SOME THERMAL AND PALEOMAGNETIC CONSEQUENCES OF TILTING A BATHOLITH

If a rock body becomes fully magnetized and then is folded, faulted, or otherwise tilted, its vector of remanent magnetization will tilt with it. However, if a rock body is tilted while it is very hot, and then cools and becomes magnetized, its vector of magnetization will show little if any of the effects of tilt. It is shown, for a wide variety of circumstances, that tilting a large batholith should not be expected to result in equal tilt of that batholith's magnetic direction. This follows from several elementary considerations: (1) Batholiths cool slowly, and slow cooling lowers the blocking-temperature range for acquisition of remanent magnetization. (2) If a tilted block is large (of the order of 25 km in width) and has been tilted through even a moderate angle (25-degrees will do), its "lower" end originally must have been very hot; quite possibly too hot to retain a remanent magnetization. (3) Tilt entails uplift, and uplift rearranges the geothermal gradient. Rapid uplift accompanied by erosion is particularly effective in raising the temperature of the upper levels of the crust, thereby delaying acquisition of magnetization until tilting is nearly complete. Other thermal constraints on the tilting of remanence vectors in batholiths are discussed; in general, it appears that quite special circumstances are required to tilt a large pluton without leaving a clear magnetic record. Few, if any, Cordilleran batholiths studied to date show this kind of evidence of tilt.