TRACKING CRUSTAL THICKNESS VARIATIONS IN ACCRETONARY OROGENS

Arc and backarc basalts are relatively common in many accretionary orogens, indicating that volcanic arcs are nascent accretionary orogens. This means that we can combine geodynamic and geochemical aspects of present-day arcs to understand the tectonic evolution of orogenic belts from a new perspective. A study of over 60 active volcanic suites, where Moho depths have been measured seismically, shows that a relation exists between Moho depth and trace element composition of basalts. The best correlation exists for maximum light/heavy REE ratios versus Moho (0.90), and Ce/Y is used to show the exponential function with increasing depth. This relation is a response to the changing proportions of crystallising minerals with increasing pressure. It allows us to monitor crustal thickness variations during the evolution of an accretionary orogen. Basaltic rocks from lava flows, synplutonic dykes and gabbro complexes can be used. Geochemical analysis of basaltic rocks in the Paleozoic Lachlan Fold Belt (LFB) of eastern Australia, and the Mesozoic Fold Belt of New Zealand (NZFB), show systematic increases in maximum Ce/Y ratios which mimic periods of “orogenesis”. These orogenies are typically crustal thickening events, but the datasets also discriminate crustal thinning events, some of which were not suspected. In the retreating LFB, crustal thickness did not exceed 35 km, but the crust occasionally thinned to <20 km, consistent with the lack of foreland basins and generation of turbidite-filled basins, respectively, during the orogenic history. Contrary to popular thought, most granites were emplaced during crustal extension events. In the advancing NZFB, crustal thickness exceeded 40 km at 380 Ma (Tuhua orogeny) and reached ~50 km at 120 Ma (Rangitaka orogeny), the latter associated with deep crustal burial (to ~16 kbar) and adakite generation. Thus, fundamental differences between advancing and retreating acccretionary orogens are also recorded in basalt compositions, which allow the thickness of the crust to be quantified during orogenic evolution.