Evolution of Volcanic Rifted Margins: Synthesis of Leg 81 Results on the West Margin of Rockall Plateau
Evolution of Volcanic Rifted Margins: Synthesis of Leg 81 Results on the West Margin of Rockall Plateau
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火山裂谷边缘的演化：罗卡尔高原西缘第 81 段结果的综合
DOI：
10.2973/dsdp.proc.81.139.1984
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发表时间：
1984
期刊：
影响因子：
3.5
通讯作者：
J. Keene
中科院分区：
文献类型：
--
作者：
D. Roberts;J. Backman;A. Morton;J. Keene
在罗科尔高原西缘的第81航次钻探结果，结合现有的地球物理数据，提供了“倾斜反射”型被动边缘的第一条横断面。与以大型倾斜断块为特征的被动边缘不同，这种类型的特征是向海洋倾斜的反射层组，可能是裂谷边缘的主要类型。“倾斜反射层”边缘可分为四个结构区：洋壳（I区）、外部高压（II区）、倾斜反射层区域（III区）和近水平反射层的“向陆”区（IV区）。第81段样带在四个地点对区域II、III和IV进行了采样。在553和555两个地点穿透了倾斜反射层。在这些地点，倾斜反射体主要由陆上拉斑玄武岩流玄武岩和少量的夹层沉积物组成;流/流包和沉积物之间的高声阻抗对比可能导致反射。构成该序列大部分的流动玄武岩很可能完全在异常24 B-25的极性反转间隔内喷发，而且可能少得多。喷出玄武岩喷发之后是一个主要的火山碎屑火山活动阶段，该阶段在异常-24 B之前停止，并记录为整个东北大西洋的广泛灰落存款。人们提出了两种假说来解释倾斜反射层。Mutter等人（1982年）认为它们是由某种陆上海底扩张形成的。相反，Roberts等人（1979年）和Hinz（1981年）提出，它们是由伸展的大陆地壳上的大量喷发形成的。第81航次的结果证实了倾斜反射体下伏于洋壳或陆壳之下，但也没有否定这一点，尽管初步的重力解释倾向于后者。然而，第81航次的结果提供了数据，反过来又限制了形成这类余量的合理模式。
Leg 81 drilling results on the west margin of Rockall Plateau, combined with available geophysical data, provide the first transect of the "dipping reflector" type of passive margin. Unlike passive margins characterized by large tilted fault blocks, this type is characterized by an oceanward dipping suite of reflectors and may be the predominant type of rifted margin. The "dipping reflector" margin can be divided into four structural zones: the ocean crust (Zone I), an outer high (Zone II), the area of dipping reflectors (Zone III), and a "landward" zone (Zone IV) of subhorizontal reflectors. The Leg 81 transect sampled Zones II, III, and IV at four sites. The dipping reflectors were penetrated at two sites, 553 and 555. At these sites, the dipping reflectors consist mainly of subaerial tholeiitic flow basalts and minor interbedded sediments; high acoustic impedance contrasts between flows/ packets of flows and sediments may cause the reflections. The flow basalts making up much of the sequence were very probably erupted entirely within the Anomaly 24B-25 reversed polarity interval and possibly a good deal less. Effusive basalt eruption was succeeded by a major phase of pyroclastic volcanism that ceased just prior to Anomaly -24B and is recorded as a widespread ash-fall deposit throughout the NE Atlantic. Two hypotheses have been proposed to explain the dipping reflectors. Mutter et al. (1982) consider that they are formed by some kind of subaerial seafloor spreading. By contrast, Roberts et al. (1979) and Hinz (1981) propose that they have formed by voluminous eruption on stretching continental crust. The Leg 81 results neigher prove nor disprove that the dipping reflectors are underlain by oceanic or continental crust, although preliminary gravity interpretation favors the latter. However, the Leg 81 results provide data and in turn constraints on reasonable models for the formation of this type of margin.