Comparative orogenesis of accretionary orogens

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Accumulated field evidence suggest that there was a secular change in tectonic style from the Archean to present. Some of the answers how and why orogenesis seems to have changed in style over time may come from insight into why tectonic processes, such as arc-arc and/or arc-continent collisions change in nature along and across strike in some mountain belts. Accretionary orogens provide the best opportunity to research this topic, because accretions of arcs and other tectonic elements are commonly short-lived and associated deformation is typically localized. My ongoing tectonic research in the Appalachian-Caledonide mountain belt and the North- and South American Cordillera has documented that the arc-continent collisions that form part of these accretionary orogens, locally changed from hard (penetrative shortening and metamorphism of upper plate extends beyond the arc-trench gap) and hot to soft (upper plate shortening localised in arc-trench gap, but not beyond) and cold along and/or across strike (e.g Taconic, Famatina and Klondike orogenies), whereas others show little or no variation and remained soft and cold (e.g. Penobscot, Salinic, Antler and Pampean orogenies). The controlling tectonic parameters and processes responsible for these variations in tectonic style are poorly known. Comparative research based on multidisciplinary studies of important aspects of these orogens showing major variations in tectonic style in critical areas with relatively good exposure and geochronological constraints, are geared to identify the characteristic structures and/or other tectonic elements shared by such mountain belts. The results will stimulate developing hypotheses capable of explaining the differences in tectonic style, which will be further tested by research in other mountain belts showing large variations in tectonic style along and/or across strike, such as the Beishan orogen of the Central Asian Orogenic Belt (CAOB) and comparisons with existing numerical modelling results and 3-D laboratory-based experiments.****Student research proposed herein focusses on investigating the ridge-subduction model for forming the Liuyuan ophiolite complex in the Beishan orogen and its potential impact on the observed variations in tectonic style (hard vs soft) in this mountain belt (PhD1) and the syn-collision exhumation of eclogites in the Baie Verte area of the Newfoundland Appalachians (PhD2), a process extremely rare or absent elsewhere in the Northern Appalachians. The eclogites occur in a segment of the Laurentian margin that was hyperextended prior to subduction, whereas the adjacent upper plate forearc basin and accretionary prism are largely absent, probably due to tectonic removal by forearc underthrusting. These and/or other factors may have had a bearing on the along strike variations in tectonic style of the Taconic orogeny.**