COMMONLY NEGLECTED FACTORS IN OROGENIC BELT EVOLUTION INVITE FURTHER STUDY

Geoscientists have continually developed tools that can be applied to the analysis of orogenic belt history. In spite of such advances, some factors that may strongly influence the geologic record do not receive adequate attention in many studies. For example, studies commonly assume that only surface erosion removes the subduction-accretion record, yet subduction erosion, rather than accretion, occurs along a significant fraction of modern subduction zones. This suggests that many formerly accreted units and parts of continental margins have been removed from the geologic record. Ridge subduction and other triple junction interactions may profoundly impact structural and metamorphic relations and may create arc-like field relations. The signature of such plate boundary interactions in the geologic record may be mistakenly attributed to the effects of collisional orogenesis or arc development. Similarly, hot spot interactions and delamination (distinct from slab detachment related to subduction and collision) events have likely left a mark on the rock record in some orogenic belts. In addition, tectonic reconstructions of orogenic belts such as the North American Cordillera interpret far fewer tectonic transitions per unit time than has occurred in the Cenozoic in the SW Pacific, often considered the modern analog of such orogenic belts prior to assembly. This suggests oversimplification in our tectonic reconstructions. Finally, preliminary studies of detrital zircon geochronology suggest that many macrofossils used to date clastic rocks along convergent plate margins may have been reworked. The interpreted timing of clastic deposition may require reexamination in many subduction complexes. New plate-boundary-scale tectonic reconstructions of orogenic belts such as the Cordillera seem to have declined in popularity in the last decade or so, as if to suggest that we have solved most of the major problems associated with them. On the contrary, the above-mentioned factors suggest that we should renew our interest in the plate tectonic history orogenic belts, and continue the quest begun by Moores and others.