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題 名 | The Evolution of Scientific Thinking in the Mantle Plume Concept: Criticism and Non-Plume Synthesis=地函柱學說科學概念的演進:評論和非地函柱模式 |
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作 者 | Smith,Alan D.; Lewis,Charles; | 書刊名 | 中國地質學會會刊 |
卷 期 | 42:1 1999.02[民88.02] |
頁 次 | 頁1-40 |
分類號 | 354.1 |
關鍵詞 | 板內火山活動; 非地函柱模型; 西向板塊遲滯; 大理石紋蛋糕模式; 應力場; Intraplate volcanism; Non-plume synthesis; Westward plate lag; Marblecake mantle; Stress fields; |
語 文 | 英文(English) |
中文摘要 | 地函柱模型不僅直接解釋了板內火山的成因和提供板塊構造學說的參考架構,它 同時也間接扮演了驅使板塊運動、地函對流及地殼一地函演化的原動力。地函柱模式看起來 很有彈性,故常被當成唯一的解釋,為了推展地函柱模式,常常會將其他模式視而不見。本 苯將結合在地函柱學說建立的過程中被摒棄的各種模式,以不同的觀點來建立現今的地球。 在這些模式中,板內火山活動導因於淺層軟流圈中含揮發性成分的礦物聚集(濕點 ),這些 含揮發性成分的礦物源自於受到拆層作用及熱侵蝕作用的大陸地函,而這些大陸地函在先前 隱沒或張裂事件中已遭受換質作用影響。在開闊洋盆中,鍊狀分佈的板內火山向外延伸均指 向大陸的縫合帶或區域的斯裂線,這指出這些板內火山的來源為受侵蝕的大陸地函,而由地 函對流帶往洋脊。在古老洋盆中的板內火山活動,其來源可追朔到大陸張裂或碰撞時所形成 的富化軟流圈,其往大陸東方移動乃是板塊有西向遲滯效應的結果。這種「轉動差異」為應 力傳經軟流圈時在地球內部產生的效應。含揮發成份的橄欖岩的固相線與軟流圈的絕熱線極 為相似,這使富化的軟流圈在受到剪力時會融熔,這種作用在整個軟流圈上的剪力,其來源 為中層圈和板塊因邊界力的驅使而往東移動,並不需要有熱異常(熱點 ) 的存在。 當板塊 運動方向與地函流動方向相反時,較流圈的上下層會分別往不同的方向流動,此時固定不動 的融熔聚積居則在上下層的交界形成,當岩石田應力軌跡與融熔聚積層相交時,岩漿上湧, 因而形成在地函深部有固定不動的岩漿來源的假象。當板塊運動方向與地函流動方向一致時 ,形成火山鍊。而由洋脊玄武岩的欽一鉿同位素來看,從元古代到現代,隱沒的海洋板塊已 經與虧損地函重新混合(大理石紋蛋糕模式),從科馬提岩的釹同位素中,我們可以知道在 太古代時存在有更虧損的地函,這與太古代時,隱沒的地般和地函問的混合並不均勻的說法 相符,這是由於當時地函溫度較高,隱沒的岩石圍在較淺的深度(<200 公里) 就融化,也 因此,隱沒的板塊並無法冷卻地函,而形成自我穩定的對流形式。由出現在洋脊周圍科馬提 片麻岩,顯示較高的地函溫度,這也可以說明為何太古代時的地殼體積較現代為大。本文所 提出的模式,比地函柱模式更簡單,並且沒有互相矛盾之處,顯示本模式並不是一個缺乏資 料佐證的模式,更具有發展的潛力。 |
英文摘要 | While directly offering a partial explanation for the origin of intraplate volcanism and a reference frame for plate tectonics, the mantle plume model also exerts an indirect control on models for the driving forces of plate motions, mantle convection and crust-mantle evolution. The apparent flexibility of the plume model has led to an impression that it represents the most viable explanation; however, promotion of the model has been at the expense of self-evaluation and consideration of non-plume models. The latter is illustrated by the fact that a contrasting picture of the Earth can be made by constructing a synthesis of alternatives dismissed at various stages in the evolution of the plume model. In this alternative synthesis, intraplate volcanism is derived from concentrations of volatile-bearing minerals (wetspots) at shallow levels in the asthenosphere which originate from delamination or thermal erosion of continental mantle which has been subjected to metasomatism during subduction or failed rifting events. In opening ocean basins, extrapolation of intraplate tracks into continental sutures/lineaments indicates an origin from continental mantle eroded and cycled toward the ridge axis. Intraplate volcanism in long-lived ocean basins can be traced to enriched domains of asthenosphere created during continental rifting or collision events, displaced to the east of the continent as a result of westward plate lag. This "differential rotation" is an internal effect within the Earth arising from the transmission of stresses through the asthenosphere. The proximity of the solidus for volatile-bearing peridotite to the asthenosphere adiabat makes the enriched domains susceptible to melting in response to pervasive shearing which results throughout the asthenosphere from flow imposed by eastward movement of the mesosphere and plate motion induced by boundary forces, without need to invoke thermal anomalies (plumes). Stationary melt-collection layers are formed at the intersection of opposing asthenospheric flow regimes when plate motion occurs in the opposite direction to mantle flow. Melt is released when lithospheric stress trajectories intersect the stationary layer, creating an illusion of fixed deep-seated melting anomalies. Formation of volcanic lines is favoured when plate motion occurs in the same direction as mantle flow. Correspondingly, the fate of subducted oceanic crust from the Proterozoic to Recent, as modelled from the Nd-Hf isotopic variation in MORB, has been re-mixing with the depleted mantle (marble-cake model). The existence of a very depleted mantle reservoir in the Early Archean is suggested from the Nd isotopic variation in komatiites, and is compatible with a tectonic scenario where crust was less efficiently re-mixed with the mantle on account of subducting lithosphere melting at shallow (<200km) depth due to high mantle temperatures. Slabs would therefore be less efficient for cooling the mantle, such that the thermal regime is self-stabilising. A large early crust is thus both a consequence, and cause of, hot mantle temperatures, and is compatible with a thermal regime marked by komatiite genesis along ocean ridges. That this alternative model is simpler than, and lacks the internal contradictions and paradoxes associated wi@h the plume model, suggests it was not an absence of viable alternative models nor inadequate data which led to the monopoly on geodynamic interpretations exerted by the plume model. |
本系統中英文摘要資訊取自各篇刊載內容。