Granitic intrusions enhance strain localization and rapid mantle exhumation along an oceanic detachment fault

Poulaki, E. M.*; 安邊 啓明   ; 他34名*

Poulaki, E. M.*; Abe, Noriaki; 34 of others*

IODP第402次航海ではティレニア海の若い海洋底を掘削し、地殻伸長に伴うマントルの上昇過程を調査した。掘削コアは主に貫入した花崗岩と蛇紋岩化したかんらん岩から成り、花崗岩は約400万年前に深度7kmで結晶化後、約50万年で急速に海底へ上昇した。構造観察から花崗岩は変形の集中を助長し、断層の局所化を促進したと示される。さらに安定同位体分析から約200Cでの蛇紋岩化が変形後に生じたことが判明した。これらは珪長質マグマが断層の局所化を誘発し、大陸分裂とマントル上昇を可能にすることを示す。

Serpentinization and magmatism weaken the lithosphere and facilitate mantle exhumation during magma-poor rifting and seafloor spreading. However, the competing mechanisms that control exhumation along detachment faults remain poorly constrained. IODP Expedition 402 drilled a young oceanic basin in the Tyrrhenian Sea and recovered in-situ mantle sections. Drilled cores consist of one sequence of variably deformed granitic intrusions intercalated with slivers of peridotites and basalts, and another of primarily serpentinized peridotites with heterogeneous deformation and local granitic intrusions. Geochronologic and geochemical data constrain that the granites crystallized at 4 Ma at ~7 km and rapidly exhumed to the seafloor within ~0.5 Myrs, requiring extension rates of ~2 cm/year. Structural observations show that these granites accommodated significant post-crystallization strain during exhumation and enhanced localization of detachment faulting. Stable isotopes record serpentinization temperatures of 200C, suggesting serpentinization occurred after the onset of deformation. These observations, demonstrate that felsic melts, when present, can nucleate strain localization along detachment faults and enable continental breakup and mantle exhumation.