USING BASALT COMPOSITIONS TO QUANTIFY CRUSTAL THICKNESS VARIATIONS IN OROGENIC BELTS

A global study of over 60 active volcanoes, where Moho depths have been measured seismically, shows that a relation exists between Moho depth and the Rare Earth Element (REE) composition of basalts. The best correlation (0.90) exists for maximum light/heavy REE ratios versus Moho, and we use Ce/Y to show the exponential function with increasing depth to 50 km. Basaltic rocks from lava flows, synplutonic dykes and gabbro complexes can be used for the crustal thickness estimation.

Application to the Circum-Pacific orogenic system in New Zealand shows that increases in maximum Ce/Y ratios correspond to the two major Devonian and Cretaceous orogenic events. Another thickening event at 250 Ma is evident, but has not been unrecognised in the NZ geological record, although it was a major orogenic phase along-strike in eastern Australia at that time. The datasets also discriminate crustal thinning events in the Permian, Jurassic and Late Cretaceous, the latter associated with formation of the vast backarc basin system of the present-day SW Pacific. The Jurassic event was also not previously recognised, but is associated with a major flare-up in magmatic activity.

This new tool for monitoring crustal thickness in orogenic belts has the potential to refine tectonic models for the North and South America Cordillera. Crustal thickening is readily recognised as fold-thrust development, but extensional events are generally less obvious, particularly if overprinted by later orogenesis. The method outlined here not only allows thickening and thinning events to be discerned, but allows the amount of extension and contraction to be quantified. Moreover, detailed studies of coeval basalts along strike in orogenic belts will help discern if these events are diachronous or synchronous, thereby helping to elucidate the mechanisms and driving forces of orogenesis.