Controls on the Apparent Thermal and Baric Structure of Mountain Belts

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The apparent thermal and baric structures of metamorphosed terranes are expressed in the map patterns of isograds and bathograds. Potential controls on such map patterns include: (1) thermal properties of the lithosphere, (2) mechanism, amount, and history of crustal thickening, and (3) variation in the rates of uplift and erosion. Simplified tectonic models of a single burial and uplift event, combined with one-dimensional models for the conductive transfer of heat, have been used to examine the effect of these controls on the relationship of apparent isotherms and isobars of peak metamorphic conditions in hypothetical mountain belts. Intersecting apparent isotherms and isobars are difficult to explain unless there are important variations in uplift times or thermal parameters across a metamorphic belt. Differences in original burial depth alone are unlikely to produce such intersections. Variation in thermal parameters across an orogen may lead to large variations in peak metamorphic temperature with little variation in the pressure at peak metamorphism. Differential uplift across a metamorphic belt is capable of producing profound differences in the pressure at the same peak metamorphic temperature and should be considered as a potential control on the baric type or bathozones of metamorphism. Testing whether differential uplift rates or variations in thermal parameters were dominant controls on the geometry of apparent isotherms and isobars is difficult, because most isotopic methods record post-peak metamorphic cooling history whereas uplift prior to and at peak metamorphism has the most important effect on the pattern of apparent isotherms and isobars. The available isotopic data do not suggest that differences in uplift rates across the Appalachians or the Scottish Highlands were the most important controls on the metamorphism. In the Pennine domain of the Alps, however, differential uplift may have been important.