Paleomagnetism of the North Patagonian batholith, southern Chile. An exercise in shape analysis

Abstract

Over 100 sites in granitic country rock, mafic inclusions within granitic rock, and mafic dikes were sampled in the northernmost 550 km of the Patagonian batholith. After magnetic cleaning, some sites were rejected as magnetically unstable, and others were combined because they were contiguous and carried similar directions, hence probably representing duplicate samplings of the same direction of the geomagnetic field. A total of 42 well-determined directions — 29 from granitic country rock and inclusions and 13 from dikes — survived to be included in the interpretation. Most of these had a normal magnetic polarity, probably indicating that a disproportionate fraction of the North Patagonan batholith formed during the mid-Cretaceous long normal interval. Tectonic interpretation of paleomagnetic directions from intrusive rocks often is difficult because paleohorizontal indicators in such rocks are usually lacking. Rather than struggle against this limitation, we yielded to it and elected to concentrate on the shapes of our various data sets. The method used is simple and straightforward, involving Bingham statistics and small-circle fitting. From this analysis, it appears that the main structural feature within the North Patagonian batholith (NPB) — the Liquiñe-Ofqui fault zone — is a site of crustal detachment; a sliver of South America west of the fault is detached from the rest of the continent and has moved relatively northward. Because the main strands of the Liquiñe-Ofqui fault are curved and concave to the west, northward movement was accompanied by counterclockwise rotation. East of the fault, the NPB appears to have been the locus of distributed shear, mainly dextral. The principal period of deformation of the batholith appears to have been mid- or late Cenozoic, and may have been initiated by the arrival of the South America–Nazca (Farallon)–Phoenix (Aluk) triple junction.