Contrasting crustal production and rapid mantle transitions beneath back-arc ridges

摘要

位于斐济和汤加之间的“Lau弧后盆地热液喷rn孔系统”上的“Eastem Lau”扩散中心，是NSF：rn资助的RIDGE2000研究项目的一个重点研究地rn点。对这个区域所做的第次大规模地震研究rn显示了地壳性质大尺度的、突发的横向和纵向rn变化，这表明向“山脊轴”供应岩浆的地幔的rn性质相应地存在大尺度的不连续性。本文作者rn们得出的结论是，稳定的、宽大的三角上涌区rn域(如对中海洋山脊所推断的那样)不可能从地rn幔楔体的角部附近形成。相反，观测结果表明，rn可能存在一个动态过程，在该过程中，山脊上rn涌区域当其在弧附近时优先俘获富含水的低粘rn度地幔。当山脊远离该弧时，会达到一个临界rn点，在这个点上，物质会被迅速释放，导致在rn山脊上形成的地壳的性质发生迅速变化。%The opening of back-arc basins behind subduction zones pro-gresses from initial rifting near the volcanic arc to seafloor spread-ing. During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate). Current models predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In con-trast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5 km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9 km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge.