RELATIONSHIPS BETWEEN EXTENSIONAL DEFORMATION AND ANATEXIS IN COLLISIONAL OROGENS

In the Himalaya there are close spatial and temporal relationships between deformation and crustal melting, but opinions differ among geologists regarding the nature of these relationships. Some emphasize the importance of major shortening features in creating an appropriate thermal structure for melting, either through shear heating or the progressive accretion of radioactive, heat-producing material. Others view major extensional features as having triggered decompression melting. Such divergent opinions are possible because major deformational structures in the Himalaya have complex slip histories, and anatectic melts appear to have been produced during both contractional and extensional fault movement.

In the East Greenland Caledonides, the relationship between extension and anatexis is more straightforward. The Forsblad Fjord-Alpefjord region (72-72°30’N) offers remarkable exposures of a >20 km-thick section of the orogen. Three low-angle detachments, comparable in tectonic significance to the South Tibetan system, omit tens of kilometers of section in the region. These structures range in age from syn-contractional (ca. 425 Ma) to post-Caledonian (Early Carboniferous). The oldest and structurally deepest of these detachments places upper amphibolite facies rocks with volumetrically minor anatexites over high-pressure granulite facies rocks containing large volumes of granitic rocks. Compositional considerations and the spatial distribution of melt in footwall rocks suggests that the granulite facies unit represents the source region for most granite plutons in the region. Although anatectic leucosomes in the amphibolite facies rocks developed prior to extensional deformation, the relationship between larger granitic plutons and the older detachment clearly indicate that decompression related to tectonic denudation was responsible for most of the total volume of melt in this region. The structural position of a higher detachment, separating the amphibolite facies unit from a greenschist facies hanging wall, was determined by a zone of ponding of granitic melt derived from much deeper levels, reinforcing the notion that granites also play an important role in localizing zones of extensional strain in an orogenic system capable of such deformation.