly mudstone, and rare chert, all of which are depositionally interrelated. Sedimentary structures and stratal coherence are generally well preserved. The internal structure of these melanges consists of a series of dismembered depositional sequences. Individual sequences are commonly more than 75 m thick. Some are upright and others are overturned. Paleoecological evidence indicates that some of these sequences were originally deposited in an infra-slope basin. Thus, these melanges are interpreted to have formed mainly by the accumulation of large slump sheets at the base of a submarine slope. Previous interpretations have considered the Pacific Rim Complex to be a late Mesozoic subduction complex constructed along the western margin of the Wrangellia terrane (Vancouver Island). Several factors argue against a subduction-melange interpretation. (1) The melanges were deposited on an older, arc-related basement, not oceanic crust; (2) exotic blocks were introduced as rock slides and not by subsurface faulting; and (3) at least some of the strata originated in an infra-slope basin. The heterogeneous deformational style is more compatible with an origin by near-surface mass-movement processes, including submarine slides, rock falls, debris flows, and in situ liquefaction. The amount of mass wasting recorded in the Pacific Rim melanges probably requires a seismically active setting, where frequent ground shaking produced large transients in pore fluid pressure. Similarities in stratigraphy and metamorphism suggest that during Late Cretaceous time, the Pacific Rim Complex was part of the San Juan-Cascades thrust system, located 220 km to the southeast in northwestern Washington State. If this is correct, the Pacific Rim melanges would have formed on the inboard side of Wrangellia, about 30 m.y. prior to the collision of Wrangellia with the American continent. The present location of the Pacific Rim, outboard of Wrangellia, is attributed to younger (latest Cretaceous or early Tertiary) transcurrent displacement.