Crustal Growth Model for IBM: Arc Crust Evolution, Continental Crust Formation, and Crust-Mantle Transformation across The Transparent Moho IBM crust/mantle structure IBM crust/mantle structure Petrologic modeling Petrologic modeling New insights into New insights into Moho formation Moho formation and arc evolution and arc evolution Y. TATSUMI IFREE/JAMSTEC

IBM Arc-Trench System - an intra-oceanic arc - with backarc basins - colliding with Japan arc AA’BB’ C C’ - best surveyed arc esp. seismic structure

Seismic Structure of Crust/Mantle Seismic Structure of Crust/Mantle

The Moho Discontinuity A sharp seismological boundary exhibiting Vp jump from 6-7 to >8 km/s defining the crust/mantle boundary ◆ Generally accepted as gabbro/peridotite boundary

Sub-IBM Moho Identification Sub-IBM Moho Identification Sub-arc Moho: continuity from sub-BB ‘normal’ Moho seismic reflectors

Characteristic Seismic Structure Characteristic Seismic Structure   km/s middle crust similar to intermediate average continental crust  & km/s lower crust layers  km/s low-V uppermost mantle  reflectors near Moho and within upper mantle

Origin of Characteristics Middle Crust  Mafic plutonics →too high-T  Boninitic plutonics: mantle-derived andesite model ← boninite magmatism in the initial IBM arc  Intermediate plutonics: mantle-derived basalt model ・ anatexis of pre-existing basaltic crust ・ mixing of mafic and felsic magmas

Two Possible Models Mantle-derived basalt model Crustal melt or mixed magma restite of m-crust melting Remaining initial basaltic crust restites of basaltic crust melting Mantle-derived andesite model boninite restite of m-crust melting Remaining initial basaltic crust high-T peridotite Basaltic Crust High-V Lower crust normal mantle Upper crust Middle crust Low-V low-V mantle Moho

Arc Crust Evolution (basalt model): 1st Stage Crustal Anatexis Initial Arc Crust Man tle Basaltic Crust Moho Basaltic Underplating Basaltic Magma Melt Migration Restite Partial Melt upper middle lower low-V Melting front Sub-arc Moho = Fossil melting front Crust Component

Arc Crust Evolution (basalt model): 2nd Stage Arc Crust Evolution (basalt model): 2nd Stage Remelting of middle crust to create a restetic low-V lower crust layer Restite Basaltic magma Man tle Basaltic Crust Upper crust Middle crust Low-V low-V Restite High-V Lower crust Moho mormal Crustul Component

Intermediate Middle Crust Composition Tanzawa Pluton: Obducted IBM Crust?

IBM Magma Compositions IBM Magma Compositions Primary/differentiated basalt and felsic magmas

Volume of Restite/Cumulate Volume of Restite/Cumulate required for creating middle and low-V lower crust layers N. Izu Cumulate Restite for andesitic MC Felsic UC 246 Basaltic UC 486 Andesitic MC km/s LC 578 (restite for felsic UC) (Unit volume: km 3 /km) Observed 1882 Anatexis Mixing Crustal Component >> Seismic L-Crust Based on experimental constraints on melting regime

Volume of Restite/Cumulate Volume of Restite/Cumulate required for creating middle and low-V lower crust layers Upper Middle Lower ‘Excess’ ‘Excess’ crustal component ↓ Transformed into mantle across the transparent Moho

Uncertainty in degree of melting Uncertainty in degree of melting Andesitic melt: F~ <F<0.5

Restite+Cumulate Volume Restite+Cumulate Volume Andesitic melt: 0.15<F<0.5 calculated observed transformed

◆ Phase equilibria along inferred geotherm ← Perple_X (Connolly, 2005) ・ low GT, 800 and 0°C at Moho and surface; 1400°C at 70km ・ high GT, 200° higher T; 1400°C at 70km ◆ Vp and  calculation ← Hacker et al. (2003) ◆ H 2 O: basalt, 0.1; intermediate partial melt, 0.3 wt% ← IBM tonalite; vol% hornblende Vp &  Estimation for Petrologically Inferred Crust

Phase Assemblages: basalt model Low-T: Garnet within the lower crust High-T: Melting at the base of lower crust Medium-T: Most plausible

Vp &  Estimation Consistent with the observed seismic structure Density inversion at the base of low-V upper mantle

Phase Assemblages: boninite model Uppermost mantle: peridotite not restite Middle crust: abundant hornbrende Uppermost lower crust: abundant pyroxenes

Vp &  Estimation (boninite model) Inconsistent with observed seismic structure Extremely high-T is needed

Oceanic Crust Moho Initial Arc Crust Creation of the initial arc crust with a mafic composition Middle Crust Upper Crust Transformed Crustal Component Arc Evolution & Transparent Moho Formation of the mature arc crust with an intermediate composition Crust-mantle transformation across the transparent Moho → Arc crust evolution from mafic to intermediate compositions

Conclusion Sub-IBM seismic crust/mantle structure can be reasonably explained by melting and differentiation regime of arc basalt magmas Mafic crustal component, i.e., restites, will be transformed to the mantle during arc evolution Sub-arc Moho is transparent Sub-arc Moho represents the fossil melting front Delamination of mafic restite will take place at its base Mafic initial arc crust will differentiates into intermediate ‘continental crust’ Restite Partial Melt Petrologic Remaining Initial Crust Peridotites CRUST MANTLE upper middle lower low-V Seismic normal Moho Crustal Component

Thanks How to test this model? Direct sampling of deep crust with Chikyu…..