Structural styles in Mesozoic and Cenozoic mélanges in the western Cordillera of North America

Abstract

The term “melange” is currently used to describe several different kinds of mudstone-rich rocks that are broadly characterized by an obscure stratigraphy, stratal disruption, or a chaotic, “block-in-matrix” fabric. Four types of melange, which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition, are particularly widespread and distinctive. Type I includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer-parallel extension. Type II consists of similarly deformed, thin layers of green tuff, radiolarian ribbon chert, and minor sandstone originally interbedded with black mudstone. Disruption in both types I and II, which probably occurred while the sediments were incompletely consolidated, has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation. Type III comprises inclusions of diverse shapes, sizes, and compositions enveloped in a locally scaly, pelitic matrix. The ultimate source of fragments is obscure, because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks. Although some type III melanges may have originated deep within accretionary prisms, final emplacement as olistostromes (muddy debris-flow deposits) or mud diapirs seems likely. Type III melanges are mechanically analogous to scaly, “sheared” serpentinites; many probably have been tectonically remobilized or even intruded into shallow-level fault zones. Type IV consists of lenticular inclusions bounded by an anastomosing network of subparallel faults. Their fabric records progressive slicing in brittle fault zones. Each of the four types of melange described here could, in theory, have formed in a variety of settings on or within an accretionary wedge at an active convergent margin; none can yet be singled out as a uniquely diagnostic “subduction melange.”