Structure and growth of the Izu‐Bonin‐Mariana arc crust: 2. Role of crust‐mantle transformation and the transparent Moho in arc crust evolution

Abstract

Evolution of arc crust and subarc mantle in the Izu‐Bonin‐Mariana (IBM) intraoceanic arc‐trench system is examined by petrological modeling of arc magma generation and differentiation. Characteristic seismic structural features of the IBM arc highlighted in this modeling include the presence of (1) a middle crust with a P wave velocity (Vp) of 6.0–6.5 km/s, (2) a 6.5–6.8 km/s Vp layer at the top of the lower crust, (3) a high‐velocity (Vp = 6.8–7.2 km/s) lower crust, and (4) an uppermost mantle exhibiting rather low velocities (Vp = 7.4–7.7 km/s). The formation of the middle crust, which is considered to have an intermediate composition, is examined by (1) the mantle‐derived basalt model including anatexis of the initial mafic lower crust or mixing of mantle‐melting‐derived basaltic magma with crust‐melting‐derived felsic magma and (2) the mantle‐derived andesite model including differentiation of boninitic magma. The Vp calculated for the inferred compositions for each layer of the IBM crust on the basis of the basalt model is consistent with the observed values, whereas the andesite model cannot account for the characteristic Vp of the middle crust, the uppermost lower crust, and the uppermost mantle. The results further suggest that the volume of mafic restite and cumulates that are “crustal residues” resulting from the evolution of middle and upper arc crust is, at least, 3 to 9 times greater than that of the seismically defined IBM lower crust. One possible explanation to overcome this apparent dilemma is that the mafic to ultramafic crustal components are transformed to subarc mantle. During this process the subarc Moho is chemically transparent and permeable to the crustal components. This crust‐mantle transformation could play the major role in the creation of mature arc crust with intermediate compositions similar to continental crust.