Lithosphere architecture characterized by crust–mantle decoupling controls the formation of orogenic gold deposits
Zengqian HouQingfei WangHaijiang ZhangBo XuNian YuRui WangDavid I. GrovesYuanchuan ZhengShoucheng HanLei GaoLin Yang
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Abstract:
This study, via combined analysis of geophysical and geochemical data, reveals a lithospheric architecture characterized by crust-mantle decoupling and vertical heat-flow conduits that control orogenic gold mineralization in the Ailaoshan gold belt on the southeastern margin of Tibet. The mantle seismic tomography indicates that the crust-mantle decoupled deformation, defined from previous seismic anisotropy analysis, was formed by upwelling and lateral flow of the asthenosphere, driven by deep subduction of the Indian continent. Our magnetotelluric and seismic images show both a vertical conductor across the Moho and high Vp/Vs anomalies both in the uppermost mantle and lowest crust, suggesting that crust-mantle decoupling promotes ponding of mantle-derived basic melts at the base of the crust via a heat-flow conduit. Noble gas isotope and halogen ratios of gold-related ore minerals indicate a mantle source of ore fluid. A rapid decrease in Cl/F ratios of lamprophyres under conditions of 1.2 GPa and 1050°C suggests that the ore fluid was derived from degassing of the basic melts. Similar lithospheric architecture is recognized in other orogenic gold provinces, implying analogous formational controls.Keywords:
Asthenosphere
The article presents the data calculated from four different viscosity structures V1, V2 [1], SH08 [2], and GHW13 [3], as well as two tomography models S40RTS [4] and SAW642AN [5], using the joint modeling of lithosphere and mantle dynamics technique [3, 6-9]. Besides, the data contain the information on the viscosity variations of the lithosphere, asthenosphere, transition zone, and D″ layer based on the viscosity structure SH08.
Asthenosphere
Low-velocity zone
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As temperature increases with depth and the creep resistance of rock decreases exponentially, a high-viscosity sub-lithospheric layer, just beneath the 'elastic' lithosphere is expected to exist. Depending on the temperature profile, a low-viscosity asthenosphere may also exist if the temperature deeper down gets high enough. Since the temperature profile is expected to change laterally – especially from below the oceans to cratonic areas underneath continents, rock properties of the lithosphere, high-viscosity sub-lithosphere and low-viscosity asthenosphere are expected to change laterally. Our aim is to constrain sub-lithospheric properties (depth, thickness and viscosity), lateral lithospheric thickness variations and asthenospheric properties using observed GIA data. A Coupled Laplace-Finite Element Method is used to compute gravitationally self-consistent sea level with time-dependent coastline and rotational feedback in addition to changes in deformation, gravity and the state of stress. We start with the VM5a-ICE-6G_C model combination and then modify the lithospheric, sub-lithospheric and asthenospheric properties (including lateral thickness variation) while keeping the mantle viscosities the same as VM5a. Through this study, we confirm that the sub-lithospheric and asthenospheric properties can significantly affect the predicted global relative sea level (RSL), present-day gravity rate-of-change (g-dot) and uplift rate (u-dot) in Laurentia and Fennoscandia. In addition, incorporating the elastic lithosphere with lateral thickness variation, sub-lithosphere and asthenosphere can improve the fit to global RSL, but the predicted peak values of g-dot and u-dot in Laurentia may decrease slightly but not significant enough to affect the fit to the observed data. Our results prefer an elastic lithosphere that has maximum thickness of 140 km under continental cratons but reduces to 60 km underneath the oceans. The results preferred depth of the asthenospheric bottom is around 190–200 km with asthenospheric viscosity around 1020Pa s. Finally, we show that the best laterally heterogeneous mantle model we found in previous publication when combined with the lithosphere with lateral thickness variaion gives the best fit to global RSL and peak g-dot and u-dot in Laurentia simultaneously.
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Lithospheric flexure
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Post-glacial rebound
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Abstract. A key element of plate tectonics on Earth is that the lithosphere is subducting into the mantle. Subduction results from forces that bend and pull the lithosphere into the interior of the Earth. Once subducted, lithospheric slabs are further modified by dynamic forces in the mantle, and their sinking is inhibited by the increase in viscosity of the lower mantle. These forces are resisted by the material strength of the lithosphere. Using geodynamic models, we investigate several subduction models, wherein we control material strength by setting a maximum viscosity for the surface plates and the subducted slabs independently. We find that models characterized by a dichotomy of lithosphere strengths produce a spectrum of results that are comparable to interpretations of observations of subduction on Earth. These models have strong lithospheric plates at the surface, which promotes Earth-like single-sided subduction. At the same time, these models have weakened lithospheric subducted slabs which can more easily bend to either lie flat or fold into a slab pile atop the lower mantle, reproducing the spectrum of slab morphologies that have been interpreted from images of seismic tomography.
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Slab window
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Asthenosphere
Lithospheric flexure
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Subduction zones consume oceanic lithosphere and are an indispensible part of plate tectonics. Unlike the oceanic lithosphere production system which can be linked as a nearly continuous, albeit sinuo
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Abstract Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of S p waves to constrain the lithosphere‐asthenosphere velocity gradient at long‐lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere‐asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere‐asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.
Asthenosphere
Low-velocity zone
Lithospheric flexure
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Asthenosphere
Collision zone
Lithospheric flexure
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Asthenosphere
Low-velocity zone
Lithospheric flexure
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在北方中国 Craton ( NCC )的东方部分基于早白垩纪 Feixian 高镁的暗岩的水内容的研究，东方 NCC 的早白垩纪 lithospheric 披风是高度含水的，这被建议了(> 1,000 ppm ， H2O wt )并且这个高水位线内容显著地减少了 lithospheric 披风的粘性并且为 NCC 的破坏提供了一个前提。向北， NCC 的东方部分从南方经历了海洋的盘子的多级式的 subduction，东方后来站在一起早古生代，并且这些事件可能引起了 NCC lithospheric 披风的强壮的水和。决定哪个 subduction 最作出贡献到这水和，我们测量了 Fushan 的早白垩纪高镁的闪长岩在 Taihang 山的南方中央的部分招待的橄榄岩捕虏体的水内容。我们的结果证明在 Taihang 山的南方部分下面的早白垩纪 lithospheric 披风的水内容是 ~ 40 ppm 并且比在 NCC 的东方部分下面的当代的 lithospheric 披风的显著地低。因此， NCC 的东方部分的早白垩纪 lithospheric 披风的水和能从西方方面被归功于到和平的板的 subduction。因此，在 NCC 的破坏的主要动态因素是可能的和平的板的 subduction。
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