MAJOR CRUSTAL BOUNDARIES IN OROGENIC SYSTEMS – HIDDEN TREASURES, MISUNDERSTOOD FEATURES AND PROGRESS MADE (Invited Presentation)

Major crustal boundaries represent a variety of geologic settings, such as plate margins, suture zones, terrane boundaries, the interface between the infrastructure and suprastructure of an orogen, or the sole thrust of thin-skinned deformation. Within orogenic settings, these boundaries represent crustal anisotropies that can have significant rheological contrasts across them. As such, they can have a fundamental control on the style of deformation within a given region and become the loci of recurring reactivation during polyphase orogenesis. They can also serve as conduit systems for the migration of partial melting, magmatism and hydrothermal fluid circulation, which can significantly obscure their original character. Thus, in order to characterize the first-order crustal architecture of a region, its tectonic evolution and the attendant orogenic processes you must be able to recognize and locate these boundaries, and characterize their geometrical, kinematic and rheological attributes.

Although major crustal boundaries can be obvious and relatively easy to place with the orogenic setting of a region, often they can be elusive or frustratingly enigmatic. Perhaps they are not exposed at surface, or they juxtapose rock packages of similar lithology, metamorphic grade and style of deformation, or they have been obliterated by younger events that may include periods of reactivation, intense deformation, magmatism, or metamorphic recrystallization. A consequence of this elusiveness is that these fundamental features can be missed, leading to significant misinterpretation, or we recognize something is amiss and use terms such as “cryptic structures” or “discontinuities”. Thankfully, we now have an array of techniques available to us, and by integrating a wide variety of data sets (e.g., structural, metamorphic, geochronologic, thermochronologic, geophysical, computer modeling), we can significantly enhance our ability to recognize and characterize these major crustal structures. Case studies from the Canadian Cordillera and Canadian Shield will be discussed that highlight how the integration of multiple techniques has been critical for recognizing major crustal boundaries, leading to a better understanding of the tectonic evolution of very complicated polyphase orogens.